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Chapter 4 Spinal Musculature: Anatomy and Function 73
3
Contra
040
-
lateral
2
1
0
−1
−2
Moment arm (cm)
−3
Ipsi-
lateral
−4
−40 −30 −20 −10
Rotation angle (degrees)
Sternocleidomastoid
Splenius capitis
Rectus capitis posterior major
FIG. 4.11 Axial rotation moment arms for the upper cervical region.
(Modied from Vasavada A, Li S, Delp S. Inuence of muscle morphometry
and moment arms on the moment-generating capacity of human neck
muscles. Spine. 1998;23:412–421.)
010203
ContralateralIpsilateral
increases in le rotated postures. However, when the head is
rotated to the right, the moment arm decreases in magnitude
and eventually changes to a le rotation moment arm. ese
results indicate that the rectus capitis posterior major has an
axial rotation moment arm appropriate to restore the head
to neutral posture from the most rotated head positions. e
moment arms of other muscles, such as the semispinalis capitis
and longissimus capitis, show the same pattern, although their
moment arms are smaller. e implication of these ndings
is that the moment arm provides a “self-stabilizing” function
to assist the central nervous system in maintaining neutrally
rotated (i.e., eyes forward) head posture. is function is particularly relevant in the upper cervical region, because most
axial rotation occurs between C1 and C2.
In the lumbar spine, posture also changes the mechanical
function of erector spine muscles. McGill and associates23
measured the ber angles of longissimus thoracic and iliocostalis lumborum with the lumbar spine in neutral and fully
exed using high-resolution ultrasound. ey found that
exion changes the line of action of these muscles, decreasing
their capacity to resist anterior shear forces. is nding is
important because anterior shear loads are related to the risk
of back injury.
24
Implications of Spinal Muscle Anatomy and Architecture for Injury and Pain
ere are at least three ways in which spinal muscles may be
implicated in mechanisms of injury and pain. First, the muscle
itself may be injured from eccentric contraction, as described
in Chapter 3. is may occur during an imposed movement
(particularly one in which the kinematics are abnormal).
Second, muscle forces may alter the load distribution within
anatomic structures that have been clinically linked to pain.
ird, muscle activity can alter spinal stiness and kinematics,
which would indirectly aect so-tissue loads and strains. e
relationship between muscles and injury can be elucidated by
biomechanical models, the validity of which depends on
accurate modeling of anatomy and architecture.
Muscle Injury Resulting From Eccentric Contraction
As noted in Chapter 3, rapid lengthening of muscle is an
important mechanism of muscle injury. An example of potential muscle injury due to imposed lengthening occurs during
whiplash. During the retraction phase of whiplash injury,
when the head translates rearward with respect to the torso,
the sternocleidomastoid muscle can experience lengthening
strains of 5% to 10% while it is active.
25,26
During the rebound
phase of whiplash injury, when the head translates forward
with respect to the torso, the splenius capitis and semispinalis
capitis muscles can experience lengthening strains of 10% to
20%. ese predictions of muscle strains, based on a biomechanical model that incorporates muscle architecture,21 are
above thresholds for strain that causes injury to activelengthening muscle.
27–29
Muscles Altering Load Distribution in Other Anatomic Structures
Because muscles are oriented primarily vertically, their activation produces axial compression of the spine. e compressive
loads on the discs and facet joints are a function of muscle
force, moment arm, and activation. When the detailed anatomy
of the lumbar erector spinae was included in a biomechanical
model,30 the predicted disc compression and shear loads were
reduced compared with a lumped extensor “muscle equivalent” commonly used in many models. is study highlights
the importance of creating an accurate representation of
muscle anatomy in biomechanical models.
Compressive loads may severely alter tissue loads, particularly
if abnormal vertebral kinematics occur. For example, the synovial
fold of the facet joint may become impinged during the abnormal
kinematics that occur during whiplash.31 Muscles may also
contribute to injury by directly loading passive structures.
For instance, the cervical multidus has direct attachments to
facet capsular ligaments
8,32
; the combined loading from joint
motion and muscle forces may lead to subcatastrophic injuries
in facet capsular ligaments. ese observations are important
because the cervical facet joints and capsular ligaments have
been clinically isolated as a source of neck pain.
