WO2008138071A1

WO2008138071A1 - Progressively flexible stem

Info

Publication number: WO2008138071A1
Application number: PCT/AU2008/000691
Authority: WO
Inventors: Lawrence Kohan
Original assignee: Orthoplan Pty Limited
Priority date: 2007-05-16
Filing date: 2008-05-16
Publication date: 2008-11-20

Abstract

A femoral component for a hip resurfacing arthroplasty comprising a femoral cap, which is adapted to engage with a cup set into a pelvic bone. The femoral cap has a convex surface and a concave surface. The femoral component also comprises a stem, which is attached to the concave surface of the femoral cap. This stem comprises a core portion that is surrounded by a sheath portion. The core portion and the sheath portion are composed of dissimilar materials. The material from which the core portion is composed is more rigid than the material from which the sheath portion is composed. The stem is flexible when a low force is applied on the femoral component and becomes progressively more rigid as gradually increasing forces are applied on the femoral component.

Description

Progressively Flexible Stem

Technical Field

The present invention relates to hip resurfacing, and in particular to a femoral component for a hip resurfacing arthroplasty. However, the invention is not limited to hip resurfacing and can be utilised for other ball and socket joint applications in humans and other mammals.

Background Art

The concept of hip resurfacing has been known since the late 1950s. Hip resurfacing relies on the fact that it is preferable to replace only the bone surfaces within a weakened or diseased hip rather than radically removing large portions of bone. This approach has the benefit of preserving the femoral head and neck. This leaves the natural off-set and anteversion of the hip joint intact and maintains approximate leg length equivalence. The larger size of the ball in the ball and socket joint diminishes the problem of dislocation. The stress loading on the bone is relatively natural. Further, metals which have a low wear rate can be used.

In a hip resurfacing arthroplasty the diseased portion of the pelvic socket is removed. A replacement cup is set into the pelvic bone. The articular surfaces of the femur and the femur head are reshaped and a femoral cap is set onto the femur and adapted to engage with the cup.

Only the bearing surface of the femur is removed, the stiffness of the implanted stem is often different to that of the bone material removed. This difference in stiffness can be sufficient to change the load distribution in the bone beneath the implant. The density of the remaining bone responds to changes in the load distribution. In some areas, there is more load than before and this can lead to an increase in the bone density. Conversely, in other areas, there can be a reduction in the compressive stress applied on the bone, which can result in some bone fading away or resorbing. This reduction in bone is highly undesirable.

In order to assist in aligning the cap with the femur during surgery, the cap includes a stem. The stem allows for alignment of the cap and also provides stability of the joint until the bone meshes with the metal and/or cement of the femoral cap.

This stem, whilst beneficial, can produce weakening of the femur, along with microfracture and unnatural stress within the femur bone. This can cause significant pain for a patient and, long term, may weaken the femur. Additionally, the stem is one of the geometric features of a resurfacing implant which can lead to a change in stiffness and bone resorption. In many cases, conventional stems are so stiff that they attract loads away from some of the bone, which then resorbs to some extent.

As indicated above, the stem is needed for both alignment and placement of the implant. However, the stem also has an important added benefit in that it increases the security of the hip by providing an alternate load path in the case of a bone fracture. This added benefit helps to protect bones against unwanted fractures when stress is applied to the bone, for example when a person stumbles or falls over.

There are known flexible stems which do help in reducing bone resorption. However, these existing flexible stems are too flexible, i.e. they are not strong. In the event of stress being applied to the bone and the stem implanted therein, if the bone fractures, these existing flexible stems do not increase the security of the hip by providing an alternate load path. Therefore, these known flexible stems do not protect bones against unwanted fractures when stress is applied to the bone.

Conventional stems are typically manufactured from materials such as stainless steel or formaldehyde resin, such as Bakelite. Conventional stems are typically homogenous, in that they are manufactured from a single type of material.

It is an object of the present invention to provide a femoral component for a hip resurfacing arthroplasty which will overcome or ameliorate at least some of the deficiencies in the prior art, or to at least provide an alternative.

It is a further object of the present invention to provide a bone component for a ball and socket resurfacing arthroplasty for a mammal, including a human, which will overcome or at least ameliorate at least some of the deficiencies in the prior art, or to at least provide an alternative.

Disclosure of Invention

According to a first aspect the present invention consists of a femoral component for a hip resurfacing arthroplasty comprising a femoral cap, adapted to engage with a cup which is set into a pelvic bone, the femoral cap having a convex surface and a concave surface, and a stem attached to the concave surface of the femoral cap, characterised in that the stem comprises a core portion surrounded by a sheath portion, said core portion and said sheath portion composed of dissimilar materials.

