Participants
Twelve women who had undergone TIO and eight healthy women in the same age range participated in this study. The indication for TIO was symptomatic hip dysplasia with a spherical femoral head, and without major damage to the cartilage [15]. All patients were operated in the Sint Maartenskliniek, Nijmegen, The Netherlands by the same experienced surgeon. The surgical procedure (modified Tönnis osteotomy) has been described in detail [20]. Patients between 18 and 70 years who had undergone a unilateral TIO between January 2003 and June 2010 were eligible for inclusion. This time window ensured that rehabilitation had been completed. Rehabilitation consisted of regular mobilization of the hip joint and strengthening exercises for the muscles of the hip region, and lasted for 3–12 months, depending on the patient’s individual speed of recovery. In total, 54 patients were screened, of whom 34 were eligible for inclusion. Of those, 18 patients were willing to participate, and 12 were included after screening for exclusion criteria. We excluded patients that had any disease or other condition that could affect their gait, including severe hip pain, as well as patients with a Body Mass Index (BMI) > 30 kg/m2 because of difficulties in marker placement for gait analysis. Selection was unspecific regarding gender, but all 12 included patients were women.
The study procedure was approved by the local ethical committee of the region Arnhem-Nijmegen, The Netherlands (study code 2012/065). A written informed consent was obtained from each subject.
Clinical assessment
The participants were invited to the gait laboratory of the Radboud university medical center, Nijmegen, The Netherlands for a combined gait and muscle strength assessment session. We also obtained the Oxford [21,22] (range 0–48) and Harris Hip Score [23] (range 0–100) from the patients during a break in the session.
Gait analysis
The participants walked barefoot on an 8 m long walkway at self-selected comfortable walking speed. An integrated data collection was performed including three-dimensional motion capture with synchronized force plate recordings. A six-camera digital optical motion capture system (Vicon MX, Oxford, UK) was used to record the position of 35 retro-reflective markers placed on the lower limb and torso (100 Hz). The standard Vicon Plug-in-Gait marker set was used, with additional markers placed on the anterior side of the thigh and lower leg at 1/3 and 2/3 segment length, and on the fifth metatarsal head of the foot. Two custom-made force plates (AMTI, Watertown, MA, USA), embedded level in the laboratory floor measured ground reaction forces (1000 Hz) during the stance phase of the gait cycle.
No specific instructions were given other than ‘walk naturally’ to prevent participants from targeting the force plates. Trials were repeated until six successful trials had been recorded, where ‘successful’ was defined as a trial in which each foot cleanly struck one of the two force plates. The gait analysis data of one patient and one control subject had to be discarded due to technical problems, leaving a group of 11 patients and 7 controls available for the gait analysis part of the study.
Data analysis
Heel strike and toe off events were identified using thresholding of ground reaction force data (heel contact when F > 20 N, toe off when F < 20 N). Spatiotemporal parameters were subsequently calculated based on a combination of these events and the heel and toe marker position data.
A 21 degrees of freedom kinematic model (‘GaitLowerExtremityModel’, as available in the AnyBody Managed Model Repository 1.5.1) consisting of trunk, pelvis, thigh, shank, talus and foot segments was scaled to each subject based on the marker trajectories using the AnyBody Modeling System (version 5.3.1, AnyBody Technology A/S, Aalborg, Denmark). The marker trajectories were initially filtered with a 5 Hz 2nd order Butterworth low-pass filter. Force plate data were low-pass filtered at 12 Hz with a 2nd order Butterworth filter. The model was based on the kinematic part of the Twente Lower Extremity Model dataset [24]. After this procedure, lower limb joint angles were obtained by solving the inverse kinematics using the optimized parameters. At each joint, ideal torque generators were added. The segment masses were scaled using common scaling laws [25], available in AnyBody. The model marker positions, segment lengths and knee joint axes were then optimized using a parameter optimization algorithm [26]. Finally, the kinematics and ground reaction forces were used as input for an inverse dynamic analysis [27], in which joint moments were calculated in the local (ISB [28]) reference frames.
Our outcome variables of interest were spatiotemporal parameters, hip joint angles and moments in the sagittal and frontal planes. Each of these variables was averaged across trials to obtain subject ‘ensemble’ averages both for the operated and the non-operated limbs. The moments were normalized to the body weight (BW) and height (Ht) of the subject (%BW*Ht). In the sagittal plane, we determined the peak flexion and extension angles and moments. In the frontal plane, we used the adduction angle at 20% of the gait cycle in the analysis, since there was a peak in the angle at that particular phase of the gait cycle. Owing to the M-shaped curve of hip abduction moments during gait, we extracted two peak values, one in the first and one in the second half of the stance phase. For the healthy controls, we used the mean of the left and right limbs in the analyses.
Muscle strength
Isometric maximum voluntary contractions were recorded for hip abduction, and knee flexion and extension. Hip abduction strength was tested in side-lying position, with the tested hip at 0° flexion and 0° adduction, and the knee extended (Figure 1A). The non-tested hip was flexed at 45°, and the knee was flexed at 90° in order to prevent the contralateral limb from contributing to the maximum strength effort. The end piece (soft Velcro strap) of a force transducer was applied just proximal to the femoral epicondyles perpendicular to the limb, and the other end of the transducer was rigidly attached to the testing bench.
For maximum knee flexion strength testing, subjects sat on the edge of a testing bench positioned close to a wall (Figure 1B). The knee and hip were flexed at 90°, and the end piece of the force transducer was applied just proximal to the malleoli. The other end was rigidly fixed to the wall, level with the end piece. During the contractions, manual pressure was applied by an assistant proximal to the top of the tested knee to prevent it from rising. Knee extension was tested in a custom-built chair with vertical backrest, with the knee and hip flexed at 90°, and with the end piece of the force transducer applied just proximal to the malleoli (Figure 1C). The subject was strapped to the chair to prevent the waist from rising.
Participants performed three maximum isometric contractions for four seconds each, separated by 30 seconds of rest. All subjects received the same verbal encouragements during the contractions to achieve maximum effort. The highest recorded maximal force was multiplied by the moment arm to the joint centre of the tested joint to obtain peak torque values. Raw data was low-pass filtered at 6 Hz with a 2nd order Butterworth filter and the peak torque was normalized by body weight.
Statistical analysis
Student’s t-tests were used for comparing group characteristics, spatiotemporal parameters, kinematics and muscle strength tests between the patient and control groups. Paired samples t-tests were used for comparisons between operated and non-operated limbs. Hip joint moments during gait were tested with repeated-measures ANOVAs; one with phase of gait cycle (early and late stance) and limb (operated and non-operated) as within-subjects factors, and another with phase of gait cycle (early and late stance) as within-subjects factor and group (operated limb of patients and controls) as between-subjects factor. Post-hoc paired samples t-tests were used if the ANOVA indicated significant differences between the operated and the non-operated limb. The significance level was set at P ≤ 0.05. For the muscle strength data, significance was set at P ≤ 0.01 because of the number of t-tests performed (12). IBM SPSS Statistics 20.0.0.1 was used for all statistical analyses.