To evaluate the biological characteristics of hip joint in infants with developmental dislocation of the hip (DDH).

From January 2013 to June 2016, 30 patients (age from 1 to 8 months, 25 females and 5 males) who were diagnosed as DDH, underwent ultrasound examination in Shenzhen Children′s Hospital, in these patients 20 cases were treated surgically, and 10 cases were treated with non-surgical treatment. There were 34 hips (26 unilateral dislocation and 4 bilateral dislocation) dislocation in 60 hips. All the patients were confirmed by X-ray, magnetic resonance imaging (MRI) examination or operation. All the patients were examined by ultrasound through the coronal and transverse plane of the hip joint. The α angle, femoral head coverage ratio by acetabulum (FHC), femoral head length and width, distance from pubis to femoral head (P-H) and distance from ischium to femoral head (I-H) were measured. The dislocation joints were compared with contralateral joints.

The α angle in the hip dislocation group was smaller than the contralateral group [(50.5±3.75)^° vs (64.8±3.38) ^°], and there was significant difference between the two groups (t=-15.181, P<0.001). The FHC, femoral head length and width in the hip dislocation group were all smaller than the contralateral group [(23.4±17.63)% vs (64.3±6.45)%, (0.98 ±0.15) cm vs (1.19 ±0.11) cm, (1.38±0.21) cm vs (1.61±0.16) cm ], and there were significant differences between the two groups (t= -12.469, -6.034, -4.568, all P<0.001). The P-H and I-H in the hip dislocation group were larger than the contralateral group [(0.97±0.45) cm vs (0.27±0.05) cm, (0.75±0.30) cm vs (0.17±0.05) cm], and there were significant differences between the two groups (t= 8.805, 10.696, both P<0.001). The α angle, femoral head length and width in bilateral dislocation of hip group were slightly smaller than the unilateral dislocation of hip group [(50.3±2.75)^° vs (51.3±4.77)^°, (0.90 ±0.15) cm vs (0.97 ±0.12) cm, (1.25±0.20) cm vs (1.37 ±0.17) cm], but there were no significant differences between the two groups. The P-H and I-H in bilateral dislocation of hip group were slightly larger than the unilateral dislocation of hip group [(0.97±0.49) cm vs (0.80±0.31) cm, (0.92±0.26) cm vs (0.68±0.18) cm], but there were no significant differences between the two groups. The value of acetabular index in the ultrasound group was slightly larger than the X-ray group [(33.13±7.82)^° vs (31.20±8.31)^°], and there were no significant differences between the two groups.

The characteristics of DDH includes acetabulum and femoral head dysplasia, and femoral head and acetabulum position relationship abnormalities. Quantitative ultrasonography of the hip can be used to quantitatively evaluate the structural features of DDH, and it is helpful to the early diagnosis and follow-up of DDH.