Vertebral bodies or discs: which contributes more to human-like lumbar lordosis?

Abstract

Background: The attainment of upright posture, with its requisite lumbar lordosis, was a major turning point in human evolution. Nonhuman primates have small lordosis angles, whereas the human spine exhibits distinct lumbar lordosis (30 degrees -80 degrees ). We assume the lumbar spine of the pronograde ancestors of modern humans was like those of extant nonhuman primates, but which spinal components changed in the transition from small lordosis angles to large ones is not fully understood.

Questions/purposes: We wished to determine the relative contribution of vertebral bodies and intervertebral discs to lordosis angles in extant primates and humans.

Methods: We measured the lordosis, intervertebral disc, and vertebral body angles of 100 modern humans (orthograde primates) and 56 macaques (pronograde primates) on lateral radiographs of the lumbar spine (humans-standing, macaques-side-lying).

Results: The humans exhibited larger lordosis angles (51 degrees ) and vertebral body wedging (5 degrees ) than did the macaques (15 degrees and -25 degrees , respectively). The differences in wedging of the intervertebral discs, however, were much less pronounced (46 degrees versus 40 degrees ).

Conclusions: These observations suggest the transition from pronograde to orthograde posture (ie, the lordosis angle) resulted mainly from an increase in vertebral body wedging and only in small part from the increase in wedging of the intervertebral discs.

Fig. 1

The lateral lumbar radiograph of an adult macaque is shown. The small lordosis angle of the macaque’s spine is evident, especially compared with that of the human.

Fig. 2

The method for measuring lumbar lordosis angle (LA) is shown on a lateral radiograph of an adult human. The human spine has a large lordosis angle, especially compared with that of the macaque.

Fig. 3

The method for measuring segmental measurements (B, D) is shown on a lateral radiograph of an adult human, using the example of PS₃ D, intervertebral disc angle of the third presacral vertebra, and PS₂ B, vertebral body wedge angle of the second presacral vertebra. Lordotic wedging of the intervertebral disc can be seen.

Fig. 4

The vertebral bodies (B) of the modern humans showed more lordotic wedging (p < 0.0001) than those of the macaques. ▲ = modern human and ● = macaque. Angles greater than 0° indicate lordotic wedging, angles of 0° indicate no wedging, and angles smaller than 0° indicate kyphotic wedging.