Patellar dislocation

Lore & Background

A patellar dislocation typically occurs when the knee is straight and the lower leg twists outward (external rotation) relative to the thigh. Occasionally, it occurs when the knee is bent and the patella is struck directly. Commonly associated sports include soccer, gymnastics, and ice hockey. Dislocations nearly always occur away from the midline. Diagnosis is typically based on symptoms and supported by X-rays.

Reader's Guide

Reduction is generally done by pushing the patella towards the midline while straightening the knee. After reduction, the leg is generally splinted in a straight position for a few weeks, followed by physical therapy. Surgery after a first dislocation is generally of unclear benefit. Surgery may be indicated in cases where a fracture occurs within the joint or where the patella has repeatedly dislocated. Patellar dislocations occur in about 6 per 100,000 people per year, making up about 2% of knee injuries. It is most common in those 10 to 17 years old, with similar rates in males and females. Recurrence after an initial dislocation occurs in about 30% of people.

Did You Know?

• Patellar dislocations make up about 2% of knee injuries.
• 24% of people whose patellas have dislocated have relatives who have experienced patellar dislocations.

The Anatomy of the Slipping Joint

The patella is a small triangular bone nestled within the quadriceps tendon, sitting in a cartilage-lined channel at the lower end of the femur where the thigh bone meets the tibia. This groove, called the patellofemoral groove, guides the kneecap as the leg bends and straightens. The bone is tethered at its superior edge by the quadriceps tendon, at its sides by the vastus medialis obliquus and vastus lateralis muscles, and at its inferior edge by the patellar tendon running down to the tibial tuberosity. The medial patellofemoral ligament, which connects horizontally to the adductor magnus tendon, is the structure most frequently torn when the patella is forced laterally out of its groove. The lateral and medial collateral ligaments provide additional side-to-side stability. When a valgus twist—accounting for roughly 93% of cases—drives the kneecap away from the midline, the medial stabilizing tissues strain and often rupture at their femoral attachment. In severe traumatic events, the medial wing of the patella may fracture, and the weight-bearing surface of the lateral femoral condyle is broken in about a quarter of such injuries.

Recognizing the Injury

A patellar dislocation presents with a knee that is partially bent, visibly swollen, and acutely painful. Patients commonly describe the pain as deep, located "inside the knee cap," and the displaced patella can often be both seen and palpated sitting out of its normal groove. The leg may remain in a flexed position even at rest, while in other cases the torn ligaments make it impossible to bend the knee at all. Clinicians confirm the diagnosis primarily through the patient's history and physical examination, with X-rays serving as a supporting tool to rule out associated fractures. A specific maneuver called the Patellar Apprehension Test involves the examiner gently rocking the kneecap side to side while the patient holds the knee at roughly 30 degrees of flexion; a positive response suggests instability. Another bedside check is the patella tracking assessment, where the individual performs a single-leg squat or slowly extends the knee from a bent position while lying down. If the patella visibly drifts laterally during early flexion—a finding known as the J sign—it points to a muscular imbalance around the joint. Long-term complications to watch for include patellar fracture and the development of arthritis.

Restoring the Kneecap and Managing Recovery

The immediate goal after a patellar dislocation is reduction—manually guiding the kneecap back toward the midline while the knee is straightened. Once the patella is seated, the leg is typically immobilized in a splint held in full extension for several weeks to allow the damaged soft tissues to heal. Physical therapy follows the splinting period to restore range of motion and strength. Whether surgery offers clear benefit after a first-time dislocation remains uncertain in the medical literature. However, operative intervention becomes more clearly indicated when a fracture has occurred within the joint or when the patella has dislocated repeatedly. For athletes, surgical repair of the medial patellofemoral ligament and the vastus medialis muscle may be necessary to restore the stabilizing structures that were torn. The risk of a second episode is significant: roughly 30% of individuals experience a recurrence after their initial dislocation. Interestingly, a quarter of those affected report that a family member has also suffered a patellar dislocation, hinting at a hereditary component to the underlying anatomy or ligament laxity.

Who Is Most Vulnerable

Patellar dislocations affect approximately 6 out of every 100,000 people each year and account for about 2% of all knee injuries. The peak age group spans from 10 to 17 years old, and while overall rates between males and females are broadly similar, the athletic population tells a more nuanced story. Young athletes under 20 who participate in sports involving sudden knee rotation—soccer, gymnastics, ice hockey—are particularly at risk, and young military trainees show a notably elevated incidence compared with the general population. Anatomical predispositions play a major role: a Q angle exceeding 25 degrees between the patellar tendon and quadriceps muscle increases vulnerability, and since normal female Q angles can reach 20 degrees versus under 15 in males, women face a higher baseline risk. Tightness in the tensor fasciae latae and iliotibial tract, combined with an imbalance between the vastus lateralis and vastus medialis, further destabilizes the kneecap. A shallow trochlear groove, known as trochlear dysplasia, and a condition called patella alta—where the kneecap sits higher than usual—also reduce the mechanical constraints that keep the patella tracking correctly.