Patellar Instability and Maltracking
The back of the patella is V-shaped, and it sits in a groove at the front of the knee called the trochlear groove. The surfaces of the bones are each covered in a layer of articular cartilage – the smooth white shiny layer of tissue that makes the joint surfaces very low friction. As the knee bends and straightens, the patella slides up and down the middle of the trochlear groove.
Patellar instability is where the kneecap doesn’t sit securely in the trochlear groove, and it has a tendency to ‘pop out’. If a kneecap comes all of the way out of the trochlear groove then this is called a patellar dislocation. A partial dislocation is called subluxation. Unless there is very severe trauma to the knee, pretty much all patellar dislocations are lateral, i.e. the kneecap pops out to the outer side of the knee.
Patellar dislocation is severely painful; partly because of the bone bruising that occurs and partly because there is always at least some associated damage to the soft tissues around the medial (inner) side of the kneecap (the medial retinaculum and the medial patellofemoral ligament). For the kneecap to be able to pop out laterally, there has to be either stretching or tearing of these miedial structures. In addition to all this, when the patella dislocates there is often bleeding into the knee as well, which causes pain. Furthermore, there can often be damage to the articular cartilage on the joint surfaces too.
Most people who suffer a patellar dislocation and possible subsequent recurrent dislocations tend to have at least some underlying predisposing factors that make them vulnerable to patellar dislocation. These can include:-
- patellofemoral dysplasia (a shallow trochlear groove and/or a flat patella)
- lateral patellar maltracking
- tight lateral structures (lateral retinaculum and ITB)
- weak VMO (the inner part of the quads muscle)
- weak/stretched/torn/deficient medial retinaculum and medial patellofemoral ligament
- patella alta (a patella that sits too high)
- increased Q-angle
The Q-angle is the angle formed between the combined line of pull of the quads and the line of the patellar tendon. The bigger the Q-angle, the more of a ‘bowstring’ effect there will be on the patella, with the patella being pulled sideways to the lateral side. The things that can lead to an increased Q-angle are:-
- internally rotated hips (excessive femoral anteversion)
- knock knees (increase valgus angle at the knee)
- external rotation of the tibia (shin bone)
- poor foot posture (flat feet)
Any one (or sometimes even all!) of these potential factors can cause a predisposition to patellar instability. However, even if the patella never actually dislocates or subluxes, these factors can cause lateral patellar maltracking, which is where the kneecap doesn’t line up properly with the middle of the trochlear groove, but instead sits too far over to the lateral side. This means that the back surface of the patella can rub unevenly on the front of the knee, and this can cause pressure overload in the patellofemoral joint and anterior knee pain. Eventually, with time, this can lead to damage and wear and tear of the articular cartilage, and eventually patellofemoral arthritis.
Whereas patellar dislocation is, thankfully, relatively uncommon, lateral patellar maltracking is extremely common.