WO2010012048A1 - Orthopaedic device - Google Patents

Orthopaedic device Download PDF

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Publication number
WO2010012048A1
WO2010012048A1 PCT/AU2009/001169 AU2009001169W WO2010012048A1 WO 2010012048 A1 WO2010012048 A1 WO 2010012048A1 AU 2009001169 W AU2009001169 W AU 2009001169W WO 2010012048 A1 WO2010012048 A1 WO 2010012048A1
Authority
WO
WIPO (PCT)
Prior art keywords
stem
orthopaedic implant
stem member
orientation
load
Prior art date
Application number
PCT/AU2009/001169
Other languages
French (fr)
Inventor
William Lindsay Walter
Shane Mccarthy Donohoo
Ronald Mark Gillies
Original Assignee
Silesco Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2008903855A external-priority patent/AU2008903855A0/en
Application filed by Silesco Pty Ltd filed Critical Silesco Pty Ltd
Publication of WO2010012048A1 publication Critical patent/WO2010012048A1/en

Links

Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
    • A61F2/00—Filters implantable into blood vessels; Prostheses, i.e. artificial substitutes or replacements for parts of the body; Appliances for connecting them with the body; Devices providing patency to, or preventing collapsing of, tubular structures of the body, e.g. stents
    • A61F2/02—Prostheses implantable into the body
    • A61F2/30—Joints
    • A61F2/32—Joints for the hip
    • A61F2/36—Femoral heads ; Femoral endoprostheses
    • A61F2/3662—Femoral shafts
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
    • A61F2/00—Filters implantable into blood vessels; Prostheses, i.e. artificial substitutes or replacements for parts of the body; Appliances for connecting them with the body; Devices providing patency to, or preventing collapsing of, tubular structures of the body, e.g. stents
    • A61F2/02—Prostheses implantable into the body
    • A61F2/30—Joints
    • A61F2002/30001—Additional features of subject-matter classified in A61F2/28, A61F2/30 and subgroups thereof
    • A61F2002/30003—Material related properties of the prosthesis or of a coating on the prosthesis
    • A61F2002/30004—Material related properties of the prosthesis or of a coating on the prosthesis the prosthesis being made from materials having different values of a given property at different locations within the same prosthesis
    • A61F2002/30014—Material related properties of the prosthesis or of a coating on the prosthesis the prosthesis being made from materials having different values of a given property at different locations within the same prosthesis differing in elasticity, stiffness or compressibility
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
    • A61F2/00—Filters implantable into blood vessels; Prostheses, i.e. artificial substitutes or replacements for parts of the body; Appliances for connecting them with the body; Devices providing patency to, or preventing collapsing of, tubular structures of the body, e.g. stents
    • A61F2/02—Prostheses implantable into the body
    • A61F2/30—Joints
    • A61F2002/30001—Additional features of subject-matter classified in A61F2/28, A61F2/30 and subgroups thereof
    • A61F2002/30316—The prosthesis having different structural features at different locations within the same prosthesis; Connections between prosthetic parts; Special structural features of bone or joint prostheses not otherwise provided for
    • A61F2002/30535—Special structural features of bone or joint prostheses not otherwise provided for
    • A61F2002/30594—Special structural features of bone or joint prostheses not otherwise provided for slotted, e.g. radial or meridian slot ending in a polar aperture, non-polar slots, horizontal or arcuate slots
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
    • A61F2/00—Filters implantable into blood vessels; Prostheses, i.e. artificial substitutes or replacements for parts of the body; Appliances for connecting them with the body; Devices providing patency to, or preventing collapsing of, tubular structures of the body, e.g. stents
    • A61F2/02—Prostheses implantable into the body
    • A61F2/30—Joints
    • A61F2/30721—Accessories
    • A61F2002/30733—Inserts placed into an endoprosthetic cavity, e.g. for modifying a material property
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
    • A61F2/00—Filters implantable into blood vessels; Prostheses, i.e. artificial substitutes or replacements for parts of the body; Appliances for connecting them with the body; Devices providing patency to, or preventing collapsing of, tubular structures of the body, e.g. stents
    • A61F2/02—Prostheses implantable into the body
    • A61F2/30—Joints
    • A61F2/32—Joints for the hip
    • A61F2/36—Femoral heads ; Femoral endoprostheses
    • A61F2/3662—Femoral shafts
    • A61F2002/3678—Geometrical features
    • A61F2002/3694—Geometrical features with longitudinal bores
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
    • A61F2250/00—Special features of prostheses classified in groups A61F2/00 - A61F2/26 or A61F2/82 or A61F9/00 or A61F11/00 or subgroups thereof
    • A61F2250/0014—Special features of prostheses classified in groups A61F2/00 - A61F2/26 or A61F2/82 or A61F9/00 or A61F11/00 or subgroups thereof having different values of a given property or geometrical feature, e.g. mechanical property or material property, at different locations within the same prosthesis
    • A61F2250/0018—Special features of prostheses classified in groups A61F2/00 - A61F2/26 or A61F2/82 or A61F9/00 or A61F11/00 or subgroups thereof having different values of a given property or geometrical feature, e.g. mechanical property or material property, at different locations within the same prosthesis differing in elasticity, stiffness or compressibility

