Journal of Orthopaedic & Sports Physical Therapy 2OOl;31(4):174-182

Joints of the Cervical Vertebral Column

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Susan R. Mercer, PhD, PT Nikolai Bogduk, PhD, DSC, M D *

The developing understanding of the morphology of the cervical spine has revealed the complexity of the system. A review of selected literature reported that a number of the joints have an unusual nature and exhibit complicated and even paradoxical motions. For the practicing therapist, the significance of these observations is that assessment and treatment procedures of the cervical spine must be very carefully analyzed. There are significant differing behaviors of some of the cervical joints in response to small changes in movement patterns or initial positioning. Therefore it is not possible to broadly classify results of assessment procedures as normal or pathological without a clear and detailed understanding of the underlying morphology. ) Orthop Sports Phys Ther 2OOl;3l:l74-l82.

Key Words: atlas, axis, cervical vertebrae, kinematics

n many circles of clinical practice it is conventional to treat the cervical spine as a single organ (ie, as if it is a single joint between the head and thorax). Indeed, the AMA Guides for the Assessment of Impairment stipulate that range of motion of the head be used for the determination of impairment of the neck.2 This approach belies the subtleties and complexities of cervical spine structure and function. The cervical spine consists of 7 segments, each bearing at least 3 joints. The segments are not isomorphic, and they contribute to total spinal function neither equally nor regularly. For example, the total range of motion of the neck is not the arithmetic sum of segmental ranges of motion." Indeed, total range of motion can be as much as 10 or 30 degrees less than the sum of the maximum segmental ranges of motion.J2 In people with nonimpaired necks, the apparent range of motion on one day may be considerably different from that on another day. Measuring range of motion from flexion to extension may yield a different value than when measured from extension to flexion.32In patients with neck pain, dysfunctional segments can occur at levels other than those responsible for the pain.3 These idiosyncrasies indicate that clinicians should appreciate and assess the cervical spine not as a single or homogeneous unit, but as a series of separate, yet linked individual segments that may contribute to symptoms and signs in a variety of complex ways. Fundamental to understanding the behavior of the cervical spine is an appreciation of how each segment of the neck contributes to the to-

I

Department of Anatomy and Structural Biology, University of Otago, Dunedin, New Zealand. Newcastle Bone and)oint Institute, University of Newcastle, Royal Newcastle Hospital, Newcastle, Australia. Please send correspondence to Susan Mercer, Department of Anatomy and Structural Biology, University of Otago, Dunedin, New Zealand. E-mail: [email protected]

tal function of the cervical spine and how each segment is designed anatomically to subserve these functions. This commentary outlines an introduction to biomechanics of the cervical spine from an anatomical perspective. Our commentary includes observations that explain why the cervical spine operates the way it does; these observations have not been formally tested and cannot be supported by citations of the literature. They are offered, nonetheless, in an effort to explain cervical biomechanics with a sensible anatomical basis, and to serve as conjectures worthy of future investigation.

Atlanto-occipital joint The first cervical vertebra, the atlas, arguably does not belong to the cervical spine. It has more in common with the occiput than with the rest of the neck. It is designed to cradle the occiput and to transmit forces from the head to the cervical spine. Quintessential features are its 2 lateral masses; each is a stout pillar of bone whose long axis is aligned vertically below the corresponding occipital condyle. The lateral masses are united by anterior and posterior arches of bone that function as outriggers to maintain the relative positions of the lateral masses and allow them to act in parallel and give the atlas its characteristic ring shape. The superior surface of each lateral mass bears a concave socket