Preferably, the material from which the core portion is composed is more rigid than the material from which the sheath portion is composed. In this way, the core portion is preferably substantially composed of metal and the sheath portion is substantially composed of a polymer material, namely polyethylene. There is preferably a void between the core portion and the sheath portion. The void is filled with an inert material, such as a saline solution.

Preferably, the stem is flexible when a low force is applied thereto and conversely, the stem is rigid when a high force is applied thereto. The stem is flexible when a low force is applied thereto and becomes progressively more rigid as gradually increasing forces are applied thereto. The stem is usually permanently attached to the femoral cap. In this way, the composite stem has a maximum rigidity when a high force is applied thereto.

According to a second aspect of the present invention there is provided a femoral component for a hip resurfacing arthroplasty comprising a femoral cap, adapted to engage with a cup which is set into a pelvic bone, the femoral cap having a convex surface and a concave surface, and a composite stem attached to the concave surface of the femoral cap, the composite stem comprising two parts, a core portion composed of a rigid material and a sheath portion composed of a more flexible material surrounding the core portion.

According to a third aspect of the present invention there is provided a femoral component for a hip resurfacing arthroplasty comprising a femoral cap and a progressively flexible composite stem. The femoral cap is adapted to engage with a cup which is set into a pelvic bone and the femoral cap having a convex surface and a concave surface. The progressively flexible stem is attached to the concave surface of the femoral cap and the progressively flexible composite stem comprises two parts, a core portion composed of a rigid material and a sheath portion composed of a more flexible material surrounding the core portion. The progressively flexible composite stem is flexible when a low force is applied thereto and becomes progressively more rigid as gradually increasing forces are applied thereto, whereby the composite stem has a maximum rigidity when a high force is applied thereto.

According to a fourth aspect of the present invention there is provided a femoral component for a hip resurfacing arthroplasty comprising a femoral cap and a progressively flexible composite stem. The femoral cap is adapted to engage with a cup which is set into a pelvic bone and the femoral cap having a convex surface and a concave surface. The progressively flexible stem is attached to the concave surface of the femoral cap and the progressively flexible composite stem comprises two parts, a core portion composed of metal and a sheath portion composed of a polymer material surrounding the core portion. There is a void between the core portion and the surrounding sheath portion. The progressively flexible composite stem is flexible when a low force is applied thereto and becomes progressively more rigid as gradually increasing forces are applied thereto, whereby the composite stem has a maximum rigidity when a high force is applied thereto.

Brief Description of Drawings

A preferred embodiment of the invention will now be described by way of example only, with reference to the accompanying figures in which:

FIG 1 depicts a vertical cross sectional view of an embodiment of the femoral component for a hip resurfacing arthroplasty in accordance with the present invention;

FIG 2 depicts a horizontal cross sectional view of the stem of the femoral component for hip resurfacing arthroplasty depicted in FIG 1;

FIG 3 is a graphical representation of the force v deflection of existing femoral components for hip resurfacing arthroplasty; and

FIG 4 is a graphical representation of the force v deflection of an embodiment of the femoral component for hip resurfacing arthroplasty in accordance with the present invention. Best Mode for Carrying Out the Invention

Generally the present invention relates to a femoral component for hip resurfacing arthroplasty. FIGS 1 and 2 show the femoral component 1 of the present invention.

Referring to FIG 1, the femoral component 1 comprises a femoral cap 10 having a concave surface 12 and a convex surface 14. The femoral cap 10 is adapted to engage with a cup 8. The femoral cap 10 and cup 8 are composed of metal, such as steel. The convex surface 14 of the femoral cap 10 allows the surface to be bearing between the cup 8 and the femoral cap 10. The concave surface 12 of the femoral cap 10 is there to minimise bone removal, improve stability and stiffness compatibility with the underlying pelvic bone of the hip.

The femoral cap 10 is adapted to be set onto a femoral bone and also adapted to engage with the cup 8 that is set into the pelvic bone of the patient. It can be seen that in use this engagement of the femoral cap 10 and cup 8 comprises a ball and socket joint.

As best shown in FIG 2, the femoral component 1 further comprises a stem 3 that is adapted to be attached to femoral cap 10. The stem 3 is of composite construction, having two parts (2, 4) separated by a void 6. The stem 3 comprises a core portion 2 surrounded by a sheath portion 4. The core portion 2 is composed of steel, whilst the surrounding sheath portion 4 is composed of a polyethylene. The void 6 is filled with saline solution or any other suitable inert material.

When a low stress is applied to the femoral component 1, the overall stiffness or rigidity of the stem 3 will be the deflection value of the sheath portion 4. In this specification, a "low stress" or "low force" would be the load experienced by a hip joint during normal level walking. The sheath portion 4, being composed of polyethylene, has a relatively high deflection value, even under the influence of a "low stress" or "low force". Therefore, the stem 3 is relatively flexible when a "low force" is exerted on the femoral component 1. This helps to ensure that bone resorption does not occur or at least is minimised.