Definitions

  • the present invention relates to orthopaedic implants and particularly to intramedullary stem components of certain orthopaedic implants.
  • Joint replacement is widely used to correct various types of joint injury, such as those caused by osteoarthritis, rheumatoid arthritis, necrosis and trauma to name a few.
  • a variety of prosthetic devices to replace joints in Hie body are known.
  • Some prostheses have an elongate stem which is implanted into the medullary cavity of a bone adjacent the joint, to anchor the prosthesis to the bone.
  • the device in the femoral component of a hip prosthesis, the device includes an elongate stem that has a ball formed at one end. The ball portion engages with an acetabular component of the prosthetic assembly.
  • the stem of the device is shaped for insertion into the medullary region of a femur adjacent to the joint and is secured therein by a number of means.
  • Such stems are also employed in tibial components of a knee or ankle joint prosthesis, the humeral component of a shoulder or elbow joint prosthesis or the ulnar component of an elbow joint replacement
  • a common problem with prostheses of this type is "stress shielding" of the surrounding bone.
  • stress shielding of the surrounding bone.
  • the implant is more rigid than the surrounding bone. Accordingly the implant takes most of the stress without transferring it to adjacent bone. Stress shielding is most problematic in the metaphyseal region.
  • a prosthesis fails to transfer the stress of a load to the metaphyseal bone and instead transfers the load along the length ' of the stem and towards the diaphysis of the bone, the metaphyseal bone may weaken.
  • a load exerted on the femoral head is typically transferred largely through the stem to the bone connection farthest from the joint, bypassing the intermediate or metaphyseal bone region surrounding the part of the prosthesis closest to the joint.
  • the gradual loss of bone support in this region of the prosthesis may lead to bone fracture or the increase in the bending load that must be borne by the stem which can lead to implant fatigue and failure.
  • the loss of bone stock makes revision surgery more difficult and less successful.
  • the present invention aims to address the problems associated with known prior art prostheses and particularly the stem components of said prostheses.
  • the present invention provides an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient, said stem member subjectable to a range of loads; and at least one modulating means to alter the relative stiffness of at least a portion of the stem member in response to a changing load on said stem member.
  • the at least one modulating means causes at least a portion of the elongate stem to change its stiffness between a relatively low stiffness and a relatively high stiffness or vice versa as the load changes.
  • stiffness includes the bending stiffness of at least a portion of the stem member.
  • the term also incorporates torsional stiffness and/or axial stiffness of at least a portion of the stem member.
  • proximal end and distal end are not limited to the anatomy of an individual such as used to define regions of bone.
  • the proximal end of the device may be positioned in a proximal region of bone (ie near the head of the bone) or at a distal region of the bone.
  • the modulating means by adjusting the stiffness of the stem member, provides a stem having variable load paths depending upon the load applied thereto.
  • the loads to which the stem member is subjected may vary.
  • the loads may include bending loads, axial loads and torsional loads.
  • the change in stiffness of the stem member may cause a change from a first load path to a second load path in the stem-bone construct when subjected to a particular load.
  • the first load path may be predominantly in a region of the stem adjacent the proximal end of the stem.
  • the second load path differs from the first load path and may comprise other regions of the stem thus distributing the load across the stem.
  • the first load path may be preferred under normal physiological loads whereas the second load path may be preferred under excessive loads.
  • At least a portion of the elongate stem adjacent to the proximal end may be positionable within a metaphyseal region of a bone.
  • the metaphyseal region of a bone is typically between the epiphyseal region and the diaphysis, or shaft, of the bone.
  • the distal end of the elongate stem may be positionable in a region of the diaphysis of the bone.
  • the load exerted on the elongate stem/bone structure is transferred to metaphyseal bone.
  • Such load transfer to the metaphyseal bone may enhance bone remodelling in said region and in the adjacent diaphysis, thereby preventing or mitigating stress shielding.
  • the modulating means causes the stiffness of the elongate stem to change to a second stiffness range.
  • the incremental load exerted on the elongate stem/bone structure is transferred to other parts of the stem/bone. That is, in said second stiffness range the stem has a different load path to the load path under normal physiological conditions.
  • the load path of the stem having said second stiffness range includes a greater length of the elongate stem towards the distal end of said stem. In this manner, when the stem is in situ in the medullary cavity of a bone, the excessive load may be transferred away from the metaphyseal region of the bone and towards the diaphyseal region of the bone.
  • the orthopaedic implant of the invention may comprise part of a joint prosthesis.
  • the orthopaedic implant may comprise part of a femoral component for a hip joint replacement prosthesis.
  • the stem may comprise part of a tibial component for either a knee or ankle joint replacement prosthesis.
  • the stem may be part of a humeral component of a shoulder or elbow joint replacement prosthesis or an ulnar component of an elbow joint replacement prosthesis.
  • the at least one modulating means may comprise a cartridge member/
  • the cartridge member may be receivable within a hollow region of the stem member.
  • the hollow region of the stem member is defined by an internal wall of the stem member.
  • the internal wall of the stem member typically defines a hollow which extends from an opening in the proximal end to a closed end positioned at a region of the stem member distal the proximal end.
  • the cartridge member is typically an elongate member which extends from a first end to a second end.
  • at least the first end of the cartridge member is securable to the stem at, or adjacent to, the proximal end of the stem.
  • at least the second end of the elongate cartridge member is spaced from the internal wall of the stem which defines the hollow.
  • only a region of the cartridge member adjacent to the first end is securable to the stem.
  • the remaining length of the cartridge of this embodiment may be spaced from the internal wall of the stem which defines the hollow.
  • a majority of the length of the elongate cartridge may be spaced from the internal wall during normal physiological loads.
  • the stifmess of the stem is within a range which allows the transfer of at least part of the load to the surrounding bone.
  • the load is transferred to a region of bone comprising the metaphyseal bone.
  • the stem When the stem is subjected to a non-physiological load including an extreme load which, in the embodiment of a femoral stem, may be associated with a stumble or fall, the stem may undergo a load induced deformation such that at least part of the previously spaced length of the cartridge is brought into engagement with the internal wall of the stem. Such engagement of the cartridge and the internal wall results in an increase in stiffness of the stem/cartridge assembly. Essentially, the stem is caused to become more rigid in nature. With the stifmess increased under the excess load, the load is largely transferred to other parts of the stem including towards the distal end of the stem. Preferably, such transfer of the excess load moves said excess load away from the metaphyseal region when the stem is in situ.
  • a non-physiological load including an extreme load which, in the embodiment of a femoral stem, may be associated with a stumble or fall
  • the stem may undergo a load induced deformation such that at least part of the previously spaced length of the cartridge is brought into engagement
  • the first end of the elongate cartridge comprises a screw thread on an outer surface thereof.
  • the cartridge may be screw threaded into the hollow by engagement of said screw thread with a complementary screw thread on the internal wall of the stem.
  • This embodiment enables a relatively straightforward positioning of the cartridge and may also facilitate replacement during subsequent revision surgery if required.
  • the modulating means of this embodiment may, therefore, be tailored for individual stems and patients.
  • the cartridge may vary in length, diameter and material to provide an optimal means to shift an excessive load on the stem.
  • the cartridge member is concentrically positioned within and relative to the stem member.
  • the cartridge member may be off-set relative to the main axis of the stem member.
  • the cartridge may comprise an elongate cylindrical member.
  • the cartridge may comprise a number of different cross-sectional configurations. A difference in cross-sectional configuration may provide different stiffness under bending about different axes.
  • the cartridge member may be solid or tubular in configuration.
  • the modulating means comprises a slotted length of the stem.
  • Said slotted length of the stem preferably comprises a tubular portion of the stem wherein the stem wall comprises at least one slot.
  • the slotted length of the stem comprises two longitudinal slots in the stem wall to define a first segment of the stem wall which is spaced from a • second segment of stem wall. Under normal physiological loads, the two segments of the tubular stem wall are spaced by the slots to provide a region of the stem having a stiffness to allow the transfer of a load to surrounding bone. Preferably under said normal physiological loads, at least part of the load is transferred to the metaphyseal region of the bone.
  • the segments of the stem wall are forced towards each other until, under an excessive load, the wall segments are forced together to form a closed, non-slotted structure which has a stiffness which is greater than the stiffness of the slotted structure.
  • the increase in the load is transferred by the now relatively more rigid structure of the stem to other regions of the stem to provide a load path that differs from the load path under normal physiological conditions.
  • an excess load is transferred away from the metaphyseal region of the stem towards the distal end of the stem when said stem is in situ.
  • the modulating means comprises an insert member.
  • the insert member may be housed within a tubular section of the stem member.
  • the insert member may engage a section of an inner wall of the tubular stem but has at least a portion which is spaced from said inner wall.
  • the insert member typically comprises a body having an engagement surface for engaging an inner surface of the stem member.
  • the body of the insert member may extend from said engagement surface and into the lumen formed by the tubular stem. While the engagement surface typically engages the inner surface of the stem member, the remainder of the insert member may be spaced from the inner surface of the tubular stem member. Under normal physiological loads, at least a portion of the insert member is spaced from an inner surface of the stem. With an increase in load and particularly a load that increases beyond the bounds of said normal physiological loads, the stem is caused to undergo a deformation such that increasingly more of the insert member engages the inner surface of the stem member. At extreme loads, the insert member may engage the entire inner surface of the stem member. The increase in the engagement interface between the insert member and the stem member results in an increase in stiffness thus providing an alternate load path for said increased or excess load.
  • the insert member may itself comprise a substantially tubular member to conform to the tubular structure of said section of the stem member.
  • the insert member is sized to fit within said tubular section of the stem but have at least a portion that is spaced from the inner wall of the stem.
  • the insert member may be connected at a discrete region to hold said insert member in position within the tubular stem member.
  • any deformation of the stem due to a load bearing on said stem may cause the engagement interface between the insert member and the stem member to increase thus increasing the stiffness of at least that length of the stem and altering the load path in the stem.
  • the stem comprises a core member positioned internal a slotted tubular length of the stem.
  • the wall sheaths of this embodiment are spaced from the core member.
  • the core member may be uniformly spaced from the wall sheaths members along the length of the slotted tubular stem. Alternatively, the spacing between said core member and the wall sheaths may increase or decrease along the length of the slotted tubular stem.
  • the wall sheaths are spaced from the core member to provide a stem having an optimal stiffness to allow the transfer of load to surrounding bone.
  • at least part of said load is transferred to the metaphyseal region of surrounding bone.
  • the wall stem is caused to undergo some degree or deformation such that the wall sheaths are caused to abut with the core member to provide at least a length of the stem which has an increased stiffness. With the stiffness increased, the relatively more rigid stem thus transfers the excess load to other regions of the stem.
  • the load is transferred away from the metaphyseal region and towards other regions of the stem and thus to other regions of surrounding bone.
  • the at least one modulating means may further comprise a tubular length of the stem comprising a series of longitudinal slots.
  • the tubular slots provide a load modulating means to alter load distribution when the stem is subjected to a varying torsional load.
  • said slotted tubular length may be configured to
  • the stem member of the present invention may comprise a single component member.
  • the stem member may be modular in nature, being made up of a plurality of modular components.
  • an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; at least one cartridge member positionable within a recess of said stem member, said recess defined by an internal wall of said stem member; wherein at least a length of said cartridge member is movable from a first orientation being spaced from said internal wall to a second orientation being in engagement with said internal wall.
  • an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising a tubular length of said stem member having opposed first and second segments of the stem member sidewaU which together define at least one slot in a sidewall thereof; wherein said first and second segments of the stem member wall are moveable between a first spaced orientation relative to one another to a second orientation wherein the first and second segments at least partially engage each other.
