Patella - patellar instability

Subluxation

The term 'subluxation' is actually used a little loosely. A surgeon may indicate 'subluxation' on an x-ray, when indicating that part of the patella habitually rides outside the underlying rim of the trochlear groove -

But a surgeon may ask if you 'sublux', in this case meaning that you periodically feel the patella suddenly - and uncomfortably - jump partly over this rim. This is a discomfort, rather than something you can see.

A surgeon may also, when examining the knee, try to push the kneecap sideways out of the groove in an attempt to physically 'sublux' the patella. This is to measure the degree of restraint by the capsule and the retinaculum (the thicker bits on the side). If the patient anxiously restrains the examiner from doing this, it is termed a 'positive apprehension test', and suggests that the patient has previously experienced pain from instability.

Dislocation

Dislocation is different. 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