POSTERIOR CRUCIATE LIGAMENT (PCL) TEARS
Background
The posterior cruciate ligament (PCL) is located within the knee joint and connects the shin bone (tibia) to the thigh bone (femur). The native PCL provides structural stability to the knee via its direct connection between the bones, and functional stability by providing information to the nervous system regarding knee joint position (proprioception).
Mechanism of injury
The PCL is the main constraint to backwards (posterior) translation of the upper (proximal) tibia on the femur; the mechanism of injury for an isolated PCL injury is typically a posteriorly directed force to the proximal tibia with the knee bent (e.g. landing on the knee). PCL injury may also occur when the knee is excessively flexed (hyperflexion) while a downward force is exerted on the thigh. The PCL plays a secondary role in resisting side to side and rotational movements, or over-straightening (hyperextension) of the knee. Therefore, any forceful sideways, rotational or hyperextension movements can injure the PCL but this is unlikely to occur without injury to another major knee ligament.
History
An individual may report some or all of the following history elements; however, patients that have injured their knee without tearing their PCL may also report a similar history.
- knee pain
- feeling/hearing a sensation/noise (e.g., pop, crack, snap)
- knee giving way/moving out and back in at the time of injury
- inability to continue the activity
- difficulty weight bearing
- rapid onset of knee joint swelling
- difficulty fully straightening the knee
- lack of trust in the knee and/or knee instability
- bruising behind the knee.
In severe injuries, the knee may appear deformed due to dislocation of the tibia; if major blood vessels are injured during the dislocation (vascular injury) there may be associated neurovascular signs and symptoms. In contrast, some individuals may be unaware that they have torn their PCL as they do not experience significant symptoms at the time of injury, or describe vague symptoms such as discomfort at the back of the knee or pain with kneeling.
Since more force is placed on the PCL as the knee bends, knee instability may not be an issue whilst the knee is relatively straight (e.g. walking or jogging) and some sports may be unaffected. PCL-related knee instability is more likely to be experienced when decelerating, or descending stairs/slopes, as these activities require deeper knee flexion angles. If instability is experienced when the knee is in a straighter position, there is likely to be additional medial (inner), lateral (outer) or anterior cruciate ligament involvement. In long-standing (chronic) cases, individuals may present with a gradual onset of anterior or medial knee pain due to the increased stress placed on these compartments secondary to PCL laxity.
Clinical tests
Clinical tests that quantify posterior translation of the tibia are used to assess the integrity of the PCL.
Posterior sagittal (‘sag’) sign/step off test, and Godfrey’s test: This test is performed with the patient lying supine (on their back), both hips flexed to 45° and knees flexed to 90°. The position of the proximal tibia is compared with the unaffected leg, assessing for a backwards ‘sag’. Godfrey’s test is a variation of the sag sign/step off test, whereby the assessor lifts the heels of the patient until the hips and knees are both at 90° and the tibiae are horizontal.
Quadriceps active test: this test is performed in the sag sign position with the limb relaxed. The patient gently contracts their quadriceps muscle to shift the proximal tibia forwards, without extending the knee. The assessor monitors for relocation (reduction) of the proximal tibia from a ‘sagged’ position.
Posterior drawer test: This test is performed with the patient supine, hip flexed to 45°, knee flexed to 90° and foot in a neutral position (i.e., straight). The examiner sits on the subject’s foot, with fingers behind the proximal tibia and thumbs on the tibial plateau. A posterior force is applied to the proximal tibia, assessing for increased posterior displacement and an ‘end point’.
Due to a lack of high-quality evidence, determining the most accurate test for PCL injury is difficult. According to the available evidence, the sag sign is best test for ruling out a PCL injury, while the quadriceps active test is the best test for ruling in a PCL injury. The posterior drawer test is the most frequently evaluated test, but evidence regarding its diagnostic accuracy is limited.
Imaging
A PCL avulsion fracture is a ‘pull off’ fracture of the PCL’s attachment to the tibia and is the most common type of isolated PCL lesion. These fractures may be evident on X-ray but computed tomography (CT) scans may be required for greater detail and to guide treatment.
Although the PCL cannot be directly visualised on X-ray, indirect signs may be evident, which raise the suspicion of PCL injury. In PCL injured knees, the tibia may be translated posteriorly on side (lateral) views, indicating a lack of support from the ligament. Kneeling PCL stress X-rays can be used to measure a side-to-side difference in posterior tibial translation and differentiate isolated PCL injury from combined PCL and posterolateral corner injury.
MRI has high diagnostic accuracy for PCL injury, with 6mm considered the maximum front to back (anterior-posterior) diameter for a normal PCL on a T2-weighted sagittal view; a diameter >6mm indicates a torn PCL. MRI is also useful for identifying ‘peel off’ lesions (PCL avulsion without bony involvement) or associated injuries (e.g. traumatic meniscal tears, cartilage injury), which may influence management in certain clinical presentations.
Management
The management of a PCL tear is determined by the type of injury and the presence or absence of additional injuries. This section discusses the management of an isolated PCL injury.
PCL avulsion injury:
PCL avulsion fractures may be managed conservatively or surgically depending on the size of the fracture, whether the fracture is displaced, and the degree of instability on clinical assessment. Non-displaced, small bony fragments with minimal laxity are often treated without surgery, while large, displaced fragments with laxity are usually fixed surgically. Open and arthroscopic procedures produce similar results, but are associated with specific risks (e.g., arthrofibrosis, infection, non-union of fracture, persistent instability, symptomatic hardware). PCL avulsions without bony involvement are unlikely to heal, but the optimal treatment of these lesions is not clear.
