Chronic anterolateral instability of the knee. A roentgen stereophotogrammetric evaluation

The rotatory stability of the knee was investigated in 20 patients with a previous tear of the ACL. The three-dimensional movements of the tibia during the testing procedure were registered using roentgen stereophotogrammetric analysis. Changes of the tibial movements between an intermediate, an anterior, or a posterior tibial position were registered using tibial tractions in combination with a simultaneous external or internal rotatory torque. At 20 degrees of flexion the internal rotatory laxity was increased on the injured side. The external rotatory laxity did not significantly differ between the two sides. With anterior traction, the internal rotatory laxity increased on both the injured and the normal sides and became almost equal. The external rotatory laxity manifested a decrease which was most pronounced on the injured side. With posterior traction, the rotatory laxities decreased and did not significantly differ between the two sides. Analysis of the simultaneously occurring translations of the tibial plateau disclosed abnormal displacements of both the medial and the lateral tibial condyles on the injured side.