Knee cap dislocation or subluxation

Feeling your kneecap slip out of place is a frightening experience. This sudden displacement, followed by sharp knee pain, often leaves people in shock. As physiotherapists specializing in knee rehabilitation, we regularly see patients who are worried after a kneecap dislocation. You are not alone: this injury affects approximately 5 to 7 people per 100,000 each year. It primarily affects active teenagers and young adults.

Good news: in 50-70% of cases, the kneecap repositions itself when the leg extends.1 The vast majority of patients recover completely with physiotherapy treatment. Surgery is not needed in most cases.

What clinical research shows:

• Self-reduction (when the kneecap returns to place on its own) is normal and common. This does not mean the injury is less serious or that it will heal without follow-up.1
• Physiotherapy is the first-line treatment for the majority of first-time dislocations.2
• Surgery is rarely needed after a first dislocation. Studies show similar long-term results between conservative treatment and surgery.3
• Recovery follows predictable and well-documented phases, allowing for a gradual return to activities.

This guide explores the causes, symptoms, diagnosis, and treatment of kneecap dislocation. To better understand how physiotherapy can help you with your recovery, consult our complete guide to physiotherapy.

What is a dislocated kneecap?

Kneecap dislocation occurs when the kneecap completely moves out of its groove on the thigh bone (femur). Generally, it shifts towards the outside of the knee. This injury primarily affects young athletes. In 50 to 70% of cases, the kneecap repositions itself on its own.

The kneecap is a small, triangular-shaped bone located at the front of the knee. It slides in a V-shaped groove called the trochlea (the channel at the end of the thigh bone). This groove guides the movement of the kneecap when you bend or straighten your knee.

During a dislocation, the kneecap comes out of this groove, much like a train coming off its tracks. In over 95% of cases, it moves towards the outside of the knee, which is called a lateral dislocation.1

This affects approximately 5.8 people per 100,000 each year.1 Adolescents between 10 and 17 years old are at higher risk, with 29 cases per 100,000.2 Women are affected twice as often as men. Why? Their wider pelvis increases the pulling angle on the kneecap. They also naturally have greater ligament laxity (more flexible ligaments).2

A reassuring phenomenon is spontaneous reduction. In 50% to 70% of cases, the kneecap returns to its place on its own when the leg extends.1 Patients often describe a "pop" sensation followed by a return to a more normal position. This spontaneous reduction is normal and does not mean the injury is less serious.

What are the symptoms of a dislocated kneecap?

Symptoms include sudden sharp pain at the front of the knee, a sensation of displacement or a "pop," rapid swelling within minutes, and difficulty moving. After the kneecap returns to its place, the pain tends to focus on the inner side of the knee.

Rapid swelling often comes from bleeding inside the joint, known as hemarthrosis. This bleeding originates from the structures that tear during the dislocation.3

In 30% to 50% of first dislocations where the kneecap remains displaced, a visible deformity can be observed. The kneecap appears shifted towards the outside of the knee.1

An important detail: after the kneecap returns to its place, pain typically concentrates on the inner side of the knee. This indicates stretching or tearing of the medial patellofemoral ligament (MPFL), which is the main stabilizer preventing the kneecap from moving outwards. This ligament is torn in approximately 90% of kneecap dislocations.4

When should you seek immediate medical attention for a dislocated kneecap?

Seek immediate medical attention if your kneecap remains displaced, if you cannot move your knee, or if you experience rapid and significant swelling. Numbness or changes in leg color also require urgent consultation.

Severe symptoms are rare (less than 1% of cases). However, you should seek immediate medical attention if you experience:

• Persistent deformity: the kneecap does not return to its place on its own
• Total inability: impossible to move or put weight on the knee
• Rapid and significant swelling: within minutes after the incident
• Intense pain: does not decrease with rest and ice
• Numbness or tingling: in the leg or foot
• Color change: leg becomes pale or bluish

If your kneecap has returned to its place on its own, a consultation within 24 to 48 hours is still recommended.2 Even if the pain is tolerable, a complete evaluation helps check for damage to the stabilizing structures and rule out a fracture.

The good news is that serious complications are rare. Most kneecap dislocations can be effectively treated with a well-structured physiotherapy program.

What causes a dislocated kneecap?

Dislocation occurs when a force exceeds the capacity of the stabilizing structures to keep the kneecap in place. Risk factors include a shallow trochlear groove, a high-riding kneecap, quadriceps weakness, and sudden pivoting movements.

