Continuing Education Activity

The Posterior Cruciate Ligament (PCL) is one of the four major ligaments of the knee joint that functions to stabilize the tibia on the femur. It originates from the anterolateral aspect of the medial femoral condyle in the area of the intercondylar notch and inserts onto the posterior aspect of the tibial plateau. It functions to prevent posterior translation of the tibia on the femur. To a lesser extent, the PCL functions to resist varus, valgus, and external rotation forces. It is approximately 1.3 to 2 times as thick and about twice as strong as the anterior cruciate ligament (ACL) and, consequently, less commonly subject to injury. This activity outlines the evaluation, and treatment of posterior cruciate ligament knee injuries, and highlight the role of the interprofessional team in managing the care of patients with this type of injury.

Objectives:

• Identify the etiology and most common causes of posterior cruciate ligament injuries.
• Review the anatomy of the posterior cruciate ligament.
• Describe the function of the posterior cruciate ligament and the common associated injuries.
• Summarize the evaluation, and treatment of posterior cruciate ligament knee injuries, and highlight the role of the interprofessional team in managing the care of patients with this type of injury.

Introduction

The Posterior Cruciate Ligament (PCL) is one of the four major ligaments of the knee joint that functions to stabilize the tibia on the femur. It originates from the anterolateral aspect of the medial femoral condyle in the area of the intercondylar notch and inserts onto the posterior aspect of the tibial plateau. It functions to prevent posterior translation of the tibia on the femur. To a lesser extent, the PCL functions to resist varus, valgus, and external rotation forces. It is approximately 1.3 to 2 times as thick and about twice as strong as the anterior cruciate ligament (ACL) and, consequently, less commonly subject to injury.[1][2][3]

Etiology

Injuries to the PCL are caused by an extreme anterior force applied to the proximal tibia of the flexed knee. Common causes include dashboard injuries where the knee is forced into the dashboard during a motor vehicular collision. The PCL also can be injured secondary to falling forward onto a flexed knee. The most common sports where PCL injuries occur are football, skiing, soccer, and baseball. Less commonly, damage can occur due to a rotational hyperextension injury to the knee joint.[4][5]

Epidemiology

According to a 2003 study by Schulz et al., the mean age of PCL injuries was 27 years. The two most common causes of PCL injury were car accidents (45%) and athletic injuries (40%). Specifically, the two most common causes of injury are motorcycle accidents (28%) and soccer-related injuries (25%). The most common injury mechanism was dashboard injuries (35%) and falls on a flexed knee with the foot in plantar flexion (24%). The male to female ratio is 2:1. [6]

Pathophysiology

The PCL originates from the anterolateral aspect of the medial femoral condyle in the area of the intercondylar notch and inserts extra-articularly onto the posterior aspect of the tibial plateau. It is 1.3 to 2 times as thick and about two times the strength of the ACL. It can be further separated into anterolateral and posteromedial bundles. The anterolateral and posteromedial bands are respectively 65% and 35% of the body of the PCL. The anterolateral bundle is taught in knee flexion and lax in knee extension where the posteromedial bundle is tight in knee extension and lax in knee flexion. The function of the PCL is to prevent posterior translation of the tibia on the femur. Because a majority of the injuries occur in knee flexion, the anterolateral portion is more commonly injured. The PCL is the primary restraint to posterior tibial translation between 30 degrees and 90 degrees. At 90 degrees, the PCL accepts 95% of posterior translational forces. It resists posterior translation with the assistance of the posterolateral joint capsule, popliteus, medial collateral ligament, and posterior oblique ligament.[7][8]

The Posterior cruciate ligament receives blood supply from the middle geniculate artery and is innervated by the tibial nerve.  

History and Physical

Patients often will present with complaints of acute onset of posterior knee pain, swelling, and instability.  A thorough history includes the mechanism of injury, such as trauma from falling onto a flexed knee or recent motor vehicle accident.  There may or may not be a complaint of a “pop” with PCL tears like those frequently reported with ACL tears.    

