Introduction
The posterior cruciate ligament (PCL) is the largest and strongest ligament in the knee, with a unique innate healing capacity. Therefore, as opposed to other ligaments such as the anterior cruciate ligament (ACL), injuries to the PCL less commonly require surgical intervention. Nevertheless, when PCL tears are suspected, a conscientious clinical approach must be taken to not only diagnose these injuries, but to accurately discriminate between those that can be managed conservatively versus those that require surgery. The foundations of the diagnosis begin with a history and physical examination and are augmented by increasingly specific imaging techniques including stress radiography and magnetic resonance imaging (MRI). Once diagnosed, PCL tears are categorised into those amenable to nonoperative management and those requiring reconstruction. Evidence has demonstrated that grade I and II injuries can be successfully treated with dynamic PCL bracing, whereas acute grade III PCL injuries and combined ligament injuries are optimally addressed with double bundle PCL reconstruction (DB-PCLR) to restore native biomechanics of the knee. The goal of this chapter is to provide detailed descriptions of the surgically relevant anatomy; the biomechanical properties of the PCL and its bundles; the diagnostic approach, including physical examination and imaging; the operative and nonoperative options with corresponding indications; and the updated literature on clinical outcomes after PCL injuries.
Demographic Characteristics
Injuries to the PCL can occur both in isolation or in the context of multiligament and meniscal injuries. Isolated injuries are usually the result of forced posterior translation of the tibia that can occur with the knee flexed, such as in the classic dashboard injury pattern, or with forced hyperextension of the knee. , Epidemiological data have reported the incidence of isolated injuries to be approximately 2 in 100,000 annually, with more tears occurring in men than women in the general population. Among all knee ligament injuries, PCL injuries are relatively uncommon. Data published from the Danish Knee Ligament Reconstruction registry reported that of 23,253 knee ligament reconstructions from 2005 to 2015, only 581 were registered as PCL reconstructions. It must be acknowledged that this does not account for PCL injuries managed nonoperatively; nonetheless, the relative rarity of isolated PCL injuries makes the natural history and epidemiology difficult to study. Furthermore, isolated injuries may in fact comprise a minority of all PCL injuries, with reported rates of multiligament PCL injuries approaching 60% or higher. These injury mechanisms often include additional rotational or varus/valgus stress resulting in multiligament and concurrent meniscal injuries. ,
Whether in the context of isolated injury or multidirectional instability, PCL deficiency contributes to altered kinematics and increased contact pressures, specifically in the medial compartment and patellofemoral joint. In the long term, patients with isolated PCL tears have a significantly increased risk (hazard ratio (HR) 6.2) of symptomatic arthritis and the subsequent need for total knee arthroplasty (HR 3.2). This is corroborated by others who have reported on arthroscopic evaluation of chronic PCL-deficient knees (≥5 years from index injury) documenting frequencies of degenerative cartilage lesions of 77.8% and 46.7% for the medial femoral condyle and patella, respectively. Incompetence of the PCL also increases risk to posterolateral knee structures, placing them at risk of subsequent injury. Because of these long-term consequences, it is imperative that these injuries are accurately diagnosed and adequately treated in a timely manner.
Anatomy
The PCL originates from the lateral aspect of the medial femoral condyle, along the medial roof and wall of the notch. It courses posteriorly and laterally to its attachment nestled between the posterior aspects of the medial and lateral tibial plateaus. The surgically relevant anatomy of the PCL is fundamentally centred on a discussion of the two-bundle structure and the detailed qualitative and quantitative descriptions of the femoral and tibial attachments. The two distinct bundles of the PCL are the anterolateral bundle (ALB) and posteromedial bundle (PMB), named for the relationship of their attachment sites to one another. The two bundles serve distinct and codominant functions as a consequence of these attachment sites, which have been described in exquisite detail in relationship to reproducibly identifiable bony landmarks and neighbouring attachments.
In regards to the femoral origin of the PCL, several bony landmarks have been identified to describe the location of the footprints, including the trochlear point, medial arch point, medial intercondylar ridge and posterior point ( Figs 7.1 and 7.2 ). , These landmarks have been described in great detail both graphically and descriptively to facilitate the accurate and reproducible identification of the PCL bundle footprints during reconstruction. The trochlear point references the point of intersection of the distalmost aspect of the trochlear cartilage, the medial arch articular cartilage along the lateral wall of the medial condyle and the medial aspect of the apex notch. Moving medially down the intercondylar notch, the medial arch point represents the transition from the notch roof to the wall. Continuing to follow the arch of articular cartilage distally down the notch and then posteriorly along the lateral aspect of the medial condyle, the next point encountered is the posterior point, which has been aptly defined as the most posterior point of the articular cartilage margin.
