Retropatellar contact stress in simulated patella infera.

Six fresh-frozen cadaver knee joints were used to study changes in retropatellar contact mechanics accompanying patella infera. The knees were tested on a servohydraulic testing machine under conditions simulating stair descent at 10 degrees, 30 degrees, 60 degrees, and 90 degrees of knee flexion. A slotted metallic block mechanism embedded in the region of the tibial tubercle allowed selective distal offset of the patellar tendon insertion so as to model conditions of 0, 6, 13, 19, and 25 mm of patella infera. Patellofemoral and quadriceps tendofemoral contact areas and contact stresses were recorded using Pressensor contact film and quantitated using digital image analysis. Patella infera significantly altered retropatellar contact mechanics. Contact areas migrated proximally on the patella and decreased in size with progressive severity of patella infera. However, the peak and spatial mean retropatellar contact stresses were not elevated correspondingly. Apparently, quadriceps tendofemoral contact was initiated at progressively lower angles of knee flexion as the patella infera progressed. Under conditions of extreme infera at high flexion angles, the magnitude of tendofemoral contact force approached that of retropatellar contact force. These data indicate that in patella infera, patellofemoral contact stresses are not elevated appreciably. Therefore, the disabling symptoms associated with patella infera may be due to factors other than local mechanical overload.