A thorough knee exam should be performed, including overall limb alignment and gait assessment. The neurovascular integrity of the lower extremity distal to the injury should also be assessed. Intimal tear to the popliteal artery should be suspected in case of weak distal pulses or Ankle-Brachial Index< 0.8 with a further investigation with arteriography. Arteriography should not delay treatment in case of an evident vascular deficit [9].

1. Inspection: Affected knee will often present with mild to moderate joint effusion. Swelling is usually less than an ACL tear. Patients may present with antalgic gait on examination with obvious favoring of the affected knee. They may have difficulty walking up or downstairs or at an incline.  During the stance phase of the gait, external rotation recurvatum or varus thrust could be suggestive of associated PLC injury. There may be a positive sag test; The sag test is performed with the patient supine, hip flexed to 45 degrees, and knee flexed to 90 degrees.  The tibia will be noted to sag distally relative to the femur as compared to the opposite knee.
2. Palpation: There may be an effusion on physical examination. Palpate the joint lines for tenderness which could be suggestive of meniscal tears.
3. Muscle strength testing: Strength should be normal, but there may be a weakness with knee extension and flexion secondary to guarding.
4. Range Of Motion: The passive range of motion may be limited 10 to 20 degrees with flexion. It may be further decreased with other concomitant injuries such as meniscal, muscular, or ligamentous etiology.
5. Special Testing:

• The posterior drawer test: This is the most accurate test for assessing PCL integrity. It is performed with the patient in a supine position with the hip flexed to 45 degrees and knee flexed to 90 degrees. A posterior force is applied to the proximal tibia whilst the femur is stabilized. In PCL injury, the tibia will be posteriorly translated relative to the femur; therefore, reduction to the neutral position is usually needed before posterior force application. Internal rotation tightens the superficial medial collateral ligament and posterior oblique ligaments which decreases posterior tibial translation as they act as secondary restraints to posterior tibial translation. So, in isolated PCL injuries, the tibia translates >10-12 mm in neutral and 6-8 mm in internal rotation, whilst in combined ligamentous injuries the tibia translates >15 mm in neutral and >10 mm in internal rotation.
• The quadriceps active test: helpful for diagnosis of complete PCL tear. The patient is positioned supine with the knee flexed to 90°. The examiner stabilizes the foot and asks the patient to attempt extending the knee, this results in isometric contraction of the quadriceps. In complete PCL injury, a posteriorly translated tibia will reduce to relative to the femur. 
• Dial test or external rotation test: This test is used to diagnose combined PCL and PLC injuries. The patient can be positioned either prone or supine. The examiner externally rotates both feet at 30°and then 90° of knee flexion, and compare both sides. Asymmetry of 10° or more is considered abnormal. Increased external rotation at 30° only is suggestive of isolated PLC injury, while increased external rotation at both 30 and 90° is suggestive of a combined PCL and PLC injury. 
• Varus/Valgus stress: This assesses combined PCL and collateral ligament injury versus isolated collateral ligament injury. Laxity at 0° is suggestive of combined injury either MCL or LCL and PCL injury. While laxity at 30° alone is suggestive of either MCL or LCL injury.

Evaluation

Initial imaging should be limited to plain X-rays. Include ipsilateral standing AP, 45-degree flexion weight-bearing, and merchant patellar views with contralateral views to assess for associated fractures, arthritis, joint effusion, and the 45-degree flexion view may show a positive sag compared to the contralateral knee.[10][11][12]

MRI is the gold standard for the evaluation of a PCL injury. It has been reported to have accuracy in the range of 96% to 100%. It can also assess for concomitant injuries such as meniscal, Ligamentous, posterolateral joint capsule, popliteus, medial collateral ligament, and posterior oblique ligament injuries. On T1- and T2-weighted sagittal MRI images,  increased signal within PCL substance or disruption in the continuity of the ligament fibers suggests acute injury. Chronic PCL injuries can be shown as posterior tibial translation. Posterior translation < 8 mm can potentially heal with the restoration of ligament continuity on MRI [13].

