Morphological Development of the Hip in Normal Infants Under Six Months of Age by the Graf Ultrasound Method

ObjectiveThis large-sample observational study aims to analyze the morphological development of the hip joint in Chinese normal infants under 6 months of age by the Graf ultrasound method. MethodsThe clinical and ultrasound data of infants who underwent early screening for developmental dysplasia of the hip (DDH) in the authors' clinic from January 2011 to December 2019 were analyzed retrospectively. The standard Graf method was used to measure the hip joint alpha angle, beta angle and femoral head coverage (FHC). The infants with Graf type I or IIa hips were included in this study. All infants were grouped by age. FHC, alpha and beta angles were compared among different study groups. ResultsA total of 3,067 infants (6,134 hips) were included in the study. There were 1,164 males and 1,903 females with an average age of 77 days (1-180 days). The mean alpha angle was 62.4 +/- 3.6 degrees on the left and 63.2 +/- 3.5 degrees on the right (P < 0.001). The mean beta angle was 55.6 +/- 4.5 degrees on the left and 54.8 +/- 4.5 degrees on the right (P < 0.001). The average FHC was 54.2 +/- 4.6% and 54.8 +/- 4.2%, accordingly (P < 0.001). The alpha angle and FHC of females was significantly smaller than that of males (P < 0.001). While the left beta angle in females was slightly larger than males (P = 0.014), there were no significant differences in the right beta angle between the two sexes (P = 0.150). During the first 3 postnatal months, the alpha angle and FHC increased while beta angle decreased with age. However, the alpha and beta angles and FHC were stable at a relatively constant level from the 4 to the 6(th) postnatal month. ConclusionsThe normal infant hip reveals progressive maturation during the first 3 months after birth, and then enters a plateau period during the 4 to 6(th) month. The development of hip joint in females and of the left side slightly lags behind that in males and of the right side, which is consistent with the observation that DDH is more common in females and on the left hips.