Level-wise differences in in-vivo lateral bending moment are associated with microstructural alterations in bovine caudal intervertebral discs

Abstract

Despite its common use as a laboratory model, little is known about the in-vivo forces and moments applied to the bovine caudal intervertebral disc. This is particularly critical, as intervertebral disc tissue is known to remodel in response to repeated loading. We hypothesized that the magnitude of loading from muscle contraction during a typical lateral bending motion varies between caudal levels and is accompanied by variations in tissue microstructure. This hypothesis was tested by estimating level-wise forces and bending moments using two independent approaches: a dynamic analytical model of the motion and analysis of muscle cross sections obtained via computed tomography. Microstructure was assessed by measuring collagen fiber crimp period in the annulus fibrosus, and composition was assessed via quantitative histology. Both the analytical model and muscle cross-sections estimated peak bending moments of over 3Nm and peak compressive force of over 125N at the c1c2 level, decreasing distally. There was a significant downward trend from proximal to distal in outer annulus fibrosus collagen crimp period only in the anterior and posterior regions, suggesting remodeling in response to the highest lateral bending moments. There were no observed trends in composition. Our results suggest that while the proximal discs in the bovine tail are subjected to forces and moments from muscle contraction which are comparable (relative to disc size) to those acting on human lumbar discs, the distal discs are not. The resulting pattern of microstructural microstructure suggests that level-wise differences should be considered when using bovine discs as a research model.