Download MECH5221M Spinal Biomechanics and Instrumentation Unit 1

School of Mechanical
Engineering
FACULTY OF ENGINEERING
MECH5221M Spinal Biomechanics
and Instrumentation
Unit 1: Functional Anatomy of the
Spine
Module Leader: Professor R M Hall
Contents
1.1 Introduction to the Spine................................................................................... 3
1.2 The Spinal Column........................................................................................... 4
1.3 Components of the Spine ................................................................................. 6
1.4 The Vertebrae .................................................................................................. 7
1.4.1 The Vertebral Body (VB)............................................................................ 9
1.4.2 Variation in Vertebral Body Morphology................................................... 11
1.4.3 The Pedicles ............................................................................................ 14
1.4.4 The Neural Arch and Processes .............................................................. 15
1.4.5 The Cervical Vertebrae ............................................................................ 16
1.4.6 The Thoracic Vertebrae ........................................................................... 19
1.4.7 The Lumbar Vertebrae............................................................................. 22
1.4.8 The Sacral Vertebrae............................................................................... 24
1.5 The Intervertebral Disc ................................................................................... 26
1.5.1 Nucleus Pulposus .................................................................................... 27
1.5.2 Annulus Fibrosus and Endplate ............................................................... 27
1.5.3 Biomechanics of the Disc......................................................................... 28
1.5.4 Disorders of the Disc and Affects of Aging............................................... 30
1.6 The Spinal Ligaments..................................................................................... 33
1.6.1 Mechanical Behaviour of the Ligaments .................................................. 34
1.6.2 Anterior Longitudinal Ligament ................................................................ 37
1.6.3 Posterior Longitudinal Ligament............................................................... 38
1.6.4 Intertransverse and Capsular Ligaments ................................................. 38
1.6.5 Supraspinous and Interspinous Ligaments .............................................. 39
1.6.6 Ligamentum Flavum ................................................................................ 39
1.7 The Spinal Muscles ........................................................................................ 40
1.7.1 Post-vertebral – Erector Spinae ............................................................... 41
1.7.2 Post-vertebral – Transversospinalis......................................................... 42
1.7.3 Post vertebral - Deep Segmental Muscles............................................... 43
1.7.4 Pre-vertebral ............................................................................................ 44
1.7.5 Key Biomechanics Considerations of Spinal Muscles.............................. 45
1.8 The Synovial Joints of the Spine .................................................................... 47
1.8.1 Sacro-iliac Joint........................................................................................ 47
1.8.2 Zygapophysial (facet)............................................................................... 49
1.8.3 Atlanto-occipital and Atlanto-axial Joints (C0-C1-C2) .............................. 50
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1.1 Introduction to the Spine
The spine is a complex multifunctional system located within the torso linking the
head with the pelvis. Biomechanically it is composed of a number of segmental units
comprising two vertebrae, an intervening intervertebral disc and associated paraspinal tissues including muscles and ligamentous tissues. Harms and Tabasso
(1999) eloquently outlined the biomechanical functions of the spine into 4 main
themes (see Figure 1.1). Cotler (2000) developed a more goal and patient orientated
list (see Figure 1.2).
The first unit covers the basic functional anatomy of the spine by first looking at the
individual components and then building on this to look later at the behaviour of the
function spinal unit (FSU). The FSU, which comprises two vertebra, the intervening
disc and the attached ligaments, is the smallest section of the spine that represents
the biomechanical behaviour of this system. Given this simplicity it is the unit most
often used in the assessment of spinal behaviour in-vitro.
Functions of the spine:
 To protect the spinal cord
 To provide stability
 To provide mobility
 To control the transmittance of movement of the upper and lower extremities
Figure 1.1
Harms’ 4 main biomechanical functions of the spine
Look at the list below - what is particular about the ordering of the
Question 1 list?
Functions of the spine:
 To protect the spinal cord
 To provide stability
 To provide mobility
 To control the transmittance of movement of the upper and lower extremities
The answer to this question is available from the distance learning courseware
available in the VLE, http://vlebb.leeds.ac.uk/.
Goals of treatment:
 Protect neural elements
 Maximise neural return
 Produce a painless, stable spine
 Return patient to work
Figure 1.2
Goals of spinal interventions - according to Cotler
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1.2 The Spinal Column
The human spine is a vertical column of typically 33 individual bony units called
vertebrae. It is typically divided into five regions with varying numbers of vertebrae
(Figure 1.3). These are, from superior to inferior, 7 cervical, 12 thoracic, 5 lumbar,
4/5 fused sacral and 4 fused coccygeal segments. In the pre-sacral vertebrae motion
between the vertebrae is limited by two posterior synovial facet joints per vertebra,
intervertebral discs, intervertebral ligaments and a complex arrangement of muscles.
The vertebrae within the sacral and coccygeal regions are fused.
Figure 1.3 Spinal regions and curvature
The measurement next to the representation of the FSU for each of the regions is
the anterior height of the disc. The shaded areas on the vertebrae are the regions,
which are visible in this lateral perspective, that are covered by articular cartilage.
Curvature
The spinal column features a characteristic ‘S’ curve in the sagittal plane: anterior
convexity (lordosis) in the cervical and lumbar regions, anterior concavity (kyphosis)
in the thoracic and sacro-coccygeal regions (Figure 1.3). In the thoracic and sacral
regions the curvature is due to vertebral geometry, while in the cervical and lumbar
regions it is due to intervertebral disc wedging. The curve is acquired during early
sitting and walking development. The curvature is thought to provide a flexible,
shock-absorbent structure that has adequate stiffness and stability at the level of
individual intervertebral joints. The following more specific reasons have been
postulated for this curvature:
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


Increased rotational stability in the sagittal plane by distributing mass away from
the skull-pelvis line and thereby increasing the moment of inertia about the axis.
Lumbar lordosis increases motion coupling (e.g. lateral bending and axial
rotation) which is thought to contribute to pelvic rotation during gait.
Cyclic change in curvature during gait cycle, (1-2° in the lumbar region) indicates
some energy absorption and release by the tendons of the spinal musculature.
This reduces the loads transmitted to the skull.
In the frontal plane the spine is normally straight and symmetrical, in some cases
with a slight thoracic curve to the right. In individuals with scoliosis a large lateral
curvature and axial rotation of the vertebrae are observed.
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1.3 Components of the Spine
The spinal column is composed of a number of components that to a large degree
determine the behaviour of the spine. These individual components, which include
the vertebrae, ligaments, discs, spinal cord and muscles, need to be characterised
so that the spine can be modelled and the contribution of these structures to the
stability of the spine, for example, can be determined for different activities. This data
can then be used to determine suitable treatment regimes as well as for the design
of new devices that replace or augment these tissues.
Figure 1.4 The passive elements of the spine that comprise a functional spine unit
(FSU). The muscles envelop the spine and are responsible for providing motion of all
or part of the spine and are the principle agent in this structure’s stability.
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1.4 The Vertebrae
While the vertebrae of the different spinal regions exhibit some features unique to
that region, it is useful to begin with a description of the general vertebra structure
and function. Although a single bone, each vertebra may be divided into anterior and
posterior segments. The anterior vertebra consists of the vertebral body, while the
posterior section is comprised of the neural arch, which is in turn a composite of the
pedicles, laminae and the various bony processes (Figure 1.5). The vertebral bodies
are responsible for transmission of most of the compressive load within the spinal
column, while the posterior processes mainly provide attachment points for the
muscles and ligaments that control the movements of the spine. Under changes in
posture the degree to which load is partition between the vertebral body/discs and
the posterior elements (facets joints) also varies.
