DO SPINOPELVIC CHARACTERISTICS NORMALIZE POST-TOTAL HIP ARTHROPLASTY? A PROSPECTIVE, CASE-CONTROL, COMPARATIVE STUDY USING 3D MOTION CAPTURE ANALYSIS

Study of the hip-spine interaction has received a lot of attention in total hip arthroplasty (THA), as adverse spinopelvic characteristics can influence surgical outcomes (e.g. dislocation risk). Surgeons have been advised to consider individual characteristics as part of pre-operative planning. In the presence of hip osteoarthritis, the stiff hip leads to reduced movement and associated compensatory movements in the pelvis and spine. These compensatory mechanisms have been suggested to resolve post-THA. However, to date, evidence is limited because i) these mechanisms have only been studied quasi-statically (i.e. radiographic assessments performed in different positions); and ii) only the sagittal plane has been studied. Study of these mechanisms via motion analysis allows for dynamic assessment during different activities and is a validated methodology. The aim of this study was to determine whether, and to what extent hip, pelvic and spinal ranges of motion (ROM) normalize following THA surgery, during tasks of daily living. This was a prospective, IRB-approved, case-control study. Fifteen patients (65.7±6.8yrs, M=7, F=8, BMI=26.7±3.7kg/m2) who received THA for the treatment of hip osteoarthritis, underwent 3D motion-capture analysis pre-operatively and at one-year post-THA. Controls (47.5±16.5yrs, M=4, F=5, BMI=23.2±2.8kg/m2) were gender-matched healthy volunteers (health workers). All participants performed a seated bend and reach (SBR) task and a seated trunk rotation (STR) task. A full-body, custom set of 45 markers was used, which allowed kinematic modeling of the pelvic segment, and hip and spinal joints. The SBR task was used to determine sagittal-plane (flexion/extension) kinematics. The STR task was used to determine transverse-plane (rotation) kinematics. Comparisons were made pre- and post-operatively (paired T-tests), and between the patient and control group (independent T-tests), for both tasks. The patient group exhibited significant deficits in sagittal-plane movement pre-THA during the SBR; pelvic tilt by 12.6° (p=0.010); hip flexion by 14.3° (p=0.003), and spinal flexion by 8.7° (p=0.079). Amongst patients, post-THA, pelvic (10.9°,p=0.010), hip (9.4°,p=0.011), and spinal (7.7°,p=0.002) ROM significantly increased. As a result, the differences detected pre-operatively between patients and controls, were no longer present post-THA, with similar sagittal-plane maximal ranges exhibited by both groups. The patient group displayed significantly less transverse plane motion pre-THA during STR compared to controls; 7.9° less pelvic rotation. Dynamic spinal, pelvic and hip characteristics significantly change post-THA. These changes occur in all three planes and influence/improve dynamics during activities of daily living. As a result, incorporation of these characteristics into pre-operative planning is likely to be sub-optimal. We would thus recommend considering individual, static characteristics into pre-operative planning, which takes into consideration age and sex and more reliably assesses sagittal posture and dynamics.