Bone prosthesis fixation device and methods of using same

Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone

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Details

623 2034, 623 2036, 623 2326, 623 2327, 606 63, 606 68, A61F 230, A61F 232, A61F 238, A61B 1756, A61B 1758

Patent

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061266914

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a bone prosthesis fixation device, a bone prosthesis comprising the device, and a method of implanting such a bone prosthesis. This invention can be applied to a variety of orthopaedic and dental implants. A particular application of the invention is to the fixation of intramedullary fixation devices within a bone cavity, such as total hip stem prostheses, or intramedullary rods.


BACKGROUND OF THE INVENTION

Existing prostheses fixation mechanisms use cement, screw, or press-fit techniques to obtain initial stability between the bone and implant. Adequate initial stability using these fixation mechanisms may not be achieved due to loosening of the implant, or fracturing of the bone. For example, the stability of a non-cemented total hip stem prosthesis is achieved by carefully impacting the implant to the broached, contoured proximal femur. This primary fixation may be followed by secondary fixation such as osseointegration and porous in-growth. However, considerable force is usually required to securely seat a press-fit prosthesis. Thus at surgery there is a concern about fracturing the femur. Furthermore, excessive hydrostatic stress or excessive shear strain in mesenchymal tissue between a newly implanted material and bone results in mesenchymal cells differentiating into fibrous tissue (D. R. Carter et al, Journal of Orthopaedic Research, 6:726-748, 1988). Therefore, no bone ingrowth happens, and eventually the implant becomes loose. Excessive hydrostatic stress can be caused by a very tight fit (press-fit) inhibiting capillary blood flow to the tissue and decreasing tissue oxygen tension. Excessive shear strain is caused by an inadequate press-fit which permits large micromotions. Also when an intramedullary stem is used, the medullar cavity is cleaned, broached, and reamed to some extent. In this procedure a part of the vascularity system that nourishes the bone is, at least temporarily disturbed, especially in the diaphyseal region. Therefore, it is better to keep the reaming and broaching to a minimum level. Theoretically, for bone ingrowth to occur, a porous coated implant must be rigidly fixed with close apposition to the bone, without causing excessive stresses and strains at the bone-implant interface. However, this would require the medullar cavity to be prepared to very tight tolerances. In practice, this press fit technique requires an interference fit between the prosthesis and the bone, thus creacting large stresses. Accordingly, it is important that the broaching and contouring of the femur be carried out carefully and exactly. This results in operations being more lengthy and more complex. Additionally the severe impacts required to achieve a press fit for the prosthesis result in this technique being suitable only for younger patients. For older patients with thinner or more brittle bones, this technique is therefore not applicable.
Instead, for older patients, where there is a higher demand for hip prostheses and the like, the prosthesis is often fixed with cement. However, this has its own problems. The adhesive commonly used, PMMA, is strongly exothermic and shrinks after it is set. There are also significant problems in obtaining a good bond between the bone cement and the bone. After a period of use, the cement can sometimes tend to break down or parts of it can break away. While researchers have looked for other, more suitable cements, no truly suitable cements have been found.
Another problem is obtaining a proper stiffness of prosthesis stem which is compatible with that of the bone. For example, choosing a stem with a low stiffness will result in poor fixation, while a stem with high stiffness causes bone necrosis due to stress shielding. With high stiffness, loads can be transmitted from the lower end of the prosthesis, in a hip prosthesis, and bone around the head of the prosthesis is largely unloaded, an effect known as "stress shielding". Studies assuming concentric cylindrical geometries for the bone and prosthesis

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