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