Spine and Spinal Orthoses SK Jain, Gautam Jain
INDEX
×
Chapter Notes

Save Clear


Anatomy and Biomechanics of Normal SpineCHAPTER 1

The developmental changes, the human spine had to undergo for the orthograde (erect and bipedal) type of posture replacing the quadrupedal, are based upon the requirements to adjust the weight-bearing pattern to a vertical line instead of a horizontal line. The spine in quadrupeds is in a single curve starting from the midpoint on the posterior aspect of the skull, whereas in bipeds it has ‘S’ shaped curvatures and it starts at the midpoint on the base of the skull. The head is balanced directly above the spine. The distraction force in the mid-spine in the quadrupeds is changed to compression force in the bipeds (two footed animal).
The bipedal posture of human establishes three essential postulates: (i) It becomes necessary to develop certain articulations and musculatures to provide static and dynamic balance while standing, walking, running, and jumping or jolting, (ii) It also becomes necessary to provide protection to the spinal cord inside the spinal canal from shock by absorbing jerks and permitting movements, and (iii) Permit weight bearing during all these activities.
 
VERTEBRAL COLUMN
The vertebral column, also called spine or backbone, serves the purpose of (i) Protection of the spinal cord, (ii) Supporting and stabilizing the trunk, (iii) Transmitting the load of head, upper extremity and trunk to the pelvis and lower extremities, and (iv) Mobility of spine in all the three planes.
The vertebral column, consists of vertebrae with the intervening discs and forms two fifth of the total height of an individual (Fig. 1.1).2
zoom view
Fig. 1.1: Vertebral column
The length of the column in an average adult male is about 71 cm (28 inch) and about 61 cm (24 inch) in an average adult female. It functions as a strong but flexible hollow pipe, which can bend forward (flexion), backward (extension), sideways (lateral flexion) and can also twist (rotation). It supports weight of the head, gives attachment to the ribs, pelvic girdle and muscles of the back and encloses the spinal cord and protects it during the course of various movements including running and jumping.
 
The Vertebrae
During the developmental stage the total numbers of vertebrae are 33. These are grouped according to the body regions as 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal. Later on vertebrae at sacral and coccygeal region fuse with some variations. 5th lumbar vertebra may fuse with sacrum, called sacralization of 5th lumbar vertebra. 1st sacral vertebra may develop as 6th lumbar vertebra called lumbarization of 1st sacral vertebra.
3As a result, in adult there are typically 26 vertebrae, which are:
7 cervical vertebrae—in the neck,
12 thoracic vertebrae—in the chest,
5 lumbar vertebrae—in the lower back,
1 sacrum (5 sacral vertebrae fused into 1) in the pelvic area, and
1 coccyx (4 coccygeal vertebrae fused into 1) at the lower end of the spine.
Cervical, thoracic and lumbar vertebrae are known as true vertebrae, whereas sacral and coccygeal are called false vertebrae.
 
Embryology
 
Development of Spinal Cord
During the third week of embryonic growth, a longitudinal furrow develops on the dorsal aspect (towards the back) of the embryo, called neural fold (Fig. 1.2). The walls and floor of the fold thicken, forming the neural groove. By the fourth week the margins of the neural groove unite, thus the groove gets converted into neural tube that subsequently gives origin to whole of the nervous system. The distal part of the neural tube continues to become the spinal cord.
zoom view
Fig. 1.2: Development of neural tube
By the end of the third fetal month, the cord extends throughout the whole length of the vertebral canal. The relatively faster growth of the vertebral column causes a pull on the cord forming the conus medullaris, lower conical end of the spinal cord. It is located at the L2-3 level at birth and by 3 months of age shifts to the level of L1-2, and remains at this 4location through the life. Thus, the spinal cord ends at L1-2 and below this level remains the bunch of spinal nerves called cauda equina.
 
