New radiographic standards for age at appearance of the ossification center of the femoral head in Japanese: Appearance at ≤12 months of age is normal in Japanese infants.

BACKGROUND: The ossification center of the femoral head reportedly appears at 7-8 months in 95% infants in the radiographs, but its appearance often delays in Japanese normal infants. The purpose of this study was to survey the age at appearance of the femoral head in the radiographs, evaluate the normal standards of its appearance in Japanese infants, and determine whether the acetabular state affects the time of its appearance. MATERIALS AND METHODS: The patients comprised 436 infants consulted with our institution because of a limitation of abduction in flexion (LA), acetabular dysplasia, or suspected developmental dysplasia of the hip (DDH). Among these patients, 111 infants (222 hips) without radiographic appearance of the femoral head at the first visit were reviewed. The 222 hips were classified into three groups: α angle ≥30° (DDH + group), <30° with LA (DDH-/LA + group), or < 30° without LA (DDH-/LA-group). The age at appearance of the femoral head was investigated in the radiograph in each group. RESULTS: The 50th percentile of the age at appearance of the femoral head was 6 months in the DDH-/LA- and DDH-/LA + groups and 8 months in the DDH + group. The femoral head significantly appeared earlier in the DDH-/LA- and DDH-/LA + groups than in the DDH + group (P < 0.01). The 95th percentile was 12 months in all groups. CONCLUSIONS: Radiographic appearance of the ossification center of the femoral head by 12 months of age is normal in Japanese infants. When the α angle is ≥ 30°, the age at appearance of the femoral head is sometimes delayed.