Gradual low riding of the patella during postoperative course after total knee arthroplasty in osteoarthritis and rheumatoid arthritis.

Patellar height in the lateral view of radiograms was measured on 94 knees of 61 women before and after total knee arthroplasty in order to study postoperative changes of position. The mean age of patients at surgery was 58.4 years (range, 30-77 years). Eighty-one knees were rheumatoid and 13 osteoarthritic. Inserted prostheses were Kinematic anteriorly joined type in 49 knees, Kinematic posterior cruciate retention type in 19, and UCI type in 26. The observation period ranged from 6 months to 9 years, with an average of 2.6 years after surgery. The ratio of length of the patellar ligament to that of the patella (Insall-Salvati's ratio) decreased by more than 10% of preoperative value in 61 knees, remained unchanged in 31, and increased in only 2. The average ratio was 0.93 +/- 0.16 before surgery and 0.75 +/- 0.20 at the final follow-up examination (P less than .01).