  • an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising an insert member housed within a tubular section of the stem member, said insert member having a first orientation relative to said stem member; wherein in said first orientation at least a portion of an outer surface of the insert member is in engagement with an inner wall of said stem member and wherein at least a portion of said outer surface of the insert member is spaced from said inner wall; and wherein as said stem member is subjected to an increasing load an increasing portion of said outer surface of said inert member is brought into engagement with said inner wall of the stem member.
  • an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising a length of said stem member, said length comprising a tubular length having a plurality of longitudinal slots in an outer wall to thereby define spaced sheaths of the stem member, said modulating means further comprising a core member positioned internal said length of the stem member; wherein said sheaths of the stem member are each spaced from said core member in a first orientation of the stem member and wherein as the stem member is subjected to an increase in load at least part of at least one sheath is caused to engage said core member.
  • the present invention provides a modulating member for use with a stem component of an orthopaedic implant, said modulating member comprising an elongate body extending from a first end to a second end and having securing means at or adjacent to the first end to secure said modulating member to a portion of the stem component, wherein at least a portion of said modulating member is positionable within a portion of the stem such that when said stem is subjected to a first load range the modulating member is in a first orientation relative to the stem component and when said stem component is subjected to a second, different load range, the modulating member is in a second orientation relative to the stem component and wherein when the modulating member is in said first orientation said stem component has a first stiffness and when the modulating member is in said second orientation said stem component has a second, different stiffness.
  • the modulating member of the sixth aspect may be receivable within a hollow region of the stem component.
  • the hollow region of the stem component is defined by an internal wall of the stem component
  • the internal wall of the stem component typically defines a hollow which extends from an opening in a proximal end of the stem to a closed end positioned at a region of the stem component distal the proximal end.
  • At least the first end of the modulating member is securable to the stem at, or adjacent to, the proximal end of the stem component Further, preferably at least the second end of the elongate modulating member is spaced from the internal wall of the stem which defines the hollow.
  • only a region of the modulating member adjacent to the first end is securable to the stem component.
  • the remaining length of the modulating member of this embodiment may be spaced from the internal wall of the stem which defines the hollow.
  • a majority of the length of the elongate modulating member may be spaced from the internal wall during normal physiological loads.
  • the stiffness of the stem is within a range which allows the transfer of at least part of the load to the surrounding bone.
  • the load is transferred to a region of bone comprising the metaphyseal bone.
  • the stem component When the stem component is subjected to a non-physiological load including an extreme load which, in the embodiment of a femoral stem, may be associated with a stumble or fell, the stem may undergo a load induced deformation such that at least part of the previously spaced length of the modulating member is brought into engagement with the internal wall of the stem.
  • a load induced deformation such that at least part of the previously spaced length of the modulating member is brought into engagement with the internal wall of the stem.
  • Such engagement of the modulating member and the internal wall results in an increase in stiffness of the stem component
  • the stem component is caused to become more rigid in nature.
  • the load is largely transferred to other parts of the stem component including towards a distal end of the stem.
  • such transfer of the excess load moves said excess load away from the metaphyseal region when the stem component is in situ.
  • the first end of the modulating member comprises a screw thread on an outer surface thereof.
  • the modulating member may be screw threaded into the hollow by engagement of said screw thread with a complementary screw thread on the internal wall of the stem component.
  • This embodiment enables a relatively straightforward positioning of the modulating member and may also facilitate replacement during subsequent revision surgery if required.
  • the modulating member of this embodiment may, therefore, be tailored for individual stem component and for individual patients.
  • the modulating member may vary in length, diameter and material to provide an optimal means to shift an excessive load on the stem.
  • the modulating member may comprise an elongate cylindrical member.
  • the modulating member may comprise a number of different cross- sectional configurations. A difference in cross-sectional configuration may provide a different stiffness under bending about different axes.
  • the modulating member may be solid or tubular in configuration.
  • Figure 1 is a schematic view of the stem member of the present invention in situ within a femur of a patient;
  • Figure 2a is a further schematic view of another embodiment of the invention.
  • Figure 2b is a cross-sectional view through I-I of Figure 2a;
  • Figure 3 is a further schematic representation of the embodiment of Figure 2a when under an extreme load
  • Figures 4a to 4d provide various cross-sectional configurations of the cartridge member
  • FIGS 5a to 5d provide further embodiments of the cartridge member
  • Figure 6a is a side view of a further embodiment of the invention.
  • Figure 6b is a cross-sectional view through II-II of Figure 6a;
  • Figure 7 is a schematic view of a further embodiment of the invention.
  • Figure 8a and 8b are cross-sectional views of a further embodiment of the invention.
  • Figures 8c, 8d and 8e are side sectional views of further embodiments of the invention.
  • Figure 9 is a further schematic view of another embodiment of the invention.
  • Figure 10 is a cross-sectional representation of the embodiment of Figure 9 through IV-IV, V-V and VI-VI.
  • the modelling threshold of bone is the genetically determined minimum effective strain range for mechanically controlled bone modelling. Where strains exceed this, modelling turns on; where strains stay below it modelling turns off.
  • the threshold is centred near 1000 microstrain in young adults which corresponds to a stress of about 20 MPa;
  • the microdamage threshold - the strain above which new microdamage escapes repair and begins to accumulate - centres around 3000 microstrain (60 Mpa).
  • the ultimate strength of bone is the load or strain that, when applied once, usually fractures bone. Normal lamellar bone has a fracture strength of 25000 microstrain, lower in the elderly and higher in the young. 25000 microstrain corresponds to 120 MPa in a healthy young adult.
  • the strength-safety factor is defined as how much stronger a bone is than needed to carry the typical largest voluntary loads.
  • the strength safety factor is estimated to be around 6 in a young healthy adult in terms of stress. This safety factor is significantly diminished in the elderly resulting in the high incidence of fracture in this age group.
  • a stem member of an orthopaedic implant according to a first aspect of the present invention is generally depicted as 10 in the accompanying drawings.
  • the stem member 10 comprises an elongate body 11 extending from a proximal end 12 to a distal end 13. As shown, the stem member 10 is positionable within a bone 100 of a patient. In Figure 1, the stem member 10 is positioned within the medullary region 101 of femur 102.
  • the elongate stem member 10 is, therefore, subjected to a range of loads when implanted within a bone 100 of a patient.
  • the stem member of this invention has a range of stiffness in response to said loads, ranging between a relatively low stiffness and a relatively high stiffness.
  • the stem member has at least one modulating means 9 to alter the stiffness of at least a portion of the elongate stem in response to a particular load.
  • modulating means 9 will be discussed below.
  • the stem member 10 is typically positioned with the medullary region 101 of a bone such that the proximal end and a region adjacent thereto are within the metaphyseal region 103 of the bone.
  • the metaphyseal region of a bone is the growing region of bone between the epiphyseal region 104 and the diaphysis 105, or shaft, of the bone 100.
  • the distal end 13 of the elongate stem is positioned in a region of the diaphysis 105 of the bone.
  • the modulating means of (he first aspect or the modulating member of the second aspect comprises a cartridge member 50.
  • the cartridge member 50 is receivable within a hollow region 14 of the stem member 10.
  • the hollow region 14 is defined by an internal wall 15 of the stem member which extends from an opening 16 in the proximal end 12 to a closed end 17 positioned at a region of the stem member distal the proximal end 12.
  • the cartridge member 50 is an elongate member which extends from a first end
  • the first end 31 is securable to the stem at, or adjacent to, the proximal end 12 of the stem.
  • the second end 32 of the elongate cartridge member 50 is spaced from the internal wall 15 of the stem.
  • the stiffness of the stem is within a range which allows • the transfer of at least part of the load to the surrounding bone 100.
  • the load is transferred to the metaphyseal region 103 of the bone 100.
  • the stem member 10 undergoes a load induced deformation part of the previously spaced length of the cartridge 50 is brought into engagement with the internal wall 15 of the stem, member 10.
  • Such engagement of the cartridge 50 and the internal wall 15 results in an increase in st ⁇ fiiess of the stem/cartridge assembly to transfer the load to other parts of the stem including towards the distal end 12 of the stem 10.
  • the excess load is spread from the metaphyseal region towards the diaphyseal region of bone as indicated by arrows B.
  • cartridge member 50 may comprise an elongate cylindrical member with the cross section depicted in Figure 4a, it may comprise other cross-sectional configuration such as those depicted in Figure 4b, 4c and 4d.
  • the cartridge member 50 may also vary along its length with examples provided in Figure 5a to 5d.
  • the modulating means 9 of the embodiment depicted in Figure 6 comprises a slotted length of the stem 18.
  • Said slotted length 18 of the stem comprises a tubular portion of the stem comprising two longitudinal slots 19a and 19b in the stem wall 20.
  • the slots 19a and 19b define a first segment 21a of the stem wall 20 which is spaced from a second segment 21b of stem wall 20.
  • the two segments 21a and 21b of the tubular stem wall 20 are spaced by the slots 19a and 19b to provide a region of the stem having a stiffness to allow the transfer of a load to surrounding bone.
  • the insert member 22 is positioned within a tubular section of the stem member.
  • the insert member 22 engages a section of an inner wall 23 of the tubular stem 10 but has at least a portion which is spaced from said inner wall by gap 24.
  • the insert member 22 comprises a body 25 having an engagement surface 26 for engaging an inner wall 23 of the stem member 10. At least part of the insert member is spaced from the inner wall 23 of the tubular stem member under normal physiological loads.
  • the insert member 22 is substantially linear and is engaged to a lateral wall of the stem member 10.
  • the insert member 22 is substantially tubular in configuration and is shown in its configuration under normal physiological loads. In this configuration, part of the insert member 22 is spaced from the inner wall 23 by gap 24. Under extreme loads, the gap narrows until the entire insert member 22 engages the inner wall 23 as depicted in Figure 8b.
  • the stem member 10 comprises a core member 27 positioned within a slotted tubular length of the stem 10.
  • the wall sheaths 28 are spaced from the core member 27.
  • the core member 27 may be uniformly spaced from the wall sheaths along the length of the slotted tubular stem or as shown by the cross- sectional representations of the stem in Figure 10, the spacing between said core member 27 and the wall sheaths 28 increases along the length of the slotted tubular stem 10 from a proximal region shown as IV-IV to the distal end 13 shown as VI-VL
  • the wall sheaths 28 are spaced from the core member to provide a stem having an optimal stiffness to allow the transfer of load to surrounding bone. Again, it is preferred that at least part of the load is transferred to the metaphyseal region 103 of the surrounding bone.
  • the wall stem is caused to undergo some degree or deformation such that the wall sheaths 28 are caused to abut with the core member 27 to provide at least a length of the stem which has an increased stiffness. With the stiffness increased, the relatively more rigid stem thus transfers the excess load to other regions of the stem.
  • a further advantage of the embodiment depicted in Figure 10 is that the spacing between the sheaths and the core towards the distal end 13 is greater than the spacing further up the stem. This provides a relatively "soft tip" to the stem and prevents tip pain associated with conventional stem inserts.
  • the modulating means 9 is a tubular length of the stem comprising a series of longitudinal slots 31 as depicted in Figure 11.
  • the tubular slots 31 alter load distribution when the stem is subjected to a varying torsional load.
  • the slotted tubular length of the stem transfers at least some of the load to the surrounding bone and preferably to the metaphyseal bone region. If the torsional load increases beyond normal physiological levels and becomes an extreme torsional load, the slotted tubular length of the stem may undergo deformation to close the slots thus creating a solid, non-slotted tubular length of the stem.
  • the non-slotted structure provides a transfer of the applied torsional load to other regions of the stem. The transfer of torsional load over a greater area reduces the likelihood of warping associated with the transfer of an extreme torsional load over a small distance of the stem.
  • a further feature of the embodiment depicted in Figure 11 is that the region adjacent the distal end and labelled 32 has a roughened surface.
  • the roughened surface provides a means to better control the location of force transfer.

