Orthopedic Surgery
The term dysplasia means "abnormal tissue development". Therefore, Hip Dysplasia means improper growth or development of the coxofemoral (hip) joint. This improper growth is usually characterized by laxity of the hip joints, which allows excessive movement in the hip joint which in turn leads to arthritis and lameness. Hip dysplasia is common in many of the large and giant breeds of dogs but can also affect small dogs and cats.
Canine hip dysplasia is characterized by varying degrees of hip joint laxity (looseness of the hip joint), subluxation (partial dislocation), and ultimately, severe arthritic change. Clinically, the symptoms range from having no clinical signs in some affected dogs to a crippling disease in others. In addition, the severity of the clinical signs does not necessarily correlate with the degree of radiographic (x-ray) or pathologic change seen. Breed and individual differences in temperament may also affect the amount of discomfort an individual exhibits.
Hip dysplasia is a genetically transmitted disease that has been seen in over 82 recognized breeds of dogs. Since first diagnosed in 1935, it has had the highest incidence in the larger breed animals.
Hip dysplasia is an inherited, developmental disease with a polygenic mode of inheritance. This means that there are multiple genes which must be present for this disease to express itself. Thus, the abnormal development of the hip (coxo-femoral) joint requires a combination of multiple genes for each individual to develop the syndrome. Suprisingly, the hip joints of dogs which ultimately develop dysplasia are normal at birth. In most cases dysplasia will occur in both legs (bilaterally). However, in approximately 7% of the cases only one hip will be affected.
One of the earliest programs to reduce hip dysplasia was begun in 1959 by the Swedish Kennel Club. They required that a certificate of normal hips, diagnosed by radiographs, be provided for any German Shepherd to be awarded championship, compete in a best-of-sire class, be given a special prize for working dogs, or if imported, to be registered in Sweden. Breeders were thus encouraged to breed only dogs found to be free of hip dysplasia.
Unfortunately, the incidence of hip dysplasia in Swedish German Shepherds did not decline as rapidly as had been hoped when the scheme was put into operation. In fact, ten years of selective breeding failed to reduce the number of moderate and severe cases. The common belief that, if no dysplastic dogs became parents, the defect would then disappear was simply not true. Two radiographically sound dogs may still produce dysplastic offspring if the genes responsible for this disease are present.
In one study, a 37.5% incidence of dysplastic puppies from normal parents was observed. This indicates that the genes tending to induce hip dysplasia are widespread in certain breeds, and that radiographs showing normal hips, although highly desirable, are no guarantee of normal hips in offspring. Similarly, parents that are superior with respect to any character (champions, perhaps) are more likely to produce superior progeny than are parents less fortunate, but they can not be counted on to do so.
Several secondary factors also influence the development of dysplasia. These include body size, body conformation and growth patterns. When the genetic potential for dysplasia is present, feeding a high calorie, high protein diet which produces rapid weight gain will increase the incidence and severity of the disease.
The degree of hind leg muscle mass is also related to the prevalence of dysplasia. In German Shepherd dogs, the greater the amount of musculature of the rear limbs, the lower the incidence of dysplasia. A well-balanced, muscular support is necessary to maintain proper joint alignment. Generalized or specific weakness of the hip muscles can lead to adverse changes in the developing hip joint.
The incongruency between muscular support and the rapid rate of skeletal growth means that if the muscles of the hind limbs fail to develop and reach functional maturity at the same rate as the bones, joint instability may result. This eventually leads to degenerative arthritis. This theory is supported by the fact that Greyhounds, with tremendous muscular support for a fine, bony skeleton, rarely have hip dysplasia.
