Abstract
Introduction
Total hip replacement (THR) patients are often considered a homogenous group whereas in reality, patients are heterogeneous. Variation in revision rates between patient groups suggest that implants are exposed to different environmental conditions in different patients [1]. Previous reports suggest that for every unit increase of BMI, there is a 2% increased risk of revision of a THR [2]. The aim of this study was to better understand the effect of patient-specific characteristics such as BMI on hip motions and to explore the possible impact on wear.
Methods
137 THR patients, at least 12 months post-surgery, underwent 3D kinematic (Vicon, Oxford, UK) and kinetic (AMTI, USA) analysis whilst walking at self-selected walking speed. 3D kinematic data were then mapped onto a modelled femoral cup at 20 pre-determined points to create pathways for femoral head contact, which were then quantified by deriving the aspect ratio (AR). Patients were stratified into three groups determined by BMI scores; healthy weight (BMI ≤25 kg/m2) (n=34); overweight (BMI >25kg/m2 to ≤ 30 kg/m2) (n=66) and obese patients (BMI > 30 kg/m2) (n=37). Comparisons were made using 95% confidence intervals (CI) and one way ANOVAs.
Results
The healthy weight strata demonstrated a minimum flexion angle of 0.59°(CI −2.15 to 3.32), compared to overweight 1.12°(CI 0.99 to 2.11) and obese strata 1.37°(−0.72 to 3.46). The healthy weight strata exhibited a lower frontal ROM 7.91° (CI 7.02 to 8.80) (p<0.000) compared to the overweight (9.42°, CI 8.76 to 10.08) and obese strata (9.79°; CI 9.08 to 10.50). No differences between strata were observed in the transverse plane. The real-world gait inputs resulted in a lower aspect ratio for all three patient groups compared to the ISO standard AR of 3.86. There was a trend towards a higher AR in patients with a lower BMI. Obese patients had a reduced AR of 3.33 (CI 3.08 to 3.58) compared to the overweight and healthy weight patients, demonstrating AR of 3.36(CI 3.21 to 3.52) and 3.48 (CI 3.25 to 3.70), respectively.
Discussion
There were few hip kinematic differences between BMI strata, except for a lower frontal ROM in the healthy weight patients. There was a resulting trend towards an increased AR in the healthy weight group. Notwithstanding the effect of contact force which was not modelled in this study, increased AR in the healthy weight group might assist long molecule entrainment and hence reduce risk of polyethylene wear for equivalent levels of activity. These results highlight the conservative nature of the ISO standard ISO-14242 and provide a possible link between kinematics and the observed increased in revision rates in patients with high BMI.