33
Muscle Eects on Spinal Stiness and Stability
It has long been recognized that muscles are necessary for
spinal stability. However, it is unclear which muscles contribute
SECTION
I
74 BASIC SCIENCE
ABC
most to spinal stability; this question has been addressed
in several theoretical and experimental studies. Crisco and
Panjabi34 examined the role of gross muscle anatomy (e.g., the
number of joints crossed by a muscle) in lateral stabilization of
the lumbar spine using a mathematical model. ey calculated
minimal muscle stiness necessary for spinal stability and
found that muscles spanning only one vertebral body required
the highest stiness (i.e., activation) for stability, whereas
those muscles that spanned the largest number of vertebrae
were most ecient (required the least activation). Ecient
stabilization (less muscle activation) is important because it
implies lower disc loads. Electromyographically driven modeling by Cholewicki and McGill35 suggested that large muscles
may provide the bulk of stiness to the spinal column, as
suggested by Crisco and Panjabi,34 but that the activity of short
intrinsic muscles was also necessary to maintain stability. In
fact, biomechanical models have shown that buckling (loss of
stability) can occur from a temporary reduction in activation
to one or more intersegmental muscles.35 Presumably, small
intrinsic muscles are better suited to stabilize displacements at
a single joint with a minimum increase in joint loads at other
levels. Similarly, Winters and associates used both computer
and physical models of the cervical spine to demonstrate
that activating only large, long muscles resulted in instability,
especially around the upright posture.
36,37
e authors also
concluded that activation of deep muscles was necessary
for spinal stability. ese types of analyses demonstrate
the importance of both gross anatomy and architecture of
spinal muscles on spinal stability. However, many important
questions remain, such as the eect of muscle fatigue on
spinal stability and the best muscle activation patterns for
stability in the prevention and rehabilitation of low back and
neck pain.
Muscle fatigue has been implicated in low back and neck
pain38; the mechanism may be related to altered loads in
other structures, decreased spinal stability, accumulation
of metabolites, or involvement of peripheral and central
mediation of pain. Patients with neck pain or cervical
radiculopathy demonstrate altered neck muscle endurance
and myoelectric evidence of fatigue.
39,40
In patients with
pain, as well as healthy subjects, fatigue can lead to dierences in neuromuscular control, including altered activation
patterns or exion–relaxation, which can increase the loads
in passive tissues.
39,41
ere is some evidence for ber type
transformations (from type 1 toward type 2) in patients with
back or neck pain,
42,43
but other studies have found that spine
musculoskeletal disorders are not related to a change in ber
44,45
type.
Fiber type studies in these muscles are extremely
dicult due to their complex architectural design and limited
ability to perform muscle biopsies.
Although the average architectural features of the major
muscles in the lumbar spine have been documented (see
earlier discussion and associated tables), there is an increasing need to generate patient-specic architectural data for
diagnostic purposes, surgical planning, and musculoskeletal
modeling. Recent advances in MRI and image processing
allow these muscles to be rapidly visualized and quantied
in three dimensions in an unprecedented fashion previously
impossible with ultrasound, MRI, or CT scanning (Fig. 4.12).
Additionally, these tools allow muscle tissue to be fractionated
into contractile and fat compartments, which is extremely relevant clinically, as “muscle quality” appears to be an important
feature in the lumbar spine and other joint systems subjected
to chronic disease. As can be seen in Fig. 4.13, the fraction of
muscle contained within “normal” muscle boundaries can be
substantially lower than anticipated. Further, fatty inltration
is found in neck muscles in cases of whiplash injury with
poor functional recovery46 and the amount decreases with
exercise47; disc injury is associated with increased adipose
and connective tissue in lumbar multidus, and a decrease in
adipose tissue on biopsy was associated with positive outcome
aer surgery.48 Last, in Chapter 3 we discussed the need for
(and diculty of) quantifying muscle ber or fascicle lengths
in individual patients. Magnetic resonance diusion tensor
imaging (MR-DTI) now allows fascicle length estimates on a
subject-by-subject basis (Fig. 4.14). However, it is important
to note that these are nonnormalized fascicle (not ber)
lengths; thus, they cannot be used to predict muscle excursion or velocity, nor can they be used to calculate PCSA.
Future developments in methods to measure sarcomere length
are required to make these normalizations. Nevertheless,
these are emerging scientic tools that should be considered
in clinical and scientic work as they are more rigorously
validated.
FIG. 4.12 Magnetic resonance imaging–based, patient-specic three-dimensional lumbar spine muscle
volumes view from (A) posterior, (B) anterior, and (C) inferior. Multidus (red), erector spinae (blue), quadratus
lumborum (yellow), and psoas (green) are easily visualized.
ABC
FIG. 4.13 Magnetic resonance imaging–based, patient-specic three-dimensional lumbar spine muscle
volume fractions (muscles in colors and fat in white) view from (A) posterior, (B) anterior, and (C) inferior.
