Posterior cruciate ligament (PCL) injuries are much less common than anterior cruciate ligament injuries.
The PCL is a primary restraint to posterior drawer and a secondary restraint to external rotation. Therefore the mechanisms of injury producing a PCL rupture include a fall on a flexed knee or a dashboard injury with direct blow to the anterior aspect of the tibia.
It is a common injury in rugby league where two players tackle with one around the shoulders and the other round the knees.
PCL injuries can be associated with injury to the posterolateral corner (PLC) of the knee. The PLC of the knee is a complex structure consisting primarily of the biceps femoris, the popliteus muscle complex, the iliotibial tract, the lateral collateral ligament and the arcuate ligament.
The mechanism of injury will be suggestive of PCL injury. Patients typically complain of pain in the popliteal fossa. Usually, there is no significant swelling of the knee as the PCL is extrasynovial. Patients sometimes dismiss the injury as minor because there is minimal swelling.
Classically, patients present with a posterior sag on the tibia appreciated on examining the knee from the lateral side. The sag of the tibia is marked if the PCL is associated with PLC injury.
If you elevate the patient's leg from the toes, in patients with PCL and PLC injury the knee goes into varus recurvatum. Always check the function of the peroneal nerve in these patients because of the high risk of injuries to this nerve.
The dial test can elicit injury to the PLC - the patient lies in the prone position, and the external rotation of tibia is compared by flexing both the knees.
Normally, the anterior aspect of the tibia is approximately 1cm anterior to the femoral condyle in the semi-flexed position. This relationship is lost in PCL injuries and injuries are graded based on the position of the tibia to the femur (see box).
Management is dependent on the nature of the injury. Imaging is needed to confirm the diagnosis. A plain X-ray of the knee will show any avulsion injury of the PCL that need fixation.
Chronic injuries of the PCL are usually seen in the midsubstance and do not need surgery.
Acute injuries of the PCL can be associated with PLC injuries. These patients should be referred for an orthopaedic opinion and would need further imaging of the knee with special MRI sequences for the PLC.
Isolated PCL injuries seldom need surgery and are usually managed with staged physiotherapy for the quadriceps.
In grade III injuries sometimes bracing in full extension for two to four weeks is needed before physiotherapy is commenced. In the short term patients can get back to sports, but in the long-term they have a risk of early osteoarthritis after 15-25 years.
PCL graft repair is still in its infancy and it is still too early to assess if it makes a difference in the long-term prevention of osteoarthritis.
Mr Mohan is orthopaedic registrar at the Royal Surrey County Hospital, Guildford
Types of posterior cruciate ligament injuries
- Isolated acute injuries
- Midsubstance injuries.
- - Avulsion injuries.
- - Isolated chronic injuries.
- - Injuries associated with posterolateral corner injuries.
Grade I (mild) tibia still anterior to distal femur but slightly diminished. Step-off present but decreased (0-5mm).
Grade II (moderate) 5-10mm of posterior translation.
Grade III (severe) greater than 10mm of posterior translation.