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  • Prostheses (AREA)

Abstract

An orthopaedic implant which has a modulating means to alter the stifihess of part of the implant when subjected to varyiαg loads.

Description

"Orthopaedic device"
Field of the Invention
The present invention relates to orthopaedic implants and particularly to intramedullary stem components of certain orthopaedic implants.
Background Art
Joint replacement is widely used to correct various types of joint injury, such as those caused by osteoarthritis, rheumatoid arthritis, necrosis and trauma to name a few. A variety of prosthetic devices to replace joints in Hie body are known. Some prostheses have an elongate stem which is implanted into the medullary cavity of a bone adjacent the joint, to anchor the prosthesis to the bone. For example, in the femoral component of a hip prosthesis, the device includes an elongate stem that has a ball formed at one end. The ball portion engages with an acetabular component of the prosthetic assembly. The stem of the device is shaped for insertion into the medullary region of a femur adjacent to the joint and is secured therein by a number of means. Such stems are also employed in tibial components of a knee or ankle joint prosthesis, the humeral component of a shoulder or elbow joint prosthesis or the ulnar component of an elbow joint replacement
The introduction of a foreign prosthesis into a bone has a number of adverse effects on the surrounding bone. For example, a common problem with prostheses of this type is "stress shielding" of the surrounding bone. To allow the bone surrounding the prosthesis to continue to grow, the bone must be exposed to some level of stress. Without such exposure the natural balance of bone growth and resorption is disrupted and the surrounding bone is gradually resorbed over time. Such stress shielding of the surrounding bone results from the fact that the implant is more rigid than the surrounding bone. Accordingly the implant takes most of the stress without transferring it to adjacent bone. Stress shielding is most problematic in the metaphyseal region. If a prosthesis fails to transfer the stress of a load to the metaphyseal bone and instead transfers the load along the length' of the stem and towards the diaphysis of the bone, the metaphyseal bone may weaken. For example, in some conventional femoral stems, a load exerted on the femoral head is typically transferred largely through the stem to the bone connection farthest from the joint, bypassing the intermediate or metaphyseal bone region surrounding the part of the prosthesis closest to the joint. The gradual loss of bone support in this region of the prosthesis may lead to bone fracture or the increase in the bending load that must be borne by the stem which can lead to implant fatigue and failure. Furthermore, the loss of bone stock makes revision surgery more difficult and less successful.
Another problem resulting from stiff stems in the bone canal is "thigh pain' or 'stem tip pain'. The stiffness mismatch between the prosthesis and the bone causes a stress riser at the tip of the stem which produces pain.
Some attempts have been made to reduce the stiffness of a stem to allow for the transfer of physiological loads to surrounding bone (US Patent No 5,176,710). However, with such a stem, a problem arises when an excessive load or non- physiological load is applied. Such an extreme load would also be transferred to the surrounding bone increasing the risk of fracture to the bone.
While load transfer to the surrounding bone at normal physiological loads is clearly desirable, it is important that an excessive load such as the stress exerted on the femoral component of a hip prosthesis during a fall or stumble is not substantially transferred to the surrounding bone as this could result in facture of the bone.
The present invention aims to address the problems associated with known prior art prostheses and particularly the stem components of said prostheses.
Summary of the Invention
In a first aspect, the present invention provides an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient, said stem member subjectable to a range of loads; and at least one modulating means to alter the relative stiffness of at least a portion of the stem member in response to a changing load on said stem member.' Typically, the at least one modulating means causes at least a portion of the elongate stem to change its stiffness between a relatively low stiffness and a relatively high stiffness or vice versa as the load changes.
The term "stiffness" includes the bending stiffness of at least a portion of the stem member. The term also incorporates torsional stiffness and/or axial stiffness of at least a portion of the stem member.
The terms "proximal end" and "distal end" are not limited to the anatomy of an individual such as used to define regions of bone. The proximal end of the device may be positioned in a proximal region of bone (ie near the head of the bone) or at a distal region of the bone.
The modulating means, by adjusting the stiffness of the stem member, provides a stem having variable load paths depending upon the load applied thereto.
The loads to which the stem member is subjected may vary. The loads may include bending loads, axial loads and torsional loads.
The change in stiffness of the stem member may cause a change from a first load path to a second load path in the stem-bone construct when subjected to a particular load. In this embodiment, the first load path may be predominantly in a region of the stem adjacent the proximal end of the stem. The second load path differs from the first load path and may comprise other regions of the stem thus distributing the load across the stem. The first load path may be preferred under normal physiological loads whereas the second load path may be preferred under excessive loads.
At least a portion of the elongate stem adjacent to the proximal end may be positionable within a metaphyseal region of a bone. The metaphyseal region of a bone is typically between the epiphyseal region and the diaphysis, or shaft, of the bone. The distal end of the elongate stem may be positionable in a region of the diaphysis of the bone. When the elongate stem/bone structure is subjected to normal physiological loads, the elongate stem typically has a first stiffness range at which at least some of the load is shifted to the surrounding bone.
Preferably, at least some of the load exerted on the elongate stem/bone structure is transferred to metaphyseal bone. Such load transfer to the metaphyseal bone may enhance bone remodelling in said region and in the adjacent diaphysis, thereby preventing or mitigating stress shielding. If the load increases substantially beyond said normal physiological loads, the modulating means causes the stiffness of the elongate stem to change to a second stiffness range. At said second stiffness range, the incremental load exerted on the elongate stem/bone structure is transferred to other parts of the stem/bone. That is, in said second stiffness range the stem has a different load path to the load path under normal physiological conditions. Typically, the load path of the stem having said second stiffness range includes a greater length of the elongate stem towards the distal end of said stem. In this manner, when the stem is in situ in the medullary cavity of a bone, the excessive load may be transferred away from the metaphyseal region of the bone and towards the diaphyseal region of the bone.
The orthopaedic implant of the invention may comprise part of a joint prosthesis. The orthopaedic implant may comprise part of a femoral component for a hip joint replacement prosthesis. Alternatively, the stem may comprise part of a tibial component for either a knee or ankle joint replacement prosthesis. Still further, the stem may be part of a humeral component of a shoulder or elbow joint replacement prosthesis or an ulnar component of an elbow joint replacement prosthesis.
The at least one modulating means may comprise a cartridge member/ The cartridge member may be receivable within a hollow region of the stem member. Typically, the hollow region of the stem member is defined by an internal wall of the stem member. The internal wall of the stem member typically defines a hollow which extends from an opening in the proximal end to a closed end positioned at a region of the stem member distal the proximal end.
The cartridge member is typically an elongate member which extends from a first end to a second end. Preferably, at least the first end of the cartridge member is securable to the stem at, or adjacent to, the proximal end of the stem. Further, preferably at least the second end of the elongate cartridge member is spaced from the internal wall of the stem which defines the hollow.
In a preferred embodiment, only a region of the cartridge member adjacent to the first end is securable to the stem. The remaining length of the cartridge of this embodiment may be spaced from the internal wall of the stem which defines the hollow. In this embodiment, a majority of the length of the elongate cartridge may be spaced from the internal wall during normal physiological loads. In such a configuration, the stifmess of the stem is within a range which allows the transfer of at least part of the load to the surrounding bone. Preferably, the load is transferred to a region of bone comprising the metaphyseal bone.
When the stem is subjected to a non-physiological load including an extreme load which, in the embodiment of a femoral stem, may be associated with a stumble or fall, the stem may undergo a load induced deformation such that at least part of the previously spaced length of the cartridge is brought into engagement with the internal wall of the stem. Such engagement of the cartridge and the internal wall results in an increase in stiffness of the stem/cartridge assembly. Essentially, the stem is caused to become more rigid in nature. With the stifmess increased under the excess load, the load is largely transferred to other parts of the stem including towards the distal end of the stem. Preferably, such transfer of the excess load moves said excess load away from the metaphyseal region when the stem is in situ.
Preferably, the first end of the elongate cartridge comprises a screw thread on an outer surface thereof. The cartridge may be screw threaded into the hollow by engagement of said screw thread with a complementary screw thread on the internal wall of the stem. This embodiment enables a relatively straightforward positioning of the cartridge and may also facilitate replacement during subsequent revision surgery if required. The modulating means of this embodiment may, therefore, be tailored for individual stems and patients. The cartridge may vary in length, diameter and material to provide an optimal means to shift an excessive load on the stem.
Other means of securing the cartridge to the walls of the hollow include press- fitting the cartridge member into the hollow and moulding a first end of the cartridge to the internal wall surrounding the opening in the proximal end of the stem. Preferably, the cartridge member is concentrically positioned within and relative to the stem member. Alternatively, the cartridge member may be off-set relative to the main axis of the stem member.
The cartridge may comprise an elongate cylindrical member. Alternatively, the cartridge may comprise a number of different cross-sectional configurations. A difference in cross-sectional configuration may provide different stiffness under bending about different axes. The cartridge member may be solid or tubular in configuration.