Multidus (red), erector spinae (blue), quadratus lumborum (yellow), and psoas (green) are easily visualized and
quantied.
FIG. 4.14 Magnetic resonance diusion tensor imaging (MR-DTI) can be
used to generate tractography maps of individual muscles. Based on
imaging resolution, these tracts likely represent muscle fascicles (or larger),
but the diusion properties themselves are heavily inuenced by muscle
ber geometry. In this example, a posterior view of three-dimensional
muscle volumes demonstrates bilateral erector spinae (blue) and multidus
muscles (red). The right multidus muscle has MR-DTI tractography results
superimposed on the multidus muscle volume depicting fascicle
orientations and lengths.
Summary
Muscular architecture is an important, and oen overlooked,
determinant of muscle function. Because muscle architecture
interacts with the skeletal and nervous systems in complex
ways, all of these factors must be examined together to fully
understand the biomechanical function of a muscle and its
contribution to any pain or injury mechanisms. Detailed
anatomic and architectural studies have yielded insights into
spinal muscle functions, but the architecture of many spinal
muscles remains to be examined. ese data are necessary for
accurate biomechanical models, which must be used in conjunction with experimental studies to elucidate the function
of spinal muscles and their role in pathologic processes of the
spine. is information can ultimately be used to develop
improved prevention and rehabilitation strategies.
Chapter 4 Spinal Musculature: Anatomy and Function 75
KEY REFERENCES
1. Macintosh JE, Bogduk N. The biomechanics of the lumbar
multidus. Clin Biomech (Bristol, Avon). 1986;1:205-213.
2.
MacIntosh JE, Bogduk N. The morphology of the lumbar erector
spinae. Spine. 1987;12:658-668.
3.
MacIntosh JE, Valencia F, Bogduk N, Munro RR. The morphology
of the human lumbar multidus. Clin Biomech (Bristol, Avon).
1986;1:196-204.
This series of papers describes the complex but highly reproducible
anatomy of the lumbar and thoracic spine musculature. Progressing
from supercial to deep and thoracic to lumbar, the extraordinary
level of organization of this musculature is clearly apparent.
4.
Kamibayashi LK, Richmond FJR. Morphometry of human neck
muscles. Spine. 1998;23:1314-1323.
This paper represents the rst quantitative study of muscle
architecture in the cervical spine. The work highlights unique
architectural features of the neck muscles and provides data for
comparative studies and the development of biomechanical
models.
5.
Cholewicki J, McGill SM. Mechanical stability of the in vivo
lumbar spine: implications for injury and chronic low back pain.
Clin Biomech (Bristol, Avon). 1996;11(1):1-15.
This paper combines a detailed anatomic model of the lumbar
musculature with passive tissue properties, cross-bridge modeling,
and electromyography to estimate muscle forces and spine stability.
The authors relate spine stability to potential mechanisms of low
back injury and pain.
6.
Ward SR, Kim CW, Eng CM, et al. Architectural analysis and
intraoperative measurements demonstrate the unique design of
the multidus muscle for lumbar spine stability. J Bone Joint Surg
Am. 2009;91(1):176-185.
This paper combined architectural measurements from cadaver
specimens, in vivo intraoperative sarcomere length measurements
in exed and extended postures, and passive mechanical property
measurements from biopsy. The results highlight the high
force-generating capacity of multidus in exed lumbar spine
positions, indicating a design for spine stabilization.
REFERENCES
1. Kalimo H, Rantanen J, Viljanen T, Einola S. Lumbar
muscles: structure and function. Ann Med. 1989;21(5):
353-359.
2. Macintosh JE, Bogduk N. 1987 Volvo award in basic science.
e morphology of the lumbar erector spinae. Spine.
1987;12(7):658-668.
SECTION
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76 BASIC SCIENCE
3. Delp SL, Suryanarayanan S, Murray WM, Uhlir J, Triolo RJ.
Architecture of the rectus abdominis, quadratus lumborum,
and erector spinae. J Biomech. 2001;34(3):371-375.
4. Kamibayashi LK, Richmond FJ. Morphometry of human neck
muscles. Spine. 1998;23(12):1314-1323.
5. Ward SR, Kim CW, Eng CM, et al. Architectural analysis and
intraoperative measurements demonstrate the unique design
of the multidus muscle for lumbar spine stability. J Bone Joint
Surg Am. 2009;91(1):176-185.
6. Regev GJ, Kim CW, acker BE, et al. Regional myosin heavy
chain distribution in selected paraspinal muscles. Spine.
