Biomechanics of the cervical spine. I Normal kinematics.pdf

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Clinical Biomechanics 15 (2000) 633±648
Review paper
Biomechanics of the cervical spine. I: Normal kinematics
Nikolai Bogduk a, * , Susan Mercer b
Newcastle Bone and Joint Institute, University of Newcastle, Royal Newcastle Hospital, Level 4, David Maddison Building, Newcastle, NSW 2300,
Department of Anatomy, University of Otago, Dunedin, New Zealand
This review constitutes the ®rst of four reviews that systematically address contemporary knowledge about the mechanical
behavior of the cervical vertebrae and the soft-tissues of the cervical spine, under normal conditions and under conditions that result
in minor or major injuries. This ®rst review considers the normal kinematics of the cervical spine, which predicates the appreciation
of the biomechanics of cervical spine injury. It summarizes the cardinal anatomical features of the cervical spine that determine how
the cervical vertebrae and their joints behave. The results are collated of multiple studies that have measured the range of motion of
individual joints of the cervical spine. However, modern studies are highlighted that reveal that, even under normal conditions,
range of motion is not consistent either in time or according to the direction of motion. As well, detailed studies are summarized that
reveal the order of movement of individual vertebrae as the cervical spine ¯exes or extends. The review concludes with an account of
the location of instantaneous centres of rotation and their biological basis.
The facts and precepts covered in this review underlie many observations that are critical to comprehending how the cervical
spine behaves under adverse conditions, and how it might be injured. Forthcoming reviews draw on this information to explain how
injuries might occur in situations where hitherto it was believed that no injury was possible, or that no evidence of injury could be
detected. Ó 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Cervical spine; Biomechanics; Movements; Anatomy
1. Introduction
therefore, predicated by the anatomy of the bones that
make up the neck and the joints that they form.
Amongst its several functions, the head can be re-
garded as a platform that houses the sensory apparatus
for hearing, vision, smell, taste and related lingual and
labial sensations. In order to function optimally, these
sensory organs must be able to scan the environment
and be delivered towards objects of interest. It is the
cervical spine that subserves these facilities. The cervical
spine constitutes a device that supports the sensory
platform, and moves and orientates it in three-dimen-
sional space.
The movements of the head are executed by muscles
but the type of movements possible depend on the shape
and structure of the cervical vertebrae and interplay
between them. The kinematics of the cervical spine are,
2. Functional anatomy
For descriptive purposes, the cervical spine can be
divided and perceived as consisting of four units, each
with a unique morphology that determines its kine-
matics and its contribution to the functions of the
complete cervical spine. In anatomical terms the units
are the atlas, the axis, the C2±3 junction and the re-
maining, typical cervical vertebrae. In metaphorical,
functional terms these can be perceived as the cradle, the
axis, the root, and the column.
2.1. The cradle
Corresponding author.
E-mail address: (N. Bogduk).
The atlas vertebra serves to cradle the occiput. Into
its superior articular sockets it receives the condyles of
the occiput. The union between the head and atlas,
0268-0033/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.
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N. Bogduk, S. Mercer / Clinical Biomechanics 15 (2000) 633±648
through the atlanto-occipital joints, is strong, and allows
only for nodding movements between the two struc-
tures. In all other respects the head and atlas move and
function essentially as one unit.
The stability of the atlanto-occipital joint stems
largely from the depth of the atlantial sockets. The side
walls of the sockets prevent the occiput from sliding
sideways; the front and back walls prevent anterior and
posterior gliding of the head, respectively. The only
physiological movements possible at this joint are ¯ex-
ion and extension, i.e. nodding. These are possible be-
cause the atlantial sockets are concave whereas the
occipital condyles are convex.
Flexion is achieved by the condyles rolling forwards
and sliding backwards across the anterior walls of their
sockets (Fig. 1). If the condyles only rolled, they would
roll up and over the anterior wall of their sockets. Axial
forces exerted by the mass of the head or the muscles
causing ¯exion prevent this upward displacement and
cause the condyles to slide downwards and backwards
across the concave surface of the socket. Thereby the
condyles remain within their sockets, and the composite
movement is a rotation, or a spin, of each condyle across
the surface of its socket. A converse combination of
movements occurs in extension. This combination of
roll and contrary glide is typical of condylar joints.
