Kinematics• The motions available to the column as a whole areflexion and extension, lateral flexion, and rotation.• These motions appear to occur independently of eachother; however, at the level of the individual motionsegment, these motions are often coupled motions.• Coupling is defined as the consistent association of onemotion about an axis with another motion around adifferent axis.
• The most predominant motions that exhibitcoupled behaviors are lateral flexion androtation.• Pure lateral flexion and pure rotation do notoccur in any region of the spine. In order foreither motion to occur, at least some of theother must occur as well.
• Coupling patterns, as well as the types and amounts ofmotion that are available, are complex, differ fromregion to region, and depend on:– the spinal posture,– curves,– orientation of the articulating facets,– fluidity,– elasticity, and– thickness of the intervertebral disks and– extensibility of the muscles, ligaments, and joint capsules.
• Motions at the interbody and zygapophysealjoints are interdependent.• The amount of motion available is determinedprimarily by the size of the disks, whereas thedirection of the motion is determinedprimarily by the orientation of the facets.
• The intervertebral disks increase movement between twoadjacent vertebrae. If the vertebrae lay flat against eachother, the movement between them would be limited totranslation alone.• The vertebrae are also allowed to rock or tilt on each otherbecause the soft, deformable disk is between them.• This arrangement adds tremendous range of motion (ROM)(Fig. 4-17).• The fibers of the anulus fibrosus behave as a ligamentousstructure and act as restraints to motion.
• The motions of flexion and extension occur as aresult of the tilting and gliding of a superiorvertebra over the inferior vertebra.• As the superior vertebra moves through a ROM, itfollows a series of different arcs, each of whichhas a different instantaneous axis of rotation.• The nucleus pulposus acts like a pivot but, unlikea ball, is able to undergo greater distortionbecause it behaves as a fluid.
• Regardless of the magnitude of motion createdby the ratio of disk height to width, a glidingmotion occurs at the interbody andzygapophyseal joints as the vertebral body tilts(rotates) over the disk at the inter-body joint.• The orientation of the zygapophyseal facetsurfaces, which varies from region to region,deter-mines the direction of the tilting andgliding within a particular region.
• If the superior and inferior zyga-pophysealfacet surfaces of three adjacent vertebrae liein the sagittal plane, the motions of flexionand exten-sion are facilitated (Fig. 4-18A).• On the other hand, if the zygapophyseal facetsurfaces are placed in the frontal plane, thepredominant motion that is allowed is lateralflexion (Fig.4-18B).
• Flexion• In vertebral flexion, anterior tilting and gliding ofthe superior vertebra occur and cause wideningof the intervertebral foramen and separation ofthe spinous processes (Fig. 4-19A).• Although the amount of tilting is dependentpartly on the size of the disks, tension in thesupraspinous and interspinous ligaments resistsseparation of the spinous processes and thuslimits the extent of flexion.
• Passive tension in the zygapophyseal jointcapsules, ligamentum flavum, PLL, posterioranulus fibrosus, and the back extensors alsoimposes controls on excessive flexion.• With movement into flexion, the anterior portionof the anulus fibrosus is compressed and bulgesanteriorly, whereas the posterior portion isstretched and resists separation of the vertebralbodies.
• Extension• In extension, posterior tilting and gliding of thesuperior vertebra occur and cause narrowing of theinter-vertebral foramen, and the spinous processesmove closer together (see Fig. 4-19B).• The amount of motion available in extension, inaddition to being limited by the size of the disks, islimited by bony contact of the spinous processes andpassive tension in the zygapophyseal joint capsules,anterior fibers of the anulus fibrosus, anterior trunkmuscles, and the anterior longitudinal ligament.
• In general, there are many more ligaments that limit flexionthan there are ligaments that limit extension.• The only ligament that limits extension is the anteriorlongitudinal ligament.• This is likely the reason that this ligament is so strong incomparison with the posterior ligaments.• The numerous checks to flexion follow the pattern ofligamentous checks to motion where bony limits areminimal.