Figure 1.5
The major regions or features of a typical vertebra (L5 to C3)
Figure 1.6 shows three typical vertebrae from each of the pre-sacral spinal regions.
As indicated previously there are common features to each such as vertebral body
and spinal canal, there are also peculiarities that pertain to regions. There are clear
differences in size particularly of the vertebral body itself which is the main load
carrying structure.
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Figure 1.6 Differences in vertebral body features between each of the main spinal
regions
Copyright Primal Pictures
The following link provides a video of describing the vertebrae in the cervical,
thoracic and lumbar vertebrae.
http://www.youtube.com/watch?v=7Wt6gWrggUk
The vertebrae (or spinal segments) are also commonly divided into “columns” for
fracture and stability classification. Columnar categorisation was first devised by
Holdsworth (1963), who proposed that vertebrae be divided into the anterior
longitudinal ligament column and the posterior ligamentous complex. This was later
revised to a three-column model by Denis (1983) and McAfee (1983) to include a
osteoligamentous complex between the aforementioned columns (Figure 1.7). These
column theories are used to describe and categorise fracture type and severity, as
well as the resultant stability. Assessment of spinal stability following trauma is an
important determinant for surgical or nonsurgical intervention.
a) Holdsworth (1963)
b) Denis (1983)
c) Ferguson
(1984)
and
Allen
Figure 1.7 Various columnar models of the spine developed mainly to indicate the
stability following trauma (A=anterior, M=middle, P=posterior).
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1.4.1 The Vertebral Body (VB)
The vertebral body is roughly cylindrical in cross-section, with the long-axis in line
with the vertical axis of the spine. Similar to long bones, the body consists of a
cancellous inner structure with an outer layer of more dense bone often referred to
as the vertebral shell, providing a good strength-to-weight ratio. The cancellous
trabeculae are oriented primarily in the vertical direction to transmit load from the
superior to the inferior surface of the vertebral body. Horizontal cross-member
trabeculae provide resistance to compressive buckling. The slightly concave superior
and inferior surfaces of the body are referred to as endplates, although there is some
disagreement on the delineation between vertebral body and intervertebral disc.
a) Sagittal view
b) Transverse view
Figure 1.8 MicroCT cross section of a lumbar vertebra body. Note the concave
nature of the endplates and the highly osteoporotic nature of this specimen.
The vertebral body carries the majority of the compressive load in the spinal column.
The actual proportion shared with the facet joints and spinal ligaments varies with
region, spinal posture, loading history and age. In the lumbar spine with a neutral
stance then approximately 80% of the compressive load will be transferred through
the vertebral body. This proportion will reduce on extension and increase on flexion.
The literature concerning the relative share of load carried by the cortical and
cancellous bone indicates variable results. This is due, in part, to the variation in
strength seen with degeneration of both types of bone with age. Experimental
models have indicated that removal of the cortical bone weakens the body structure
by between 10% and 35-44%, providing some evidence that much of the load is
carried by the inner trabeculae.
The slightly concave osseus endplates contain perforations for nutrient and waste
flux. It is postulated that these channels weaken the endplate, making it particularly
susceptible to fracture. It is regarded as the “weak-link” in the lumbar spine with a
high proportion of spinal damage being identified in the endplate or its supporting
trabeculae. Recently, considerable emphasis has been placed on investigating the
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mechanical integrity of these endplates. Their ability to withstand both static and
cyclic loading is a critical factor in the subsidence of spinal devices implanted to
augment or replace spinal tissues. Redesign of such devices to exploit the stronger
regions of vertebral end plates should lead to increased success. It has been
reported that the endplates are strongest at their periphery, particularly in the
postero-lateral regions (Figure 1.9 and Figure 1.10). It has been noted that the
properties demonstrated by endplates in sheep and pigs varies from that observed in
the human spine. In these animals the anterior portion of the endplates tends to be
stronger than that observed in the posterior zones; opposite to that observed in
Figure 1.10. This should be taken into consideration when undertaking preliminary
experiments of new devices or interventions within invitro animal models which
depends on the mechanical properties of the endplates.
Figure 1.9 The grid points used to determine the spatial variation in the endplate
properties of the lumbar vertebral bodies (Grant 2001)
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a)
b)
Figure 1.10 The relationship between vertebral body endplate stiffness/strength and
position. Note that the increased failure load posteriorly and that the highest loads
are found in the vicinity of the attachments of the pedicles to the vertebral body
(Grant 2001)
Weakness in the endplates as a result of osteoporosis is a significant issue when
undertaking spinal surgery in the elderly. Numerous studies have demonstrated the
direct correlation between screw pull-out strength and BMD as well as the increased
chances of subsidence with interbody fusion devices or total disc replacements. In
the latter cases, osteoporosis is usually a contra-indication for this type of surgery.
1.4.2 Variation in Vertebral Body Morphology
One obvious feature of the spine is the change in size of the vertebrae as we go
from one region to the next. Even within regions there are distinct differences
particularly at the junction between the five zones. Vertebral bodies are progressively
smaller, lighter and consequently weaker from the lumbar to cervical regions,
reflecting the loads they experience (Figure 1.11 and Figure 1.12).
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Figure 1.11 Variation in the cross sectional area of the upper vertebral endplate
with spine level position
Figure 1.12 Variation of failure load with spinal level position
As well as the shape and size of the vertebral body the bone quality is an important
determinant of vertebral strength. Variations in the bone mineral density can also
occur between levels although this tends to be greatest between the cervical and
lower regions (Figure 1.13). Moving from the mid thoracic to lumbar regions the rate
of change in BMD with position is relatively small although there is some scatter
arising from differences between adjacent vertebrae. This VB to VB variation in BMD
tends to increase as bone becomes more degenerate (osteoporotic) with age. These
two factors, vertebral size and BMD, contribute to the strength of individual vertebra.
Indeed a simple relationship has been developed between strength at low loading
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rates and the cross sectional area of the VB together with its bone mineral density as
assessed from quantitative Computed Tomography (qCT).
Figure 1.13 The variation of BMD with spinal level. The reduction in BMD from the
cervical to the lumbar region is of the order of 50 %, although the change in the mid
thoracic and lumbar regions is relatively small. The increase in the size of the
vertebrae in lumbar spine more than compensates for this reduction in BMD when
resisting the high compressive loads observed in this region.
Reductions in bone quality within the vertebral body can cause significant reductions
in the compressive failure load required to fracture the vertebra. This reduction in
bone quality is most often caused by the reductions in BMD through osteoporosis,
which may be considered, to a general approximation, as a generalised bone loss.
However, similar drops in failure load can be generated with metastatic bone disease
which are reflective of the osteolytic lesions formed as the secondary tumour invades
the bone tissue. Figure 1.14 demonstrates the weakness observed when cadaveric
vertebrae are loaded to failure for three pathologies; multiple myeloma, metastatic
bone disease arising from bladder cancer and osteoporosis.
Figure 1.14 Initial failure load arising from three pathologies
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Minimally invasive techniques namely vertebroplasty and kyphoplasty are now used
frequently to treat chronic and/or severe pain related to vertebral compression
fractures in the three conditions outlined above. The following video links describes
the two techniques:
Vertebroplasty:
http://www.youtube.com/watch?v=2phEd8kZOOU
Kyphoplasty:
http://www.youtube.com/watch?v=vbdyMgDKhQ8&feature=related
The following video demonstrates the effect of the balloon within the vertebra when it
is inflated.
The effect of the balloon within the vertebra (this video is available to watch in
the unit on the VLE, http://vlebb.leeds.ac.uk).
What are the potential biomechanical advantages that are often
suggested for the use of kyphoplasty over and above those from
Question 2 vertebroplasty.
