NL9500256A - Prosthesis - Google Patents

Abstract

The invention relates to a prosthesis, in particular a load-bearing endoprosthesis for hard tissue, provided with an anchor element for anchoring the prosthesis in the bone, which anchor element comprises a rigid core, the outer surface of which is coated with a substantially continuous elastomeric covering layer. The thickness of the elastomeric covering layer is preferably matched to the local stress conditions at the interface between the anchor element and the tissue surrounding the element. If appropriate, means which promote bone growth may be incorporated in the elastomeric covering layer.

Description

PROSTHESIS

The present invention relates to a prosthesis, in particular a load-bearing hard tissue endoprosthesis, provided with an anchor element for anchoring the prosthesis in bone. The invention further relates to the anchor element as such.

The anchoring of prostheses, such as hip prostheses, knee prostheses or dental prostheses, in bone or jaw bone is of great importance for the optimal functioning of the prosthesis and the comfort for the patient. Badly anchored prostheses result in pain and other problems.

In daily practice, three classes of hip prostheses are currently in use. On the one hand, the prostheses that are fixed with bone cement, such as, for example, poly-methylmetacrylate (PMMA), on the other hand, the cementless prostheses and the cementless custom prosthesis.

The first type has the advantage that the patient can resume normal activities fairly early, but there are also some serious drawbacks. In practice, the cement is never evenly distributed over the interface between bone and prosthesis. This can cause cracks when shrinking during the curing of the cement. Due to the uneven voltage distribution, zones can arise where excessive voltage concentrations occur and zones that are shielded from load transfer (called "stress shielding"). As a result, the bone modeling no longer takes place adeguately. For example, callus formation or absorption takes place. In addition, wear particles caused by erosion of the polyethylene cup in hip or knee prostheses, or the metal prosthesis stem itself, in places where the cement is broken cause resorption (Simon, 1994, Mallooney et al.

1990, Anthony et al. 1990). These and related phenomena give rise to aseptic loosening of the prosthesis. 10 years after the procedure, valuation problems occur in 30 to 50% of the patients. This leads to revision interventions at 10 to 30% (Naulaers 1990). Furthermore, when the monomer is added to the bone cement during implantation, a blood pressure drop often occurs with a minimal but real chance of fatal outcome. In addition, the curing of PMMA is an exothermic process, which causes a local temperature rise, which can lead to a local bone necrosis. The toxic monomer of the PMMA will often not fully polymerize, so that the toxic monomer will affect the body in the long term (Tountas 1987). In connection with the above-described problems, cementless prostheses have increasingly been used in the past 20 years.

Two subclasses can be distinguished within cementless prostheses. On the one hand, there are prostheses with biological anchoring, where bone ingrowth ensures adhesion. On the other hand, there are prostheses with mechanical anchoring (called "self-locking") in which the prosthesis is designed so that the surface is perpendicular to the force to be transferred. This is the case, for example, with a conically rising prosthesis, which will sink into the medullary bone cavity as a result of pressure load until it is fixed therein.

The main drawback of the biological anchored type is the long immobilization period required for bone ingrowth. With self-locking prostheses, removal of the prosthesis is often problematic during revision surgery, and the requirements for precision implantation and preparation of the medullary canal are much stricter.

In addition, even with cementless prostheses, the problem of uneven load distribution remains, because the anchor element of the prosthesis (the femur shaft) and the human thigh (femur) are non-elastic. This applies in particular to hip prostheses, but to a certain extent also applies to other types of endoprostheses. The unevenly distributed load transfer is caused, among other things, by the difference in E-modulus between bone and prosthesis. Namely, the material of which the anchor element of the prosthesis is made has an elastic modulus of about 200 GPa, while bone has an E-modulus ranging from 6 to 17 GPa. The consequences of this have been described, for example, by Simon (1994). Because the bone is loaded unevenly, the anchor element of the prosthesis absorbs a disproportionate amount of the load. Among other things, this results in remodeling as an adjustment to the new tax situation. In addition, the prosthesis can come loose and micro-movements can occur in the interface between bone and anchor. The micro movements are suspected to be the cause of pain. In addition, wear particles can be formed by these phenomena. The uneven load transfer is an unavoidable consequence of the implantation of a bone anchor in a femur.

The iso-elastic hip prosthesis was launched to solve these problems (Morscher and Mathys, 1975, 1976). With such a prosthesis, it is intended that the bone and the prosthesis deform as a whole. To this end, a prosthesis made of polyacetal resin reinforced with a metal wire core is used. Polyacetal resin has an E-modulus of 5 to 10 GPa, like human bone, and further meets the requirements of durability, biocompatibility, wear resistance and pressure resistance (Wolter 1976, Kinzl 1976). When using a prosthesis that is constructed almost entirely of a material of such a low E modulus, the tendency for relative deformation between bone and prosthesis anchor is quite large and the shear and shear stresses at the interface become more important than with the conventional, rigid cementless prostheses. As a result, the proximal part comes off causing pain and the distal part grows into the bone which is a serious problem in revision surgery.

