Minor endplate damage as an initiator of systemic biomechanical disruption of the lumbar disc: a finite element analysis of the 'mechanical tipping point' in disc failure.

BACKGROUND

Vertebral end plates (EP) are essential for disc homeostasis due to their dual role of structural load distributor and a nutritional interface. Focal EP defects are frequently observed clinically, but their true biomechanical significance in vivo is unknown. Finite Element (FE) analysis offers a unique opportunity to simulate specific anatomical changes and can quantify the consequences of the mechanical disruption.

METHODS

We employed a validated FE Model of L4-L5 functional unit and investigated the effects of physiological compression and pure moments on models simulating: (1) Healthy discs with intact EP, (2) Four EP defect models, and (3) Graded anatomical EP damage based on Integrated Total Endplate Score (I-TEPS). Stress distribution on cartilage end plate (CEP), bony end plate (BEP), annulus fibrosus (AF), nucleus pulposus (NP), and subchondral bone was documented.

RESULTS

In the intact healthy model, stress was uniformly distributed without focal concentrations. In contrast, all EP defect models demonstrated significant, non-linear stress elevation across the motion segment. Incremental analysis based on I-TEPS revealed a 'mechanical tipping point' phenomenon: stresses remained relatively stable until score three but increased sharply once the score reached four, coinciding with the onset of BEP involvement.

CONCLUSION

Focal EP defects, irrespective of their location, cause stress extensions beyond the defect zone, causing cascading mechanical disruption of the disc environment. Combined CEP and BEP defects (I-TEPS ≥ 4) represent synergistic mechanical failure, characterized by abnormal NP pressure, annular tension, and subchondral bone stress. These findings demonstrate that EP integrity is one of the primary determinants of segmental spine stability.