What is patellar instability?
The kneecap normally glides up and down a groove (the trochlea) at the front of the thigh bone. In patellar instability, it slides out of that groove — usually towards the outer side — either partially (a subluxation) or completely (a dislocation).
A first dislocation often follows a twist or a direct blow, but some people are predisposed by the shape of their anatomy. Once it has happened, the soft tissues that hold the kneecap in place can be stretched, making recurrence more likely.
Signs and symptoms
- checkThe kneecap visibly shifting to the side, then often snapping back.
- checkA sudden “giving way” with the knee buckling under load.
- checkPain and rapid swelling after a dislocation episode.
- checkApprehension and a fear that the kneecap will slip again.
Causes & risk factors
- A shallow trochlear groove (trochlear dysplasia).
- A kneecap that sits high or too far to the outer side.
- Generally loose ligaments and muscle imbalance.
- A previous dislocation, which raises recurrence risk.
How it is diagnosed
Examination assesses how freely the kneecap can be pushed sideways and reproduces the patient’s apprehension. X-rays and an MRI then reveal the underlying anatomy — the depth of the groove, the position of the kneecap, and damage to the stabilising ligament (the MPFL) — which together guide whether surgery is needed.
Treatment options
Rehabilitation
A first dislocation without major damage is usually treated with strengthening and bracing to restore control of the kneecap.
MPFL reconstruction
For recurrent instability, the key stabilising ligament is rebuilt with a graft to hold the kneecap in its groove.
Bony realignment
Where anatomy is the driver, the attachment of the kneecap tendon or the groove itself may be reshaped.
Because instability is often driven by anatomy, the right operation is chosen for each patient — addressing the specific reason their kneecap slips, rather than a one-size-fits-all repair.
Recovery & outlook
After rehabilitation or surgery, a staged programme restores motion, then strength, then confidence in the knee. Most patients return to sport within three to six months. Modern stabilising surgery is highly effective at preventing further dislocations when the underlying cause is correctly addressed.