Abstract
Introduction
Total Hip Arthroplasty (THA) devices are now increasingly subjected to a progressively greater range of kinematic and loading regimes from substantially younger and more active patients. In the interest of ensuring adequate THA solutions for all patient groups, THA polyethylene acetabular liner (PE Liner) wear representative of younger, heavier, and more active patients (referred to as HA in this study) warrants further understanding.
Previous studies have investigated HA joint related morbidity [1]. Current or past rugby players are more likely to report osteoarthritis, osteoporosis, and joint replacement than a general population.
This investigation aimed to provide a preliminary understanding of HA patient specific PE liner tribological performance during Standard Walking (SW) gait in comparison to IS0:14242-1:2014 standardized testing.
Materials and Methods
Nine healthy male subjects volunteered for a gait lab-based study to collect kinematics and loading profiles. Owing to limitations in subject selection, five subjects wore a weighted jacket to increase Body Mass Index ≥30 (BMI). An induced increase in Bodyweight was capped (<30%BW) to avoid significantly effecting gait [3] (mean=11%BW).
Six subjects identified as HA per BMI≥30, but with anthropometric ratios indicative of lower body fat as previously detailed by the author [2] (Waist-to-hip circumference ratio and waist circumference-to-height ratio). Three subjects identified as Normal (BMI<25). Instrumented force plate loading profiles were scaled (≈270%BW) in agreement with instrumented hip force data [4].
A previously verified THA (Pinnacle® Marathon® 36×56mm, DePuy Synthes) Finite Element Analysis wear model based on Archard's law and modified time hardening model [5] was used to predict geometrical changes due to wear and deformation, respectively (Figure 1). Subject dependent kinematic and loading conditions were sampled to generate, for both legs, 19 SW simulation runs using a central composite design of response surface method.
Results
HA group demonstrated comparable SW gait characteristics and Range of Motion (RoM) to the Normal group (p>0.1) (Figure 2) but statistically greater SW peak loads, PE liner wear rates, deformation, and penetration after 3Mc (Million cycles) of SW (p<0.01).
HA group demonstrated comparable RoM (p>0.4) and peak loading to ISO-14242-1:2014 (p>0.1) although, up to 8° increase in flexion-extension angle was observed. The HA group demonstrated statistically greater wear rates (mean 7.5% increase) to ISO-14242-1:2014 (p<0.05) (Figure 3). No difference in PE liner deformation or penetration was observed (p>0.4).
Discussion
This study detailed only a 19th percentile within a broader HA population (BW=91kg, n=485) [6] however, were statistically worst-case compared to a Normal group and ISO-14242-1:2014. A 95th percentile HA population (BW=127kg) may produce lower PE liner tribological performance than reported in this investigation and therefore, warrants further investigation.
Further studies would be beneficial to determine whether the increase in PE liner wear rate for HA patients is predictable based on kinematics and loading alone, or whether influences exist in design inputs and surgical factors.
Conclusion
The HA population detailed in this study (representative of a 19th percentile) demonstrated statistically greater SW PE liner wear rates compared to ISO-14242-1:2014. This study may have implications for the test methods considered appropriate to verify novel designs.
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