The ultimate restraint to ¯exion and extension of the
atlanto-occipital joint is impaction of the rim of the
socket against the base of the skull. Under normal
conditions, however, ¯exion is limited by tension in the
posterior neck muscles and by impaction of the sub-
mandibular tissues against the throat. Extension is lim-
ited by the occiput compressing the suboccipital
Axial rotation and lateral ¯exion are not physiologi-
cal movements of the atlanto-occipital joints. They
cannot be produced in isolation by the action of mus-
cles. But they can be produced arti®cially by forcing the
head into these directions while ®xing the atlas. Axial
rotation is prohibited by impaction of the contralateral
condyle against the anterior wall of its socket and si-
multaneously by impaction of the ipsilateral condyle
Fig. 2. Right lateral views of axial rotation of the atlanto-occipital
joints. Rotation requires forward translation of one condyle and
backward translation of the other. Translation is possible only if the
condyles rise up the respective walls of the atlantial sockets. As a re-
sult, the occiput rises relative to its resting position (centre ®gure).
against the posterior wall of its socket. For the head to
rotate, the condyles must rise up their respective walls.
Consequently, the occiput must separate from the atlas
(Fig. 2). This separation is resisted by tension in the
capsules of the atlanto-occipital joints. As a result, the
range of motion possible is severely limited. Lateral
¯exion is limited by similar mechanisms. For lateral
¯exion to occur the contralateral condyle must lift out of
its socket, which engages tension in the joint capsule.
2.2. The axis
Carrying the head the atlas sits on the atlas, with the
weight being borne through the lateral atlanto-axial
joints. After weight-bearing, the cardinal function of the
atlanto-axial junction is to permit a large range of axial
rotation. This movement requires the anterior arch of
the atlas to pivot on the odontoid process and slide
around its ipsilateral aspect; this movement being
accommodated at the median atlanto-axial joint
(Fig. 3(A)). Meanwhile, at the lateral atlanto-axial joint
the ipsilateral lateral mass of the atlas must slide back-
wards and medially while the contralateral lateral mass
must slide forwards and medially (Fig. 3).
Radiographs of the lateral atlanto-axial joints belie
their structure. In radiographs the facets of the joint
appear ¯at, suggesting that during axial rotation the
lateral atlanto-axial joints glide across ¯at surface. But
radiographs do not reveal cartilage. The articular car-
tilages both of the atlantial and the axial facets of the
Fig. 1. Right lateral views of ¯exion and extension of the atlanto-oc-
cipital joints. The centre ®gure depicts the occipital condyle resting in
the atlantial socket in a neutral position. The dots are reference points.
In ¯exion the head rotates forwards but the condyle also translates
backwards, as indicated by the displacement of the references dot. A
converse combination of movements occurs in extension.
Fig. 3. Atlanto-axial rotation. A: top view. The anterior arch of the
atlas (shaded) glides around the odontoid process. B: right lateral view.
The lateral mass of the atlas subluxates forwards across the superior
articular process of the axis.
N. Bogduk, S. Mercer / Clinical Biomechanics 15 (2000) 633±648
Fig. 4. Lateral view of a right lateral atlanto-axial joint (centre ®gure)
showing the biconcave structure of the articular cartilages. Upon
forward or backward displacement, the lateral mass of the atlas settles
as it slips down the slope of the cartilage.
Fig. 5. The mechanism of paradoxical movements of the atlas. In the
neutral position (centre ®gure) the atlas is balanced on the convexities of
its articular cartilages. If the atlas is compressed anterior to the balance
point, it ¯exes. If compressed behind the balance point, it extends.
joint are convex, rendering the joint biconvex [1] (Fig. 4).
The spaces formed anteriorly and posteriorly, where the
articular surfaces diverge, are ®lled by intra-articular
meniscoids [2]. In the neutral position the summit of the
atlantial convexity rests on the convexity of the axial
facet. As the atlas rotates, however, the ipsilateral at-
lantial facet slides down the posterior slope of its axial
fact, and the contralateral atlantial facet slides down the
anterior slope of its facet. As a result, during axial ro-
tation the atlas descends, or nestles into the axis (Fig. 4).
Upon reversing the rotation the atlas rises back onto the
summits of the facets.
Few muscles act directly on the atlas. The levator
scapulae arises from its transverse process but uses this
point of suspension to act on the scapulas; it does not
move the atlas. Obliquus superior and rectus capitis
posterior minor arise from the atlas and act on the oc-
ciput, as do rectus anterior and rectus lateralis. At-
taching to the anterior tubercle, longus cervicis is the
one muscle that acts directly on the atlas, to ¯ex it. But
paradoxically there is no antagonist to this muscle.
