Mechanical age-related differences in the human cadaveric annulus fibrosus

As humans age, the intervertebral disc begins to deteriorate and lose structural integrity. The purpose of this study was to examine age-related mechanical differences of the annulus fibrosus in a human cadaveric model.

Twenty-two discs were removed from eight soft fixed human cadaveric spine segments (T10-S1) ranging from 53 to 90 years of age; 5 male, 3 female. All discs were a degenerative grade of 3 or higher. Single layer (n = 22), bilayer (n = 22), and multilayer annulus samples (n = 37) were mechanically tested from of the excised discs. 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 tested via a 180° peel test to investigate the interlamellar matrix. From these tests, numerous mechanical properties were quantified.

Age was found to significantly affect single and bilayer stiffness and numerous stress properties including single layer failure stress, bilayer end of toe-region stress, and bilayer stress at 15 % strain such that as age increased, the magnitude of these mechanical properties decreased. In contrast, age did not affect any peel test mechanical property (p > 0.05).

This study demonstrated that, with increasing age, the annulus fibrosus becomes more compliant and weaker. However, the adhesive matrix found between the lamellae of the annulus does not appear to be impacted by age.