Biomechanics ofVertebral Column : Structure Dr. D. N. Bid
Introduction• The vertebral column is an amazingly complex structure that must meet the seemingly contradictory demands of mobility and stability of the trunk and the extremities and of providing protection for the spinal cord.• Although the pelvis is not considered to be part of the vertebral column, the pelvic attachment to the vertebral column through the sacroiliac joints (SIJs) will be included in this discussion because of the interrelationship of these joints to those of the lumbar region.
Structure• The vertebral column resembles a curved rod, composed of 33 vertebrae and 23 intervertebral disks.• The vertebral column is divided into the following five regions: cervical, thoracic, lumbar, sacral, and coccygeal (Fig. 4-1).• The vertebrae adhere to a common basic structural design but show regional variations in size and configuration that reflect the functional demands of a particular region.
• The vertebrae increase in size from the cervical to the lumbar regions and then decrease in size from the sacral to coccygeal regions.• Twenty-four of the vertebrae in the adult are distinct entities.• Seven vertebrae are located in the cervical region, 12 in the thoracic region, and 5 in the lumbar region.• Five of the remaining nine vertebrae are fused to form the sacrum, and the remaining four constitute the coccygeal vertebrae.
• In the frontal plane, the vertebral column bisects the trunk when viewed from the posterior aspect.•• When viewed from the sagittal plane, the curves are evident (see Fig. 4-1).• The curve of the vertebral column of a baby in fetal life exhibits one long curve that is convex posteriorly, whereas secondary curves develop in infancy.• However, in the column of an adult, four distinct anteroposterior curves are evident (Fig. 4-2).
• The two curves (thoracic and sacral) that retain the original posterior convexity throughout life are called primary curves, whereas the two curves (cervical and lumbar) that show a reversal of the original posterior convexity are called secondary curves.• Curves that have a posterior convexity (anterior concavity) are referred to as kyphotic curves; curves that have an anterior convexity (posterior concavity) are called lordotic curves.• The secondary or lordotic curves develop as a result of the accommodation of the skeleton to the upright posture.
• A curved vertebral column provides significant advantage over a straight rod in that it is able to resist much higher compressive loads.• According to Kapandji, a spinal column with the normal lumbar, thoracic, and cervical curves has a 10-fold ability to resist axial compression in comparison with a straight rod.• The vertebral column functions as a closed chain with both the head and the ground.
• We can easily see how this occurs through contact of the feet to the ground, but we often forget the need for the head to remain in a somewhat stable position as we move to allow the sensory organs, particularly the eyes and ears, to be optimally positioned for function.• Each of the many separate but interdependent components of the vertebral column is designed to contribute to the over-all function of the total unit, as well as to perform specific tasks.
• The first section of this chapter will cover the general components of the mobile segment, followed by regional variations and the SIJs.• The second section of the chapter will cover the muscles of the vertebral column and pelvis.
• The Mobile Segment• It is generally held that the smallest functional unit in the spine is the mobile segment; that is, any two adjacent vertebrae, the intervening intervertebral disk (if there is one), and all the soft tissues that secure them together.
• A Typical Vertebra• The structure of a typical vertebra consists of two major parts: an anterior, cylindrically shaped vertebral body and a posterior, irregularly shaped vertebral or neural arch (Fig. 4-3).• The vertebral body is designed to be the weight- bearing structure of the spinal column.• It is suit-ably designed for this task, given its block-like shape with generally flat superior and inferior surfaces.
• In order to minimize the weight of the vertebrae and allow dynamic load- bearing, the vertebral body is not a solid block of bone but a shell of cortical bone surrounding a cancellous cavity.• The cortical shell is reinforced by trabeculae in the cancellous bone, which provide resistance to compressive forces.
• The neural arch can be further divided into the pedicles and the posterior elements.• The pedicles are the portion of the neural arch that lie anterior to the articular processes on either side and serve as the connection between the posterior elements and the vertebral bodies.• Their function is to transmit tension and bending forces from the posterior elements to the vertebral bodies.• They are well designed for this function, inasmuch as they are short, stout pillars with thick walls.
• In general, the pedicles increase in size from the cervical to lumbar regions, which makes sense inasmuch as greater forces are transmitted through the pedicles in the lumbar region.