2010;35(13):1265-1270.
7. Macintosh JE, Valencia F, Bogduk N, Munro RR. e
morphology of the human lumbar multidus. Clin Biomech
(Bristol, Avon). 1986;1(4):196-204.
8. Anderson JS, Hsu AW, Vasavada AN. Morphology,
architecture, and biomechanics of human cervical multidus.
Spine. 2005;30(4):E86-E91.
9. Macintosh JE, Bogduk N. e biomechanics of the lumbar
multidus. Clin Biomech (Bristol, Avon). 1986;1(4):205-213.
10. orstensson A, Carlson H. Fibre types in human lumbar back
muscles. Acta Physiol Scand. 1987;131(2):195-202.
11. Donisch EW, Basmajian JV. Electromyography of deep back
muscles in man. Am J Anat. 1972;133(1):25-36.
12. Gray H. Gray’s Anatomy. New York: Gramercy Books; 1977.
13. Cornwall J, Kennedy E. Fiber types of the anterior and
lateral cervical muscles in elderly males. Eur Spine J.
2015;24(9):1986-1991.
14. Mayoux-Benhamou MA, Revel M, Vallee C, et al. Longus colli
has a postural function on cervical curvature. Surg Radiol Anat.
1994;16(4):367-371.
15. McGill S, Juker D, Kropf P. Quantitative intramuscular
myoelectric activity of quadratus lumborum during a wide
variety of tasks. Clin Biomech (Bristol, Avon). 1996;11(3):170-172.
16. Bogduk N, Pearcy M, Hadeld G. Anatomy and biomechanics
of psoas major. Clin Biomech (Bristol, Avon). 1992;7(2):109-119.
17. McGill SM, Patt N, Norman RW. Measurement of the trunk
musculature of active males using CT scan radiography:
implications for force and moment generating capacity about
the L4/L5 joint. J Biomech. 1988;21(4):329-341.
18. Santaguida PL, McGill SM. e psoas major muscle:
a three-dimensional geometric study. J Biomech.
1995;28(3):339-345.
19. Gans C, Bock WJ. e functional signicance of
muscle architecture—a theoretical analysis. Ergeb Anat
Entwicklungsgesch. 1965;38:115-142.
20. Juker D, McGill S, Kropf P, Steen T. Quantitative
intramuscular myoelectric activity of lumbar portions of psoas
and the abdominal wall during a wide variety of tasks. Med Sci
Sports Exerc. 1998;30(2):301-310.
21. Vasavada AN, Li S, Delp SL. Inuence of muscle morphometry
and moment arms on the moment-generating capacity of
human neck muscles. Spine. 1998;23(4):412-422.
22. Bogduk N, Johnson G, Spalding D. e morphology and
biomechanics of latissimus dorsi. Clin Biomech (Bristol, Avon).
1998;13(6):377-385.
23. McGill SM, Hughson RL, Parks K. Changes in lumbar lordosis
modify the role of the extensor muscles. Clin Biomech (Bristol,
Avon). 2000;15(10):777-780.
24. Norman R, Wells R, Neumann P, et al. A comparison of peak
vs cumulative physical work exposure risk factors for the
reporting of low back pain in the automotive industry. Clin
Biomech (Bristol, Avon). 1998;13(8):561-573.
25. Brault JR, Siegmund GP, Wheeler JB. Cervical muscle response
during whiplash: evidence of a lengthening muscle contraction.
Clin Biomech (Bristol, Avon). 2000;15(6):426-435.
26. Vasavada AN, Brault JR, Siegmund GP. Musculotendon and
fascicle strains in anterior and posterior neck muscles during
whiplash injury. Spine. 2007;32(7):756-765.
27. Lieber RL, Friden J. Muscle damage is not a function
of muscle force but active muscle strain. J Appl Physiol.
1993;74(2):520-526.
28. Macpherson PC, Schork MA, Faulkner JA.
Contraction-induced injury to single ber segments from
fast and slow muscles of rats by single stretches. Am J Physiol.
1996;271(5 Pt 1):C1438-C1446.
29. Patel TJ, Das R, Friden J, Lutz GJ, Lieber RL. Sarcomere strain
and heterogeneity correlate with injury to frog skeletal muscle
ber bundles. J Appl Physiol. 2004;97(5):1803-1813.
30. McGill SM, Norman RW. Eects of an anatomically detailed
erector spinae model on L4/L5 disc compression and shear.
J Biomech. 1987;20(6):591-600.
