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The kneecap (patella) is normally positioned over the front of the knee joint at the base of the thighbone (femur). A kneecap can be dislocated, or moved out of its normal position, when:

• The kneecap is out of place (patellar tracking disorder) and force is applied, causing dislocation.
• The foot is firmly planted pointing outward, and the knee is bent with the thigh turned inward. This kind of injury is common during many sports activities.
• The inner edge of the kneecap is hit, pushing it toward the outer side of the leg.

Symptoms of a dislocated kneecap may include:

• Severe pain.
• A misshapen knee that looks like a bone is out of position.
• A popping sensation, followed by a feeling that something is out of place.
• Inability to bend or straighten the knee.
• Knee swelling.
• Cool, pale skin or numbness and tingling in or below the affected knee.

A dislocation can cause other problems even if the bone pops back into place.

• If the dislocation was due to a preexisting malalignment, the knee may dislocate again.
• Ligaments, tendons, muscles, and cartilage in or around the joint may stretch or tear.
• A piece of bone may break off somewhere in the knee joint.