Sonographic diagnosis of developmental dysplasia of the hip: importance of increased thickness of acetabular cartilage.

OBJECTIVE

Dynamic and static sonography has become the imaging technique of choice in the diagnosis and follow-up of developmental dysplasia of the hip. The purpose of this study was to determine the value of using the thickness of acetabular cartilage as measured on sonograms as a discriminating factor in the diagnosis of developmental dysplasia.

SUBJECTS AND METHODS

We evaluated 220 consecutive hips in 110 infants referred for sonography because of possible developmental dysplasia (referred group). We also evaluated 66 hips in 33 infants who had normal findings on physical examination and no clinical or sonographic evidence of the abnormality (control group). The thickness of the acetabular cartilage, the alpha angle, and the presence of subluxation or dislocation on sonograms were recorded. Follow-up clinical and sonographic examinations were performed between 6 weeks and 6 months after the initial examinations in 19 patients in whom the thickness of the acetabular cartilage was increased (> 3.5 mm) and in 20 patients in whom the thickness was in the normal range (< 3.5 mm). All 39 patients who had follow-up examinations were from the referred group, and all these hips were normal according to conventional sonographic criteria on the initial examination.

RESULTS

Of the 220 hips in the referred group, 170 were initially classified as normal according to traditional sonographic standards, and 50 were classified as dysplastic (alpha angle < 55 degrees with or without subluxation or dislocation). The mean thickness of the acetabular cartilage in the control group was 2.6 mm (1 SD = 0.37) as compared with 4.6 mm (1 SD = 0.71) for the dysplastic hips in the referred group. The mean thickness in the hips classified as normal in the referred group was 3.0 mm. Nineteen hips in the referred group in which the thickness of the acetabular cartilage was greater than in the control group (i.e., > 3.4 mm) were imaged again during follow-up examinations. Sixteen of these 19 hips had become clinically unstable or dysplastic according to conventional sonographic criteria.

CONCLUSION

The results suggest that sonographic evidence of increased thickness of the acetabular cartilage is an early sign of developmental dysplasia of the hip and that close follow-up of infants with this abnormality is indicated.