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Patellar dislocation

Also called kneecap dislocation, dislocated kneecap, lateral patellar dislocation

The kneecap coming fully out of its groove on the thigh bone. How a European consensus defines it, who it affects, how often it happens again, and what trials of surgery, braces and exercise found.

Region
Knee
Updated
October 22, 2026
Reviewed
October 22, 2026 · how entries are checked

What it is

How it is defined

A Cochrane review is a systematic review done to the Cochrane Collaboration’s published standards. One describes patellar dislocation as the kneecap coming fully out of the trochlear groove, the groove at the lower end of the femur (thigh bone)5. A European formal consensus, ESSKA, defines a first-time dislocation the same way1. The event must be confirmed by examination, imaging or both1. Of the panel’s 55 experts, 54 were orthopaedic surgeons1.

Two views of the bones of the right knee with the kneecap coloured orange and a band of tissue on its inner side coloured blue. From the front, the kneecap sits on the lower end of the thigh bone, above the shin bone, and the blue band runs from its inner edge across the inner side of the knee. From above, cut through the kneecap, the kneecap rests in the groove between the two rounded knuckles at the front of the thigh bone, and the blue band sweeps back from its inner edge along the inner side of the bone.
The kneecap in its groove on the right thigh bone, from the front and from aboveRendered by Your Basic Trainer from the Open 3D Model by AnatomyTOOL, which builds on Z-Anatomy and BodyParts3D (Database Center for Life Science). (source, CC BY-SA 4.0). Rendered from the 3D model with only the thigh bone, kneecap, shin bones and the medial patellar retinaculum shown, the kneecap coloured orange and the retinaculum blue, the bones cut above and below the knee, and the second view cut level through the kneecap with the shin bones hidden

One population study describes most episodes as a twisting injury on a bent knee, without contact3. The kneecap moves outward over the thigh bone, often also damaging the soft tissues that hold it on the inner side and the joint cartilage3.

What is injured with it

Reviews of sudden outward dislocations report these injuries9,10:

  • The MPFL: injured in 94.7%, in a meta-analysis of 35 articles (2,558 people)9. It most often tore at the kneecap end at age 16 or under, and at the thigh-bone end over 169.
  • Bone bruises (injuries inside a bone, without a break): pooled at 89.6% in a review of 39 studies (3,354 people)10.
  • Osteochondral injury (damage to joint cartilage and the bone just beneath it): pooled at 48.8% in the same review10. The inner side of the kneecap was the most common site10.

In the population cohort of 609 people, imaging found osteochondral injury in 16.7%3. ESSKA states that cartilage damage starts with the first episode and that its severity tracks how much instability persists1. It also states that, under age 15, a first dislocation is the most common cause of bleeding into the knee joint1.

Who it affects

How common it is

Studies give incidence, the number of new cases, per 100,000 person-years, a count of people multiplied by the years each was followed1,2,3.

Group studiedWhat was found
Denmark nationwide, 1994 to 2013, 24,154 first dislocations42 per 100,000 person-years on average; highest in females aged 10 to 17, at 1082
A population cohort, 609 first outward dislocations over 21 years23.2 per 100,000 person-years, adjusted for age and sex3; similar in males and females, highest at ages 14 to 18, at 147.73
Studies summed up by the ESSKA consensusReported rates of 10 to 150 per 100,000 person-years, which may differ by region1

The ESSKA consensus states that first dislocations are most common in the growth spurt and usually happen before age 181. It describes a tendency toward females, and calls first dislocations after age 25 uncommon1.

Features linked with it

The ESSKA consensus lists risk factors in the knee’s shape, and others1:

  • Trochlear dysplasia: a shallow or badly shaped groove at the lower end of the thigh bone1.
  • Patella alta: a kneecap that sits higher on the knee than usual1.
  • Alignment: malalignment of the leg, and an outward-placed tibial tubercle, a bump on the front of the shinbone1.
  • Other factors: age, hyperlaxity (joints looser than usual), and the force of the injury1.

A systematic review of 18 studies judged its evidence low in quality11. In one study it included, general joint looseness was six times as common in 104 people with dislocation as in 110 without11. The figures were 64.4% against 10.9%11.

The other knee

In the Danish registry, the risk of a dislocation in the other knee was 5.8% overall and 11.1% at ages 10 to 172. In a trial of children under 16 after a first dislocation, instability of the other kneecap was noted in 48%12.

What studies found

How often it happens again

Studies of repeat dislocation after a first one report these rates2,4:

  • A meta-analysis of 17 studies: 33.6% overall4.
  • The population cohort of 609, followed for a mean of 12.3 years: 30.0%3. Most people in it had a single episode, and 10.8% had surgery for repeat instability3.
  • The Danish registry, over 10 years: 22.7% overall and 36.8% in girls aged 10 to 172.
  • A cohort of 189 people, over 2 to 5 years: 17% of first-time cases had more instability, against 49% of those with earlier instability13.
  • The ESSKA consensus: at least 25% after non-surgical care, and up to 70% in children and teenagers14.

The meta-analysis linked younger age, open growth plates, trochlear dysplasia and patella alta with a higher chance of repeat dislocation4. A larger distance between the groove and the tibial tuberosity was linked too, while sex and earlier dislocation of the other knee were not4. In studies within that review, rates were 7.7% to 13.8% with no risk factors, and 70.4% to 78.5% with three4. In the trial of children under 16, by contrast, a family history of kneecap instability was the only significant predictor12.

