Overview

Patellar dislocation is when the kneecap slips out of its groove at the front of the knee — almost always toward the outside. It usually happens during a twist, a fall onto a bent knee or a sudden change of direction, and it stretches or tears the soft-tissue tether on the inner side of the kneecap (the medial patellofemoral ligament).

Many first-time dislocations slip back into place on their own as the knee straightens. When one does not, it should be put back by a clinician without delay — not by pulling on the leg yourself.

Patellar instability is the broader pattern: a kneecap that threatens to slip, partially slips (subluxation) or dislocates repeatedly.

Typical Pattern

A pop or "out of place" feeling at the front of the knee
Pain that can be severe at first, often with rapid swelling from bleeding inside the joint
A kneecap that visibly sits to the outside when the dislocation does not reduce
Difficulty straightening the leg or bearing weight right after the injury
Later, a sense of giving way, catching or the kneecap feeling unstable with twisting, stairs or sport

A pop with swelling can look like an ACL injury, and the two are sometimes confused. The mechanism and where it hurts help, but they do not confirm either diagnosis on their own.

Why It Develops

Some people have a higher baseline risk: teenagers, a shallower groove for the kneecap, general joint looseness, certain leg alignments, or a previous dislocation on either side. Skeletal immaturity also raises the risk in younger people.

After a first dislocation, the kneecap is at higher risk of slipping again. Studies of non-operative care report recurrence in roughly 15% to 44% of people, with the highest rates in adolescents. That is why instability is taken seriously — and why rehabilitation is not optional.

Treatment

Right after the injury

An unreduced dislocation should be put back promptly by a clinician
Early care focuses on protecting the knee, settling swelling, restoring comfortable movement and beginning gentle strengthening as pain allows
Brief bracing or taping helps some people early on; long-term immobilization is not the answer

Rehabilitation

A structured program — early motion and strengthening, then progressive loading, balance and sport-specific training — is the first-line approach for most first-time dislocations. Strengthening the quadriceps, hip muscles and lower-leg control helps the kneecap stay in its groove during movement.

Most first-time dislocations do not need surgery. Surgery (most often reconstruction of the medial patellofemoral ligament) is considered when instability keeps recurring despite good rehabilitation, when a loose bone or cartilage fragment is present, or when the kneecap will not stay in place. The decision depends on your age, anatomy, activity demands and how the knee responds.

When to Get Assessed

Get prompt care if the kneecap is visibly out of place and does not go back when you straighten the leg, if swelling is rapid and severe, if you cannot bear weight, or if the knee looks deformed. Even a dislocation that popped back in should be assessed — to check for associated damage and to plan rehabilitation. Recurrent slipping or giving way also warrants assessment. Instability is not something to just live with, but it is not a reason to fear movement.

References

British Orthopaedic Association. "BOASt — Assessment and Management of First Time Lateral Patellar Dislocation (FTLPD)." https://www.boa.ac.uk/resource/boast-assessment-and-management-of-first-time-lateral-patellar-dislocation.html
"Nonoperative Treatment of Patellar Instability." https://pubmed.ncbi.nlm.nih.gov/28459749/
"Predictors of Recurrent Patellar Instability in Children and Adolescents After First-time Dislocation." https://pubmed.ncbi.nlm.nih.gov/26491910/
"Consensus-Based Guidelines for Management of First-Time Patellar Dislocation in Adolescents." Journal of Pediatric Orthopaedics. 2024. https://pubmed.ncbi.nlm.nih.gov/38258884/

This information is for education only and is not a medical diagnosis. InjuryAtlas does not replace an assessment by a qualified health professional. If you have severe pain, visible deformity, numbness, loss of function, or symptoms that are getting worse, seek medical care promptly.

Patellar Dislocation and Instability