The answer to this question is available from the distance learning courseware
available in the VLE, http://vlebb.leeds.ac.uk/.
1.4.3 The Pedicles
The pedicles form the bony interface between the vertebral body and the posterior
elements such as the spinous and articular processes. The role of the pedicles is to
transmit forces between the vertebral bodies, which are loaded by adjacent vertebral
bodies, and the posterior elements, which are loaded by muscle and ligament
attachments. Recently, the development of pedicle screws systems has focused
attention on the pedicle in terms of both geometric properties and strength. This has
been further heightened by the widespread uptake of vertebroplasty procedures in
which cement is injected into the vertebral body by a transpedicular approach.
Measurements indicate a gradual increase in size from the cervical spine to the
lower lumbar region but with dip in the mid-thoracic region (Figure 1.15 and Figure
1.16).
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Figure 1.15 Pedicle width and height at selected vertebra in the spine
Figure 1.16 Pedicle inclination with respect to the sagittal and transverse planes
1.4.4 The Neural Arch and Processes
The neural arch is the bony semicircular structure that goes beyond the pedicles on
to which are attached the spinous, transverse and articular (facet) processes (Figure
1.17). The spinous and transverse processes provide additional mechanical
advantage to the muscle and ligament attachments whilst on the articular processes
resides one side of the facet joint. The superior and inferior articular processes of
adjacent vertebrae form the zygapophysial (facet) joints. The movements of the
spine are partially dictated by the shape, position and orientation of the processes
that form these joints, all of which exhibit inter- and intra-regional variation. The
spinous processes of adjacent vertebrae can come into contact in full extension or if
there is significant disc narrowing. In this case these processes would share some
compressive load with the vertebral body and facet joints.
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a)
b)
Figure 1.17 Location of the spinous, transverse and facet processes in the lumbar
spine. The thoracic and lower cervical vertebrae have a similar arrangement of
processes.
1.4.5 The Cervical Vertebrae
The cervical vertebrae are the smallest and lightest of all the vertebrae since they
are normally required to carry only the weight from the head and neck above the
index level and, more importantly, muscular loads that stabilise the spine and
generate movement in this region. The distinguishing feature of the cervical
vertebrae is the foramen transversarium which allow passage of the vertebral artery
and vein (Figure 1.19).
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Figure 1.18 Superior view of the seven cervical vertebrae
Copyright Primal Pictures
Figure 1.19 A schematic representation of the C3 to C6 vertebrae
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Note the bifid spinous processes that are characteristic of these vertebrae - the
seventh cervical vertebra has just a single node at the tip if the spinal process more
in keeping with the neighbouring thoracic spine.
The cervical vertebrae are best divided into several groups for the description of form
and function.
The atlas (C1)
Except for its circular formation, the atlas is barely distinguishable as a vertebra.
Anterior and posterior arches are joined by lateral mass with superior and inferior
facets, and lateral transverse processes.
Figure 1.20 The atlas
The lack of a vertebral body common to the other vertebra requires alternative
medical devices when dealing with pathologies or trauma in this part of the spine.
The loads transferred from the head to the lower spine are mainly carried through
the two 'lateral masses' that are located between the foramen transversarium and
the canal.
The axis (C2)
This vertebrae is predominantly cervical in nature, however, the superior surface
differs notably due to the odontoid process. The anterior and superior surfaces also
feature facets for articulation with the atlas.
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Figure 1.21 The axis
The axis (C2) vertebra has the unique 'odontoid process' (dens). It is this dens which
articulates with the anterior tubercle of the atlas anteriorly and the cartilage layer on
transverse ligament posteriorly. Further articulations between C1 and C2 occur
between the superior facets of C2 and the lower sides of the lateral masses.
Third to 6th vertebrae
The vertebral body is kidney shaped in the transverse plane (Figure 1.19). Articular
facets are formed by slight projections on the superior surface and corresponding
bevels on the inferior surface, lateral to the spinal canal. The intervertebral discs do
not extend fully to the lateral edge of the vertebral body in the cervical region. At this
lateral edge, and anterior to the intervertebral foramen, the small synovial
uncovertebral joints provide additional control and stability in the neck.
The vertebral canal in this region is larger than the other regions, and triangular in
cross-section, since the pedicles extend in a lateral-posterior direction. The spinous
process is “bifid” providing two nodules for muscle/ligament attachment. The short
transverse processes are located anterior to the spinal canal, in contrast with that of
the lower vertebrae. There are articular processes on the inferior and superior
surface of each vertebra. The superior facet is oriented up- and backwards, with the
adjoining inferior facet facing down- and forwards. There is a general trend for a
more vertical orientation within the lower cervical spine.
7th vertebra
This vertebra is distinguished by a long single-lobe spinous process. It is also
considerably larger than the other cervical vertebrae and closer in shape to those in
the thoracic region in terms of pedicle and inferior facet orientation and size of spinal
canal. It does, however, feature small foramen transversarii for the passage of the
vertebral vein.
1.4.6 The Thoracic Vertebrae
The thoracic vertebrae are unique due to their interaction with the ribs via the
costovertebral and costotransverse joints (Figure 1.22). The ribs are important
biomechanically for two reasons; firstly the additional ligamentous structures at the
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costovertebral joints tends to stiffness the spine in this region and secondly, the
ribcage increases the moment of inertia within this zone.
Figure 1.22 Rib attachments in the thoracic region of the spine
Figure 1.23 shows the superior of the twelve vertebrae located within the thoracic
region of the spine. Note the change from a relatively small cervical like vertebrae at
T1 to a substantially larger lumbar like vertebrae at T12.
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Figure 1.23 Superior view of the twelve lumbar vertebrae
Copyright Primal Pictures
The thoracic vertebral bodies are generally described as heart-shaped in the
transverse plane. The vertebral canal formed is smaller than the upper and lower
regions, and more circular in cross-section. The pedicles face in a latero-posterior
direction. The long spinous processes face almost directly downwards, with those of
the upper and lower vertebrae being shorter. The transverse processes are long and
thick, projecting in a latero-posterior and slightly superior direction. They have a
bearing at the end facing anterolaterally for articulation with the tubercle of the
corresponding rib. These bearings are not present on the T11 and T12 vertebra,
hence the ribs are “floating” at this level. The superior and inferior articular processes
are almost vertical, originating medial to the base of the transverse process. The
joints formed between the articular processes sit on the periphery of a circle with it’s
centre within the vertebral body, thus the structure of the thoracic region is
particularly amenable to rotational motion.
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a)
b)
Figure 1.24 Typical thoracic vertebra. In b) the photograph of a mid-thoracic
vertebra note the relatively high asymmetry in the bony structures emphasised by
the transverse processes.
1.4.7 The Lumbar Vertebrae
The lumbar vertebrae (Figure 1.25) carry the largest loads of all the spinal regions,
and hence have a lowest height-to-width ratio. The vertebral body is relatively large
and kidney shaped (Figure 1.26). The endplates are parallel except in for the L5
suggesting that for the most part spinal curvature, in this case the lordosis, is derived
from the discs. The size of the vertebral canal is between that of the two upper
regions – it is to be remembered that the spinal cord ends at approximately L1 with
the canal being occupied by the corda equina at the levels below.
