What are the Risk Factors for Repeat Patellar Dislocations?
There are many factors that need to be assessed when one is looking at patellar instability. First, a tear of the ligaments that hold the kneecap towards the inside of the knee, primarily the medial patellofemoral ligament, or MPFL, is important to assess. Other factors are if the patella is high riding, called patella alta, and of the thigh bone, called the trochlea, is flat or is in a more V or U shaped.
A flat trochlea is known as trochlea dysplasia and because the bone is flat there is not much bony inherent stability and these patients have a higher risk of having a patellar dislocation. In addition, if one is knocked knee or in valgus alignment, there is also a higher risk of having the patellar dislocate laterally.
Therefore, all of these factors need to be assessed in determining ones risk of a recurrent lateral patellar dislocation.
In general, the redislocation rate for patients who have normal bony anatomy, indicating the patella is not too high riding, the trochlea is V-shaped rather than relatively flat, and who have neutral or a bowlegged alignment, the risk of redislocation can be less than 10% with a proper rehabilitation program. However, if one does have some patella alta, trochlea dysplasia and/or a knocked knee appearance (valgus alignment), there can be up to a 40% or more rate of kneecap redislocation, even with a well-directed rehabilitation program.
In general, for patients who do not have any significant pieces of cartilage and bone knocked off due to the kneecap dislocation, we would recommend proceeding with a rehabilitation program first, but also recognizing and following these patients closely who do have underlying bony anatomy which puts them at a higher risk of having the patella redislocate.