Patellar Dislocation

What is patellar dislocation?

Patellar dislocation is an injury in which the kneecap (the patella) slips out of its normal position at the front of the knee. The patella normally sits in a shallow groove at the lower end of the thigh bone, called the trochlear groove. When the knee bends and straightens, the kneecap glides up and down within this groove. In a patellar dislocation, the kneecap is pushed out of the groove, almost always toward the outer side of the knee. When this happens, the knee often looks visibly deformed, cannot move normally, and is usually very painful.

Understanding what is patellar dislocation also means knowing the difference between a full dislocation and a related injury called subluxation. In a full dislocation, the kneecap comes completely out of the groove. In a subluxation, the kneecap slips partway out and then returns to place on its own. Both injuries can damage the soft tissues that normally hold the kneecap in position, especially a band of tissue on the inner side of the knee called the medial patellofemoral ligament (often shortened to MPFL).

Patellar dislocation is most common in adolescents and young adults, particularly those who play sports that involve jumping, pivoting, or sudden changes of direction. It also occurs in people whose anatomy makes the kneecap naturally less stable, such as those with a shallow trochlear groove or looser-than-average ligaments. In many hospital systems, this condition is managed by orthopedic specialists; the condition is coded in medical records as ICD-10 S83.0.

Symptoms of patellar dislocation

Patellar dislocation symptoms are usually sudden and dramatic when the injury first happens. Many people describe feeling or hearing a pop, followed by intense pain and the sense that the knee has “given way.” Common signs include:

• Visible deformity: the kneecap sits out of place, often clearly shifted toward the outer side of the knee
• Sudden, severe pain at the front and inner side of the knee
• A popping or tearing sensation at the moment of injury
• Inability to straighten or bend the knee normally
• Rapid swelling, often within the first hour, as blood collects inside the joint
• A feeling of instability, as if the knee will not support your weight
• Tenderness along the inner edge of the kneecap, where the stabilizing ligament may be torn

Symptoms can differ depending on the type and stage of the injury. In a first-time (acute) dislocation, pain and swelling are usually the most prominent features. Sometimes the kneecap slides back into place on its own before the person reaches medical care; in that case the knee may look normal but still be swollen, painful, and unstable, which can make the injury harder to recognize.

In people who have had more than one dislocation — a pattern called recurrent patellar instability — each episode may be less painful than the first, but the sense of the kneecap slipping or “giving way” tends to happen more easily, sometimes with everyday movements such as turning or climbing stairs. Between episodes, some people notice aching at the front of the knee, a grinding sensation, or apprehension (a fearful feeling) when the knee is moved in certain directions.

Causes and risk factors

Patellar dislocation causes fall into two broad groups: a forceful injury in a previously normal knee, and dislocation that occurs more easily because of underlying anatomy.

The most common mechanism is a twisting movement on a planted foot — for example, a sudden pivot during basketball, soccer, dancing, or gymnastics — often combined with the knee bending and rotating inward while the kneecap is pulled outward. A direct blow to the inner side of the kneecap, such as during a fall or a sports collision, can also force the patella out of its groove.

Several factors can make a person’s kneecap less stable and increase the risk of dislocation:

• A shallow trochlear groove (trochlear dysplasia): if the groove that guides the kneecap is flatter than usual, the patella can slip out more easily.
• A high-riding kneecap (patella alta): when the kneecap sits higher than normal, it engages the groove later as the knee bends, leaving it less protected.
• Generalized ligament laxity: people whose joints are naturally more flexible (“loose-jointed”) have less soft-tissue restraint around the kneecap.
• Alignment of the leg: knock-kneed alignment or an increased outward pull on the kneecap from the thigh and shin bones can encourage lateral (outward) slipping.
• Weakness or imbalance of the thigh muscles: the inner part of the quadriceps muscle helps keep the kneecap centered; weakness here can contribute to instability.
• Previous dislocation: after a first dislocation, the stabilizing ligament on the inner knee is often stretched or torn, making repeat episodes more likely.
• Age and activity: adolescents and young adults, especially those active in pivoting sports, have the highest rates of first-time dislocation.
• Family history: patellar instability sometimes runs in families, often because anatomical features are inherited.

In many cases, several of these factors act together. A person with a shallow groove and loose ligaments, for instance, may dislocate the kneecap with a relatively minor twist that would not injure a knee with typical anatomy.

