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Biomechanics ofVertebral Column: Regional Structure &FunctionDr. D. N. BidSarvajanik College of Physiotherapy,Rampura, Surat – 395003.
• The complexity of a structure that must fulfill manyfunctions is reflected in the design of its componentparts.• Regional structures are varied to meet different butequally complex functional requirements.• Structural variations evident in the first cervical andthoracic vertebrae, fifth lumbar vertebra, and sacralvertebrae represent adaptations necessary for joiningthe vertebral column to adjacent structures.
• Differences in vertebral structure are alsoapparent at the cervicothoracic,thoracolumbar, andlumbosacral junctions,at which a transition must be made betweenone type of vertebral structure and another.
• The vertebrae located at regional junctions are calledtransitional vertebrae and they usually possesscharacteristics common to two regions.• The cephalocaudal increase in the size of the vertebralbodies reflects the increased proportion of bodyweight that must be supported by the lower thoracicand lumbar vertebral bodies.• Fusion of the sacral vertebrae into a rigid segmentreflects the need for a firm base of support for thecolumn.
• In addition to these variations, a large numberof minor alterations in structure occurthroughout the column.• However, only the major variations arediscussed here.
Structure of the Cervical Region• The cervical vertebral column consists of sevenvertebrae in total.• Morphologically and functionally, the cervical columnis 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 occipitalcondyles and the first two cervical vertebrae, C1 andC2, 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 characteristicsand are therefore considered to be the typical cervicalvertebrae.• The atlas, axis, and C7 exhibit unique characteristics andare considered the atypical cervical vertebrae.• All of the cervical vertebrae have the unique feature of aforamen (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 washersitting between the occipital condyles and the axis.• The functions of the atlas are to cradle the occiput and totransmit forces from the occiput to the lower cervicalvertebrae.• These functions are reflected in the bony structure. Theatlas is different from other vertebrae in that it has novertebral body or spinous process and is shaped like a ring(Fig. 4-22).
• There are two large lateral masses that have avertical alignment under each occipital condylethat reflect the function of transmitting forces.• The lateral masses are connected by an anteriorand posterior arch that form the ring structureand also create large transverse processes formuscle attachments.8• The lateral masses include four articulatingfacets: two superior and two inferior.
• The superior zygapophyseal facets are large, typicallykidney-shaped, and deeply concave to accommodate thelarge, convex articular surfaces of the occipital condyles.• There is, however, large variation in the size and shape ofthese facets.• The inferior zygapophyseal facets are slightly convex anddirected inferiorly for articulation with the superiorzygapophyseal facets of the axis (C2).• The atlas also possesses a facet on the internal surface ofthe anterior arch for articulation with the dens (odontoid• process) of the axis.
• Axis• The primary functions of the axis are to transmitthe combined load of the head and atlas to theremainder of the cervical spine and to providemotion into axial rotation of the head and atlas.• The axis is atypical in that the anterior portion ofthe body extends inferiorly and a verticalprojection called the dens arises from thesuperior surface of the body (Fig. 4-23).
• The dens has an anterior facet for articulation with theanterior arch of the atlas and a posterior groove forarticulation with the transverse ligament.• The arch of the axis has inferior and superiorzygapophyseal facets for articulation with the adjacentinferior vertebra and the atlas, respectively.• The spinous process of the axis is large and elongatedwith a bifid (split into two portions) tip.• The superior zygapophyseal facets of the axis faceupward and laterally. The inferior zygapophyseal facetsface anteriorly.
• Articulations• The two atlanto-occipital joints consist of thetwo concave superior zygapophyseal facets ofthe atlas articulating with the two convexoccipital condyles of the skull.• These joints are true synovial joints with intra-articular fibroadipose meniscoids and lienearly in the horizontal plane.
• There are three synovial joints that compose theatlantoaxial joints:– the median atlantoaxial joint between the dens andthe atlas and– two lateral joints between the superior zygapophysealfacets of the axis and the inferior zygapophyseal facetsof the atlas (Fig. 4-24).• The median joint is a synovial trochoid (pivot)joint in which the dens of the axis rotates in anosteoligamentous ring formed by the anteriorarch of the atlas and the transverse ligament.
• The two lateral joints appear, on the basis ofbony structure, to be plane synovial joints;however, the articular cartilages of both theatlantal and axial facets are convex, renderingthe zygapophyseal facet joints biconvex.• The joint spaces that occur as a result of theincongruence of the biconvex structure arefilled with meniscoids.
