Function of the Posterior Cruciate Ligament and Posterolateral Ligament Structures

Effect on Limits of Extension

The normal fully hyperextended knee position averaged 5.6 ± 3.8 degrees. After sectioning the PCL or PLS alone, only slight and clinically insignificant increases in hyperextension were noted (such as 2.6 ± 0.09 degrees when the PLS was cut). When both the PCL and PLS were sectioned, the increase in hyperextension was 4.0 ± 1.4 degrees.

The data indicate that there is no primary restraint in the knee that resists hyperextension. Many structures work in synergy to resist hyperextension, and it is therefore not surprising that sectioning the PLS or PCL results in only a small increase. Other structures anatomically positioned to resist hyperextension include the entire posterior knee capsule (femoral-meniscal tibial attachments), oblique popliteal ligament, fabellofibular ligament, and posterior fibers of the PCL and anterior cruciate ligament (ACL). A reconstructive procedure is described in Chapter 17 for knees with severe knee hyperextension. This procedure is performed in addition to other required ligament reconstructive procedures.

Effect on Anteroposterior Translation Limits

The normal anterior and posterior translation knee limits are shown in Figure 15-1, A . The increase in these limits when the PCL is cut are shown in Figure 15-1, B , and the further increase in these limits when the PLSs are also sectioned is shown in Figure 15-1, C . The values for the increase in motion limits are given in Table 15-1 .

The data show that the PCL is a primary restraint to posterior tibial translation throughout knee flexion, with the exception of a small increase in posterior translation at full extension when the PLSs are cut. The clinical finding of a knee with increased posterior translation at 30 to 45 degrees of knee flexion, similar to the posterior translation limit at 90 degrees ( Fig. 15-2 ), indicates associated injury to the PLSs and the medial structures.

It should be noted that a complete description of the translation limits requires an anatomic measurable point on the tibia, which is selected at the central coronal point on the tibia. The limits of posterior translation to the medial and lateral tibiofemoral joints are described in a subsequent study.

In a knee with a combined deficiency of the PCL and PLS, the abnormal posterior tibial translation is at least four to five times the normal limit throughout knee flexion.

Effect on Internal-External Tibial Rotation Limits

The normal knee motion limits to internal and external tibial rotation are shown in Figure 15-3, A . There is no increase in external tibial rotation when the PCL is cut alone (see Fig. 15-3, B ). This finding demonstrates that the PLSs are the primary restraint for external tibial rotation throughout knee flexion. When only the PLSs are sectioned, an increase in external tibial rotation occurs (see Fig. 15-3, C ), which is greatest at 30 degrees of knee flexion. The amount of external tibial rotation decreases as the knee is flexed, showing the influence of the PCL in limiting external tibial rotation after the PLSs are sectioned.

These data indicate that the PLSs provide the primary restraint to external tibial rotation at low knee flexion positions and therefore should be tested in this range (20-40 degrees of flexion). Increases in external tibial rotation at 90 degrees indicate injury to both the PLS and PCL, consistent with the dial test (see Chapter 17 ). The data are in disagreement with the classic interpretation of the posterolateral drawer test at 90-degree flexion, indicating injury to only the PLS. The resisting function of the FCL, popliteus tendon, and PFL is described in separate studies mentioned later in this chapter.

Gollehon and coauthors reported similar results showing an increase in external tibial rotation of 20-degree flexion after sectioning the PLS at 30 degrees of flexion. Sectioning of the PCL along with the PLS did not produce further significant increases in external tibial rotation ( Fig. 15-4 ).