A metal-backed, rotating-bearing patellar prosthesis to lower contact stress. An 11-year clinical study.
A congruent-contact, metal-backed, rotating-bearing patellar prosthesis has been used clinically over an 11-year period. The features of this implant include a congruent-contact, rotating bearing attached to a thin metallic anchoring plate fixed to the bone by cruciate fixturing fins. The device was initially used with methylmethacrylate and later used as a cementless, porous-coated, ingrowth fixation implant. The benefits of congruent contact with mobility to adjust its tracking position during flexion and extension help to eliminate axial shear stresses at the bone-prosthesis interface and lower the contact stresses at the bearing-metal interface to improve wear properties. Prostheses were implanted in 515 knees over a period of six months to 11 years. Of these 515 knees, 331 patellar prostheses have been followed for 24 to 132 months (mean, 73 months). Of that group of 331 implants, 141 were cemented and 190 were cementless. Specific complications involving the use of this implant were as follows. One intraoperative vertical midpatellar fracture in a cemented case went on to uneventful healing with an excellent result. Two displaced postoperative transverse midpatellar fractures (0.39%) occurred in two revision total knee arthroplasties that involved major lateral releases. One of those was a cemented revision and required removal of the implant and a partial patellectomy to gain an excellent result. In the other, a noncemented revision, the patient lacks 45 degrees of quadriceps extension and has a poor result following traumatic fracture at three years postsurgery. One patellar dislocation (0.19%) occurred in a 270-pound, osteoarthritic man in whom undersized components were used. No polyethylene wear-through, separation from the metal anchoring plate, or implant breakage was seen in this series. Overall, the performance of this congruent-contact, metal-backed, rotating-bearing patellar prosthesis indicates the prosthesis can provide long-term function and stability with few short-term or long-term complications.