The unloaded knee behaves in a predictable fashion because its articular surfaces do not alter their shapes and the ligaments do not stretch during passive motion. When significant loads are applied, both these things happen. Ligaments stretch under tension and articular surfaces indent under compression, straining the constraints to movement and profoundly modifying, even reversing, the underlying patterns of movement described above.
The role of the ligaments in controlling this pattern of movement is readily demonstrated. Ligament sectioning studies (Butler et al., 1980; Grood et al., 1981; Piziali et al., 1980; Seering et al., 1980) have established that the anterior cruciate ligament (ACL) is the primary constraint upon anterior tibial translation (and a secondary constraint on internal tibial rotation) and the posterior cruciate ligament (PCL) is the primary constraint on posterior tibial translation (and a secondary constraint on external tibial rotation). The collateral ligaments are the primary constraints on abduction and adduction and on internal rotation (MCL) and external rotation (LCL). This and similar evidence was not confronted by Freeman’s group in dismissing the contribution of the PCL to the kinematics and mechanics of the knee, concluding that the strength of the PCL in man “may represent an evolutionary vestige, not a contemporary necessity”, Nakagawa et al. (2004).
The role of the articular surfaces is difficult to study because it is not possible to alter the shape of an articular facet without simultaneously making some ligament fibres slack (causing instability) or tightening others (causing limitation of movement). Therefore, unlike the ligaments, no particular movement constraint can be attributed to any particular feature of joint surface shape, the function of the articular surfaces being mainly to keep the ligaments at their appropriate tension by resisting interpenetration. As we shall see, prosthetic articular surfaces, if they reproduce only this function, can restore normal movement even if they are not shaped exactly like the natural surfaces.
Perturbation tests
We return to the study of cadaver specimens described earlier in Figures 3.1 to 3.4. During some of those tests, the specimens were held stationary at a number of positions within the flexion range and medial and lateral forces applied by the experimenter’s finger-tip to the proximal end of the intramedullary rod attached to the femur. The object was to establish the extent to which the passive path of motion could be perturbed by the application of external force. Figure 3.19 shows how the tight hysteresis loops of Figures 3.2 and 3.3 were readily perturbed by the application of even such light forces but that the perturbations to motion were immediately removed and the unique passive path of motion restored when the perturbing forces were removed. The preferred path of motion is therefore dynamically stable.
The passive laxity of the joint exhibited by these perturbations arises mainly from the deformation of the ligaments and articular surfaces.
