Patellofemoral instability

In subluxation the kneecap may feel uncomfortably about to come right out of the underlying groove, but then it pops back into place.

With dislocation the kneecap comes right out of its groove, creating a painful and ugly deformity. In some cases it pops back but it other cases it stays like that and the person cannot straighten the knee without help.

What constitutes patellar instability?

Patellar instability is a continuum. Patients may progress from one level of instability to another:

• Anterior knee pain - pain in the front of the knee
• Giving way - a sudden inability to hold weight, so that the knee collapses
• Subluxation - when the kneecap pops to one side, and then pops back again
• One-time dislocation - when the kneecap completely derails, but this does not recur
• Recurrent dislocation - when de-railing of the kneecap occurs quite often
• Habitual dislocation - when the kneecap de-rails all the time

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Structural causes of patellar instability

Factors which contribute to an unstable kneecap include one or more of -

• Trochlear dysplasia
• Patella alta
• Valgus deformity (knock-knees)
• Medialised trochlear groove
• Lateralised tibial tubercle

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