Biomechanics of vertebral column regional structure & function dnbid 2013-3Presentation Transcript
Biomechanics ofVertebral Column: Regional Structure & Function Dr. D. N. Bid Sarvajanik College of Physiotherapy, Rampura, Surat – 395003.
• The complexity of a structure that must fulfill many functions is reflected in the design of its component parts.• Regional structures are varied to meet different but equally complex functional requirements.• Structural variations evident in the first cervical and thoracic vertebrae, fifth lumbar vertebra, and sacral vertebrae represent adaptations necessary for joining the vertebral column to adjacent structures.
• Differences in vertebral structure are also apparent at the cervicothoracic, thoracolumbar, and lumbosacral junctions, at which a transition must be made between one type of vertebral structure and another.
• The vertebrae located at regional junctions are called transitional vertebrae and they usually possess characteristics common to two regions.• The cephalocaudal increase in the size of the vertebral bodies reflects the increased proportion of body weight that must be supported by the lower thoracic and lumbar vertebral bodies.• Fusion of the sacral vertebrae into a rigid segment reflects the need for a firm base of support for the column.
• In addition to these variations, a large number of minor alterations in structure occur throughout the column.• However, only the major variations are discussed here.
Structure of the Cervical Region• The cervical vertebral column consists of seven vertebrae in total.• Morphologically and functionally, the cervical column is divided into two distinct regions: – the upper cervical spine, or craniovertebral region, and – the lower cervical spine (Fig. 4-21).• The craniovertebral region includes the occipital condyles and the first two cervical vertebrae, C1 and C2, or, respectively, the atlas and axis.
• The lower cervical spine includes the vertebrae of C3 to C7.• The vertebrae from C3 to C6 display similar characteristics and are therefore considered to be the typical cervical vertebrae.• The atlas, axis, and C7 exhibit unique characteristics and are considered the atypical cervical vertebrae.• All of the cervical vertebrae have the unique feature of a foramen (transverse foramen) on the transverse process, which serves as passage for the vertebral artery.
Craniovertebral Region• Atlas• The atlas (C1) is frequently described to be like a washer sitting between the occipital condyles and the axis.• The functions of the atlas are to cradle the occiput and to transmit forces from the occiput to the lower cervical vertebrae.• These functions are reflected in the bony structure. The atlas is different from other vertebrae in that it has no vertebral body or spinous process and is shaped like a ring (Fig. 4-22).
• There are two large lateral masses that have a vertical alignment under each occipital condyle that reflect the function of transmitting forces.• The lateral masses are connected by an anterior and posterior arch that form the ring structure and also create large transverse processes for muscle attachments.8• The lateral masses include four articulating facets: two superior and two inferior.
• The superior zygapophyseal facets are large, typically kidney-shaped, and deeply concave to accommodate the large, convex articular surfaces of the occipital condyles.• There is, however, large variation in the size and shape of these facets.• The inferior zygapophyseal facets are slightly convex and directed inferiorly for articulation with the superior zygapophyseal facets of the axis (C2).• The atlas also possesses a facet on the internal surface of the anterior arch for articulation with the dens (odontoid• process) of the axis.
• Axis• The primary functions of the axis are to transmit the combined load of the head and atlas to the remainder of the cervical spine and to provide motion into axial rotation of the head and atlas.• The axis is atypical in that the anterior portion of the body extends inferiorly and a vertical projection called the dens arises from the superior surface of the body (Fig. 4-23).
• The dens has an anterior facet for articulation with the anterior arch of the atlas and a posterior groove for articulation with the transverse ligament.• The arch of the axis has inferior and superior zygapophyseal facets for articulation with the adjacent inferior vertebra and the atlas, respectively.• The spinous process of the axis is large and elongated with a bifid (split into two portions) tip.• The superior zygapophyseal facets of the axis face upward and laterally. The inferior zygapophyseal facets face anteriorly.
• Articulations• The two atlanto-occipital joints consist of the two concave superior zygapophyseal facets of the atlas articulating with the two convex occipital condyles of the skull.• These joints are true synovial joints with intra- articular fibroadipose meniscoids and lie nearly in the horizontal plane.
• There are three synovial joints that compose the atlantoaxial joints: – the median atlantoaxial joint between the dens and the atlas and – two lateral joints between the superior zygapophyseal facets of the axis and the inferior zygapophyseal facets of the atlas (Fig. 4-24).• The median joint is a synovial trochoid (pivot) joint in which the dens of the axis rotates in an osteoligamentous ring formed by the anterior arch of the atlas and the transverse ligament.
