OBJECTIVETo investigate characteristics and the differences of the anatomical parameters of the proximal femur of the developmental dysplasia of the hip (DDH).

METHODSA total of 38 patients(47 hips) diagnosed as DDH with CT scan data and the pelvis radiograph from January 2012 to December 2014 in China-Japan Union Hospital of Jilin University were retrospectively analyzed. All the hips were divided into 3 groups according to Crowe classification method. Thirty normal hips were selected as controls who admitted at the same time. CT data of the patients were imported into Mimics 17.0. The three-dimensional models of the proximal femur were then reconstructed, and the following parameters were measured: neck-shaft angle, neck length, offset, height of the centre of femoral head, height of the isthmus, height of greater trochanter, the medullary canal diameter of isthmus (Di), the medullary canal diameter 10 mm above the apex of the lesser trochanter (DT+ 10), the medullary canal diameter 20 mm below the apex of the lesser trochanter (DT-20), and then DT+ 10/Di, DT-20/Di and DT+ 10/DT-20 were calculated.Variance discrepancy analysis was used to compare the difference among the four groups, and LSD method was used to compare the difference between either two groups.

RESULTSThe parameters of neck-shaft angle of DDH with Crowe I, Crowe II-III, Crowe IV and the control group were (131.8°±7.1°), (131.7°±6.5°), (122.8°±11.4°) and (131.8°±5.9°), respectively; the parameters of neck-shaft angle of DDH with Crowe IV was smaller than that of DDH with Crowe I, Crowe II-III and control group (all P<0.05). The parameters of the neck length of DDH with Crowe IV ((44.6±6.6) mm) was smaller than that of DDH with Crowe I ((48.6±6.7) mm), Crowe II-III ((50.4±4.7) mm) (all P<0.05). There is no statistic difference in the offset among the groups (F=2.392, P>0.05). The parameters of the height of greater trochanter of DDH with Crowe IV ((12.1±6.1) mm) was bigger than that of DDH with Crowe I ((8.9±7.2) mm), Crowe II-III ((7.5±3.3) mm) and control group ((6.1±3.9) mm) (all P<0.05). The parameters of the height of the centre of femoral head of DDH with Crowe I, Crowe II-III, Crowe IV were (39.6±6.5) mm, (39.1±4.2) mm, (38.8±8.6) mm, which were smaller than that of the control group ((46.5±6.2) mm) (all P<0.05). The parameters of Di of DDH with Crowe I, Crowe II-III, Crowe IV and the control group were (9.9±2.2) mm, (8.3±1.8) mm, (8.7±1.7) mm and (10.1±1.4) mm; the parameters of Di of DDH with Crowe II-III and Crowe IV were smaller than that of the control group (all P<0.05). The parameters of DT+ 10 ((17.2±5.3) mm) and DT-20 ((12.2±3.0) mm) of DDH with Crowe IV were smaller than that of DDH with Crowe I ((25.2±3.4) mm, (17.1±2.3) mm) and Crowe II-III ((21.9±4.2) mm, (16.3±3.2) mm) (all P<0.05). The parameter of the height of the isthmus of DDH with Crowe IV ((94.1±19.7) mm) was smaller than that of DDH with Crowe I ((106.2±13.8) mm), Crowe II-III ((108.8±10.5) mm) and control group ((116.5±10.6) mm), respectively (P=0.010, 0.008, 0.000). The parameters of DT+ 10/Di (2.0±0.4) and DT-20/Di (1.4±0.2) of DDH with Crowe IV were smaller than that of DDH with Crowe I (2.6±0.5, 1.8±0.3), Crowe II-III (2.7±0.60, 1.9±0.3) (all P<0.05).

CONCLUSIONSComparing to DDH with Crowe I-III and control group, DDH with Crowe IV has a dramatic change in the intramedullary and extramedullary parameters. The isthmus and the great trochanter are higher and there is apparent narrowing of the medullary canal around the level of the lesser trochanter.