Diagnosis

Patellar dislocation diagnosis begins with a careful conversation and physical examination. Your doctor will ask how the injury happened, whether you felt a pop, whether the kneecap looked out of place, and whether the knee has given way before. If the kneecap is still dislocated when you are examined, the diagnosis is usually obvious from the appearance of the knee.

If the kneecap has already returned to place, the doctor looks for clues such as swelling inside the joint, tenderness along the inner edge of the kneecap, and a positive “apprehension test” — a gentle sideways push on the kneecap that produces a feeling that it is about to slip out again. The examiner may also assess how far the kneecap can be moved side to side and check the overall alignment and flexibility of the leg.

Imaging tests are used to confirm the injury and look for complications:

• X-rays are usually taken first. They show whether the kneecap is in place, reveal certain bone fractures, and can demonstrate anatomical features such as a high-riding kneecap or a shallow groove.
• Magnetic resonance imaging (MRI) — a scan that shows soft tissues in detail — is often recommended after a first dislocation. It can show tears of the medial patellofemoral ligament, bruising of the bone in a typical pattern, and, importantly, fragments of cartilage or bone (called osteochondral fragments) that may have broken off inside the joint.
• Computed tomography (CT), a detailed cross-sectional X-ray scan, is sometimes used when surgeons need precise measurements of bone shape and alignment, particularly when planning surgery for repeated dislocations.

Identifying loose cartilage or bone fragments is one of the most important goals of diagnosis, because these fragments can damage the joint and often change the treatment plan. Your doctor may also aspirate (draw fluid from) a very swollen knee to relieve pressure and check for blood in the joint, which suggests significant internal injury.

Treatment options

Patellar dislocation treatment depends on whether this is a first-time injury or a repeated one, whether any bone or cartilage fragments have broken loose, and the person’s age, activity level, and underlying anatomy. Care for this injury is typically coordinated by an orthopedic team, such as the Orthopedics & Joint Center, where sports medicine and knee specialists manage both nonsurgical and surgical care.

Immediate care and reduction

If the kneecap is still out of place, the first step is reduction — gently guiding the patella back into its groove. This is usually done by a trained clinician, often after pain medication is given, by slowly straightening the knee while guiding the kneecap inward. Trying to force the kneecap back without training can cause further injury, so emergency evaluation is recommended whenever the kneecap remains visibly out of place.

Nonsurgical treatment

For most first-time dislocations without loose bone or cartilage fragments, treatment is nonsurgical. This typically includes:

• Rest, ice, and elevation in the first days to reduce swelling
• A brace or knee immobilizer for a limited period to protect the healing tissues; prolonged immobilization is generally avoided because it can cause stiffness and muscle wasting
• Pain relief, often with over-the-counter medications such as acetaminophen or anti-inflammatory drugs, as advised by your doctor
• Crutches at first, with gradual return to putting weight on the leg as comfort allows
• Physical therapy, which is the cornerstone of recovery — a structured program to restore knee motion, strengthen the quadriceps and hip muscles, and retrain balance and movement patterns

This “watchful waiting plus rehabilitation” approach allows the stretched or torn ligament to heal while the muscles are strengthened to help stabilize the kneecap. Many people return to their previous activities over a period of weeks to months, though the exact timeline varies from person to person.

Surgical treatment

Surgery may be recommended in certain situations, most commonly when:

• A fragment of bone or cartilage has broken off and is loose inside the joint
• The kneecap keeps dislocating despite a proper rehabilitation program (recurrent instability)
• Significant anatomical problems make future dislocations very likely

Common surgical options include arthroscopy (keyhole surgery using a small camera) to remove or fix loose fragments; reconstruction of the medial patellofemoral ligament, in which the torn stabilizing ligament is rebuilt, often using a small piece of tendon; and bone realignment procedures, in which a portion of the shin bone where the kneecap tendon attaches is repositioned to improve the pull on the kneecap. In selected cases with a very shallow groove, surgeons may reshape the groove itself. The choice of procedure is individualized, and your surgeon will explain the expected benefits, risks, and recovery for your specific situation.

After surgery, rehabilitation with a physical therapist is essential and usually continues for several months before a return to sports is considered.

Living with patellar dislocation and outlook

The outlook after a patellar dislocation is generally reasonable, but it depends heavily on the individual injury and anatomy. Many people who complete a full rehabilitation program after a first-time dislocation return to their usual activities without further problems. However, a meaningful proportion of people — particularly adolescents and those with risk factors such as a shallow groove or loose ligaments — experience repeat dislocations or ongoing feelings of instability.

