Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

Introduction

As humans age, there is a natural progression of degeneration of the intervertebral disc. Disc degeneration is often accompanied by a loss of structural integrity of the annulus fibrosus. Annular lamellae work together to resist tensile strain but over time can experience microdamage resulting in degeneration. Many studies have examined various properties of single [1] and/or multilayer annular samples often using ovine, porcine, and bovine tissue [2], and in some rare cases, human cadaveric tissue [3,4]. Although these studies provide insight into the mechanics of the disc, it is necessary to more comprehensively investigate these properties in a human cadaveric model. The primary purpose of this study was to examine age-related mechanical differences of the annulus in a human cadaveric model.

Methods

Twenty-two discs were removed from eight soft fixed human cadaveric spine segments (T10-S1); 78 years +/- 13.4 years; 5 male, 3 female. Discs were dissected into single layer (n=22), bilayer (n=22), and multilayer samples (n=37). Single layer and bilayer samples were mechanically tested in tension; single layer testing isolated the intralamellar matrix while bilayer testing provided a more holistic measure of the annular mechanical properties. The multilayer samples were mechanically tested via a 180˚ peel test to investigate the interlamellar matrix. From these tests, numerous properties were examined using normalized stress and strain data.

Results

For the single layer samples, age was found to be negatively correlated with tissue stiffness (r=-0.558; p=0.011) and stress at initial failure (r=-0.568; p=0.009). Bilayer sample correlation analyses also revealed that age was negatively correlated with stress at end of toe-region (r=-0.604; p=0.003), stress at 15% strain (r=-0.478; p=0.033); and stress at 30% strain (r=-0.495; p=0.037). Further analyses examined the effect of age by grouping the samples into <75 years old and >75 years old. For the bilayer samples, Young’s modulus (p=0.024), stress at the end of toe-region (p=0.002), stress at 15% strain (p=0.009), and stress at 30% strain (p=0.011) were found to be significantly greater for the <75-year-old group compared to >75-year-old group. Age did not affect the mechanical properties from the peel tests; however, annular sample depth (superficial versus deep) affected peel stiffness (p<0.001) and peel strength (p=0.002) such that samples obtained from the superficial layers of the annulus were significantly stronger compared to the deep layers.

Discussion

The findings of this study support current literature regarding age-related differences in the annulus fibrosus. All significant findings for single and bilayer samples followed a similar trend of older spines (>75 years) experiencing lower stress properties than younger spines (<74 years), suggesting the annulus weakens with age. Additionally for multilayer samples, there was a main effect of annular depth for all peel properties, suggesting the outer annulus is stronger than the inner annulus, likely due to its collagenous composition.