Knee cap dislocation or subluxation

Feeling your kneecap pop out of place is a frightening experience. That sudden sensation of displacement, followed by sharp pain in the knee, often leaves people in shock. As physical therapists specializing in knee rehabilitation, we regularly see patients who are concerned after dislocating their kneecap. You are not alone: this injury affects approximately 5 to 7 people per 100,000 each year. It mainly affects active teenagers and young adults.

Good news: in 50 to 70% of cases, the kneecap pops back into place on its own when the leg is extended.1 The vast majority of patients make a full recovery with physical therapy. Surgery is not necessary in most cases.

What clinical research shows:

• Spontaneous reduction (when the kneecap pops back into place on its own) is normal and common. This does not mean that the injury is less serious or that it will heal without follow-up.
• Physical therapy is the first-line treatment for most initial dislocations.2
• Surgery is rarely necessary after a first dislocation. Studies show similar long-term results between conservative treatment and surgery.
• Recovery follows predictable and well-documented phases. This allows for a gradual return to activities.

This guide explores the causes, symptoms, diagnosis, and treatment of patellar dislocation. To better understand how physical therapy can help you in your recovery, check out our comprehensive guide to physical therapy.

What is a dislocated kneecap?

Patellar dislocation occurs when the kneecap slips completely out of its groove on the femur. It usually moves toward the outside of the knee. This injury mainly affects young athletes. In 50 to 70% of cases, the kneecap pops back into place 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 groove at the end of the femur). This groove guides the movement of the kneecap when you bend or extend your knee.

During a dislocation, the kneecap slips out of this groove. It's like a train coming off its tracks. In more than 95% of cases, it moves toward the outside of the knee. This is called a lateral dislocation.

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

A reassuring phenomenon: self-reduction. In 50 to 70% of cases, the kneecap returns to its normal position on its own when the leg is extended. Patients often describe a "pop" sensation followed by a return to a more normal position. This self-reduction is normal. It does not mean that the injury is less serious.

What are the symptoms of a dislocated kneecap?

Symptoms include sudden sharp pain in the front of the knee, a feeling of displacement or "pop," rapid swelling within minutes, and difficulty moving. After the kneecap returns to its normal position, the pain is concentrated on the inner side of the knee.

Rapid swelling is often caused by bleeding inside the joint. This is called hemarthrosis. The bleeding comes from structures that tear during dislocation.

In 30 to 50% of first dislocations where the kneecap remains displaced, visible deformation can be seen. The kneecap appears to be displaced toward the outside of the knee.

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

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

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

Serious 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 normal position on its own.
• Total disability: unable to move or put weight on the knee
• Rapid and significant swelling: within minutes of the incident
• Severe pain: does not decrease with rest and ice
• Numbness or tingling: in the leg or foot
• Color change: leg turning pale or bluish

If your kneecap has popped back into place on its own, it is still recommended that you see a doctor within 24 to 48 hours.2 Even if the pain is tolerable, a complete evaluation will allow for any damage to the stabilizing structures to be checked. It will also rule out a fracture.

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

What causes a dislocated kneecap?

Dislocation occurs when a force exceeds the ability of the stabilizers to keep the kneecap in place. Risk factors include a shallow trochlear groove, a kneecap that is too high, weak quadriceps, and sudden pivots.

Trigger 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 such as basketball, soccer, and volleyball present a higher risk.

Direct contact to the inner side of the kneecap (hockey, soccer, falls) can also cause dislocation.1

Some people dislocate their kneecap during harmless movements. This can happen when getting up from a chair or walking down stairs. These situations often indicate significant anatomical factors that predispose the individual to instability.

Anatomical factors

Muscular and ligament factors

• Quadriceps imbalance: weakness of the vastus medialis obliquus (VMO, the internal stabilizer of the patella) compared to other parts of the muscle
• Ligamentous 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. Your anatomy may create a vulnerability, but there are modifiable factors. Strength training and improving technique can significantly reduce the risk of recurrence.

How is a dislocated kneecap diagnosed?

