The stifle joint (knee) is composed of two bones (fig 1) the femur (thigh bone) and the tibia (shin bone). The patella (knee cap) is a small bone that is incorporated in the tendon of the quadriceps muscle group and lies in the trochlear groove of the femur (Fig 3). The tendon is called the straight patella tendon and inserts on the tibial tuberosity of the tibia (Fig 2).

The function of the patella is to prevent friction between the tendon and the trochlear groove of the femur. If the patella was not present then the tendon would rub directly on the bone causing it to
degenerate and finally rupture. Luxation means 'popping out' or 'sliding out.' The patella can luxate medially (inside) or laterally (out-side) (fig 5). 

It usually luxates medially and one or both stifle joints can be affected. The cause is usually developmental but can follow trauma.
In those breeds where developmental medial luxating patella is common, i.e. the Staffordshire Bull Terrier, a significant hereditary component is suspected.
Whether the hereditary component is ten, thirty, fifty, or sixty percent, is unknown.
Other factors involved include conformational problems such as shallow trochlear groove (fig 3). Obviously if the trochlear groove is shallow the patella is far more likely to luxate. A straight stifle, added to a shallow trochlear groove, predispose to patella luxation. However, I have seen the condition in SBT's that have a well bent stifle. Therefore, a lot more work needs to be done in our breed before we have a definite answer. Other factors include diet, congenital factors and exercise.

At Glasgow Veterinary School they have reported a manifestation of the disease that was first reported in the Staffordshire Bull Terrier. In these cases the underside of the patella ulcerates as it rubs on the
medial femoral condyle (fig 3) when the patella luxates medially. This is a very painful condition and the patella must be removed. In my clinic we have only seen the condition once and that was in a
Cavalier King Charles Spaniel.
The severity of the medially luxating patella reflects how these dogs cope. In mild cases where the patella only luxates occasionally the affected dogs will cope very well and do not require surgery.

However, affected dogs develop secondary osteo-arthritis in the stifle. In later life it is the secondary osteo-arthritis that is more likely to cause lameness. However, a lot of dogs will develop a characteristic mechanical lamesness. They tend to hop for a few strides then flick the patella back into its normal position and then walk normally. In severely affected cases where both patellas are permanently luxated the dogs may be unable to stand. These cases can be confused with dogs with spinal problems.

The treatment of medially luxating patella
The treatment of the condition is surgical where the condition causes a clinical problem. I deepen the trochlear grove (fig 6) via a wedge sulcoplasty. Here we cut out a small piece of the articular cartilage
from the trochlear groove. Then we deepen the grove and then replace the piece of articular cartilage so that, in fact, we have effectively deepened the grove.
We also perform a tibial tuberosity (fig 7). Here we cut away the tibial tuberosity and move it across laterally on the tibial holding it in place in its new position with a pin. As the straight patella tendon inserts on the tibial tuberosity we are in fact moving the patella across laterally. Therefore, it is less likely to luxate medially. We can also tighten up the lateral side of the joint with sutures (stitches). This
has the effect of again pulling the patella across laterally thus making it less likely the luxate medially.

To sum up, patella luxation is a developmental orthopeadic disease of young dogs that affects the stifle
joint. It commonly affects the SBT.
The resulting characteristic lameness usually requires surgery The cause is multifactoral with a significant hereditary component. The condition has been proven to be hereditary in the Yorkshire Terrier. For these reasons I believe that it is very unwise to breed from affected dogs.

However, the condition can follow trauma to a normal stifle joint with a normal trochlear groove. These cases respond very well to surgery. It is safe to breed from these dogs as there is no hereditary component. Indeed, this is the same as luxation of the hip or any other joint following trauma.

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