Osteoporosis is actually irrelevant to silent vertebral fractures, judging from a new study by researchers at Case Western Reserve and Cornell universities. Their biomechanical comparisons between the hominoid species show that the T8 thoracic vertebrae in healthy young men are weaker and more porous than those in other hominoid species, probably an inevitable consequence of the fact that our ancient forebears began walking on two feet.

Anatomist Meghan Cotter and her coworkers at Case Western's Musculoskeletal Mechanics and Materials Laboratory used CT imaging to assess the bone mass of the T8 vertebrae from gibbons, orangutans, gorillas and chimpanzees from the collections of natural history museums in Cleveland, Chicago, and Washington DC. The human vertebrae came from 14 young men who died suddenly at a known age and had no history of bone disease.

They also collected microcomputed images of vertebral bodies for a subset of these samples, and used them to create biomechanical models of bone structure by methods often applied to human bones in the medical literature.

Relative to overall body mass, humans and apes have similar bone mass, the team reports. But vertebral body compressive strength was lower in the human vertebrae, relative to both bone mass and body mass, and the human vertebrae were more porous, even among men who had yet to reach the age of 40.

The relatively low bone strength in human thoracic vertebrae is caused by distinctive structural differences, they say: Compared to the (other) apes, we have reduced vertebral trabecular volume and a thinner vertebral shell, long before the expected onset of osteoporosis. Given the vagaries of human posture compared to that of the other hominoids, they speculate that loading on the T8 vertebrae is actually greater than in apes, even though the vertebrae are comparatively weaker.

This sorry state of affairs is probably an inevitable consequence of the ancestral "decision" to walk upright on two feet, which had numerous evolutionary advantages including energy efficiency and manual dexterity. But it causes relatively high impact on the bones during the heel strike phase of walking, the team observes.

The bones and joints of human legs and feet evolved as relatively more massive and porous than those of the other apes presumably because it helps to protect the brain from this impact. The increased porosity and size of the thoracic vertebrae may be analagous adaptations for the same purpose.

The fact that this leads to silent vertebral fractures among people well past reproductive age is, from the evolutionary perspective, inconsequential. Of course osteoporosis increases the risk, but biomechanics creates it in the first place.

The article "Human Evolution and Osteoporosis-Related Spinal Fractures"  appeared yesterday in the online journal PLoS One, a publication of the Public Library of Science.

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