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Neutral

--

Flexlon

Extension

physiological movement of the atlanto-occipital joint. Axial rotation requires anterior translation of the contralateral occipital condyle and posterior translation of the ipsilateral condyle. These movements are essentially prevented by the respective anterior and posterior walls of the atlantial sockets. Nevertheless, axial rotation can be induced. The atlantal sockets FIGURE 1. Side view of atlantmcipital flexion and extension. Left lateral are cupshaped and wider at their mouth than at view of flexion and extension at the atlanto-occipital joint where the dots their depths. Consequently, if sufficient axial torque indicate reference points. The neutral position, where the atlantial socket cradles the occipital condyle (a); in flexion, as the head nods forward, the is applied to the head, the occipital condyles will be occipital condyles roll forward and slide backward (b);and the converse forced up the walls of the sockets, and axial rotation is achieved at the cost of vertical displacement of the combination of movements during extension (c). occiput. Ultimately this movement will be limited by tension developed in the capsules of the atlanto-occipital joints and in the alar ligaments. The range of that receives the ipsilateral occipital condyle. The movement possible is therefore limited. Axial rotadeep walls of this socket preclude translation of the tion has been documented only in cadavers, in which condyle laterally, anteriorly, or posteriorly, but the observed range of motion is only seven degrees.2H concave shape permits nodding movements of the Other studies stipulate a value of 0 degree^.^." In head. the light of such small values, clinicians should take Nodding is achieved by a combination of rolling care if seeking to comment on restrictions of axial and contrary sliding, as occurs in any condylar joint rotation of the atlanto-occipital joints. (Figure 1). As the head nods forward, the occipital Lateral flexion of the atlanto-occipital junction is condyles roll forward in the sockets of the atlas, but as they do so they tend to roll up the concave anteri- limited by a similar mechanism. For lateral flexion to occur, either the contralateral occipital condyle must or wall of each socket. Concomitant compression loads, exerted by the mass of the head, the flexor rise out of its socket while pivoting on the ipsilateral muscles, or by tension in capsules of the atlanto-occondyle, or both condyles must slide in parallel up the contralateral walls of their respective sockets in cipital joints, cause the condyles to slide downward and backward along the anterior wall. As a result, an- order to tilt the atlas. Such movements are not physiterior rotation is coupled with downward and posteri- ological, but may be induced. In cadavers the total 1.6 derange of motion has been measured as 3.9 o r sliding, and the condyles effectively remain n e s tled on the floor of the atlantial sockets. This combi- greesS5and 11.0 degrees.2nWhen induced, lateral flexion may be coupled with flexion, extension, or nation of movements results in an axis of movement passing transversely through the bodies of the occipi- axial rotation. The pattern of coupling depends on the exact shape of the atlantial sockets, and any comtal condyles, around which the head essentially spins bination of coupling is possible in the face of variatangentially to the curvature of the atlantial sockets. tions in the geometry of the sockets.S5These variaA converse combination of movements occurs when tions preclude defining any single rule as to the patthe head is extended on the atlas (Figure 1). terns of coupled motion of this joint. Lateral flexion The absolute limit to flexion and extension of the under physiological conditions has not been systematlantoaxipital joints would be impaction of the occiatically demonstrated and studied. put against the rim of the atlantial sockets, but under physiological conditions, other factors limit the motion before such impaction occurs. Flexion is limited by ten- Atlanto-axial Joints sion of the posterior neck muscles and by impaction of The foremost role of the second cervical vertebra, the submandibular tissues against the throat. Extension the axis, is to bear the axial load of the head and atis limited by compression of the suboccipital muscles las and to transmit that load into the remainder of against the occiput. the cervical spine. For this function, the axis presents The total normal range of flexion and extension broad, laterally placed superior articular facets that at the atlanto-occipital joint has been described as support the lateral masses of the atlas and form the having a mean value between 14 and 35 degreeS,7.~~.1~.~.4~.2~ a range from 0 to 25 degrees,' o r a lateral atlanto-axial joints. From these facets, the load of the atlas and the head are transmitted both infermean value of 14 degrees with a standard deviation iorly and anteriorly to the C2-3 intervertebral disc of 15 degrees.21With such large variations in what constitutes the normal range of motion for this joint, and inferiorly and posteriorly to the C2-3 zygapophysial joints. clinicians need to take care with what they consider Otherwise, the axis is designed to allow the axial normal and abnormal when assessing the movement rotation of the head and atlas. The axis presents a of occiput on the atlas. Axial rotation about a vertical axis is not a true centrally-placed odontoid process that acts as the piv-