Endosteal anchors for permanent dental prostheses are used in dentistry. Implant detachment usually occurs when the interface between implant anchor and bone is exposed to harmful bone modeling and / or bone resorption processes. This is usually accompanied by the formation of an intermediate soft tissue (Barbier 1994, Bremarkmark 1985). In addition to factors such as the biocompatibility of the implant material, the carefulness of the surgical procedure and the aftercare performed, the biomechanical anchoring of the implant is also very important. Occlusion forces must be tolerated by the implant without fracture and these forces must be transferred to the surrounding bone without damaging the tissues at the implant-bone interface. In fact, a positive stimulus on the surrounding bone is expected from the load transmission through a good implant.

Therefore, there is a great need for an implant capable of absorbing shocks and spreading the load evenly over the bone-implant interface. Better clinical results are expected from such a type of implant than from the currently in use all-metal high-rigidity implants.

The object of the present invention is to provide a prosthesis which avoids the above drawbacks and achieves the desired positive effects.

This is achieved by the invention by a prosthesis, in particular a load-bearing hard tissue endoprosthesis, provided with an anchor element for anchoring the prosthesis in bone, the anchor element comprising a rigid core, the outer surface of which is coated with a substantially continuous elastomer coating.

The prosthesis according to the invention has a number of advantages over the conventional cemented or cementless botanicals. The elastic properties of the covering layer enable a so-called press-fit fixation of the prosthesis in the surrounding bone. The use of bone cement is then unnecessary. The elastomer coating fulfills the role of a mechanical buffer between the bone tissue and the rigid material of the anchor core. This dampens the shocks resulting from the cyclic load during walking.

In addition, in hip prostheses, the risk of splitting fractures of the femur due to excessive inhalation when implanting the prosthesis is reduced.

Another consequence of the elastic properties of the coating is that point contacts between bone and the anchor core of the prosthesis are spread into surface contacts. As a result, minor irregularities that persist in the wall of the intramedullary canal after surgeon reaming do not result in point contacts.

Micro movements between the anchor element and the bone are absorbed in a shear deformation of the elastomer coating. With a perfect functioning of the system, which is formed by the bone, the covering layer and the anchor core, there should be no relative movement in the interfaces between bone and covering layer and between the covering layer and anchor core. In this way, the creation of wear particles as a result of friction is avoided. Due to the complete covering with a sealing elastomer layer, the core of the anchor element itself is shielded from the surrounding physiological environment, so that any still-formed wear particles are not given the opportunity to contaminate this surrounding environment.

When the core consists of metal, the elastomer coating also prevents metal ions, which arise from corrosion of the metal surface, from contaminating the physiological environment surrounding the bone. It is known that when titanium is exposed to a physiological environment it dissolves in the form of titanium ions (Bruneel and Helsen, 1988).

Preferably, the layer thickness of the elastomer coating is adapted to the local stress state at the interface between the anchor element and the tissue surrounding the element. In practice, for an anchor element of a hip prosthesis, this means that the cover layer is thicker proximally-medially and disto-laterally. The contact forces between bone anchor and bone are greatest in those places.

In an advantageous embodiment of the invention, the elastomer layer can be coated on the outside with a bone growth-promoting layer, such as, for example, a hydroxyapatite layer. A layer of a biodegradable polymer, in which bone growth factors may be included, is also suitable. The purpose of such a layer is to prevent negative tissue reactions and to obtain a certain bone ingrowth.

Preferably, the core of the anchor element is made of a material of high strength and rigidity, such as, for example, a titanium alloy, or corrosion-resistant steel. Slots or recesses may optionally be provided in the surface, which, in addition to adhesion, also help to immobilize the elastomer coating around the metal core.

The structural elastomer topcoat is preferably elastic, cushioning and bio-inert. The elastomer has a suitable E-modulus at 100% from 1 to 15 MPa and a hardness between shore A65 and 100. Suitable materials are, for example, Sarlink ™ 3280, a thermoplastic vulcanizate from DSM, or other thermoplastic polyolefins. In addition, polyurethane rubber, for example of the polyether type, such as Estane ™ (Goodrich) can be used. Polyphosphazene rubber is also suitable, in particular when the boundary layer of the elastomer coating has to be biologically active, such as when it contains bone growth promoting substances. However, it will be clear to the skilled person that any material which provides the anchor element with the properties desired according to the invention is in principle suitable.

