Surgical interventions for the treatment of chronic neck pain, which affects 330 million people globally [1], include fusion and cervical total disc replacement (CTDR). Most of the currently clinically available CTDRs designs include a metal-on-polymer (MoP) bearing. Numerous studies suggest that MoP CTDRs are associated with issues similar to those affecting other MoP joint replacement devices, including excessive wear and wear particle-related inflammation and osteolysis [2,3]. A device with a metal-on-metal (MoM) bearing has been investigated in the current study. Six MoM CTDRs made from high carbon cobalt-chromium (CoCr) were tested in a six-axis spine simulator, under standard ISO testing protocol (ISO-18192-1) for a duration of 4 million cycles (MC). Foetal bovine calf serum (25%v/v), used as a lubricant, was changed every 3.3×10. 5. cycles and saved for particle analysis. Components were taken down for measurements after each 10. 6. cycles; surface roughness, damage modes and gravimetric wear were assessed. The mean wear rate of the MoM CTDRs was 0.24mm. 3. /MC (SD=0.03), with the total volume of 0.98mm. 3. (SD=0.01) lost over the test duration. Throughout the test, the volumetric wear was linear; no significant bedding-in period was observed. The mean pre-test surface roughness decreased from 0.019μm (SD=0.005) to 0.012μm (SD=0.002) after 4MC of testing. Prior to testing, fine polishing marks on the bearing surfaces were observed using light microscopy. Following 4MC of testing, these polishing marks had been removed. Consistently across all components, surface discolouration and multidirectional, criss-crossing, circular wear tracks, caused by abrasive wear, were observed. The wear results showed low wear rates exhibited by MoM CTDRs (0.24mm. 3. /MC), when compared CTDR designs incorporating metal-on-polymer bearings (0.56mm. 3. /MC) [4] as well as MoM lumbar CTDRs [5,6] (0.76mm3/MC – 6.2mm. 3. /MC). These findings suggest that MoM CTDRs are more wear resistant than MoP CTDRs, however the particle characterisation and biological consequences of wear remain to be determined

Introduction. It is believed that wear of replacement joints vivo in is strongly dependent on input motions (kinematics) and loading. There is difficulty in accurately measuring total disc replacement (TDR) kinematics in vivo. It is therefore desirable to ascertain the sensitivity of implant wear in vitro to perturbations of the standard testing parameters. An anterior-posterior (AP) shear force input is not currently included in the present ISO and ASTM testing standards for lumbar TDRs but is known to exist in in vivo. Other joint-replacement wear tests have shown that the phasing of input motions influences the ‘cross-shear’ process of polyethylene wear. Polyethylene bearing materials do not behave linearly to axial loading changes and so the effect on wear rate is difficult to predict. The study aim was to assess the effects on wear of a ProDisc-L TDR under the following conditions: ISO 18191-1 standard inputs; an additional input AP shear; input kinematics phasing changes; axial loading changes. Methods. A five active degree of freedom (DOF) spine simulator was used to compare the effects of varying the kinematic and loading input parameters on a ProDisc-L TDR (Synthes Spine). A four DOF standard ISO (ISO18192-1) test was followed by a five DOF test which included the AP shear force. The standard ISO test was repeated on a second simulator (of identical design) but with the phasing of the lateral bend (LB) and flexion extension (FE) motions changed to be in-phase, creating a low cross-shear motion pattern. The standard ISO test was then modified to give half the ISO standard axial loading. All tests conducted were based on the ISO18192-1 standard for lumbar implants with 15 g/l protein lubricant and modified as described. Gravimetric wear measurements were taken every million cycles (mc) in units of milligrams (mg). Six discs were tested to give statistically significant results. Results. When the fifth DOF AP input force was added, the wear rate showed a non-significant (p=0.78) change in mean wear rate from 12.7 ± 2.1 mg/mc (± standard deviation) to 11.6 ± 1.2 mg/mc. For the repeated test, on the second simulator, changing from standard ISO to in-phase FE-LB conditions (producing a low cross-shear wear pattern) showed a significant mean wear rate fall of 16.1 ± 1.4 mg/mc to 6.0 ± 1.3 mg/mc. The low-load test showed a marginally non-significant (p=0.18) difference in mean wear rate from 16.0 ± 0.8 mg/mc to 15.1 ± 0.8 mg/mc. Conclusion. When comparing the standard ISO test with the modified five DOF AP input test no significant difference in mean wear rate was observed. Comparing the standard ISO test with the modified in-phase (low cross-shear) test produced a significant 62% reduction in wear rate. Reducing the loading by half did not produce a significant fall in mean wear rate. The wear of lumbar TDRs is strongly dependant on input phasing kinematics and perhaps not so dependent on axial loading and AP shear. This counter-intuitive result is important for in vivo wear performance estimation

