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Review
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The Spine: A Strong, Stable, and Flexible Structure With Biomimetics Potential

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Review

The Spine: A Strong, Stable, and Flexible Structure With Biomimetics Potential

Fabio Galbusera et al. Biomimetics (Basel).

Abstract

From its first appearance in early vertebrates, the spine evolved the function of protecting the spinal cord, avoiding excessive straining during body motion. Its stiffness and strength provided the basis for the development of the axial skeleton as the mechanical support of later animals, especially those which moved to the terrestrial environment where gravity loads are not alleviated by the buoyant force of water. In tetrapods, the functions of the spine can be summarized as follows: protecting the spinal cord; supporting the weight of the body, transmitting it to the ground through the limbs; allowing the motion of the trunk, through to its flexibility; providing robust origins and insertions to the muscles of trunk and limbs. This narrative review provides a brief perspective on the development of the spine in vertebrates, first from an evolutionary, and then from an embryological point of view. The paper describes functions and the shape of the spine throughout the whole evolution of vertebrates and vertebrate embryos, from primordial jawless fish to extant animals such as birds and humans, highlighting its fundamental features such as strength, stability, and flexibility, which gives it huge potential as a basis for bio-inspired technologies.

Keywords: energy consumption; flexibility; lordosis; spine; stability; vertebrates.

Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Schematic representation of a vertebra. Adapted from Reference [6].
Figure 2
Figure 2
Schematic representation of the spine and vertebral anatomy of a ray-finned fish. Adapted from Reference [32].
Figure 3
Figure 3
Axial skeleton of a reptile, the extinct Titanophoneus (top, tail not completely shown) and a mammal, Felis catus (bottom), highlighting the presence of cervical, thoracic, and lumbar ribs in the former, and of the rib cage and sternum in the latter.
Figure 4
Figure 4
Zygapophyses (in red) in the human thoracic spine (left) and in a snake vertebra (right). In the latter, the zygosphene–zygantrum joints are also highlighted (in green and blue). Top: cranial view; bottom; caudal view. Part of the figure was adapted from Reference [49].
Figure 5
Figure 5
Schematic view of the trunk skeleton in a bird (left). On the right, from top to bottom: cranial, sagittal, and dorsal views of a cervical vertebra of a seagull. The facies articularis cranialis articulates with the facies articularis caudalis of the next vertebra; their shapes allow for a large range of motion. Adapted from Reference [62].
Figure 6
Figure 6
The sprawling (left) and erect (right) postures. In the sprawling posture, the long bones of the limbs are mostly loaded in bending, whereas the load becomes mostly compressive when standing erect. Adapted from Reference [76].
Figure 7
Figure 7
Simplified representation of the action of the body weight in quadrupeds (left) and in bipeds with a vertical spine (right). In quadrupeds such as the sheep, the body weight generates an extension load on the spine resulting in sag, whereas, in a vertical spine, body weight and muscles induce a mostly axial loading in compression.
Figure 8
Figure 8
Anatomical protection mechanisms to avoid hyperextension in mammal quadrupeds and non-bipedal primates (macaque, left) in comparison with humans (right). Ventral transverse processes determine tensile stress in the intertransverse ligament in extension, which limits the vertebral rotations. Lumbar vertebrae commonly possess a styloid process which acts as an osseous block to facet sliding in extension. In humans, these protection mechanisms were lost; transverse processes are dorsally located, and the styloid process is absent. Furthermore, facet joints have fewer encompassing shapes and do not strongly interlock in extension.
Figure 9
Figure 9
Sagittal profile of the spine in chimpanzees in standing posture and humans. Chimpanzees have an almost horizontal sacral plate, which forces the spine to assume a C-shape. In humans, the higher sacral slope determines the development of an S-shape.
Figure 10
Figure 10
Adaptations of the back musculature of humans (in comparison with macaques) to allow for bipedal stance and gait. In humans and all other hominoids, the erector spina are dorsally located and functions, therefore, as an extensor (top), whereas, in the macaque, it is more ventrally located and has only a minor extensor function. The caudal insertion of the iliocostalis lumborum moved from the lumbar spine to the iliac crest (middle), thus enhancing its lateral flexor function. Specifically, in humans, the posterior superior iliac spine which acts as the iliac insertion of the multifidus is markedly more posterior, thus providing an additional lever arm and room for muscle mass (bottom).
Figure 11
Figure 11
Schematic representation of the development of vertebrae and intervertebral discs from the notochord and the sclerotome. Left: mouse embryo at day 10.5–11.5 of development; right: mature axial skeleton. The nucleus pulposus develops from the notochord, whereas the annulus fibrosus and the other tissues including vertebral body, ligaments, and endplates originate from the sclerotome. NT: neural tube; SC: sclerotome; NC: notochord; VB: vertebral body; IVD: intervertebral disc; NP: nucleus pulposus; iAF: inner annulus fibrosus; oAF: outer annulus fibrosus; CEP: cartilaginous endplate. Reprinted with permission from Reference [106].

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