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Orthopaedic Proceedings
Vol. 101-B, Issue SUPP_5 | Pages 76 - 76
1 Apr 2019
Vasiljeva K Al-Hajjar M Lunn D Chapman G Redmond A Flatters I Thompson J Jones A
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Introduction

One of the known mechanisms which could contribute to the failure of total hip replacements (THR) is edge contact. Failures associated with edge contact include rim damage and lysis due to altered loading and torques. Recent study on four THR patients showed that the inclusion of pelvic motions in a contact model increased the risk of edge contact in some patients. The aim of current study was to determine whether pelvic motions have the same effect on contact location for a larger patient cohort and determine the contribution of each of the pelvic rotations to this effect.

Methods

Gait data was acquired from five male and five female unilateral THR patients using a ten camera Vicon system (Oxford Metrics, UK) interfaced with twin force plates (AMTI) and using a CAST marker set. All patients had good surgical outcomes, confirmed by patient-reported outcomes and were considered well-functioning, based on elective walking speed. Joint contact forces and pelvic motions were obtained from the AnyBody modelling system (AnyBody Technologies, DK). Only gait cycle regions with available force plate data were considered. A finite element model of a 32mm head on a featureless hemispherical polyethylene cup, 0.5mm radial clearance, was used to obtain the contact area from the contact force. A bespoke computational tool was used to analyse patients' gait profiles with and without pelvic motions. The risk of edge contact was measured as a “centre proximity angle” between the cup pole and centre of the contact area, and “edge proximity angle” between the cup pole and the furthest contact area point away from the pole. Pelvic tilt, drop and internal-external rotation were considered one at a time and in combinations.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_3 | Pages 149 - 149
1 Feb 2017
Groves D Vasiljeva K Al-Hajjar M Fisher J Williams S
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Introduction

Contact between the femoral head and rim of the acetabular liner in total hip replacements has been linked to adverse tribological performance that may potentially shorten the lifespan of the prosthesis. Predicting the size and location of the contact area can be done computationally, however, experimental validation of these models is challenging due to the conforming nature of the bearing surfaces.

This study aimed to develop a method of accurately determining the in-vitro contact area between the femoral head and acetabular cup in metal-on-polyethylene and ceramic-on-polyethylene bearings under different component orientations.

Method

Metal-on-polyethylene and ceramic-on-polyethylene samples, with a nominal diameter of 36mm (DePuy Synthes, Leeds, UK), were tested with the cups orientated using a combination of inclination (equivalent to 45°, 55° and 65° in-vivo) and version (−20°, 0°, 20° and 40°) angles. The liners, which were first gold hard-coated (EMSCOPE SC 500, Quarum Technologies, UK), were inserted into a Pinnacle® titanium shell, and femoral heads were mounted on a vertical spigot (Figure 1). A single-station multi-axis electromechanical hip joint simulator (Prosim, Simulator Solutions, UK) was used to position the samples with 18.7° flexion, 6.2° adduction and 8.3° external rotation, congruous with just after heel strike (ISO 14242-1), and apply a 3kN static axial load through the centre of the femoral head.

The contact area was generated by manually turning the head about the vertical axis of the centre of rotation of the applied load, removing the gold hard-coating from the contacting areas. The contact area was determined from photographs of the acetabular cup using SolidWorks (Dassault Systèmes, US) and ImageJ (National Institutes of Health, US) software packages. Three repeats under each combination of cup angles were completed, and the mean contact area and 95% confidence limits were determined for each bearing under all cup angle combinations.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_3 | Pages 8 - 8
1 Feb 2017
Al-Hajjar M Vasiljeva K Heiner A Kruger K Baer T Brown T Fisher J Jennings L
Full Access

Introduction

Previous studies have shown that third body damage to the femoral head in metal-on-polyethylene hip replacement bearings can lead to accelerated wear of the polyethylene liners. The resulting damage patterns observed on retrieved metal heads are typically scratches and scrapes. The damage created in vitro must represent the third body damage that occurs clinically. A computational model was developed to predict the acceleration of wear of polyethylene articulating against in vitro damaged femoral heads. This involved using a damage registry from retrieval femoral heads to develop standardized templates of femoral head scratches statistically representative of retrieval damage

The aim of this study was to determine the wear rates of polyethylene liners articulating against retrievals and artificially damaged metal heads for the purpose of validating a computational wear prediction model; and to develop and validate an in vitro standardised femoral head damage protocol for pre-clinical testing of hip replacements.

Materials and Methods

Twenty nine, 32mm diameter, metal-on-moderately cross-linked polyethylene bearings (MarathonTM) inserted into Ti-6Al-4V shells (Pinnacle®) were tested in this study. All products were manufactured by DePuy Synthes, Warsaw, Indiana, USA. Following a retrieval study seven different damage patterns were defined, and these were applied to the femoral heads using a four-degree-of-freedom CNC milling machine (Figure 1). The ProSim 10-station pneumatic hip joint simulator (Simulation Solutions, UK) was used for experimental wear simulation using standard gait cycles and testing each experimental group for 3 million cycles. The acetabular cups were inclined at 35° on the simulator (equivalent to 45° in vivo). The wear volumes were determined using a microbalance (Mettler-Toledo XP205, Switzerland) at one million cycle intervals. Statistical analysis used was one way ANOVA followed by a post hoc analysis with significance taken at p<0.05.