Nucleus replacement: an investigation into biomechanical assessment protocols and surgical techniques

Abstract

Nucleus replacement devices (NRDs) have the potential to be a treatment for degenerative disc disease and to be used to replace herniated intervertebral disc (IVD) material. However, presently NRDs are not clinically used. This is predominantly due to the high occurrence of device expulsion through the annular implantation route. Subjecting NRDs to more rigorous in vitro testing may enable common failure mechanisms to be identified and avoided prior to clinical trials. Therefore, the aim of this thesis is to propose more thorough biomechanical assessment protocols for NRDs. Firstly, this thesis investigated two critical NRD assessment protocols which are not clearly defined by the current international testing standards. Secondly, it investigated the potential of using digital volume correlation (DVC) to non-invasively assess NRD’s interaction with the surrounding IVD tissues. Finally, it investigated different nuclectomy techniques to recommend the least damaging (to the AF and endplates) surgical procedure. The first study recommended two expulsion protocols based on a comparison study made by assessing four different expulsion protocols reported in the literature. The results obtained indicated that there were no significant difference between the protocols if the loading parameters and the NRDs’ biomechanical properties (e.g., stiffness and viscoelastic recovery) are similar. However, it is recommended that partial rotation hula hoop and ramp to failure tests are performed to assess the number of cycles and the load at which NRD failure occurs. The second study investigated the role of the nucleus in recovery following daily activity simulating protocols. This enabled the role of the NP during the viscoelastic recovery to be delineated. The results obtained can be used as a benchmark for future NRD designs. The third study demonstrated the potential of DVC to be used as a tool to noninvasively obtain 3D strain distribution maps of the entire IVD to assess NRD devices. The results showed the effect of nuclectomy on the various components of strain and how the NRD interacted with the surrounding IVD tissues post-treatment. The last study compared three clinically used nuclectomy techniques; rongeurs, automated-shaver and laser. The automated shaver demonstrated to be superior to the other two techniques in terms of the location of material removal and the effects on the disc’s biomechanics. The novel experimental work conducted in this thesis has developed a greater understanding of the comparability between different expulsion protocols, on the role of the nucleus in disc height recovery, and how IVD strain distribution changes with nuclectomy, thus, providing benchmark data for future NRD designs. Meanwhile, the comparative experimental work on nuclectomy techniques will allow clinicians to select the least disruptive nucleus removal technique.