Triggering Mechanisms

The most common mechanism is pivoting: the foot remains planted on the ground during a sudden change of direction, with the knee slightly bent.1 This is why sports like basketball, soccer, and volleyball carry a higher risk.

Direct contact on the inner side of the kneecap (hockey, football, falls) can also cause a dislocation.1

Some people dislocate their kneecap during seemingly minor movements. This can happen when standing up from a chair or going down stairs. These situations often point to significant anatomical factors that predispose individuals to instability.

Anatomical factors

Muscular and Ligamentous Factors

• Quadriceps imbalance: weakness of the vastus medialis obliquus (VMO, the inner stabilizer of the kneecap) compared to other parts of the muscle
• Ligament hyperlaxity: naturally more elastic ligaments offer less resistance
• Family component: the risk increases if parents or siblings are affected

An important message: you are not simply unlucky. While your anatomy may create a vulnerability, there are modifiable factors. Muscle strengthening and improved technique can significantly reduce the risk of recurrence.

How is a dislocated kneecap diagnosed?

Diagnosis is based on your clinical history and a physical examination, including the apprehension test. X-rays can identify possible fractures. An MRI evaluates soft tissue damage and anatomical factors that predict recurrence.

Clinical History

The patient's account is often sufficient to guide the diagnosis. The typical description (pivot or impact, sensation of displacement, immediate pain, rapid swelling) is so characteristic that the diagnosis is often clear.

Physical examination

Your physiotherapist or doctor will evaluate:

• Palpation: marked tenderness on the inner side of the knee, where the MPFL attaches
• Apprehension test: with the knee bent at 30 degrees, a lateral push on the kneecap causes an apprehension reaction (fear and resistance) if a recent dislocation has occurred

Imaging

X-rays : initial examinations to rule out a fracture. An osteochondral fracture (a detached piece of bone and cartilage) occurs in 25 to 30% of first-time dislocations.6 MRI : not routine, but strongly recommended after a first dislocation in a young patient.5 MRI allows for evaluation of:

• The condition of the MPFL (torn in 90% of cases) and the location of the tear4
• The TT-TG distance (a measure of misalignment): a distance greater than 20mm indicates significant misalignment5
• The presence of osteochondral fragments

Predictors of recurrence

Age at the time of the first dislocation is the strongest predictive factor:

The presence of trochlear dysplasia significantly increases the risk of further dislocations.

How does physical therapy treat a dislocated kneecap?

Physiotherapy is the first-line treatment for kneecap dislocation. It includes four phases: initial protection, restoring movement and strengthening, proprioceptive re-education, and a gradual return to sport based on objective criteria.

A conservative approach is recommended for most first-time dislocations.2 Studies show that immediate surgery does not offer better long-term results for most patients.3

Phase 1: Protection (0-2 weeks)

The RICE protocol (Rest, Ice, Compression, Elevation) helps control initial inflammation. Important: rest does not mean complete immobilization.

An extension brace is worn for a few days to a week.2 Early exercises are crucial: passive movements as soon as tolerated, and isometric quadriceps contractions (contractions without movement). These contractions help prevent muscle atrophy, which can start as early as 48 hours after the injury.

Phase 2: Range of Motion and Strengthening (2-6 weeks)

The goal is to regain the same flexion and extension as the uninjured knee. The physiotherapist performs patella mobilizations to maintain its mobility in all directions.

Strengthening progresses from exercises without movement to those with movement: leg extensions with weights, mini-squats, and step-ups. Emphasis is placed on the vastus medialis obliquus (VMO), which is the inner stabilizer of the kneecap.2

Strengthening the hips is just as important. The abductors (gluteus medius) and external rotators control the position of the femur. This improves patellar alignment.

Phase 3: Proprioception (6-12 weeks)

Proprioception is the ability to sense the position and movement of your joint without looking at it. The receptors responsible for this perception are damaged after a dislocation. It's as if your knee has lost some of its "awareness."

Exercises include: single-leg balance, unstable surfaces (cushion, balance board), eyes closed, catching a ball while balancing.

Dynamic stabilization introduces more complex movements: jumping in place, side jumps, controlled changes of direction, gradual pivots.

