The patella (kneecap) is the bone at the front of your knee. The patella slides backwards and forwards in the trochlear groove of the femur (the V shaped joint surface at the front of the lower end of the upper bone in the knee) as the knee bends and straightens. The patella is held in the groove by a combination of the shape of the patella and the shape of the groove assisted by the thigh muscles which help to maintain stability. The diagram should help you to understand this more easily.
Patella dislocation usually results in the patella moving towards the outside (lateral aspect) of the knee joint. This situation can arise as a result of abnormal architecture of the groove in which the patella sits (trochlear dysplasia). This can also result from an abnormal positioning of the patella. Sometimes the muscular pull on the patella drags the patella from the groove. Sometimes this can be associated with damage to the ligaments around the patella of which the medial patellofemoral ligament (the ligament which runs between the knee cap and the upper bone in the knee) is the most important. If this ligament is damaged then the patella can be prone to dislocation and reconstruction of this ligament is the appropriate option if physiotherapy is not effective. Details of medial patellofemoral ligament reconstruction can be found here.
Sometimes patients have have instability of the patella as a result of long-standing abnormality within the patellofemoral joint (joint between the kneecap and the upper bone). Deciding on a solution to restore stability in the joint may be difficult. A consultation with Simon and appropriate investigation will assist in understanding the cause of the dislocation and finding the best treatment option for you.