This paradox underscores the fact that the atlas acts
as a passive washer, interposed between the head and
the cervical spine proper. Its movements are essentially
passive and governed essentially by the muscles that act
on the head. Accordingly, rotation of the atlas is
brought about by splenius capitis and sternocleidomas-
toid acting on the head. Torque is then transferred from
the head, though the atlanto-occipital joints, to the at-
las. The ®bres of splenius cervics that insert into the
atlas supplement this eect.
The passive movements of the atlas are most evident
in ¯exion/extension of the neck where, indeed, the atlas
exhibits paradoxical motion. At full ¯exion of the neck
the atlas can extend, and usually does so [3]. This arises
because the atlas, sandwiched between the head and
axis, and balanced precariously on the summits of the
lateral atlanto-axial facets, is subject to compression
loads. If the net compression passes anterior to the
contact point in the lateral atlanto-axial joint, the lateral
mass of the atlas will be squeezed into ¯exion (Fig. 5).
Conversely, if the line of compression passes behind the
contact point, the atlas will extend; even if the rest of the
cervical spine ¯exes (Fig. 5). If, during ¯exion, the chin is
tucked backwards, paradoxical extension of the atlas is
virtually assured, because retraction of the chin favours
the line of weight-bearing of the skull to fall behind the
centre of the lateral atlanto-axial joints.
The restraints to ¯exion/extension of the atlas have
never been formally established. No ligaments are dis-
posed to limit this motion. The various atlanto-occipital
membranes are fascial in nature and would not consti-
tute substantive ligamentous restraints. Essentially, the
atlas is free to ¯ex or extend until the posterior arch hits
either the occiput or the neural arch of C2, respectively.
The restraints to axial rotation are the capsules of the
lateral atlanto-axial joints and the alar ligaments. The
capsules contribute to a minor degree; the crucial re-
straints are the alar ligaments [4]. Dislocation of the
atlas in rotation does not occur while so long as the alar
ligaments remain intact. This feature further under-
scores the passive nature of the atlas, for the alar liga-
ments do not attach to the atlas; rather, they bind the
head to the odontoid process of the axis. By limiting
the range of motion of the head they secondarily limit
the movement of the atlas.
Backward sliding of the atlas is limited absolutely by
impaction of the anterior arch of the atlas against the
odontoid process, but there is no bony obstruction to
forward sliding. That movement is limited by the
transverse ligament of the atlas and the alar ligaments.
As long as either ligament remains intact, dislocation of
the atlas is prevented [5].
Lateral gliding involves the ipsilateral lateral mass of
the atlas sliding down the slope of its supporting supe-
rior articular process while the contralateral lateral mass
slides upwards. The movement is primarily limited by
the contralateral alar ligament, but is ultimately blocked
by impaction of the lateral mass on the side of the
odontoid process [6].
2.3. The root
The C2±3 junction is commonly regarded as the
commencement of the typical cervical spine, where all
N. Bogduk, S. Mercer / Clinical Biomechanics 15 (2000) 633±648
segments share the same morphological and kinematic
features. However, the C2±3 junction diers from other
segments in a subtle but obscure way.
The dierences in morphology are not readily ap-
parent and, for this reason, have largely escaped notice.
A pillar view of the region reveals the dierence. (A
pillar view is obtained by beaming X-rays upwards and
forwards through the cervical spine, essentially along the
planes of the zygapophysial joints.) In such a view
the body of the axis looks like a deep root, anchoring the
apparatus, that holds and moves the head, into the
typical cervical spine (Fig. 6). Moreover, in such view,
the atypical orientation of the C2±3 zygapophysial joints
is seen. Unlike the typical zygapophysial joints whose
planes are transverse, the superior articular processes of
C3 face not only upwards and backwards but also me-
dially, by about 40° [7]. Together, the processes of both
sides form a socket into which the inferior articular
processes of the axis are nestled. Furthermore, the su-
perior articular processes of C3 lie lower, with respect
to their vertebral body, than the processes of lower
segments [8].
These dierences in architecture imply that the C2±3
joints should operate in a manner dierent from that of
lower, typical cervical segments. One dierence is that
during axial rotation of the neck, the direction of cou-
pling with lateral ¯exion at C2±3 is opposite to that seen
at lower segments (see Table 4). Instead of bending to-
wards the same side as rotation, C2 rotates away from
that side, on the average. The lower location of the su-
perior articular process of C3 correlates with the lower
location of the axis of sagittal rotation of C2 (see
Fig. 14). Other dierences in how C2±3 operates have
not been elaborated, but the unique architecture of C2±3
suggests that further dierences are open to discovery.
2.4. The column
At typical cervical segments, the vertebral bodies are
stacked on one another, separated by intervertebral
discs. The opposing surfaces of the vertebral bodies,
however, are not ¯at as they are in the lumbar spine.
