Overview
Posterior cruciate ligament (PCL) injury is typically caused by direct trauma to a bent knee. Limited information exists regarding the optimal management of a PCL injury but conservative management (physiotherapy and bracing) is often effective for isolated partial tears or complete ruptures. Surgery may be required for individuals with persistent symptoms despite appropriate conservative treatment, avulsion fractures of the PCL or multi-ligament knee injuries.
Anatomy
The posterior cruciate ligament (PCL) is located within the knee joint and consists of two separate bundles (anterolateral and posteromedial) that connect the shin (tibia) to the thigh bone (femur) (figure 1-2). The tension in each bundle changes depending on knee position and whether movement is being performed passively (e.g. bending the knee with your hands) or actively (e.g. forward lunge). Overall, greater force is placed on the PCL when the knee is bent (flexed) or over-straightened (hyperextended) (figure 3).
Figure 1: back view of the knee showing the anterolateral (ALB) and posteromedial bundle (PMB) of the PCL. From Anderson et al (2012).
Figure 2: PCL force at different knee flexion angles. From Markolf et al (2006).
The PCL is thought to provide 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).
Causes
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 additional injury to another major knee ligament.
Presentation
PCL injuries account for approximately 2-3% of sporting injuries, occurring most often in sports that involve contact/tackling (e.g. rugby, American football). Up to 40% of major trauma patients (e.g. road traffic accidents) presenting with swelling inside the knee joint (effusion) will have sustained a PCL injury.
Considering most isolated PCL injuries are caused by a blunt force to the front of the proximal tibia, a cut/graze, swelling and/or bruising may be evident at the point of contact. 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.
Individuals may describe a noise (pop, snap or crack), tearing sensation, significant pain or immediate swelling within the joint (haemarthrosis), with possible bruising at the back of the knee; X-rays are indicated for individuals with a positive Ottawa Knee Rule or a tense haemarthrosis, to confirm or exclude knee fracture. 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. If present, individuals may 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 straight, 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 and/or medial knee pain due to the increased stress placed on these compartments secondary to PCL laxity.
Diagnosis
PCL injuries often occur during knee dislocation; 18% of knee dislocations involve injury to vascular structures, which can become limb or life threatening. As a priority, it is therefore important to assess for vascular injury in known or suspected cases of knee dislocation.
Clinical tests that quantify posterior translation of the tibia are used to assess the integrity of the PCL, as described below. Due to a lack of high quality evidence, determining the most appropriate test for PCL injury is difficult. According to the available evidence, the sag sign is best test for ruling out a PCL injury when negative, while the quadriceps active test is the best test for ruling in a PCL injury when positive. The posterior drawer test is the most frequently evaluated test but determining the value of the test is difficult. For details on the diagnostic accuracy of individual tests, please visit the statistics section.
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’ (figure 3 and video 1). 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 (figure 4-5).
Figure 3-5: positive sag sign (left leg), Godfrey’s test and positive Godfrey’s test (left leg).
Interpretation:
The medial tibial plateau should sit 1cm anterior to the medial femoral condyle. If the PCL is torn, the proximal tibia ‘sags’ posteriorly under the force of gravity and this ‘step off’ is lost. A positive test, indicates disruption of the PCL.
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 (video 1).
Video 1: positive sag sign and quadriceps active tests.
Interpretation:
If the PCL is ruptured, the proximal tibia sags into posterior subluxation and the patellar tendon is directed anteriorly. Contraction of the quadriceps muscle in the PCL deficient knee results in an anterior shift of the proximal tibia of 2 mm or more. The test is qualitative (i.e. not officially measured).
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’ (video 2).
Video 2: positive posterior drawer test.
Interpretation:
Increased posterior displacement of the proximal tibia, as compared to the uninvolved side, is indicative of a partial or complete tear of the PCL. Laxity with an end point is suggestive of a grade I-II injury, while laxity with no end point indicates a rupture of the PCL.
It is important to ensure that the tibia and foot are not externally rotated during the posterior drawer test, as this would produce a posterolateral force on the tibia, which assesses for combined PCL and posterolateral corner injury.
Imaging
X-rays are often normal in isolated PCL ruptures and are therefore not indicated unless there is a suspicion of knee fracture. Although the PCL cannot be directly visualised on X-ray, indirect signs may be evident, which raise the suspicion of PCL injury.
A PCL avulsion fracture (figures 6-8) 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.
Figure 6: posterior cruciate ligament avulsion fracture (arrow).
Figures 7-8: lateral and axial CT scans of a PCL avulsion fracture.
In PCL injured knees, the tibia may be translated posteriorly on side (lateral) views, indicating a lack of support from the ligament (figure 9). Kneeling PCL stress X-rays can be used to measure a side-to-side difference in posterior tibial translation and thereby PCL laxity; 8-12mm difference indicates an isolated PCL injury while a difference more than 12mm is suggestive of a combined PCL and posterolateral corner injury.