In a further embodiment, the modulating means comprises a slotted length of the stem. Said slotted length of the stem preferably comprises a tubular portion of the stem wherein the stem wall comprises at least one slot. In a preferred embodiment, the slotted length of the stem comprises two longitudinal slots in the stem wall to define a first segment of the stem wall which is spaced from a • second segment of stem wall. Under normal physiological loads, the two segments of the tubular stem wall are spaced by the slots to provide a region of the stem having a stiffness to allow the transfer of a load to surrounding bone. Preferably under said normal physiological loads, at least part of the load is transferred to the metaphyseal region of the bone. As the load increases on the stem, so the segments of the stem wall are forced towards each other until, under an excessive load, the wall segments are forced together to form a closed, non-slotted structure which has a stiffness which is greater than the stiffness of the slotted structure. .The increase in the load is transferred by the now relatively more rigid structure of the stem to other regions of the stem to provide a load path that differs from the load path under normal physiological conditions. Preferably, as before, an excess load is transferred away from the metaphyseal region of the stem towards the distal end of the stem when said stem is in situ.
In another embodiment, the modulating means comprises an insert member. The insert member may be housed within a tubular section of the stem member. The insert member may engage a section of an inner wall of the tubular stem but has at least a portion which is spaced from said inner wall.
In one embodiment, the insert member typically comprises a body having an engagement surface for engaging an inner surface of the stem member. The body of the insert member may extend from said engagement surface and into the lumen formed by the tubular stem. While the engagement surface typically engages the inner surface of the stem member, the remainder of the insert member may be spaced from the inner surface of the tubular stem member. Under normal physiological loads, at least a portion of the insert member is spaced from an inner surface of the stem. With an increase in load and particularly a load that increases beyond the bounds of said normal physiological loads, the stem is caused to undergo a deformation such that increasingly more of the insert member engages the inner surface of the stem member. At extreme loads, the insert member may engage the entire inner surface of the stem member. The increase in the engagement interface between the insert member and the stem member results in an increase in stiffness thus providing an alternate load path for said increased or excess load.
In a further embodiment, the insert member may itself comprise a substantially tubular member to conform to the tubular structure of said section of the stem member. In this embodiment, the insert member is sized to fit within said tubular section of the stem but have at least a portion that is spaced from the inner wall of the stem. In one embodiment, there may be a gap between substantially the entire circumference of the insert member and the inner surface of the stem member. In this embodiment, the insert member may be connected at a discrete region to hold said insert member in position within the tubular stem member. As in the embodiment above, any deformation of the stem due to a load bearing on said stem may cause the engagement interface between the insert member and the stem member to increase thus increasing the stiffness of at least that length of the stem and altering the load path in the stem.
In a still further embodiment, the stem comprises a core member positioned internal a slotted tubular length of the stem. The core member and a plurality of separate, spaced wall sheaths formed by longitudinal slots in the wall of the stem together comprise the modulating means. The wall sheaths of this embodiment are spaced from the core member. The core member may be uniformly spaced from the wall sheaths members along the length of the slotted tubular stem. Alternatively, the spacing between said core member and the wall sheaths may increase or decrease along the length of the slotted tubular stem.
In the above embodiment, under normal physiological loads, the wall sheaths are spaced from the core member to provide a stem having an optimal stiffness to allow the transfer of load to surrounding bone. Again, it is preferred that at least part of said load is transferred to the metaphyseal region of surrounding bone. Under increasing load, the wall stem is caused to undergo some degree or deformation such that the wall sheaths are caused to abut with the core member to provide at least a length of the stem which has an increased stiffness. With the stiffness increased, the relatively more rigid stem thus transfers the excess load to other regions of the stem. Preferably the load is transferred away from the metaphyseal region and towards other regions of the stem and thus to other regions of surrounding bone.
The greater the distance of the wall sheaths from the core member, the greater the deflection (and therefore the greater the load) required to cause the wall sheaths to engage the core member.
In a further embodiment, the at least one modulating means may further comprise a tubular length of the stem comprising a series of longitudinal slots. In this embodiment, the tubular slots provide a load modulating means to alter load distribution when the stem is subjected to a varying torsional load. When subjected to a normal physiological torsional load, said slotted tubular length may be configured to
" transfer at least some of the load to the surrounding bone and preferably to the metaphyseal bone region. If the torsional load increases beyond normal physiological levels and becomes an extreme torsional load, the slotted tubular length of the stem may undergo deformation to close the slots thus creating a solid, non-slotted tubular length of the stem. The non-slotted structure provides a transfer of the applied torsional load to other regions of the stem. The load path of the slotted length of stem therefore differs from the load path of the non-slotted length of stem.
The stem member of the present invention may comprise a single component member. Alternatively, the stem member may be modular in nature, being made up of a plurality of modular components.
In a second aspect, there is provided an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; at least one cartridge member positionable within a recess of said stem member, said recess defined by an internal wall of said stem member; wherein at least a length of said cartridge member is movable from a first orientation being spaced from said internal wall to a second orientation being in engagement with said internal wall.
hi a third aspect, there is provided an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising a tubular length of said stem member having opposed first and second segments of the stem member sidewaU which together define at least one slot in a sidewall thereof; wherein said first and second segments of the stem member wall are moveable between a first spaced orientation relative to one another to a second orientation wherein the first and second segments at least partially engage each other.
In a fourth aspect, there is provided an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising an insert member housed within a tubular section of the stem member, said insert member having a first orientation relative to said stem member; wherein in said first orientation at least a portion of an outer surface of the insert member is in engagement with an inner wall of said stem member and wherein at least a portion of said outer surface of the insert member is spaced from said inner wall; and wherein as said stem member is subjected to an increasing load an increasing portion of said outer surface of said inert member is brought into engagement with said inner wall of the stem member.
In a fifth aspect, there is provided an orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising a length of said stem member, said length comprising a tubular length having a plurality of longitudinal slots in an outer wall to thereby define spaced sheaths of the stem member, said modulating means further comprising a core member positioned internal said length of the stem member; wherein said sheaths of the stem member are each spaced from said core member in a first orientation of the stem member and wherein as the stem member is subjected to an increase in load at least part of at least one sheath is caused to engage said core member.
In a sixth aspect, the present invention provides a modulating member for use with a stem component of an orthopaedic implant, said modulating member comprising an elongate body extending from a first end to a second end and having securing means at or adjacent to the first end to secure said modulating member to a portion of the stem component, wherein at least a portion of said modulating member is positionable within a portion of the stem such that when said stem is subjected to a first load range the modulating member is in a first orientation relative to the stem component and when said stem component is subjected to a second, different load range, the modulating member is in a second orientation relative to the stem component and wherein when the modulating member is in said first orientation said stem component has a first stiffness and when the modulating member is in said second orientation said stem component has a second, different stiffness.
The modulating member of the sixth aspect may be receivable within a hollow region of the stem component. Typically, the hollow region of the stem component is defined by an internal wall of the stem component The internal wall of the stem component typically defines a hollow which extends from an opening in a proximal end of the stem to a closed end positioned at a region of the stem component distal the proximal end.
Preferably, at least the first end of the modulating member is securable to the stem at, or adjacent to, the proximal end of the stem component Further, preferably at least the second end of the elongate modulating member is spaced from the internal wall of the stem which defines the hollow.
In a preferred embodiment, only a region of the modulating member adjacent to the first end is securable to the stem component. The remaining length of the modulating member of this embodiment may be spaced from the internal wall of the stem which defines the hollow. Li this embodiment, a majority of the length of the elongate modulating member may be spaced from the internal wall during normal physiological loads. In such an orientation, the stiffness of the stem is within a range which allows the transfer of at least part of the load to the surrounding bone. Preferably, the load is transferred to a region of bone comprising the metaphyseal bone.
When the stem component is subjected to a non-physiological load including an extreme load which, in the embodiment of a femoral stem, may be associated with a stumble or fell, the stem may undergo a load induced deformation such that at least part of the previously spaced length of the modulating member is brought into engagement with the internal wall of the stem. Such engagement of the modulating member and the internal wall results in an increase in stiffness of the stem component Essentially, the stem component is caused to become more rigid in nature. With the stiffness increased under the excess load, the load is largely transferred to other parts of the stem component including towards a distal end of the stem. Preferably, such transfer of the excess load moves said excess load away from the metaphyseal region when the stem component is in situ.
