Hip dysplasia is the most common cause of rear leg lameness in dogs. Large breed dogs experience hip dysplasia more frequently than other breeds of dogs, however, small breed dogs can also be affected, but are less likely to show symptoms.
Hip dysplasia is a genetic disorder which is a polygenetic trait. In other words, there is more than one gene in the individual animal's genetic makeup which determines if the dog will develop the disorder. The hip is a ball-and-socket type of joint. The ball part of the joint is the head of the femur bone and the socket portion of the joint is the recessed portion of the pelvis (or the acetabulum) where the femur fits. When the head of the femur rests loosely in the acetabulum, it is said to be a dysplastic hip. Normally the femur fits snugly in its resting place in the pelvis with only enough room to allow the joint to move freely in its normal range of motion. The pelvis where the acetabulum is located is shallower and more poorly developed in a dysplastic hip. This causes the joint to be unstable because the muscle growth and development lags behind the skeleton's growth rate. As the dog begins to stand and walk and bear weight on the hip, the connecting ligaments and muscles become loose and unstable since the skeleton is unable to help carry the weight. This situation allows the femoral head in the acetabulum to move more freely than it should which promotes abnormal wear and tear on the joint.

Note in the figure below, the differences in the normal hip in comparison to the hip with moderate and severe hip dysplasia. You can see how with the more severe case of hip dysplasia, there is almost no bone left to hold the femur in place and how much space there is between the pelvis and the femur.

Feeding a very high-calorie diet to a growing dog can cause it to develop hip dysplasia, if it is predisposed to the condition. This is because the rapid weight gain places undue increased stress on the hips. Being overweight supports the genetic potential for hip dysplasia, as well as other skeletal diseases. Also inappropriate exercise (such as jumping up and down from heights and from standing up on their back legs) during the early periods of rapid bone growth can bring on the symptoms of hip dysplasia.

Dogs with hip dysplasia are born with normal appearing hips, but which progressively undergo changes structurally. The age of onset tends to be between 4 to 12 months. Affected puppies may show pain in the hip, walk with a limp, have a swaying gait, bunny hop when running or experience difficulty in the hindquarters when getting up. If you put a puppy on its back, the rear legs may not extend into the frog-leg position without causing it some pain.

X-rays of the hips and pelvis is the only reliable way to diagnose hip dysplasia. Good X-rays require heavy sedation or anesthesia. Moderate dysplasia is when the hips are loose and partially out of their sockets, and the femoral heads are beginning to flatten. Severe dysplasia is seen with degenerative arthritis and the femurs are beginning to or have rotated outwardly from the pelvis. Usually bone spurs can be seen on the femoral heads and the rims of the sockets.

Feeding a very high-calorie diet to a growing dog can cause it to develop hip dysplasia, if it is predisposed to the condition. This is because the rapid weight gain places undue increased stress on the hips. Being overweight supports the genetic potential for hip dysplasia, as well as other skeletal diseases. Also inappropriate exercise (such as jumping up and down from heights and from standing up on their back legs) during the early periods of rapid bone growth can bring on the symptoms of hip dysplasia.

Dogs with hip dysplasia are born with normal appearing hips, but which progressively undergo changes structurally. The age of onset tends to be between 4 to 12 months. Affected puppies may show pain in the hip, walk with a limp, have a swaying gait, bunny hop when running or experience difficulty in the hindquarters when getting up. If you put a puppy on its back, the rear legs may not extend into the frog-leg position without causing it some pain.

X-rays of the hips and pelvis is the only reliable way to diagnose hip dysplasia. Good X-rays require heavy sedation or anesthesia. Moderate dysplasia is when the hips are loose and partially out of their sockets, and the femoral heads are beginning to flatten. Severe dysplasia is seen with degenerative arthritis and the femurs are beginning to or have rotated outwardly from the pelvis. Usually bone spurs can be seen on the femoral heads and the rims of the sockets.

After your vet has reviewed your dog's X-rays, he may recommend hip surgery. Hip surgery in select puppies can prevent some cases of degenerative joint disease. Surgery is also indicated in dogs where medical intervention has failed to provide relief from pain and lameness

Treatment
There are 5 surgical options available to you. Below I have listed the individual operations and the purpose(s) for each one. Technical factors govern which choice is best for the situation.

Triple Pelvic Osteotomy and Femoral Osteotomy

Both of these operations are usually done on puppies that do not have degenerative joint changes yet. The goal of both procedures is to position the femoral head more deeply into the acetabulum. By doing this, normal joint function is maintained and the chances of developing arthritis is much less.

Pectineus Myectomy

This surgery involves removing one of the muscles from both sides. This surgery will only help to relieve pain for some time, but will not prevent or slow the progress of joint disease.

Femoral Head and Neck Excision Arthroplasty

In this procedure the head and neck of the femur is removed allowing fibrous tissue to grow into the pelvis. This growth becomes strong and replaces the ball and socket joint . This is one of the options for Legg-Calve-Perthes Disorder as well. This procedure is reserved for dogs under 35 pounds. It is very affective in relieving hip pain.

Total Hip Replacements

This is the surgical procedure of choice for larger dogs, or dogs 9 months or older that have disabling degenerative joint disease in one or both hips. The old joint is removed and is replaced with a new artificial hip joint not unlike those done for humans. The procedure requires special equipment and is performed by and orthopedic specialist. Of the cases done, 95% have a good prognosis

Prevention: As in most things, an ounce of prevention is worth a pound of cure. The same is true when it comes to hip dysplasia. During a the time that a puppy is growing up, if you can prevent excessive weight gain and keep them from unduly stressing their hip joints this will delay the onset of hip dysplasia in dogs that are genetically predisposed to the disorder. It may also lead to a less severe form of dysplasia.