PCL injuries are graded on a scale of I-III.

• Grade I (partial tear) - 1-5mm posterior translation. Tibia remains anterior to femoral condyles.
• Grade II (complete isolated) – 6-10mm posterior tibial translation. A complete tear of PCL without another injury. Anterior tibia flush with femoral condyles.
• Grade III (complete PCL with combined capsular and/or Ligamentous injury) - > 10mm posterior tibial translation. Tibia posterior to femoral condyles which may indicate a concomitant capsuloligamentous injury.

Treatment / Management

The main variables to consider in managing a PCL injury is whether it is an acute or chronic injury, isolated or combined injury. Historically, nonoperative management was the first line of treatment for isolated PCL injuries regardless of how severe the injury is. This was partially due to the unreliable results of PCL reconstruction in restoring knee function and kinematics with residual laxity being commonly reported postoperatively. With advances in arthroscopy techniques, PCL reconstruction is becoming a more reliable procedure, although there is a lack of long term outcomes[14]. 

Non-operative treatments are indicated for:

• Acutely isolated grade I and II injuries with posterior tibial translation between 8 mm and 12 mm.
• Those with grade III who are mildly symptomatic or participate in low demand activities [15]. 

PCL has an intrinsic ability to heal. This usually accomplished in a lax position [16][17]. Acute treatment involves RICE (rest, ice, compression, and elevation). Initial knee bracing controls pain, posterior tibial translation, and has well managed isolated PCL injuries [18][19]. Crutches could be helpful until quadriceps recovers strength.  Rehabilitation is important with a focus on knee extensor strengthening. The estimated return to play is 2 to 4 weeks. Grade III injuries can be treated with knee immobilization x 4 weeks with a transition to rehabilitation focusing initially on passive range of motion followed by knee extensor strengthening.

Operative treatment is indicated for:

• Acute PCL injuries with tibial translation > 12 mm, associated repairable meniscal tears, knee dislocation or bony avulsions, and combined capsuloligamentous injuries.
• Chronic PCL injuries with posterior tibial translation > 8 mm [20][21], symptomatic chronic PCL injuries e.g symptoms with deceleration or incline descending, chronic PCL injuries with instability [22][23] and combined capsuloligamentous injuries.

Operative techniques for PCL reconstruction:

• Most are performed arthroscopically including ORIF for bony avulsion.
• Single bundle vs double bundle techniques.
• Autograft e.g Bone-patellar tendon-bone or hamstrings vs allograft e.g Achilles, hamstring, bone-patellar tendon-bone, and anterior tibialis. Clinically there is no difference in failure rate between the two graft types. But, with autograft, there is an increase in surgical time and donor site complications[24]. 
• Tibial inlay vs Transtibial techniques.

The single-bundle technique (isometric point or anatomical): This is the classic PCL reconstruction technique. The anatomical technique relies on radiographic and arthroscopic references and is more preferred versus the isometric point technique which tends to result in laxity in the postoperative period as time passes due to over constraint of the joint.[25][26].

The double-bundle reconstruction technique:

Theoretically, this should restore the normal knee kinematics in comparison with single-bundle reconstruction which restores kinematics only between the first 0° to 60° of flexion[27]. This technique uses one tibial tunnel and two femoral tunnels with the anterolateral bundle tunnel being bigger than the one for the posteromedial bundle[28]. 

There are two main types of graft fixation; the transtibial tunnel versus the tibial-inlay techniques.

