Pain on the inside of the knee when bending the joint is a very common complaint, often pointing toward irritation or injury of the medial structures. This movement, known as flexion, places load and tension on the tissues along the inner side of the joint. The knee is a complex hinge joint that relies on surrounding soft tissues for stability. Understanding the anatomy of the inner knee helps determine the source of this discomfort.
Key Structures on the Inside of the Knee
The medial, or inner, side of the knee joint houses several distinct structures that can contribute to pain during flexion. The Medial Collateral Ligament (MCL) is a broad band of tissue spanning the distance from the thighbone to the shinbone. Its primary function is to resist forces that push the knee inward, maintaining stability against side-to-side stress.
Deep within the joint lies the medial meniscus, a C-shaped piece of fibrocartilage that functions as a shock absorber and load distributor. This structure is connected to the MCL, meaning injury to one can involve the other. During knee flexion, the meniscus moves slightly backward, and a tear can become pinched or stressed.
Just below the joint line is the pes anserine complex, the combined insertion point for the tendons of three muscles: the sartorius, gracilis, and semitendinosus. These tendons assist in knee flexion and provide rotational stability. The pes anserine bursa, a small fluid-filled sac, sits beneath these tendons to reduce friction. Inflammation of this bursa can also cause pain upon bending.
Acute Injuries Causing Medial Knee Pain
Acute injuries on the inner knee typically result from a sudden, traumatic event, often involving forceful twisting or impact. A frequent cause is a sprain or tear of the Medial Collateral Ligament (MCL). This usually happens when a direct blow strikes the outside of the knee, forcing the joint inward and excessively stretching the MCL. This causes immediate pain and swelling along the inner knee.
MCL injuries range from a mild stretch (Grade 1) to a complete tear (Grade 3). All grades result in localized tenderness and pain that is often worsened by bending the knee. A person may also feel a sense of instability or a feeling that the knee might “give way” when bearing weight or pivoting. The resulting stiffness and swelling further restrict the ability to flex the knee comfortably.
Another common acute cause of sharp, flexion-related pain is a tear in the medial meniscus, typically occurring from a sudden, forceful twisting motion while the foot is planted. When the knee is bent under load, a fragment of the torn cartilage can become displaced or trapped within the joint. This mechanical obstruction leads to a sharp, stabbing pain when flexing the knee, especially when squatting or twisting.
A characteristic symptom of a significant meniscal tear is mechanical locking, where the knee temporarily gets stuck and cannot fully straighten or bend due to the trapped cartilage. The pain is often localized directly to the joint line on the inside of the knee. The inability to perform deep flexion movements without a sharp catch is a strong indicator of this structural damage.
Overuse and Chronic Conditions
Medial knee pain during bending can develop gradually due to repetitive strain or degenerative changes. Pes anserine bursitis is a common overuse condition involving inflammation of the bursa located below the joint line on the inner side of the shinbone. This condition frequently affects runners, individuals with tight hamstring muscles, or those with underlying knee osteoarthritis.
The pain from bursitis is typically a dull ache or burning sensation, exacerbated by activities involving repetitive knee flexion, such as climbing stairs or running. Tenderness is highly localized when pressure is applied directly over the bursa. Discomfort may increase when rising from a chair after sitting, as irritation occurs when the hamstring tendons slide over the inflamed bursa during bending.
Medial compartment osteoarthritis is a degenerative condition where the protective cartilage within the inner section of the knee joint gradually wears away. This loss narrows the space between the thigh and shin bones on the medial side, leading to bone-on-bone friction. This friction generates pain strongly aggravated by weight-bearing activities and deep knee flexion.
The pain associated with this arthritis is often worse in the morning or after prolonged periods of rest, which is a hallmark of degenerative joint disease. Bending the knee under load, such as during walking or squatting, increases compressive forces on the damaged medial compartment surfaces. This chronic condition causes persistent, aching pain and stiffness that worsens progressively, limiting the overall range of flexion.
Immediate Care and Warning Signs
When medial knee pain begins after an activity, immediate steps can alleviate symptoms and prevent further irritation. Resting the knee by avoiding activities that cause pain, especially deep bending or twisting movements, is the first measure. Applying ice packs to the painful inner knee for fifteen to twenty minutes helps reduce localized swelling and inflammation.
Gentle compression with an elastic bandage can provide support and minimize fluid buildup, but care should be taken not to wrap it too tightly. Elevating the leg above the level of the heart also helps manage swelling by assisting fluid return. Over-the-counter anti-inflammatory medications may be used to manage pain and inflammation in the short term, following dosage instructions carefully.
Certain warning signs require prompt medical evaluation, as they suggest a potentially serious injury. An inability to bear any weight on the affected leg is a significant red flag. Severe swelling that develops rapidly within an hour or two of injury also warrants immediate attention.
Any sensation that the knee is mechanically locking, catching, or buckling indicates an unstable internal structure, possibly a displaced meniscal tear or severe ligamentous injury. If the pain persists or worsens after forty-eight hours of self-care, or if an audible “pop” was heard at the time of injury, a full clinical assessment is necessary. These symptoms suggest a need for diagnostic imaging and a structured treatment plan.