What is the Posterior Cruciate Ligament (PCL)?
A ligament is a strong fibrous tissue that attaches one bone to another.
The anterior and posterior cruciate ligaments form an “X” in the centre of the knee joint. The posterior cruciate ligament (PCL) being the larger of the 2 and situated behind the anterior cruciate ligament (ACL).
What is the Function of the PCL?
The PCL resists backwards motion of the lower leg. Unlike the ACL, which is mainly functional during certain high risk athletic activities, the PCL is important and is functioning almost all the time even during simple walking.
How Does One Injure the PCL?
A PCL tear occur when a direct blow to the front of the knee or leg just below the knee (tibia) creates a large sudden force directed backwards. This puts a significant amount of stress on the PCL. The stress in the ligament is even higher when the knee is flexed (bent) closed to 90°. The posterior cruciate ligament then stretches to the point of mechanical failure which is considered a tear.
This can happen when someone is tackled in football below the knee from the front or when someone in any sport lands forcefully directly onto their knee with their knee simultaneously bent.
The PCL can also tear in this manner when in a head-on motor vehicle collision the vehicle’s dashboard strikes directly against the knee.
Sometimes the PCL can be stretched and subsequently torn by forceful hyperextension (bending backwards beyond straight) occurring to the athlete’s knee. This may occur when, in football, a player is hit on the legs just below the knee from the front and their knee hyperextends because their foot is firmly planted in the playing surface. This mechanism, especially when the knee twists during the injury, can lead to tearing of other important knee structures beyond simply the PCL.
What are the Symptoms of PCL Tear?
Signs and symptoms of a posterior cruciate ligament injury may include:
- Mild to moderate pain in the knee
- Rapid onset of knee swelling and tenderness (within three hours of the injury)
- Pain with kneeling or squatting
- A slight limp or difficulty walking
- Feeling of instability or looseness in the knee, or the knee gives way during activities
- Pain with running, slowing down, or walking up or down stairs or ramps
- Sometimes the patient may have little or no complaints until much later
When Does It Become More Serious?
It is more serious when the PCL tear is associated with other knee ligament tears. It can be combined with an ACL tear, medial collateral ligament (MCL) or more commonly a lateral collateral ligament (LCL) or posterolateral corner of the knee injuries.
These knees tend to be more unstable and problematic compared to an isolated mild PCL tear.
How to Diagnose PCL Tear?
The best way to diagnose a PCL tear is to examine the knee for posterior sag at 90 degrees of knee flexion with the patient lying on the examination couch. The loss of the usual medial tibial plateau anterior step-off will imply the presence of a posterior tibial sag from a PCL tear.
A posterior drawer test can also be done.
This is the left knee of a patient. Notice that there is no posterior laxity on posterior drawer test.
This is the right knee of the same patient. Notice the posterior sag of the tibia on posterior drawer test.
This implied a PCL tear in this patient’s right knee.
What are the Useful Investigations?
A knee x-ray should be done. This is to exclude a bony avulsion of the PCL off the tibial bone insertion. This can be treated with open reduction and internal fixation using screws.
I find the stress views of the knee in the lateral position with the knee at 90 degrees of flexion useful. I will request that each knee be subjected to an anterior and posterior drawer at the time of the x-rays. The knee with the PCL tear will show increased posterior translation of the tibial with respect to the femur.
Note the increased posterior sag in the left knee compared to the right knee during posterior drawer test:
How to Classify the Severity of PCL Tears?
A PCL tear can be classifed as either an isolated PCL tear, where only the PCL is injured, or as a combined ligament injury. A combined ligament injury would involve a tear of the PCL and at least one other injured ligament. A common example would be a PCL and lateral-sided ligament injury.
Injuries to the PCL can also be graded as I, II or III.
Grade I and II injuries are partial PCL tears. Grade I refers to only a few mm of sag of the tibia backwards while Grade II injuries refer to sagging of the tibia to the level flush with the end of the thigh bone (femur). This roughly corresponds to 1 cm of backwards sag.
A grade III injury signifies a complete rupture and the tibia sags backwards even further. It is likely that when a grade III injury occurs, there are other ligaments torn along with the PCL. It is important to scrutinize the type of PCL injury an athlete sustains because there are significant treatment implications, especially for a Grade III or combined ligament injury.
How to Decide on Treatment?
A partial PCL tear, grade I and II, are typically treated non-operatively with a long course of intensive physical therapy to strengthen the surrounding muscles controlling the knee.
Complete PCL tears often require surgical treatment to regain knee stability.
When the PCL pulls off a small piece of bone from the back of the lower leg (tibial avulsion), the PCL may be surgically repaired. If the bone fragment is large enough a screw can be place to secure the piece of avulsed bone back to where it was originally.
However in the majority of PCL injuries, the ligament tears in the middle of the structure. In this case, the PCL must be reconstructed which refers to replacing the entire ligament with what is known as a graft.
I perform both single bundle and double bundle PCL reconstructions.
My graft of choice can be either autogenous hamstrings or allografts (usually tibialis anterior tendons).
I have a video of how I perform PCL reconstructions in YouTube. I cannot show it here due to Ministry of Health’s guidelines. Do email me at email@example.com if you would like a link to see this video.