Orthopedic Polymer Implant
Problems with current implant designs stem from the difference in mechanical properties between the materials used in the implant system and the bone itself.
The isoelastic bone-implant system of the invention minimizes, if not eliminates, the stress shielding effect created by a metal implant, thus, leading to a longer implant lifetime in the body. In one embodiment, a thermoplastic polymer with an elastic modulus approximating the modulus of bone is used for the implant.
Since bone is a natural composite material composed of a matrix with organic and inorganic substances, composites are also an excellent choice of materials to use for implants, specifically for those situations where material properties have a large impact on the implant's success, such as replacement of a hip. Hence, a second embodiment comprises a composite comprising a thermoplastic polymer and a reinforcing material, the composite having an elastic modulus approximating the elastic modulus of bone. The composite preferably comprises polyetheretherketone (PEEK), a high temperature thermoplastic, containing preferably 10% by volume of chopped E-glass fibers which results in a material having approximately the same stiffness as bone and, therefore, in a significant improvement with respect to the stress-shielding problem.
The final step is the application or formation of a porous coating on the surface of the implant to create the porous environment for bone ingrowth. The coating can comprise hydroxy-apatite applied to the surface, a roughness formed on the surface, or a biocompatible material such as titanium embedded in the surface.
A two piece embodiment of an intramedullary implant is joined and locked together, after the opposite ends of each piece are inserted in the medullary canal, using an interlocking mechanism comprising a fluted protrusion on one piece and a corresponding fluted cavity in the other piece with the fluted portions being complementarily tapered.
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