Kneecap dislocation and instability

The kneecap (patella) normally glides in a groove at the end of the thighbone as the knee bends and straightens. In patellar instability the kneecap slips partly out of that groove (subluxation) or comes all the way out (dislocation), almost always toward the outer side of the knee. After a dislocation, the tissues that hold the kneecap in place may be stretched or torn, which can make another episode more likely.

• A kneecap that is visibly out of place toward the outer side of the knee
• A knee that is stuck in a bent position
• Sudden pain, a pop, and rapid swelling
• A feeling that the kneecap is slipping or that the knee is giving way
• Nervousness about twisting or pivoting on the leg
• Aching at the front of the knee between episodes

A kneecap dislocation often happens without contact, when the body twists or pivots over a planted foot; a direct blow can also push the kneecap out. It is most common in teenagers and young adults who play sports. Some features make the kneecap easier to dislocate:

• A shallow or flat groove at the end of the thighbone
• A kneecap that sits higher than usual
• Knock-knees or other differences in leg alignment
• Ligaments that are naturally loose
• Damage to the medial patellofemoral ligament (MPFL), the main restraint on the inner side of the kneecap
• A previous dislocation

The kneecap often slides back into place on its own when the knee straightens. If it does not, a clinician moves it back (reduction). The examination checks for tenderness along the inner side of the kneecap and for apprehension when the kneecap is gently pushed outward.

X-rays look for fractures and show the position of the kneecap and the shape of the groove. An MRI can reveal pieces of cartilage or bone that were knocked loose and damage to the supporting ligaments.

A first-time dislocation without significant cartilage or bone damage is usually treated without surgery. A brace may be worn for a short time for comfort and protection, followed by physical therapy to restore motion and strengthen the quadriceps and hip muscles, with a gradual return to sport. Taping or a stabilizing brace is sometimes used during activity.

Surgery may be recommended when a loose piece of cartilage or bone needs to be removed or fixed, or when the kneecap keeps dislocating despite rehabilitation. Procedures are chosen to fit the cause and may include:

• MPFL reconstruction: rebuilding the ligament on the inner side of the kneecap with a graft
• Tibial tubercle osteotomy: moving the bony bump on the shinbone where the kneecap tendon attaches, to improve the kneecap's path
• Less often, deepening the groove at the end of the thighbone

The choice depends on the shape and alignment of the knee and, in children and teens, on whether the growth plates are still open.

With non-surgical care, return to sport is based on regaining strength, motion, and confidence in the knee. After surgery, a brace and crutches are common at first, followed by staged physical therapy over several months before a full return to sports.

Strong quadriceps and hip muscles and good landing and cutting technique help keep the kneecap on track. Some dislocations cannot be prevented, particularly when the shape of the knee makes the kneecap less stable.