Arguments for Sparing the Posterior Cruciate Ligament in Total Knee Arthroplasty

Advocates for PCL sparing total knee arthroplasty generally cite five theoretical advantages: (1) increases passive range of motion, (2) preserves the quadriceps lever arm, (3) enhances joint stability, (4) reduces bone-implant interfacial stress on the tibial component, and (5) improves joint proprioception. A review of the normal function of the PCL in the knee is required to understand these arguments.

As the knee is flexed, the PCL tethers the femur to the posterior tibia. This causes the femoral condyles to roll backward on the tibial plateau. The resulting anterior-to-posterior translation of the femur on the tibia has been termed femoral roll back.² The medial femoral condyle is more constrained than the lateral femoral condyle and, therefore, undergoes less roll back. This causes internal rotation of the tibia on the femur with knee flexion.

Preservation of normal femoral roll back is suggested as the mechanism for increased passive range of motion. Because of roll back, the femur moves backward on the tibia by 10 mm from full extension to full flexion. This posterior translation of the femur prevents it from impinging on the posterior portion of the tibial plateau during flexion. Posterior translation of the femur by 10 mm also increases the quadriceps lever arm by approximately 40% (Fig. 88-1).¹ This improves quadriceps power for knee extension from a flexed position. Proponents of PCL sparing have argued that this leads to improved stair climbing. They have suggested that patients without normal femoral roll back need to lean forward while climbing stairs so that their center of gravity falls anterior to the knee (counteracting the reduced quadriceps lever arm).¹

FIGURE 88-1 An illustration of increased quadriceps lever arm due to femoral roll back.

(From Andriacchi TP, Galante JO: Retention of the posterior cruciate in total knee arthroplasty. J Arthroplasty 3(Suppl): 13-19, 1988, p.S17.)

The PCL is the primary stabilizer for posteriorly directed forces on the tibia. These forces may be quite high during normal activities (e.g., approaching body weight during walking).¹ Without the native PCL, these forces may be transferred to the bone-implant interface. Two types of PCL-sacrificing implants recreate the posterior stabilizing effect of the PCL using different designs: (1) posterior stabilized (PS) implants have a post on the tibial component and a cam on the femoral component, and (2) ultracongruent (UC) implants have deep troughs in the tibial component to accommodate the femoral condyles. In these PCL-sacrificing implants, a posterior-directed force on the tibia will pass through the tibial component and result in a shear force at the bone-implant interface. This may accelerate loosening of the tibial component.

Because of its location in the middle of the knee, the PCL is also a secondary stabilizer for both varus and valgus forces placed on the tibia.¹ The typical varus thrust that occurs with walking on level ground results in an adduction moment about the knee joint. This is resisted by the PCL through a relatively short lever arm (Fig. 88-2). This function of the PCL may not be fully replicated by PS and UC implants. As a result, there may be greater forces through the medial compartment than the lateral compartment when the PCL is sacrificed.

FIGURE 88-2 The PCL produces a stabilizing moment (force in PCL) that acts at a distance (lever arm) from the medial compartment. The PCL is therefore a secondary stabilizer acting against the normal adduction moment (or varus thrust) that occurs with walking.

(From Andriacchi TP, Galante JO: Retention of the posterior cruciate in total knee arthroplasty. J Arthroplasty 3(Suppl): 13-19, 1988, p.S17.)

The PCL, like the anterior cruciate ligament (ACL), contains mechanoreceptors that detect knee joint position (i.e., proprioception). These mechanoreceptors sense stress placed on the ligaments and act through a reflex arc to stimulate the dynamic stabilizers of the knee (i.e., muscles that cross the knee joint). Proprioceptive deficiency in the knee may lead to knee instability, gait abnormalities, and difficulties with balance.³ Improved knee joint proprioception through retention of the PCL may result in better patient function after total knee arthroplasty.⁴

Arguments for Sacrificing the Posterior Cruciate Ligament in Total Knee Arthroplasty

Advocates for PCL-sacrificing total knee arthroplasty generally cite three possible advantages: (1) it facilitates correction of deformity, (2) it enhances range of passive motion, and (3) it decreases stress at the bone-implant interface. Several authors suggest that release of the PCL allows for improved correction in knees with more than 15 degrees of varus or valgus deformity and knees with more than 15 degrees of fixed flexion deformity.^5,6 Matsueda and colleagues⁷ showed in a cadaver study that release of the PCL led to the ability to achieve significantly greater tibiofemoral coronal angles during both medial and lateral releases. They also found that release of the PCL produced extension gaps of 10.9 mm and flexion gaps of 6.6 mm. This would allow for correction of significant flexion deformities.

The PS implant recreates the femoral roll back through the articulation between the cam on the femoral component and the post on the tibial component. Femoral roll back, as stated earlier, allows for increased passive range of motion and quadriceps mechanical function. Through fluoroscopic analysis, femoral roll back has been shown to be more consistent with PS implants than cruciate-sparing implants.⁸ Some cruciate-sparing implants exhibited paradoxical anterior femoral translation. The UC implant has an anterior lip and tibial surface that conforms to the femoral condyles to substitute for the PCL. This does not recreate femoral roll back; however, the axis of rotation of the femur on the tibia is placed posterior on the tibial plateau. This allows for improved range of motion and quadriceps mechanical function in the absence of femoral roll back.²

The PCL is thought to impart posterior stability to the flexed knee through two mechanisms. First, it is taut in flexion and orientated somewhat horizontal to the tibia, so that it may directly resist posterior displacement of the tibia on the femur.² Second, the PCL holds the femoral condyles down on the tibial surface. The tibial surface slopes upward anteriorly, and this is further enhanced by the menisci.² With the PCL securing the femoral condyles to the tibial surface, the condyles will impinge on the anterior sloping tibial surface if the tibia experiences a posterior-directed force. For the PCL to provide posterior stability through these mechanisms, it must have the appropriate tension in flexion. This is achieved by varying bone cuts on the superior tibia and posterior femur (i.e., varying the flexion gap).