Testing for isometry during reconstruction of the posterior cruciate ligament. Anatomic and biomechanical considerations.

The change in the distance of linear separation between each pair of osseous fiber attachment sites of the posterior cruciate ligaments was measured and plotted as a function of the knee flexion angle from 0 degree to 120 degrees. Data were collected under four sequential test conditions that had in common quadriceps relaxation, absence of tibial rotation forces, and horizontal femoral stabilization. The posterior cruciate ligament fibers were intact or transected (excursion wires left intact) with gravitational joint distraction of the lower leg unconstrained or constrained. The small, posterior oblique fiber region was the most isometric of the four tested fiber regions. Progressively increasing deviations from isometry were seen in the posterior longitudinal, central, and anterior fiber regions, in that order. Transection of the posterior cruciate ligament, combined with unconstrained gravitational distraction of the knee joint, further increased the magnitude of deviation from isometry of the anterior and central fibers, but only changed the pattern of deviation for the more nearly isometric posterior fibers. Under simulated operative conditions, most of the posterior cruciate ligament's anatomic attachment sites exhibit nonisometric behavior, with near isometry demonstrated only by the relatively small posterior fiber attachment sites. If isometry alone is used for bone tunnel placement, the large anterior and central fiber regions will be left largely unreconstructed. Because the normal behavior of most of the fibers of the posterior cruciate ligament involves 4 to 6 mm of end-to-end length increase with progressive knee flexion, this pattern and degree of deviation from isometry should be sought to approximate an anatomic reconstruction of the anterocentral bulk of the ligament.