A BIOMECHANICAL STUDY OF REPLACEMENT OF THE POSTERIOR CRUCIATE LIGAMENT WITH A GRAFT .1. ISOMETRY, PRE-TENSION OF THE GRAFT, AND ANTERIOR-POSTERIOR LAXITY

Twelve fresh-frozen knee specimens from cadavera were subjected to ant erior-posterior laxity testing with 200 newtons of force applied to th e tibia; testing was performed before and after a femoral load-tell wa s connected to a mechanically isolated cylindrical cap of subchondral femoral bone containing the femoral origin of the posterior cruciate l igament. The posterior cruciate ligament then was removed, the proxima l end of a thin trial isometer wire was attached to one of four points designated on the femur, and displacement of the distal end of the wi re relative to the tibia was measured over a 120-degree range of motio n. The potted end of a ten-millimeter-wide bone-patellar ligament-bone graft was centered over the femoral origin of the ligament and attach ed to the femoral load-cell. Isometry measurements were repeated with the wire attached to the bone block of the free end of the graft in th e tibial tunnel. Force was recorded at the load-cell (representing for ce in the intra-articular portion of the graft) as pre-tension was app lied, with use of a calibrated spring-scale, to the tibial end of the graft. A laxity-matched pre-tension of the graft was determined such t hat the anterior-posterior laxity of the reconstructed knee at 90 degr ees of flexion was within one millimeter of the laxity that was measur ed after installation of the load-cell. Anterior-posterior testing was repeated after insertion of the graft at the laxity matched pre-tensi on. The least amount of change in the relative displacement of the tri al wire over the 120-degree range of flexion occurred when the wire wa s attached to the proximal point on the femur (a point on the proximal margin of the femoral origin of the posterior cruciate ligament, midw ay between the anterior and posterior borders of the ligament). The gr eatest change in the relative displacement was associated with the ant erior point (a point on the anterior margin of the femoral origin of t he ligament, midway between the proximal and distal borders). The mean relative displacements of the trial wire when it was attached to a po int at the center of the femoral origin of the ligament were not signi ficantly different from the corresponding mean displacements of the di stal end of the graft when the proximal end of the graft was centered at this point. At 90 degrees of flexion, the force recorded by the loa d-cell averaged 64 to 74 per cent of the force applied to the tibial e nd of the graft. The laxity-matched pre-tension of the graft at 90 deg rees of flexion (as recorded by the load-cell) ranged from six to 100 newtons (mean and standard deviation, 43.0 +/- 33.4 newtons). With the numbers available, the mean laxities after insertion of the graft wer e not significantly different, at any angle of flexion, from the corre sponding mean values after installation of the load-cell. CLINICAL REL EVANCE: Isometer readings front a trial wire attached to a point on th e femur provided an accurate indication of the change in the length of a graft subsequently centered at that point. Anteriorly placed femora l tunnels should be avoided, as the isometer readings indicated increa sed tension, with flexion of the knee, in a graft placed in this regio n. The force in the intra-articular portion of the graft was always le ss than the force applied to the bone block in the tibial tunnel. Ther efore, the femoral end of the graft should be tensioned to avoid frict ional losses from the severe bend in the graft as it passes over the p osterior tibial plateau. With correct pre-tensioning of a graft, norma l anterior-posterior laxity at 0 to 90 degrees of flexion can be resto red. However, because of the considerable range in the laxity-matched pre-tensions, we recommend that the pre-tension be greater than forty- three newtons for all patients to ensure that normal laxity is restore d.