+

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ot which the anterior arch of the atlas spins and glides around in order to achieve axial rotation. Anteriorly, this movement is accommodated by a synovia1 joint between the odontoid process and the anterior arch of the atlas, known as the median atlanto-axial joint, at which the anterior arch can pivot or slide across the surface of the odontoid process. Inferiorly, the movement is accommodated by the lateral atlanto-axial joints. Radiographs and skeletal material belie the structure of the lateral atlanto-axial joints. Although the osseous articular facets of this joint are flat, they are covered by articular cartilages that are convex in the sagittal plane.lR Each joint is, therefore, biconvex in structure. At rest, the apex of the cartilage of the inferior facet of the atlas balances on the apex of the superior articular cartilage of the axis. Anteriorly and posteriorly, where the surfaces of the articular cartilage diverge, the spaces between the cartilages are filled with large intra-articular m e n i s c o i d ~ . ~Not ~,~' simply space-fillers, these meniscoids serve to keep a film of synovial fluid applied to those surfaces of the articular cartilages that are not in contact with one another. h i a l rotation of the atlas requires anterior displacement of one lateral mass and a reciprocal posterior displacement of the opposite lateral mass. As this occurs, the inferior articular cartilages of the atlas must slide down the respective slopes of the convex superior articular cartilages of the axis. As a result, the atlas screws down onto the axis as it rotateslR (Figure 2A-C). As the atlas moves, its articular facets assume the space previously occupied by the intra-articular meniscoids. Meanwhile the meniscoids are withdrawn from the space as the lateral mass draws the capsule of the joint anteriorly or posteriorly on each side. Upon reversal of the movement, the meniscoids return passively to resume the space. If the articular cartilages are asymmetrical, a small amplitude of sidebending may accompany axial rotation of the atlas, and the coupling may be ipsilateral or contralateral depending on the bias of the a ~ y m m e t r y . ~ The alar ligaments are the principal structures that restrain axial rotation, with the lateral atlanto-axial joint capsules playing a minor role."J2 The normal range of movement is 43 f 5.5 degrees in each direction." At the limits of rotation, the lateral atlantoaxial joints are almost subluxated. Axial rotation at the atlantoaxial level is extremely important functionally for movement at this level accounts for 50% of the total range of rotation of the neck. Indeed, the first 45 degrees of rotation of the head to either side occurs at the C1-C2 level before any lower cervical segments move in this plane. The odontoid process is curved slightly posteriorly. This shape allows the anterior arch of the atlas to slide upwards and slightly backwards, thereby allowing the atlas to extend." Flexion occurs by reciprocal 176

motion but also involves anterior translation of the atlas during which the anterior arch separates from the odontoid process. The total range of flexionextension is about 10 degrees.!'!' Although not a physiological movement, lateral translation at the atlanto-axial joint is assessed in some schools of manual therapy. Because the superior articular facets of the axis slope inferiorly and laterally, lateral translation of the atlas must be accompanied by ipsilateral sidebending (Figure 3). Recipre cally, lateral translation occurs passively during sidebending of the cervical ~ p i n e . ~ V hmovement is is primarily resisted by the contralateral alar ligament and ultimately by bony impaction of the contralateral lateral mass onto the lateral aspect of the odontoid proce~s:~ Posterior translation of the atlas is limited by impaction of the anterior arch of the atlas against the odontoid process, which blocks this movement. In anterior translation, there is no bony block. This movement is limited by the transverse and alar ligaments.12J4Either ligament alone is enough to ensure integrity of the atlanto-axial joint. Subluxation or dislocation implies destruction of both ligaments.14

