When are intervertebral discs stronger than their adjacent vertebrae?

Study Design: Mechanical testing of cadaveric tissues.
Objective: To compare the strength of discs and vertebrae from the same spines in order to determine which are more vulnerable to injury, and to determine how their relative vulnerability depends on age and gender.
Summary of Background Data: Vertebrae can strengthen and weaken according to mechanical demands, but the avascular intervertebral discs may be unable to "keep up." Little is known about the relative strength of discs and vertebrae.
Methods: Forty-seven thoracolumbar motion segments were obtained from 30 cadavers 48 to 91 years of age. Each was compressed until a vertebra fractured, and vertebral yield compressive stress (force per unit area) was calculated. Adjacent undamaged intervertebral discs were removed, and circumferential slices, 2.2 mm thick, were cut from the inner, middle, and outer regions of the anterolateral anulus. Slices were stretched to failure to determine their ultimate tensile stress.
Results: Yield compressive stress of male and female vertebrae decreased by 69% and 75%, respectively, in the age range of 48 to 91 years (P < 0.001). In contrast, the ultimate tensile stress of the adjacent anulus did not fall significantly with age, except in the outer region of male discs, where it fell by 66% (P < 0.01). Disc strength was proportional to vertebral strength, but only for the outer anulus, and in male spines (r2= 24%, P = 0.019, n = 22).
Conclusion: The outer anulus can adapt to mechanical demands because it is the most metabolically active region of the disc. Disc and bone properties are better matched in male spines because male vertebrae are less affected by variable hormonal changes. The low adaptive potential of intervertebral discs makes them relatively weak in the strengthening spines of young men but relatively strong in the weakening spines of elderly women.