Isometry and graft placement in posterior cruciate ligament reconstructive surgery

Sagittal plane motion of the knee may be modeled as a four-bar linkage. The point where the cruciate links cross is the instant center of rotation or flexion axis. The axis moves posteriorly as the knee flexes. The use of the four-bar linkage model to explain the effect of graft placement on isometric measurements is presented. To perform valid isometric measurements in the posterior cruciate ligament (PCL)-disrupted knee, an anterior force must be applied to the knee to prevent posterior tibial subluxation, and an axial force must be applied to prevent joint distraction.