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

Danielle M. Stewart, Lauren A. Monaco, Diane E. Gregory

To determine the effect of age on the biomechanical properties of the intralamellar matrix of single annulus fibrosus (AF) lamellae. 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. 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). 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.