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 inner edge of the kneecap is hit, pushing it toward the outer side of the leg. This can happen more easily if there is patellar tracking disorder (the kneecap is already slightly out of place).
- 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.
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 weak thigh muscles or a problem with the alignment or structure of the knee bones, 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.