Cranio-cervical Motion Although certain characteristic movements occur selectively at the atlanto-occipital and atlanto-axial joints, the head, atlas, and axis normally function as a composite unit. During axial rotation of the head, the head and atlas move in concert on the axis at the atlanto-axial joints, when there is no relative motion between the head and the atlas. It is the head that primarily moves, and the atlas is passively drawn with it. For that reason it should not be suprising that the alar ligaments essentially bypass the atlas and ' ~is~the ~~~'~~~~~.~ bind the axis to the o c ~ i p u t . ~ . ~ . It movement of the head that they resist, not the movement of the atlas. During sagittal rotation, the head moves on the axis with the atlas functioning as an interposed washer with essentially passive movements. Certain muscles, such as rectus capitis posterior minor, obliquus superior, and the rectus capitis anterior and lateralis, arise from the atlas and act on the skull; few muscles act on the atlas itself in order to move it. Although the atlas does provide an origin for 1 of the 4 fascicles of levator scapulae this fascicle is directed downwards and so is not responsible for physiological movement of the atlas. The obliquus inferior aids in axial rotation, but the terminal fibres of longus cervicis is the only other muscle that inserts the atlas. Acting on the anterior tubercle of the atlas, the longus cervicis is able to flex the atlas, but conspicuously there is no reciprocal extensor of the atlas. This lack of an extensor predicates the passive nature of the kinematics of the atlas in the sagittal plane. J Orthop Sports Phys Ther-Volume 31 .Number 4.April2001

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Anterior +

Posterior +

Flexion of the head and neck is executed by the longus capitis and cervicis and by the sternocleidomastoid. Extension is executed by semispinalis capitis in concert with other posterior neck muscles. All of these muscles bypass the atlas, but nevertheless exert compression loads on it. This effect combined with the biconvex structure of the lateral atlanto-axial joints accounts for what is known as the paradoxical motion of the atlas during flexionextension of the head and neck. During flexion of the neck the atlas may flex or extend (Figure 4). During extension of the neck, the atlas may extend or flex.%'." This paradox arises because the atlas is perched on the convexities of the superior facets of the axis and is therefore susceptible to small variations in the eccentricity of compression loads exerted on its lateral masses. If the compression load is exerted anterior to the contact point between the facets of the lateral atlanto-axial joint, the effect will be to tilt the atlas into flexion. If the compression load is exerted behind the contact point, the atlas will extend. In essence, the atlas tilts because its lateral masses are squeezed between the occiput and the axis; it tilts backwards if the compression force runs posterior to the center of the lateral mass and forwards if the compression load is anterior to the center of the lateral mass. This occurs irrespective of how the other cervical vertebrae move. Protruding the chin as the neck flexes favors an anterior displacement of the compression load on the atlas, and the atlas will flex in concert with the other cervical vertebrae. On the other hand, tucking the chin backwards favors a posterior displacement of the compression load, and the atlas will extend when the other cervical vertebrae flex. The movement, however, is no longer paradoxical if it is understood that it is a reflection of the line of transmission of compression loads across the atlas (Figure 4). It also highlights how the atlas moves essentially passively under the load of the head. During sidebending of the head, an involved series of events occur^.^'^.'^ Radiography reveals that upon bending to the left, C1 rotates to the right but C2 rotates to the left. Students are sometimes taught these combinations as clinically relevant rules of thumb, but without e ~ p l a n a t i o nInspection .~ of the anatomy of the upper cervical vertebrae explains why these movements occur. Lateral bending exerts an axial compression force along the ipsilateral side of

FIGURE 2. Atlanto-axial rotation. At rest, the apex of the cartilage of the inferior facet of the atlas balances on the apex of the superior articular cartilage of the axis (a). As the inferior articular process of the atlas is displaced anteriorly on the axis, it slides down the anterior slope of the axis (b). As the inferior articular process of the atlas is displaced posteriorly on the axis, it will slide down the posterior slope of the axis (c). J Orthop Sports Phys Ther.Volume 31. Number 4.April 2001

J Orthop Sports Phys Ther*Volume 31 .Number 4.April 2001

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FIGURE 4. Pdradoxical motion of the atlas during neck motion. Lateral view of the convex inferior facet of the atlas perched on the convex superior facet of the axis (a). Line of transmission of compression loadexerted anterior to the contact point, resulting in flexion of the atlas (b). Line of transmission of compression load posterior to the contact point resulting in extension of the atlas (c).

