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 o sseous fiber attachment sites of the posterior cruciate ligaments was measured and plotted as a function of the knee flexion angle from 0 de grees to 120 degrees, Data were collected under four sequential test c onditions that had in common quadriceps relaxation, absence of tibial rotation forces, and horizontal femoral stabilization, The posterior c ruciate ligament fibers were intact or transected (excursion wires lef t intact) with gravitational joint distraction of the lower leg uncons trained or constrained, The mall, posterior oblique fiber region was t he most isometric of the four tested fiber regions, Progressively incr easing deviations from isometry were seen in the posterior longitudina l, central, and anterior fiber regions, in that order, Transection of the posterior cruciate ligament, combined with unconstrained gravitati onal 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 posteri or fibers. Under simulated operative conditions, most of the posterior cruciate ligament's anatomic attachment sites exhibit nonisometric be havior, with near isometry demonstrated only by the relatively small p osterior fiber attachment sites, If isometry alone is used for bone tu nnel placement, the large anterior and central fiber regions will be l eft largely unreconstructed. Because the normal behavior of most of th e 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.