• Craniovertebral Ligaments• Besides the longitudinal ligaments mentionedearlier in the chapter, a number of otherligaments are specific to the cervical region.• Many of these ligaments attach to the axis, atlas,or skull and reinforce the articulations of theupper two vertebrae.• Four of the ligaments are continuations of thelongitudinal tract system; the four remainingligaments are specific to the cervical area.
• The posterior atlanto-occipital and atlantoaxialmembranes are the continuations of the ligamentumflavum (Fig. 4-25A).• Their structure, however, varies from the ligamentumflavum in that they are less elastic and thereforepermit a greater ROM, especially into rotation.• The anterior atlanto-occipital and atlantoaxialmembranes are the continuations of the anteriorlongitudinal ligament (see Fig. 4-25B).
• The tectorial membrane is the continuation of the PLLin the upper two segments and is a broad, strongmembrane that originates from the posterior vertebralbody of axis, covers the dens and its cruciate ligament,and inserts at the anterior rim of the foramenmagnum53 (Fig. 4-26).• The thick ligamentum nuchae, which extends from thespinous process of C7 to the external occipitalprotuberance, is an evolution of the supraspinousligaments (see Fig. 4-13).• The ligamentum nuchae serves as a site for muscleattachment and likely helps to resist the flex-ionmoment of the head.
• Transverse Ligament• The transverse ligament stretches across the ring of theatlas and divides the ring into a large posterior section forthe 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 articularcartilage on its anterior surface for articulation with thedens.• Longitudinal fibers of the transverse ligament extendsuperiorly to attach to the occipital bone, and inferior fibersdescend to the posterior portion of the axis.
• The transverse ligament and its longitudinal bands aresometimes referred to as the atlantal cruciform ligament(Fig. 4-27).• The transverse portion of the ligament holds the dens inclose approximation against the anterior ring of the atlasand serves as an articular surface.• Its primary role, however, is to prevent anteriordisplacement of C1 on C2.• This ligament is critical in maintaining stability at the C1/C2segment.• Its superior and inferior longitudinal bands provide someassistance in this role.
• The transverse atlantal ligament is very strong,and the dens will fracture before the ligamentwill tear.• Integrity of the transverse ligament can becompromised, however, particularly with suchdiseases as rheumatoid arthritis and withother conditions such as Down syndrome.
• Alar Ligaments• The two alar ligaments are also specific to the cervicalregion (see Fig. 4-27).• These paired ligaments arise from the axis on either side ofthe dens and extend laterally and superiorly to attach toroughened areas on the medial sides of the occipitalcondyles55 and to the lateral masses of the atlas.• The ligaments are approximately 1 cm in length and abouta pencil width in diameter and consist mainly of collagenfibers arranged in parallel.• These ligaments are relaxed with the head in midpositionand taut in flexion.
• Axial rotation of the head and neck tightens both alarligaments.• The right upper and left lower portions of the alarligaments limit left lateral flexion of the head and neck.6• These ligaments also help to prevent distraction of C1 onC2.• The alar ligaments are weaker than the transverse atlantalligament.• The apical ligament of the dens connects the axis and theoccipital bone of the skull.• It runs in a fan-shaped arrangement from the apex of thedens to the anterior margin of the foramen magnum of theskull.
The Lower Cervical Region• Typical Cervical Vertebrae• Body• The body (Fig. 4-28) of the cervical vertebra is small,with a transverse diameter greater thananteroposterior diameter and height.• The upper and lower end plates from C2 to C7 alsohave transverse diameters (widths) that are greaterthan the corresponding anteroposterior diameters.• The transverse and anteroposterior diameters increasefrom C2 to C7 with a significant increase in bothdiameters in the upper end plate of C7.
• The posterolateral margins of the upper surfaces of thevertebral bodies from C3 to C7 support uncinateprocesses that give the upper surfaces of thesevertebrae a concave shape in the frontal plane.• The uncinate processes are present prenatally andafter birth gradually enlarge from 9 to 14 years of age.• The anterior inferior border of the vertebral bodyforms a lip that hangs down toward the vertebral bodybelow, which produces a concave shape of the inferiorsurface of the superior vertebra in the sagittal plane.
• Arches• Pedicles. The pedicles project posterolaterally and arelocated halfway between the superior and inferiorsurfaces of the vertebral body.• Laminae. The laminae are thin and slightly curved.They project posteromedially.• Zygapophyseal Articular Processes (Superior andInferior). The processes support paired superior facetsthat are flat and oval and face supero-posteriorly.• The width and height of the superior zygapophysealfacets gradually increase from C3 to C7.• The paired inferior facets face anteriorly and lie closerto the frontal plane than do the superior facets.27• The superior facets of C3 and C7 are more steeplyoriented than the others.