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Figure 1.25 Superior view of the five lumbar vertebrae
Copyright Primal Pictures
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a) Transverse plane
b) Sagittal plane
c) Transverse view
Figure 1.26 Typical lumbar vertebra. In a) and b) the facets show the hooked
morphology that is observed in the lower lumbar region, which resists both forward
slip of the upper vertebra with respect to the lower one as well as axial rotation. In c)
the transverse view is of a upper lumbar vertebrae which does not have the hook
shaped facets and, as such, resists on the axial rotation
The pedicles are short and thick, and face posteriorly. The spinous processes project
horizontally backwards and are wider at the top than the base. The laminae of
adjacent vertebra are widely spaced to accommodate the ligamenta flava. The
articular processes project from the junction of the pedicle and lamina. The superior
facets are concave as they cut the transverse plane and face posteromedially. The
inferior processes are more narrowly spaced than the superior, and their facets face
anterolaterally to receive the superior facets of the vertebra below. This arrangement
prevents significant rotational motion as well being resistant to anterior slippage. The
transverse processes project laterally and only slightly to the posterior. In the L4 and
L5 they are directed upwards, and those of the 5th may be fused with the lateral
sacrum.
1.4.8 The Sacral Vertebrae
The five sacral vertebrae and their associated transverse processes are fused to
form the sacrum. It is triangular in shape, tapering from a broad upper section to a
blunt inferior tip. In line with position of fusion between each vertebral pair are the
pelvic and dorsal sacral foramina for the passage of the first four ventral and dorsal
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rami, respectively (Figure 1.27). The fused transverse processes of the lateral
sacrum form a bridge with the ilium of the pelvis via the ligamentous sacro-iliac joint.
The posterior sacrum contains the sacral foramen and sacral canal (vertebral canal)
for the passage of the corda equine to the sacral hiatus at the caudal end. On the
dorsal margins, the sacrum is convex and irregularly contoured, featuring
components equivalent to that of the posterior lumbar vertebrae, such as the spinous
processes (also called median crests) for the attachment of ligaments and muscles.
a) Anterior view
b) Posterior view
Figure 1.27 Anterior (a) and posterior (b) views of the sacrum
The superior vertebra, S1, receives the complete upper body load via the superior
articular facets and L5-S1 intervertebral disc, and transmits it to the lower limbs via
the pelvic girdle. In the erect position, the sacrum is tilted forwards at approximately
50° to the horizontal, which would result in anterior and downwards motion of the
lumbar vertebrae across the superior surface if it were not for three essential
features:



The “wedging” of the sacrum into the space between the ilia, and action of the
associated sacro-iliac ligaments.
The L5-S1 intervertebral disc has a greater anterior height, resulting in a
posterior-anterior angle of approximately 16°, thus reducing the angle of
inclination of the L5 vertebral body relative to S1.
A stable hook-type interaction at the L5-S1 facet joint is facilitated by orientation
of the superior sacral articular processes at between 45 to 90° to the horizontal.
The coccyx
The coccyx consists of three or four vertebrae-type elements, which curve
kyphotically as they descend. The lower elements are fused in adult life, with only
the first articulating against the sacrum via the coccygeal cornua on its superior
surface. The dorsal surface of the coccyx is characterised by a row of rudimentary
articular processes. The coccyx, together with the sacrum and hip bones, form the
pelvis.
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1.5 The Intervertebral Disc
The intervertebral discs connects the surfaces of the inferior and superior endplates
of adjacent vertebral bodies. Thus each adjacent pre-sacral vertebra bar the C0-C1C2 joint is part of a tri-joint complex (Figure 1.28). The primary roles of the discs are
to transmit load, allow movement and provide stability.
Figure 1.28 The tri-joint nature of the articulation between adjacent pre-sacral
vertebra
The intervertebral discs both facilitate and restrict relative bending, rotation and
translation between adjacent vertebrae and thereby resist compressive, shear,
torsional and tensile stresses. The summation of small movements allowed by
individual discs results in the mobility of the spinal column as a whole. The
intervertebral discs constitute some 20-33% of spinal column height. In the cervical
and lower lumbar regions, the intervertebral discs have greater anterior height, thus
contributing to the lordotic curve observed in these regions. Lumbar intervertebral
discs are approximately 10 mm in height, therefore increasing the overall height of
the region by some 5 cm.
The intervertebral disc can be separated into three components by both form and
function (Figure 1.29):
 Annulus fibrosis – this outer ring provides resistance to relative rotation and
translation of adjacent vertebral bodies
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

Nucleus pulposus – this inner gel-like material ensures even distribution of
compressive stress over the vertebral bodies
Endplates – these cartilaginous plates sandwich the other two components.
It should be noted that while these regional distinctions are made for descriptive
purposes, the actual in vivo boundary between each component is not clear.
Figure 1.29 Schematic representation of the intervertebral disc components
The disc forms the anterior boundary of the intervertebral foramen which allows the
spinal nerves to exit from the cord at each vertebral level. It is also part of the
anterior wall of the spinal canal. The physiological loads imparted on the disc are
relatively high being equivalent to those transferred through the vertebral bodies.
1.5.1 Nucleus Pulposus
The nucleus pulposus is a circular gel-like mass encircled by the annulus fibrosis,
with its centre slightly offset posteriorly. It consists of large protein and sugar
molecules called proteoglycans in a network of fine fibres. The proteoglycans have a
high affinity for water, which makes up some 70-80% of the nucleus, making it
essentially incompressible and facilitating resistance to axial compressive loads. The
water content of the nucleus varies with load. During upright activities in which the
disc is compressively loaded, water seeps from the disc and disc height is reduced.
With a reduction in load, particularly in the supine position, water is re-adsorbed and
disc height is restored.
1.5.2 Annulus Fibrosus and Endplate
The annulus fibrosus is a peripheral ring of fibro-cartilaginous material. It is made up
of 10-20 tightly packed but weakly bound concentric collagen lamellae. Their
orientation provides resistance to axial compressive loading, via the nucleus, while
the elastic properties of the collagen allow for bending between vertebral bodies.
The collagen fibres within each lamella are generally parallel and oriented at
approximately 60° to 70° with respect to the vertical, with successive lamellae
alternating between forward and backward sloping fibres (Figure 1.30).
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Figure 1.30 Schematic representation of disc fibre orientation
Note that different investigators may observe slightly different fibre orientations
The advantage of this alternation in the fibre arrangement is twofold:


Prevents formation of a preferential cleavage plane, which if interrupted may
allow passage of the nucleus material.
Facilitates multi-directional resistance to tensile forces.
The outer fibres are primarily type I collagen to resist the greater tensile forces, while
the inner fibres are predominantly type II collagen due to their shielded state.
Reflecting this distribution, the outer fibres of the annulus fibrosus are connected to
bone via the ring apophysis of each vertebra and hence resist bending and torsion
between vertebral pairs. The inner fibres insert into the cartilage of the vertebral
endplates. Axial compressive stiffness is higher in the anterior annulus than the
posterior, with a decreasing trend with age and degeneration.
End Plate
The end plates are sometimes considered part of the vertebral body due to their
development from the epiphysial plates, however some texts delineate two separate
entities: the cartilaginous disc endplate and the osseus vertebral body endplate. The
cartilaginous plates bind the disc to the inferior and superior vertebral bodies, leaving
only a narrow periphery of uncovered bone called the ring apophysis. Disorders of
the endplates invariably involve both the cartilaginous and osseus bodies.
Innervation, blood supply and nutrition
The disc has considerable innervation of the outer third of the annulus layers. There
is minimal innervation of the middle third and a complete lack of nerve fibres in the
inner third and the nucleus. Most nerve endings in the disc are free, and are thought
to be partially nociceptive in function (pain sensors). Receptor nerve endings in the
superficial annulus layers are thought to be proprioceptive.
The intervertebral disc receives little direct blood supply, with only extremely small
vessels on the external surface of the annulus. Flux of nutrients and waste is instead
facilitated by diffusion to and from the vessels on the periphery of the nucleus and
within the vertebral bodies via the endplates. The bulk flow of water during loading
and unloading cycles also aids this transfer.
