Retrieval analysis of an explanted Mobi-C cervical disc replacement: A case study

• Published in: Medical engineering & physics 2021-04, Vol.90, p.54-65
• Main Authors: Kandemir, Göksu, Pitsika, Marina, Nissen, Justin J., Bowey, Andrew, Joyce, Thomas J.
• Format: Article
• Language: English
• Subjects: Cervical spine; Explant analysis; Metal-on-polymer; Non-constrained artificial cervical disc; Total disc replacement
• ISSN: 1350-4533; 1873-4030
• DOI: 10.1016/j.medengphy.2021.02.007

•This is the first reported explant analysis of a Mobi-C cervical disc.•It was shown that migration of the insert occurred, leading to compression of the spinal cord, pain and eventually a revision operation.•While they may provide more normal biomechanics, non-constrained designs, such as the Mobi-C, may also be associated with excessive migration of the insert in vivo.•In common with explanted metal-on-polymer joint replacements, the metal articulating surfaces of the disc became rougher while the polyethylene insert became smoother in vivo. Ex vivo analysis of artificial discs is essential to better understand their ability to replace degenerated intervertebral discs. The Mobi-C differs from some other contemporary disc designs in that it has a mobile polyethylene insert that is sandwiched between superior and inferior cobalt chromium endplates. While some studies claim the Mobi-C to have restored normal cervical spinal biomechanics, others have noted high levels of migration. Our objective was to contribute to this debate by, for the first time, analysing an explanted Mobi-C cervical disc which was removed due to worsening myelopathy at the nano and macro scales. Intraoperatively, the insert was found to have excessively migrated and it compressed the spinal cord. Roughness was measured as 0.016 ± 0.006 μm (Sa) and 0.055 ± 0.020 μm (Sa) for the superior and inferior plates, and 1.210 ± 0.154 μm (Sa) and 0.446 ± 0.083 μm (Sa) for the superior and inferior surfaces of the insert. Compared to unworn surfaces, the roughness increased for the superior and inferior plates and decreased for both surfaces of the insert. However, the only statistically significant change occurred on the articulating surface of the inferior plate (p = 0.04). At the nanoscale, valleys dominated the articulating surfaces. The superior plate had a burnished appearance whereas the inferior plate appeared matt. Impingement was observed on the endplates. The insert was severely damaged, burnished and had scratches. Additionally, subsurface whitening and internal cracking were observed on the insert.