Knee pain felt specifically on the side and triggered by bending is a common complaint, often pointing toward irritation or injury of the structures flanking the joint. The knee relies on surrounding ligaments, tendons, and cartilage for smooth movement, and stress during flexion can cause pain. Determining whether the discomfort originates from the outer (lateral) or inner (medial) side is the first step in identifying the underlying cause.
Causes of Pain on the Outer Side of the Knee
Pain felt on the outer, or lateral, side of the knee during bending is frequently linked to conditions involving the iliotibial band (IT band). Iliotibial Band Syndrome (ITBS) is a common diagnosis, especially among individuals involved in repetitive activities like running or cycling. When the IT band is tight or inflamed, the repetitive friction as the knee flexes and extends can cause a sharp or burning sensation on the outside of the joint.
The pain from ITBS often becomes noticeable at a specific point in the activity, worsening with continued movement. This occurs as the band slides over the bony prominence on the outer femur. ITBS is often associated with muscle imbalances, such as weakness in the hip abductor muscles, which increases tension on the IT band.
A tear in the lateral meniscus, the C-shaped cartilage that acts as a shock absorber, is another source of outer knee pain. This injury often occurs following a sudden twisting motion while the foot is planted, common in sports like soccer or basketball. When the knee bends, the damaged cartilage can be pinched between the femur and tibia, resulting in localized pain, clicking, or a catching sensation.
Strain or sprain of the Lateral Collateral Ligament (LCL) can also cause outer knee pain. The LCL runs along the outside of the knee, providing stability against forces that try to push the joint inward. An LCL injury typically results from a direct impact to the inside of the knee or a sudden hyperextension, leading to pain and possible instability that is aggravated by flexion.
Causes of Pain on the Inner Side of the Knee
Discomfort on the inner, or medial, side of the knee when bending can point to issues with the structures that stabilize the joint against outward forces. The Medial Collateral Ligament (MCL) is a strong band of tissue running along the inner knee, and a strain to this ligament is a frequent cause of medial-sided pain. MCL sprains, which involve stretching or a partial tear, typically result from a blow to the outside of the knee or a forceful twisting motion.
Bending the knee, especially under load, can aggravate the strained MCL, causing localized tenderness and pain along the inner aspect of the joint line. While athletes in contact sports often experience this injury, it can also occur during awkward movements in daily life. The pain may be accompanied by mild swelling and a feeling of instability.
A common cause of inner knee pain, particularly aggravated by deep bending or climbing stairs, is Pes Anserine Bursitis or Tendinitis. The pes anserinus is the combined insertion point of the sartorius, gracilis, and semitendinosus tendons, located on the inner, lower part of the tibia. A bursa, a small fluid-filled sac, is located beneath these tendons to reduce friction.
Inflammation of this bursa or the surrounding tendons causes tenderness approximately two inches below the joint line. This condition is often seen in individuals with tight hamstrings, obesity, or knee osteoarthritis, as these factors increase mechanical stress on the area. Repetitive stress, such as from running, can also lead to the irritation and swelling that causes pain with knee flexion.
A medial meniscus tear is another structural issue that causes pain on the inner knee when bending. Damage to this crescent-shaped cartilage can occur from twisting or degenerative changes over time. When the knee is bent, the torn fragment can get caught or pinched, leading to pain, swelling, and sometimes a locking sensation.
Immediate Relief and When to Seek Medical Attention
For acute pain on the side of the knee, initial self-care strategies focus on reducing inflammation and protecting the injured area. The R.I.C.E. principles—Rest, Ice, Compression, and Elevation—are the standard first steps for managing a musculoskeletal injury. Resting the knee involves avoiding activities that trigger the specific pain, such as deep squats or running.
Applying ice to the painful area for 15 to 20 minutes several times a day can help minimize swelling and dull the pain. Compression with an elastic bandage helps control swelling, and elevating the leg above the level of the heart encourages fluid drainage. Activity modification requires a temporary switch to low-impact movements, such as swimming or cycling, to maintain fitness without stressing the joint.
Several “red flag” symptoms indicate the need for professional diagnosis, suggesting a more serious injury or condition. Immediate medical attention is necessary if the knee joint appears visibly bent or deformed, or if a distinct “popping” sound was heard at the moment of injury. Urgent care should also be sought if the person is unable to bear weight on the leg or experiences sudden, severe swelling.
Other concerning symptoms that warrant a medical appointment include the knee feeling like it is constantly giving out or buckling, which suggests ligament instability. A persistent locking or catching sensation may indicate a significant cartilage or meniscus tear that needs assessment. Pain that persists for more than a few days, worsens despite home care, or is accompanied by warmth, redness, or fever, requires a professional evaluation.