1.5.3 Biomechanics of the Disc
The biomechanics of the intervertebral disc are highly dependent on their state of
degeneration. Disc degeneration is generally observed in the second and third
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decades for men and women, respectively. The disc exhibits viscoelasticity,
hysteresis and creep. When in compression a single disc will creep by as much as
0.5 millimetres. Similarly, the annulus fibrosis exhibits greater failure stresses at
higher strain rates. For example, at a strain rate of 0.01%/sec in the horizontal
direction, failure of the outer annulus occurs at 1-3 MPa, while at faster rates this
rises to 3.5 MPa horizontally.
The intervertebral disc is often said to act as a shock-absorber for repetitive low
impacts such as those experienced during walking and running. However, they are
too stiff to be effective in this mode, and in addition, another primary requirement for
absorption of shock loading is the dissipation of heat, which is not optimal in the disc.
This shock-absorbing mechanism is falsely attributed to the ability of the nucleus to
expand radially under compressive loading. In fact the surrounding muscle are
tissues rich in vascular structures and it is these entities that are the shock
absorbers. An excellent paper by Smeathers et al. provides an excellent overview of
this issue.
Measurement of spinal loads have been carried out within the disc using a pressure
transducer in a number of studies. These studies can be used to give an indication of
the relative loads during different activities (Figure 1.31).
Figure 1.31 Relationship between activity and the pressures recorded within the
disc
In most, if not, all activities the loads are greater than those recorded in upright
standing and are relatively large even in a seated position. Only when the spine is in
a horizontal position, in which the flexion moment generated by the upper body
weight is effectively reduced to zero, does the intradiscal pressure reduce
significantly.
The following video provides further information on the relationship between posture
and intradiscal pressure.
http://www.youtube.com/watch?v=_T-eQJnZRRk
The annulus fibrosis is the primary load bearing structure in the disc. The hydrostatic
pressure within the nucleus pulposus produces a tensile hoop stress in the annulus.
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Reciprocally, swelling of the nucleus is resisted by the annulus and the longitudinal
ligaments such that a pressure of around 0.05 MPa is present in the nucleus of an
unloaded cadaveric disc. Intervertebral joint stiffness is proportional to increased
pressure in the nucleus, thus providing a pre-stress in the annulus. During flexion of
the spine, the disc pivots about a centre of rotation near the anterior nucleus
pulposus, resulting in a reduction of annulus height at the anterior edge and
increased height at the posterior margins. The changes in height are accompanied
by thickening and thinning of the wall, respectively. Torsional (axial rotation) loading
engages only those fibres of the annulus oriented in the direction of the rotation.
1.5.4 Disorders of the Disc and Affects of Aging
Disorders of the disc that affect proteoglycan concentration or integrity within the
nucleus inhibit the flux of water, causing a reduction in disc height. A herniated,
prolapsed or “slipped” disc occurs when the nucleus protrudes through a defect in
the annulus. Posterior herniation is more likely than an anterior one due to the
thinner annulus in this region, and this may impinge on the spinal cord. Posterolateral herniations in the lower lumbar and cervical regions are the most common.
The spinal nerve roots branch from the spinal cord at each level, passing downwards
and laterally just below the pedicle, in close proximity to the upper posterior portion
of the intervertebral disc. Thus disc prolapse can also result in pressure on nerve
roots, and associated radicular pain; for example, impingement of the sciatic nerve in
the lower lumbar vertebrae causes pain in the leg and foot. Additionally discs show
only a limited ability to heal following injury or disease, probably as a result of a lack
of remodelling of the collagen fibres within the annulus.
The biochemical and functional effects of aging can be significant in the
intervertebral disc, resulting in a degradation of mechanical function (Figure 1.32).
Biochemical Change
Reduced water content of the
nucleus due to reduced
proteoglycan concentration
Reduced elasticity of collagen fibres
in annulus and endplate
Ossification of endplate
Increased collagen content and
crosslinking, but also increased
gross defects in annulus lamellar
structure
Effect on Function and Biomechanics
Nucleus becomes dry and stiff, with
reduced volume, therefore more load taken
by posterior annulus
Increased brittleness, greater susceptibility
to failure
Possible reduced nutrient/waste flux
Overall weakening of annulus
Figure 1.32 Biochemical and the resulting changes in the biomechanics that occur
in the disc
Disc degeneration can be demonstrated through a myriad of techniques. These
techniques include the use of MRI to investigate the dehydration of the disc whilst
the loss of disc space can be observed on lateral radiographs. MRI can be used to
investigate the degree of canal stenosis if a herniated disc is suspected. The
following video provides a short tutorial on disc degeneration.
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http://www.youtube.com/watch?v=nVwrmgO-ikI
In the laboratory, a number of biomechanical investigations have taken place that
have led to a greater understanding of the effects of degeneration. It has been
shown that as the disc degenerates there is a loss in pressure within the nucleus
(Figure 1.33). This reduction in pressure leads to a compressive load being applied
to the annulus in which the inner layers now have a compressive component rather
than a tensile one. The tensile forces in the outer layer remain but are lower in size.
This has important consequences for the way the vertebra fractures under a
relatively low loading rate regime. In the normal disc the failure would be most likely
be through endplate fracture whilst in the degenerate disc with the load transferred
peripherally then vertebral body failure has a significantly greater likelihood (Figure
1.34).
a) Normal disc
b) Degenerated disc
Figure 1.33 The transfer of load across a) a normal functional disc and b) a
degenerated disc. In a) there is a substantial pressure within the nucleus when
loaded, which is equal to or greater than that generated in the annulus. In b) there is
a reduction in pressure within the nucleus and more of the load appears to be carried
by the annulus.
Figure 1.34 The mechanism of the failure in a normal (a) and degenerate disc (b).
In the normal case the endplate deflection is considerable whilst in the degenerated
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case the loss of water in the nucleus reduces the load carrying capacity in this
component.
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1.6 The Spinal Ligaments
The spinal ligaments are passive tissue elements of the spine that play an important
role in both motion and stability particularly towards the ends of the range of motion
(Figure 1.35). They protect the spinal cord and to some extent the vertebrae, by
restricting motion to “safe” limits or particular ranges of motion. This is certainly true
when high loads are applied at high speed, for example during trauma. The
ligaments are also highly innervated and may be involved with proprioceptive
feedback. Similar to all other skeletal ligaments, spinal ligaments are cartilaginous
fibres connecting two bony elements. They are most effective when the loads they
are resisting are in the direction of collagen fibre alignment. As with all fibres of this
type they resist motion in tension not compression.
1) Allowing motion of the vertebrae while ensuring efficient muscular activation.
2) Protecting the spinal cord by acting as a direct physical barrier, by limiting the
range of vertebral movement in multiple directions, and by absorbing energy in the
event of high speed application of load.
3) Providing stability to the spine.
4) Providing proprioceptive information to the peripheral and central nervous system.
Figure 1.35 Functions of the spinal ligaments
Altogether there are seven sets of ligaments associated with the spine in the region
C2 to L5 (Figure 1.36). Whilst there is this common collection these structures vary
between different segments in terms of structural properties as well as there size,
fixation points and orientation (Figure 1.37). The ligaments may also vary in
geometry within a spinal level. The ligaments above C2 differ significantly and are
not explored here. For details on these structures see Palastanga, Field and Soames
Anatomy and Human Movement (4th Edition, Butterworth Heinemann 2002).
Figure 1.36 The generalised anatomical position of the seven ligaments associated
with the spine between C2 and L5
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Figure 1.37 Variations in the ligamentous structures between different pre-sacral
segments of spine.
