Pain on the side of the knee during movement, particularly bending or flexing, points toward irritation or injury of the structures that stabilize the joint laterally. The knee is a complex hinge joint stabilized by ligaments, cushioned by cartilage, and surrounded by soft tissues like tendons and fascia. Pain localized to the side of the knee during the act of bending often suggests a mechanical issue where one of these structures is being compressed or stretched. Identifying the exact source of the discomfort requires careful evaluation, as the pain could originate from an acute traumatic event or from chronic overuse.
Understanding Iliotibial Band Syndrome
Iliotibial Band Syndrome (ITBS) is one of the most frequent causes of pain felt on the outside, or lateral aspect, of the knee, especially in athletes like runners and cyclists. The iliotibial band (IT band) is a thick, fibrous strip of fascia that runs along the entire outside of the thigh, extending from the hip down to the tibia just below the knee. Its primary function involves stabilizing both the hip and the knee joint during movement.
The mechanism of pain in ITBS is friction or compression where the band passes over a bony prominence on the thigh bone called the lateral femoral epicondyle. This friction generates inflammation in the underlying tissues, causing pain. Symptoms are typically exacerbated by repetitive knee flexion and extension, such as during running or cycling, which repeatedly slides the band over the bone.
Pain is often described as sharp, stinging, or burning and is usually pinpointed to an area one or two inches above the knee joint line. The pain is often most intense when the knee is bent to about 30 degrees of flexion. Unlike acute trauma, the discomfort often begins after a certain distance or duration of activity and may resolve shortly after the activity is stopped.
Underlying causes of ITBS are frequently linked to poor biomechanics, such as weakness in the hip and gluteal muscles. This muscle weakness can lead to improper tracking of the leg, increasing tension on the IT band and causing excessive compression at the knee. Other contributing factors include a sudden increase in training volume or intensity, excessive downhill running, and training on uneven surfaces.
Structural Damage: Ligament Sprains and Meniscus Tears
While ITBS is an overuse injury, lateral knee pain during bending can also signal structural damage from an acute event, such as a ligament sprain or a meniscal tear. These injuries typically result from a specific, high-impact force or an awkward movement rather than repetitive strain.
The Lateral Collateral Ligament (LCL) is a cord-like structure that runs along the outside of the knee, helping to prevent the joint from bowing outward. An LCL sprain commonly occurs when a force strikes the inner side of the knee, pushing the joint outward and overstretching the ligament. Patients often recall hearing a distinct “pop” at the time of injury, followed by swelling and localized pain on the outside of the knee. A defining symptom of LCL damage is a feeling of instability or the knee “giving way,” particularly when the leg is put under stress.
The meniscus is a C-shaped piece of cartilage that acts as a shock absorber and stabilizer between the thigh bone and the shin bone. A tear in the lateral meniscus can cause pain on the side of the knee, especially when twisting or bending deeply. Unlike the burning pain of ITBS, a meniscal tear often presents with mechanical symptoms, such as the knee locking, catching, or a sharp limitation in the ability to fully straighten or bend the leg.
Meniscal injuries can occur suddenly during sports that involve cutting or pivoting, but they can also develop slowly through wear and tear over time. Pain from a meniscal tear is typically felt along the joint line itself, which is lower than the common pain location for ITBS. The presence of locking or a sudden loss of full motion strongly suggests cartilage damage rather than a soft tissue friction syndrome.
Localized Inflammation: Bursitis and Tendinopathy
Knee pain aggravated by bending may also stem from localized inflammation of fluid sacs (bursae) or tendons (tendinopathy) near the joint. On the lateral side, tendinopathies of the biceps femoris or popliteus muscles can mimic the symptoms of other conditions. Biceps femoris tendinopathy involves irritation of the hamstring tendon that attaches to the fibula on the outer-back side of the knee. This discomfort can be felt during knee flexion and is often tender to direct touch.
Pes Anserine Bursitis (Inner Knee Pain)
Pes Anserine Bursitis, though located on the medial (inner) side of the knee, is a common source of bending pain and a frequent differential diagnosis for knee issues. The pes anserine bursa and its surrounding tendons are situated about two to three inches below the joint line on the inner side of the tibia.
Inflammation here is often due to overuse, tight hamstrings, or obesity, causing pain that worsens when climbing stairs or rising from a seated position. The bursa acts as a cushion to reduce friction between the three attached tendons and the bone. Repetitive stress can cause it to become irritated and swell. The resulting pain is localized to the inner side of the leg just below the joint, differentiating it from the lateral pain of ITBS or LCL injuries. Both bursitis and tendinopathy involve a gradual onset of pain that increases with activity involving knee flexion.
When to Seek Professional Diagnosis and Treatment
For minor knee discomfort, immediate self-care can often help manage the initial symptoms and prevent further irritation. Initial treatment should involve the R.I.C.E. principles: Rest, Ice, Compression, and Elevation. Resting the knee by avoiding activities that cause pain and applying ice for 15 to 20 minutes several times a day can help reduce inflammation and swelling.
However, certain symptoms indicate a more serious underlying injury that requires immediate professional evaluation from a healthcare provider. You should seek medical attention if you experience a sudden inability to bear weight on the affected leg. Visible deformity of the knee joint or sudden, rapid swelling are also signs of a potentially severe structural injury, such as a fracture or acute ligament tear.
Mechanical symptoms, including the knee locking in a certain position or the sensation of the joint buckling or giving way, are important red flags that suggest damage to the meniscus or a significant ligament instability. Furthermore, if the pain is so intense that it disrupts your sleep or if the discomfort persists without improvement after several days of rest and home care, a medical consultation is necessary for an accurate diagnosis and treatment plan.