Multiplane loading of the extensor mechanism alters the patellar ligament force/quadriceps force ratio

Since the direction of the quadriceps force and location of the patellofemoral contact point likely differ between axial and multiplane loadings, the force and moment balance solutions for a multiplane loading condition may not yield the same patella ligament force/quadriceps force ratio (F(PL)/F(Q) ratio) when compared with an axial loading condition. The purpose of this study was to compare the effects of an axial loading condition and an anatomical, multiplane loading condition on the F(PL)/F(Q) ratio at various knee flexion angles. Ten cadaver knees were used in this investigation. Each was mounted on a custom jig that was fixed to an Instron frame. Quadriceps muscle loads were applied with same resultant force magnitudes under two force directions, as follows: (1) axial loading (central quadriceps tendon loading parallel to the femoral axis), and (2) an anatomically based, multiplane loading condition (individual vasti loaded, taking into consideration physiologic muscle fiber orientation). Patellar ligament tension was measured using a buckle transducer. The patellar ligament force/quadriceps force ratio (F(PL)/F(Q) ratio) was calculated for both loading conditions at 0 deg, 20 deg, 40 deg, and 60 deg of knee flexion. Across the range of knee motion evaluated, the F(PL)/F(Q) ratio for the axial loading condition was significantly greater than the F(PL)/F(Q) ratio for the multiplane loading condition. Our results suggest that loading orientation affects the transfer of forces from the quadriceps tendon to the patellar ligament.