Lateral Imbrication (Lateral Reinforcement)
This involves tightening the joint capsule. As the kneecap slips in and out of place the joint capsule that surrounds it is stretched. The surgeon simply takes a tuck in the joint capsule so that the kneecap is better confined.
Although this may be sufficient in very mild cases, in most cases this is done in addition to other surgical modifications, ensuring that the kneecap does not continue to slide in and out of place and enlarge the joint capsule again in the future.
As demonstrated in the image to the right, the patella rests in the trochlear notch at the base of the femur.
In a correctly functioning joint, the patella slides up and down this groove as the knee bends. In an abnormal joint, the walls on either side of the trochlear notch are flattened, allowing the patella to move out of this notch and to either side of the knee.
In a trochlear modification surgery, the surgeon deepens the groove so the patella stays where it belongs. The cartilage that lines the notch is peeled away from the bone, the groove deepened and the cartilage replaced.
Tibial Crest Transposition:
Tibial crest transposition surgery is required in extreme cases, where long term, severely displaced kneecaps have resulted in a knock-kneed conformation. In these cases, the patella rests outside of the trochlear notch so frequently that it has caused the tibias to rotate.
The rotation is actually due to the thigh muscle (the quadriceps pictured to the left) attaching inwardly. In this case, the tibial crest will have to be removed (where the quadriceps attach) and pinned back where it belongs to straighten out the leg.