Phase 4: Return to Sport (3-6 months)

Returning to sport is based on objective criteria, not just on the time elapsed:

Progression follows a sequence: straight-line running, then changes of direction, sprints, cutting drills, and finally sport-specific training.

Patellar taping can be used during the transition. However, the goal remains to develop sufficient muscular stability to no longer need it.

How long does it take to recover from a dislocated kneecap?

Recovery generally takes 8 to 16 weeks for daily activities and 3 to 6 months for a full return to sport. Acute pain decreases within 2-4 weeks. Swelling resolves within 4-8 weeks.

Typical progression

Weeks 1-2 Pain is significant but gradually decreases. Swelling peaks in the first week then subsides. Mobility is very limited. Walking with crutches is necessary. Walking without crutches may be possible towards the end of the second week if tolerated. Weeks 3-6 Pain becomes intermittent. Swelling is almost completely gone by week 6. Range of motion should return to normal. Strength reaches approximately 60-70% of pre-injury levels. Return to normal daily activities is possible between weeks 4 and 6. This includes grocery shopping, climbing stairs, and walking normally. Weeks 7-12 Pain is minimal or absent for daily activities. Strength reaches approximately 80-85% of pre-injury levels. Straight-line running becomes possible towards the end of this period. Months 4-6 This period is dedicated to a gradual return to sports activities. Progression moves from straight-line movements to changes of direction, sprints, and then sport-specific movements. High-risk sports like basketball, soccer, and volleyball are not recommended before 4 to 6 months.

Factors that can slow down recovery

• Osteochondral fracture: a loose fragment often requires arthroscopic surgery, which extends recovery time.
• Cartilage lesions: can cause persistent symptoms.
• Lack of adherence to the physiotherapy program: this is the most significant modifiable factor. Pushing too hard too soon or not doing your exercises can slow down healing.
• Significant anatomical factors: increase the risk that physiotherapy alone may not be enough.

When should you consult a specialist for a dislocated kneecap?

Consult an orthopedic surgeon if you have a fracture with a mobile fragment, recurrent dislocations despite complete rehabilitation, persistent instability that prevents sport after 6 months, or severe anatomical factors requiring correction.

Important Context

Surgery is rarely necessary after a first dislocation.3 Studies comparing conservative treatment to surgery show similar long-term results for most patients.

Indications for Surgical Consultation

• Osteochondral fracture with loose body: fragments larger than 1 cm floating in the joint cause locking, pain, and progressive cartilage damage. Arthroscopy is generally necessary.6

• Recurrent dislocations: the recurrence rate is 15 to 40% within 2 to 5 years. It can reach 50 to 70% in young people under 16 years old.3 After a second dislocation, a surgical evaluation is strongly recommended, even if rehabilitation has been completed.

• Risk of progression: each dislocation further damages the stabilizers. This can lead to progressive cartilage lesions, resulting in early osteoarthritis.

Surgical Options

High-level athletes

For high-level athletes, MPFL reconstruction may sometimes be recommended even after a first dislocation. This is especially true if the tear is complete with retraction.5

The decision to proceed with surgery is a collaboration between you, your physiotherapist, and an orthopedic surgeon specializing in femoropatellar pathology. A comprehensive evaluation of anatomical factors guides the choice of the best approach for your situation.

References

• Fithian DC, Paxton EW, Stone ML, et al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med. 2004;32(5):1114-1121.

• Petri M, Liodakis E, Hofmeister M, et al. Operative vs conservative treatment of traumatic patellar dislocation: results of a prospective randomized controlled clinical trial. Arch Orthop Trauma Surg. 2013;133(2):209-213.

• Lewallen LW, McIntosh AL, Dahm DL. Predictors of recurrent instability after acute patellofemoral dislocation in pediatric and adolescent patients. Am J Sports Med. 2013;41(3):575-581.

• Balcarek P, Walde TA, Frosch S, et al. Patellar dislocations in children, adolescents and adults: a comparative MRI study of medial patellofemoral ligament injury patterns and trochlear groove anatomy. Eur J Radiol. 2011;79(3):415-420.

• Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surg Sports Traumatol Arthrosc. 1994;2(1):19-26.

• Nomura E, Inoue M. Cartilage lesions of the patella in recurrent patellar dislocation. Am J Sports Med. 2004;32(2):498-502.

• Menetrey J, Putman S, Gard S. Return to sport after patellar dislocation or following surgery for patellofemoral instability. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2320-2326.