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 corner injuries occur in about 60 per cent of cases [15]. Combined injuries therefore require closer attention to bony structures and even the risk of neurovascular injuries resulting from occult knee dislocation. When isolated, however, PCL injuries can become chronic with an associated posterior capsular stretching. The severity of PCL disruption is graded in relation to tibiofemoral posterior translation or ‘posterior sag sign’ from the original 10mm anterior position of the medial tibial plateau to the medial femoral condyle. Grade of injury is described with reference to this original position: grade I is 0–5mm, grade II is 5–10mm and grade III is greater than 10mm posterior sag. It is estimated that 15–20 per cent of isolated PCL injuries represent an avulsion from its distal tibial attachment [16]. Bone fragment fracture with the attachment of the PCL can be divided into three types: PCL rupture with involvement of an osteochondral sleeve; true PCL avulsion resulting from a posterior dislocation or subluxation of the knee; and shear fracture of the posterior tibial plateau involving PCL attachment [11,16].
Clinical evaluation
Clinical presentation can differ widely, given the varying level of severity, types of injury and associated trauma. A thorough history of injury mechanism and subsequent symptoms is central to the clinical evaluation of PCL injuries. Knee pain, swelling and stiffness might be the only presenting symptoms. Initial signs of trauma that might be identified include abrasions and contusions about the knee. A popliteal haematoma can be indicative of a PCL avulsion injury as opposed to a more contained intrasubstance tear. Signs of associated injuries need to be identified, therefore a clear documentation of initial and current symptoms is important. The traditional feeling of a ‘pop’ associated with anterior cruciate ligament (ACL) injuries is unlikely to manifest itself at the time of injury. Equally, signs of mechanical instability may not be readily identifiable.
A complete clinical examination is required and it must also include examination of the ACL and collateral ligaments. The posterior drawer test is used to assess posterior laxity associated with a deficient PCL; similarly, this is also apparent in the posterior sag sign (Godfrey’s), which is graded as outlined above. Further clinical evaluation can be obtained with a reverse pivot shift test. External rotation and varus thrust exacerbate to posterior translation of the tibia, the posteriorly subluxed lateral plateau reduces as the knee is brought into extension between 20 and 30 degrees. A positive quadriceps active test can also help detect the presence of posterior laxity. The examiner must be attentive to the false positive anterior drawer test and pseudo-positive Lachman test that can result from posterior laxity in cases of chronic PCL-deficient knee; gait needs to be assessed for posterior translation, external rotation and varus thrust during the stance phase, and varus malalignment might also be present and exacerbate the instability symptoms. Varus malalignment and varus recurvatum may indicate associated PLC injury.
Initial imaging will include standard anteroposterior and lateral plain radiographs. The aim of these primary images is to look for concomitant fractures and ensure congruity of the joint. More elaborate plain radiographs, including stress views, have been superseded by the advent of magnetic resonance imaging (MRI) availability. MRI has therefore become the modality of choice with the highest sensitivity and specificity [17]. MRI is equally useful in assessing chondral surfaces, menisci and other ligaments. A computerised tomography (CT) scan can be a useful adjunct in the assessment of PCL avulsion injuries, providing a better outline of the bony element of the injury. Reformatted and three-dimensional CT scans may also assist with the pre-operative planning of surgical approach and method of fixation. MRI scans have been shown to be less optimal in the assessment of chronic PCL injuries [18], since a chronically ruptured PCL may still assume a normal anatomic position.
Management
Non-operative treatment is often advocated in isolated grade I and grade II injuries, with activity modification, quadriceps rehabilitation and functional bracing. In contrast, evidence from single-arm medium-to long-term studies reported favourable results following the surgical treatment of isolated PCL injuries [19]. This body of research needs to be taken with caution due to the absence of control or adequate comparators. PCL-deficient knees belonging to a spectrum of multi-ligament injuries are usually diverted towards operative treatment. Marked instability and resulting malalignment with this type of injury have been put forward as indicators for surgical reconstruction due to generally poorer prognosis if not stabilised. The evidence from such a series has been variable, owing to the marked heterogeneity of this type of patient population.
Surgical approaches
Fixation of the avulsed tibial attachment of the PCL can be carried out either by open exposure or by arthroscopically assisted means. Despite adequate experience, Kim and co-workers [22] believed that the technique of arthroscopy-assisted reduction and fixation was difficult and had a steep learning curve. In our experience, a similar fixation can be achieved by open exposure through the postero-medial approach.
A posterior approach to the knee is required for fixation of PCL avulsion or reconstruction using a tibial inlay technique. This technique gained popularity in the previous decade due to the concerns resulting from the acute bend associated with transtibial graft insertion. Clinical series have reported satisfactory results using a direct posterior approach for either the repair of PCL tibial bony avulsion or graft reconstruction of the PCL using the tibial inlay technique [20–26]. Although relatively uncommon and associated with concerns over iatrogenic injuries to important neurovascular structures, the posterior approach to knee has had renewed interest.
