Hip dysplasia is a common condition in dogs. It predominantly affects larger breeds of dog and is particularly common in German Shepherds, Labrador retrievers, Golden Retrievers, Rottweilers, Great Danes and St Bernards. However, any breed of dog and even cats can be affected. To fully understand it we must first consider the normal hip joint.
ANATOMY OF THE NORMAL HIP JOINT
The hip joint is formed between the acetabulum of the pelvis and the head of the femur (thigh bone). The head of the femur is a ball shape that fits snugly into the socket on the acetabulum. All the bony surfaces of the hip joint are covered in smooth articular cartilage to allow pain free movement of the joint. The joint is also lubricated via a special viscous fluid called synovial fluid, which is produced from the synovial membrane. The femur is kept in position by the round ligament which runs from the head of the femur to attach to the acetabulum. The joint is surrounded by a tough fibrous layer called the joint capsule; there are two hip joints the right and left.
WHAT IS HIP DYSPLASIA
Hip dysplasia is an abnormal development of the hip joint. Usually both hips are affected. The hip joint is unstable so that the head of the femur (the ball) does not fit snugly in the acetabulum (the socket). This is known as subluxation. Hip dysplasia causes pain and lameness because the articular cartilage gets worn away, the joint capsule becomes stretched, the joints become inflamed and eventually arthritis develops.
WHAT CAUSES HIP DYSPLASIA
Hip dysplasia is called a multifactorial disease. This means that there are more than one potential cause. Both hereditary (genetic) and environmental factors play a part in the development of the disease. Environmental Factors:
- Excessive food consumption resulting in rapid weight gain and growth increases the risk of developing hip dysplasia. Overfeeding itself does not cause hip dysplasia but will increase the risk of it developing in genetically susceptible individuals. Furthermore, studies have shown that restricted feeding regimes have reduced the amount of hip dysplasia seen in the at risk breeds. Why excess food causes a problem is unclear. It may be simply that excess food produces excess weight gain and this produces an excessive force on a potentially susceptible hip. However, there may be more complex and subtle reasons than this relating to the effect of food intake on various chemicals and hormones in the body.
- Repeated mild trauma such as excess exercise while growing such as running up and down stairs and jumping on and off furniture may also be important. It is thought that in a susceptible hip, mild trauma can cause inflammation of the synovial membrane (synovitis) which results in excessive production of synovial fluid. This may then reduce the stability of the joint worsening the hip dysplasia.
Genetic factors There is a strong hereditary basis to hip dysplasia in dogs . The exact number and location of the genes is not known at present. As technology advances and gene mapping is performed the exact genes responsible for hip dysplasia will become apparent. Dogs produced from dogs with hip dysplasia tend to be dysplastic and vice versa. However, some dogs with perfect hips can carry the genes for hip dysplasia and if they are mated with another dog with the genes the puppies have a high risk of developing hip dysplasia. Due to this genetic factor most countries have some form of hip xray screening programme to detect dogs with hip dysplasia. Dogs or bitches that prove to have radiographic (Xray) signs of hip dysplasia should not be bred from. When selecting a puppy it is vital to know if the dam (mum) and father (sire) have been tested. Your vet can advise you more as to what the test results mean as they vary from country to country. No screening system is perfect and possibly due to this many breeders do not get their breeding dogs tested. Breeders may also be concerned that the results of such tests could hurt their business and reputation. Finally, some dogs with hip dysplasia can appear to the untrained eye as normal either because the lameness is very subtle, both hips are equally affected and so no overt lameness is present or because the problem is intermittent. Even mildly affected dogs should ideally not be used for breeding.
CLINICAL SIGNS
(What are the signs that may tell you your dog has hip dysplasia?) The age at which hip dysplasia becomes apparent is usually 5-10 months of age due to hip instability and pain, or 3-12 years due to secondary osteoarthritis. Males & females are equally affected. In the younger dogs signs may include difficulty getting up after rest; not wanting to exercise; lameness (both intermittent and continuous) in one or both hindlegs; a change in jumping behaviour; a “bunny hopping” walk or run; clicking while walking. Remember not all signs will be present in all dogs. In the older dogs signs are due to osteoarthritis including difficulty getting up, reduced ability to exercise; stiffness; a waddling walk; muscle wastage; lameness. Next week we’ll look at the Diagnosis and Treatment of Hip Dysplasia.