31. Kaneoka K, Ono K, Inami S, Hayashi K. Motion analysis
of cervical vertebrae during whiplash loading. Spine.
1999;24(8):763-769.
32. Winkelstein BA, McLendon RE, Barbir A, Myers BS. An
anatomical investigation of the human cervical facet capsule,
quantifying muscle insertion area. J Anat. 2001;198(Pt 4):455-461.
33. Barnsley L, Lord SM, Wallis BJ, Bogduk N. e prevalence of
chronic cervical zygapophysial joint pain aer whiplash. Spine.
1995;20(1):20-25, discussion 26.
34. Crisco JJ 3rd, Panjabi MM. e intersegmental and
multisegmental muscles of the lumbar spine. A biomechanical
model comparing lateral stabilizing potential. Spine.
1991;16(7):793-799.
35. Cholewicki J, McGill SM. Mechanical stability of the in vivo
lumbar spine: implications for injury and chronic low back
pain. Clin Biomech (Bristol, Avon). 1996;11(1):1-15.
36. Daru K. Computer Simulation and Static Analysis of the Human
Head, Neck and Upper Torso. Tempe, AZ: Arizona State
University; 1989.
37. Winters JM, Peles JD. Neck muscle activity and 3-D head
kinematics during quasi-static and dynamic tracking
movements. In: Winters JM, Woo SL-Y, eds. Multiple Muscle
Systems: Biomechanics and Movement Organization. New York:
Springer-Verlag; 1990. Multiple Muscle Systems: Biomechanics
and Movement Organization.
38. Hamberg-van Reenen HH, Ariens GA, Blatter BM, et al.
Physical capacity in relation to low back, neck, or shoulder
pain in a working population. Occup Environ Med.
2006;63(6):371-377.
39. Zabihhosseinian M, Holmes MW, Ferguson B, Murphy B. Neck
muscle fatigue alters the cervical exion relaxation ratio in
sub-clinical neck pain patients. Clin Biomech (Bristol, Avon).
2015;30(5):397-404.
40. Halvorsen M, Abbott A, Peolsson A, Dedering A. Endurance
and fatigue characteristics in the neck muscles during
sub-maximal isometric test in patients with cervical
radiculopathy. Eur Spine J. 2014;23(3):590-598.
41. Nimbarte AD, Zreiqat MM, Chowdhury SK. Cervical
exion-relaxation response to neck muscle fatigue in males and
females. J Electromyogr Kinesiol. 2014;24(6):965-971.
42. Uhlig Y, Weber BR, Grob D, Muntener M. Fiber composition
and ber transformations in neck muscles of patients
with dysfunction of the cervical spine. J Orthop Res.
1995;13(2):240-249.
Chapter 4 Spinal Musculature: Anatomy and Function 77
43. Mazis N, Papachristou DJ, Zouboulis P, et al. e eect of
dierent physical activity levels on muscle ber size and type
distribution of lumbar multidus. A biopsy study on low back
pain patient groups and healthy control subjects. Eur J Phys
Rehabil Med. 2009;45(4):459-467.
44. Brown SH, Gregory DE, Carr JA, et al. ISSLS prize winner:
Adaptations to the multidus muscle in response to
experimentally induced intervertebral disc degeneration. Spine.
2011;36(21):1728-1736.
45. Crossman K, Mahon M, Watson PJ, Oldham JA, Cooper
RG. Chronic low back pain-associated paraspinal muscle
dysfunction is not the result of a constitutionally determined
“adverse” ber-type composition. Spine. 2004;29(6):628-634.
46. Elliott JM, Courtney DM, Rademaker A, et al. e rapid and
progressive degeneration of the cervical multidus in whiplash:
an MRI study of fatty inltration. Spine. 2015;40(12):
E694-E700.
47. O’Leary S, Jull G, Van Wyk L, Pedler A, Elliott J.
Morphological changes in the cervical muscles of women with
chronic whiplash can be modied with exercise—a pilot study.
Muscle Nerve. 2015;52(5):772-779.
48. Rantanen J, Hurme M, Falck B, et al. e lumbar multidus
muscle ve years aer surgery for a lumbar intervertebral disc
herniation. Spine. 1993;18(5):568-574.
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The Intervertebral Disc: Normal,
SECTION
5
CHAPTER
e intervertebral disc is a structure interposed between
the bodies of the vertebral column that acts as the shock
absorber of the spine, transmitting compressive loads between
bony segments. Its structure is generally brocartilaginous,
consisting of several anatomic segments with distinct functional importance in both the native and pathologic spine.
