Mechanical Function of the Patella

The patella contributes to the knee extension moment arm through the entire range of knee motion. Its contribution increases with progressive extension. In terms of extension moment, special modifications of transverse repair have no advantage over simple transverse approximation of the patellar ligament and quadriceps tendon, but transverse repair is superior to longitudinal repair. Depending on the technique of patellectomy defect repair, full postpatellectomy extension may require as much as a 30 per cent increase in quadriceps force. Tibial tubercleplasty can lengthen the extension moment arm and permit full extension with no greater force than that required in the knee before patellectomy. A 30 per cent increase in quadriceps force may be beyond the capacity of some patients, particularly those with long-standing intra-articular disease, those in advanced age, or those who have extension lag prior to operation. If patellectomy is indicated for this select group of patients, tibial tubercleplasty ought to be considered.