• The two lateral joints appear, on the basis of bony structure, to be plane synovial joints; however, the articular cartilages of both the atlantal and axial facets are convex, rendering the zygapophyseal facet joints biconvex.• The joint spaces that occur as a result of the incongruence of the biconvex structure are filled with meniscoids.
• Craniovertebral Ligaments• Besides the longitudinal ligaments mentioned earlier in the chapter, a number of other ligaments are specific to the cervical region.• Many of these ligaments attach to the axis, atlas, or skull and reinforce the articulations of the upper two vertebrae.• Four of the ligaments are continuations of the longitudinal tract system; the four remaining ligaments are specific to the cervical area.
• The posterior atlanto-occipital and atlantoaxial membranes are the continuations of the ligamentum flavum (Fig. 4-25A).• Their structure, however, varies from the ligamentum flavum in that they are less elastic and therefore permit a greater ROM, especially into rotation.• The anterior atlanto-occipital and atlantoaxial membranes are the continuations of the anterior longitudinal ligament (see Fig. 4-25B).
• The tectorial membrane is the continuation of the PLL in the upper two segments and is a broad, strong membrane that originates from the posterior vertebral body of axis, covers the dens and its cruciate ligament, and inserts at the anterior rim of the foramen magnum53 (Fig. 4-26).• The thick ligamentum nuchae, which extends from the spinous process of C7 to the external occipital protuberance, is an evolution of the supraspinous ligaments (see Fig. 4-13).• The ligamentum nuchae serves as a site for muscle attachment and likely helps to resist the flex-ion moment of the head.
• Transverse Ligament• The transverse ligament stretches across the ring of the atlas and divides the ring into a large posterior section for the spinal cord and a small anterior space for the dens.• The transverse length of the ligament is about 21.9 mm.• The transverse ligament has a thin layer of articular cartilage on its anterior surface for articulation with the dens.• Longitudinal fibers of the transverse ligament extend superiorly to attach to the occipital bone, and inferior fibers descend to the posterior portion of the axis.
• The transverse ligament and its longitudinal bands are sometimes referred to as the atlantal cruciform ligament (Fig. 4-27).• The transverse portion of the ligament holds the dens in close approximation against the anterior ring of the atlas and serves as an articular surface.• Its primary role, however, is to prevent anterior displacement of C1 on C2.• This ligament is critical in maintaining stability at the C1/C2 segment.• Its superior and inferior longitudinal bands provide some assistance in this role.
• The transverse atlantal ligament is very strong, and the dens will fracture before the ligament will tear.• Integrity of the transverse ligament can be compromised, however, particularly with such diseases as rheumatoid arthritis and with other conditions such as Down syndrome.
• Alar Ligaments• The two alar ligaments are also specific to the cervical region (see Fig. 4-27).• These paired ligaments arise from the axis on either side of the dens and extend laterally and superiorly to attach to roughened areas on the medial sides of the occipital condyles55 and to the lateral masses of the atlas.• The ligaments are approximately 1 cm in length and about a pencil width in diameter and consist mainly of collagen fibers arranged in parallel.• These ligaments are relaxed with the head in midposition and taut in flexion.
• Axial rotation of the head and neck tightens both alar ligaments.• The right upper and left lower portions of the alar ligaments limit left lateral flexion of the head and neck.6• These ligaments also help to prevent distraction of C1 on C2.• The alar ligaments are weaker than the transverse atlantal ligament.• The apical ligament of the dens connects the axis and the occipital bone of the skull.• It runs in a fan-shaped arrangement from the apex of the dens to the anterior margin of the foramen magnum of the skull.
The Lower Cervical Region• Typical Cervical Vertebrae• Body• The body (Fig. 4-28) of the cervical vertebra is small, with a transverse diameter greater than anteroposterior diameter and height.• The upper and lower end plates from C2 to C7 also have transverse diameters (widths) that are greater than the corresponding anteroposterior diameters.• The transverse and anteroposterior diameters increase from C2 to C7 with a significant increase in both diameters in the upper end plate of C7.
• The posterolateral margins of the upper surfaces of the vertebral bodies from C3 to C7 support uncinate processes that give the upper surfaces of these vertebrae a concave shape in the frontal plane.• The uncinate processes are present prenatally and after birth gradually enlarge from 9 to 14 years of age.• The anterior inferior border of the vertebral body forms a lip that hangs down toward the vertebral body below, which produces a concave shape of the inferior surface of the superior vertebra in the sagittal plane.
• Arches• Pedicles. The pedicles project posterolaterally and are located halfway between the superior and inferior surfaces of the vertebral body.• Laminae. The laminae are thin and slightly curved. They project posteromedially.• Zygapophyseal Articular Processes (Superior and Inferior). The processes support paired superior facets that are flat and oval and face supero-posteriorly.• The width and height of the superior zygapophyseal facets gradually increase from C3 to C7.• The paired inferior facets face anteriorly and lie closer to the frontal plane than do the superior facets.27• The superior facets of C3 and C7 are more steeply oriented than the others.
