Pathogenesis of Canine Hip Dysplasia

A recently invented diagnostic method for canine hip dysplasia (1) was tested in controlled studies and found to be accurate at 8-months of age. The dorsolateral subluxation of femoral heads (DLS) test was devised to evaluate displacement of femoral heads from acetabula when a hip is radiographed in a load-bearing position (Fig. 1).

Hip Dysplasia
 Figure 1: Illustration of a tranquilized dog in the sternal-abdominal position on a foam rubber matt for the DLS test. The stifles and the ankles are bound with tape. Care is taken to check for symmetry as viewed from the top and side. Careful initial positioning is needed to avoid superimposition of the stifles over the hip joints on the radiographic projection. The pad is about 5 inches high for many dogs; a 4 inch or 3 inch thick pad may be used for smaller dogs to ensure weight bearing on the stifles. The dotted line with the arrow head represents the direction of the X-ray beam during the dorsoventral radiographic procedure

The test is done on anesthetized or tranquilized dogs positioned on their abdomen in a kneeling position in a foam rubber pad so that body weight is transferred to the bent stifles on the tabletop. A DLS score is calculated as the percent of the femoral head covered by the acetabulum on a dorsoventral radiograph of hips (Fig. 2).

DLS Score

 Figure 2: Illustration showing how the DLS score is calculated from a dorsoventral radiograph. A straight horizontal line is drawn between the acetabular lateral margins. A perpendicular line is dropped from this line at the inside edge of the femoral head and from the lateral edge of the acetabulum. The distance (d) between these two perpendicular lines is measured in millimeters. The DLS score expressed as percent is determined by dividing "d" by the diameter of the femoral head (q, in mm) and multiplying by 100. The DLS score represents the percent of the femoral head covered by the acetabular rim.

In two published reports (1, 2) dysplastic joints had a DLS score of 40% or less, whereas normal hip joints had scores greater than 60% (see Figs. 3 and 4).

Canine Xray

Canine Xray

Figure 3: A photograph of a dorsoventral radiograph of the position used for the DLS test. The DLS score is determined on clear plastic overlays with the radiograph. This dog had a DLS score of 40% for the left hip and 34% for the right hip. "L" marks the left side of this dysplastic dog as viewed from above the dog

Figure 4: A photograph of a dorsoventral radiograph of the weight bearing position used for the DLS test. The DLS score is determined on a clear plastic overlay with the radiograph. This dog had a DLS score of 63% for the left hip and 66% for the right hip. "L" designates the left side of the disease-free dog as viewed from above the dog.

We examined dogs at 8-months of age and related the DLS score to the appearance of osteoarthritis (OA) at a later age (3). The presence of hip OA is an outcome assessment of the dysplastic trait. Eight months was the preferred age since the DLS score was stable by age 8-months, and 8-months might be early enough for decision making for many breeding programs. Data suggested that the likelihood ratio for developing an osteoarthritic hip joint for a DLS score > 55% = 0.2, DLS score 45-55% = 2.6, and DLS score <45% = 8.0. Thus the likelihood ratios for the presence of OA for the three DLS categories varied by a factor of 40 (3). We have proposed that a dog with a DLS score of less than 55% at 8 months of age has the dysplastic trait, In a follow-up study it was substantiated that dogs with DLS > 55% at 8-months had normal hip joints on radiographs at two years of age; 67% of dogs with DLS <50% at 8-months had hip OA on radiographs at two years of age.

We compared the DLS score and the PennHipTM distraction index (DI) at 8-months of age to predict hip OA (4). Less than 40% of the variation in DLS score was accounted for by DI; r2 = 0.36. A logistic multiple regression analysis was done on the data and suggested that the DLS score had a better odds ratio for OA joints to normal joints by a factor greater than two. The DLS score was more accurate (had better sensitivity) than the DI for identifying hip OA, since the DLS score gave low percentages of false-negative tests in the abnormal dogs. Accuracies for predicting disease-free hip joints were similar for the two methods (similar specificities) since both DLS score and DI gave low percentages of false-positive tests in normal dogs (4).

Taken together our results led us to conclude that the DLS score at 8-months of age was a good predictor of hip osteoarthritis. Since both sensitivity and specificity were high, the DLS score might have broad application for identification of both disease-free and osteoarthritic hips. The DLS score is useful at 8 months of age for programs selecting for either hip dysplastic or normal dogs.

Literature References

1) Farese, J.P., Todhunter, R.J., Lust, G., Williams, A.J., and Dykes, N.L. 1998. Dorsolateral subluxation of hip joints in dogs measured in a weight-bearing position with radiography and computed tomography. Vet. Surg. 27:393-405.

2) Farese, J.P., Lust, G., Williams, A.J., Dykes, N.L., Todhunter, R.J. 1999. Comparison of measurements of dorsolateral subluxation of the femoral head and maximal passive laxity for evaluation of the coxofemoral joint in dogs. Am. J. Vet. Res. 60:1571-1576.

3) Lust, G., Todhunter, R.J., Erb, H.N., Dykes, N.L., Williams, A.J., Burton-Wurster, N., and Farese, J.P. 2001. Repeatability of dorsolateral subluxation scores in dogs and correlation with macroscopic appearance of hip osteoarthritis. Am. J. Vet. Res. 62:1711-1715.

4) Lust, G., Todhunter, R.J., Erb, H.N., Dykes, N.L., Williams, A.J., Burton-Wurster, N., and Farese, J.P. 2001. Comparison of three radiographic methods for diagnosis of hip dysplasia in 8-month-old dogs. Jour. Am. Vet. Med. Assoc. 219:1242-1246.