Note the inclusion of ligamentum nuchae in the cervical spine which replaces
principally, the supraspinous ligament for which some people believe it is continuous.
The transverse ligaments are not shown for purposes of clarity.
Copyright Primal Pictures
Most of the intervertebral ligaments are situated behind (posterior to) the centre of
rotation of the FSU in the sagittal plane. This means their main role is to prevent
excessive flexion, but in doing so they also compress the IVD thereby increasing
internal disc pressure. The exception here is the anterior longitudinal ligament,
which, as its name suggests, is forward of the typical axis of rotation, and plays a
major role in resisting excessive extension. The exact function of a specific ligament
is determined by those factors given in Figure 1.38.



Insertion points (including orientation).
Material and geometric properties of the tissue.
Type of motion being undertaken.
Figure 1.38 The factors on which the exact function of a given ligament depends
1.6.1 Mechanical Behaviour of the Ligaments
Similar to all collagen-based tissues, ligamentous tissue exhibits non-linear
relationship between stress and strain (or load and displacement) (Figure 1.39). The
ligaments differ not in the shape of the curve but in the exact stress-strain (or loaddeformation) values producing that shape.
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Figure 1.39 Generic ligamentous stress-strain curve. Within the neutral zone the
ligaments provide little resistance to the motion around the anatomical (upright)
position of the spine. As the excursion proceeds towards the ends of the range of
motion then the resistance increases.
The stress-strain curve of the ligament progresses through three zones: neutral,
elastic and traumatic (or plastic). The first two regions are deemed within the
physiological deformation range while the third occurs in traumatic loading and
results in near-permanent deformation of the ligament usually through microscopic
tears in the fibres. Catastrophic failure occurs when the failure load is exceeded and
the ligament snaps. The ligaments maybe under a residual tension whilst the spine is
in the neutral (or anatomical) position. This is certainly true of the ligamentous flavum
(LF) and the posterior longitudinal ligament (PLL), which are under approximately 8
% strain in this position. Only at the limits of extension does the ligamentum flavum
become slack (Figure 1.40)
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Figure 1.40 Strains predicted from a mathematical model of the lumbar spine
Question 3
What particular advantage is there of having the LF and PLL
under tension in the neutral position?
The answer to this question is available from the distance learning courseware
available in the VLE, http://vlebb.leeds.ac.uk/.
The degree of strain under the action of four motions is shown in Figure 1.40. As
expected for flexion there is a decreasing trend in strain from the supraspinous
ligament (the most posterior) to the anterior longitudinal ligament (the most anterior)
with the latter being effectively zero. Similarly for extension the only significant
contributions to resist this motion are from the anterior longitudinal ligament, which is
anterior of the instantaneous axis of rotation, and the capsular ligaments that
surround the facet joints. Further, note that in rotational motion one of the capsule
ligaments had a significant role to play whilst in lateral bend the PLL was shown to
have the greatest strain followed by the LF. However, in this latter case a large
proportion of the strain may be attributable to the pre-strain nominally observed in
these ligaments whilst in the neutral position.
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1.6.2 Anterior Longitudinal Ligament
The anterior longitudinal ligament (ALL) is a broad band of fibrous tissue,
approximately semi-circular in cross-section, which encircles the anterior vertebral
body (Figure 1.41). It runs from the sacrum to the occipital bone of the skull. It
attaches firmly to each vertebral body, but only superficially to the annulus of each
intervertebral disc. There are regional variations in the geometry of the ALL with the
lumbar region being widest (up to 25 mm in breadth). The strength of the ligament is
derived from its thickness and its construction from overlapping fibres of varying
length. Deep fibres span adjacent vertebrae, while the more superficial extend over
four to five vertebrae.
Figure 1.41 Anterior Longitudinal ligament
Note the fibre direction and the bulging of the ALL over the discs.
The ALL primarily resists distraction of the anterior column, which occurs through
axial separation of the vertebral bodies, or extension. The ligament may form a
protective barrier between the inferior vena cava and dorsal aorta, and any
osteophyte formation on the antero-lateral vertebral body. In the lumbar spine the
strength of the ALL is usually found to be greater than the PLL by virtue of its greater
cross sectional area with the failure stress being approximately the same (~11 MPa).
In the cervical spine observations on the failure load are roughly equal due to the
fact the cross sectional areas are similar between ALL and PLL.
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1.6.3 Posterior Longitudinal Ligament
Similar to the ALL, the posterior longitudinal ligament (PLL) is a long band of parallel
collagen fibres of varying length with the superficial fibres spanning several
vertebrae and the deeper ones only the adjacent VBs. It runs posterior to the
vertebral body within the spinal canal, between the occipital bone and the coccyx.
The width of the ligament is quite variable, being broad in the cervical region,
reducing in width over vertebral bodies and expanding over the discs of the thoracic
and lumbar regions (Figure 1.42 a & b). In contrast to the ALL, the PLL attaches
firmly to the posterior surface of the intervertebral discs, but separates at the midline
of each vertebral body for the passage of arteries and veins. The PLL is in tension
during flexion, however due to its close proximity to the centre of rotation (low lever
arm) it is less able to protect the spine from excessive flexion than some of the other
ligaments. The PLL has been suggested to act in the protection of the spinal cord
from herniated disc material, and due to its rich nerve ending population it may
detect abnormal deformations of the disc.
a) The PLL in the thoracic region of b) Schematic representation of the
the spine b) schematic representation PLL showing the attachments within
of the PLL showing the attachments the disc
within the disc
Figure 1.42 The PLL in the thoracic region of the spine
Note the variability in the width of the ligament as one motion segment is traversed.
1.6.4 Intertransverse and Capsular Ligaments
Intertransverse ligaments
As their name suggests, these small ligaments stretch between the transverse
processes of adjacent vertebra. They are only significantly present in the lumbar or
thoracic regions (sometimes said to take the place of the intertransverse muscles
which are present in the cervical region). These ligaments don’t appear to contribute
significantly to the biomechanics of the motion segment although they may stretch
Page 38
up to 20% during lateral bending, which indicates that they may function to resist this
mode of motion.
Capsular ligaments
The capsular ligaments are sometimes considered to be part of the facet joint
capsule. They are very short but strong ligaments that extend between the margins
of adjacent articular processes with fibres oriented perpendicular to the plane of the
joint. These ligaments provide resistance to flexion, lateral bending and
hyperextension. They are made up of two primary bundles of fibres, at the anterolateral and posterior margins, which due to their differing lengths become taut at the
same degree of flexion. Vertical distraction causes failure of the shorter, anterolateral, fibres first. Failure strength of these ligaments is of the order of 200 N being
slightly stronger in the cervical and lumbar regions.
1.6.5 Supraspinous and Interspinous Ligaments
The supraspinous ligament (SSL) is a single slender entity that extends between the
posterior tips of the spinous processes from the ligamentum nuchae to the sacrum.
This latter point is currently a matter of debate as it is sometimes observed to
terminate at L4. The SSL is also continuous with the interspinous ligaments. As with
other spinal ligaments the deeper fibres connect adjacent vertebrae (spinous
process tips) whilst superficial ones may traverse over several motion segments.
The interspinous ligaments stretch between the root and apex of two adjacent
spinous processes. Similar to the supraspinous they are larger in both dimensions in
the lumbar region. Given that the supra- and interspinous ligaments form a
continuous ligament sheet they operate in tandem to resist distraction of the spinous
processes and limit spinal flexion. The strength of the this ligament complex was
shown to reduce with age.