When a high stress is applied to the femoral component 1, the void 6 between the core portion 2 and the sheath portion 4 would close up. The sheath portion 4 would bear up on the core portion 2, and there would be a rapid increase in the overall stiffness or rigidity of the stem 3 because the strength of the rigid core portion 2 would become utilised. In this specification, a "high stress" or "high force" would be the load experienced by a hip joint during a stumble or fall. The core portion 2, being composed of steel, has a relatively low deflection value, even under the influence of a "high stress" or "high force". Therefore, the stem 3 is relatively rigid when a "high force" is exerted on the femoral component 1. This helps to ensure the security of the hip by providing an alternate load path in the case of a bone fracture.

It should be understood by those skilled in the relevant art that the femoral component 1 would experience loads in between a "low force" and "high force". For example, when a person climbs a set of stairs or rises from a chair. When under the influence of these forces in between the "low" and "high" forces, the stem 3 would act in between the "low" and "high" stiffness ranges, and these would be at a lower frequency than normal level walking.

With reference to FIG 3, which is a graphical representation of the force v deflection under a stress load of existing femoral components, it can be seen that existing femoral components for use in hip resurfacing arthroplasty exhibit undesirable properties when subjected to stress or force. FIG 3 shows the deflection properties of different existing or prior art femoral components when subjected to stress or force. The deflective properties of existing solid "stiff" stem femoral components, existing solid "flexible" stem femoral components and existing femoral components having no stem are shown. Line A represents existing solid "stiff" stem femoral components and shows that existing solid "stiff" components have a low deflection value, even when subjected to relatively high stress loads. The area of the graph marked "D", shows the where it is desirable for the component to provide security to the hip under a stress load. Whilst the relatively low deflection value, even when subjected to relatively high stress loads of existing solid "stiff" stem components is desirable for security of the hip by providing an alternate load path in the case of bone fracture, it is undesirable as the stem is so stiff that it attracts load away from some of the bone, which then resorbs to some extent.

Line B represents existing solid "flexible" stem femoral components have a high deflection value, even when subjected to relatively low stress loads. The area of the graph marked "D", shows the where it is desirable for the component to provide security to the hip under a stress load. Whilst the relatively high deflection value, even when subjected to relatively low stress loads of existing solid "flexible" stem components is desirable for bone density and helps to minimise the possibility of bone resorption, it is undesirable for security of the hip, as the flexible stem does not provide an alternate load path in the case of bone fracture.

Line C represents existing femoral components that do not comprise a stem. Whilst the relatively high deflection value, even when subjected to relatively low stress loads of existing components without stems is desirable as these "stemless" components do not promote bone resorption, as with existing solid "flexible" stem components, these "stemless" components are undesirable as they do not provide any support under high stress loads. An additional problem with femoral components that do not comprise a stem, is that they are much more difficult to implant in a person's hip.

With reference to FIG 4, which is a graphical representation of the force v deflection under a stress load of the femoral component according to the present invention, it can be seen that the femoral component of the present invention for use in hip resurfacing arthroplasty exhibits highly desirable properties when subjected to stress or force. FIG 4 shows the deflection properties of the femoral component of the present invention when subjected to stress or force.

With reference to FIG 4, it can be seen that the properties of the femoral component for use in hip resurfacing arthroplasty of the present invention, as represented by the dashed line E, when subjected to stress or force, are beneficial for both security and helping to prevent bone resorption. Under a low stress load, the component of the present invention exhibits a high deflection value, which is desirable for bone density and helps to minimise the possibility of bone resorption. Conversely, under a high stress load, the component of the present invention exhibits a low deflection value, which is desirable for security of the hip by providing an alternate load path in the case of bone fracture.

Whilst the embodiment of the present invention depicted in FIGS 1 and 2 shows the core portion 2 and sheath portion 4 being composed of steel and polyethylene respectively, it should be understood that other suitable materials can be used. For example, any biologically suitable materials having suitable deflective values can be used. The core portion 2 may be composed of a rigid plastic, polymer, suitable polycarbon or any similar biologically stable rigid material. The sheath portion 4 may be composed of different plastics or any other similar biologically stable rigid material.

Whilst the void 6 of the preferred embodiment is filled with saline, in other embodiments not shown in the accompanying drawings, any other suitable inert material may be used. Further still, in other embodiments, the void 6 may remain unfilled.

Whilst the femoral component 1 has been described for use in hip surfacing arthroplasty, the present invention could be used for any ball and socket joint replacement in humans or other mammals. Industrial Applicability

The component of the present invention can be used in relation with hip resurfacing and in particular for a hip resurfacing arthroplasty. However, the component is not limited to hip resurfacing and can be utilised for other ball and socket joint applications in humans and other mammals.

