Analysis of the vacuum phenomenon in plain hip radiographs in children

摘要

Objectives: To find the accurate incidence of the vacuum phenomenon in the hip and the best projection position for producing the vacuum phenomenon in plain hip radiographs in children. Methods: All pediatric anteroposterior (AP)-view and frog-leg-position plain hip joint radiographs obtained in our hospital between January 2003 and March 2013 were examined. The subjects’ ages ranged between 0 and 14 years (mean, 4.2 years). All of the plain radiographs showing crescent-, linear-, and irregular-shaped lucencies between the femoral head and acetabulum were included in the present study. Results: A total of 16,749 cases, including 12,422 cases (5,912 boys and 6,510 girls) with only AP-view plain radiographs and 4,327 cases (1,537 boys and 2,790 girls) with both AP-view and frog-leg-position plain radiographs that were assessed in our hospital between January 2003 and March 2013, were examined. None of the AP-view plain hip radiographs exhibited the vacuum phenomenon. Vacuum phenomenon of the hips was found in only 258 cases (321 hips) in the frog-leg-position plain radiographs of 4,327 cases, resulting in a constituent ratio of 5.96% (258/4327). A total of 1,738 normal children were assessed in the 4,327 frog-leg-position radiographs, and 150 cases of the vacuum phenomenon were found in the normal children; thus, the incidence of the vacuum phenomenon in normal children was 8.63% (150/1,738). In 2,360 children with developmental dysplasia of the hip (DDH) who were assessed in the 4,327 frog-leg-position radiographs, 98 cases of vacuum phenomenon were found, yielding an incidence of 4.15% in children with DDH (98/2,360). Thus, the 258 cases with vacuum phenomenon included 150 normal hips (58.14%), 98 cases with DDH (37.98%), 5 cases with Legg-Calvé-Perthes disease (1.94%), and 1 case each of solitary eosinophilic granuloma in the left ischium (0.39%), polyostotic fibrous dysplasia of the left and right proximal femurs (0.39%), 1 case of hereditary multiple exostoses (0.39%), 1 case of congenital coxa vara (0.39%), and 1 fracture of the femoral neck after surgery (0.39%). The 321 hips in the 258 cases were classified as complete (121 hips, 37.69%) or partial (200 hips, 62.31%) types according to the proportion of the lucency area in the hip joints and as linear (159 hips, 49.53%), crescent (151 hips, 47.04%), or irregular (11 hips, 3.43%) lucencies according to the shape of the lucency area in the hips. Conclusions: The vacuum phenomenon of the hip in children is found in frog-leg-position plain radiographs. It is easier to find the vacuum phenomenon in normal hips compared to cases with DDH. Frog-leg-position plain radiographs provide a better projection position for obtaining the vacuum phenomenon of the hip in children compared to AP-view plain radiographs.