The aging disc: using an ovine model to examine age-related differences in the biomechanical properties of the intralamellar matrix of single lamellae.

PURPOSE: To determine the effect of age on the biomechanical properties of the intralamellar matrix of single annulus fibrosus (AF) lamellae. METHODS: One intervertebral disc (IVD) was excised from five young (<12 months), five middle-aged (2-4 years) and five older (5-7 years) ovine lumbar spines. From each IVD, a maximum of four single AF lamellae samples were harvested: two from the anterior region and two from the posterior region. Tissues were mounted in a tensile testing apparatus such that tension was applied perpendicular to the orientation of the collagen fibers to isolate the intralamellar matrix. Variables of interest from the stress-strain relationship were: end of toe-region strain and corresponding stress, initial failure stress and strain, and elastic stiffness. RESULTS: When compared to the middle-aged and old samples, the intralamellar matrix of young AF samples displayed significantly higher stress values at the end of the end of toe-region (p = 0.008) and at initial failure (p = 0.002). Further, the young samples were stiffer than both middle-aged and old samples (p = 0.04). CONCLUSIONS: This study was the first to show that the intralamellar matrix of single AF lamellae is weaker and more compliant in middle-aged and old ovine IVDs compared to young IVDs. These findings are likely a result of the remarkable age-related changes that occur that ultimately weaken the IVD as a whole.

A comparison between porcine, ovine, and bovine intervertebral disc anatomy and single lamella annulus fibrosus tensile properties.

This project aimed to compare gross anatomical measures and biomechanical properties of single lamellae from the annulus fibrosus of ovine and porcine lumbar vertebrae, and bovine tail vertebrae. The morphology of the vertebrae of these species differ significantly both from each other and from human, yet how these differences alter biomechanical properties is unknown. Geometric parameters measured in this study included: 1) absolute and relative intervertebral (IVD) and vertebral body height and 2) absolute and relative intervertebral disc (IVD) anterior-posterior (AP) and medial-lateral (ML) widths. Single lamella tensile properties included toe-region stress and stretch ratio, stiffness, and tensile strength. As expected, the bovine tail IVD revealed a more circular shape compared with both the ovine and porcine lumbar IVD. The bovine tail also had the largest IVD to vertebral body height ratio (due to having the highest absolute IVD height). Bovine tail lamellae were also found to be strongest and stiffest (in tension) while ovine lumbar lamellae were weakest and most compliant. Histological analysis revealed the greatest proportion of collagen in the bovine corroborating findings of increased strength and stiffness. The observed differences in anatomical shape, connective tissue composition, and tensile properties need to be considered when choosing an appropriate model for IVD research.