Correlation between biomechanical properties of the annulus fibrosus and magnetic resonance imaging (MRI) findings.

Abstract

To investigate the regional tensile properties of human annulus fibrosus (AF) and relate them to magnetic resonance imaging (MRI) findings. 44 human cadaveric lumbar spines were harvested (24 male, 20 female, aged 25-64 years). MRI was used to identify Pfirrmann grade of disc degeneration, and Modic changes (MCs). Intervertebral discs were then removed and dissected into five regions: nucleus pulposus, anterior AF, anterolateral AF, lateral AF, and posterolateral AF. Samples for tensile testing (1.5 mm × 1.5 mm × 5 mm) were removed from inner, middle and outer parts of each region. 1969 specimens from 189 discs were stretched to failure. Average tensile stiffness (modulus) increased from 4.80 MPa in the inner AF to 13.0 MPa in the outer AF. Strength (UTS) increased similarly, from 1.18 to 3.29 MPa, whereas elongation at failure decreased, from 49 to 38%. The only significant change with age was a reduction in UTS in the middle annulus. In contrast, severe grades of disc degeneration were associated with consistent and highly significant reductions in tensile properties. Effects were greatest in the outer AF, where stiffness and strength fell by 29 and 43%, respectively. Modic changes also were associated with reduced stiffness and strength, but here the effects were greatest in the inner and middle AF. Weakening of degenerated AF may be caused by accumulating structural defects, and enzymatic degradation. MRI has the potential to identify local weakening of the AF.