Rather, they are gently curved in the sagittal plane. The
anterior inferior border of each vertebral body forms a
lip that hangs downwards like a slight hook towards the
anterior superior edge of the vertebra below. Mean-
while, the superior surface of each vertebral body slopes
greatly downwards and forwards. As a result, the plane
of the intervertebral disc is set not perpendicular but
somewhat oblique to the long axes of the vertebral
bodies. This structure re¯ects, and is conducive to,
¯exion±extension being the cardinal movement of typi-
cal cervical segments.
The vertebral bodies are also curved from side-to-
side, but this curvature is not readily apparent. It is re-
vealed if sections are taken through the posterior ends of
the vertebral bodies, either parallel to the planes of the
zygapophysial joints, or perpendicular to these planes.
Such sections reveal that the inferior surface of the hind
end of the vertebral body is convex, and that convexity
is received by a concavity formed by the body below and
its uncinate processes (Fig. 7). The appearance is that of
an ellipsoid joint (like the wrist). This structure suggest
that vertebral bodies can rock side-to-side in the con-
cavity of the uncinate processes. Further consideration
reveals that this is so, but only in one plane.
If sections are taken through the cervical spine along
planes perpendicular to the zygapophysial joints, and if
the sections through the uncinate region and through
the zygapophysial joints are superimposed, the appear-
ance is revealing [9,10] (Fig. 8). The structure of the
interbody junction is ellipsoid and suggests that rocking
could occur between the vertebral bodies. However, in
this plane the facets of the zygapophysial joints are di-
rectly opposed. Therefore, any attempted rocking of the
vertebral body is immediately prevented by the facets
(Fig. 8).
If sections are taken through the plane of the zyga-
pophysial joints, the ellipsoid structure of the interbody
joint is again revealed, but the zygapophysial joints
Fig. 6. A tracing of a pillar view of the upper cervical spine, showing
the unique morphology of C2 (shaded). (A pillar view is a radiographic
projection of the cervical spline obtained by directing the beams up-
wards and forwards from behind the cervical spine, essentially along
the planes of the lower cervical zygapophysial joints.) Note how the
zygapophysial joints at lower levels (arrowed) are orientated trans-
versely whereas at C2±3 they are inclined medially, cradling the pos-
terior elements of the axis while its vertebral body dips like a deep root
into the cervical vertebral column.
N. Bogduk, S. Mercer / Clinical Biomechanics 15 (2000) 633±648
Fig. 9. The appearance, viewed from above, of superimposed sections
of a C5±6 cervical intervertebral joint taken through the uncinate
region and through the zygapophysial joints, along a plane parallel to
that of the zygapophysial joints. In this plane, if the C5 vertebral body
rotates, its inferior articular facets (iaf) are free to glide across the
surface of the superior articular facets of C6.
Fig. 7. A sketch of a section taken obliquely through the posterior end
of a C5±6 interbody joint, along a plane parallel to the plane of the
zygapophysial joints. Between the uncinate processes (u) the C6
vertebral body presents a concave articular surface that receives the
convex inferior surface of C5.
Fig. 8. The appearance, viewed from above, of a section of a C6±7
cervical intervertebral joint taken through the uncinate region and the
zygapophysial joints, along a plane perpendicular to the zygapophysial
joints. In this plane, if the C6 vertebral body rotates to the left, its right
inferior articular process (iap) immediately impacts, en face, into the
superior articular process (sap) of C7; which precludes lateral rotation
of C6.
present en face. Consequently the facets do not impede
rocking of the vertebral bodies in this plane. Indeed, the
facets slide freely upon one another (Fig. 9).
These observations indicate that the cervical inter-
vertebral joints are saddle joints: they consist of two
concavities facing one another and set at right angles to
one another [9,10]. Across the sagittal plane the inferior
surface of the vertebral body is concave downwards,
while across the plane of the zygapophysial joints the
uncinate region of the lower vertebral body is concave
Fig. 10. The saddle shape of cervical intervertebral joints. The inferior
surface of the upper vertebral body is concave downwards in the
sagittal plane (s). The superior surface of the lower vertebral body is
concave upwards in the transverse plane (t).
upwards (Fig. 10). This means that the vertebral body is
free to rock forwards in the sagittal plane, around a
transverse axis, and is free to rock side-to-side in the
place of the facets, around an axis perpendicular to the
facets (Fig. 11). Motion in the third plane ± side-to-side
around an oblique anterior ± posterior axis is precluded
by the orientation of the facets.
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