Degeneration of the disc is thought to be the leading cause
of low back pain worldwide and is associated with multiple
other conditions, such as spinal stenosis, herniated nucleus
pulposus, and deformity.1 roughout the human life span,
intervertebral discs undergo progressive but highly variable
degeneration, oen beginning in early life.
is chapter discusses the basic structure of the intervertebral disc and surrounding tissues, followed by a review
of the degenerative cascade leading to intervertebral disc
degeneration (IDD) as well as clinical consequences. Despite
the ubiquity of IDD and its association to low back pain, the
exact mechanisms of discogenic low back pain IDD are not
well dened. What has been established is that the process
is multifactorial, involving a complex interaction of genetics,
aging, mechanics, and environment biology.
Normal Disc
Disc Anatomy
e intervertebral disc is composed of three main structures:
the cartilaginous endplates (CEP), the central nucleus pulposus (NP), and the peripherally located anulus brosus (AF)
(Fig. 5.1).
Cartilaginous Endplates
e endplates are cartilaginous structures that serve as the
superior and inferior margins of the intervertebral disc. In
early life, the endplates are analogous to epiphyseal plates
elsewhere in the body, and serve as the growth centers of the
intervertebral bodies.3 Similar to epiphyses elsewhere, the
hyaline cartilage of the endplate initially occupies a signicant
portion of the disc. As aging occurs, this cartilage layer thins,
1,2
Aging, and Pathologic
Adam S. Olsen
James D. Kang
Nam Vo
Gwendolyn Sowa
and by adulthood consists of about a 1-mm-thick layer of
avascular tissue composed of rounded chondrocytes and type
II collagen.4 Cartilaginous endplates (along with surrounding
subchondral bone) do undergo some degree of elastic deformation during loading, but their contribution to the shockabsorptive properties of the disc is minimal. Instead, the
endplates functionally allow force transmission along the
vertebral axis via the discs and act as semipermeable barriers
for nutrient and waste exchange.
Nucleus Pulposus
e nucleus lies between adjacent endplates and forms the
gel-like core of the disc. e nucleus consists of a proteoglycan
and water matrix held together by an irregular network of
collagen type II and elastin bers. Proteoglycans have numer-
ous highly anionic glycosaminoglycan (GAG) side chains (i.e.,
chondroitin sulfate and keratan sulfate), which attract countercations and allow the NP to imbibe water. is composition
is similar to articular cartilage, and the ability of the matrix to
imbibe and release water in relation to applied stresses allows
the disc to cushion against compressive loads. e primary
proteoglycan is aggrecan; the high concentration of this
hydrophilic molecule provides the osmotic properties needed
to resist compression.
Cells in the NP are initially notochordal, but their numbers
decline aer birth and eventually become undetectable at
about age 4 to 10 years in humans.6 e NP is gradually
replaced during growth by smaller and rounded cells resembling the chondrocytes of articular cartilage.7 ese
chondrocyte-like cells synthesize mostly proteoglycans and
collagen type II in response to changes in hydrostatic pressure.
ese cells are also able to survive in the hypoxic environment
of the intervertebral disc and contain inducible hypoxiaresponsive transcription factors.8 e NP functions as a shock
absorber, acting in essence as a pressurized, deformable sphere
that dissipates compressive forces to the AF and the adjacent
vertebral bodies. As compressive forces on the spine increase,
hydrostatic pressure within the nucleus pushes outward from
its center in all directions.
5
I
79
80 BASIC SCIENCE
Apophyseal
Bone
Capillary
bed
Cartilage
endplate
Nucleus pulposus
Capsule
Nucleus
Cell
Spinal
cord
Nerve
roots
joint
Anulus fibrosus
FIG. 5.1 The intervertebral disc is a pivotal part of the spinal column; its properties inuence behavior of
adjacent tissues. There is great variation in matrix organization, composition, and cell morphology and activity
in dierent regions of the disc.
Anulus Fibrosus
e AF surrounds the NP and is composed of approximately
20 concentric rings (lamellae) of highly organized collagen
bers, primarily collagen type I. e collagen bers are
oriented approximately 60 degrees to the vertical axis of the
spine and run parallel within each lamella but perpendicular
between adjacent lamellae, allowing for maximal tensile
strength.9 Individual lamellae are connected to one another
by radially oriented elastin bers, which account for approximately 2% of the anulus’ dry weight. A network of bridging
tissues that span multiple lamellae, containing both elastic
and vascular elements, has also been described.10 Fibers of the
outer anulus attach to the periphery of the vertebral bodies,
whereas inner bers pass from one endplate to another.
