The current wear-testing standard (ISO18192-1) for total disc replacement (TDR) requires only four degrees of freedom (DOF) inputs: axial load, flexionextension, lateral bending and axial rotation. The study aim was to assess the effect of an additional DOF, anterior-posterior (AP) shear on the wear of the ProDisc-L TDR. A 5DOF simulator was used to test ProDisc-L implants under 4DOF and 5DOF conditions. The 4DOF conditions were defined by ISO18192-1 whilst the 5DOF used ISO18192-1 conditions with the addition of an AP load of ?175 and -140 N (anterior and posterior, respectively), extrapolated from in vivo data. The implants were mounted such that the polyethylene insert could be removed for gravimetric measurements. Tests were run using bovine serum (15 g/l protein concentration) as a lubricant for five million cycles (MC), with measurements repeated every 1 MC. The mean wear rate in the 4DOF test was 12.7 ± 2.1 mg/MC compared to 11.6 ± 1.2 mg/MC in the 5DOF test. There were marked differences in the wear scars between 4DOF and 5DOF simulations. With 4DOF, wear scars were centralised on the dome of the insert, whilst 5DOF scars were larger, breaching the anterior rim of the dome causing deformation at the edge. The 4DOF wear test showed similar gravimetric wear rates to previously published ISO-tested TDRs. The addition of AP load was found to have no significant effect on the overall wear rate. However, there were pronounced differences in the respective wear scars, which highlights the need for more research in order to understand the factors that influence wear of TDR.

Study Design An in-vitro study of the wear rates of the Charité lumbar total disc replacement. Objective To investigate the effect of anterior-posterior shear on the in-vitro wear rates of the Charité lumbar total disc replacement. Summary of Background Data Current standards prescribe only 4 degree of freedom (DOF) inputs for evaluating the in-vitro wear of total disc replacements, despite the functional spinal unit incorporating 6DOF. Anterior-posterior shear has been highlighted as a significant load, particularly in the lumbar spine. A previous study investigated the effect of an anterior-posterior shear on the ProDisc-L, finding that wear rates were not significantly different from 4DOF wear tests. Methods 6 Charité lumbar discs were mounted in a 5 active DOF spine wear simulator and tested under 4DOF (ISO18192) conditions. 6 further Charité lumbar discs were tested under 5DOF conditions, consisting of 4DOF conditions plus an anterior-posterior shear displacement of +2/-1.5mm. The displacement was decreased and then increased by a factor of 2, to investigate the effect of the magnitude of displacement. µCT scans were taken of the discs before and after wear testing, and the height loss of the discs calculated. These were compared to the same measurements taken from explanted Charite discs, µCT scanned at another institution. Results 4DOF wear rates (12.2±1.0mg/MC) were not significantly different from 4DOF tests on the ProDisc-L. Wear rates were significantly increased (p<0.01) for ‘standard’ 5DOF conditions (22.3±2.0 mg/MC), decreased 5DOF (24.3±4.9 mg/MC) and increased 5DOF (29.1±7.6mg/MC). The height loss of the explants and in-vitro tested discs were not significantly different (p>0.05). Conclusion The addition of anterior-posterior shear to wear testing inputs of the Charité lumbar total disc replacement increases the wear rate significantly, which is in direct contrast to the previous 5DOF testing on the ProDisc. This study highlights the importance of clinically relevant testing regimens, and that test inputs may be different for dissimilar design philosophies.

The challenges of measuring in vivo total disc replacement (TDR) kinematics are well recognized, meaning that it is difficult to establish appropriate input conditions for wear simulation. Therefore it is desirable to ascertain the sensitivity of implant wear in vitro to perturbations of the kinematics and other testing parameters. It has previously been demonstrated in other metal-on-polyethylene joint replacements that cross-shear strongly influences wear rate. This study investigates this phenomenon by altering the phasing of the inputs by making the lag in the flexion-extension and lateral bend displacements zero. Further, the effect of an additional anterior-posterior shear, which has been reported in vivo, was investigated for two different TDR designs using an extra load or displacement input in addition to those prescribed by the standard ISO 18192-1. Altering the standard ISO 18192-1 waveform phasing significantly reduced the mean wear rate of the constrained polyethylene disc. The addition of an anterior-posterior input showed no significant change in the rate of wear for the constrained TDR but was increased for the unconstrained device. These data demonstrate the strong dependency of the wear in these types of joints to the input conditions as well as the devices design parameters. Hence, these factors should be given prime consideration when designing both the device itself and the assessment regime in which the construct is to be tested.

Total disc replacements (TDRs) in the spine have been clinically successful in the short term, but there are concerns over long-term failure due to wear, as seen in other joint replacements. Simulators have been used to investigate the wear of TDRs, but only gravimetric measurements have been used to assess material loss. Micro computer tomography (microCT) has been used for volumetric measurement of explanted components but has yet to be used for in-vitro studies with the wear typically less than < 20 mm3 per 10(6) cycles. The aim of this study was to compare microCT volume measurements with gravimetric measurements and to assess whether microCT can quantify wear volumes of in-vitro tested TDRs. microCT measurements of TDR polyethylene cores were undertaken and the results compared with gravimetric assessments. The effects of repositioning, integration time, and scan resolution were investigated. The best volume measurement resolution was found to be +/- 3 mm3, at least three orders of magnitude greater than those determined for gravimetric measurements. In conclusion, the microCT measurement technique is suitable for quantifying in-vitro TDR polyethylene wear volumes and can provide qualitative data (e.g. wear location), and also further quantitative data (e.g. height loss), assisting comparisons with in-vivo and ex-vivo data. It is best used alongside gravimetric measurements to maintain the high level of precision that these measurements provide.

Laboratory wear simulations of the dual-bearing surface Charité total disc replacement (TDR) are complicated by the non-specificity of the device's center of rotation (CoR). Previous studies have suggested that articulation of the Charité preferentially occurs at the superior-bearing surface, although it is not clear how sensitive this phenomenon is to lubrication conditions or CoR location. In this study, a computational wear model is used to study the articulation kinematics and wear of the Charité TDR. Implant wear was found to be insensitive to the CoR location, although seemingly non-physiologic endplate motion can result. Articulation and wear were biased significantly to the superior-bearing surface, even in the presence of significant perturbations of loading and friction. The computational wear model provides novel insight into the mechanics and wear of the Charité TDR, allowing for better interpretation of in vivo results, and giving useful insight for designing future laboratory physical tests.