• Transverse Processes. A foramen is located in the transverse processes bilaterally for the vertebral artery, vein, and venous plexus. Also, there is a groove for the spinal nerves.• Spinous Processes. The cervical spinous processes are short, slender, and extend horizontally. The tip of the spinous process is bifid. The length of the spinous processes decreases slightly from C2 to C3, remains constant from C3 to C5, and undergoes a significant increase at C7.56• Vertebral Foramen. The vertebral foramen is relatively large and triangular.
• Intervertebral Disk• The structure of the intervertebral disk in the cervical region is distinctly different from that in the lumbar region (Fig. 4-29).• Mercer and Bogduk, in several works, contributed most of the information known about the structure of the cervical disks.• They reported that instead of a fibrous ring completely surrounding a gelatinous center, there is a discontinuous ring surrounding a fibrocartilaginous core.
• The fibers of the anulus fibrosus are not arranged in alternating lamellar layers as in the lumbar region.• In addition, they do not surround the entire perimeter of the nucleus pulposus.• Instead, the anular fibers in this region have a crescent shape when viewed from above, being thick anteriorly and tapering later-ally as they approach the uncinate processes (see Fig. 4-29A).• Anteriorly, the anulus fibrosus is thick with oblique fibers in the form of an inverted “V” whose apex points to the location of the axis of rotation on the anterior end of the upper vertebra.
• Laterally, there is no substantive anulus fibrosus, and posteriorly, it is only a thin layer of vertically oriented fibers.• Posterolaterally, the nucleus is contained only by the PLL.
• Fissures in the disk develop along with the uncinate processes and become clefts by approximately 9 years of age (see Fig. 4-29B).• These clefts become the joint cavity of what has been known as the uncovertebral joints or “joints of Luschka.”
• Interbody Joints of the Lower Cervical Region (C3 to C7)• The interbody joints of the lower cervical region are saddle joints, and motion therefore occurs in only two planes (Fig. 4-30).• In the frontal plane, the inferior surface of the cranial vertebra is convex and sits in the concave surface of the caudal vertebra created by the uncinate processes.• In the sagittal plane, the inferior surface of the cranial vertebra is concave and the superior surface of the caudal vertebra is convex because of the uncinate processes.• The motions that occur are predominantly rocking motions with few translatory motions available.
• Zygapophyseal Joints• The zygapophyseal joints in the cervical spine, as in other regions, are true synovial joints and contain fibroadipose meniscoids.• The joint capsules are lax to allow a large ROM; however, they do restrict motion at the end of the available ranges.• The joints that are oriented approximately 45º from the frontal and horizontal planes lie midway between the two planes.
Function of the Cervical Region• Although the cervical region demonstrates the most flexibility of any of the regions of the vertebral column, stability of the cervical region, especially of the atlanto-occipital and atlantoaxial joints, is essential for support of the head and protection of the spinal cord and vertebral arteries.• The design of the atlas is such that it provides more free space for the spinal cord than does any other vertebra.
• The extra space helps to ensure that the spinal cord is not impinged on during the large amount of motion that occurs here.• The bony configuration of the atlanto-occipital articulation confers some stability, but the application of small loads produces large rotations across the occipito-atlanto-axial complex and also across the lower cervical spine.
• Kinematics• The cervical spine is designed for a relatively large amount of mobility.• Normally, the neck moves 600 times every hour whether we are awake or asleep.• The motions of flexion and extension, lateral flexion, and rotation are permitted in the cervical region.• These motions are accompanied by translations that increase in magnitude from C2 to C7.• However, the predominant translation occurs in the sagittal plane during flexion and extension.• Excessive anteroposterior translation is associated with damage to the spinal cord.
• The atlanto-occipital joints allow for only nodding movements between the head and the atlas (Fig. 4-31).• In all other respects, the head and atlas move together and function as one unit.• The deep walls of the atlantal sockets prevent translations, but the concave shape does allow rotation to occur.• In flexion, the occipital condyles roll forward and slide backward.• In extension, the occipital condyles roll backward and slide forward. Axial rotation and lateral flexion are not physiological motions at these joints, inasmuch as they cannot be produced by muscle action.
• There is little agreement about the extent of the range of motion (ROM) available at the atlanto- occipital joints.• The combined ROM for flexion-extension reportedly ranges from 10 to 30.• The total ROM available in both axial rotation and lateral flexion is extremely limited by tension in the joint capsules that occurs as the occipital condyles rise up the walls of the atlantal sockets on the contralateral side of either the rotation or lateral flexion.
