Causes of Posterolateral Knee Pain
Posterolateral knee pain has a broad differential diagnosis that includes injuries to the lateral meniscus, popliteus tendon pathology, iliotibial band syndrome, lateral collateral ligament injury, posterolateral corner injuries, popliteal (Baker's) cyst, biceps femoris tendinopathy, common peroneal nerve irritation, and less commonly ganglion cysts or tumors.
Primary Structural Causes
Ligamentous and Capsular Injuries
The posterolateral corner represents a complex anatomical region where the fibular collateral ligament, popliteus tendon, and popliteofibular ligament serve as the main static stabilizers against varus and posterolateral rotational forces 1. These injuries are frequently missed on initial examination and can be severely debilitating if not properly diagnosed. When evaluating for posterolateral corner pathology, look specifically for:
- Varus instability at 30 degrees of flexion
- Increased external rotation (dial test) at 30 and 90 degrees
- External rotation recurvatum test positivity
- Varus thrust during gait
Meniscal Pathology
Lateral meniscus tears are a common cause of posterolateral pain, particularly tears involving the posterior horn 2. However, a critical caveat: meniscal tears are often incidental findings in older patients, with the majority of people over 70 having asymptomatic tears 2. The presence of a tear on imaging does not automatically establish causation for the pain.
Tendinous Causes
Popliteus tendinopathy and ganglion cysts of the popliteus tendon sheath represent overlooked causes of deep posterolateral pain 3. The pain from proximal popliteus tendon injury is deeply located and worsens with knee flexion. Proximal lateral gastrocnemius tendon sprains also cause posterior knee pain that can be aggravated by flexion 4.
Iliotibial band syndrome presents with lateral knee pain and can be effectively evaluated and followed with ultrasound 2.
Cystic Lesions
Popliteal (Baker's) cysts are readily identified on MRI and ultrasound, with both modalities showing comparable accuracy for detecting cyst rupture 2. These cysts represent fluid-filled extensions from the knee joint into the popliteal fossa.
Ganglion cysts arising from the lateral meniscus or popliteus tendon sheath can cause focal posterolateral pain 3.
Bone and Cartilage Pathology
Lateral compartment osteoarthritis with associated bone marrow lesions (BMLs) and synovitis can manifest as posterolateral pain 2. New or increasing BMLs correlate with increased knee pain, particularly in males and those with family history of osteoarthritis 2.
Subchondral insufficiency fractures and stress fractures of the lateral tibial plateau can present with posterolateral pain, though these more commonly affect the medial femoral condyle 2.
Neurological Causes
Common peroneal nerve irritation at the fibular head can produce posterolateral knee pain, often with associated paresthesias in the lateral leg and dorsum of the foot 5, 6.
Diagnostic Approach
Initial Imaging
Start with standard knee radiographs including anteroposterior, lateral, and tangential patellar views 2. If radiographs are unremarkable but clinical suspicion remains high, consider referred pain from the hip or lumbar spine 2.
Advanced Imaging
When radiographs are normal or show only effusion and pain persists, MRI without contrast is the next appropriate study 2. MRI accurately depicts:
- Meniscal tears and associated ganglion cysts
- Popliteal cyst presence and rupture
- Posterolateral corner ligament injuries
- Bone marrow lesions
- Popliteus and gastrocnemius tendon pathology
- Iliotibial band abnormalities
Ultrasound serves as a targeted alternative for evaluating popliteal cysts, guiding aspiration, assessing iliotibial band syndrome, and performing sonopalpation for tendon pathology 2, 4.
Critical Pitfalls
Do not assume all meniscal tears seen on MRI are symptomatic, especially in patients over 45 years old 2. Clinical correlation is essential.
Posterolateral corner injuries are frequently missed on initial examination 1. Perform specific stress tests (varus at 30°, dial test, external rotation recurvatum) when posterolateral instability is suspected.
Consider referred pain from hip or lumbar spine pathology when knee imaging is unremarkable 2.