Posterolateral knee pain can be a challenging problem for active people. The outside corner of the knee is complex, made up of multiple ligaments, tendons, and muscles — so an accurate diagnosis is essential for effective treatment. This guide covers the possible causes of lateral posterior knee pain, and how to work out the likely cause.
Causes of posterolateral knee pain
Posterolateral corner (PLC) injury
One of the most significant causes of posterolateral knee pain is an injury to the posterolateral corner (PLC). The PLC comprises several structures, including the lateral collateral ligament (LCL), the popliteus tendon, and the popliteofibular ligament. These injuries often result from sudden knee hyperextension or an outward force, common in contact sports and traumatic falls.
PLC injuries frequently cause pain on the outside of the knee and difficulty turning or pivoting. Your doctor should examine the knee for looseness, using specific tests that stress the posterolateral corner. Because PLC injuries rarely occur in isolation — they’re often associated with an ACL or PCL tear — it’s essential to test all the knee ligaments. MRI is the key investigation for identifying all damaged structures.
Isolated LCL injury
An isolated injury to the lateral collateral ligament (LCL) can cause posterolateral knee pain, although these injuries rarely occur on their own. The LCL is the main ligament on the outside of the knee, and it’s usually injured by a force directed toward the inside of the knee.
Popliteus tendinitis
Popliteus tendinitis occurs when the popliteus tendon — which wraps around the outside of the knee — is overused, particularly in downhill running. It causes pain in the back and outside of the knee, worse with activity. Treatment is usually with relative rest, anti-inflammatory measures, and rehabilitation.
Biceps femoris tendinopathy
The biceps femoris is the outer hamstring, attaching to the fibular head on the outside of the knee. Overuse — especially explosive sprinting and acceleration — leads to biceps femoris tendinopathy, causing pain at the back and outside of the knee that stiffens with rest and warms up with activity.
Iliotibial band (ITB) syndrome
Iliotibial band syndrome is a common cause of lateral posterior knee pain, particularly in runners. It develops from friction between the iliotibial band and the lateral femoral condyle, causing inflammation and knee pain. The pain usually starts during a run and increases the longer the athlete runs. There is focal tenderness over the ITB on the outside of the knee. Tight hamstrings and weak hip muscles increase the risk.
Treatment involves anti-inflammatory measures and rehabilitation to reduce pressure on the ITB at the knee. Sometimes, a cortisone injection deep into the ITB helps. Some centres have used Botox on the tensor fascia lata (TFL) muscle in the hip to reduce tension in the band.
Lateral meniscus tear
A tear of the lateral meniscus can cause pain on the outside of the knee, sometimes with catching, locking, or swelling. A lateral meniscal cyst can also form and, if it sits posteriorly, may cause posterolateral fullness and occasionally irritate the nearby common peroneal nerve.
Fabella syndrome
The fabella is a small sesamoid bone found in the posterolateral capsule of some knees. When present, it can occasionally become a source of persistent posterolateral pain, especially with exercise — a condition known as fabella syndrome.
How is posterolateral knee pain diagnosed?
Because the outer corner of the knee is complex, a careful clinical assessment is essential. Your doctor takes a history and examines the knee — testing the ligaments (including the PLC), the tendons, and the menisci, and checking for instability. MRI is usually the key investigation, as it shows the ligaments, tendons, menisci, and any associated injuries. Ultrasound can assess the tendons (popliteus, biceps femoris, ITB) and guide an injection where needed.
Frequently asked questions about posterolateral knee pain
What is the most common cause of posterolateral knee pain?
In runners, ITB syndrome and popliteus or biceps femoris tendinopathy are common. After trauma — a hyperextension injury or a blow to the knee — a posterolateral corner (PLC) injury is the important one to exclude, as it rarely occurs alone.
When should I worry about posterolateral knee pain?
See a doctor promptly if the knee feels unstable or gives way (which can indicate a PLC or combined ligament injury), or if you develop numbness, tingling, or weakness in the foot (which can indicate common peroneal nerve involvement). Pain after significant trauma should always be assessed.
How is posterolateral knee pain diagnosed?
Through a careful examination of the ligaments, tendons, and menisci, which MRI usually supports. Ultrasound helps assess the tendons and guide injections. The complexity of the region is exactly why an accurate diagnosis matters.
Can posterolateral knee pain be treated without surgery?
Most causes — ITB syndrome, popliteus and biceps femoris tendinopathy — are managed without surgery, using rehabilitation, load modification, and sometimes an injection. A significant PLC injury (particularly grade 3, or combined with an ACL/PCL tear) may need surgical reconstruction.
Final word from Sport Doctor London about posterolateral knee pain
Posterolateral knee pain is complex because the outer corner of the knee contains so many structures. Getting to the right diagnosis — distinguishing a tendon problem from a ligament injury or a meniscal tear — is the key to effective treatment. Most causes respond well to rehabilitation and, where needed, a guided injection, with surgery reserved for significant ligament injuries.
If you have persistent pain in the back and outside of the knee, Dr Masci can assess you in London, including an ultrasound in the clinic. Contact the team here or call +44 (0) 203 488 0350.
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