Hip dysplasia is the most common inherited orthopaedic disease in large and giant breed dogs, and occurs in many medium-sized breeds as well. The mode of inheritance is polygenic (caused by many different genes). Scientists do not yet know which genes are involved, or how many genes. Factors that can make the disease worse includes excess weight, a fast growth rate, and high-calorie or supplemented diets. The hip joint is a "ball and socket" joint; the "ball" (the top part of the thigh bone or femur) fits into a "socket"(acetabulum) formed by the pelvis. If there is a loose fit between these bones, and the ligaments which help to hold them together are loose, the ball may slide part way out of the socket (subluxate). The disease is progressive. The degree to which hips are dysplastic does not always correlate with the amount of pain. Some dogs with very bad hips radiographically are less painful than others whose x-rays show only minor changes. Tilting of the x-ray to the side or forwards towards the camera will adversely affect the score to some degree. There may be uneven scoring in terms of right and left side, and an extra 3 or 4 points depending on the degree of tilt. The effect will be mainly on the first three sections. Tilting is unlikely to influence the last six sections to any degree. It is felt that excessive exercise in early puppyhood is unwise. If one has bred for bad hips then a failure to exercise will not necessary prevent HD occurring. Hip dysplasia will only be reduced if breeders deliberately select firstly from the best hip animals (without ignoring other features) and follow this up by using the best progeny tested sires. In all breeds some 80% or more of dogs differ by 4 points or less between hips and thus HD is largely a bilateral state. However some dogs score unevenly. At times this is a positional feature but in some it is not. Dogs can score unevenly because of trauma or due to transitional segments on the spine, but a proportion of dogs are uneven and will breed this feature on. The x-ray is scored out of 6 on 8 sections on each hip and 1 section out of 5. Worst score is 106 - the highest for rottweilers to date is 99. The Mean score for the breed at present is 13. Some dogs who are 'clinically sound' that is move normally, when x-rayed can have major changes, and likewise a dog with unsound movement can have minor changes only. THE ONLY WAY TO DETERMINE THE HIP STATUS IS BY X-RAY.