Quantification of Volume Mismatch of Acetabulum and Femoral Head in Developmental Dysplasia of Hip.

Objectives: The sustained subluxation or dislocation of the femoral head over time does not permit normal development of acetabulum and results in predictable pattern of acetabular growth disturbance that is termed hip dysplasia. The primary aim of this study is to analyze and quantify the volume mismatch between acetabulum and femoral head of affected side as compared to normal hip.

Methods: A prospective observational study was conducted by including isolated untreated unilateral idiopathic developmental dysplasia of hip (DDH). After routine clinical and radiographic examination, computed tomography (CT) of both hips was done with pre-determined radiation dosage within safe limits for the pediatric age group in 18 patients of median age 2 years (range 1-5 years).

Results: A significant difference was noted between acetabular index (p<0.001), acetabular volume (p<0.001), femoral head volume (p<0.001), and acetabular anterior sectoral angle (p=0.002) of the affected and the normal hips. As compared to the normal side, the acetabulum is 2.6 times smaller than the normal side and femoral epiphysis volume by 3.8 times. A significant negative correlation (r=-0.66, p=0.04) was noted between posterior acetabular sectoral angle and acetabular volume of affected hip.

Conclusion: CT is an important investigation in evaluation of late-presenting DDH. The absence of femoral head in its orthotopic location affects the volume of acetabulum as well as that of femoral head. The abnormality of the volume of acetabulum which is seen as related to the dysplasia should be studied and assessed in detail in a child of late-presenting DDH. This would guide us toward the coverage defect and type of osteotomy to be performed.