07/11/2022
Lumbar Spine Disc Herniation: Mechanisms of Reabsorption
“Clinically, the phenomenon of spontaneous shrinkage or disappearance of a herniated lumbar IVD without surgical intervention is called reabsorption.
👉 The spontaneous resorption rate of lumbar disc herniation (LDH) is over 60% according to a meta-analysis by Zhong et al. (2017) and Chiu et al. (2015) https://pubmed.ncbi.nlm.nih.gov/28072796/, https://pubmed.ncbi.nlm.nih.gov/25009200/ with extrusion and sequestration being more prone to regression than other types. https://pubmed.ncbi.nlm.nih.gov/24630494/
But how does this actually happen?
👉 Resorption of the intervertebral disc tissue can be explained by 3 theories, but the exact mechanism is not conclusively understood:
1. Reduction in disc material size due to gradual dehydration and shrinkage, which could explain the decrease in the signal of the disc on MRI. https://pubmed.ncbi.nlm.nih.gov/11901758/
https://pubmed.ncbi.nlm.nih.gov/11750011/
2. The tension on the posterior longitudinal ligament causes the herniated disc fragment to be retracted back into the intervertebral disc space. This can be a mechanism when the annulus fibrosus (AF) is not damaged, but not with extruded and migrated fragments.
https://pubmed.ncbi.nlm.nih.gov/24316264/
https://pubmed.ncbi.nlm.nih.gov/3875236/
3. The third theory, the most well studied and with the best clinical evidence, is gradual resorption of the disc material by enzymatic degradation and phagocytosis induced by an inflammatory response and neovascularisation: s. figure
📍When protruding IVD tissue squeezes out of the epidural space, it disrupts immune privilege, triggering an autoimmune response, then lymphocytes activate macrophages.
📍Related factors secreted by IVD cells and macrophages further drive the recruitment of macrophages to the intervertebral disc in paracrine and autocrine forms. Macrophages undergo differentiation from M1 to M2 types.
📍M1-type macrophages secrete pro-inflammatory factors to initiate angiogenesis, promote the expression of matrix metalloenzymes and apoptosis of herniated IVD nucleus pulposus cells.
📍M2-type macrophages secrete anti-inflammatory factors to relieve pain response, promote new blood vessel formation, and are responsible for tissue remodeling and repair, and absorb the protruding debris to reduce the total volume of the intervertebral disc, and reduce the mechanical compression of the nerve.
📍Throughout the process, infiltration and activation of macrophages mediate inflammatory responses, matrix metalloenzyme activation, and neovascularization.”
Analysis by Physio Meets Science
Photo Credits
https://pubmed.ncbi.nlm.nih.gov/16688039/
https://pubmed.ncbi.nlm.nih.gov/19333096/
https://pubmed.ncbi.nlm.nih.gov/15626982
https://pubmed.ncbi.nlm.nih.gov/35999644/
