THE INFLUENCE OF MUSCLE FORCES AND EXTERNAL LOADS ON CRUCIATE LIGAMENT STRAIN

We know it is important to avoid excessive strain on reconstructed lig aments, but we do not know how individual muscles affect cruciate liga ment strain. To answer this, we studied the effect of muscle forces an d external loads on cruciate ligament strain. Nine cadaveric knee join ts were tested in an apparatus that allowed unconstrained knee joint m otion. Quadriceps, hamstring, and gastrocnemius muscle forces were sim ulated. Additionally, external loads were applied such as varus-intern al or valgus-external rotation forces. Cruciate ligament strain was re corded at different knee flexion angles. Activation of the gastrocnemi us muscle significantly (P < 0.05) strained the posterior cruciate lig ament at flexion angles larger than 40 degrees. Quadriceps muscle acti vation significantly strained the anterior cruciate ligament when the knee was flexed 20 degrees to 60 degrees (P < 0.01) and reduced the st rain on the posterior cruciate ligament in the same flexion range (P < 0.05). Activation of the hamstring muscles strained the posterior cru ciate ligament when the knee was flexed 70 degrees to 110 degrees (P < 0.05). Combined varus and internal rotation forces significantly incr eased anterior cruciate ligament strain throughout the flexion range ( P < 0.05). The results suggest that to minimize strain on the ligament after posterior cruciate ligament surgery, strong gastrocnemius muscl e contractions should be avoided beyond 30 degrees of knee flexion. Th e study also calls into question the use of vigorous quadriceps exerci ses in the range of 20 degrees to 60 degrees of knee flexion after ant erior cruciate ligament reconstruction.