Effects of nucleotomy on segmental flexibility: A numerical analysis

Nucleotomy, a common treatment for disc herniations, aims to relieve pressure on spinal structures. While effective in alleviating symptoms, this intervention can compromise spinal stability. However, previous in vivo studies in sheep have demonstrated conflicting results with significant long-term stiffening of the spine following nucleotomy, with occasional spontaneous fusion of the affected motion segment. The objective of this study was to investigate the mechanical regulation of tissue adaptation processes post-nucleotomy using computational modeling. A parametric finite element model of the L4-L5 ovine spinal motion segment, developed previously, was modified to simulate surgical procedures that have been performed in prior in vivo studies. An iterative approach was used to simulate post-surgical tissue healing and adaptation processes. Two loading scenarios were simulated: one with combined axial compression and flexion moments, and the other incorporating axial rotation. An initial decrease in stability, with stiffness reduced by up to 50% due to disc decompression and nucleus removal, was followed by a gradual increase in stiffness over time as a consequence of bone healing and remodeling, with the most pronounced stiffening - up to 350% of the intact state - observed in axial rotation. The findings align with previous in vivo observations, suggesting that spontaneous fusion and increased rigidity may be natural consequences of mechano-biological adaptation. The results of this study highlight that healing processes accompanied by adaptive bone remodeling are directed towards restoration of spinal stability after nucleotomy. These findings align with previous in vivo observations, suggesting that spontaneous fusion and increased rigidity may be a natural consequence of post-nucleotomy mechano-biological adaptation. On the other hand, the results indicate a critical role of an appropriate loading regime on the outcome of these processes.