Using these landmarks, the femoral attachments of the ALB and PMB have been quantitatively defined. The centre of the femoral attachment of the ALB is 7.4 mm from the trochlear point, 11.0 mm from the medial arch point and 7.9 mm from the distal margin of the femoral articular cartilage. The PMB centre is located 12.1 mm from the ALB centre, bordered by the medial intercondylar ridge, which runs directly in the anteroposterior direction, passing through the posterior point ( Fig. 7.3 ).
The tibial attachment in the PCL facet has been similarly defined using reproducible bony and soft tissue landmarks. , Within the facet, the ALB bundle centre has been described in relation to the shiny white fibres of the medial meniscal posterior root, residing 6.1 mm from the point at which these fibres pass closest to the PCL tibial attachment ( Fig. 7.4 ). The fibres of the PMB can be qualitatively divided into thick and thin portions, the former of which has been deemed to be the anatomical centre because of its robust attachment and biomechanical importance. The centre of the thick portion of the PMB is located 8.9 mm from the centre of the ALB, 11.1 from the shiny white fibre point, 12.6 mm from the lateral articular margin and 3.1 mm from the medial groove. These footprint locations are subsequently used as the basis for anatomical PCL reconstruction.
Biomechanics
The biomechanical properties of the PCL and the individual contributions of the individual bundles have been rigorously defined. The PCL in its entirety is the primary restraint to posterior tibial translation. The PCL has also been demonstrated to provide significant resistance and stability with isolated internal and external rotation and combinations of torsional and posteriorly directed loads. However, perhaps the more important piece of information is the codominant relationship of the ALB and PMB. Illustrating this, Kennedy et al. reported a 2.6-mm increase in posterior translation after ALB sectioning, and a 0.9-mm increase after PMB sectioning and a 11.7-mm increase in translation at 90 degrees with combined transection. The bundles have also been described to serve different functions throughout the range of motion, with the ALB serving as the primary restraint to posterior translation at 90 degrees , and the PMB functioning similarly near extension in addition to resisting internal rotation at greater flexion angles. ,
These same biomechanical principles and testing have been applied to both single and double bundle reconstruction techniques, which both attempt to reestablish the codominant roles of the ALB and the PMB. The ALB is approximately twice as large, twice as stiff, three times as strong and capable of maintaining near normal knee kinematics after PMB sectioning. Therefore preferential reconstruction of the ALB has been the understandable focus of prior single bundle reconstruction. , , However, after the establishment of the codominant relationship of the bundles, multiple biomechanical studies have since demonstrated that a DB-PCLR better restores native graft forces and knee kinematics, including restraint to posterior translation and internal rotation. ,
Diagnosis
The diagnosis of PCL injuries, as with any orthopaedic injury, begins with the foundation of a history and physical examination, followed by increasingly complex imaging. A careful history, including details of injury mechanisms such as dashboard injuries or similarly oriented falls on a flexed knee or forced hyperextension, should elevate suspicion of a possible PCL injury. , Examination of a suspected PCL injury proceeds most often with a posterior drawer performed at 90 degrees ( Fig. 7.5 ). Reported sensitivity and specificity has been as high as 90% and 99%, respectively; however, sensitivities may in fact vary from 20% to 100%. Grading of injury severity can be included as well: Grade I: 0 to 2 mm of posterior translation; grade I: 3 to 5 mm; grade II: 6 to 10 mm; and grade III: 10 mm or greater. ,
The posterior drawer is augmented with other physical examination manoeuvres that use both gravity and the patient’s own musculature to assess the integrity of the PCL. These tests are the posterior sag sign and quadriceps active test, respectively. The posterior tibial sag sign is evaluated with the patient lying flat on their back, with the hips flexed to 45 degrees and the knees flexed to 90 degrees bilaterally. When viewed laterally, a high-grade partial to complete PCL tear will allow the tibia to ‘sag’ posteriorly ( Fig. 7.6 ). This visual assessment has a similar variability in sensitivity ranging from 46% to 100%. The quadriceps active test can be performed in the same position; however, the foot of the affected extremity is secured by the examiner firmly to the bed. The patient is then asked to attempt to straighten the knee, which pulls the posteriorly sagged tibia anteriorly. With an observed anterior translation greater than 2 mm, the quadriceps active test has sensitivities and specificities ranging from 53% to 98% and 96% to 100%, respectively. Aside from assessment of posterior subluxation and its correction, internal rotation examination manoeuvres have also proved fruitful in assisting diagnosis. The supine internal rotation test, in which an internal rotation torque is applied to the foot at varying degrees of flexion (60 to 120 degrees), has reported a sensitivity of 95.5% for grade III tears ( Fig. 7.7 ).
