The main ligament behind the knee is the posterior cruciate ligament, or PCL. It sits deep inside the knee joint, connecting the thighbone to the shinbone, and acts as the primary restraint preventing your lower leg from sliding backward. But the PCL isn’t alone back there. Several other ligaments and structures reinforce the back of the knee, and understanding the full picture helps explain why posterior knee problems can feel and behave so differently depending on what’s involved.
The Posterior Cruciate Ligament (PCL)
The PCL is the strongest ligament in the knee, roughly twice as thick as the more commonly torn ACL. It runs from the back of the shinbone (tibia) up and forward to attach on the inner surface of the thighbone (femur), forming a diagonal cross with the ACL inside the joint. Anatomically, it looks like a single flat ribbon of tissue rather than a round cord. Its femoral attachment sits close to the cartilage surface of the thighbone, just a few millimeters from the joint surface when the knee is bent to 90 degrees.
The PCL’s primary job is keeping the tibia from shifting backward relative to the femur. Every time you walk downhill, descend stairs, or decelerate from a run, the PCL is under tension. It also helps control how the knee rotates during movement, working together with structures on the outer back corner of the knee to limit excessive twisting.
Other Ligaments Behind the Knee
Beyond the PCL, a network of smaller ligaments reinforces the back of the knee capsule and prevents it from buckling under stress.
The oblique popliteal ligament is a broad, flat band that stretches diagonally across the back of the knee capsule. It tightens when you fully straighten your leg, helping prevent hyperextension. Think of it as a safety strap across the back wall of the joint.
The arcuate popliteal ligament sits on the outer back corner of the knee and is part of a group of structures collectively called the posterolateral corner, or PLC. This corner includes the fibular collateral ligament (the main ligament on the outer side of the knee), the popliteus muscle and its tendon, the popliteofibular ligament, and the arcuate ligament itself. Together, these structures control external rotation and work with the PCL to prevent the shinbone from shifting backward and twisting outward. Injuries to the posterolateral corner often happen alongside PCL tears, and missing a PLC injury during diagnosis is one of the main reasons PCL repairs sometimes fail.
How PCL Injuries Happen
The classic PCL injury is the “dashboard injury.” In a car accident, the front of the shinbone strikes the dashboard while the knee is bent, driving the tibia straight backward and stretching or tearing the PCL. The same mechanism happens in sports when an athlete falls on a bent knee with the foot pointed downward, or when a direct blow hits the front of the upper shin during a tackle.
Unlike ACL tears, which often produce a dramatic pop and immediate swelling, PCL injuries can be surprisingly subtle. You might notice a vague ache behind the knee, mild swelling, and a feeling of instability when decelerating or going downstairs. Many people walk on a torn PCL for weeks before getting diagnosed because the initial pain isn’t severe enough to send them to a doctor.
Signs of a Posterior Ligament Injury
The hallmark finding is called the posterior sag sign. When you lie on your back with both hips and knees bent to 90 degrees, a healthy shinbone sits about 10 millimeters in front of the thighbone at the inner side of the knee. With a PCL tear, that normal step-off disappears or reverses, and the shinbone visibly drops backward compared to the uninjured side. This is something a clinician checks during a physical exam, but you can sometimes notice it yourself by looking at both knees side by side in that position.
Other signs include pain that worsens with deep knee bending, difficulty with stairs (especially going down), and a feeling that the knee is “loose” or unreliable during quick changes of direction. If the posterolateral corner is also damaged, you may notice the knee giving way when pivoting or twisting.
PCL Injury vs. Baker’s Cyst
Not all pain behind the knee comes from a ligament. A Baker’s cyst (also called a popliteal cyst) is a fluid-filled sac that forms in the hollow at the back of the knee. It typically causes a visible bulge, a feeling of tightness, and stiffness that makes it hard to fully bend or straighten the leg. The pain tends to worsen after prolonged standing or activity.
Baker’s cysts usually develop as a secondary problem. Osteoarthritis, rheumatoid arthritis, or a cartilage tear causes excess fluid production inside the joint, and that fluid pushes into the back of the knee capsule. Rarely, a Baker’s cyst can rupture, causing sudden sharp pain in the knee and swelling in the calf that can mimic a blood clot.
The key difference: a ligament injury usually involves a specific traumatic event (a blow, a fall, a sudden twist) and produces instability, while a Baker’s cyst develops gradually and produces a palpable lump with stiffness. Both can coexist, since a ligament tear can trigger the joint inflammation that leads to a cyst.
Treatment and Recovery After PCL Surgery
Many isolated PCL tears, especially partial ones, heal well without surgery. Physical therapy focused on strengthening the quadriceps (the muscles on the front of the thigh) can compensate for a lax PCL and restore functional stability. The quadriceps pull the tibia forward, effectively doing the PCL’s job from the outside.
Surgery becomes necessary when the PCL is completely torn, when multiple ligaments are involved, or when nonsurgical rehab fails to resolve instability. PCL reconstruction replaces the torn ligament with a graft, and the recovery timeline is long and structured.
For the first four to six weeks after surgery, you’ll be in a brace locked in a straight position and on modified weight-bearing, gradually working toward full weight-bearing around week six. Range of motion is initially limited to 90 degrees of bending, increasing by about 15 degrees per week after the second week. The brace stays locked for the first eight weeks, and you won’t be allowed to kneel until at least 16 weeks.
One of the most important restrictions: no active hamstring use for the first 12 weeks. The hamstrings pull the tibia backward, which is exactly the force the new graft needs protection from while it heals. Isolated hamstring strengthening doesn’t begin until 14 weeks post-surgery. Straight-line jogging and basic jump training typically start around four to five months, and full return-to-sport testing happens around eight months. Discharge criteria include less than a 10% strength difference between legs and at least 90% symmetry on functional hop tests.
Why Posterior Knee Anatomy Matters
The back of the knee is one of the most structurally complex areas in the body, with overlapping ligaments, tendons, muscles, nerves, and blood vessels packed into a small space. The PCL is the headline structure, but the oblique popliteal ligament, arcuate ligament, and the entire posterolateral corner all contribute to keeping the knee stable during the forces of daily life and sport. When something goes wrong back there, an accurate diagnosis matters because the treatment for a PCL tear, a posterolateral corner injury, and a Baker’s cyst are all quite different. Getting the right answer early leads to a faster and more complete recovery.