Oral Presentation Sydney Spinal Symposium 2018

Annular bulge following discectomy (for lumbar disc herniation) and concomitant replacement by a nucleus prosthesis (#6)

Uphar Chamoli 1 2 3 , Ashish Diwan 1 3
  1. Spine Service, St. George & Sutherland Clinical School, University of New South Wales, Sydney, NSW
  2. School of Biomedical Engineering, University of Technology Sydney, Ultimo, NSW
  3. Kunovus Pty Ltd, Kogarah, NSW

INTRODUCTION

It remains unknown whether limited discectomy (LD) or aggressive discectomy (AD) provides better clinical outcomes for the treatment of lumbar disc herniation patients with radiculopathy. Concomitant replacement of the nucleus tissue lost to herniation and removed during discectomy with a nucleus prosthesis has the potential to restore biomechanical characteristsics of the motion segment to intact state levels.1 The synergistic interaction between the prosthesis, remnant nucleus, annulus, and endplates is responsible for stiffness restoration of the motion segment.1,2 The main objective of this study was to systematically assess the biomechanical consequences of AD, LD, partial and complete filling of the nucleus cavity (after discectomy) with a prosthesis, using a computed tomography (CT) based finite element (FE) model of the human lumbar spine.

METHODS

High resolution lumobosacral CT data from an anonymised healthy human subject (M, 26 yo) was segmented in an image processing software (Avizo Standard) to build a geometrically accurate 3D model of an intact human lumbar spine (L1-L5).

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Figure 1. Transverse view of the L3-L4 intervertebral disc in different finite element models of the lumbar spine. The intact FE model was further manipulated to generate eight more models representing different grades of nucleotomy and implant filling in the L3-L4 intervertebral disc different grades of nucleotomy and implant filling in the L3-L4 intervertebral disc

The solid model was imported into FE software Strand7 for further pre-processing, running nonlinear solves, and post-processing of the results. An in situ curing non-hydrogel silicone based nucleus replacement implant contained within an inflatable balloon (Kunovus Pty Ltd, Kogarah, NSW) was modelled in this study.

RESULTS

At the index level (L3-L4), both partial and complete nucleotomy significantly increased annular bulge during flexion and extension loading as measured in the mid sagittal plane (greatest increase during Fx in the complete nucleotomy model, 0.63 mm at 10Nm load). The anterior and posterior annular bulges were restored to the intact state levels in all the completely filled implant models similarly, but large deviations from the intact state were observed in the partially filled implant models. In the completely filled implant models, mean annular stresses were below the intact state level (-5%, for 100% IMP model at peak flexural load) but mean stresses in the innermost endplate region were consistently above the intact state levels (+9%, for 100% IMP model at peak flexural load).

DISCUSSION AND CONCLUSIONS

Although discectomy is a clinically beneficial procedure for symptomatic lumbar disc herniation patients, revision discectomy rates range from 2-18% within the first decade of the primary surgery.3 The results of this study suggest that both LD and AD may lead to significant increases in annular bulge compared with the intact state. Filling the nucleotomised cavity with an elastomeric prosthesis significantly influences the biomechanical characetristics of the spinal motion segment. While partial filling of the cavity may result in lower implant-endplate interface stresses, annular bulge is restored to normal intact state levels only in the completely-filled models.

  1. Di Martino A et al., Spine (Phila Pa 1976) 30 (16 Suppl): S16-22, 2005.
  2. Strange DG et al., The Spine Journal 10: 602-9, 2010.
  3. Virk et al., Clinical Orthopaedics and Related Research 475:2752-62, 2017