Preferably, the first end of the modulating member comprises a screw thread on an outer surface thereof. The modulating member may be screw threaded into the hollow by engagement of said screw thread with a complementary screw thread on the internal wall of the stem component. This embodiment enables a relatively straightforward positioning of the modulating member and may also facilitate replacement during subsequent revision surgery if required. The modulating member of this embodiment may, therefore, be tailored for individual stem component and for individual patients. The modulating member may vary in length, diameter and material to provide an optimal means to shift an excessive load on the stem.
The modulating member may comprise an elongate cylindrical member. Alternatively, the modulating member may comprise a number of different cross- sectional configurations. A difference in cross-sectional configuration may provide a different stiffness under bending about different axes. The modulating member may be solid or tubular in configuration.
Other means of securing the modulating member to the walls of the hollow include press-fitting the modulating member into the hollow or moulding a first end of the modulating member to the internal wall surrounding the opening in the proximal end of the stem component. Brief Description of the Drawings
Figure 1 is a schematic view of the stem member of the present invention in situ within a femur of a patient;
Figure 2a is a further schematic view of another embodiment of the invention;
Figure 2b is a cross-sectional view through I-I of Figure 2a;
Figure 3 is a further schematic representation of the embodiment of Figure 2a when under an extreme load;
Figures 4a to 4d provide various cross-sectional configurations of the cartridge member;
Figures 5a to 5d provide further embodiments of the cartridge member;
Figure 6a is a side view of a further embodiment of the invention; .
Figure 6b is a cross-sectional view through II-II of Figure 6a;
Figure 7 is a schematic view of a further embodiment of the invention;
Figure 8a and 8b are cross-sectional views of a further embodiment of the invention;
Figures 8c, 8d and 8e are side sectional views of further embodiments of the invention;
Figure 9 is a further schematic view of another embodiment of the invention;
Figure 10 is a cross-sectional representation of the embodiment of Figure 9 through IV-IV, V-V and VI-VI.
Detailed Description of a Preferred Mode of Carrying out the Invention The modelling threshold of bone is the genetically determined minimum effective strain range for mechanically controlled bone modelling. Where strains exceed this, modelling turns on; where strains stay below it modelling turns off. The threshold is centred near 1000 microstrain in young adults which corresponds to a stress of about 20 MPa; The microdamage threshold - the strain above which new microdamage escapes repair and begins to accumulate - centres around 3000 microstrain (60 Mpa). The ultimate strength of bone is the load or strain that, when applied once, usually fractures bone. Normal lamellar bone has a fracture strength of 25000 microstrain, lower in the elderly and higher in the young. 25000 microstrain corresponds to 120 MPa in a healthy young adult. The strength-safety factor is defined as how much stronger a bone is than needed to carry the typical largest voluntary loads. The strength safety factor is estimated to be around 6 in a young healthy adult in terms of stress. This safety factor is significantly diminished in the elderly resulting in the high incidence of fracture in this age group.
Different activities produce different loads expressed as multiples of body weight. Slow walking produces a joint reactive force of 2.5 times body weight at the hip; fast walking 3.5 times body weight; stumbling can produce loads of 10 times body weight and falls produce loads that exceed this. Loading of a hip causes structural actions in the hip prosthesis which need to be transmitted to the bone in the form of bending and torsion. Bending and torsion result in different combinations of compressive and tensile stress being applied to the stem and transmitted to the bone. The embodiments of this invention aim to distribute these loads in an optimal way across the range of loads.
A stem member of an orthopaedic implant according to a first aspect of the present invention is generally depicted as 10 in the accompanying drawings. The stem member 10 comprises an elongate body 11 extending from a proximal end 12 to a distal end 13. As shown, the stem member 10 is positionable within a bone 100 of a patient. In Figure 1, the stem member 10 is positioned within the medullary region 101 of femur 102.
The elongate stem member 10 is, therefore, subjected to a range of loads when implanted within a bone 100 of a patient. The stem member of this invention has a range of stiffness in response to said loads, ranging between a relatively low stiffness and a relatively high stiffness. The stem member has at least one modulating means 9 to alter the stiffness of at least a portion of the elongate stem in response to a particular load. Various examples of modulating means 9 will be discussed below.
The stem member 10 is typically positioned with the medullary region 101 of a bone such that the proximal end and a region adjacent thereto are within the metaphyseal region 103 of the bone. The metaphyseal region of a bone is the growing region of bone between the epiphyseal region 104 and the diaphysis 105, or shaft, of the bone 100. The distal end 13 of the elongate stem is positioned in a region of the diaphysis 105 of the bone.
As shown in Figure 2, the modulating means of (he first aspect or the modulating member of the second aspect comprises a cartridge member 50. The cartridge member 50 is receivable within a hollow region 14 of the stem member 10. The hollow region 14 is defined by an internal wall 15 of the stem member which extends from an opening 16 in the proximal end 12 to a closed end 17 positioned at a region of the stem member distal the proximal end 12.
The cartridge member 50 is an elongate member which extends from a first end
31 to a second end 32. The first end 31 is securable to the stem at, or adjacent to, the proximal end 12 of the stem. The second end 32 of the elongate cartridge member 50 is spaced from the internal wall 15 of the stem.
As shown in Figure 2, only a region of the cartridge member adjacent to the first end 31 is securable to the stem member 10. The remaining length of the cartridge is spaced from the internal wall 15. In this embodiment, a majority of the length of the elongate cartridge is spaced from the internal wall 15 during normal physiological loads. In such a configuration, the stiffness of the stem is within a range which allows • the transfer of at least part of the load to the surrounding bone 100. Preferably, the load is transferred to the metaphyseal region 103 of the bone 100.
When the stem is subjected to a non-physiological load depicted by arrow A in
Figure 3, the stem member 10 undergoes a load induced deformation part of the previously spaced length of the cartridge 50 is brought into engagement with the internal wall 15 of the stem, member 10. Such engagement of the cartridge 50 and the internal wall 15 results in an increase in stϋfiiess of the stem/cartridge assembly to transfer the load to other parts of the stem including towards the distal end 12 of the stem 10. In Figure 3, the excess load is spread from the metaphyseal region towards the diaphyseal region of bone as indicated by arrows B.
While the cartridge member 50 may comprise an elongate cylindrical member with the cross section depicted in Figure 4a, it may comprise other cross-sectional configuration such as those depicted in Figure 4b, 4c and 4d.
The cartridge member 50 may also vary along its length with examples provided in Figure 5a to 5d.
The modulating means 9 of the embodiment depicted in Figure 6 comprises a slotted length of the stem 18. Said slotted length 18 of the stem comprises a tubular portion of the stem comprising two longitudinal slots 19a and 19b in the stem wall 20. The slots 19a and 19b define a first segment 21a of the stem wall 20 which is spaced from a second segment 21b of stem wall 20. Under normal physiological loads, the two segments 21a and 21b of the tubular stem wall 20 are spaced by the slots 19a and 19b to provide a region of the stem having a stiffness to allow the transfer of a load to surrounding bone. As the load increases on the stem member 10, so the segments 21a and 21b of the stem wall 20 are forced towards each other until, under an excessive load, the wall segments are forced together to form a closed, non-slotted structure which has a stifmess which is greater than the stiffness of the slotted structure.
In the embodiment of the invention depicted in Figure 7, the modulating means
9 comprises an insert member 22. The insert member 22 is positioned within a tubular section of the stem member. The insert member 22 engages a section of an inner wall 23 of the tubular stem 10 but has at least a portion which is spaced from said inner wall by gap 24.
The insert member 22 comprises a body 25 having an engagement surface 26 for engaging an inner wall 23 of the stem member 10. At least part of the insert member is spaced from the inner wall 23 of the tubular stem member under normal physiological loads. In Figure 7, the insert member 22 is substantially linear and is engaged to a lateral wall of the stem member 10. In Figure 8a, the insert member 22 is substantially tubular in configuration and is shown in its configuration under normal physiological loads. In this configuration, part of the insert member 22 is spaced from the inner wall 23 by gap 24. Under extreme loads, the gap narrows until the entire insert member 22 engages the inner wall 23 as depicted in Figure 8b.
In a further embodiment, the stem member 10 comprises a core member 27 positioned within a slotted tubular length of the stem 10. The core member 27 and a plurality of separate, spaced wall sheaths 28 formed by longitudinal slots 29 in the wall of the stem together comprise the modulating means 9. The wall sheaths 28 are spaced from the core member 27. The core member 27 may be uniformly spaced from the wall sheaths along the length of the slotted tubular stem or as shown by the cross- sectional representations of the stem in Figure 10, the spacing between said core member 27 and the wall sheaths 28 increases along the length of the slotted tubular stem 10 from a proximal region shown as IV-IV to the distal end 13 shown as VI-VL
Under normal physiological loads, the wall sheaths 28 are spaced from the core member to provide a stem having an optimal stiffness to allow the transfer of load to surrounding bone. Again, it is preferred that at least part of the load is transferred to the metaphyseal region 103 of the surrounding bone. Under increasing load, the wall stem is caused to undergo some degree or deformation such that the wall sheaths 28 are caused to abut with the core member 27 to provide at least a length of the stem which has an increased stiffness. With the stiffness increased, the relatively more rigid stem thus transfers the excess load to other regions of the stem.
The greater the distance of the wall sheaths 28 from the core member, the greater the load required to cause the wall sheaths to engage the core member. A further advantage of the embodiment depicted in Figure 10 is that the spacing between the sheaths and the core towards the distal end 13 is greater than the spacing further up the stem. This provides a relatively "soft tip" to the stem and prevents tip pain associated with conventional stem inserts.