• Motions at the atlantoaxial joint include rotation, lateral flexion, flexion, and extension.• Approximately 55% to 58% of the total rotation of the cervical region occurs at the atlantoaxial joints (Fig. 4- 32).• The atlas pivots about 45 to either side, or a total of about 90.• The alar ligaments limit rotation at the atlantoaxial joints.• The remaining 40% of total rotation available to the cervical spine is distributed evenly in the lower joints.
• The shape of the zygapophyseal joints and the interbody joints dictates the motion at the lower cervical segments.• Pure anterior translation does not occur, because it would cause the zygapophyseal joints to abut one another.• Flexion of these segments must include anterior tilt of the cranial vertebral body coupled with anterior translation.• Given the 45 slope, tilt of the vertebral body, in addition to anterior translation, is necessary to get full motion from these joints (Fig. 4-33).
• Extension includes posterior tilt of the cranial vertebral body, coupled with posterior translation.• Lateral flexion and rotation are also coupled motions, because movement of either alone would cause the zygapophyseal joints to abut one another and prevent motion.
• Lateral flexion is coupled with ipsilateral rotation, and rotation is coupled with ipsilateral lateral flexion.• These motions are also a combination of vertebral tilt to the ipsilateral side and translations at the zygapophyseal joints.
• Mercer and Bogduk suggested that the notion of lateral flexion and horizontal rotation are an artificial construct.• In their view, movement should be viewed as gliding that occurs in the plane of the zygapophyseal joints (Fig. 4-34).• In this plane, the coupled motions are evident.• Lower cervical segments generally favor flexion and extension ROM; however, there is great variability in reported ranges of motion in the individual cervical segments.
• In general, the range for flexion and extension increases from the C2/C3 segment to the C5/C6 segment, and decreases again at the C6/C7 segment.• The zygapophyseal joint capsules and the ligaments, in addition to the shape of the joints, dictate motions at all of the cervical segments.• The zygapophyseal joint capsules are generally lax in the cervical region, which contributes to the large amount of motion available here.• The height in relation to the diameter of the disks also plays an important role in determining the amount of motion available in the cervical spine.
• The height is large in comparison with the anteroposterior and transverse diameters of the cervical disks.• Therefore, a large amount of flexion, extension, and lateral flexion may occur at each segment, especially in young persons, when there is a large amount of water in the disks.
• The disk at C5/C6 is subject to a greater amount of stress than other disks because C5/C6 has the greatest range of flexion- extension and is the area where the mechanical strain is greatest.
• Kinetics• Although the cervical region is subjected to axial compression, tension, bending, torsion, and shear stresses as in the remainder of the spinal column, there are some regional differences.• The cervical region differs from the thoracic and lumbar regions in that the cervical region bears less weight and is generally more mobile.
• No disks are present at either the atlanto-occipital or atlantoaxial articulations; therefore, the weight of the head (compressive load) must be transferred directly through the atlanto-occipital joint to the articular facets of the axis.• These forces are then transferred through the pedicles and laminae of the axis to the inferior surface of the body and to the two inferior zygapophyseal articular processes.• Subsequently, the forces are trans-ferred to the adjacent inferior disk.
• The laminae of the axis are large, which reflects the adaptation in structure that is necessary to transmit these compressive loads.• The trabeculae show that the laminae of both the axis and C7 are heavily loaded, whereas the intervening ones are not.• Loads diffuse into the lamina as they are transferred from superior to inferior articular facets.
• The loads imposed on the cervical region vary with the position of the head and body and are minimal in a well-supported reclining body posture.• In the cervical region from C3 to C7 compressive forces are transmitted by three parallel columns: – a single anterocentral column formed by the vertebral bodies and disks and – two rodlike posterolateral columns composed of the left and right zygapophyseal joints.• The compressive forces are transmitted mainly by the bodies and disks, with a little over one third transmitted by the two pos-terolateral columns.
• Compressive loads are relatively low during erect stance and sitting postures and high during the end ranges of flexion and extension.• Cervical motion segments tested in bending and axial torsion exhibit less stiffness than do lumbar motion segments but exhibit similar stiffness in compression.• In an experiment with cadaver specimens, combinations of sagittal loads in vitro demonstrated that the midcervical region from C2 to C5 is significantly stiffer in compression and extension from C5 to T1.
• Specimens that were axially rotated before being tested in flexion and compression failed at a lower flexion angle (17) than at the mean angle (25) of nonaxially rotated specimens.• The implication is that the head should be held in a nonrotated position during flexion/extension activities to reduce the risk of injury.70