What happens over the years

ESSKA states that about 25% of people have repeat instability, and about 50% other problems, such as pain, giving way and less sport1. It states these lasting problems are more common in people still growing than in those who have stopped14.

One retrospective cohort, which looked back at past cases, had 111 people managed without surgery after a first dislocation15. It excluded 7 who later had surgery15. Of the rest, 73.1% reported no further dislocation at about 3 years15. Of those, only 26.4% said they had returned to their chosen sport without limits15.

A 2017 study followed a cohort of 609 people after a first dislocation, with matched controls16. In it, 48.9% had arthritis behind the kneecap at 25 years, against 8.3% of controls16. Repeat dislocation, osteochondral injury and trochlear dysplasia were linked with that arthritis16.

Surgery or non-surgical care

The Cochrane review of 10 trials (519 people) was uncertain whether surgery improves outcome, on very low-certainty evidence5. In 8 of its studies, it took an assumed 348 per 1000 dislocating again without surgery5. Against that, it estimated 157 fewer per 1000 two to nine years after surgery5. That estimate also rested on very low-certainty evidence5.

A review of 7 trials (282 people) found all of them relatively low in quality17. It linked surgery with fewer repeat dislocations, but whether surgery gives better function remained unclear17.

Studies comparing surgery with non-surgical care give results that differ by age group18,19:

StudyWhat it found
Review of 9 trials, 433 peopleFewer repeat dislocations after surgery overall; lower after surgery at a mean age over 20, with no significant difference at 20 or under18
Meta-analysis of 4 studies of a recent dislocation, age 18 or underNo significant difference in outcomes, repeat dislocation included; better pain scores without surgery19
Trial in children under 16, followed for a mean of 14 yearsRepeat dislocation in 71% of knees without surgery and 67% with it12; early surgery did not improve long-term outcome12
Trial in 74 children aged 9 to 14, over 2 years22% redislocated after MPFL repair, against 43% with a knee brace20; function no better than with the brace; most in both groups satisfied20

A 2026 meta-analysis pooled trials in 306 people with MPFL injuries, of mean age about 20, followed for about 22 months21. It found repeat instability in 10.69% after surgery and 29.93% without it21. It found no significant difference in knee function scores21. A further check in that review found the current data too few for firm conclusions on each comparison21. A network meta-analysis of 13 trials (789 people) found the lowest rate of repeat instability after MPFL reconstruction, then MPFL repair, then non-surgical care22.

Braces after a first dislocation

A trial took 79 people who had stopped growing, after a first dislocation, and 64 completed it24. Redislocation at 3 years was 34.4% with a brace that limited knee motion and 37.5% with a neoprene sleeve24. The limiting brace brought less knee motion and more thigh muscle wasting early on, and worse function at 6 months24. No clinically relevant difference remained at 3 years24. Its authors note it was sized to detect a large difference in redislocation, not modest ones24.

ESSKA states there is no evidence that any brace is better than no brace after a first dislocation14.

After repeat dislocations

One trial took 61 people aged 12 to 30 with repeat dislocations and no major risk factors in the bone25. Both groups had a knee arthroscopy (keyhole surgery inside the joint, using a small camera) and an active exercise programme; one group also had MPFL reconstruction25,26. Ongoing instability was reported by 41.9% of the group without reconstruction and 6.7% of the reconstruction group at 12 months25. At 3 years, the figures were 53.6% and 16.7%26. Both groups improved on self-reported scores, with no significant difference between them26.

Its oversight group stopped it early, after 61 of 100 planned people, because outcomes differed widely25. Its authors say blinding was not possible, and the results cannot be generalised to everyone with kneecap instability26. The 12-month and 3-year reports of this Norwegian trial list funding from foundations, a sports medicine association, a regional health authority and a hospital25,26.

Results after MPFL reconstruction

Three reviews of return to sport after MPFL reconstruction give these rates6,7,8:

ReviewReturned to sport
23 articles, 930 people, for kneecap instability92.8%; 71.3% to the same level or higher6
27 studies, 1,278 people, for repeat instability85.1%; 68.3% to the same level, at an average of 7.0 months7
14 articles, MPFL alone, for repeat dislocation84.1%, with repeat instability in 1.2%, in short-term results8

A review of 53 studies of MPFL surgery looked at how each decided when people went back to activity27. Only 10 (18.9%) used objective or subjective criteria; most used time-based criteria27. The review found that safe guidelines for return to play were not established27.

A 2023 review of 28 studies (1,478 people) found complications in 0% to 32.3% of knees28. Most were lasting pain at the front of the knee28. Failure ranged from 0% to 10.7%, and kneecap fracture from 0% to 8.3%28.

In a Danish registry of 1,956 reconstructions, 20.9% had a new kneecap-related visit to the health service more than a year after surgery29. That was counted over eight years, and was 28.9% at ages 10 to 1729. The risk was higher after other stabilising surgery (33.8%) and after non-surgical care (29.4%)29.

Connections

The patella page covers the kneecap itself, which slides up and down in the thigh bone’s groove30. The kneecap and thigh bone form one of three joints inside the knee30.

The joint cartilage that ESSKA says is damaged from the first episode is covered on the articular cartilage page1. Arthritis behind the kneecap can occur alone or together with arthritis of the main knee joint31. That wider picture is on the knee osteoarthritis page16.

Patellar dislocation involves

Patellar dislocation can damage

Patellar dislocation is followed in some people by arthritis covered in

Sources

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Every source the library cites

Changes

  • First published.