In a further embodiment, the modulating means 9 is a tubular length of the stem comprising a series of longitudinal slots 31 as depicted in Figure 11. In this embodiment, the tubular slots 31 alter load distribution when the stem is subjected to a varying torsional load. When subjected to a normal physiological torsional load, the slotted tubular length of the stem transfers at least some of the load to the surrounding bone and preferably to the metaphyseal bone region. If the torsional load increases beyond normal physiological levels and becomes an extreme torsional load, the slotted tubular length of the stem may undergo deformation to close the slots thus creating a solid, non-slotted tubular length of the stem. The non-slotted structure provides a transfer of the applied torsional load to other regions of the stem. The transfer of torsional load over a greater area reduces the likelihood of warping associated with the transfer of an extreme torsional load over a small distance of the stem.
A further feature of the embodiment depicted in Figure 11 is that the region adjacent the distal end and labelled 32 has a roughened surface. The roughened surface provides a means to better control the location of force transfer.
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:
1. An orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient, said stem member subjectable to a range of loads; and at least one modulating means to alter the relative stiffness of at least a portion of the stem member in response to a changing load on said stem member.
2. The orthopaedic implant of claim 1 wherein said at least one modulating means causes at least said portion of the stem member to change from a relatively low stiffness to a relatively high stiffness as the load changes.
3. The orthopaedic implant of claim 1 wherein said at least one modulating means alters the bending and/or the torsional and/or the axial stiffness of at least said portion of the stem member.
4. The orthopaedic implant of any one of the preceding claims being part of a joint prosthesis.
5. The orthopaedic implant of claim 4 comprising part of a femoral component for a hip joint replacement prosthesis.
6. The orthopaedic implant of claim 4 comprising part of a tibial component for a knee or ankle j oint replacement prosthesis.
7. The orthopaedic implant of claim 4 comprising part of a humeral component of a shoulder or elbow joint replacement prosthesis.
8. The orthopaedic implant of claim 4 comprising part of an ulnar component of an elbow joint replacement prosthesis.
9. The orthopaedic implant of any one of the preceding claims wherein the at least one modulating means comprises a cartridge member.
10. The orthopaedic implant of claim 9 wherein said stem member has a receiving region to receive said cartridge member.
11. The orthopaedic implant of claim 10 wherein said receiving region comprises a 5 recess within said stem member, said recess defined by an internal wall of the stem member.
12. The orthopaedic implant of claim 11 wherein the internal wall defines a blind- ended recess which extends from an opening in the proximal end of the stem member.
10
13. The orthopaedic implant of claim 11 wherein at least a length of said cartridge is spaced from said internal wall of the stem member in a first orientation of the cartridge relative to the stem member.
15 14. The orthopaedic implant of claim 13 wherein the cartridge member comprises an elongate body extending from a first end to a second end said first end securable to the stem member at, or adjacent to, the proximal end of the stem.
15. The orthopaedic implant of claim 14 wherein, in said first orientation said first 0 end of said cartridge is secured to the stem member at said proximal end of the stem member and the remainder of said cartridge is spaced from said internal wall of the stem member.
16. The orthopaedic implant of claim 15 wherein said cartridge member is in said 5 first orientation when said stem member is subjected to a first load.
17. The orthopaedic implant of claim 16 wherein said cartridge adopts a second, different orientation to said first orientation relative to said stem member when said stem member is subjected to a second load. 0
18. The orthopaedic implant of claim 17 wherein said second orientation comprises displacement of the cartridge member such that a length of the cartridge member spaced from said internal wall of the stem member in said first orientation engages said internal wall of said stem member in said second orientation. 5
19. The orthopaedic implant of claim 17 or claim 18 wherein said first load is relatively smaller than said second load.
20. The orthopaedic implant of any one of claims 14 to 19 wherein said first end of 5 said elongate cartridge member comprises a screw thread on an outer surface thereof for mating with a complementary screw thread on said internal wall of the stem member.
21. The orthopaedic implant of any one of claims 9 to 20 wherein said cartridge 10 member comprises an elongate cylindrical member.
22. The orthopaedic implant of any one of claims 1 to 8 wherein said modulating means comprises a tubular length of the stem member having at least one slot in a sidewall thereof.
15
23. The orthopaedic implant of claim 22 comprising opposed first and second segments of stem member sidewall which define two opposing longitudinal slots in the stem member sidewall.
20 24. The orthopaedic implant of claim 23 wherein said first and second segments of the stem member sidewall are moveable between a first spaced orientation relative to one another to a second orientation wherein the first and second segments at least partially engage each other.
25 25. The orthopaedic implant of claim 24 wherein, in said second orientation said first and second segments come together in full abutment
26. The orthopaedic implant of claim 24 or claim 25 wherein said first and second segments of the stem member move from said first spaced orientation relative to one
30 another towards said second orientation when the load on the stem member increases.
27. The orthopaedic implant of any one of claims 1 to 8 wherein the modulating means comprises an insert member housed within a tubular section of the stem
. member.
35
28. The orthopaedic implant of claim 27 wherein the insert member comprises a body having an outer surface and wherein at least a portion of the outer surface is in . engagement with an inner wall of said stem member and at least a portion of the outer surface is spaced from said inner wall when said insert member is in a first orientation relative to the stem member.
29. The orthopaedic implant of claim 28 wherein, upon an increase in load on the stem member, more of the outer surface of said inert member is brought into engagement with said inner wall of the stem member.
30. The orthopaedic implant of claim 29 wherein in a second orientation, the entire outer surface of said insert member engages said inner wall of said stem member.
31. The orthopaedic implant of any one of claims 27 to 30 wherein the insert member comprises a substantially tubular member.
32. The orthopaedic implant of any one of claims 1 to 8 wherein the modulating means comprises a length of said stem member, said length comprising a tubular length having a plurality of longitudinal slots in a sidewall, said modulating means further comprising a core member positioned internal said length of the stem member.
33. The orthopaedic implant of claims 32 wherein said longitudinal slots in said sidewall of the stem member are defined by spaced sheaths of the stem member.
34. The orthopaedic implant of claim 33 wherein said sheaths of the stem member are each spaced from said core member in a first orientation of the stem member and wherein at least part of at least one sheath is in engagement with said core member in a second orientation of the stem member.
35. The orthopaedic implant of claim 34 wherein said sheaths are each uniformly spaced from said core member.
36. The orthopaedic implant of claim 34 or claim 35 wherein in said second orientation each of said sheaths are at least partially in engagement with said core member.
37. The orthopaedic implant of any one of claim 1 to 8 wherein the at least one modulating means comprises a tubular length of the stem member comprising a series of longitudinal slots defining spaced strips of the stem member when the stem member is in a first orientation, said tubular length of the stem member moveable to a second orientation wherein the spaced strips are brought into substantial abutment with one another to define a substantially continuous cylindrical wall of the stem member.
38. An orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; at least one cartridge member positionable within a recess of said stem member, said recess defined by an internal wall of said stem member; wherein at least a length of said cartridge member is movable from a first orientation being spaced from said internal wall to a second orientation being in engagement with said internal wall.
39. An orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising a tubular length of said stem member having opposed first and second segments of the stem member sidewall which together define at least one slot in a sidewall thereof; wherein said first and second segments of the stem member wall are moveable between a first spaced orientation relative to one another to a second orientation wherein the first and second segments at least partially engage each other.
40. An orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising an insert member housed within a tubular section of the stem member, said insert member having a first orientation relative to said stem member; wherein in said first orientation at least a portion of an outer surface of the insert member is in engagement with an inner wall of said stem member and wherein at least a portion of said outer surface of the insert member is spaced from said inner wall; and wherein as said stem member is subjected to an increasing load an increasing portion of said outer surface of said inert member is brought into engagement with said inner wall of the stem member.
41. An orthopaedic implant including: an elongate stem member extending from a proximal end to a distal end and positionable within a bone of a patient; a stiffness modulating means comprising a length of said stem member, said length comprising a tubular length having a plurality of longitudinal slots in an outer wall to thereby define spaced sheaths of the stem member, said modulating means further comprising a core member positioned internal said length of the stem member; wherein said sheaths of the stem member are each spaced from said core member in a first orientation of the stem member and wherein as the stem member is subjected to an increase in load at least part of at least one sheath is caused to engage said core member.
42 A modulating member for use with a stem component of an orthopaedic implant, said modulating member comprising an elongate body, extending from a first end to a second end and having securing means at or adjacent to the first end to secure said modulating member to a portion of the stem component, wherein at least a portion of said modulating member is positionable within a portion of the stem component such that when said stem is subjected to a first load Tange the modulating member is in a first orientation relative to the stem component and when said stem component is subjected to a second, different load range, the modulating member is in a second orientation relative to the stem component and wherein when the modulating member is in said first orientation said stem component has a first stiffness and when the modulating member is in said second orientation said stem component has a second, different stiffness.
PCT/AU2009/001169 2008-07-28 2009-09-07 Orthopaedic device WO2010012048A1 (en)