The attachment or fixation of the elastomer cover layer with respect to or on the anchor element can be achieved in various ways. For example, fixation is accomplished when the core has at least one bore through which the elastomer extends. In this way, opposite sides of the elastomeric cover layer, for example, the ventral and dorsal sides, are joined together. This leads to immobilization of the coating with respect to the core.

In another embodiment of the invention, the elastomer cover layer may be glued to the anchor core.

Preferably, a bio-inert adhesive is used for this.

The elastomeric coating of the invention provides a more physiological load transfer between prosthesis core and surrounding bone tissue. This improves wearing comfort for the patient by reducing pain and extending the life of the implant. Namely, there is less chance of bone atrophy due to shielding of the stresses in the bone by an overly rigid prosthesis and less chance of inflammatory reactions caused by corrosion products and / or wear particles.

The present invention will be further elucidated with reference to the annexed drawings, in which: figure 1 shows the bone anchor with elastomer coating of a hip prosthesis; figure 2 shows a tibial plateau according to the invention as part of a knee prosthesis; and Figure 3 shows an anchor for a dental prosthesis coated with an elastomer coating.

Figure 1 shows a femur 1 (femur) in which a bone anchor 2 is incorporated. The bone anchor consists of a core 3, which is covered over its entire surface with a layer of elastic and damping biocompatible material 4. For this, Sarlink ™ 3280 can be used.

The anchor has three bores 5 which ensure a good fixation of the elastomer coating 4 relative to the core 3. The core 3 itself can for instance consist of a Ti-Al-Nb alloy which combines high strength and corrosion resistance with less rigidity and density than, for example, a stainless steel equivalent. The bores 5 are sufficiently rounded to avoid crack initiation sites. In this way, the fatigue resistance is kept optimal.

The anchor 2 is coupled to a conically shaped neck 6 (also called "Morse taper") on which a sphere 7 is applied, which is preferably made of a corrosion-resistant alloy with good tribological properties, such as, for example, Co-Cr or stainless -steel 316L. This sphere 7 together with an acetabulum (not shown) of, for example, ultra-high molecular weight polyethylene (UHMWPE) replaces the hip joint.

The thickness of the elastomer top layer 4 is adapted to the tension condition in the thigh. Proximo-medial (9) and disto-lateral (8), the cover layer 4 is thicker because the contact forces between bone anchor 2 and bone 1 are greatest there.

The bone anchor can be implanted in the medullary cavity of the femur using the standard cementless press-fit prosthesis technique. The use of bone cement is therefore unnecessary. Achieving a good press fit is facilitated by the elastic properties of the cover layer, so that the reaming of the medullary cavity can be performed less aggressively. The damping properties of the elastomeric layer will help prevent femur splitting fractures when inhaling the bone anchor.

In Fig. 2, a sectional view shows a tibial plateau as part of a knee prosthesis. The bone anchor 10 is provided on its side facing the shin 11 with an elastomer coating 12. On top of this is a disc 13, for example of polyethylene, which replaces the joint surface. On the opposite side of the artificial joint there is a replacement joint surface 14. This replaces the joint of the femur (femur) 15. The condyles 16 slide off the femur and rotate over the disc 13. The tibial plateau is surrounded by cancellous bone 17. Here, too, the elastomer coating will assist in damping the impact loads that occur when stepping.

Fig. 3 finally shows an anchor 18 of a dental prosthesis. The portion 19 of the prosthesis in contact with the jawbone 20 and the cancellous bone or marrow 21 of the jawbone 20 is covered with an elastomeric coating 22. The gum 23 closes about the upper portion 24 of the anchor. In the embodiment shown, a crown 26 is attached to the anchor by means of a fastening screw 25. Due to the elastomer coating, the load that occurs during chewing will be transferred more evenly to the jawbone. In addition to crowns, this anchoring can also be used for other elements from prosthetic dentistry, such as bridges and even whole or parts of dental prostheses.

The present invention facilitates anchoring of a prosthesis in bone tissue by obviating the use of bone cement. The life of the implant and the wearing comfort for the patient are enhanced by the presence of the elastomer cover layer that separates the core of the prosthetic anchor from the surrounding bone or jawbone. Due to its damping and elastic properties, the layer forms a mechanical buffer between bone and core. In addition, the mostly metal core is protected against the surrounding aggressive physiological environment. Because the elastomer coating can deform, movements of the core relative to the surrounding bone are absorbed. Due to the press-fit, the layer remains immobile with respect to the wall of the bone, so that no wear particles are formed, even on the metal core. Due to the possibility of deformation of the elastomer coating, no harmful point contacts are created during load transfer between bone and anchor.

References

Anthony, P.P., Gie, G.A., Howie. C.R., Ling, R.S.M., "Localized femoral osteolysis in relation to the femoral components of cemented total hip arthroplasties." Journal of Bone and Joint Surgery, 1990, 72-B, pp. 971-979.