Imaging
After physical examination, a diagnosis of a PCL tear can be supplemented with imaging, beginning with stress radiography in which radiographs are taken while a posterior tibial load is applied. Stress views are obtained of both the normal and injured knees to establish a side-to-side difference (SSD) in posterior translation. Normal variability in uninjured individuals can range from 0 to 4 mm SSD, with 5 to 12 mm typically being classified as isolated PCL tears and more than 12 mm being classified as a concurrent ligamentous injury of the posterolateral or posteromedial structures. , , , ,
To assess these SSDs, various methods of load application have been described in the literature; however, the most common include the Knee Translation 1000 (KT-1000), the Telos device and gravity (kneeling). Comparisons have found the latter two have the greatest sensitivity because of the magnitude of SSDs that are produced and observed. The Telos device applies an instrumented 150 N/m force on the tibial tubercle, whereas kneeling views place a posteriorly directed load on the tibial tubercle using the patient’s own body weight. Although both are valid techniques, kneeling stress views are often preferred because of cost and simplicity ( Figs 7.8 and 7.9 ).
Lastly, MRI can be used to assess PCL injuries with a high sensitivity for acute injuries (96% to 100%). This robust sensitivity is diminished, even to as low as 62.5%, with respect to chronic injuries because of healing. , However, other quantitative imaging metrics, including posteromedial tibial translation measured on MRI, can be used to assess the underlying PCL integrity and function in the chronic injured state.
Treatment
Unlike tears of the ACL, tears of the PCL have an innate healing capacity that make it important to distinguish between those that require surgical intervention and those that can be managed nonoperatively. Regardless, both have their technical considerations. There is general agreement that isolated partial PCL tears can be managed nonoperatively. However, when conservative management is indicated, as is the case with grade I and II tears, precautions must be taken to ensure that the PCL does not heal with residual laxity. Over time, altered knee kinematics and abnormal joint loading caused by posterior instability, including increased medial compartment and patellofemoral contact pressures, can significantly increase the risk of arthritis and subsequent need for total knee arthroplasty. Residual laxity and further progression is most easily prevented with dynamic bracing that prevents excess posterior translation, which can be done from the time of diagnosis and does not require any period of immobilisation. , , Beyond grade II injuries, surgical intervention is indicated to restore joint stability and improve patient function and outcomes. Once a decision is made to proceed with surgery, several surgical options exist.
Surgical Reconstruction Techniques
Surgical reconstruction of the PCL can be generally divided into single and double bundle reconstructions. Among single bundle techniques, two primary variations have been described, including the anatomical transtibial and tibial inlay techniques. , The anatomical transtibial single bundle technique preferentially reconstructs the larger ALB by centring reconstruction tunnels on the femoral and tibial footprints of the ALB. , , , The tibial inlay technique differs primarily in the method of tibial fixation. In the transtibial approach the graft courses posterolaterally from the femoral tunnel to the aperture of the tibial tunnel and is pulled anteromedially, where it is then fixed in the tibial tunnel. This course of the graft as it enters the tibial tunnel has been coined the killer turn because of the acute angle and concern for graft failure from increased mechanical wear based on biomechanical studies. , , The tibial inlay method avoids this concern by creating a trough rather than a tunnel at the tibial attachment where a bone plug is subsequently fixed, typically with cannulated screws and washers. The inlay method also requires a different surgical approach, historically using a posteromedial incision between the semitendinosus and medial gastrocnemius; however, entirely arthroscopic approaches have been described. , Additional variations in technique are observed in the choice of graft, both in source (autograft versus allograft) and type. For single bundle transtibial reconstructions, common autograft choices include four-stranded hamstring grafts, bone–patellar tendon–bone (BTB) and quadriceps tendon, whereas the most common allograft choice reported has been Achilles tendon. Given the need for a bone plug, graft choices for the tibial inlay technique are limited to BTB, quadriceps and Achilles allograft. , , ,
Double bundle reconstructions have gained increasing attention based on biomechanical and clinical outcomes research demonstrating that DB-PCLR is necessary to recreate the native anatomy and function of the PCL. , In an anatomical double bundle reconstruction, two femoral tunnels are used to recreate the ALB and PMB. An 11-mm closed socket tunnel is reamed for the ALB, with the distal border abutting the edge of articular cartilage. The ALB footprint centre is identified by the earlier described landmarks of the notch (e.g., medial arch point, posterior point, trochlear point). Because of the broad femoral bundle attachments across the roof and wall of the notch, a second 7-mm closed socket tunnel is used to recreate the PMB, posterior to the ALB, leaving a 2-mm bone bridge between tunnels ( Fig. 7.10 ). On the tibial side, the anatomical centre of the ALB is preferentially recreated with a single 12-mm tunnel centred in the PCL facet along the bundle ridge that demarcates the ALB and PMB attachments. Anteriorly and posteriorly, the tibial tunnel is flanked by the shiny white fibres of the medial meniscus posterior root and the champagne-glass drop-off, respectively ( Fig. 7.11 ). Because of difficulty of direct arthroscopic visualisation, tibial tunnel placement is often verified with fluoroscopy after guide pin placement, but before reaming ( Fig. 7.12 ). Similar to single bundle techniques, tibial inlay techniques have also been described for double bundle reconstructions, in which a bone trough is created for the tibial footprint.