the vertebral column (Figure 5). The ipsilateral lateral mass of the atlas is compressed and transmits the compressive force caudally to C2-3 and subsequent facet joints. Subjected to this vertical load, the ipsilatera1 inferior articular process of C2 is driven downwards but also backwards along the sloping superior articular process of C3. This backward displacement causes C2 to rotate towards the direction of the sidebending. If simultaneously there was no movement of the atlanto-axial joints, the head and face would also rotate to the same side. However, if the face is to be directed forward, a compensatory derotation must occur at the lateral atlanto-axial joints and the atlas must rotate to the opposite side. The overall result is that in lateral flexion of the head, C2 is squeezed into ipsilateral rotation, while the atlas undergoes contralateral rotation. To understand this seemingly complex pattern of movement, it should be realized that the contralateral rotation of the atlas is not a direct consequence of the lateral torque applied to the head. It is artificially produced by the examiner for aesthetic reasons. Sidebending naturally induces ipsilateral rotation of the head; however, to create the illusion of bending only in the coronal plane, the examiner or the subject must subtly apply a torque to the head in order to keep the face looking forwards. It is this subtle torque that rotates the atlas to the opposite side.

Motion of the Lower Cervical Vertebrae The cervical vertebrae C3 through C7 form a column whose functions are to support the head, keep it upright, yet allow mobility. The vertebrae accordingly exhibit features that reflect these load-bearing, stabilizing, and mobility functions. Each vertebra consists of 3 pillars set in a triangu-

FIGURE 5. Sidebendingof the head (panels a and d, posterior view; panels b and c, lateral view). A vertical compression load is exerted down the ipsilateral side of the vertebral column during sidebending of the head (a). Compressive load transmitted caudally to C2-3 and subsequent facet joints (b). Subjected to a vertical load, the inferior articular process of C2 is driven downward and backward along the sloping superior articular process of C3 (c). The backward displacement causes C2 to rotate towards the direction of the sidebending (dl.

lar arrangement that when stacked form 3 parallel columns. The anterior pillars are the vertebral bodies, which are united by interposed intervertebral discs to form the anterior column. The 2 posterior columns are formed by the articular pillars of the cervical vertebrae. The superior and inferior articular processes of consecutive vertebrae are opposed to one another and united by a joint capsule to form the zygapophysial joints. In a longitudinal sense, these 3 columns subserve the load-bearing functions of the cervical spine. The articular facet of each superior articular process faces superiorly and posteriorly at an angle about 45 degrees to the transverse plane.2" The superior orientation allows the articular process to bear the weight of the pillar above. The posterior orienta-

FIGURE 3. Lateral translation at the atlanto-axial joint. Front view of the bony facets of the lateral atlanto-axial joint. The joint slopes downward and laterally (a). If lateral translation is attempted, the contralateral inferior articular process of the atlas will impact the sloping surface of the axis (b). Further translation will be accommodated if the contralateral inferior articular process rides up the slope of the superior articular process while the ipsilateral inferior process rides down the slope of the opposite superior articular process (c). J Orthop Sports Phys Ther-Volume 31 .Number 4.April 2001

