Intervertebral Disc Degeneration in a Naturally Occurring Primate Model: Radiographic and Biomechanical Evidence

We hypothesized that Macaque monkeys ( macaca fasicularis ) undergo natural intervertebral disc degeneration similar to humans, based on radiographic appearance and mechanical response. 18 macaques (22.3 ± 0.9 years) born in captivity and singly caged were investigated with lateral and antero‐posterior radiographs taken less than 18 months prior to death. Lateral radiograph of the thoracic and lumbosacral areas of the spine was read for disc space narrowing and osteophytosis at each anterior vertebral margin by a single observer using an atlas method (grade 0–3). Specimens were then dissected free of its musculature and posterior elements, maintaining only a vertebral body‐disc‐vertebral body preparation intact for testing (T10‐11, T12‐L1, L6‐7). 54 specimens were wired superiorly and inferiorly on the vertebral bodies, embedded in poly‐methylmethacrylate, bathed in 0.9% saline, and frozen at −20°C until testing. Each specimen was then loaded into a Material Test System and preconditioned (100‐N, 50 cycles at 1 Hz), followed by an impulse test (350‐N over 50‐msec rise). Stiffness was found by fitting the impulse data to 20% – 80% peak force with a linear regression (r 2 > 0.90). Hysteresis energy was calculated as the area between the loading and unloading curves. Results show a link between stiffness and radiographic disc grading where advanced degeneration exhibited higher stiffness and absorbed less energy.