1.6.6 Ligamentum Flavum
The ligamenta flava are paired ligaments (left and right) that run from the lower inner
surface of the laminae to the upper outer laminae surface of the subjacent vertebrae.
The collagen fibres in the ligamentum flavum are unusually un-crimped and loosely
arranged. With a high proportion of elastin fibres, this ligament is highly elastic and
has a yellow hue. Extending during spinal flexion and resuming its shape when the
spine is upright, it provides and maintains a smooth and unbuckled surface for
contact with the spinal cord meninges. The failure loads of the LF rises from
approximately 130 N in the cervical spine to nearly 250 N in the lumbar spine which
equates to a failure stress of between 3 and 8 MPa.
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1.7 The Spinal Muscles
The primary role of the spinal muscles is to provide postural stability to the inherently
unstable spinal column. A ligamentous spine devoid of musculature is capable of
carrying only 20 N (2 kg) before bucking if it has been placed in a balanced upright
position. The muscles achieve this stability by isometric contraction which increases
the stiffness of the system by effectively “clamping” the vertebrae together. The
muscles also provide active control of movement, and provide shock absorbency by
dissipating the heat generated by repetitive motion and loads. The spinal muscular
system and its activation patterns are complex, and involves not only those muscles
with direct attachment to the vertebrae, but also the muscles surrounding the
abdominal walls. The muscle arrangement varies significantly between segments
(cervical, thoracic, lumbar and sacral) and even within segments so as to
accommodate the differing motions and postures that are achieved by an active
human.
The muscles contributing to lumbar spinal motion are commonly categorised by
location: post-vertebral (in general extensors) and pre-vertebral (flexors) (Figure
1.43) with only two of the group not contributing to either significantly. The postvertebral muscles are those situated behind the spinal column, and are further
divided by location into the deep, intermediate and superficial groups. The prevertebral muscles are the four abdominal muscles anterior to the spinal column
which although they may not be connected directly to the vertebrae contribute
considerably to spinal motion and stability. To this we must add the Psoas major
Muscle group
Flexion
Rectus Abdominis
External Oblique
Internal Oblique
Psoas Minor
Psoas Major
Multifidus
Quadratus
Lumborum
Semispinalis
Erector Spinae
Interspinales
Rotatores
Intertraversarii
X
X
X
X
X
Extension
Rotation
Lateral
Bend
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Figure 1.43 Muscles grouped into primary actions in the lumbar and thoracic spine
Page 40
1.7.1 Post-vertebral – Erector Spinae
The post-vertebral muscles are arranged as a series of layers which are
distinguished by the direction and length of their fibres. The length of the fibres
decreases with depth so that gross motor control is obtained by the outer muscles
while fine movements are achieved by the inner ones.
Erector Spinae
The superficial erector spinae muscles all originate as a single unit from the posterior
sacrum, iliac crest, and spinous processes of lumbar spine and the thoracolmbar
junction (Figure 1.44). As it travels upward, it divides into three groups:
1) The iliocostalis group is the most lateral, and is further divided into a group for
each spinal region.
2) The longissimus group is situated medial to the iliocostalis, and further divides into
thoracic, cervical and head sections.
3) The spinalis is most medial, and is usually the smallest of the group. It consists
primarily of the spinalis thoracis, and sometimes spinalis cervicis.
Given the muscles mass of the erector spinae (and the significant mechanical
advantage of the lever arm) this group is the most important in terms of spinal
extension and is the one that is incorporated into many of the more simplistic spinal
models used in task analysis for instance. In addition, this muscle group plays a
significant role in counteracting the tendency to forward flex due to the moment
generated by the weight of the upper torso. Bilateral activation of the erecter spinae
muscles results in flexion/extension of the spine, while unilateral action causes
lateral flexion to the activated side.
Page 41
a)
b)
Figure 1.44 The erector spinae muscles of the spine. There are separate groups for
the lumbo-thoracic and thoraco-cervical spine. In simplified task analysis when safe
handling loads are being analysed then the erector spinae is considered to be just
one muscle and, indeed, it is often the only muscle to be considered.
1.7.2 Post-vertebral – Transversospinalis
The intermediate transversospinalis muscle group is deep to the erector spinae. It is
comprised of three smaller groups, the semispinalis, multifidis and rotatores (Figure
1.45a and b). The semispinalis muscles (Figure 1.45a) originate in transverse
processes, travelling upward and medially to insert into one or more spinous process
some three to six vertebra higher. The thoracis group run from T6-T12 to T1-T4 and
C6-C7. The cervicis group run from T1-T6 to C2-C7. The function of the semispinalis
thoracis and cervicis is to the spine. The capitis group runs from T1-T7 and C6-C7 to
the occipital bone of the skull and acts to extend the head.
The continuous radiating mass of the multifidus muscle (Figure 1.45a) is deep to the
semispinalis. Originating from the sacrum and transverse processes of L1 up to C6,
the groups run in an upward and medial direction to insert at the spinous processes
of L1-C2. Their size, and consequently their strength, reflects the contributions the
muscle makes in the regions of the spinal column, being thicker in the lumbar and
cervical regions than the thoracic. The path of each muscle is such that it pulls down
on the spinous process, therefore bilaterial activation leads to extension of the spine
or control of flexural movement. While unilateral action may contribute to lateral
flexion to the same side. The contribution to axial rotation is not agreed as the
muscle path is only very slightly transverse.
Page 42
The rotatores (Figure 1.45b), as suggested by their name, axially rotate the spine,
and are also associated with flexion/extension movements. The presence of this
muscle group is best defined in the thoracic spine. They have single origins from the
sacrum or a transverse process and travel upwards to the base of a spinous process
or lamina of the superior vertebra. The short rotatores pass between adjacent
vertebrae (and hence are sometimes classed as deep segmental muscles), while the
long rotatores pass to the second above.
a)
b)
Figure 1.45 The intermediate muscle groups posterior of the spine. These are the
multifidus, semispinalis and the rotatores.
1.7.3 Post vertebral - Deep Segmental Muscles
The deep (segmental) muscles (Figure 1.46) pass only between adjacent vertebrae,
and are made up of two groups. The interspinales insert and originate at the spinous
processes while the intertransversarii run between transverse processes in
anterior/posterior (cervical) or medial/lateral (lumbar) pairs. Both are generally only
present in the cervical and lumbar regions. Due to their size, these muscles are not
thought to contribute significantly to spinal movement, providing only for small
adjustments between vertebrae. However, it has been hypothesised that their
primary role is proprioceptive.
Page 43
Figure 1.46 The deep muscle groups posterior of the spine. These are the
interspinales and the intertransversarii.
1.7.4 Pre-vertebral
The pre-vertebral muscles include two oblique and a transverse that surround the
abdominal cavity in layers, and a fourth anterior abdominal muscle (rectus
abdominous). In addition to supporting the abdominal wall, they aid spinal movement
via their attachment at the sternum and ribs. The external and internal oblique
muscles, and the transverse abdominal muscles deep to the obliques (Figure 1.47),
have similar origin and insertion points. Origins are at the six lower ribs, anterior iliac
crest and transverse processes of lumbar vertebrae, while insertions are at the iliac
crest, pubis, and the centre of the abdominal fascia. Acting bilaterally, the obliques
aid flexion/extension of the spine. Unilateral action, dominated by the internal,
causes axial rotation to the same side.
The rectus abdominus pair is located either side of the anterior abdomen midline. It
originates at the pubic crest and symphysis, and inserts at the xiphoid process and
5-7 ribs. The rectus abdominis aids in spine flexion/extension, and in anterior rotation
of the pelvis relative to the spine. The psoas major is a particularly strong muscle
which is attached to the lesser trochanter of the femur and the lateral sides of the
vertebral bodies (typically T12 to L5). With the femur fixed the psoas major flexes the
spine on contraction.
