Abstract
Introduction
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.
Method
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.
Results
The wear tests showed that under solely flexion and loading there was a low wear rate of 1.1±0.5 mm³/MC. With the addition of rotation and a 9mm AP displacement in Stage two the wear rate increased to 25.8±3.1 mm³/MC. When the displacement was removed in Stage three the wear rate decreased significantly to 15.2±2.5 mm³/MC (Figure 3).
Discussion
Wear of the TAR was shown to vary significantly with kinematic input. As observed with other polymer total joint replacement articulations, unidirectional motion of the ankle yielded minimal wear of the Zenith TAR. Using an extreme anterior/posterior displacement motion from literature significantly increased the wear in combination with the rotation. After these wear stages the TiN tibials show obvious signs of wear in the region of polyethylene contact. The counter surface of the polyethylene insert showed both linear and radial scratches whereas as the curved surface of the insert and the talar component solely showed unidirectional wear lines.
Conclusions
The design allows a large range of motion within the simulator. The wear rate presented by the Zenith TAR is similar to previous tests on unconstrained bearings and is highly dependent on the kinematic conditions.