Figure 9: posteriorly translated proximal tibia in a combined tibial plateau fracture and PCL injured knee.
Figure 10: MRI appearance of a grade 2 PCL injury with >6mm AP diameter.
MRI has high diagnostic accuracy for PCL injury, with 6mm considered the maximum front to back (anterior-posterior) diameter for a normal PCL on T2 sagittal imaging; a diameter >6mm indicates a torn PCL (figure 10). MRI is also useful for identifying ‘peel off’ lesions (PCL avulsion without bony involvement) or associated injuries that may influence management.
Classification
IKDC grading scale: the side-to-side difference in posterior joint translation during the posterior drawer test is graded as follows.
| Test | Normal | Grade 1+ | Grade 2+ | Grade 3+ |
|---|---|---|---|---|
| Posterior Drawer | 0-2mm | 2-5mm | 5-10mm | >10mm |
Table 1: IKDC grading scale.
Treatment
The optimal management of an isolated or combined PCL injury is unclear, with current guidelines based on limited evidence or expert opinion. Current evidence indicates that surgical management of multi-ligament injuries (involving the PCL) provides superior outcomes (e.g. stability, return to work and sport) when compared with non-surgical management.
The following information describes the current management principles for an isolated PCL injury. For an example of a conservatively managed PCL injury protocol, please click here.
Conservative management:
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 but 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.
In the initial stages following injury, any impairments related to knee joint pain, swelling/effusion and range of motion (ROM) should be addressed before focussing on fully weight bearing, muscle strengthening and other neuromuscular exercises. Strengthening exercises target the quadriceps muscle, which actively prevent posterior translation of the tibia, while proprioceptive exercises promote dynamic stability of the knee.
The following principles have been proposed, which encourage optimal loading of the injured PCL; this information is presented in table 2 and demonstrated in video 3.
| For the first two weeks avoid: | For the first 12 weeks avoid: |
|---|---|
| Knee flexion past 90° | Effects of gravity causing the proximal tibia to 'sag' |
| Fully weight bearing | Knee hyperextension |
| Isolated hamstrings exercises | |
| Closed chain exercises >70° | |
| Kneeling |
|
| Jogging |
Table 2: restrictions for the first 12 weeks afer PCL injury. From Pierce et al (2013).
Video 3: acute PCL injury rehabilitation principles.
Surgical management:
Individuals with an isolated PCL injury, that have persistent PCL-related symptoms despite appropriate non-surgical treatment, should be considered for surgery. The optimal rehabilitation following surgical treatment has not been determined; post-operative restrictions may be set by the surgeon and any guidelines or protocols should be clearly communicated to the treating therapist.
For an example of an evidence-based, post-operative rehabilitation please click here.
PCL avulsion injury:
PCL avulsions (‘peel off’) without bony involvement are unlikely to heal but the optimal treatment of these lesions is not clear. PCL avulsion fractures may be managed conservatively or surgically depending on the size of the fracture and instability noted on clinical assessment. Non-displaced, small bony fragments with grade I-II laxity are often treated without surgery, while large, displaced fragments with grade 3 laxity are usually fixed surgically. Open and arthroscopic procedures produce similar results and are associated with specific risks (arthrofibrosis, infection, non-union of fracture, persistent instability, symptomatic hardware).
The optimal rehabilitation following conservative or surgical treatment of PCL avulsion injuries has not been defined but the principles are similar to those described above for PCL rupture.
Bracing
Numerous braces are available that resist posterior translation of the proximal tibia, thereby protecting the PCL during the healing phase.
The PCL-Jack brace (Albrecht, GmbH) provides a constant (static) support to the proximal tibia (figure 11) and is the only brace that has been validated on PCL-injured patients, reducing an initial sag of 7.1mm to 2.3mm at 12 months and 3.2mm at two years. 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. A recent study describes the successful return to sports for grade 2 and 3 PCL injuries using a PCL-Jack brace and the rehabilitation principles described above.
The Össur Rebound PCL brace (figure 12) 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 (figure 13) 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.
Figures 11-13: PCL-Jack brace, PCL rebound brace and M.4 PCL brace.
Recovery
For individuals that start appropriate conservative treatment within 4 weeks of PCL 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. Clear return to sports criteria is lacking for PCL injury, with recent studies describing an individualised approach, depending on how athletes respond during the rehabilitation process. There should be clinical evidence of PCL healing (a firm end point on posterior drawer testing), while some experts have suggested that individuals should pass the anterior cruciate ligament injury return to sports criteria.
There are no clear factors identifying individuals that will experience disability, pain or knee osteoarthritis as a result of long-standing PCL instability.
Acknowledgements
Written by: Richard Norris, The Knee Resource
Reviewed by: Prof. Dr Christian Kopkow
Hochshule für Gesundheit (University of Applied Sciences)
Department of Applied Health Sciences, Bochum, Germany.
Twitter: @CKopkow
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