Biomechanics of the Patellofemoral Joint

Abstract

The main biomechanical function of the pa-tella is to improve the quadriceps efficiency by increasing the lever arm of the extensor mechanism. The patella displaces the patellar tendon away from the femorotibial contact point throughout range of motion, therefore increasing the patellar tendon moment arm.1-3 There are other important functions. The patella is necessary to centralize the divergent forces coming from the four heads of the quadriceps, and to transmit tension around the femur in , a frictionless way, to the patellar tendon and tibial tuberosity. The hyaline articular cartilage provides an insensitive (aneural), thick, avascular tissue that is specifically adapted to bearing high compressive 10ads. 2 ,3 The patella also functions as a bony shield, not only for the trochlea, but also for the distal femoral condyles with the knee in flexion. Finally , the patella plays an important role in the cosmetics of the knee. This can be readily appreciated by observing the squared appearance of the flexed knee in patellectomized patients. Knee Extension The patella is an important contributor to knee extension power. An important mechanical study designed to define and quantitate the extensor function of the patella and the effects of its removal was performed on ca-daver limbs by Kaufer (1971-1979).4,5 The extension moment arm was determined at 120°, 90°, 60°, 30°, and 0° of flexion before patellectomy; after patellectomy, performed through a longitudinal tendon-splitting incision , and before tendon repair; after patel-lectomy and longitudinal repair, and after pa-tellectomy and transverse repair of the quad-riceps tendon to the patellar ligament. The force required for full extension of the knee was applied with a rope sutured to the quad-riceps tendon. The rope was passed over a pulley positioned to simulate the direction of the quadriceps pull and traction weights were added until full extension was reached. The intact knee had a longer quadriceps moment arm than the patellectomized knee (Ta-ble 3.1). In the intact knee, the quadriceps moment arm and the contribution of the patella increased almost linearly with knee extension. At 120°, the patella accounted for 0.4 cm (ap-proximately 10%) of the quadriceps moment arm. At full extension, the patella's contribution increased to 1.8 cm (approximately 30%) ofthe quadriceps moment arm. The longitudinal repair after patellectomy had no effect on the quadriceps moment arm. The transverse repair slightly increased the quad-riceps moment arm. The force necessary for full extension of the intact knee, an average of 21 kg, was termed the 100% force. After patellectomy and longitudinal closure, this same force failed to produce full extension by almost 30°; full extension required an average force of 28.5 kg 25 G. R. Scuderi (ed.), The Patella