It is most common in young people (16-20 years) and most frequent in the female sex.
Sometimes there is a story of a direct blow but mostly it follows a simple twisting injury in people who have an inherent instability of the kneecap due to a shallow femoral groove (trochlear dysplasia), a high-riding patella (patella alta), very angled femurs (high Q angle) (valgus deformity) or excessive joint laxity (eg. Ehlers Danlos Syndrome).
The dislocation is almost invariably lateral (ie the patella flips over to the outer side of the knee). Medial dislocation is very rare and then is often a consequence of a surgeon's previous exhuberance in dealing with recurrent lateral dislocation.
A clunk may be heard at the time of injury with a lot of pain and swelling afterwards. The kneecap area will look grossly deformed. Sometimes there are fractures of the two involved bones (kneecap and femur) at the point at which they gave way.
The first time a knee dislocates it may be treated 'conservatively', eg. splintage, with later physiotherapy. If the dislocation becomes recurrent, the surgeon may, depending on his clinical findings, do a 'proximal or distal realignment procedure' - operations like an Elmslie-Trillat, Maquet or Fulkerson procedure, perhaps combined with lateral release.
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