Figure A5 shows how the model ACL responds when the tibia is moved backwards and forwards relative to the femur (as in a drawer test) at a fixed flexion angle (Zavatsky & O’Connor, 1992b; Lu & O’Connor, 1996b). With the unloaded knee flexed to 50°, all but the anterior fibres of the ACL are slack (Fig. A5(b)). When the tibia is pushed backwards 5 mm from the neutral position, the anterior fibre slackens and all other fibres slacken further (Fig. A5(a)). When the tibia is pulled forwards 5 mm from its neutral position, ACL fibres are progressively tightened and stretched to bear load (Fig. A5(c)). A 5 mm anterior translation tightens and stretches about half the model ACL (see Animation 5).
Figure A5 Fibres of the model ACL, with the knee at 50º flexion, (a) slacken and (c) tighten when the tibia is pulled backward and forward from the neutral unloaded position (b).
Figure A6 (and Animation 6) show similar diagrams of the model knee with all four ligaments modelled as arrays of fibres. Posterior translation of the tibia (Fig. A6(a)) tightens the PCL and the LCL, and slackens the ACL and the MCL. Anterior translation (Fig A6(c)) tightens the ACL and the MCL, and slackens the PCL and the LCL. The ligaments offer increasing resistance to anteroposterior displacement from the neutral position (Fig. A6(b)) as more and more fibres are recruited to bear load, giving the knee its characteristic laxity. The laxity allowed by ligament strain is further increased by indentation of the articular surfaces under load (Huss et al., 1999). The calculation of the anteroposterior laxity of the model joint under a drawer force of 67 N by Huss et al. (1999) agrees well with measurements made by Grood and Noyes (1988), providing a validation of the model (see Fig. 3.21).
Discussion
The laxity allowed by stretching of the ligaments should be recovered after unconstrained unicompartmental arthroplasty which retains all the ligaments and restores them to their natural tensions. The contribution to laxity attributed to deformation of the surfaces will be lost when they are replaced by more rigid prosthetic components. However, the contribution of surface deformation in the intact joint is relatively small (Huss et al., 1999).
