Biomechanics of the Spine

Basic Concepts, Spinal Disorders and Treatments

Fabio Galbusera, Hans-Joachim Wilke, Fabio Galbusera, Hans-Joachim Wilke

• Editorial: Academic Press
• Año: 2018
• ISBN:
  9780128128527
• Categoría: Tecnología e ingeniería
• Categoría: Ciencias biomédicas

Biomechanics of the Spine encompasses the basics of spine biomechanics, spinal tissues, spinal disorders and treatment methods. Organized into four parts, the first chapters explore the functional anatomy of the spine, with special emphasis on aspects which are biomechanically relevant and quite often neglected in clinical literature. The second part describes the mechanics of the individual spinal tissues, along with commonly used testing set-ups and the constitutive models used to represent them in mathematical studies. The third part covers in detail the current methods which are used in spine research: experimental testing, numerical simulation and in vivo studies (imaging and motion analysis). The last part covers the biomechanical aspects of spinal pathologies and their surgical treatment.

This valuable reference is ideal for bioengineers who are involved in spine biomechanics, and spinal surgeons who are looking to broaden their biomechanical knowledge base. The contributors to this book are from the leading institutions in the world that are researching spine biomechanics.

• Includes broad coverage of spine disorders and surgery with a biomechanical focus
• Summarizes state-of-the-art and cutting-edge research in the field of spine biomechanics
• Discusses a variety of methods, including In vivo and In vitro testing, and finite element and musculoskeletal modeling

Section 1

The Human Spine

Chapter 1

The Spine: Its Evolution, Function, and Shape

Fabio Galbusera IRCCS Galeazzi Orthopedic Institute, Milan, Italy

Abstract

From its first appearance in early vertebrates as local densifications of the notochord, or rudimentary vertebrae, the spine’s main function has been to protect the spinal cord from external forces and traumas, thus avoiding excessive straining during body motion. In addition, in humans as well as in all other tetrapods, the spine supports the body’s weight, through its flexibility enables the motion of the trunk, and provides the trunk and limbs with robust origins, insertions, and movements. In this chapter, the evolution of the spine from fish, to mammals, to modern humans and the spine’s functions and shape are briefly described. A detailed analysis of the individual spine regions and their biomechanics are covered in the subsequent chapters.

Keywords

Spine curvature; Tetrapod; Bipedalism; Upright posture; Standing; Pelvis; Lordosis

The Evolution of the Spine

During the Silurian Period (444 to 419 million years ago), the only animals with a bony skeleton were fish, which were structurally similar to the modern ray-finned fish that now comprise more 30,000 extant species. The skeleton of ray-finned fish includes a vertebral column consisting of a series of rigid bony vertebrae that protect the spinal cord (Fig. 1) (Nelson, 2006). For locomotion, ray-finned fish utilize fins consisting of bony rays covered by a layer of skin, which are not directly connected to the vertebral column (except for the tail) but are supported only by muscles. Indeed, ray-finned fish do not have a pelvic girdle, and the pectoral fins, from which the forelimbs evolved, are directly connected to the skull (Rockwell et al., 1938).

Fig. 1 Skeletons of a ray-finned fish (left, European perch) and of Ichthyostega (right). In the tetrapod design, the shoulder and the pelvic girdles connect the limbs to the spine and facilitate terrestrial life. Adapted from Ahlberg, P.E., Clack, J.A., Blom, H., 2005. The axial skeleton of the Devonian tetrapod Ichthyostega. Nature 437, 137–140.

Anatomical structures similar to limbs appeared in lobe-finned fish, whose fins are fleshy and partially covered with scales (Benton, 2015). Fins in most lobe-finned fish did not, however, have a rigid connection to the spine; these features progressively emerged during the Devonian Period when the first amphibians with a tetrapod body plan appeared, such as the Acanthostega and the Ichthyostega (Fig. 1) (Pierce et al., 2013; Pierce and Clack, 2012). As these animals underwent a gradual transition from an aquatic to a terrestrial life, the design principle of a spine detached from the limbs fell short of being able to support their body weight outside an aquatic environment, and the pelvic girdle emerged as a mobile but sturdy connection between the spine and the lower limbs. Recent studies suggest that limbs appeared before the transition to terrestrial life began (Pierce and Clack, 2012; Pierce et al., 2013). The usefulness of the tetrapod plan for aquatic life remains, however, unclear (Benton, 2015). Besides, it is interesting to note that in later animals that reacquired an aquatic life, such as marine reptiles, the direct connection between pelvis and spine disappeared.

At the same time, the forelimbs detached from the skull and moved posteriorly forming the shoulder girdle and the neck (i.e., the region of the spine between the skull and the shoulders), allowing for head motion as well as for disconnecting the skull and the brain from the anatomical structures involved in locomotion, thus reducing the mechanical loads on the brain (Pierce et al., 2013). With these latter evolving steps, it can be said that the principles of the tetrapod body design with the vertebral column as its core, which was then maintained in all reptiles, birds, and mammals, were established.

Comparative Spinal Anatomy in Vertebrates

The global structure of bony fish vertebrae and that of the subsequently evolved terrestrial animals share many features. Fish vertebrae have a round vertebral body called centrum, endplates with a marked biconcavity, and a dorsally located neural arch that encloses the spinal cord (Rockwell et al., 1938). In most cases, a ventral vertebral arch called the hemal arch, or chevron, is also present (Fig. 2). The two arches commonly protrude in a spine. Together with the transverse processes, the neural and hemal spines serve as muscle insertions and articulate with the ribs (Bone and Moore, 2007). Zygapophysial joints limiting bending and flexion motions are also present in most fish. Instead of intervertebral discs, a segmentally constricted cartilage-like notochord running through the entire length of the spine provides flexibility to the body. The fish spine is generally subdivided into a pre-caudal region and a caudal region; no major regional differentiation is observable in the pre-caudal spine.

Fig. 2 Comparative vertebral anatomies. From left to right: a fish pre-caudal vertebra (pike), in which a hemal arch is also present; a reptile vertebra (python); a lumbar vertebra of a mammal quadruped (deer); a human lumbar vertebra.

In comparison with fish and early amphibians, vertebrae of extant amphibians do not show any specific advancements and even tend to exhibit a simpler structure. However, amphibians show for the first time in the evolutionary scale the presence of a sacral bone, which consists of a series of fused vertebrae providing a solid interface to the pelvis and in turn to the lower limbs, and has therefore a fundamental mechanical role in terrestrial weight bearing and locomotion (Schoch, 2014; Romer and Parsons, 1977). Paleontological studies showed that the sacrum evolved in transitional animals such as the Ichthyostega that were still conducting an aquatic life (Pierce et al., 2013). Thus the development of the sacrum may not be related to terrestrial locomotion; however, this trait developed when the transition to terrestrial life initiated.

Vertebrae of reptiles show a very close resemblance to those of mammals and birds, the most evident difference being the concave socket of the anterior endplate fitting to the convex surface of the posterior endplate of the next vertebra (Fig. 2) (Romer and Parsons, 1977). Intercentra (i.e., small bony elements located between adjacent vertebrae that are also found in fish and amphibians but have been lost in mammals except in their tails) are often present in reptiles, as well as hemal arches. In birds, the only flexible region of the spine is the neck, whereas thoracic vertebrae are partially fused to provide for a robust attachment for the wings. Lumbar vertebrae and sacrum are fused into the synsacrum as well as the caudal vertebrae.

The anatomy of the spine of mammals exhibits a relatively low variability among the class; the number of vertebrae is almost constant, with all mammals having seven cervical vertebrae apart from sloths and manatees, and with all spine regions ha...