Study design: Radiographic evaluation of lumbar total disc replacement (TDR).
Objectives: To assess radiographically segmental angulation and mobility after lumbar TDR, to determine the rate of posterior component impingement, and to investigate the influence of implantation level and mono- versus bi-segmental implantations.
Summary of background data: Polyethylene (PE)-wear can lead to inferior outcome after lumbar TDR due to aseptic loosening. One contributing factor might be increased segmental lordosis with component impingement.
Methods: Fifty-six consecutive patients with 66 ProDisc-L (Synthes Spine, Solothurn, CH) prostheses (46 mono-segmental, 10 bi-segmental) were evaluated radiographically. All prostheses had 6 degrees intrinsic angulation and a 10 mm PE-inlay. Segmental angulation and extension range of motion was measured twice on standing radiographs (neutral position and maximum extension) using the spike method. Component impingement was assumed if angulation of the prosthesis fins was >16 degrees . Intraobserver variability was assessed using Pearson correlation coefficient and 95% confidence interval (95% CI).
Results: The average angulation in neutral position was 9.9 degrees (+/-4.8 degrees ) and 9.9 degrees (+/-4.9 degrees ) at first and second measurement, respectively. In maximum extension it was 11.3 degrees (+/-4.9 degrees ) and 11 degrees (+/-4.9 degrees ). Pearson correlation coefficient suggested near perfect agreement (0.99) for measurement of angulation and good agreement for range of motion measurement (0.85). Ninety-five percent CI was +/-1.2 degrees and +/-1.4 degrees , respectively. Data were interpreted using absolute measurements (AM) and 95% CI, suggesting impingement if segmental angulation was >16 degrees (AM) and >17.2 degrees (95% CI). Regarding neutral position, 11% (AM) and 5% (95% CI) of the artificial discs showed component impingement. In extension, this increased to 15% (AM) and 9% (95% CI), respectively. Impingement was more frequent at L4/5 and in bi-segmental implantations. Extension according to AM was maintained in 52 prostheses (79%) with on average 1.4 degrees +/- 1.1 degrees . According to the 95% CI, which required a change of >1.4 degrees , extension was seen in 21 prostheses (32%) with on average 2.5 degrees +/- 1 degrees.
Conclusion: Posterior component impingement was seen in a considerable number of implants. With regard to potential consequences like PE-wear, further studies are needed to investigate the correlation between radiographic and clinical findings.