Barbier, L., "Adaptive Bone remodeling around dental implants under load-bearing conditions", Ph. D. Thesis, K.U. Leuven, School of Dentistry, Dept. Prosthetic Dentistry, 1994.

BrAnemark, P.-I., "Introduction to osseointegration." In: Tissue-integrated prostheses: Osseointegration in Clinical Dentistry (Eds .: P.-I. Brinemark, G. Zarb & T. Albrekts-son). Quintessence Publ. Co. Inc. Chicago: 11-76.

BRUNEEL, N. & Helsen, J.A., "In vitro simulation of biocompatibility of Ti-Al-V", Journal of Biomedical Materials Research, Vol 22, 203-214 (1988)

Kinzl, L., Burri, C., Mohr, W., Paulini, K., Wolter, D., "Gewebevertragligkeit der Polymere Polyathylen, Polyester und Polyacetalharz", Zeitschrift fiir Orthopadie und ihre Grenzgebiete, 1976, 114 (5), pp. 777-784.

Maistrelli, GL, Fornassier, V., Binnington, A., McKenzie, K., Sesse, V., Harrington, I., "Effect of stem modulus in a total hip arthroplasty model," Journal of Bone and Joint Surgery [Br .], 1991, 73-B, pp. 43-46.

Maloney, W.J., Jasty, M., Rosenbergh, A., Harris, W. H., "Bone lysis in well-fixed cemented femoral components", Journal of Bone and Joint Surgery, 1990, 72-B, pp. 966-970.

Morscher, E., Mathys, R., "Erste Erfahrungen mit einer zementlosen isoelastischen Totalprothese der Hüfte", Z. Orthop., 1975, 113, pp. 745-749.

Naulaers, Dr., "Total Hip Prostheses", Doctors Newspaper, 1990, 513, pp. 16-17.

Simon, J.-P., "Restoration of bone stock with impacted cancellous allografts and cement in revision of the femoral component in total hip arthroplasty. A clinical and biomechanical study." Ph. D. Thesis, K.U. Leuven, Orthopedic Hospital, 1994, p. 41.

Tountas, A., EVANS, J., Mcgoey, P., Waddel, J., "Long-Term Review of the McGoey-Evans High-Friction Uncemental Total Hip Arthroplasty," The Canadian Journal of Surgery, March 1987, 30 ( 2), pp. 81-85.

Vandersloten, J. & Vanderperre, C., "Wolffs law and the trabecular structure of bones: two case studies", Tissus calcifiés et biomatériaux, Biomat'87, Bordeaux, 1987.

Claims (15)

A prosthesis, in particular load-bearing endo-prosthesis for hard tissues, provided with an anchor element for anchoring the prosthesis in bone, characterized in that the anchor element comprises a rigid core, the outer surface of which is coated with a substantially continuous elastomer coating. Prosthesis according to claim 1, characterized in that the layer thickness of the elastomer coating is adapted to the local stress state at the interface between the anchor element and the tissue surrounding the element. Prosthesis according to claim 1 or 2, characterized in that bone growth-promoting agents are included in the elastomer coating. Prosthesis according to claim 3, characterized in that the bone growth promoting agents are selected from a layer consisting of one or more calcium phosphates, a biodegradable polymer, which includes bone growth factors, or combinations thereof. Prosthesis according to any one of claims 1 to 4, characterized in that the anchor element has at least one bore through which the elastomer extends to effect fixation of the elastomer coating with respect to the anchor element by opposite sides of the elastomer coating together. A prosthesis according to any one of claims 1 to 4, characterized in that the elastomer cover layer is attached to the anchor core through an adhesive layer. Prosthesis according to any one of claims 1-6, characterized in that it is a hip prosthesis. Prosthesis according to any one of claims 1-6, characterized in that it is a knee prosthesis. A prosthesis according to any one of claims 1-6, characterized in that it is a dental prosthesis. Prosthesis anchor member comprising a rigid core, the outer surface of which is coated with a substantially continuous elastomer topcoat. Anchor element according to claim 10, characterized in that the layer thickness of the elastomer coating is adapted to the local stress condition at the interface between the anchor element and the tissue surrounding the element. Anchor element according to claim 10 or 11, characterized in that bone growth-promoting agents are incorporated in the elastomer coating. An anchor element according to claim 12, characterized in that the bone growth promoting agents are selected from a layer consisting of one or more calcium phosphates, a biodegradable polymer, which includes bone growth factors, or combinations thereof. Anchor element according to any one of claims 10-13, characterized in that the anchor element has at least one bore, through which the elastomer extends to effect fixation of the elastomer coating with respect to the anchor element by opposite sides of the elastomer coating together. A prosthesis according to any one of claims 10-13, characterized in that the elastomer top layer is attached to the anchor core through an adhesive layer.