In a study on ten fresh human cadavers we examined the change in the height of the intervertebral disc space, the angle of lordosis and the geometry of the facet joints after insertion of intervertebral total disc replacements. SB III Charité prostheses were inserted at L3-4, L4-5, and L5-S1. The changes studied were measured using computer navigation sofware applied to CT scans before and after instrumentation. After disc replacement the mean lumbar disc height was doubled (p < 0.001). The mean angle of lordosis and the facet joint space increased by a statistically significant extent (p < 0.005 and p = 0.006, respectively). By contrast, the mean facet joint overlap was significantly reduced (p < 0.001). Our study indicates that the increase in the intervertebral disc height after disc replacement changes the geometry at the facet joints. This may have clinical relevance

Summary Statement. To evaluate carbon-fiber-reinforced PEEK as alternative biomaterial for total disc arthroplasty a closed loop between biotribology (in vitro), application of sterile particle suspensions in the epidural space of rabbits and biological response in vivo was established. Introduction. To prevent adjacent level degeneration in the cervical spine, total disc arthroplasty (TDA-C) remains an interesting surgical procedure for degenerative disc disease. Short- or midterm complications are migration, impaired post-operative neurological assessment due to artefacts in x-ray and MRI diagnosis and substantial rates of heterotopic ossification. The idea was to create a TDA-C design based on a polymer-on-polymer articulation to overcome these limitations of the clinically established metal-on-polyethylene designs. The objective of our study was to characterise the biotribological behaviour of an experimental cervical disc replacement made out of carbon-fiber-reinforced (CFR) PEEK and evaluate the biological response of particulate wear debris in the epidural space in vivo. Materials & Methods. In vitro wear simulation acc. to ISO 18192-1 was performed for 10 million cycles on a clinically established TDA-C device (Aesculap, Tuttlingen) made of cobalt-chromium-on-polyethylene in a direct comparison to an experimental disc prototype made of CFR-PEEK. An estimation of particle size and morphology was done acc. to Affatato et al. [5] and sterile suspensions of comparable particles (size 90% < 1 µm) in phospate buffered saline (PBS) were produced [6] for the application in the epidural space of 36 white new zealand rabbits. The particle concentration was 1 mg/ml with a volume of 0.2 ml injected percutaneously using fluoroscopic guidance and the inflammatory response was assessed 3 and 6 months post-operatively in a direct comparison between the groups PBS (control), UHMWPE and CFR-PEEK. Results. The gravimetric wear rate was for the cobalt-chromium-on-polyethylene TDA-C device as a clinical reference 1.0 ± 0.1 mg/ million cycles, compared to 0.02 ± 0.02 mg/ million cycles for the experimental CFR-PEEK articulation (p < 0.001), whereas the cumulative amount of wear of the CFR-PEEK TDA-C prototypes (0.5 ± 0.23 mg/ million cycles) was decreased by an order of a magnitude compared to cobalt-chromium-on-polyethylene (12.1 ± 1.46 mg/ million cycles) (p < 0.001). For CFR-PEEK and UHMWPE most of the particles were observed in a submicron size range and the morphology was comparable. Histopathological examination demonstrated wear debris in the vertebral canal of injection sites surrounded by inflammatory cells. The inflammation was limited to the epidural space around the particles and polymer particles were associated by a low grade foreign body reaction comprising macrophages and multi-nucleated giant cells. CFR-PEEK particulate wear debris showed at least similar histopathological reactions than UHMWPE in the cervical epidural space. Conclusion. A closed loop between biotribology (in vitro), application of sterile particle suspensions in the epidural space of rabbits and biological response in vivo was established to evaluate carbon-fiber-reinforced PEEK as alternative biomaterial for total disc arthroplasty

Background. Oblique implantable total disc replacements (TDR) have been developed in an attempt to partially resect the anterior longitudinal ligament (ALL), together with additional partial resection of lateral annulus fibres. To date, the literature has not addressed the impact of the TDR oblique implantation on the lumbar spine sagittal alignment. The hypothesis of this study was that TDR at the L4-L5 level does not change the sagittal alignment and the range of motion of the lumbar spine when the implant is placed in accurate position. Methods. Prospective single-center radiological investigation of L4/5 TDR inserted through an oblique approach for the treatment of disc disease. A series of 52 patients with a minimum of 2-year FU after oblique TDR at L4/L5 level was analysed for radiological changes in sagittal alignment and range of motion of the lumbar spine. The total sagittal lumbar lordosis (TSLL), the segmental sagittal lumbar lordosis (SSLL) of the operated level, and the range of motion of the TDR implant were determined in pre- and postoperative functional X-rays. The accuracy of the implant position was also evaluated. Results. A total of 52 patients (mean age, 42.7) were available. There were no revision surgeries for general and/or device-related complications. Only a 28.8% of cases (n=15) showed a satisfactory position. Off-center lateralised implants were the most common misplacements. Axial malrotated TDR accounted for the 28.1% of cases. From 3 to 24 months of FU, differences in range of motion were found in the total L1-S1 flexion, and in the mean range of motion of the implant both improving significantly. TDRs showing unsatisfactory implantation in the radiological studies (71.8%) demonstrated similar lumbar and segmental range of motion in comparison to properly implanted TDRs. Conclusions. Oblique implanted L4/L5 TDR significantly increases total lordosis while retaining segmental lordosis, independently of the accuracy of its intervertebral position. Oblique TDR maintains antero-posterior segmental and total balance in most cases. Further studies should evaluate whether this finding has any implication for the long-term outcome. Level of Evidence. Level III

