We compared the ability of three different posterior cruciate ligament (PCL) reconstructions to restore normal anteroposterior laxity to the knee from 0 to 130 degrees of knee flexion. Cadaver knees were tested intact, after PCL rupture or after bone-patellar tendon-bone grafting. Grafts were performed isometrically or with a single bundle representing the anatomical anterior PCL fibre bulk (aPC) or with a double bundle that added the posterior PCL fibre bulk (pPC). The grafts were tensioned to restore normal knee laxity at 60 degrees of flexion, except for the pPC which was tensioned at 130 degrees. The isometric graft led to overconstraint as the knee extended resulting in high graft tension in extension and excess laxity in flexion. The aPC graft matched normal laxity from 0 to 60 degrees of flexion but was lax from 90 to 130 degrees of flexion. Only the double-bundled graft could restore normal knee laxity across the full range of flexion.
This paper reviews and updates our knowledge of the anatomy and biomechanics of the posterior cruciate ligament, and of the posterolateral, posteromedial and meniscofemoral ligaments of the knee. The posterior cruciate ligament is shown to have two functional fibre bundles that are tight at different angles of knee flexion. It is the primary restraint to tibial posterior draw at all angles of knee flexion apart from near full extension. In contrast, the posterolateral and posteromedial structures are shown to tighten as the knee extends, and to be well-aligned to resist tibial posterior draw. These structures also act as primary restraints against other tibial displacements. Tibial internal rotation is restrained by the medial and posteromedial structures, while tibial external rotation is restrained by the lateral and posterolateral structures. They are also the primary restraints against tibial abduction-adduction rotations. The meniscofemoral ligaments are shown, for the first time, to contribute significantly to resisting tibial posterior draw, and to have a strength of approximately 300 N. Taken together, this evidence shows how the posterolateral and posteromedial structures are responsible for posterior knee stability near extension, and this, along with the action of the meniscofemoral ligaments, may explain why an isolated rupture of the posterior cruciate ligament does not often lead to knee instability
Anterior cruciate ligament (ACL) reconstruction depends critically on isometric graft placement. Unfortunately, different supposedly isometric points have been published, and no prior work has compared them to find out which are really isometric. The purpose of this study was to compare the isometry of previously published 'isometric' points for ACL reconstruction. The isometric points and knee loadings of previous studies were reproduced accurately in 12 fresh cadaveric knees. The length changes were measured through 140 degrees knee flexion, using an intra-articular suture attached to a displacement transducer. Six points had less than 1 mm length change and were located proximally in the natural ACL attachment at the posterior end of Blumensaat's line. The other seven points had length change patterns that would cause ACL graft tightening or slackening with knee flexion if they were used as the sites of bone tunnels for graft placement. This study confirms the existence of an isometric zone close to the posterior end of Blumensaat's line under several loading conditions. Other graft attachment points are less suitable for ACL reconstruction.
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