• The remaining posterior elements are the laminae, the articular processes, the spinous process, and the transverse processes (Fig. 4-4).• The laminae are centrally placed and serve as origination points for the rest of the posterior elements.• The laminae are thin, vertically oriented pieces of bone that serve as the “roof” to the neural arch, which protects the spinal cord.
• In addition, the laminae transmit forces from the posterior elements to the pedicles and, through them, onto the vertebral body.• This force transfer occurs through a region of the laminae called the pars interarticularis.
• The pars interarticularis, as its name suggests, is the portion of the laminae that is between the superior and inferior articular processes (Fig. 4-5).• The pars interarticularis is subjected to bending forces as forces are transmitted from the vertically oriented lamina to the more horizontally oriented pedicles.• The pars interarticularis is most developed in the lumbar spine, where the forces are the greatest in magnitude.
• Typically, an increase in cortical bone occurs to accommodate the increased forces in this region.• However, in some individuals, the cortical bone is insufficient, making them susceptible to stress fractures.
• The spinous processes and two transverse processes are sites for muscle attachments and serve to increase the lever arm for the muscles of the vertebral column.• The articular processes consist of two superior and two inferior facets for articulation with facets from the cranial and caudal vertebrae, respectively.• In the sagittal plane, these articular processes form a supportive column, frequently referred to as the articular pillar.• Table 4-1 summarizes the components of a typical vertebra.
• The vertebrae are subjected to a wide variety of forces; however, they have a typical bony architecture that suggests a typical loading pattern.• Vertebral bone trabecular systems that develop in response to the stresses placed on the vertebral bodies and the neural arch are found within the spongy bone (Fig. 4-7).• The vertebrae have vertically oriented trabeculae with horizontal connections near the end plate and with denser bone areas near the pedicle bases.
• The vertical systems within the body help to sustain the body weight and resist compression forces (Fig. 4-8).• There are also fan-shaped trabeculae introduced into the vertebral body at the area of the pedicle in response to bending and shearing forces transmitted through this region.
• The Intervertebral Disk• The intervertebral disk has two principle functions: to separate two vertebral bodies, thereby increasing available motion, and to transmit load from one vertebral body to the next.• Therefore, the size of the intervertebral disk is related to both the amount of motion and the magnitude of the loads that must be transmitted.• The intervertebral disks, which make up about 20% to 33% of the length of the vertebral column, increase in size from the cervical to the lumbar regions.
• The disk thickness varies from approximately 3 mm in the cervical region, where the weight- bearing loads are the lowest, to about 9 mm in the lumbar region, where the weight-bearing loads are the greatest.• Although the disks are smallest in the cervical region and largest in the lumbar region, it is the ratio between disk thickness and vertebral body height that determines the available motion.
• The greater the ratio, the greater the mobility.• The ratio is greatest in the cervical region, followed by the lumbar region, and the ratio is smallest in the thoracic region.• This reflects the greater functional needs for mobility found in the cervical and lumbar regions and for stability in the thoracic region.
• The majority of the information regarding structure and function of the intervertebral disks has been gleaned from studies of the lumbar region.• It was long thought that the disks of the cervical and thoracic regions had a structure similar to those of the lumbar region.• It appears that this is not the case, particularly with regard to the intervertebral disks of the cervical region.• This section will describe the general structure and function of the intervertebral disk. Specific variations will be described with the regional structure.
• The intervertebral disks are composed of three parts: (1) the nucleus pulposus, (2) the anulus fibrosus, and (3) the vertebral end plate (Fig. 4-9).• The nucleus pulposus is the gelatinous mass found in the center, the anulus fibrosus is the fibrous outer ring, and the vertebral end plate is the cartilaginous layer covering the superior and inferior surfaces of the disk, separating it from the cancellous bone of the vertebral bodies above and below.
• All three structures are composed of water, collagen, and proteoglycans (PGs); however, the relative proportions of each vary.• Fluid and PG concentrations are highest in the nucleus and lowest in the outer anulus fibrosus and the outer vertebral end plate (closest to the vertebral body).
