1Anthropology, University of Washington, 2Orthopaedic and Sports Medicine, University of Washington, 3Orthopaedic Surgery, University of California, San Francisco, 4Intramural Research Program of the NIH, National Institue of Aging
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Age-related intervertebral disc degeneration is prevalent among humans and is often attributed to orthograde posture, but whether or not posture is the cause of disc degeneration is difficult to test. Rhesus macaques (Macaca mulatta) are a comparable model because they sit upright, which vertically loads their spines, and exhibit polyarticular osteoarthritis, including spinal osteophytosis and decreased disc height. Under similar compressive loading, therefore, the rhesus macaque disc should demonstrate similar degenerative pathology as humans.
We analyzed mid-sagittal disc histology of thoracolumbar spines from six rhesus macaques (T9-L1, n=18 discs, ages 19-35 years) and lumbar spines from seven humans (T12-S1, n=44 discs, ages 51-67). Histology images of discs were rated for Thompson grade (a grade of disc degeneration based on gross morphology) and scored for specific degenerative features of the disc and adjacent bone (e.g. osteophytosis, disc tears, bony endplate lesions). While Thompson grade is significantly correlated with most degenerative features (p values range from < 0.001 to 0.048) in both humans and rhesus macaques, radial disc tears correlated with degeneration in humans, (p < 0.001), but not in rhesus macaques (p=0.99). This difference could derive from functional differences due to the different spinal curvature of humans, where T12-S1 is lordotic, and rhesus macaques, where T9-L1 is kyphotic.
Spinal osteoarthritis is prevalent with age in modern humans and also found among hominins (e.g. Homo neanderthalensis). Relating differences among extant species in posture to spinal osteoarthritis (e.g., location of osteophyte formation) may facilitate understanding the unique postures of extinct hominins.
Research was supported by NIH grant U01-AG021379 and the Debs Chair in Orthopaedic Research at the University of Washington. This study was supported in part by the Intramural Research Program of the NIH, National Institute on Aging.