The Risk of Developing DJD
For more than 40 years, it has been empirically accepted that hip joint laxity is related to the development of DJD. However, prior to the research conducted at the University of Pennsylvania, there existed little or no scientific evidence to support this view. The problem was approached in two ways: First, the relationship of hip joint laxity with the coexistence of DJD in a cross-section of adult dogs was examined (cross-sectional study). Next, the relationship of laxity at an early age with the appearance of DJD at a later time was investigated (longitudinal study). The results are summarized below.
Cross-sectional Study
An analysis of 142 dogs (mean age of 20 months) showed a direct relationship of hip laxity (as determined by the DI) to the radiographic existence of DJD. Hips with low DI's, i.e. "tight hips", were very unlikely to exhibit DJD. In this study only one hip with a DI less than 0.30 exhibited any evidence of DJD (DI = 0.29). The converse was not true; that is, not all hips with a DI greater than 0.30 necessarily showed radiographic evidence of DJD. However, as the DI increased, there was an increase in the frequency of DJD. (see the graph below)
It is interesting to note that more than 50% of the hips in this study had distraction indices below 0.30, yet only one hip in this group < 0.30 showed any radiographic evidence of DJD. There appears to be a cut-off point - DI approximately 0.30 - below which the canine hips is not susceptible to getting DJD.
Longitudinal Study
In this study, dogs were radiographed at 4 months, 12 months, 24 months of age. A logistic regression model was invoked to determine the contribution of factors such as DI, Norberg Angle (NA), subjective score (OFA), weight and sex at 4, 12 or 24 months to the risk of developing DJD at or before 3 years. The analysis indicated that the DI at all age groups was the most significant prognostic factor and that the strength of the predictive power improved with age. The sex, weight, NA and subjective (OFA) score were not found to be significant factors in this study.
The graph at left is from a study involving German Shepherd dogs. It demonstrates how the DI at a given age is associated with the probability for developing DJD later on. The DI can be thought of as a risk factor for the development of DJD. Tight-hipped dogs (those with small DI's) are at a low risk and loose-hipped dog's (those with high DI's) are at a high risk. In other words, the larger the DI (at 4 months), the greater the risk for developing DJD by three years of age.
The results represented in this graph are for German Shepherds. For example, a separate probability curve exists for Rottweilers. However, regardless of the breed, the "tight hip / loose hip" susceptibility to DJD still holds.
Note: The DI as a risk factor for the development of DJD is analogous to the association of serum cholesterol with the risk of developing heart disease. The higher a person's cholesterol level, the greater is the risk that the person will develop heart disease. Not all individuals, however, with high cholesterol level will necessarily have heart disease. Yet to be on the safe side most people would choose to have low rather than high serum cholesterol levels because the odds are in their favor that low cholesterol means better cardiovascular health. Similarly tighter hips equate to greater DJD resistance.