• Conversely, collagen concentrations are highest in the vertebral end plate and outer anulus and lowest in the nucleus pulposus.• Although the nucleus pulposus is clearly distinct from the anulus fibrosus in the center and the anulus fibrosus is clearly distinct from the nucleus in the outer rings, there is no clear boundary separating the two structures where they merge.• They are distinct structures only where they are furthest apart.
Nucleus Pulposus• The nucleus pulposus is 70% to 90% water, depending on age and time of day.• PGs make up approximately 65% of the dry weight, which, as you recall, have an ability to attract water molecules because of the presence of glycosaminoglycans, hence the high water content.• Collagen fibers contribute 15% to 20% of the dry weight, and the remainder of the dry weight contains many cells, including elastin, proteins, proteolytic enzymes, chondrocytes, and other types of collagen.
• The nucleus pulposus has both type I and type II collagen; however, type II predominates because of its ability to resist compressive loads.• In fact, very little if any type I collagen is present in the center portion of the nucleus pulposus.• The nucleus pulposus has been frequently likened to a water balloon.• When compressed, it deforms, and the increased pressure stretches the walls of the balloon in all directions (Fig. 4- 10).
Anulus Fibrosus• In general, the anulus fibrosus is 60% to 70% water, also depending on age and time of day.• Collagen fibers make up 50% to 60% of the dry weight, with proteoglycans contributing only 20% of the dry weight.2• Clearly, the relative proportions of these components are different from the nucleus pulposus, reflecting the difference in structure.• The remainder of the dry weight is made up of approximately 10% elastin and other cells such as fibroblasts, and chondrocytes.
• Again, type I and type II collagen are present; however, type I collagen predominates in the anulus fibrosus, particularly in the outer portions.• This makeup reflects the need for the anulus fibrosus rather than the nucleus pulposus to resist greater proportions of tensile forces.• The anulus fibers are attached to the cartilaginous end plates on the inferior and superior vertebral plateaus of adjacent vertebrae and to the epiphyseal ring region by Sharpey fibers.
Vertebral End Plates• The vertebral end plates are layers of cartilage 0.6 to 1mm thick that cover the region of the vertebral bodies encircled by the ring apophysis on both the superior and inferior surfaces.• They cover the entire nucleus pulposus but not the entire anulus fibrosus.• The vertebral end plate is strongly attached to the anulus fibrosus and only weakly attached to the vertebral body, which is why it is considered to be a component of the disk rather than the vertebral body.
• The vertebral end plates consist of proteoglycans, collagen, and water, as in the rest of the disk.• In addition, there are cartilage cells aligned along the collagen.• As in the other regions of the disk, there is a higher proportion of water and proteoglycans closest to the nucleus pulposus and a higher proportion of collagen closest to the anulus fibrosus and the subchondral bone of the vertebral body.• The cartilage of the vertebral end plates is both hyaline cartilage and fibrocartilage.
• Hyaline car-tilage is present closest to the vertebral body and is found mainly in young disks.• Fibrocartilage is present closest to the nucleus pulposus and, with increasing age, becomes the major component of the vertebral end plate, with little or no hyaline cartilage remaining, reflecting the need to tolerate high compressive forces.
Innervation and Nutrition• The intervertebral disks are innervated in the outer one third to one half of the fibers of the anulus fibrosus.• In the cervical and lumbar regions, the innervation has been demonstrated to be by branches from the vertebral and sinuvertebral nerves.• The sinuvertebral nerve also innervates the peridiskal connective tissue and specific ligaments associated with the vertebral column.
• The intervertebral disks do not receive blood supply from any major arterial branches.• The metaphyseal arteries form a dense capillary plexus in the base of the end plate cartilage and the subchondral bone deep to the end plate, and small branches from these metaphyseal arteries do supply the outer surface of the anulus fibrosus.• The remainder of the disk receives its nutri-tion via diffusion through these sources.
Articulations• Two main types of articulations are found in the vertebral column: cartilaginous joints of the symphysis type between the vertebral bodies, including the interposed disks, and diarthrodial, or synovial, joints between the zygapophyseal facets located on the superior articular processes of one vertebra and the zygapophyseal facets on the inferior articular processes of an adjacent vertebra above.
