Figure 3.20 Fibres of the model ACL, with the knee at 50° flexion, (a) slacken and (c) tighten when the tibia is pulled backward and forward from the neutral unloaded position (b). Fibres shown buckled are slack, fibres shown straight are either isometric or stretched (see website, Animation 5) (Lu & O’Connor 1996a).
Figure 3.20 shows how the ACL fibres of the model knee slacken (Fig. 3.20(a)) or tighten (Fig. 3.20(c)) when the tibia is pushed backwards or pulled forward distances of 5 mm from the neutral unloaded position (Fig. 3.20(b)), in a simulation of the Lachman or Drawer test. Fig. A6(a) in the Appendix shows that backward movement of the model tibia on the femur of 5 mm is sufficient to relax the ACL completely and to stretch all fibres of the LCL, all fibres of the posterior bundle of the PCL and about half the anterior bundle. Ligament stretch leads to anterior/posterior translation of the femur on the tibia, a manifestation of passive laxity. In a further elaboration of the model, Huss et al. (1999) showed that deformation of the cartilage in the model knee adds only modestly to the calculated passive laxity. The successive recruitment of ligament fibres to resist increasing load makes the ligaments increasingly resistant to elongation and increases their effective extensional stiffness. This largely explains why the effective resistance to imposed displacement of the bones, as in the drawer test, increases as the displacement increases.
Figure 3.21 Total A/P tibial translation for applied A/P forces of 67 N plotted against flexion angle for the model knee with both cruciate and collateral ligaments intact, compared with the results of experiments by Grood and Noyes 48.
Figure 3.21 plots the total anteroposterior translation (anterior plus posterior) induced by an anteroposterior force of 67 N against flexion angle, calculated using the knee model with extensible ligaments, Zavatsky and O’Connor (1992b), compared with the results of measurements on human knees reported by Grood and Noyes (1988). 67 N was Grood’s estimated value of the force applied by the typical examiner in performing the drawer test. The calculation predicts the value of the maximum total laxity at about 30° quite well but overestimates the measured laxity at 60° and 90°. Feikes (1999) also estimated the total anteroposterior laxity using her 3-D knee model (Fig. 3.13), with similar conclusions.
Passive rotational laxity of the knee
O’Connor, Zavatsky and Gill (2003) described work by Zavatsky on the path of motion when the otherwise unloaded joint was flexed and extended in the presence of either internally or externally rotating torques of 1, 2 and 3 Nm (Fig. 3.22). The applied torque rotated the knee in its own direction and, if anything, tightened further the hysteresis loops. The internally rotating torques further increased the internal tibial rotation exhibited during purely passive motion (the loop marked 0), while the external rotation produced near extension by the externally rotating torque tended to be cancelled out during further flexion of the joint. The curves for torques of ±3 Nm are similar to those reported by Blankevoort et al. (1988) and described by them as the “envelopes of passive motion”. Figure 3.22 shows that the paths of motion in the presence of smaller torques or none are very well defined and ordered, lying within the Blankevoort envelopes. The 0 loop is almost identical to that of Figure 3.2, though obtained in a different apparatus.
Figure 3.22 External (curves marked -1, -2 and -3) and Internal rotation (curves marked 1, 2 and 3) of the tibia of a specimen knee when flexed and extended in the presence of an externally or internally rotating torque, compared to the path of passive motion of the wholly unloaded specimen (loop marked 0).
These effects quantify the results of the perturbation tests reported in Figure 3.19 and are consistent with the reports of others, both in vitro (Iwaki et al., 2000) and in vivo, Hill et al. (2000).
Quadriceps/ligament interactions, dynamic laxity
Many of the activities of daily living require action from the quadriceps muscles. We now discuss how quadriceps action alters the path of passive motion by stretching and slackening the ligaments and by indentation of the articular surfaces, giving the joint its dynamic laxity.
Figure 3.23 plots axial rotation of 10 cadaver specimens against flexion angle during a simulation of deep squats (mean values ± one standard deviation) and compares the path of passive motion when the specimens were flexed and extended in the absence of external load (marked P) with movements observed in the presence of a vertical load on the hip/ankle axis, balanced by tension in a wire sewn to the quadriceps tendon (marked Q).
Effect of muscle force on coupled axial rotation
Figure 3.23 demonstrates how tension in the muscle tendons changes the preferred path of motion. It compares the path recorded in ten specimens during passive deep squats (as in Fig. 3.5 above) and during deep squats in the presence of vertical load on the hip/ankle axis balanced by tension in a wire sewn to the quadriceps tendon (O’Connor et al. 2003). The maximum coupled internal tibial rotation was reduced from about a mean value of 22° to about 15°, rotation occurring mainly near extension and further rotation ceasing once the knee was bent to about 50°, perhaps the reason why it is commonly called “terminal rotation”. Tension in the patellar tendon near extension induces increased tibial rotation and in the flexed knee offers increasing resistance to rotation, with the patella now firmly lodged in the trochlear groove.