• Fewer ligamentous checks to extension arenecessary, given the presence of numerousbony checks.
• Lateral Flexion• In lateral flexion, the superior vertebra laterally tilts,rotates, and translates over the adjacent vertebrabelow (Fig. 4-20).• The anulus fibrosus is compressed on the concavity ofthe curve and stretched on the convexity of the curve.• Passive tension in the anulus fibers, inter-transverseligaments, and anterior and posterior trunk muscles onthe convexity of the curve limits lateral flexion.• The direction of rotation that accompanies lateralflexion differs slightly from region to region because ofthe orientation of the facets.
• All interbody and zygapophyseal joint motion thatoccurs between the vertebrae from L5 to S1adheres to the general descriptions that havebeen presented.• Regional variations in the structure, function, andmusculature of the column are covered in thefollowing sections.• Table 4-3 summarizes the regional variations inthe structure of the vertebrae.
Kinetics• The vertebral column is subjected to axialcompression, tension, bending, torsion, andshear stress not only during normal functionalactivities but also at rest.• The column’s ability to resist these loads variesamong spinal regions and depends on:– the type, duration, and rate of loading;– the person’s age and posture;– the condition and properties of the various structuralelements (vertebral bodies, joints, disks, muscles, jointcapsules, and ligaments); and– the integrity of the nervous system.
• Axial Compression• Axial compression (force acting through the longaxis of the spine at right angles to the disks)occurs as a result of the force of gravity, groundreaction forces, and forces produced by theligaments and muscular con-tractions.• The disks and vertebral bodies resist most of thecompressive force, but the neural arches andzygapophyseal joints share some of the load incertain postures and during specific motions.
• The compressive load is transmitted from the superior endplate to the inferior end plate through the trabecular boneof the vertebral body and the cortical shell.• The cancellous body contributes 25% to 55% of thestrength of a lumbar vertebra before the age of 40 years,and the cortical bone carries the remainder.• After age 40, the cortical bone carries a greater proportionof the load as the trabecular bone’s compressive strengthand stiffness decrease with decreasing bone density.
• Depending on the posture and region of thespine, the zygapophyseal joints carry from 0%to 33% of the compression load.• The spinous processes also may share some ofthe load when the spine is in hyperextension.
• The nucleus pulposus acts as a ball of fluid that can bedeformed by a compression force.• The pressure created in the nucleus pulposus actuallyis greater than the force of the applied load.• When a weight is applied to the nucleus pulposus fromabove, the nucleus pulposus exhibits a swellingpressure and tries to expand outward toward theanulus fibrosus and the end plates (see Fig. 4-10).
• As the nucleus attempts to distribute the pressure in alldirections, stress is created in the anulus fibrosus, andcentral compressive loading occurs on the vertebralend plates.• The forces of the nucleus pulposus on the anulusfibrosus and of the anulus fibrosus on the nucleuspulposus form an interaction pair.• Normally, the anulus fibrosus and the end plates areable to provide sufficient resistance to the swellingpressure in the nucleus pulposus to reach and maintaina state of equilibrium.• The pressure exerted on the end plates is transmittedto the superior and inferior vertebral bodies.
• The disks and trabecular bone are able toundergo a greater amount of deformationwithout failure than are the cartilaginous endplates or cortical bone when subjected to axialcompression.• The end plates are able to undergo the leastdeformation and therefore will be the first to fail(fracture) under high compressive loading.• The disks will be the last to fail (rupture).
• The intervertebral disks, like all viscoelastic materials,exhibit creep.• This phenomenon produces typical diurnal changes in diskcomposition and function.• When the intervertebral disks are subjected to a constantload, they exhibit creep.• Under sustained compressive loading such as that incurredin the upright posture, the rise in the swelling pressurecauses fluid to be expressed from the nucleus pulposus andthe anulus fibrosus.
