Hip dysplasia was first described in 1937. It is a disease of complex inheritance. Accordingly, veterinarians and dog breeders have attempted to eliminate CHD through selective breeding strategies. However, the reduction of CHD frequency in pure-breed dogs has been disappointing.
The hip is a ball and socket joint. The "ball" is known as the femoral head. The "socket" part of the joint is known as the acetabulum. The femoral head is attached to the inside of the acetabulum by the round ligament or ligamentum teres and to the periphery of the acetabulum by the joint capsule (see illustration). The joint capsule is a continuous envelope that surrounds the joint from the neck of the femur to pelvic bones around the acetabulum, this closed compartment is filled with a viscous, lubricating fluid called synovial fluid. Both the joint capsule and the synovial fluid play an important role in the stability of the hip joint and ultimately in the development of CHD.
The muscles that cause rotation of the hip (not shown) are also extremely important for hip stability. Each individual muscle develops a force component that pulls the femoral head into the acetabulum. For optimal function and stability of the hip, these forces require complex dynamic coordination and balance.
Dysplasia comes from the Greek words dys, meaning "disordered" or "abnormal", and plassein meaning "to form". The expression hip dysplasia can be interpreted as the abnormal or faulty development of the hip. Abnormal development of the hip causes excessive wear of the joint cartilage during weight bearing, eventually leading to the development of arthritis, often called degenerative joint disease (DJD) or osteoarthritis. The terms DJD, arthritis and osteoarthritis are used interchangeably.
CHD was first described in 1937 by Dr. Gerry B. Schnelle. In a paper entitled Bilateral Congenital Subluxation of the Coxofemoral Joints of a Dog Schnelle writes: "The condition described herein, rare though it may be, should be recognized as being congenital and potentially hereditary, and the dog or bitch in which it occurs should be destroyed or sterilized in the eugenic interests of the breed."
In 1966, Henricson, Norberg and Olsson refined the definition of CHD describing it as: "A varying degree of laxity of the hip joint permitting subluxation during early life, giving rise to varying degrees of shallow acetabulum and flattening of the femoral head, finally inevitably leading to osteoarthritis." Today, the general veterinary consensus is that hip dysplasia is hip joint laxity resulting in osteoarthritis.
As described above, CHD is a developmental disease meaning that it is not present at birth, but develops with age. The series of radiographs below illustrate how a loose hip gradually develops DJD.
|At 6 months, this dog's hips exhibit extreme laxity, but no DJD.|
|At 15 months, laxity is accompanied by the development of "mild" to "moderate" DJD: the femoral heads appear slightly "flattened", the femoral necks are beginning to thicken and the acetabular rims are in the early stages of remodeling.|
|At 6 years, DJD has progressed into a "severe" form, marked by extreme bony remodeling of the acetabular cups and the femoral head and necks.|
An affected dog may have one or any combination of the following clinical signs:
Clinical signs by themselves do not necessarily mean that a dog has hip dysplasia, other conditions of the hip can mimic CHD. A radiograph is essential for a more accurate assessment of the dog's hip joint integrity.
More information about radiographic procedures can be found in the PennHIP Method section.