Dislocation is different from subluxation. Here the restraints have failed, usually torn, and the patella jumps right out of the groove, often locking in this position with the whole knee looking severely deformed.
At this level the problem usually has a large structural component and is not simply due to a functional problem such as a lazy VMO muscle. Structural problems may include -
- trochlear dysplasia - abnormally low side walls in the trochlear groove - the groove in which the patella rides during bending and straightening of the knee
- patellar dysplasia - abnormal patella shape - this may be a small patella, or a flat one, or one in which there is inequality of the facets(the flat under-surfaces)
- patella alta - a high riding patella, which makes contact with the groove at a high level where the groove is flat, and is therefore more prone to being forced out of the groove