• Transverse Processes. A foramen is located in thetransverse processes bilaterally for the vertebral artery,vein, and venous plexus. Also, there is a groove for thespinal nerves.• Spinous Processes. The cervical spinous processes areshort, slender, and extend horizontally. The tip of thespinous process is bifid. The length of the spinousprocesses decreases slightly from C2 to C3, remainsconstant from C3 to C5, and undergoes a significantincrease at C7.56• Vertebral Foramen. The vertebral foramen is relativelylarge and triangular.
• Intervertebral Disk• The structure of the intervertebral disk in the cervicalregion is distinctly different from that in the lumbarregion (Fig. 4-29).• Mercer and Bogduk, in several works, contributed mostof the information known about the structure of thecervical disks.• They reported that instead of a fibrous ring completelysurrounding a gelatinous center, there is adiscontinuous ring surrounding a fibrocartilaginouscore.
• The fibers of the anulus fibrosus are not arranged inalternating lamellar layers as in the lumbar region.• In addition, they do not surround the entire perimeterof the nucleus pulposus.• Instead, the anular fibers in this region have a crescentshape when viewed from above, being thick anteriorlyand tapering later-ally as they approach the uncinateprocesses (see Fig. 4-29A).• Anteriorly, the anulus fibrosus is thick with obliquefibers in the form of an inverted “V” whose apex pointsto the location of the axis of rotation on the anteriorend of the upper vertebra.
• Laterally, there is no substantive anulusfibrosus, and posteriorly, it is only a thin layerof vertically oriented fibers.• Posterolaterally, the nucleus is contained onlyby the PLL.
• Fissures in the disk develop along with theuncinate processes and become clefts byapproximately 9 years of age (see Fig. 4-29B).• These clefts become the joint cavity of whathas 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 saddlejoints, and motion therefore occurs in only two planes (Fig.4-30).• In the frontal plane, the inferior surface of the cranialvertebra is convex and sits in the concave surface of thecaudal vertebra created by the uncinate processes.• In the sagittal plane, the inferior surface of the cranialvertebra is concave and the superior surface of the caudalvertebra is convex because of the uncinate processes.• The motions that occur are predominantly rocking motionswith few translatory motions available.
• Zygapophyseal Joints• The zygapophyseal joints in the cervical spine, as inother regions, are true synovial joints and containfibroadipose meniscoids.• The joint capsules are lax to allow a large ROM;however, they do restrict motion at the end of theavailable ranges.• The joints that are oriented approximately 45º fromthe frontal and horizontal planes lie midway betweenthe two planes.
Function of the Cervical Region• Although the cervical region demonstrates themost flexibility of any of the regions of thevertebral column, stability of the cervical region,especially of the atlanto-occipital and atlantoaxialjoints, is essential for support of the head andprotection of the spinal cord and vertebralarteries.• The design of the atlas is such that it providesmore free space for the spinal cord than does anyother vertebra.
• The extra space helps to ensure that the spinalcord is not impinged on during the large amountof motion that occurs here.• The bony configuration of the atlanto-occipitalarticulation confers some stability, but theapplication of small loads produces largerotations across the occipito-atlanto-axialcomplex and also across the lower cervical spine.
• Kinematics• The cervical spine is designed for a relatively large amountof mobility.• Normally, the neck moves 600 times every hour whetherwe are awake or asleep.• The motions of flexion and extension, lateral flexion, androtation are permitted in the cervical region.• These motions are accompanied by translations thatincrease in magnitude from C2 to C7.• However, the predominant translation occurs in the sagittalplane during flexion and extension.• Excessive anteroposterior translation is associated withdamage to the spinal cord.
• The atlanto-occipital joints allow for only noddingmovements between the head and the atlas (Fig. 4-31).• In all other respects, the head and atlas move together andfunction 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 slidebackward.• In extension, the occipital condyles roll backward and slideforward. Axial rotation and lateral flexion are notphysiological motions at these joints, inasmuch as theycannot be produced by muscle action.
• There is little agreement about the extent of therange of motion (ROM) available at the atlanto-occipital joints.• The combined ROM for flexion-extensionreportedly ranges from 10 to 30.• The total ROM available in both axial rotation andlateral flexion is extremely limited by tension inthe joint capsules that occurs as the occipitalcondyles rise up the walls of the atlantal socketson the contralateral side of either the rotation orlateral flexion.
