Pathophysiology and current understanding of degenerative disc disease

Table 2 Multifactorial mechanisms of intervertebral disc degeneration

Category	Key mechanisms	Consequences for IVD	Representative factors
Mechanical stress and load	Repetitive microtrauma to AF and NP. Collagen/elastin disorganization. Loss of NP hydration. Altered biomechanics → increased shear forces	Decreased disc height. Structural failure. Instability. Herniation and nerve compression	Heavy labor, torsion, bending, vibration exposure
Genetic and environmental factors	SNPs affecting ECM proteins and inflammatory mediators. Altered ECM synthesis or stability. Epigenetic changes due to lifestyle factors	Early weakening of ECM. Increased inflammatory signaling. Catabolic microenvironment	Gene variants (COL1A1, COL9A2, ACAN, IL-1, IL-6), smoking, obesity, vibration, repetitive loading
Nutrition and metabolism	Impaired nutrient diffusion across CEP. Local hypoxia + acidic microenvironment. Accumulation of AGEs. Oxidative stress and systemic metabolic dysregulation	Reduced cell viability. Inhibited proteoglycan synthesis. ECM stiffening. Impaired permeability and degeneration	Endplate calcification, atherosclerosis, diabetes, metabolic syndrome, obesity
Cellular senescence	Oxidative stress, DNA damage, mitochondrial dysfunction. SASP production: Proinflammatory cytokines, MMPs, ADAMTS. NF-κB and p38 MAPK activation	ECM degradation. Increased inflammation. Loss of regenerative capacity. Accumulation of non-functional cells	Increased ROS, mitochondrial dysfunction, SASP factors (IL-1β, IL-6, TNF-α), MMPs, ADAMTS
Aging and microenvironment	Loss of proteoglycans and GAGs → decreased hydration. Increased collagen cross-linking, fragmentation. CEP calcification and sclerosis. Accumulation of waste products	Reduced elasticity and load-distribution. Hypoxia and acidity. Increased apoptosis and catabolism. Progressive irreversible degeneration	Age-related CEP thickening, decreased metabolic activity, reduced nutrient diffusion
Lifestyle and comorbidities	Smoking-induced hypoxia. Obesity increasing axial load. Sedentary behavior reducing beneficial mechanical stimuli. Metabolic disorders increasing AGEs	Oxidative stress. Matrix degradation. Increased stiffness and reduced tensile strength. Accelerated degeneration	Smoking, obesity, poor posture, inactivity, diabetes

IVD: Intervertebral disc; AF: Annulus fibrosus; NP: Nucleus pulposus; CEP: Terminal endplate; ECM: Extracellular matrix; SNPs: Single-nucleotide polymorphisms; COL: Collagen; ACAN: Aggrecan; IL: Interleukins; AGEs: Advanced glycosylation end products; ROS: Reactive oxygen species; SASP: Senescence-associated secretory phenotype; TNF-α: Tumor necrosis factor alpha; MMPs: Matrix metalloproteinases; ADAMTS: A disintegrin and metalloproteinase with thrombospondin motifs; MAPK: Mitogen-activated protein kinase; GAG: Glycosaminoglycan; NF-κB: Nuclear factor kappa B.