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Application Number Priority Date Filing Date Title
AU2008903855A AU2008903855A0 (en) 2008-07-28 Orthopaedic device
AU2008903855 2008-07-28

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Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4292695A (en) * 1980-06-25 1981-10-06 Lord Corporation Prosthesis stem
US4314381A (en) * 1980-06-25 1982-02-09 Lord Corporation Hip joint prosthesis
WO1986006954A1 (en) * 1985-05-23 1986-12-04 Labor Für Experimentelle Chirurgie Schweizerisches Self-locking stemmed component for a joint endo-prosthesis
WO1988000031A1 (en) * 1986-07-03 1988-01-14 Habley Medical Technology Corporation Impact dissipating and load diverting hip prosthesis
EP0543099A2 (en) * 1991-11-19 1993-05-26 Bristol-Myers Squibb Company Implant fixation stem
WO2008036831A2 (en) * 2006-09-20 2008-03-27 Smith & Nephew, Inc. Variable stiffness stem for prosthetic implants
WO2008138071A1 (en) * 2007-05-16 2008-11-20 Orthoplan Pty Limited Progressively flexible stem

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4292695A (en) * 1980-06-25 1981-10-06 Lord Corporation Prosthesis stem
US4314381A (en) * 1980-06-25 1982-02-09 Lord Corporation Hip joint prosthesis
WO1986006954A1 (en) * 1985-05-23 1986-12-04 Labor Für Experimentelle Chirurgie Schweizerisches Self-locking stemmed component for a joint endo-prosthesis
WO1988000031A1 (en) * 1986-07-03 1988-01-14 Habley Medical Technology Corporation Impact dissipating and load diverting hip prosthesis
EP0543099A2 (en) * 1991-11-19 1993-05-26 Bristol-Myers Squibb Company Implant fixation stem
WO2008036831A2 (en) * 2006-09-20 2008-03-27 Smith & Nephew, Inc. Variable stiffness stem for prosthetic implants
WO2008138071A1 (en) * 2007-05-16 2008-11-20 Orthoplan Pty Limited Progressively flexible stem

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