Mechanical function of the annulus fibrosus is preserved following quasi-static compression resulting in endplate fracture.

Background: Vertebral fractures in young populations are associated with intervertebral disc disorders later in life. However, damage to the annulus fibrosus has been observed in rapidly loaded spines even without the subsequent occurrence of a fracture. Therefore, it may not be the fracture event that compromises the disc, but rather the manner in which the disc is loaded. The purpose of this study was to quantify the mechanical properties of the annulus fibrosus following quasi-static compressive loading of the motion segment either to sub-fracture or fracture-inducing magnitude.

Methods: Porcine cervical motion segments were axial compressed at 0.1 mm/s, either until endplate fracture occurred ('fracture group'), or until segments reached 75 % of average fracture stress as determined from the fracture group ('sub-fracture group'). An unloaded control group was also included. Post-loading, three samples of the annulus were excised. The first was mounted in a 180 peel test configuration in order to quantify lamellar adhesion. The other two samples were excised from the superficial and midspan region of the annulus and were exposed to uniaxial tension to 50 % strain.

Findings: Lamellar adhesion and tensile annulus mechanics did not differ between the fracture and sub-fracture group, nor between the unloaded controls.

Interpretation: Given the lack of differences in annular mechanical properties across the three conditions, it was concluded that under very slow, quasi-static compressive loading conditions, the annulus appeared undamaged even in the group that sustained a fracture; this is likely because a significant viscoelastic response was not generated in the disc under these slow loading conditions.