Spinal total disc replacement (TDR) designs rely heavily on total hip replacement (THR) technology and it is therefore prudent to check that typical TDR devices have acceptable friction and torque behaviour. For spherical devices friction factor (f) is used in place of friction coefficient (mju). The range of loading for the lumbar spinal discs is estimated at perhaps 3 times body weight (BW) for normal activity rising to up to 6 times BW for strenuous activity. [1]. For walking this equates to around 2000 N, which is the maximum load required by the ISO standard for TDR wear testing. [2]. . Three Prodisc-L TDR devices (Synthes Spine) were tested in a single station friction simulator. Bovine serum diluted to 25% was used as a lubricating medium. Flexion-extension was ±5 deg for all experiments with constant axial loading of 500, 2000 and 3000 N. The cycle run length was limited to 100 and the f and torque (T) values recorded around the maximum velocity of the cycle point and averaged over multiple cycles. Preliminary results shows that the 500 N loading produced the largest f of 0.05 ± 0.004. The 2000 N load, which approximates daily activity, gave f = 0.036 ± 0.05 and the 3000 N load gave f = 0.013 ± 0.003. The trend was for lower f with increasing loads. A lumbar TDR friction factor of 0.036 for a 2000N load and the reduction in f for increasing loads is comparable to the lower end of the range of values reported for THR in similar simulator studies using metal-on-polyethylene bearing materials. [3]. The 3000 N result showing that increasing the load above that expected in daily activity does not raise the f could be important when considering rotational stability and anchorage in a TDR device because frictional torque at the bearing surfaces is proportional to the product of load, device radius and f

Introduction. The biological response to UHMWPE particles generated by total joint replacements is one of the key causes of osteolysis, which leads to late failure of implants. Particles ranging from 0.1-1.0μm have been shown to be the most biologically active, in terms of osteolytic cytokine release from macrophages [1]. Current designs of lumbar total disc replacements (TDR) contain UHMWPE as a bearing surface and the first reports of osteolysis around TDR in vivo have appeared recently in the literature [2]. The current wear testing standard (ISO18192-1) for TDR specifies only four degrees of freedom (4DOF), i.e. axial load, flexion-extension, lateral bend and axial rotation. However, Callaghan et al. [3] described a fifth DOF, anterior-posterior (AP) shear. The aim of this study was to investigate the effect that this additional AP shear load component had on the size and morphology of the wear particles generated by ProDisc-L TDR devices over five million cycles in a spine simulator. Methods. A six-station lumbar spine simulator (Simulation Solutions, UK) was used to test ProDisc-L TDR components (Synthes Spine, USA) under the ISO 18192-1 standard inputs and with the addition of an AP load of +175 and −140N. Wear particles were isolated at 2 and 5 mc using a modified alkaline digestion protocol [4]. Particles were collected by filtration and imaged by high resolution FEGSEM. Particle number and volume distributions were calculated as described previously [4] and were compared statistically by one way ANOVA (p<0.05). Results. Similar particle morphologies were observed under 4DOF and 5DOF inputs, including flakes, fibrils and granules. No significant differences were observed between the size and volume distributions under 4DOF and 5DOF when comparisons were made at the same time point, although there was a trend towards larger particles being generated under 5DOF inputs. The mode of the frequency distribution was in the submicron size range with the volume distributions showing greater variability. Discussion. This study represents the first comprehensive wear particle analysis comparing 4DOF (ISO conditions) to 5DOF inputs on a single TDR device. Vicars et al. [5] reported no significant differences in the wear volume of ProDisc-L TDR components under 4 and 5DOF inputs. The present study has shown that the particle size distributions and particle morphologies were not significantly affected by the addition of AP shear. The wear particles were similar in size and morphology to those previously reported for total hip replacements and total knee replacements [6], indicating that wear debris produced by TDR may have a similar potential for osteolysis