• The amount of fluid expressed from the disk depends bothon the size of the load and the duration of its application.• The expressed fluid is absorbed through microscopic poresin the cartilaginous end plate.• When the compressive forces on the disks are decreased,the disk imbibes fluid back from the vertebral body.• The recovery of fluid that returns the disk to its originalstate explains why a person getting up from bed is taller inthe morning than in the evening.
• The average variation in height during the day has beendemonstrated to be 19 mm with a loss of approximately 1.5mm (almost 20%) in height from each of the lumbarintervertebral disks.• Running is a form of dynamic loading that decreases diskheight more rapidly than static loading.• The height of the vertebral column is a widely usedindicator of cumulative compression.• In a study involving 31 men, Ahrens found that the menhad a mean loss of 0.89 cm and 0.72 cm after a 6-mile run.
• Bending• Bending causes both compression and tension onthe structures of the spine.•• In forward flexion, the anterior structures(anterior portion of the disk, anterior ligaments,and muscles) are subjected to compression; theposterior structures are subjected to tension.• The resistance offered to the tensile forces bycollagen fibers in the posterior outer anulusfibrosus, zygapophyseal joint capsules, andposterior ligaments help to limit extremes ofmotion and hence provide stability in flexion.
• Creep occurs when the vertebral column is subjectedto sustained loading, such as might occur in either thefully flexed postures commonly assumed in gardeningor in the fully extended postures assumed in paintingthe ceiling.• The resulting deformation (elongation or compression)of supporting structures such as ligaments, jointcapsules, and intervertebral disks leads to an increasein the ROM beyond normal limits and places thevertebral structures at risk of injury.
• If the creep deformation of tissues occurswithin the toe region of the stress-straincurve, the structures will return to theiroriginal dimensions in either minutes or hoursafter a cessation of the gardening or paintingactivity.
• In extension, the posterior structures generallyare either unloaded or subjected to compression,whereas the anterior structures are subjected totension.• In general, resistance to extension is provided bythe anterior outer fibers of the anulus fibrosus,zygapophyseal joint capsules, passive tension inthe anterior longitudinal ligament, and possiblyby contact of the spinous processes.
• In lateral bending, the ipsilateral side of the diskis compressed; that is, in right lateral bending,the right side of the disk is compressed, whereasthe outer fibers of the left side of the disk arestretched.• Therefore, the contralateral fibers of the outeranulus fibrosus and the contralateralintertransverse ligament help to provide stabilityduring lateral bending by resisting extremes ofmotion.
• Torsion• Torsional forces are created during axial rotation thatoccurs as a part of the coupled motions that take placein the spine.• The torsional stiffnesses in flexion and lateral bendingof the upper thoracic region from T1 to T6 are similar,but torsional stiffness increases from T7/T8 to L3/L4.• Torsional stiffness is provided by the outer layers ofboth the vertebral bodies and inter-vertebral disks andby the orientation of the facets.
• The outer shell of cortical bone reinforces thetrabecular bone and provides resistance to torsion.• When the disk is subjected to torsion, half of theanulus fibrosus fibers resist clockwise rotations,whereas fibers oriented in the opposite direction resistcounter-clockwise rotations.• It has been suggested that the anulus fibrosus may bethe most effective structure in the lumbar region forresisting torsion; however, the risk of rupture of thedisk fibers is increased when torsion, heavy axialcompression, and bending are combined.
• Shear• Shear forces act on the midplane of the disk and tend tocause each vertebra to undergo translation (moveanteriorly, posteriorly, or from side to side in relation to theinferior vertebra).• In the lumbar spine, the zygapophyseal joints resist some ofthe shear force, and the disks resist the remainder. Whenthe load is sustained, the disks exhibit creep, and thezygapophyseal joints may have to resist all of the shearforce.• Table 4-4 summarizes vertebral function.