Cells in the anulus are found between lamellae, arranged in
parallel to the collagen bers. Outer anulus cells are thin,
elongated, and phenotypically similar to broblasts, whereas
cells of the innermost anulus are more spheroid, similar to
articular chondrocytes.
1,11
e anulus contains the NP and
maintains its pressurization under compressive loads. e
tensile properties of the anulus allow the nucleus to recover
its original shape and position when the compressive load is
reduced.
Blood Supply, Nutrition, and Innervation
Blood Supply
In early fetal life, vascular channels traverse the endplates, but
they diminish in size starting at birth until complete disappearance by approximately 5 years of age. In adults, the blood
supply of the disc arises from two capillary plexuses. One
plexus penetrates 1 to 2 mm into the outer anulus, supplying
only the periphery of the anulus. e other capillary plexus
begins in the vertebral body and penetrates the subchondral
bone (see Fig. 5.1), terminating in capillary loops at the bonecartilage junction.12 e density of this capillary network
varies in location across the endplate, being greatest in the
center and lowest at the periphery. Although blood ow to the
disc is minimal, it may not be entirely passive as muscarinic
receptors have been identied on the disc periphery, which
may inuence perfusion.13 Cells in the center of the human
adult lumbar NP are 8 mm from the nearest blood source,
making the disc one of the largest avascular structures in the
body.
Nutrition
e limited vascularity of the intervertebral disc has important physiologic implications—mainly that nutrition depends
almost entirely on diusion (Fig. 5.2).
environment of the cells varies throughout the disc because of
its size and architectural makeup, with cells in the NP being 6
to 8 mm from the nearest blood vessel. Small molecules necessary to maintain cellular function (i.e., glucose and oxygen)
readily leave vertebral capillaries and diuse across the thin
cartilaginous endplate and the outermost layers of the anulus
into the disc extracellular matrix (ECM). Concentration gradients of glucose, oxygen, and other nutrients and metabolites
exist across the disc, regulated by the rates of nutrient supply
and consumption as well as the net negative charge of the
NP produced by high proteoglycan concentration. e low
14–16
e nutritional
Chapter 5 The Intervertebral Disc: Normal, Aging, and Pathologic 81
Nutrient profiles across the disc
Nutrients diffuse from
capillaries into the disc
Atherosclerosis of the
arteries supplying the
vertebral bodies is
associated with disc
degeneration
1
0.75
0.5
Relative
0.25
concentrations
0
Vertebral body
Capillary bed penetrating
subchondral plate
Cartilaginous endplate
Vertebral body
Disc
0 0.25 0.5 0.75 1
Distance
SECTION
Calcification of
endplate cartilage
can cut off the nutrient
supply from capillary
bed to the disc
I
Disc
Concentrations of oxygen and glucose are lowest
and those of lactic acid highest in the disc center.
The absolute levels depend on the vascular supply,
degree of endplate calcification, disc size, and
cellular activity. Loss of cellular activity and cell
death result if nutrient levels are too low.
Oxygen Glucose Lactic
FIG. 5.2 Schematic view of routes for nutrient transport into avascular disc and resulting nutrient proles. The
diagram also shows possible regions of disturbance. (Modied from Crock HV, Goldwasser M, Yoshizawa H.
Vascular anatomy related to the intervertebral disc. In: Ghosh P, ed. Biology of the Intervertebral Disc. Boca Raton,
FL: CRC Press; 1991:109–133.)
oxygen tension in the nucleus leads to anaerobic metabolism (i.e., glycolysis), resulting in a high concentration of
lactic acid and a lower pH in the nucleus compared with
the periphery of the disc.16 Metabolic byproducts, such as
lactic acid, diuse from the disc in the opposite direction of
glucose entry.
Innervation
Under normal conditions, only the outer 1 to 2 mm of the AF
is innervated in nondegenerated human discs; the majority of
these bers are sympathetic perivascular nerves. A small
number of mechanoreceptors are also reported in the outer
several AF lamellae, likely providing some level of proprioceptive feedback.17 e sources of this innervation are variable,
but include plexuses contained in both the anterior longitudinal and posterior longitudinal ligaments; these nerves have
been found to contain various substances, such as neuropeptide Y, substance P, acetylcholinesterase, and others.18 e
remainder of the anulus and nucleus are uniquely avascular
Acid
and lacking neurons under normal, nondegenerated conditions. Several studies have described further nerve ingrowth
into degenerated lumbar discs, however, which is discussed
later in this chapter.
