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, 19 (1), 226

3D Shape Analyses of Extant Primate and Fossil Hominin Vertebrae Support the Ancestral Shape Hypothesis for Intervertebral Disc Herniation

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3D Shape Analyses of Extant Primate and Fossil Hominin Vertebrae Support the Ancestral Shape Hypothesis for Intervertebral Disc Herniation

Kimberly A Plomp et al. BMC Evol Biol.

Abstract

Background: Recently we proposed an evolutionary explanation for a spinal pathology that afflicts many people, intervertebral disc herniation (Plomp et al. [2015] BMC Evolutionary Biology 15, 68). Using 2D data, we found that the bodies and pedicles of lower vertebrae of pathological humans were more similar in shape to those of chimpanzees than were those of healthy humans. Based on this, we hypothesized that some individuals are more prone to intervertebral disc herniation because their vertebrae exhibit ancestral traits and therefore are less well adapted for the stresses associated with bipedalism. Here, we report a study in which we tested this "Ancestral Shape Hypothesis" with 3D data from the last two thoracic and first lumbar vertebrae of pathological Homo sapiens, healthy H. sapiens, Pan troglodytes, and several extinct hominins.

Results: We found that the pathological and healthy H. sapiens vertebrae differed significantly in shape, and that the pathological H. sapiens vertebrae were closer in shape to the P. troglodytes vertebrae than were the healthy H. sapiens vertebrae. Additionally, we found that the pathological human vertebrae were generally more similar in shape to the vertebrae of the extinct hominins than were the healthy H. sapiens vertebrae. These results are consistent with the predictions of the Ancestral Shape Hypothesis. Several vertebral traits were associated with disc herniation, including a vertebral body that is both more circular and more ventrally wedged, relatively short pedicles and laminae, relatively long, more cranio-laterally projecting transverse processes, and relatively long, cranially-oriented spinous processes. We found that there are biomechanical and comparative anatomical reasons for suspecting that all of these traits are capable of predisposing individuals to intervertebral disc herniation.

Conclusions: The results of the present study add weight to the hypothesis that intervertebral disc herniation in H. sapiens is connected with vertebral shape. Specifically, they suggest that individuals whose vertebrae are towards the ancestral end of the range of shape variation within H. sapiens have a greater propensity to develop the condition than other individuals. More generally, the study shows that evolutionary thinking has the potential to shed new light on human skeletal pathologies.

Keywords: Back pain; Bipedalism; Human evolution; Intervertebral disc herniation; Spine; Vertebrae.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1
Fig. 1
A Schmorl’s node on the inferior endplate of a human thoracic vertebra
Fig. 2
Fig. 2
Landmarks used in the analyses. There are 54 in total. The red ones are the 26 that were used in the first set of analyses. In the third set of analyses, the 33 landmarks on the superior surface of the vertebrae were used. The top-left image is the superior view; the top-right image is the inferior view; and the bottom image is the right lateral view
Fig. 3
Fig. 3
Shape variation in the extant penultimate thoracic vertebrae captured by PCs 1 and 3, which account for 19.7 and 7.3% of the variation, respectively. PC2 did not did not reveal differences among the taxa and therefore was replaced with PC3. The wireframes illustrate the vertebral shapes described by PC1 and PC3. The stars indicate where the wireframes are located in the scatter-plot
Fig. 4
Fig. 4
Shape variation in the extant final thoracic vertebrae captured by PCs 1 and 2, which account for 15 and 12.7% of the variation, respectively. The wireframes illustrate the vertebral shapes described by PC1 and PC2. The stars indicate where the wireframes are located in the scatter-plot
Fig. 5
Fig. 5
Shape variation in the extant first lumbar vertebrae captured by PCs 1 and 2, which account for 22.0% and 15.3% of the variation, respectively. The wireframes illustrate the vertebral shapes described by PC1. The stars indicate where the wireframes are located in the scatter-plot
Fig. 6
Fig. 6
Cartoon illustrating the differences in diameter between a heart-shaped vertebral body and a more circular vertebral body

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