• The joints between the vertebral bodies are referred to as the interbody joints.• The joints between the zygapophyseal facets are called the zygapophyseal (apophyseal or facet) joints (Fig. 4-11).• Synovial joints also are present where the vertebral column articulates with the ribs, with the skull, and with the pelvis at the SIJs.
Interbody Joints• Available movements at the interbody joints include gliding, distraction and compression, and rotation (also called tilt or rocking in the spine) (Fig. 4-12).• Gliding motions can occur in the following directions: anterior to posterior, medial to lateral, and torsional.• Tilt motions can occur in anterior to posterior and in lateral directions.• These motions, together with the distraction and compression, constitute six degrees of freedom.6
• The amounts of each of these motions are small and vary by region according to structural differences in the disks and the vertebral bodies, as well as in the ligamentous supports.• In addition, the zygapophyseal joints influence the total available motion of the interbody joints.
Zygapophyseal Articulations• The zygapophyseal joints are composed of the articulations between the right and left superior articulating facets of a vertebra and the right and left inferior facets of the adjacent cranial vertebra.• The zygapophyseal joints are diarthrodial joints and have regional variations in structure.• Intra-articular accessory joint structures have been identified in the zygapophyseal joints.
• These accessory structures appear to be of several types, but most are classified as either adipose tissue pads or fibroadipose meniscoids.• The structures are most likely involved in protecting articular surfaces that are exposed during flexion and extension of the vertebral column.
Ligaments and Joint Capsules• The ligamentous system of the vertebral column is extensive and exhibits considerable regional variability.• Six main ligaments are associated with the interverte-bral and zygapophyseal joints.• They are the anterior and posterior longitudinal ligaments and PLL; the ligamentum flavum; and the interspinous, supraspinous, and intertransverse ligaments (Figs. 4-13 and 4-14).
Anterior and Posterior Longitudinal Ligaments• The anterior longitudinal ligament (ALL) and posterior longitudinal Ligament (PLL) are associated with the interbody joints.• The anterior longitudinal ligament runs along the anterior and lateral surfaces of the vertebral bodies from the sacrum to the second cervical vertebra.• Extensions of the ligament from C2 to the occiput are called the anterior atlanto-occipital and anterior atlantoaxial ligaments.• The anterior longitudinal ligament has at least two layers that are made up of thick bundles of collagen fibers.
• The fibers in the superficial layer are long and bridge several vertebrae, whereas the deep fibers are short and run between single pairs of vertebrae.• The deep fibers blend with the fibers of the anulus fibrosus and reinforce the antero- lateral portion of the intervertebral disks and the anterior interbody joints.
• The ligament is well developed in the lordotic sections (cervical and lumbar) of the vertebral column but has little substance in the region of thoracic kyphosis.• The anterior longitudinal ligament increases in thickness and width from the lower thoracic vertebrae to L5/S1.
• The tensile strength of the ligament is greatest at the high cervical, lower thoracic, and lumbar regions, with the greatest strength being in the lumbar region.17• The ligament is compressed in flexion (Fig. 4-15A) and stretched in extension (see Fig. 4-15B).• It may become slack in the neutral position of the spine when the normal height of the disks is reduced, such as might occur when the nucleus pulposus is destroyed or degenerated.18• The anterior longitudinal ligament is reported to be twice as strong as the PLL.17
• The PLL runs on the posterior aspect of the vertebral bodies from C2 to the sacrum and forms the ventral surface of the vertebral canal.• It also consists of at least two layers: a superficial and a deep layer.• In the superficial layer, the fibers span several levels.
• In the deep layer, the fibers extend only to adjacent vertebrae, interlacing with the outer layer of the anulus fibrosus and attaching to the margins of the vertebral end plates in a manner that varies from segment to segment.• Superiorly, the ligament becomes the tectorial membrane from C2 to the occiput.• In the lumbar region, the ligament narrows to a thin ribbon that provides little support for the interbody joints.
• The PLL’s resistance to axial tension in the lumbar area is only one sixth of that of the anterior longitudinal ligament.• The PLL is stretched in flexion (Fig. 4-16A) and is slack in extension (see Fig. 4-16B).
