Total ankle replacement (TAR) is surgically complex; malalignment can arise due to surgical technique or failure to correct natural varus/valgus malalignment. Across joint replacement, malalignment has been associated with pain, component edge loading, increased wear and higher failure rates. Good component alignment is considered instrumental for long term TAR success. The conforming surface geometry of mobile bearing TARs leaves no freedom for coronal plane malalignment. The aim of this study was to investigate the biomechanical effect of coronal alignment on a mobile bearing TAR. Three TARs (Zenith, Corin Group) were tested under five coronal malalignment angles from 0–10° in a single station electromechanical knee simulator applying a typical ankle gait profile. As swing phase load is critical to TAR contact mechanics but will vary depending on the joint laxity. Swing loads of 100N, 300N and 500N were investigated. A positive control test with a swing load of 1000N was also studied, and was expected to eliminate the majority of lift off effects. Under each condition, the version was allowed to move freely while tests were performed, and the version profile under each alignment angle was recorded. Each test was carried out for 600 cycles in 25% bovine serum. Under the same loading conditions, but without lubrication, a Tekscan sensor recorded data from two cycles to assess the change in contact pressure and area at the five coronal angles.Introduction
Methods
Total ankle replacements (TAR) are a much debated alternative to ankle fusion for treatment of end stage arthritis. Compared with hip and knee replacements these are implanted in small numbers with less than 500 per year recorded by the joint registry for England and Wales. The small numbers are a likely result of typically low mid-term survival rates, as well as extensive contra-indications for surgery. There have been multiple generations of TARs consisting of both constrained and unconstrained designs but due to device classification pre-clinical testing has been minimal. Five Zenith (Corin Group PLC), Titanium Nitride (TiN) coated, unconstrained TARs with conventional polyethylene inserts (Figure 1) were tested in an adapted knee simulator (Simulator Solutions, UK) for six million cycles (MC). The input parameters (Figure 2) were taken from available literature as there is no recognised ISO standard in place. A parametric study with three conditions was conducted to understand the impact of kinematic inputs on the polyethylene wear rate. These conditions aimed to understand the effect of both linear wear with isolated flexion, then multidirectional motion by implementing a rotational input with and without anterior/posterior (AP) displacement. Each condition was run for two MC. Stage One: Flexion and Load Stage Two: Flexion, Load, Rotation and Displacement Stage Three: Flexion, Load and Displacement A lubricant of 25% bovine serum, 0.03% Sodium Azide solution was used to replicate the protein content of the natural joint capsule. The wear was measured gravimetrically every million cycles and surface measurements taken with a contacting profilometer.Introduction
Method
Total ankle replacement (TAR) has been used as a surgical intervention for arthritis since the 1970s. However, unlike clinically successful hip and knee replacements, TARs are renowned for extensive contraindications to surgery and high failure rates with an average of 83% survival at 5 years. The majority cite aseptic loosening as the reason for failure. The aim of this study wais to analyse retrieved TARs visually and through interferometry to identify potential the failure mechanisms associated with these devices. Retrieved total ankle replacements (n=11) from consecutive revision surgeries carried out at Chapel Allerton Hospital, Leeds between August 2012 and January 2014, were collected for study at the University of Leeds, under an NRES approved procedure (09/H1307/60). The bearing surfaces of the samples were visually inspected for evidence of damage and wear. The bearing surfaces between the tibial component and the flat surface of the polyethylene insert were then examined using a scanning white light interferometer (NP Flex, Bruker, USA). It was not possible to characterise the talar bearing surface or the inferior polyethylene surface at this stage through interferometry due to the curvature of the surface. The components were aligned and five sections on each of the surfaces measured. These sections represented; anterior-medial, anterior-lateral, posterior-medial, posterior lateral and central regions of the bearing surfaces. 3D roughness values were recorded, and the mean 3D surface roughness compared between implants. Measurements were taken on the medial and lateral aspects of the bearing surfaces to investigate whether damage was location specific. A coefficient of determination was calculated to assess the relationship between implantation time and surface roughness.Introduction
Methods