179

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tion stabilizes that vertebra by impeding its anterior translation (Figure 6). In the past, the slope of the articular facets of the typical cervical vertebrae has been implicated in determining patterns of segmental motion in the sagittal plane. It has been shown, however, that it is the height of the superior articular processes that is the major determinant.2Vlexion of a typical cervical spinal segment is a movement composed of anterior sagittal rotation and anterior translation to various extents. Irrespective of its slope, the taller the superior articular process, the more it impedes anterior translation for any degree of anterior sagittal rotati~n.~Th height e of the superior articular process. therefore, determines the extent of coupling between sagittal rotation and sagittal translation. The greater the ratio between rotation and translation, the closer the axis of flexionextension lies to the moving ~ e r t e b r aThe . ~ superior articular processes are taller at lower cervical levels. Consequently the axes of flexionextension at these levels lie closer to the intervertebral disc of the segment. At upper cervical levels, the superior articular processes are short; the segments exhibit a relatively greater amplitude of translation; and their axes of motion lie substantially below the disc of the segment (Figure 6).526 Traditional teaching would maintain that 2 other forms of motion occur at typical cervical segments, viz. sidebending, and axial rotation. An examination of the structure of the typical cervical joints, however, reveals that this is not the case.m The joints between the bodies of cervical vertebrae are essentially saddle joints and allow movements in only 2 planes. In the sagittal plane, the inferior surface of the vertebral body is gently concave, consistent with its freedom to rotate in that plane about a transverse axis. The posterior inferior surface of the vertebral body is rounded into a gentle convexity that is accommodated by the concavity presented by the uncinate processes of the vertebra below. This concavity, however, is oriented at approximately 45 degrees above the transverse plane (ie, in a plane parallel to the plane of the zygapophysial joints) (Figure 7). When viewed in this plane, the joint between the vertebral bodies presents the appearance of an ellip soid joint.6m That joint allows rotation of the upper vertebra of a segment but around an axis perpendicular to the plane of the zygapophysial joints. Moreover, that axis passes through the anterior inferior edge of the vertebral body (Figure 7)."vm Consequently, when rotating, the vertebra pivots about this anterior edge while its tail swings left or right in an arcuate fashion in the concavity of the uncinate processes. Meanwhile, as the vertebral body swings, its inferior articular processes glide freely across the planar surface of the superior articular processes below (Figure 7). Rotations perpendicular to this plane are not possible. Any attempt at rotation sideways results

FIGURE 6. Lateral view of the cervical vertebral column depicting the orientation of the superior articular facets and illustrating the mean location and 2-standard-deviation range of distribution of the instantaneous axes of rotation of cervical motion segments.

in immediate impaction of the ipsilateral inferior facet against its opposing superior articular facet.6 These patterns of movements are also reflected in the structure of the cervical intervertebral discs. The cervical discs are not like archetypical lumbar discs.25 Anteriorly the anulus fibrosus is thick and strong, where it binds the anterior edge of the upper vertebra of the segment. Moreover, its fibres do not assume a crisscross pattern but converge upwards in a lambdoid fashion to anchor the anterior edge of the vertebral body near the midline, at a point coincident with the path of the axis of axial rotation. In effect, the anterior anulus constitutes a strong interosseous ligament located at the pivot point of axial rotation (Figure 8). Laterally, around the perimeter of the intervertebra1 disc, the anulus progressively attenuates and dissipates, and is all but lacking opposite the anterior edge of the uncinate process on each side. Posteriorly, the anulus fibrosus is represented only by a thin bundle of longitudinal fibres restricted to the paramedian plane. Otherwise, the back of the disc is marked by a transverse fissure (Figure 8). This fissure develops as central extensions of clefts in the uncovertebral regions on each side and is a normal feature of cervical It is this fissure that effectively forms the joint cavity of the ellipsoid joint between the vertebral bodies, and which allows for the swinging movement of the upper vertebral body. This description of the morphology and movements of the typical cervical segments can be reconJ Orthop Sports Phys Ther.Volurne 31 .Number 4.April 2001

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FIGURE 7. Morphology of typical cervical joints. Sketch of the transverse concavity (t) of the superior surface of the vertebral body and the sagittal concavity (s) of the inferior surface of the vertebral body above (a). Sketch of the morphology of the interbody joint as seen in the plane of the zygapophysial joints (superior view), the convex posterior aspect of the superior vertebral body accommodated by the concavity of the uncinate processes of the vertebra below (b). Rotation in the plane of the zygapophyseal joint as the vertebral body swings and its inferior articular processes glide freely across the superior articular processes of the vertebra below (c).

