Posterior cruciate ligament avulsion injury
Tarek Boutefnouchet and Ayaz Lakdawala review current concepts and surgical approaches to deal with PCL avulsion injury
The posterior cruciate ligament (PCL) is an important stabiliser of the knee. Isolated injuries are less common and surgery on the PCL has seen less popularity compared with other knee ligaments. Recent advances in understanding of PCL anatomy, biomechanics and imaging studies have begun to shift the paradigm of PCL injuries and reconstruction. These injuries might in fact be more common than originally conceived, accounting for an estimated 3 per cent of all knee injuries and 37 per cent of soft-tissue knee injuries [1]. PCL avulsion from the posterior aspect of the proximal tibia presents a different group of injuries which has its own challenges. The aim of this article is to review the current concepts pertinent to PCL avulsion types of injury, with a particular focus on surgical approaches.
Clinical anatomy
PCL is a complex structure comprising of two bundles: the anterolateral (AL) and the posteromedial (PM), named in relation to their origin on the femur and insertion on the tibia. Each of these structures possesses an individual anatomic course which determines its function. The AL is therefore tense in knee flexion while the PM is tense in extension; the AL bundle has a greater tensile strength and the PM bundle is more isometric [2]. The overall bundle of the PCL structures originates from a broad area on the anteromedial aspect of the femoral condyle within the intercondylar notch on a crescent-shaped prominence named the medial intercondylar ridge; this position has been shown to be variable [3]. Its insertion is usually constant and located in a posterior depression on the intercondylar imminence known as the PCL fossa. This point of insertion was found to be about 7mm anterior to the posterior tibial cortex; however, a small thinner bundle of fibres inserts directly on the posterior cortex blending with the posterior capsule and periosteum [4]. The average length of the PCL is about 38mm, it is thinner in midsubstance and the overall bundle tapers from about 32mm in cross-section at its femoral origin to about 13mm at its tibial insertion [5–7]. Blood supply to the PCL is derived primarily from the middle geniculate artery, a branch of the popliteal artery. It also receives additional supply from capsular vessels and synovial sheath vasculature. Nerve endings are present within the PCL substance and consist of Ruffini corpuscles for pressure and Pacini corpuscles for velocity and Golgi tendon apparatus. Injuries to the PCL can therefore affect proprioception and afferent sensory pathway from the knee joint.
Biomechanics
The PCL has complex biomechanical properties which impact on injury, treatment, surgical reconstruction and rehabilitation. It serves as a primary posterior stabiliser and as a secondary stabiliser in external rotation. The point of maximum tensile strength is at 90 degrees of flexion; h–owever, the posterior stabiliser action is initiated at 30 degrees of knee flexion. The tensile strength therefore increases exponentially from extension to flexion. Cadaveric studies where the PCL was sectioned demonstrated an increase in posterior translation in flexion by 10–15mm, and an increase in external rotation by 21 degrees [8]. The PCL has a synergistic function with structures of the postero-lateral corner, hence a combined injury will inadvertently result in marked posterior and external rotatory instability [8]. The role played in external rotation stability has been shown to contribute to the so-called ‘screw-home’ mechanism as the knee moves from flexion to extension [9]. Biomechanical studies of PCL-deficient knees have demonstrated markedly altered biomechanics of the joint. This was particularly apparent in the increased joint reaction forces on the medial femoral condyle [10]. Therefore PCL plays a more important role in normal knee kinematics than originally anticipated, and this function has been shown to depend greatly on the anatomic attachment points rather than the isometric properties of the ligament. These observations carry important implications for surgical reconstruction techniques.
Epidemiology and injury mechanism
The true incidence of PCL injuries remains difficult to estimate due to the variation in definition of combined, as opposed to isolated, injuries. The reported incidence in the literature is therefore variable – between one and 40 per cent of all acute knee injuries [11]. These numbers depend on the type of centre reporting, with major trauma centres and sports injuries units reporting differing levels of incidence [12,13]. It is estimated that up to two-thirds of PCL injuries are associated with multiple trauma and only 6.5 per cent occurred in isolation [11]. These figures need to be taken with caution, since a proportion of isolated cases can remain undiagnosed. It is therefore more useful to consider the mechanism of injury, since isolated PCL injuries tend to occur following hyperflexion non-contact forces, whereas combined injuries result from direct, posteriorly displacing impact. In isolated injuries the PM bundle often remains intact [14], while concomitant postero-lateral