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Canine hip dysplasia   (HD) is a genetic disease which is inherited in a rather complex manner because it is influenced by more than one gene.  That is its mode of inheritance is polygenetic.     It was first recorded back in the 1930s but its incidence has increased as the popularity of breeding and showing dogs has increased.  Whether this is coincidence or whether there is some relationship has not been established.   In a number of breeds the disease is present in some degree in over 80% of individuals.

The hip joint is a ball and socket joint with the ball (femoral head) being on the proximal end of the thigh bone (femur) and the socket (acetabulum) being on the pelvis.   If the hip joint is normal the ball is a neat fit in the socket.  Generally all new born puppies have normal hip joints but in those that have HD an abnormality develops during growth.   The problem appears to be in the growing of the socket.  For the socket to grow bigger as the dog grows requires quite a complex process because it is difficult to grow a hole.  With the acetabulum this is achieved by the three bones that make up one side of the pelvis (ilium, ischium and pubis) joining at the acetabulum thus forming a complex pattern of growth zones.   If the acetabulum is to develop correctly the growth rate at these different zones has to be precisely matched.  If it is not, the acetabulum will become distorted so that the ball is no longer an accurate fit and this is hip dysplasia.  Some recent research suggests that dogs with HD have problems with other bone growth zones but because these are mainly involving single bones they do not cause problems. 

Hip dysplasia in itself does not commonly cause lameness.  It only does so if it is of such a degree that the hip dislocates and this is uncommon.  So many dogs, particularly younger dogs, with hip dysplasia show no signs of lameness.   This combined with the fact that it is a developmental abnormality is the reason why we have to resort to assessing dogs for hip dysplasia from an X-ray taken when the dog is at least 12 months of age.

With dogs that have HD the ball is not a neat fit in the socket so the joint is subject to excessive wear and tear.  This excessive wear and tear results in the development of degenerative joint disease (arthritis).  It is the arthritis that causes the lameness.  Other factors can impact of the degree of wear and tear the joint is subjected to.   The two main ones are the weight of the dog and the amount and type of exercise the dog takes.  Obviously the heavier the dog the more stress is put on the hip joint so large heavy dogs are more likely to become lame because of hip dysplasia than small light ones.   Dogs that get a lot of exercise at fast gaits such as galloping behind a car or bicycle and dogs that do a lot of jumping or turning and stoping such as when fetching balls or sticks stress the hip joints and are therefore more likely to become lame.   A normal hip joint can sustain these sorts of stresses without developing arthritis.    So the age at which a dog with HD shows signs of lameness and the severity of the lameness is influenced by an inherited component. That is the degree of developmental abnormality of the joint. Then there is the environmental influence which is the degree of stress the joint is subjected to.

When it finally became generally recognised in the 1960s that hip dysplasia was an inherited disease dogs were screened for its presence by taking an X-ray of the pelvis. From the X-ray the degree of HD  was  graded Normal, Near normal , Slight, Moderate etc. but this was a highly subjective method of assessment so there was variation in interpretation from one assessor to another.  In the late 1960s Prof DD Lawson and Dr MB Willis evolved the scoring system used in the UK, Australia, New Zealand and Singapore.  This  gives a more objective assessment of the hip joints.  The standardisation of the scores awarded is further contributed to by only accepting  scores awarded by specialist veterinary radiologists .


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