Abstract
Introduction
Preclinical testing of implants considers THR patients a homogenous group; in reality, patients are heterogeneous and previous large cohort studies have explored stratification and identified that THR patients function differently [1]. The wide- spread failure of the ASR hip highlighted the potential importance of patient characteristics [2], and a more robust pre- clinical testing procedure may have improved prediction of outcome. Therefore this study aimed to identify differences in hip contact force (HCF) in THR patients stratified by their functional ability.
Methods
133 THR patients, >12 months post-surgery, underwent 3D kinematic (Vicon, UK) and kinetic (AMTI, USA) analysis whilst walking at self-selected speed. HCF's, normalized by body weight, were computed through multibody modeling (AnyBody Technology, Denmark) during gait and a mean for each patient was calculated from three to five walking trials. Patients were stratified into three functionality groups by distribution around the mean gait speed for the full cohort of 1.1m/s. The low functioning group (LF) comprised cases with a gait speed ≤0.93 m/s (i.e. 1.1m/s ≤1SD), the mid functioning group (MF) comprised cases with a gait speed between 0.94 m/s and 1.25 m/s (cohort mean ± 1SD), while the high functioning group (HF) included cases walking ≥1.26 m/s. Differences between groups were analyzed using one- dimensional statistical parametric mapping [3]. Linear regression was used to test for significant differences across groups. The test statistic SPM{t} was evaluated at each point in the normalized time series, and a critical threshold corresponding to an error rate of α= 0.05 was calculated based on random field theory. Supra-threshold clusters with their associated p-values were then identified.
Results
Systematic differences were observed between the different functioning groups throughout the gait cycle. Four different supra-threshold clusters reached or exceeded the critical threshold of t=3.244 indicating systematic between-group differences, with the chances of observing similar clusters in repeated random samplings of p<0.001. HCF were linearly increasing with functionality from heel strike to foot flat (1st cluster), from terminal stance to initial swing (3rd cluster), and during terminal swing (4th cluster), while they were decreasing for higher functionality during midstance (2nd cluster).
Discussion
Stratification of THR patients by functionality showed significant differences in the contact forces that implants have to withstand during gait. The LF patients displayed a pathological HCF, with a flattening of the typical double hump, indicating that differences can be observed between groups of healthy THR patients. Overall, THR patients are functionally heterogeneous and preclinical testing should reflect differences better than currently required by the ISO-14242 implant testing standard.
