Lateral dislocation of the patella following Marmor and Guepar arthroplasty of the knee.

The follow-up of 50 knees treated with the modular Marmor prosthesis and 50 with the Guepar hinge joint included tangential radiography of the femoro-patellar joint. After Marmor arthroplasty lateral patellar dislocation was found in seven knees, in six causing pain. After Guepar arthroplasty lateral patellar dislocation was found in 32 knees, but pain was present in only three. There was only a questionable correlation between pre-existing valgus deformity and postoperative lateral patellar dislocation, and an external rotation deformity of the lower leg was found in only five cases after Guepar arthroplasty. Thus, neither phenomenon could explain the high incidence of lateral patellar dislocation in the Guepar series. These dislocations must be assumed to be due to the unphysiological strain on the extensor apparatus inherent in this type of prosthesis.