ciled with traditional descriptions and interpretations. Axial rotation about a longitudinal axis and lateral rotation about a sagittal axis are not natural or pure movements of the cervical spine. For this reason, they are always coupled with one another (axial rotation is always accompanied by ipsilateral lateral rotation). What might be called pure axial rotation occurs about an obliquely set axis perpendicular to the plane of the zygapophysial joints. But when axial rotation is forced artificially about a longitudinal axis, the moving vertebra encounters the 45 degree slope of the superior articular facets, and that slope drives the vertebra into lateral rotation. A reciprocal combination occurs when lateral rotation is artificially forced about a sagittal axis. The coupling between axial and lateral rotation is therefore a consequence of the artificial expectation that movements must occur about longitudinal and sagittal axes, which is not consistent with the morphology of the cervical joints. Their anatomy dictates that they are saddle joints with movements in only 2 planes. If clinicians reoriented their expectations to movements in the sagittal plane and movements in the plane of the zygapophysial joints, they would access the pure movements of the cervical segments, and the apparent complexity of coupled movements would disappear. J Orthop Sports Phys Ther.Volume 31 .Number 4-April 2001

Anterior anulus fibrosus

Posterior anulus f ibrosus FIGURE 8. Sketch of the cervical intervertebral disc. Lateral view depicting the transverse fissuring of the fibrocartilaginous core of the intervertebral disc. The posterior anulus fibrosus has been removed (a). Top view of the nucleus pulposis where the anterior anulus fibrosus (crecentric in shape) is dissipating opposite the uncinate processes and the posterior anulus fibrosus (thin) is restricted to the paramedian plane (b).

CONCLUSIONS Close attention to details of anatomy of the cervical spine provides explanations for many aspects of its biomechanics. Prominent in this regard is the passive behavior of the atlas, whose movements are predicated by the shape of the articular cartilages of 181

the lateral atlanto-axial joints and dictated more by forces applied from the head than by muscle action. Also, new observations o n the structure o f the cervical intervertebral discs dictate that the motion o f t y p ical cervical segments is more accurately viewed as saddle joints than as triaxial joints. Awareness o f anatomical and biomechanical detail should serve to prevent misinterpretation o f clinical signs and the perpetuation o f false models o f cervical pathology.

16. 17. 18. 19. 20.

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agnosis of injuries of the upper cervical vertebral column. Rontgenblatter. 1980;33:67-76. JiroutJ. Synkinetic contralateral tilting of atlas and head on lateral inclination. Part I. Manuelle Med. 1985;1:116-120. JiroutJ. Synkinetic contralateral tilting of atlas and head on lateral inclination. Part II.Manuelle Med. 1985;1:121-125. Koebke J, Brade H. Morphological and functional studies on the lateral joints of the first and second cervical vertebrae in man. Anat Embryo1 (Berl). 1982;164:265-275. Kottke FJ, Mundale MO. Range of mobility of the cervical spine. Arch Phys Med Rehabil. 1959;40:379-382. Lewit K, Krausova L. Messungen von vor-und ruckbeuge in den kopfgelenken. Fortschr Rontgenst. 1963;99:538549. Lind B, Sihlbom H, Nordwall A, Malchau H. Normal range of motion of the cervical spine. Arch Phys Med Rehabil. 1989;70:692-695. Ludwig K. Uber das ligamentum alare dentis epistrophei des menschen. Z Anat EntwGesch. 1952;116:442-445. Markuske H. Untersuchungen zur Statik und Dynamik der kindlichen Halswirbelsaule: der Aussagewert seitlicher Rontgenaufnahmen. Die Wirbelsaule in Forschung und Praxis. Stuttgart: Hippokrates; 1971. Mercer SR, Bogduk N. Intra-articular inclusionsof the cervical synovial joints. Brit ] Rheumatol. 1993;32:705-710. Mercer SR, Bogduk N. The ligaments and anulus fibrosus of human adult cervical intervertebral discs. Spine. 1999; 24:619-626. Nowitzke A, Westaway M, Bogduk N. Cervical zygapophyseal joints; geometrical parameters and relationship to cervical kinematics. Clin Biomech. 1994;9:342-348. Oda J, Tanaka H, Tsuzuki N. Intervertebral disc changes associated with aging of human cervical vertebra. From the neonate to the eighties. Spine. 1988;13:1205-1211. Panjabi M, Dvorak J, Duranceau J, et at. Three-dimensional movements of the upper cervical spine. Spine. 1988;13:726-730. Penning L. Differences in anatomy, motion, development and aging of the upper and lower cervical disk segments. Clin Biomech. 1988;3:37-47. Penning L. Normal movements of the cervical spine. Am 1 Roentgenol. 1978;130:317-326. Schonstrom N, Twomey L, Taylor J. The lateral atlantoaxial joints and their synovial folds: an in vitro study of soft tissue injuries and fractures. ] Trauma. 1993;35:886892. Van Mameren H, Drukker J, Sanches H, Beursgens J. Cervical spine motion in the sagittal plane (I).Range of motion of actually performed movements, an X-ray cinematographic study. Fur ] Morphol. 1990;28:47-68. Werne S. The possibilities of movement in the craniovertebral joints. Acta Orthop Scand. 1958;28:165-173. White AA, Panjabi MM. Clinical Biomechanics of the Spine. Philadelphia, Pa: Lipincott; 1978. Worth DR, Selvik G. Movements of the craniovertebral joints. In: Grieve G,ed. Modern Manual Therapy of the Vertebral Column. Edinburgh: Churchill Livingstone; 1994:53-68.

