Article
How does design of different total cervical disc prosthesis alter the segmental motion pattern in comparison to fusion in the cervical spine? A biomechanical analysis
Search Medline for
Authors
-
Richard Bostelmann
- Neurochirurgische Klinik, Universitätsklinikum Düsseldorf, Düsseldorf
-
Hans Jakob Steiger
- Neurochirurgische Klinik, Universitätsklinikum Düsseldorf, Düsseldorf
-
Werner Schmoelz
- Unfallchirurgische Klinik, Medizinische Universität Innsbruck, Österreich
| Published: | June 2, 2015 |
|---|
Outline
Text
Objective: Previous in vitro studies on cervical total disc replacements (TDR) showed a motion preservation effect of TDRs. While most of the investigated TDR designs are built up of a typical ball and socket design (Design P) with a fixed center of rotation (CoR), one the investigated TDRs is built up of ellipsoid bearing surfaces (Design C). This is intended to allow coupled motion and translational movement and not to have a fixed CoR. The purpose of the presented in vitro study was to determine the range of motion (RoM) and center of rotation (COR) for two TDR designs and compare it to the intact motion segment and anterior cervical discectomy and fusion (ACDF).
Method: Six human cadaveric cervical spines (C3-C7) were loaded with pure moments of ±1.5Nm in a spine tester. After intact testing, a TDR with Type P prosthesis was performed at C4-C5, followed by implantation of a Type C TDR and an ACDF with plating.
The segmental motions of the instrumented level (C4-C5) were measured using an ultra-sound based motion analysis system (Winbiomechanics, Zebris, Isny, Germany). Functional X-rays in maximum flexion and extension were taken and the COR was determined using the FXA software (ACES GmbH, Esslingen, Germany).
Results: Compared to the intact RoM in flexion/extension (9.9°; SD 5.2), Type P TDR non significantly reduced the RoM (8.5°; SD 5.3; p=0.36), while Type C TDR (6.0°; SD 4.1; p=0.049) and ACDF (3.8°; SD 2.4; p=0.011) significantly reduced the RoM. Relative to intact motion segment the height of the CoR for all instrumentations was significantly shifted in cranial directions (Figure. 1) towards the disc center (Type C, 38.5% SD 20.8 p=0.023; Type P 40%, SD 15.2, p=0.005; ACDF, 44.5%, SD 29.8, p=0.035). In anterior-posterior the COR was shifted posterior for Type C (12%, SD 18, p=0.254) and Type P (26%, SD 17, p=0.037) and remained unchanged for ACDF (0.2%, SD 11.3, p=0.974).
Conclusions: By cervical disc replacement with the tested implants CoR of a motion segment significantly changes from the center of the caudal vertebra towards the CoR of the TDRs. In anterior-posterior direction the CoR of the implanted motion segment also changes towards of the CoR of the TDR and depends on TDR design and positioning. In consequence change of CoR will lead to various modifications in load distribution and different facet loading. Corresponding the RoM in flexion / extension showed reduced flexibility for TDR Type C, protecting the segment from an non physiologically more of motion.