Diagnosis is based on clinical history and physical examination, including the apprehension test. X-rays identify possible fractures. 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 (pivoting or impact, sensation of displacement, immediate pain, rapid swelling) is so characteristic that the diagnosis is often obvious.

Physical examination

The physical therapist or doctor will evaluate:

• Palpation: marked tenderness on the inner side of the knee, where the LPFM attaches
• Apprehension test: with the knee flexed at 30 degrees, pushing sideways 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 (detached fragment of bone and cartilage) occurs in 25 to 30% of first dislocations. MRI : not systematic, but strongly recommended after a first dislocation in a young patient.5 MRI allows for the evaluation of:

• The condition of the LPFM (torn in 90% of cases) and where the tear is located.
• The TT-TG distance (misalignment measurement): a distance of more than 20 mm indicates significant misalignment.
• 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 greatly increases the risk of further dislocations.

How does physical therapy treat a dislocated kneecap?

Physical therapy is the first line of treatment for patellar dislocation. It consists of four phases: initial protection, movement recovery and strengthening, proprioceptive rehabilitation, and gradual return to sports 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 splint is worn for a few days to a week.2 Early exercises are essential: passive movements as soon as tolerated and isometric quadriceps contractions (contractions without movement). These contractions combat muscle atrophy, which begins 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 physical therapist performs mobilizations of the kneecap to maintain its mobility in all directions.

Progressive strengthening exercises without movement toward movement: weighted leg extensions, mini squats, step-ups. Emphasis is placed on the vastus medialis oblique (VMO), the internal stabilizer of the patella.

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

Phase 3: Proprioception (6–12 weeks)

Proprioception is the ability to perceive 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: balancing on one leg, unstable surfaces (cushion, balance board), eyes closed, catching a ball while balancing.

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

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

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

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

Patellar taping can be used during the transition period. However, the goal remains to develop sufficient muscle stability to do without 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 sports. Acute pain subsides within 2-4 weeks. Swelling resolves within 4-8 weeks.

Typical progression

Weeks 1-2 The pain is significant but gradually subsides. Swelling peaks during the first week and then decreases. Mobility is very limited. Walking with crutches is necessary. Without crutches, walking is possible towards the end of the second week if tolerated. Weeks 3-6 The pain becomes intermittent. The swelling is almost completely gone by week 6. Range of motion should return to normal. Strength reaches about 60-70% of the pre-injury level. A 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 during daily activities. Strength reaches approximately 80-85% of pre-injury levels. Running in a straight line becomes possible toward the end of this period. Months 4-6 This period is devoted to a gradual return to sports activities. Progression moves from running in a straight line to changes of direction, sprints, and then sport-specific movements. High-risk sports such as basketball, soccer, and volleyball are not recommended for 4 to 6 months.

Factors that can slow down recovery

• Osteochondral fracture: a loose body often requires arthroscopic surgery, which prolongs recovery.
• Cartilage damage: can cause persistent symptoms
• Lack of adherence to the physical therapy program: this is the most important modifiable factor. Pushing yourself too hard too soon or not doing your exercises can slow down recovery.
• Important anatomical factors: increase the risk that physical therapy alone will not be sufficient

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 you from participating in sports after 6 months, or severe anatomical factors that require correction.

Important background information

Surgery is rarely necessary after a first dislocation.3 Studies comparing conservative treatment with surgery show similar long-term results for the majority of patients.

Indications for surgical consultation

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

• Recurrent dislocations: the recurrence rate is 15 to 40% within 2 to 5 years. It can reach 50 to 70% in young people under the age of 16.3 After a second dislocation, surgical evaluation is strongly recommended, even if rehabilitation has been complete.

• Risk of progression: each dislocation causes further damage to the stabilizers. This can cause progressive cartilage damage, leading to early osteoarthritis.

Surgical options

High-level athletes

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

The decision to proceed with surgery is a collaborative one between you, your physical therapist, and an orthopedic surgeon who specializes in patellofemoral pathology. A comprehensive assessment 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. American Journal of Sports Medicine. 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.