I recommend you to either before or after having read this - go to the section Patellar Instability overview.
There are many reasons why the kneecap becomes unstable and the four most important will be explained beneath. Some very complicated words are involved like Trochlear Dysplasia, Patella Alta, Increased TT-TG distance and increased Femoral Antetorsion, but as said this will be explained. Occasionally the kneecap is struck by a foot or something else on the inner side so hard that it pushes the kneecap out, but mostly it is a sudden twist or turn that makes the kneecap go out.
The most common reason for the kneecap to dislocate is that the groove (trochlea) which patella runs in when the knee is bent is either shallow, not deep enough, or dome shaped (this is called trochlear dysplasia and there is a section only for this). If the kneecap is not contained (kept in place in the groove), the patella will have a tendency to jump out, to the outside of the knee. Among those who have experienced the first kneecap dislocation and who have a shallow groove (trochlear dysplasia) two-thirds will experience another dislocation. If you have a normal groove, in contrast, the risk will only be one-third.
Why the trochlear groove is flat is that it for unknown reasons at birth it is filled with too much bone underneath the cartilage.
Patella Alta means that the patella is too high and this is an important factor for loose kneecap. If the patella is too high there is a huge risk that, at the point where it should locate itself in the groove, it will instead go to the outer side of the groove, and dislocate. Below you can see an MRI scan with a high riding kneecap.
MRI and Patella Alta
Distalisation of the Tibial Turbercle for normalizing anatomy
Increased TT-TG distance means that the Tibial Tubercle is unusually far to the outer side of the shinbone and that causes the kneecap to track outwards. This will give a tendency to pull the kneecap out of the groove and thereby dislocate or subluxate. Often the increased TT-TG distance is caused by Trochlear Dysplasia since this makes the groove go towards the inner side (yes it is right inner side and that makes the TT-TG distance increase). In those cases where the Tibial Tubercle is rotated outwards, the treatment will consist of a Tibial Tubercle Osteotomy
This demonstrates and axial view on CT scan or MRI scan were both the Trochlear Groove (TG) and the Tibial Tubercle (TT) are visualised. The distance between those is normally close to 9 mm - if it is above 20 mm it is far to high.
In some cases the patella dislocates secondary to a malrotated hip which makes the knee go inwards (increased anteversion of the femur). By clinical examination this can be suspected and in these cases a CT or MRI scan can confirm the diagnosis. The malrotation can be corrected by a femoral rotational osteotomy. Beneath you see a figure demonstrating a left femur with severe increased internal rotation, causing the patella to dislocate. The patient had previously had a failed MPFL reconstruction. She responded very well to a combined femoral rotational osteotomy and a revision MPFL, and her pain resolved completely when the internal fixation device was removed a year after.
Typically the kneecap dislocate doing activities with changing direction, such as sports or dance. In others, it feels as if the kneecap is coming out just by walking on uneven surfaces. At worst you should concentrate if not kneecap is out of joint when doing squat or getting into and out of a car. Quality of life can be significantly affected because it discourages one from many activities.
MYTHS OR REALITIES?
It is a myth that it is easy to deal with by doing an operation. Quite a few patients have been operated unsuccessfully, and some even several times. It is therefore important to find the true cause of the kneecap is out of joint by an MRI scan and X-rays. Then the operation can be individualized in order to reestablish normal anatomy and then the operation will be effective.
Another myth is that it is hereditary and this is in fact often not a myth but true. Sometimes dislocating kneecap can be traced several generations back.
The next myth is that you just have to live with the problem, if it is the case that you already had surgery and the surgery has not worked as intended. Knowledge about the cause of the disorder and the treatment of it has changed radically over the last few years.