Page 44
Figure 1.47 Transverse section across the spine within the trunk. Here the relative
positions of the pre-vertebral muscles as well as the erector spinae can be observed.
Figure 1.48 The rectus abdominis muscles tends to forward flex the spine with the
leg stationary
1.7.5 Key Biomechanics Considerations of Spinal Muscles
Similar to all other skeletal muscles, the spinal ones provide a linear force over joints
to produce or restrain movement via bending moments and torques. They also
Page 45
increase the stiffness of the spine by providing a constant compressive force. Where
a muscle passes over a vertebral region, with origin and insertion at vertebra or ribs
either side, it can still exert a force on the unattached region. An example is the
action of the longissimus thoracis, acting on the lumbar spine with origins at the
sacrum and insertions at the thoracic transverse processes and ribs. This type of
action can bring challenges when formulating computation models of the spine on
where the action of muscles occurs and to what degree.
In addition, and more importantly, spinal muscles exhibit a large inherent
redundancy, such that there are many different combinations of muscles that may be
recruited to carry out the same task. How the recruitment occurs will depend on
many factors including the presence or absence of an underlying pathology, training
and genetics. The difficulty in modelling the spine arises from the fact we do not
know what the recruitment strategy is – that is, what is the spine trying to, perhaps,
maximise or indeed minimise? This strategy may itself depend on the activity or the
part of the movement being undertaken. Initial models were linear in their recruitment
in which muscles with the greatest mechanical advantage were recruited first in a
sequential manner. When a given muscle reached its maximum force (usually
defined by the cross sectional area) then the muscle with next largest lever arm
would be brought into play. However, observations indicate that muscles recruitment
does not follow this strategy and simultaneous activity is often observed. More
complex models have been developed which allow this sort of muscle activation but
we are still someway from getting the exact recruitment matched with in-vivo data.
Indeed many spinal models use electromyographic (EMG) analysis to provide
additional information on muscle activation.
Relatively early recordings of the superficial erector spinae muscles showed that the
activity of this muscle group reduced to zero when the spine became fully flexed.
This was explained by suggesting that the forward bending moment at this position
was balanced by the passive elements only, which comprise those within the
muscles and the ligaments posterior of the axis of rotation. However, more recent
evidence has shown that the deeper erector spinae muscles and the quadratus
lumborum become highly activated when full flexion is reached.
Figure 1.49 Muscle activation in forward flexion
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1.8 The Synovial Joints of the Spine
The principle feature of spinal motion is often considered to be the disc with a focus
particularly on the consequences of disc degeneration. However, there are a number
of equally important synovial joints in which underlying pathologies or trauma can
have an important effect on the overall functioning of the spine. The three set
synovial joints that we need to consider are:
1) the sacro-iliac joint.
2) the facet (Zygapophysial) joints
3) the synovial articulations associated with the Atlanto-occipital and Atlanto-axial
joints (C0-C1-C2 complex).
1.8.1 Sacro-iliac Joint
The sacro-iliac joint is formed between the auricular surface of the ilium and the ala
surface of the sacrum. It is a true synovial joint although some researchers consider
it, at least in part, to be a fibrous joint due to the extensive ligamentous structures,
which are particularly strong posteriorly. The articular surfaces themselves are
‘hockey-stick shaped’ with the surface waviness forming an interlocking mechanism
that helps resist motion of the joint when loaded (Figure 1.50). In very old the joint
maybe partially fused or show fibrous adhesions that span between the articular
surfaces.
Figure 1.50 The articular surfaces located on the surfaces of the ilium and sacrum
There are two main sets of ligaments that provide significant stability to the joint; the
posterior sacro-iliac and anterior sacro-iliac ligaments (Figure 1.51). The posterior
ligaments behind the joint are much thicker than those anteriorly giving them greater
strength. They are often categorised into two bands that comprise the interosseous
sacro-iliac ligament and the long/short posterior sacro-iliac ligaments. The
interosseous sacro-iliac ligament is deep within the gap that lies between the ilium
and the sacrum and are relatively short but extremely strong structures. As well as
these two structures other ligaments provide additional stability to the sacro-iliac
joint. These are the sacro-spinous and the sacro-tuberous ligaments, which prevent
forward tilting of the sacrum, and the ilio-lumbar ligament.
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Figure 1.51 Transverse view of the sacro-iliac joint indicating the positions of the
posterior and anterior sacro-iliac ligament
There is very little movement in the sacro-iliac joint with a maximum downwards
movement of three millimetres together with a forward flexion of up to 5 when load
bearing occurs. The stability of the sacro-iliac joint has been described as being akin
to the keystone in an arch (Figure 1.52). As the joint is loaded tension is generated in
the sacro-illiac and accessory ligaments that keeps the femora and pelvis (pillars of
the arch) together and locking the sacrum in place with the aid of the joint’s irregular
surface.
Figure 1.52 The keystone mechanism that provides stability in the sacro-iliac joint.
Loading of this joint generated tension in the surrounding ligamentous structures that
prevents the pelvis from separating and sacrum from tilting in a forward manner.
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1.8.2 Zygapophysial (facet)
The zygapophysial (facet) joints are formed between the superior and inferior
articular processes of adjacent vertebrae. There are two facet joints per vertebrae
pair, and along with the intervertebral disc, these form the tri-joint complex of the
spine. The facets are small synovial joints, consisting of two cartilage covered
articulating surfaces surrounded by synovial fluid and enclosed in a joint capsule.
The exact orientation of the superior and inferior articular processes varies along the
length of the spine (Figure 1.53) and thus the motions of the facet joints and the
functional spinal unit (FSU) varies accordingly.
Figure 1.53 The orientation of the facet joints in the cervical, thoracic and lumbar
regions of the spine
Note the different orientations the facets and the implications this has on axial motion
in that region of the spine.
A brief description of the facets is provided for in the video below:
http://www.youtube.com/watch?v=9w6NiPc8Bp8
As well as transmitting load, the facet joints may also have a role in limiting the range
of motion. In this second function, they effectively protect the intervertebral disc by
preventing excessive torsional and translational movement (dislocation) between
vertebrae. This role is particularly prominent in the lumbar spine in which a locking
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mechanism is present which prevents axial rotation and anterior translation of the
upper vertebrae relative to the lower during flexion of the spine. The variation in
precise orientation of the facet joints along the length of the spine is also responsible
for the variation in compressive load sharing. For example, in the lumbar region, the
articular surfaces progress from vertical in the upper to oblique at the lumbo-sacral
joint. This leads to an approximately 10% share of compressive load in the former,
compared with 20% in the latter. Reduced intervertebral disc height can result in
facet joint degeneration and increased loads on the joint. This feature, together with
the resurgence of total disc arthroplasty has generated considerable interest in facet
joint replacement particularly if a considerable portion of the pain arises from the
degeneration of these structures.
1.8.3 Atlanto-occipital and Atlanto-axial Joints (C0-C1-C2)
These two joints are the most complicated of the spinal column both anatomically
and kinematically, and exhibit completely unique biomechanics. They are often
considered as a single joint complex. Both joints contribute approximately equally to
total flexion/extension movements in the sagittal plane. There is a small amount of
C0-C1 axial rotation due to the cuplike geometry in the frontal and sagittal planes,
although there is little ligamentous limitation. The majority of axial rotation occurs at
the C1-C2 joint due to the convex geometry of both articulating surfaces and the lack
of ligamentous fixation. Translational motion at the occipital-atlanto-axial joint
complex is small.
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