The main ligament behind the knee is the posterior cruciate ligament, commonly called the PCL. It’s the strongest ligament in the knee joint, with a tensile strength of 2,500 to 3,000 newtons, and its primary job is to keep your shinbone from sliding backward underneath your thighbone. Several smaller ligaments also reinforce the back of the knee, but the PCL is the dominant structure and the one most people are asking about when they feel pain or instability in that area.
What the PCL Does
The PCL runs diagonally inside your knee joint, connecting the back of your shinbone (tibia) to your thighbone (femur). Its femoral attachment sits on the inner wall of the knee, starting about 6 millimeters from the edge of the cartilage surface when the knee is bent at 90 degrees. This positioning allows it to act like a taut rope that prevents your lower leg from shifting backward relative to your upper leg.
The ligament is made up of two bundles of fibers that work in a complementary way. The anterolateral bundle tightens when the knee bends and provides the most posterior stability at 90 degrees of flexion. The posteromedial bundle tightens when the knee straightens. Together, they account for roughly 95% of the resistance to backward displacement of the shinbone. The PCL also helps prevent the knee from bending too far (hyperflexion), which is why it’s under the most stress during deep squatting or kneeling.
Other Ligaments at the Back of the Knee
The PCL isn’t alone back there. The posterior knee capsule is reinforced by a network of smaller structures that provide additional stability:
- Oblique popliteal ligament: A broad band that strengthens the back of the knee capsule. It helps resist inward rotation of the shinbone and side-to-side stress when the knee is fully straight.
- Arcuate ligament: A deeper structure on the outer back corner of the knee that works as part of the posterolateral corner, a group of tissues that controls outward rotation.
- Popliteofibular ligament: Connects the popliteus muscle tendon to the head of the fibula (the smaller bone on the outer side of your lower leg), with a tensile strength of about 229 newtons.
The posterolateral corner as a whole works together with the PCL to control external rotation and backward translation of the tibia. When both the PCL and these corner structures are damaged, the knee becomes significantly more unstable than with either injury alone.
How the PCL Gets Injured
Isolated complete PCL tears are uncommon, occurring in about 2 out of every 100,000 people per year and making up roughly 1% of all acute knee injuries. The classic scenario is a “dashboard injury,” where someone in a car accident strikes their bent knee against the dashboard. The force drives the shinbone straight backward, overwhelming the PCL. The same mechanism happens in sports when an athlete falls onto a bent knee with the foot pointed downward, or when a direct blow hits the front of the upper shin.
Contact sports like football and rugby are common settings for PCL injuries, as are motorcycle accidents and any high-energy trauma that pushes the tibia posteriorly while the knee is flexed.
Symptoms of a PCL Injury
PCL injuries tend to be more subtle than ACL tears. There’s usually no dramatic “pop” at the moment of injury, and swelling can be mild enough that people initially assume they just bruised their knee. Over the following days, though, symptoms typically become more noticeable: pain that worsens over time, stiffness, a feeling of instability, and difficulty walking. Going down stairs is often the activity that feels the worst, because descending loads the knee in exactly the position where the PCL is most important (around 90 degrees of bend).
This gradual onset is part of what makes PCL injuries easy to miss or dismiss. Many people continue walking on an injured PCL for days or weeks before seeking care, unlike ACL tears that usually send someone to the emergency room the same day.
How PCL Injuries Are Diagnosed
Doctors use several hands-on tests to check whether the PCL is intact. The most commonly known is the posterior drawer test, where the examiner pushes the upper shin backward with the knee bent at 90 degrees to see how far it moves. This test has a specificity of 98%, meaning a positive result almost certainly indicates a real injury, but its sensitivity is only 69%, so it misses some tears.
The posterior sag test, where the examiner simply observes whether the shinbone sags backward under gravity when the knee is bent, performs slightly better at 79% sensitivity and 100% specificity. Another useful option is the quadriceps active test (75% sensitivity, 98% specificity), which asks the patient to tighten their thigh muscle while the examiner watches for forward movement of the tibia, indicating that the shinbone was sitting in an abnormally posterior position. MRI is typically used to confirm the diagnosis and assess the extent of the tear.
Treatment and Recovery Timeline
Most isolated PCL injuries, particularly partial tears, are managed without surgery. The rehabilitation process follows a structured progression that spans three to four months in straightforward cases, and up to five or six months when healing is slower.
The first phase focuses on protecting the knee and controlling swelling, typically lasting four to six weeks. During this time, a brace may be used to prevent the shinbone from sagging backward while the ligament heals. Weight-bearing is gradually introduced. The second phase, from roughly weeks four through eight, shifts toward strengthening the quadriceps. Strong quadriceps are especially important after a PCL injury because the thigh muscle actively pulls the shinbone forward, compensating for the loosened ligament.
Advanced strengthening begins around weeks eight through twelve, incorporating more dynamic exercises and sport-specific movements. Return to full activity, including competitive sports, is typically cleared at 12 weeks or later for people on a standard recovery track. Those who heal more slowly may not reach that milestone until 18 weeks or beyond.
Surgery is generally reserved for complete tears that cause persistent instability, injuries involving multiple ligaments, or cases where conservative treatment fails to restore adequate function. Reconstruction involves replacing the torn ligament with a graft, and the recovery timeline is substantially longer than for non-surgical management.
Why the PCL Matters Long-Term
Even after a PCL injury heals, the ligament often remains slightly looser than it was before. This residual laxity changes the way forces distribute across the knee joint, particularly at the cartilage surfaces on the inner side and behind the kneecap. Over years, this altered loading pattern can accelerate cartilage wear. Maintaining strong quadriceps through ongoing exercise is one of the most effective ways to counteract this and protect the joint over the long term, because the muscle dynamically replaces some of the stability the ligament lost.