Hip dysplasia simply stated means an "abnormal formation" of the hip joint. Think of the condition first as a looseness in a joint that should be snug - then most of the problems attendant to hip dysplasia are a result of this "looseness".  

The normal anatomy of the hip joint is a classic Ball and Socket joint.  The head of the femur (the "Ball") is supposed to match the acetabulum (the "Socket").  A good hip joint has a neat, snug fit between the ball and socket - that is, the head of the femur should not be slipping and slopping around somewhere in the neighborhood of the acetabulum!

There are infinite variations of dysplasia - ranging from only very slight changes from normal to complete dislocation. Consequently, no two dogs will be affected by HD exactly alike.

HOW IS HD ACQUIRED?  This is one disorder that has been proven, positively, to have a genetic basis.  How much of a genetic origin in each case can vary from 25% to 85%.   A condition that is completely determined by genetics, for example gender, has a Heritibility Factor of 1.  A condition totally unaffected by genetics, for example a broken leg, has a Heritibility Factor of zero.

Studies have shown that HD's Heritibility factor ranges from .25 to .85; this is a significant genetic contribution.  So the Heritibility Factor for a given dog is the result of a combination of the Heritibility Factors from each parent.  Simply put: if the parents are carrying genetic material for hip dysplasia - so will the offspring.   And the greater the genetic contribution for loose hips or malformed bone or abnormal muscle mass (Heritibility Factor) from the parents, the greater the chances for hip dysplasia in the offspring.
The expression of hip dysplasia in any dog has other determinants, though, genetics play only a varying role in the total picture.  The effect of the developing dog's environment does play a role in the clinical (observable) signs of dysplasia, although just like the genetic component the effects of environment are variable and not completely understood. 

Some of the environmental aspects that can affect the observable expression of hip dysplasia are the following:

1. Nutrition - There are reports that in puppies a restricted calorie intake could restricted the growth rate, and in turn will lessen the potential for the dog to develop hip dysplasia.  (I wouldn't suggest doing this to any pup... it makes as much sense as stealing money from your own checking account!)  The problem is that some restricted diets restrict the fat and protein content and increase the carbohydrate content of the food.  Bad! The real goal should be to keep growing pups from becoming OVERWEIGHT.  Restricting fat and protein in a growing pup can be a disaster.  A high quality, meat-based diet is absolutely necessary for growing pups, just don't feed so much of it that the pup becomes overweight.
 

2. Physical Activity - In a young, growing dog with a genotype (genetic makeup) for HD who will eventually develop some trouble because of it, will develop more arthritis and have more eventual difficulty if it is highly active physically.  Climbing stairs, jumping into and out of pick-up trucks, running with other normal dogs can all subject the growing hip structures to unwarranted stress and trauma and increase future discomfort for the dog.  The effects of this excessive activity is worsened in an overweight pup. (In a normal, growing dog, all these activities will not cause hip dysplasia!)
 

3. Bedding - There is no scientific proof,  but lots of observational conclusions, that pups reared especially during the nursing period on slippery surfaces such as newspapers will be prone to hip difficulties.  That is not to say that smooth concrete, wood or newspaper surfaces cause dysplasia, just that they can make a bad situation worse.   Better surfaces for newborn pups would be blankets or towels... something they can get a better grip on.
 

Dogs with hip dysplasia appear to be born with normal hips and then to develop the disease later. This has led to a lot of speculation as to the contributing factors which may be involved with this disease. This is an inherited condition, but not all dogs with the genetic tendency will develop clinical signs and the degree of hip dysplasia which develops does not alway seem to correlate well with expectations based on the parent's condition. Multiple genetic factors are involved and environmental factors also play a role in determining the degree of hip dysplasia. Dogs with no genetic predisposition do not develop hip dysplasia.

At present, the strongest link to contributing factors other than genetic predisposition appears to be to rapid growth and weight gain. It is likely that the laxity in the hip joints is aggravated by the rapid weight gain. If feeding practices are altered to reduce hip dysplasia in a litter of puppies, it is probably best to use a puppy food and feed smaller quantities than to switch to an adult dog food. The calcium/phosphorous to calorie ratios in adult dog food are such that the puppy will usually end up with higher than desired total calcium or phosphorous intake by eating an adult food. This occurs because more of these foods are necessary to meet the caloric needs of puppies, even when feeding to keep the puppy thin.

Once a determination is made that hip dysplasia is present, a treatment plan is necessary. For dogs that exhibit clinical signs at less than a year of age, aggressive treatment may help alleviate later suffering. In the past a surgery known as a pectineal myotomy was advocated but more recent evidence suggests that it is an ineffective surgical procedure. Surgical reconstruction of the hip joint (triple pelvic osteotomy) is helpful if done during the growth stages. For puppies with clinical signs at a young age, this surgery should be strongly considered. It has a high success rate when done at the proper time.

Hip dysplasia may not ever be eliminated by programs designed to detect it early unless some effort is made to publish the results of diagnostic tests openly. This is the only way that breeders will be able to tell for certain what the problems have been with hip dysplasia in a dog's ancestry.