Posterolateral knee pain: How to Find a Cause

Common Causes of Posterolateral Knee Pain 

Posterolateral Corner (PLC) Injury

One of the most common causes of posterolateral knee pain is an injury to the posterolateral corner (PLC). The PLC consists of structures such as the lateral collateral ligament (LCL), popliteus tendon, and popliteofibular ligament. Injuries to this region often result from sudden knee hyperextension or outward stretching, which are common in contact sports and traumatic falls.

Generally, PLC injuries frequently cause pain on the outside of the knee and difficulty in turning or pivoting. Your doctor should examine your knee to see if you have evidence of loosening of the knee with specific tests that stress the PCL. 

Treatment depends on the grade of the injury. Low-grade injuries can be treated with rehab, while higher-grade injuries require surgical repair or reconstruction. 

Iliotibial Band Syndrome (ITBS) causes posterolateral knee pain 

Iliotibial band syndrome is another common cause of lateral posterior knee pain, particularly in runners. This condition develops due to friction between the iliotibial band and the lateral femoral condyle, leading to inflammation and knee pain. Localised pain outside the knee usually starts during a run and increases as the athlete continues to run.

Individuals with ITBFS commonly have focal tenderness at the ITB on the outside of the knee. Hamstring tightness and weakness of the hip muscles often increase the risk of developing this condition. 

Treatment consists of anti-inflammatory measures and rehab to reduce pressure on the ITB at the knee. Sometimes, a cortisone injection deep into the ITB can help. Some centres have also used Botox on the TFL muscle in the hip to alter biomechanics. 

Biceps Femoris Tendonitis 

Biceps femoris tendonitis can cause posterolateral knee pain, as this key hamstring muscle inserts near the fibular head. Overuse, poor biomechanics, or sudden acceleration and deceleration movements can lead to tendon swelling. Sometimes, biceps femoris tendonitis leads to snapping or popping with knee bending, also called snapping biceps femoris tendon. 

Individuals often describe pain at the fibular head, particularly during sprinting or explosive activities. The tendon is tender to the touch, and pain occurs with resisted hamstring manoeuvres. 

Treatment consists of a progressive strengthening programme of the hamstring muscle combined with shockwave therapy, GTN patches, supplements and injections. 

Lateral Meniscus Tear

Lateral meniscus injuries are another essential consideration in active individuals with lateral posterior knee pain. The lateral meniscus plays a role in shock absorption and stability. Acute twisting injuries or degenerative changes can lead to meniscal tears.

Patients often experience pain along the posterolateral joint line, swelling, and sometimes mechanical symptoms such as locking or clicking. Activities such as squatting and jumping generally cause pain. 

Most traumatic tears are treated with surgery, while degenerative tears respond to rehab and injections. 

Lateral Compartment Osteoarthritis causing posterolateral knee pain 

Another potential cause of posterolateral knee pain is arthritis of the lateral compartment. Degenerative changes in this region often lead to stiffness, swelling, and pain that worsens with activities such as walking and running. Knee joint swelling and stiffness are common findings in arthritis. 

We treat knee osteoarthritis with simple measures such as weight loss, strengthening and injections. 

Lateral Collateral Ligament Injury

A cause of posterolateral knee pain is an isolated LCL injury although these injuries rarely occur in isolation. 

Popliteus Tendinitis 

Popliteus tendinitis occurs when the popliteus tendon, which wraps around the outside of the knee, is overused, particularly in downhill running or sudden stopping movements.

Inflammation of the popliteus tendon causes pain in the back and outer part of the knee. Often, the popliteus tendinitis is tender to the touch. 

We treat this condition with anti-inflammatory treatments and physical therapy. Intractable cases may require a cortisone injection. 

 Common Peroneal Nerve Entrapment

In rare cases, posterolateral knee pain may be caused by common peroneal nerve entrapment. The common peroneal nerve wraps around the fibular neck and can be irritated at this location due to direct trauma to the nerve outside the knee, a varus force on the knee, or an ankle sprain.

People with common peroneal nerve entrapment may report pain outside the knee, which can spread to the outside of the calf. Numbness or weakness in the lower leg and foot can also occur. Touching the nerve near the fibula neck can reproduce symptoms. 

Treatment consists of physical therapy, including neural flossing. In longstanding cases, a nerve hydrodistension can relieve symptoms. 

Proximal Tibiofibular Joint Pathology 

Proximal tibiofibular joint pathology can also contribute to pain on the outside and back of the knee. This joint near the posterolateral knee can become irritated due to instability or repetitive stress. Patients with damage to this joint often report localised pain over the fibular head. Weight-bearing or twisting movements usually exacerbate the pain.

We treat this condition initially with physical therapy. Sometimes, a cortisone injection into the joint can provide relief. 

Lumbar Spine Referred Pain

Pain in the lumbar spine can sometimes be mistaken for knee pain. Conditions such as nerve pinching in the spine can present with symptoms that mimic knee pathology, including radiating pain, lower back discomfort, numbness, and pins and needles down the leg.

Treatment includes physical therapy for the lower back and neural flossing to reduce nerve tension. Sometimes, nerve root sheath injections can help. 

Assessment of Posterolateral Knee Pain

Clinical assessment

A thorough clinical assessment is essential to diagnose the cause of posterolateral knee pain. A detailed history helps determine whether the pain began suddenly following an injury or developed gradually. Identifying activities exacerbating symptoms, such as running, pivoting, or prolonged sitting, can provide valuable clues.

A physical examination begins with examining the knee and looking for swelling, bruising, or muscle atrophy. Observing gait and knee alignment can also reveal underlying biomechanical abnormalities. Palpation is crucial for identifying tenderness over specific structures such as the lateral joint line, the fibular head, or the iliotibial band insertion. Assessing the range of motion and comparing it with the unaffected knee can help detect stiffness or restrictions. Special tests can aid in the diagnosis of posterolateral knee injuries. 

How to Investigate Posterolateral Knee Pain

Imaging studies play a crucial role in confirming the diagnosis. X-rays are often the first-line imaging modality to rule out fractures, osteoarthritis, or tibiofibular joint abnormalities. MRI is the gold standard for detecting soft tissue injuries, including meniscal tears, PLC injuries, and tendon pathology. Ultrasound can help assess tendon injuries such as biceps femoris tendinopathy or popliteus tendinitis. In cases where nerve damage is suspected, nerve conduction studies may be performed to evaluate nerve function. 

Final Word from Sportdoctorlondon about Posterolateral Knee Pain

Posterolateral knee pain is complex and has multiple potential causes. A thorough clinical assessment and imaging are essential for identifying the cause. Early and accurate diagnosis allows for appropriate management, ensuring quicker activity returns for athletes and active individuals. If persistent posterolateral knee pain limits performance, seeking an evaluation by a sports medicine specialist is essential. 

Related conditions:

• Pain at the back of the knee 
• Causes of pain on the outside of the knee 
• Knee location chart