Disc Composition
e function of the intervertebral disc depends greatly on the
properties of its ECM. e ECM provides the biomechanical
properties and acts to regulate the extracellular uid composi-
tion and the rate at which nutrients and metabolites are
exchanged. e ECM consists of a complex network of macromolecules whose composition varies in dierent regions of
the disc (Fig. 5.3).
and maintained by a small population of cells (~9000 cells/
mm3 in the anulus and ~5000 cells/mm3 in the nucleus)
occupying less than 1% of the disc volume.4 Disc cells also
produce a complex array of cytokines, growth factors, and
proteases to maintain equilibrium between the rates of synthesis and degradation of ECM components.
4,19
ECM macromolecules are synthesized
20,21
82 BASIC SCIENCE
Territorial Interterritorial
PRELP
Chondrocyte
Integrin
A
Collagen II/XI
FIG. 5.3 (A) Schematic view of dierent matrix macromolecules, their interactions with the cell and with other
matrix molecules, and their distribution within the territorial matrix (TM) and interterritorial matrix (ITM). (B)
Transmission electron micrograph of section through disc cell and its surrounding matrix. TM and ITM not only
have dierent molecular compositions, but also a dierent morphology. COMP, cartilage oligomeric matrix
protein; CILP, cartilage intermediate layer protein; CS, chondroitin sulfate; HA, hyaluronan; HS-PG, heparan
sulfate-proteoglycan; KS, keratan sulfate; NC4, N-terminal noncollagenous domain 4; PRELP, proline/arginine-rich
end leucine-rich repeat protein. (A, Modied from Heinegard D, Aspberg A, Morgelin M, et al. Extracellular
matrix of cartilage. Section for Connective Tissue Biology, University of Lund, 2003. Available at http://www.
cmb.lu.se/ctb.)
Biglycan
HS-PG
Chondroadherin
COMP
Collagen VI
CILP
Aggrecan
KS
CS
Fibulin
HA
Collagen II/XI
Decorin
Fibromodulin
Collagen IX
NC4
Link
protein
Cell
B
TM
ITM
Water
e major component by weight of the intervertebral disc is
water; its concentration is regulated by the abundance of
proteoglycan aggregates in the disc. e concentration of
water varies with age, location within the disc, and body position.22 e NP is most highly hydrated, and the water concentration can be as high as 90% in an infant, declining to
approximately 80% in nondegenerated young adult discs.23
e water content of the anulus is lower than the nucleus,
declining to 65% in the outer anulus in adult discs.
Water content varies with load, leading to diurnal changes
in disc hydration.24 During the diurnal cycle, 25% of the disc’s
water can be lost and regained in young lumbar discs.25 Some
water is expelled from the disc during the day because of the
increased forces of body weight and muscle contractions; it is
reimbibed at night when the compressive forces are removed.
is diurnal cycle results in changes in disc height and aects
the disc’s mechanical properties.
Macromolecules
Collagen is one major macromolecular component of the disc.
e collagen content of the disc is highest in the outer anulus,
and the dry weight decreases signicantly in the nucleus of
adult discs.26 e concentration of collagen type I is highest in
the outer anulus and decreases going toward the nucleus.26
Collagen type II follows the opposite gradient, with the highest
concentration located in the nucleus. Along with collagen
types I and II, the ECM contains many other minor collagens,
including types III, V, VI, IX, and XI.
e other major macromolecule of the disc is aggrecan,27
which consists of a protein core with approximately 100
anionic GAG side chains. Many aggrecan molecules covalently
attach to hyaluronan chains, forming large aggregates. ese
aggregates are trapped by the surrounding collagen network,
imparting a net negative charge to the ECM. e interstitial
water contains an excess of cations, which is directly related
to the concentration of negative charge (i.e., GAG concentration). e high concentration of cations imparts a high osmotic
pressure in the nucleus. Changes in proteoglycan concentration and GAG concentration lead to changes in osmotic
pressure, aecting the ability of the disc to maintain hydration
and turgor when loaded.
27
In addition to collagens and aggrecan, the disc contains
lower concentrations of numerous other macromolecules,14
including elastin, the smaller proteoglycans decorin and
bromodulin, cartilage oligomeric matrix protein, and cartilage intermediate layer protein. ese molecules function
either structurally or biomechanically and are important for
normal disc function.
Intervertebral Disc Degeneration
Degeneration
IDD is dened as an aberrant, cell-mediated response to
progressive damage, with combined structural failure and
accelerated or advanced signs of aging. ese proposed denitions also suggest that structurally intact discs with accelerated