PCL rupture:
The PCL has an intrinsic ability to heal and may regain continuity following rupture. Mechanical stress stimulates ligament healing, but excessive stress may cause the PCL to heal in an elongated position. Non-surgical management has traditionally been advocated for grade I-II PCL injuries; however, recent evidence suggests high level athletes with grade III instability can have excellent outcomes if appropriate conservative management is initiated within 4 weeks of injury. Conservative management is therefore recommended before considering surgery. PCL reconstruction is indicated for symptomatic instability or in specific multi-ligament injuries.
Conservative management:
Conservative management of a PCL injury is dependent on the clinical presentation. For example, addressing pain, swelling, and range of motion deficits is prioritised before introducing advanced strength or perturbation training. Rehabilitation progression is guided by pain/discomfort and swelling whilst monitoring for symptomatic knee instability. If the patient reacts adversely, the intervention should be adjusted accordingly. The following principles have been proposed to promote optimal loading of the injured PCL.
0-2 weeks:
- avoid flexing the knee >90º
- partial weight bear using crutches
0-12 weeks:
- avoid positions that cause the proximal tibia to ‘sag’ relative to the femur
- avoid hyperextending the knee
- avoid knee-dominant hamstring exercises (e.g., hamstring curls, straight knee bridging)
- avoid closed-chain exercises >70º
- avoid kneeling
- do not jog/run.
Bracing:
Several braces are available that resist posterior translation of the proximal tibia, thereby supporting the PCL during the healing phase. The PCL-Jack brace provides a constant (static) support to the proximal tibia and is the only brace that has been validated on PCL-injured patients. The PCL-Jack brace should be fitted as soon as possible, ideally within 3-4 weeks of injury, and worn at all times (except when changing clothes and washing) for 4 months. The Össur Rebound PCL brace is a dynamic brace that provides increasing support to the proximal tibia as the knee flexes. When compared with the PCL-Jack brace, the Össur Rebound PCL brace has been shown to more closely replicate the loading profile of the native PCL, whilst also reducing pressure on the patellofemoral joint. The M.4 PCL brace provides support to the calf and has been shown to restore normal positioning of the tibia following PCL injury and reconstruction in a cadaveric study.
PCL reconstruction:
Post-operative rehabilitation following PCL reconstruction has traditionally been based on graft healing times, with certain activities restricted until specific timeframes have elapsed. More recently, greater importance has been placed on meeting specific criteria before progressing rehabilitation; therefore, current guidelines are both time and criteria-based. Our PCL reconstruction protocols are available in the protocol section.
Return to play
Definitive criteria for a safe return to sport/activity following PCL injury are lacking. There should be clinical evidence of PCL healing (a firm end point on posterior drawer testing), and on-field sports specific rehab should be successfully completed before attempting to return to game situations.
Time:
Timeframes for PCL injury will depend on whether the patient has undergone surgery. For individuals that start appropriate conservative treatment within 4 weeks of injury, return to full sporting activity has been shown to be possible between 10-26 weeks (average 15 weeks) for a grade II injury and 10-40 weeks (average 18 weeks) for a grade III injury. The majority of conservatively managed PCL ruptures can return to their pre-injury sporting levels. For individuals that have undergone surgical reconstruction of the PCL, return to sport is not recommended until at least 9 months after surgery. There are no clear factors identifying individuals that will experience disability, pain or knee osteoarthritis as a result of long-standing PCL instability.
Strength and jump testing:
Limb symmetry Index (LSI) uses the unaffected leg as a reference to identify deficits in strength and jump performance. A LSI of greater than 90% (injured limb divided by uninjured limb) is recommended before returning to play. However, LSI may over-estimate the function of the affected knee, as strength deficits may also be present in the unaffected limb following injury. If available, strength and jump measurements taken before the injury can be used for reference to guide return to sports. If this information has not been recorded, the estimated pre-injury capacity (EPIC), which involves testing the uninjured limb as soon as practical after injury, may be a more suitable guideline than the LSI. However, the EPIC method also relies on measurements being recorded and may not available.
Electromechanical dynamometry is the gold-standard method for measuring muscle strength. However, this equipment is expensive and often inaccessible. Isokinetic testing allows strength to be quantified through range, and can be performed at specific angular velocities (e.g. 60 or 180 degrees per second); peak torque values will be lower with higher angular velocities.
Several jump tests have been described but the ability of these tests to predict future injury is unknown. Hopping in a horizontal direction (e.g., hop for distance, triple hop, triple cross-over hop and 6-metre timed hop) has traditionally been used to determine whether a patient has restored adequate limb symmetry but recent evidence indicates that horizontal hopping may not be an accurate measure of knee function. For example, approximately 90% of work during the propulsion phase when hopping for distance is provided by the hip and ankle; more work is done by the knee on landing but patients can compensate by flexing the hip. Vertical jumping demands more even contributions of the hip, knee and ankle during propulsion and landing and may better reflect knee function. Countermovement and drop jump metrics, can be analysed using force plates, contact mats, and smartphone/tablet applications.
Written by: Richard Norris
Peer reviewed by:
Prof. Christian Kopkow | @CKopkow
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