Ligaments are strong, dense structures made of connective tissue that stabilize a joint. They connect bone to bone across the joint. The knee joint is located where the end of the thigh bone (femur) meets the top of the shin bone (tibia) and four main ligaments connect these two bones:
- Medial collateral ligament (MCL) runs along the inside of the knee. It prevents the knee from bending in.
- Lateral collateral ligament (LCL) runs along the outside of the knee. It prevents the knee from bending out.
- Anterior cruciate ligament (ACL) is in the middle of the knee. It prevents the shin bone from sliding out in front of the thigh bone.
- Posterior cruciate ligament (PCL) works with the ACL. It prevents the shin bone from sliding backwards under the femur.
The PCL is a much larger ligament in the knee compared to the ACL. It runs from the back of the shin bone and inserts into the front of the medial aspect of the thigh bone called the medial femoral condyle.
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.
Injuries to the posterior cruciate ligament are not as common as other knee ligament injuries. In fact, they are often subtle and more difficult to evaluate than other ligament injuries in the knee.
Many times a posterior cruciate ligament injury occurs along with injuries to other structures in the knee such as cartilage, other ligaments, and bone.
Injured ligaments are considered “sprains” and are graded on a severity scale.:
- Grade I — A mild injury causes only microscopic tears in the ligament. Although these tiny tears can stretch the PCL out of shape, they do not significantly affect the knee’s ability to support the patient’s weight.
- Grade II (moderate) — The PCL is partially torn, and the knee is somewhat unstable, meaning it gives out periodically when standing, walking or going through diagnostic tests.
- Grade III (severe) — The PCL is either completely torn or is separated at its end from the bone that it normally anchors, and the knee is more unstable. Because it usually takes a large amount of force to cause a severe PCL injury, patients with Grade III PCL sprains often also have sprains of the ACL or collateral ligaments or other significant knee injuries.
A PCL injury occurs 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.
Signs & Symptoms
The symptoms during an acute PCL injury is very similar to that of an ACL injury. There will be pain, swelling and restriction of knee movement.
Subsequently when the person recovers from the acute episode, the symptoms may be one of instability or pain in the front of the knee when walking up or down stairs. The incidence of overt instability of the PCL injured knee is less frequent compared to an ACL-deficient knee.
In chronic cases, patients may also have arthritis in the kneecap and inner aspect of the knee joint. Some patients have no symptoms and can carry on with their daily activities.
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
The doctor will examine both knees, comparing the injured knee with the uninjured one. A check of the injured knee for swelling, deformity, tenderness, fluid inside the knee joint and discoloration is done. After determining the knee’s range of motion (how far it can move in all directions), the doctor will pull against the ligaments to check their strength. The patient will be asked to bend the knee while the doctor gently pushes forward on the lower leg where it meets the knee. If the PCL is torn, the lower leg can be moved backward in relation to the knee. The more the lower leg can be moved away from its normal position, the greater the amount of PCL damage and the more unstable the knee.
A Posterior Drawer test can also be done – the shinbone (tibia) is pushed back while the knee is bent 90 degrees. If the tibia moves more than five millimeters backward, it is likely that the PCL has been torn.
X-ray – while an X-ray can’t detect ligament damage, it can reveal bone fractures. Patients with posterior cruciate ligament injuries sometimes experience avulsion fractures — in which a small chunk of bone, attached to the ligament, pulls away from the main bone.
Magnetic resonance imaging (MRI) – this painless procedure uses radio waves and a strong magnetic field to create computer images of the soft tissues of the body. An MRI scan can clearly show a posterior cruciate ligament tear and determine if other knee ligaments or cartilage also are injured.
Arthroscopy – if it’s unclear how extensive the knee injury is, the doctor inserts a tiny camera into the knee. This is the best way to examine a partial tear for it allows the doctor to gently pull at the PCL and determine the extent of the damage. This procedure is done on an outpatient basis and is relatively pain-free.
In Grade I injuries, there is only a few mm of sag of the tibia backwards while in Grade II injuries, there is a 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 a patient sustains because there are significant treatment implications, especially for a Grade III or combined ligament injury.
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.
PCL Reconstruction Surgery
The PCL can be reconstructed surgically using either a piece of the patient’s own tissue (autograft) or a piece of donor tissue (allograft).
With an autograft, the surgeon typically replaces the torn PCL with part of the patient’s own patellar tendon (the tendon below the kneecap) or a section of tendon taken from a large leg muscle (eg. hamstring). With an allograft, tibialis anterior tendons is usually used.
Almost all these surgeries are performed using less-invasive arthroscopic (camera-guided) knee surgery, which uses smaller incisions, has less pain, causes less scarring than traditional surgery, and have quicker recovery times.
After surgery to reconstruct the PCL, the patient will wear a long-leg knee brace and gradually begin a rehabilitation program to strengthen the leg muscles around the knee.
Physical therapy will begin 1 to 4 weeks after the surgery. How long it takes to recover from a posterior cruciate ligament injury will depend on the severity of the injury. Combined injuries often have a slow recovery, but most patients do well over time.
It may be several weeks before the patient returns to a desk job – perhaps months if the job requires a lot of activity. Full recovery typically requires 6 to 12 months.