Biologic fixation and bone modeling with an unconstrained canine total knee prosthesis.

To study the effects of dynamic loading on biologic fixation, an unconstrained type of prosthesis was designed for total replacement of the knee joint of dogs. The femoral component was fabricated from cast cobalt-based surgical alloy. The tibial component was fabricated from surgical grade, ultra-high molecular weight, high density polyethylene. Both components were designed for initial stabilization at surgery by mechanical interlock with bone. In addition, the bone-interfacing surface of the metal component was made porous and the stem of the polymer component was grooved to permit the subsequent ingrowth of tissue. Knee arthroplasty was performed on a total of six beagles. The prostheses were monitored for periods of 20 months and demonstrated an overall excellent stability and functionality. Each tibial component became stabilized by the formation of a thin, surrounding shell of osseous tissue. Interposed between this bone and the implant was usually a thin layer of fibrous tissue, suggesting micromovement during loading. Each femoral component became solidly fixed by bone growth into the porous surface. The altered stress state in the region of the implant eventually resulted in reactive bone modeling, with both bone formation and resorption occurring along the length of the implant.