Patellofemoral problems represent a significant source of morbidity following total knee arthroplasty (TKA). Patellofemoral biomechanics and contact stress following TKA depends (among other factors) upon the position of the patella relative to the tibiofemoral joint. Patellar height may be altered either by changes in the tibio-femoral joint level (pseudo patella baja/alta) or by a change in patella tendon length (true patella baja/alta). The purpose of this study is to examine the latter. Two previous studies have described patella tendon shortening following TKA but both have significant limitations and produced differing results.

The aim of this study was to identify the incidence of true patella tendon shortening following TKA for the treatment of osteoarthritis (OA).

All patients undergoing primary TKA for OA in 2001 and 2002 were identified. Cases were excluded if they had rheumatoid arthritis, had undergone previous open knee surgery, suffered a significant post-op complication, or had less than 1 year radiological follow up. Case notes and radiographs of 50 knees in 34 patients were reviewed. The Insall-Salvati ratio was measured on immediate pre-op, initial post-op, and final follow-up lateral knee radiographs. Differences between mean pre and post-op ratios were compared using a paired t-test. There were 19 women and 15 men aged 47 to 84 (mean 70.4) years. The mean pre-op Insall-Salvati ratio was 0.99. The initial post-op ratio was unchanged (p=0.06). After a minimum of 1 year the mean ratio remained 1.0 (p=0.09). In no knee was there a significant change in patella tendon length.

In this study we found no evidence of patella tendon shortening. Two other studies have identified shortening in one third to two thirds of knees. The methodology of these studies is however open to criticism. The patients in neither study are representative of general orthopaedic practice; the surgical technique in one was unorthodox and the radiological measurement method in the other not validated. This work therefore represents the first study of patella tendon length following TKA using a validated radiological index in a representative osteoarthritic population.

In conclusion, TKA in this group of patients with osteoarthritis, employing a standard surgical technique was not associated with postoperative patella tendon shortening or true patella baja.