Patella Dislocation

The patella (knee cap) rests at the front of the knee in a v-shaped groove called the trochlea. This works in conjunction with other supportive structures to keep the patella in place during movement. The strong quadriceps muscles at the front of the thigh blend together and are attached to the patella through the quadriceps tendon, which then continues down to the tibia as the patella tendon. This chain of structures forms the extensor mechanism and when the muscles are activated, the chain is pulled in a straight line from hip to knee and the patella slides inside the trochlear groove as the knee moves.

If the patella partially leaves the groove, this is known as patella subluxation. If it moves out of the groove all together, it is known as patella dislocation. This might occur as the result of a direct blow to the knee which knocks the patella out of place and damages the supporting structures that usually keep the patella gliding within the groove. For this reason, once a patella has been dislocated, it is more likely to dislocate in the future. There are two important structures that work to keep the patella tracking centrally within the trochlea:

• the medial patellofemoral ligament (MPFL) attachesbetween the inside part of the patella and the femur (thigh bone) and acts like a guide rope to stop the knee cap moving out to the side when the knee moves
• the vastus medialis oblique (VMO) is a muscle just above the patella which also helps to prevent the patella sliding sideways during knee movement

Some people maybe more prone to dislocations because of differences in the way their bones and other structures are formed or are aligned. Some examples are:

• the v-shaped trochlear groove might not form properly (trochlea dysplasia) meaning it is too shallow to keep the patella running smoothly and centrally when the knee moves
• in some people, the patella rests higher than usual so that it does not sit well within the trochlear groove
• the alignment from a person’s hip to foot maybe such that when the extensor mechanism is activated, the centre of pull does not lie in the middle of the trochlea. An example of this is “knock-knees.” This means the patella is pulled out to the side when the knee is moved, instead of a straight line
• Some medical conditions mean that the connective tissues, like the MPFL and other ligaments are stretchier than they should be and are less able to protect patella tracking
• In some people, the muscles on one side of the thigh are too strong or too weak so that the extensor mechanism does not pull in a straight line

If the patella has dislocated once due to a traumatic cause, there is a good chance that the damaged tissues will heal up, the muscles can be made strong and the patient can return to full and normal function.

If you have dislocated your patella, non-surgical treatment might include:

• Rest and immobilisation of your knee to allow the tissues to heal
• Pain relief medication while the knee is healing
• A physiotherapy program to strengthen and balance the muscles around the knee

Your surgeon may recommend an operation if:

• You have dislocated your patella more than once
• A small piece of your patella or femur has broken off, creating a loose body inside the joint
• If the patella continues to dislocate despite good strengthening of the muscles