Living well with this condition often involves ongoing attention to the knee. Maintaining strength in the quadriceps and hip muscles, keeping a healthy body weight, warming up before sport, and following any bracing or activity advice from your care team can all help reduce strain on the kneecap. Some people choose to modify high-risk activities, especially those involving hard pivoting, at least during the recovery period.

It is honest to say that a dislocation can have longer-term consequences. Damage to the cartilage on the underside of the kneecap or in the groove can lead to persistent pain at the front of the knee and, in some cases over years, to patellofemoral arthritis — wear of the joint between the kneecap and the thigh bone. This does not happen to everyone, and careful treatment and rehabilitation are aimed at lowering this risk, but no treatment can guarantee a specific outcome. Regular follow-up allows problems to be identified and addressed early.

Frequently asked questions

What is patellar dislocation in simple terms?

Patellar dislocation means the kneecap has slipped out of the groove at the end of the thigh bone where it normally sits, almost always toward the outer side of the knee. It usually happens during a twisting movement or after a direct blow, causes sudden pain and swelling, and often makes the knee look visibly out of shape until the kneecap is guided back into place.

Can a dislocated kneecap heal on its own?

Sometimes the kneecap slides back into place by itself, and the stretched or torn tissues can heal over several weeks with rest, bracing, and physical therapy. However, “healing on its own” does not mean the injury should be ignored. Even when the kneecap returns to place, imaging is often needed to check for loose bone or cartilage fragments, and a rehabilitation program is usually important to reduce the chance of another dislocation.

How serious is a patellar dislocation?

Seriousness varies. Many first-time dislocations heal well with nonsurgical treatment. The injury becomes more concerning when a piece of bone or cartilage breaks off inside the joint, when dislocations keep recurring, or when the cartilage surface is damaged, because these situations can lead to ongoing pain, instability, or joint wear. A proper evaluation with examination and imaging is the only reliable way to judge how serious a particular injury is.

How long does recovery from a patellar dislocation take?

Recovery time varies with the injury and the treatment. After a first-time dislocation treated without surgery, many people regain comfortable daily function within a few weeks, while return to sports often takes longer — commonly a few months — and depends on regaining strength, motion, and confidence in the knee. Recovery after surgery generally takes longer, often several months of guided rehabilitation. Your care team can give you a timeline based on your specific situation.

What are the main patellar dislocation symptoms I should recognize?

The classic signs are a popping sensation, sudden severe pain at the front of the knee, a kneecap that looks shifted to the outer side, rapid swelling, and an inability to move the knee normally. If the kneecap slips back on its own, the knee may look normal but remain swollen, tender along its inner edge, and feel unstable or untrustworthy when you try to walk.

Will my kneecap dislocate again?

It might, and the risk is higher in younger people and in those with anatomical risk factors such as a shallow trochlear groove, a high-riding kneecap, or loose ligaments. Completing a full strengthening and rehabilitation program appears to help many people, and surgery is sometimes recommended when dislocations keep recurring. No one can promise the kneecap will never slip again, but risk can often be meaningfully reduced.

Is surgery always needed for patellar dislocation treatment?

No. Most first-time dislocations without loose fragments are treated with reduction, a short period of bracing, pain control, and physical therapy. Surgery is usually reserved for cases with loose bone or cartilage pieces in the joint, repeated dislocations despite rehabilitation, or significant structural problems that make future dislocations very likely. The decision is individualized and made together with an orthopedic specialist.

When to see a doctor

Any suspected patellar dislocation deserves medical evaluation, even if the kneecap seems to have gone back into place, because hidden damage inside the joint is common. Seek urgent medical care — go to an emergency department or call emergency services — if you notice any of the following red flags:

• The kneecap is visibly out of place and has not returned to its normal position
• You cannot move the knee or bear any weight on the leg
• Severe or rapidly increasing swelling of the knee
• Numbness, tingling, or coldness in the lower leg or foot, or the foot looks pale or bluish
• An open wound, obvious deformity of the bones, or suspicion of a fracture
• Signs of infection after an injury or procedure, such as fever, spreading redness, or warmth around the knee
• The knee locks or catches, which may indicate a loose fragment inside the joint

You should also arrange a non-urgent appointment if your kneecap repeatedly slips or feels unstable, if pain at the front of the knee persists despite rest, or if swelling does not settle over one to two weeks. Early assessment by an orthopedic specialist allows the right combination of imaging, rehabilitation, and, when needed, surgical planning, and gives your knee the best chance of a good long-term outcome.