Posterolateral corner (PLC)

What it is

Rather than a single ligament, the posterolateral corner is a team. Its three main stabilisers are the lateral collateral ligament, the popliteus tendon and the popliteofibular ligament, supported by the capsule, the biceps femoris tendon and the iliotibial band. They work with the cruciate ligaments, so the corner and the PCL or ACL are often injured together.

Why it matters for patients

Posterolateral corner injuries usually come from a blow to the front inner side of the knee, a hyperextension injury or a high-energy twist in sport or a road accident. Patients describe outer and back-of-knee pain, swelling, and a knee that feels unstable or bows outwards when standing and walking.

These injuries are easy to miss, and an unrecognised corner injury is one of the known reasons an ACL or PCL reconstruction can fail. They also heal poorly on their own when complete. For that reason surgeons look for them carefully after any significant ligament injury, and severe tears are usually repaired or reconstructed, ideally within the first weeks.

How it is examined

Tests include stressing the knee outwards, the dial test, which compares outward rotation of both shins, and checking the peroneal nerve. MRI and stress X-rays confirm and grade the injury.