In this specification, unless the context clearly indicates otherwise, the term "comprising" has the non-exclusive meaning of the word, in the sense of "including at least" rather than the exclusive meaning in the sense of "consisting only of". The same applies with corresponding grammatical changes to other forms of the word such as "comprise", "comprises" and so on.

It will be apparent that obvious variations or modifications may be made which are in accordance with the spirit of the invention and which are intended to be part of the invention, and any such obvious variations or modifications are therefore within the scope of the invention.

Claims

1. A femoral component for a hip resurfacing arthroplasty comprising: a femoral cap, adapted to engage with a cup which is set into a pelvic bone, the femoral cap having a convex surface and a concave surface, and a stem attached to the concave surface of the femoral cap, characterised in that the stem comprises a core portion surrounded by a sheath portion, said core portion and said sheath portion composed of dissimilar materials.

2. The femoral component for a hip resurfacing arthroplasty of claim 1 wherein the material from which the core portion is composed is more rigid than the material from which the sheath portion is composed.

3. The femoral component for a hip resurfacing arthroplasty of claim 1 wherein the core portion is substantially composed of metal and the sheath portion is substantially composed of a polymer material.

4. The femoral component for a hip resurfacing arthroplasty of claim 3 wherein the core portion is substantially composed of steel and the sheath portion is substantially composed of polyethylene.

5. The femoral component for a hip resurfacing arthroplasty of claim 1 wherein there is a void between the core portion and the sheath portion.

6. The femoral component for a hip resurfacing arthroplasty of claim 5 wherein the void is filled with an inert material.

7. The femoral component for a hip resurfacing arthroplasty of claim 6 wherein the inert material is saline.

8. The femoral component for a hip resurfacing arthroplasty of claim 1 wherein the stem is flexible when a low force is applied on the femoral component and the stem is rigid when a high force is applied on the femoral component.

9. The femoral component for a hip resurfacing arthroplasty of claim 8 wherein the stem is flexible when a low force is applied on the femoral component and becomes progressively more rigid as gradually increasing forces are applied on the femoral component.

10. The femoral component for a hip resurfacing arthroplasty of claim 1 wherein the stem is permanently attached to the femoral cap.

11. A femoral component for a hip resurfacing arthroplasty comprising: a femoral cap, adapted to engage with a cup which is set into a pelvic bone, the femoral cap having a convex surface and a concave surface; and a composite stem attached to the concave surface of the femoral cap, the composite stem comprising two parts, a core portion composed of a rigid material and a sheath portion composed of a more flexible material surrounding the core portion.

12. The femoral component for a hip resurfacing arthroplasty of claim 11 wherein there is a void filled with an inert material between the core portion and the surrounding sheath portion.

13. The femoral component for a hip resurfacing arthroplasty of claim 12 wherein the inert material is saline.

14. The femoral component for a hip resurfacing arthroplasty of claim 11 wherein the composite stem is flexible when a low force is applied on the femoral component and the composite stem becomes progressively more rigid as gradually increasing forces are applied on the femoral component, whereby the composite stem has a maximum rigidity when a high force is applied on the femoral component.

15. A femoral component for a hip resurfacing arthroplasty comprising: a femoral cap, adapted to engage with a cup which is set into a pelvic bone, the femoral cap having a convex surface and a concave surface; a progressively flexible composite stem attached to the concave surface of the femoral cap, the progressively flexible composite stem comprising two parts, a core portion composed of a rigid material and a sheath portion composed of a more flexible material surrounding the core portion; and wherein the progressively flexible composite stem is flexible when a low force is applied on the femoral component and becomes progressively more rigid as gradually increasing forces are applied on the femoral component, whereby the composite stem has a maximum rigidity when a high force is applied on the femoral component.

16. The femoral component for a hip resurfacing arthroplasty of claim 15 wherein there is a void filled with an inert material between the core portion and the surrounding sheath portion.

17. A femoral component for a hip resurfacing arthroplasty comprising: a femoral cap, adapted to engage with a cup which is set into a pelvic bone, the femoral cap having a convex surface and a concave surface; a progressively flexible composite stem attached to the concave surface of the femoral cap, the progressively flexible composite stem comprising two parts, a core portion composed of metal and a sheath portion composed of a polymer material surrounding the core portion, wherein there is a void between the core portion and the surrounding sheath portion; and wherein the progressively flexible composite stem is flexible when a low force is applied on the femoral component and the composite flexible stem becomes progressively more rigid as gradually increasing forces are applied on the femoral component, whereby the composite stem has a maximum rigidity when a high force is applied on the femoral component.

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