The relation between intervertebral disc bulging and annular fiber associated strains for simple and complex loading

Area of Science:

• Biomechanics
• Spinal Mechanics
• Biomaterials Science

Background:

• Annulus fibrosus mechanical failure, including tears and prolapses, can result from high intradiscal pressure and affect disc bulging.
• The precise relationship between disc bulging and spinal loading remains incompletely understood.

Purpose of the Study:

• To investigate annular fiber strains and disc bulging under various simple and complex spinal loads.
• To analyze the impact of defects on these biomechanical responses.

Main Methods:

• A novel laser scanner was employed to image the surfaces of six L2-L3 spinal segments.
• Specimens underwent loading (500 N axial compression or 7.5 Nm torque) in principal and combined directions.
• Disc bulging and annular fiber strains were computed for intact and defect conditions.

Main Results:

• Axial compression induced 2.7% strains (6.7% with defect), increasing disc bulging from 0.7 mm to 0.87 mm.
• Flexion caused 7.2% strains (17.5% with defect) and 1.63 mm bulge (2.21 mm with defect).
• Combined axial rotation and lateral bending yielded the highest strains (24.6%) with maximal bulging of 1.14 mm, showing no tight correlation between strain and bulge.

Conclusions:

• Combined spinal loading, particularly axial rotation with lateral bending, generates significantly higher annular fiber strains than single-axis loading.
• Disc bulging is not a reliable indicator of peak annular fiber strain, especially under complex loading scenarios.
• High fiber strains are localized in small posterolateral regions and exceed previously reported magnitudes.