All experiments of the current study were conducted under the approval of Chang Gung Memorial Hospital Institutional Review Board (IRB No. 201601982B0C501). All the experiments and procedures conformed to the Ethical Principles for Medical Research Involving Human Subjects (World Medical Association Declaration of Helsinki). Twenty patients (DDH group; 20 females; age: 7.1 ± 1.9 years; height: 119.7 ± 13.7 cm; mass: 22.5 ± 5.5 kg; leg length discrepancy: 0.5 ± 0.3 cm) who had been treated for unilateral developmental dislocation of the hip by reduction surgery at an age of 2.5 ± 1.7 years participated in the current study (Table 1). Written informed consents were obtained from the participants and their parents or guardians as approved by the Institutional Review Board. Of the 20 subjects in the DDH group, two had been treated with closed reduction, two with open reduction, and 16 had been treated by open reduction with Pemberton’s osteotomy according to the severity. All subjects were able to walk without support, and were free of pain and infection of the hip or any other neuromuscular diseases that might influence ambulation. Follow-up radiographs showed that frontal-plane acetabular coverage was within normal range at the time of the gait experiment in all the subjects. For each subject, the neck-shaft angle (NSA), acetabular index (AI), femoral offset, center-edge angle (CEA), acetabular depth ratio, articulotrochanteric distance (ATD), c/b ratio, and Alsberg angle (AA) were measured from anteroposterior X-ray images of the pelvis for both sides at the time of the gait experiment (Fig. 1 and Table 1). Participants were excluded from the study if they had other neuromusculoskeletal diseases or neurological pathology that might affect gait. Thirteen healthy children (10 girls and 3 boys; age: 7.8 ± 1.88 years; height: 125.8 ± 12.1 cm; mass: 26.6 ± 6.2 kg; leg length discrepancy: 0.3 ± 0.4 cm) were also recruited with the same consent procedure and served as the control group matching with the DDH group without significant between-group differences in sex, age, height and body weight (BW). An a priori power analysis based on pilot results using G*POWER 3 16 determined that five subjects for each group would yield a power of 0.8 at a significance level of 0.05. Thus, the number of subjects for each group was more than adequate for the main objectives of the current study.
Table 1
Means (standard deviations) of the demographic characteristics of the juvenile patients treated for unilateral developmental dysplasia of the hip (DDH) during toddlerhood.
| Affected hip | Unaffected hip | p-Value |
---|
Offset (mm) | 21.5 (8.2) | 22.3 (5.4) | 0.501 |
Neck-shaft angle (°) | 144.5 (11.1) | 143.7 (9.5) | 0.781 |
Acetabular index (°) | 18.8 (8.0) | 18.7 (6.0) | 0.939 |
Center-edge angle (°) | 21.8 (11.1) | 21.0 (9.4) | 0.792 |
Acetabular depth ratio (mm/mm) | 0.24 (0.05) | 0.26 (0.04) | 0.196 |
Articulotrochanteric distance (mm) | 19.8 (5.2) | 18.9 (5.5) | 0.470 |
c/b ratio (mm/mm) | 0.67 (0.05) | 0.68 (0.05) | 0.579 |
Alsberg angle (°) | 70.9 (10.1) | 68.5 (10.2) | 0.379 |
Age at gait experiment (years) | 6.7 (2.3) | - |
Age at surgery (years) | 2.3 (1.5) | - |
Body mass (kg) | 24.3 (8.1) | - |
Body height (cm) | 117.6 (16.2) | - |
Body mass index | 17.4 (3.5) | - |
Leg length discrepancy (cm) | 0.5 (0.27) | - |
In a hospital gait laboratory, each subject walked on a 10-m walkway at their preferred gait speed several times before data collection. To track the segmental motions of the pelvis-leg apparatus, light-weight retroreflective markers were attached to the anterior and posterior superior iliac spines, greater trochanters, mid-thighs, medial and lateral femoral epicondyles, fibular heads, tibial tuberosities, medial and lateral malleoli, navicular tuberosities, 5th metatarsal bases, big toes and heels 17. Three-dimensional trajectories of the markers were measured using a 7-camera motion analysis system (MX T-40, Vicon Motion Systems Ltd., UK) and the ground reaction forces (GRF) were measured using three forceplates (AMTI, USA). At least six successful trials, each with a complete gait cycle for each limb, were obtained for each subject for subsequent analysis.
From the measured marker and forceplate data, the angular motions and resultant forces at the lower limb joints were calculated using inverse dynamics analysis with the pelvis-leg apparatus modelled as a 7-rigid-link system 15; 18–21. Before the analysis, the measured GRF and marker data were low-pass filtered with a 4th order Butterworth filter with cut-off frequencies of 25 Hz and 10 Hz, respectively. Each body link was embedded with a local coordinate system with the positive x-, y- and z-axes directed anteriorly, superiorly and to the right, respectively 22 and a z-x-y Cardanic rotation sequence was used to calculate the joint angles 23. The center of rotation of the hip was estimated using a functional method 24, and those of the knee and ankle were defined as the mid-points between the lateral and medial femoral epicondyles and between the lateral and medial malleoli, respectively 13. Body segmental inertial properties needed for joint force calculations were obtained using an optimization-based method 25 and the effects of soft tissue artifacts were reduced using a global optimization method 26. The axial components of the joint forces were calculated by projecting the resultant forces onto the long axis of the distal segment. Loading rates of the vertical GRF and the joint axial forces during initial contact and loading response phases, and their unloading rates during the pre-swing phase, were obtained as the maximum instantaneous slope of these force curves, calculated by finding the first derivatives of the spline curves fitted to the data using GCVSPL 19; 27. All the force-related variables were normalized to body weight. Loading rate variables were averaged between left and right limbs for the healthy control group. Temporospatial gait parameters, namely stride length, stride time, step length, step width, cadence, and walking speed were also obtained.
Each of the calculated variables between affected and unaffected sides was compared using paired t-tests. Comparisons between Affected vs. Control and Unaffected vs. Control were performed using independent t-tests. The associations between each of the radiographic measurements, and the peak loading and unloading rates of the vertical GRFs, as well as the joint axial forces for both the affected and unaffected sides were obtained using Pearson’s correlation analysis. A correlation coefficient of 0.7–0.9 indicated strong correlation, 0.4–0.6 moderate, and 0.1–0.3 weak correlation. All significance levels were set at α = 0.05. All statistical analyses were performed using SPSS 20.0 (SPSS, IBM, Armonk, New York, USA).