• Motions at the atlantoaxial joint include rotation,lateral flexion, flexion, and extension.• Approximately 55% to 58% of the total rotation of thecervical region occurs at the atlantoaxial joints (Fig. 4-32).• The atlas pivots about 45 to either side, or a total ofabout 90.• The alar ligaments limit rotation at the atlantoaxialjoints.• The remaining 40% of total rotation available to thecervical spine is distributed evenly in the lower joints.
• The shape of the zygapophyseal joints and the interbodyjoints dictates the motion at the lower cervical segments.• Pure anterior translation does not occur, because it wouldcause the zygapophyseal joints to abut one another.• Flexion of these segments must include anterior tilt of thecranial vertebral body coupled with anterior translation.• Given the 45 slope, tilt of the vertebral body, in addition toanterior translation, is necessary to get full motion fromthese joints (Fig. 4-33).
• Extension includes posterior tilt of the cranialvertebral body, coupled with posteriortranslation.• Lateral flexion and rotation are also coupledmotions, because movement of either alonewould cause the zygapophyseal joints to abutone another and prevent motion.
• Lateral flexion is coupled with ipsilateralrotation, and rotation is coupled withipsilateral lateral flexion.• These motions are also a combination ofvertebral tilt to the ipsilateral side andtranslations at the zygapophyseal joints.
• Mercer and Bogduk suggested that the notion oflateral flexion and horizontal rotation are anartificial construct.• In their view, movement should be viewed asgliding that occurs in the plane of thezygapophyseal joints (Fig. 4-34).• In this plane, the coupled motions are evident.• Lower cervical segments generally favor flexionand extension ROM; however, there is greatvariability in reported ranges of motion in theindividual cervical segments.
• In general, the range for flexion and extensionincreases from the C2/C3 segment to the C5/C6segment, and decreases again at the C6/C7 segment.• The zygapophyseal joint capsules and the ligaments, inaddition to the shape of the joints, dictate motions atall of the cervical segments.• The zygapophyseal joint capsules are generally lax inthe cervical region, which contributes to the largeamount of motion available here.• The height in relation to the diameter of the disks alsoplays an important role in determining the amount ofmotion available in the cervical spine.
• The height is large in comparison with theanteroposterior and transverse diameters ofthe cervical disks.• Therefore, a large amount of flexion,extension, and lateral flexion may occur ateach segment, especially in young persons,when there is a large amount of water in thedisks.
• The disk at C5/C6 is subject to a greateramount of stress than other disks becauseC5/C6 has the greatest range of flexion-extension and is the area where themechanical strain is greatest.
• Kinetics• Although the cervical region is subjected to axialcompression, tension, bending, torsion, andshear stresses as in the remainder of the spinalcolumn, there are some regional differences.• The cervical region differs from the thoracic andlumbar regions in that the cervical region bearsless weight and is generally more mobile.
• No disks are present at either the atlanto-occipital oratlantoaxial articulations; therefore, the weight of thehead (compressive load) must be transferred directlythrough the atlanto-occipital joint to the articularfacets of the axis.• These forces are then transferred through the pediclesand laminae of the axis to the inferior surface of thebody and to the two inferior zygapophyseal articularprocesses.• Subsequently, the forces are transferred to theadjacent inferior disk.
• The laminae of the axis are large, which reflectsthe adaptation in structure that is necessary totransmit these compressive loads.• The trabeculae show that the laminae of both theaxis and C7 are heavily loaded, whereas theintervening ones are not.• Loads diffuse into the lamina as they aretransferred from superior to inferior articularfacets.
• The loads imposed on the cervical region vary with theposition of the head and body and are minimal in awell-supported reclining body posture.• In the cervical region from C3 to C7 compressive forcesare transmitted by three parallel columns:– a single anterocentral column formed by the vertebralbodies and disks and– two rodlike posterolateral columns composed of the leftand right zygapophyseal joints.• The compressive forces are transmitted mainly by thebodies and disks, with a little over one thirdtransmitted by the two posterolateral columns.
• Compressive loads are relatively low during erectstance and sitting postures and high during theend ranges of flexion and extension.• Cervical motion segments tested in bending andaxial torsion exhibit less stiffness than do lumbarmotion segments but exhibit similar stiffness incompression.• In an experiment with cadaver specimens,combinations of sagittal loads in vitrodemonstrated that the midcervical region fromC2 to C5 is significantly stiffer in compression andextension from C5 to T1.
• Specimens that were axially rotated beforebeing tested in flexion and compression failedat a lower flexion angle (17°) than at the meanangle (25°) of nonaxially rotated specimens.• The implication is that the head should beheld in a non-rotated position duringflexion/extension activities to reduce the riskof injury.