[Postero-postero-external instabilities of the knee: experimental study of an extra-articular system to protect reconstructions].

INTRODUCTION: Autologous tendon grafts still remain the best material for the reconstruction of the knee ligaments. They provide the most constant and durable results. For the cases of important postero-posterolateral instability, strong and biomechanically correct techniques of reconstruction exist, but most publications state the fact that laxities progressively reappear in the course of the rehabilitation, that is during the crucial period of fragility of the transplants due to remodeling. These grafts must therefore be protected without impinging on the functional therapy and without impeding the progressive introduction of the mechanical strains necessary for the metaplasia of the transplants. The authors present the biomechanical study of a protection system represented by a synthetic ligament placed between the posterior aspect of the fibular head and a point midway between the femoral insertions of the lateral collateral ligament and of the popliteus tendon. MATERIAL AND METHODS: Anatomical, radiological, mathematical studies and measures on cadaver knees were made. The antero-posterior laxity and the laxity in rotation were measured between 20 degrees and 120 degrees of flexion on 4 knees in the autopsy room and on 2 fresh frozen knees. 3 series of measures were made for each knee: firstly on intact articulations, secondly after cutting of each posterior and posterolateral ligamentary component successively, and finally after putting in place of the synthetic ligament, without any other reconstruction. Graphs showing the laxity in relation to flexion were established. RESULTS: The different studies confirm that the ligament does not significantly interfere with joint function, apart from reducing the automatic inwards rotation of the femur at the end of extension. The graphs concerning the intact knees are very similar to these established with the ligament in place. DISCUSSION AND CONCLUSION: Quasi normal function was restored by the protection ligament, after cutting of every posterior and posterolateral ligament, without any other reconstruction. This means that abnormal movements that could injure the autologous grafts were eliminated. Both posterior and posterolateral reconstructions should be therefore protected. We used this protection system for 12 cases since 1986. It definitely improved the results of our postero-posterolateral reconstructions.