The aim of the direct posterior approach to the knee is to gain a safe exposure of the neurovascular structures, and a broad view of the posterior femoral condyles, the tibial plateau posterior capsule and the soft tissue structures. PCL bone avulsions are some of the most widely recognised indications for the utilisation of this approach.
The patient is positioned prone on either a standard or radiolucent table with adequate padding of all pressure points. A high-thigh tourniquet is also used for a bloodless operative field and the lower limb is draped in the standard fashion. The foot is preferably left uncovered or wrapped in a clear sheath to allow continuous monitoring of vascular status. A bolster is frequently placed under the operated side ankle to produce a 20–30 degree knee flexion, relieving tension from structures within the popliteal fossa. The tourniquet is released prior to closure of the wound in order to ensure continuity of the major vessels and achieve adequate haemostasis.
The classical posterior approach to the knee as described by Abott & Carpenter [26] requires dissection of the neurovascular bundle and is time consuming. Various other approaches described by Trickey [27], McCormick [28] and Ogata [29] have failed to simplify significantly the exposure.
Burks and Schaffer [30] described a simplified posterior approach to the knee which did not require the neurovascular dissection besides giving good exposure of the PCL and posterior horns of both menisci.
The classic approach described consists of a curvilinear (lazy S) incision extending from lateral to medial. The extent of the incision depends on the exposure required and size of the patient. The superficial fascia is incised in line with the skin incision. The first structures encountered centrally are the short saphenous vein and the sural nerve lateral to the vein. The latter is followed proximally to identify the tibial nerve. The distal deep dissection is between the medial gastrocnemius and the medial border of the tibia. The long saphenous vein is encountered at this level. Care must be taken due to the presence of multiple tributaries to this vein. Once the tibial nerve is identified, the short saphenous vein and the sural nerve are retracted laterally. The popliteal vein and the popliteal artery lie direct medial to the tibial nerve within the centre of the popliteal fossa. The vein runs medial to the artery. Extra care is taken when mobilising these structures due to the presence of the geniculate branches. The popliteal artery is the deepest structure and lies just posterior to the knee joint capsule at the level of the posterior horn of the lateral meniscus. The proximal lateral part of the approach helps to identify and protect the common peroneal nerve. The distal medial part of the approach exploits the plane between the medial gastrocnemius and the hamstring tendons. The internal borders of the two heads of the gastrocnemius are identified, and the lateral border of the semimembranosus and the medial border of the biceps femoris are identified. These structures form the boundaries of the popliteal fossa. The semimembranosus muscle can be retracted medially revealing the tendon origin of the medial gastrocnemius. The latter is retracted laterally to protect the neurovascular structures. This plane offers an adequate exposure of the posteromedial joint line. For PCL avulsion repair this approach offers a safe access to the posterior part of the tibial plateau between the medial gastrocnemius retracted laterally, protecting the neurovascular bundle laterally and the semimembranosus medially.
The approach as described by Burks and Schaffer [30] uses a hockey stick-shaped incision, with the long limb of the incision extending distally along the medial border of the gastrocnemius and curved over the flexor crease proximally. The medial head of the gastrocnemius and the semimembranosus and semitendinosus are identified. The medial gastrocnemius is mobilised laterally, hence protecting the neurovascular structures. This creates a direct window onto the posteromedial joint line. If needed, periosteal reflection of the popliteus from its tibial insertion can be performed. The popliteus runs in a superolateral direction, and this exposure allows direct window onto the posterolateral joint line.
Bhattacharyya and colleagues [31] described a modification involving release of the medial gastrocnemius tendon which is retracted laterally protecting the neurovascular structures. Care must be taken to leave a cuff for tendon repair. Division of this tendon proximally protects its blood supply, which consists of a branch located about 5cm distal to its origin.
For fixation of avulsion fractures or access to the posterior aspect of the knee joint, the senior author (Ayaz Lakdawala) prefers access as described by Burks and Schaffer. It is a safe approach and provides good access to the posterior aspect of the knee (see Figures 1–4).
Conclusion
PCL injuries remain relatively rare. Developments in surgical reconstruction techniques, surgical devices and outcome measures will inadvertently continue to draw more attention to this subject. Equally, there is a renewed interest in the direct posterior approach to the knee with its reported variations. When executed carefully, this approach can be safe and offers an adequate exposure of the structure lying in the posterior part of the knee joint. This exposure facilitates anatomic reduction and fixation of PCL avulsion fractures. Equally, this approach has been safely applied in the reconstruction of the PCL using the tibial inlay technique and posterior tibial plateau fractures.
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Authors
Tarek Boutefnouchet
Tarek Boutefnouchet is at the Heart of England NHS Foundation Trust, Birmingham, UK.
Ayaz Lakdawala
Ayaz Lakdawala is a consultant orthopaedic surgeon at the George Eliot Hospital NHS Trust, Nuneaton, UK.
E: ayazl@aol.com