J Orthop Sports Phys Ther.Volume 31 .Number 4.April2001

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Invited Commentary Dr Susan Mercer's and Dr Nikolai Bogduk's treatise on joints of the cervical vertebral column has enormous clinical implications. Their description of the morphology of the cervical discs leaves little doubt that the cervical spine cannot function like the lumbar spine and, therefore, requires examination and treatment that accounts for its idiosyncrasies.' Drs Mercer and Bogduk's differentiation of the cervical spine into the craniocervical and lower cervical underscores the different morphology and therefore function between these regions. I would suggest another separation, that of cervico-thoracic, where C7-T1, T1-T2 and T2-T3 segments can be considered as a separate region with its concomitant special functions requiring special evaluations and treatments. I think of C2 as a transitional vertebra. The articulation from above is certainly part of the craniovertebral unit; however, the action of C2 confers upon C3 as the initiator of the concomitant sidebending and rotation to the same side. Therefore, C2 often requires evaluation with both the cervical and craniocervical regions. On the segmental level, range of motion can, and does, exceed the total range of motion available in the cervical spine. This can be easily demonstrated by using an inclinometer and having a subject perform flexion and extension of the cervical spine from different postures of the thoracic spine. The observations that thoracic posture (ie, slump versus military posture) alters range of motion have led us to standardize our protocol so that we place the inclinometer just once for each plane of movement (ie, we record flexion and extension, always starting in flexion, but we only record as significant changes those which encompass both sagittal plane motions). Lateral bendings and rotations are similarly affected, so that the lateral bendings are again recorded indi-

J Orthop Sports Phys Ther.Volume 31 .Number 4.April2001

vidually but considered more significant when total coronal plane movement is changed. Rotation, because it is tested in the supine position, is not as interdependent right and left as the neutral position, which is more easily obtained and stabilized; however, it is not assumed that the supine rotation is comparable to rotation in the anti-gravity postures. Segmental mobility testing, using passive forces exerted on the skull, does not necessarily follow a predictable pattern of coupled motion and, in view of the small amount of range of motion, should probably be abandoned in favor of the direct palpation of anatomical landmarks. Finally, some of the findings of Drs Mercer and Bogduk may be applied in the clinical examination. For example, sidebending of the head is accompanied by C2 rotation to the same side with rotation of C1 to the opposite, as long as the subject's transverse plane posture is maintained in neutral. By using deductive reasoning, this phenomena allows us to determine whether craniocervical or midcervical problems are productive of symptoms by utilizing the 3 degrees of freedom present in the cervical spine and recording the range of motion and the production or alteration of symptoms. In summary, I'd like to thank Drs Mercer and Bogduk for their excellent contribution to the Special Issue on the Cervical Spine in the Journal. Richard E. Erhard, DC, PT Department of Physical Therapy 6035 Forbes Tower University of Pittsburgh Pittsburgh, PA 15260

REFERENCE 1 . The Spinal Exercise Handbook (with CD ROM). Pittsburgh, Pa: Laurel Concepts; 1998.