Ligamentum Flavum• The ligamentum flavum is a thick, elastic ligament that connects lamina to lamina from C2 to the sacrum and forms the smooth posterior surface of the vertebral canal.• Some fibers extend laterally to cover the articular capsules of the zygapophyseal joints.• From C2 to the occiput, this ligament continues as the posterior atlanto-occipital and atlantoaxial membranes.• The ligamentum flavum is strongest in the lower thoracic region and weakest in the midcervical region.
• Although the highest strain in this ligament occurs during flexion when the ligament is stretched, this ligament is under constant tension even when the spine is in a neutral position, because of its elastic nature.• This highly elastic nature serves two purposes.• First, it creates a continuous compressive force on the disks, which causes the intradiskal pressure to remain high.• The raised pressure in the disks makes the disks stiffer and thus more able to provide support for the spine in the neutral position.
• Second, a highly elastic ligament in this location is advantageous because the ligament will not buckle on itself during movement.• If the ligament did buckle on itself, it would compress the spinal cord in the vertebral canal, especially with any movement into flexion.
Interspinous Ligaments• The interspinous ligament connects spinous processes of adjacent vertebra. It is described as a fibrous sheet consisting of type I collagen, proteoglycans, and profuse elastin fibers.• The interspinous ligament, along with the supraspinous ligament, is the first to be damaged with excessive flexion.• The interspinous ligament is innervated by medial branches of the dorsal rami and thought to be a possible source of low back pain.• The interspinous ligament has been found to contribute to lumbar spine stability and to degenerate with aging.
Supraspinous Ligament• The supraspinous ligament is a strong cordlike structure that connects the tips of the spinous processes from the seventh cervical vertebra to L3 or L4.• The fibers of the ligament become indistinct in the lumbar area, where they merge with the thoracolumbar fascia and insertions of the lumbar muscles.• In the cervical region, the ligament becomes the ligamentum nuchae.
• The supraspinous ligament, like the interspinous ligament, is stretched in flexion, and its fibers resist separation of the spinous processes during forward flexion.• During hyperflexion, the supraspinous ligament, along with the interspinous ligament, is the first to fail.• The supraspinous ligament contains mechanoreceptors, and deformation of the ligament appears to play a role in the recruitment of spinal stabilizers such as the multifidus muscles.
Intertransverse Ligaments• The structure of the paired intertransverse ligaments is extremely variable. In general, the ligaments pass between the transverse processes and attach to the deep muscles of the back.• In the cervical region, only a few fibers of the ligaments are found.• In the thoracic region, the ligaments consist of a few barely discernible fibers that blend with adjacent muscles.• In the lumbar region, the ligaments consist of broad sheets of connective tissue that resembles a membrane.
• The membranous fibers of the ligament form part of the thoracolumbar fascia.• The ligaments are alternately stretched and compressed during lateral bending.• The ligaments on the right side are stretched and offer resistance during lateral bending to the left, whereas the ligaments on the left side are slack and compressed during this motion.• Conversely, the ligaments on the left side are stretched during lateral bending to the right and offer resistance to this motion.
Zygapophyseal Joint Capsules• The zygapophyseal joint capsules assist the ligaments in providing limitation to motion and stability for the vertebral column.• The roles of the joint capsules also vary by region.• In the cervical spine, the facet joint capsules, although lax, provide the primary soft tissue restraint to axial rotation and lateral bending, but they provide little restraint to flexion and extension.
• The capsules are strongest in the thoracolumbar region and at the cervicothoracic junction sites where the spinal configuration changes from a kyphotic to lordotic curve and from a lordotic to kyphotic curve, respectively, and the potential exists for excessive stress in these areas.• The joint capsules, like the supraspinous and interspinous ligaments, are vulnerable to hyper- flexion, especially in the lumbar region.
• It has been suggested that the joint capsules in the lumbar region provide more restraint to forward flexion than any of the posterior ligaments because they fail after the supraspinous and interspinous ligaments when the spine is hyperflexed.• Table 4-2 provides a summary of the ligaments and their functions.
• The zygapophyseal joint capsules of the lumbar spine, in addition to the anular fibers, also provide primary restraint to axial rotation, however those of the thoracic spine do not provide primary restraint to axial rotation.