Human Evolution and Osteoporosis-Related Spinal Fractures

Abstract

The field of evolutionary medicine examines the possibility that some diseases are the result of trade-offs made in human evolution. Spinal fractures are the most common osteoporosis-related fracture in humans, but are not observed in apes, even in cases of severe osteopenia. In humans, the development of osteoporosis is influenced by peak bone mass and strength in early adulthood as well as age-related bone loss. Here, we examine the structural differences in the vertebral bodies (the portion of the vertebra most commonly involved in osteoporosis-related fractures) between humans and apes before age-related bone loss occurs. Vertebrae from young adult humans and chimpanzees, gorillas, orangutans, and gibbons (T8 vertebrae, n = 8–14 per species, male and female, humans: 20–40 years of age) were examined to determine bone strength (using finite element models), bone morphology (external shape), and trabecular microarchitecture (micro-computed tomography). The vertebrae of young adult humans are not as strong as those from apes after accounting for body mass (p<0.01). Human vertebrae are larger in size (volume, cross-sectional area, height) than in apes with a similar body mass. Young adult human vertebrae have significantly lower trabecular bone volume fraction (0.26±0.04 in humans and 0.37±0.07 in apes, mean ± SD, p<0.01) and thinner vertebral shells than apes (after accounting for body mass, p<0.01). Since human vertebrae are more porous and weaker than those in apes in young adulthood (after accounting for bone mass), even modest amounts of age-related bone loss may lead to vertebral fracture in humans, while in apes, larger amounts of bone loss would be required before a vertebral fracture becomes likely. We present arguments that differences in vertebral bone size and shape associated with reduced bone strength in humans is linked to evolutionary adaptations associated with bipedalism.

The Effect of the Achilles Tendon on Trabecular Structure in the Primate Calcaneus

“…Comparative studies characterizing the microarchitecture of trabecular bone have consistently found that humans have a lower bone volume fraction (BV/TV) than chimpanzees. Whether in the humeral head (Shaw and Ryan, 2012;Scherf et al, 2013), first metacarpal head (Lazenby et al, 2011), thoracic vertebra (Cotter et al, 2011), femoral head (Shaw and Ryan, 2012), calcaneal body (Maga et al, 2006), talar body (DeSilva and Devlin, 2012), or medial metatarsal heads (Griffin et al, 2010), humans consistently possess lower BV/TV than chimpanzees. Under the Achilles tendon insertion, however, we found a trend towards slightly higher BV/TV in humans than in chimpanzees, though this trend was not statistically significant.…”

Gracility of the modernHomo sapiensskeleton is the result of decreased biomechanical loading

“…The etiology of this relative gracility remains uncertain, and this uncertainty hinders the development of strategies for mitigating fracture risk and morbidity. The progressive gracilization of the Homo postcranial skeleton was originally detected in cortical bone structure (1, 2), but has now been demonstrated in the trabecular bone microstructure of joints (12,14,(16)(17)(18)(19), where osteoporotic fracture risk is highest (20). Most notably, in an analysis of thoracic vertebral bodies, Cotter et al (12) found that young adult humans have significantly lower trabecular bone volume fraction (BV/TV) and thinner vertebral shells than similarly sized apes.…”

“…trabecular bone | gracilization | human evolution | biomechanics | mobility C ompared with other hominoids and extinct hominin species, more recent humans possess relatively gracile postcranial skeletons (1)(2)(3)(4)(5)(6)(7)(8)(9). One of the consequences of this gracility in contemporary humans is an increased fracture risk associated with age-related bone loss and osteoporosis [hip fractures alone are projected to reach 6.26 million per year globally by 2050 (10)] (11)(12)(13)(14)(15). The etiology of this relative gracility remains uncertain, and this uncertainty hinders the development of strategies for mitigating fracture risk and morbidity.…”

“…2). In contrast, some (12,13) attribute lower trabecular bone volume in humans to the mechanical consequences of the larger vertebral cross-sectional areas and larger joint surface areas required of an obligate biped. The crux of this argument is that larger joint surfaces distribute loads across a greater area, thereby reducing strain in the underlying trabecular tissue and leading to lower bone volume.…”

“…The crux of this argument is that larger joint surfaces distribute loads across a greater area, thereby reducing strain in the underlying trabecular tissue and leading to lower bone volume. Cotter et al (12) have suggested that even if…”