• The transtibial tunnel technique simulates the origins of the anterolateral bundle both on the femur and tibia. This technique has the biomechanical disadvantage of what is known as the "killer turn" which is the sharp angle on the exit of the tibial tunnel which can result in attenuation with subsequent failure of the graft [29][30].
• The tibial-inlay techniques used for ORIF of bony avulsions and to secure bone-to-bone tibial attachment with an anchor or screw. Its disadvantage is having a screw fixation within 20 mm of the popliteal artery. The technique can be performed by a posteromedial approach between semitendinosus and medial gastrocnemius, but multiple all-arthroscopic techniques have been described[31]. Tibial inlay techniques can also be utilized for double-bundle reconstruction using a bone plug to attach the graft to the tibial origin and the tendon is split in two representing the two PCL bundles. 

High Tibial Osteotomy (HTO): It is indicated in the setting of chronic PCL deficient knees and varus malalignment.it affects the mechanical weight-bearing axis of the knee by altering both the coronal and sagittal planes[32]. Opening medial wedge osteotomy; in the coronal plane, will improve the varus malalignment and, in the sagittal plane, will increase the tibial slope which consequently increases anterior tibial translation and helps PCL deficiency by reducing posterior sagging of the tibia. In addition, HTO delays the progression of secondary osteoarthritis[33].

Differential Diagnosis

• Anterior cruciate ligament injury

• Lateral collateral knee ligament injury 

• Medial collateral knee ligament injury 

• Meniscus injuries

• Talofibular ligament injury 

Complications

Intraoperative and Postoperative complications of PCL surgery [34][35]:

Neurovascular injury (e.g Popliteal artery injury).

Fracture.

Residual instability.

Osteoarthritic progression (Patellofemoral and medial tibiofemoral).

Osteonecrosis.

Stiffness.

Failure of associated ligament reconstructions or meniscal repairs.

Revision of PCL reconstruction.

Postoperative and Rehabilitation Care

Rehabilitation is fundamental in determining patients' outcomes following PCL injuries and reconstruction [36][37][23]. There are variable rehabilitation programmes throughout the literature. But, there are main principles that should constitute the foundation for any programme. These principles include [37][38]:

• Progressive weight-bearing and range of motion exercises.
• Avoidance of posterior tibial subluxation early in rehabilitation by avoiding resisted hamstring strengthening exercises.
• Early focused quadriceps strengthening.  

Pierce proposed a 5-phase post-operative programme [23].Patients are supported with a knee immobilizer brace for 3 days then converted to a dynamic anterior drawer brace. It has been recommended that the PCL brace be worn 24/7 for 24 weeks postoperatively.

Phase I (0- 6 weeks):

• (0-2 weeks):passive prone ROM (0 to 90 degrees.
• (3-6 weeks) advance to full passive prone ROM as tolerated.

Phase II (7-12 weeks): 

• Progressive crutch weaning and weight-bearing as tolerated, 
• Restrict knee flexion to less than 70º  during weight-bearing exercises.

Phase III (13 to 18 weeks):

• Progression of ROM weight-bearing exercise past 70º of knee flexion after 16 weeks.
• Continuation of brace usage.

Phase IV (19-24 weeks):

• A gradual introduction of sport-specific drills.

Phase V (25-36 weeks):

• Brace weaning if there is sufficient healing of the graft as guided by the 6 months postoperative PCL stress radiographs.
• Start a straight-line jogging progression.
• The final goal of multiplanar agility exercises and return to preoperative activities.

Enhancing Healthcare Team Outcomes

PCL injuries are not uncommon and usually present to the emergency department. Mild injuries are usually managed non-operatively by the emergency department physician, nurse practitioner or the primary care provider. Acute treatment involves RICE (rest, ice, compression, and elevation). Rehabilitation is important with a focus on knee extensor strengthening. Grade III injuries can be treated with knee immobilization x 4 weeks with a transition to rehabilitation focusing initially on passive range of motion to knee extensor strengthening.  Operative treatment is reserved for combined capsuloligamentous injuries, grade II or grade III injuries with bony avulsion, or chronic PCL injuries with instability.

Following injury and surgery, all patients need to enter rehabilitation to regain muscle strength and function. The time to return to activity varies from 2-8 weeks. [39]