Patellar subluxation and patellar dislocation both refer to a condition in which the kneecap cannot reliably remain within the trochlear groove at the bottom of the femur (thigh bone).
Normally, the patella rests in a groove at the lower end of the femur (thighbone) called the trochlear groove. The underside of the patella has a shape like a keel of a ship. This “keel” engages with the trochlear groove to ensure vertical movement of the kneecap as the knee is flexed and extended. Patella instability can result from malformations of the trochlear groove (e.g. excessive shallowness). It can also be a result of weakness of the quadricep muscles or lateral ligaments that are also responsible for keeping the patella in place.
Another condition that falls under the heading of patella instability is patellar subluxation. This refers to to temporary partial dislocation of the kneecap from its normal position in the trochlear groove at the end of the thigh bone.
As with patella instability , this condition is usually attributable to structural abnormalities of the knee. For example, subluxation may a result of the underdevelopment of the inner thigh muscle. It may also be due to overdevelopment of the outer thigh muscle.
Another knee problem related to patellar subluxation is patellar tilt / glide. This refers to two conditions under which the patella deviates from its normal movement in response to knee flexion or extension:
With patellar tilt, the kneecap does not sit evenly in the trochlear groove at the top of the femur as it is intended to do. Instead, it tilts to the side;
If the patient is suffering from patellar glide, the kneecap moves laterally instead of vertically. This occurs in response to excessive tightness of the tissues to the side of the patella. This in turn causes them to pull the patella in a lateral direction.
Both of these patella instability conditions can give rise to patellofemoral pain syndrome, i.e. pain in response to flexing or extending the knee. In some cases, the patient may experience pain when performing any sort of weight bearing activity like walking, running or squatting.
Using knee braces like the ones below can help to relieve these problems. However, you should not assume that they are for this purpose only. These braces can also be useful in treating other knee conditions besides patellar subluxation. For example, they may also incorporate features to treat knee joint instability, a torn ACL or other knee problems.