Development of the Vertebrae
On the ventral aspect (anteriorly, towards the abdomen) of the neural tube that gives rise to spinal cord, lies the primitive skeletal axis called notochord. The paraxial bar of mesoderm on each side of the notochord and the neural tube divides into a series of segments called somites, which are separated from each other by intersegmental septa. During the fourth week, sclerotome cells, which drive from paraxial mesoderm, migrate around the neural tube to merge with the cells from the somites on the other side of the neural tube to form the membranous vertebral column (Fig. 1.3).
zoom view
Fig. 1.3: Somites in early embryo
As development continues, sclerotome portion of each somite undergoes the process of re-segmentation. It occurs when caudal (lower) half of each sclerotome grows and fuses with the cephalic (upper, towards the head) half of each sclerotome to form the individual vertebra. Thus, each vertebra is formed by the fusion of caudal half of one somite and cranial half of lower somite, and remains opposite the intersegmental septa. In the mesoderm on each side of the neural tube, a bar of cartilaginous tissue appears that grows backwards forming the vertebral arch.
During the fourth fetal month the growing arches from either side fuse with one another dorsally (towards the back) enclosing the neural tube and form the spinous process.
Occasionally, the growing cartilaginous bars fail to fuse on the dorsal aspect of the neural tube giving rise to a gap due to the nonformation of 5lamina and spinous process in the vertebral arch, the condition known as spina bifida.
 
Development of the Intervertebral Disc
The intervertebral discs develop from both the mesoderm (the annulus fibrosus) and the notochord (the nucleus pulposus).
The membranous column gradually turns cartilaginous. The notochord passes along the whole length of the column within the cartilaginous vertebrae and the intervertebral tissue. From second intrauterine month onwards, the notochord begins to shrink and finally disappears with the onset of ossification. Between adjacent vertebral bodies, a part of notochord persists that turns into the nucleus pulposus, which along with the surrounding mesoderm, that forms the annulus fibrosus, develops the intervertebral disc.
 
Functional Spinal Unit
Spine is formed by 33 somites. Each somite gives rise to lower half of the upper vertebra, inter vertebral disc and upper half of the lower vertebra and to the blood vessels supplying this unit. This is the functional spinal unit of the spine (Fig. 1.4) that also includes adjoining ligaments between them.
zoom view
Fig. 1.4: Functional spinal unit
 
Structure of a Typical Vertebra
Vertebrae in different regions of the spinal column vary in shape, size and other details. However, they are similar enough that their structure and function can be described as of a typical vertebra as shown in 6Figures 1.5 and 1.6. A typical vertebra can be divided into anterior and posterior portions. The anterior portion contains vertebral body, intervertebral disc and ligaments. The posterior portion contains pedicle, laminae, neural arches forming the vertebral foramen, facets forming intervertebral joints, transverse and spinous processes and the ligaments.
zoom view
Fig. 1.5: A typical vertebra (Superior view)
zoom view
Fig. 1.6: A typical vertebra (Lateral view)
Body: The body, which is disc-shaped, carries and transmits weight, is situated anteriorly. It is composed of a mass of spongy bone inside surrounded by compact bone on its outer side. The superior and inferior surfaces of the body are also of the spongy bone except at the periphery where there is a ring of compact bone.
The spongy bone at the upper and lower surface of vertebral body is covered with rough hyaline cartilage for the attachment of 7intervertebral disc. The spongy nature of the interior of the vertebral body makes it strong, and at the same time light and resilient.
The vertebral body is subjected to strain, which is mostly in vertical axis due to the weight of the body above that vertebra. The lamellae inside the vertebrae run in vertical as well as horizontal direction. The vertical lamellae are strong and give strength to the vertebra. These vertical lamellae are supported by relatively weak horizontal lamellae as shown in Figure 1.7.
zoom view
Fig. 1.7: Internal structure of the body of vertebra
Vertebral arch: The vertebral arch (or neural arch) as shown in Figures 1.5 and 1.6 is the posterior part of the vertebra. It consists of a pair of pedicles and a pair of laminae. It supports seven processes, four articular, two transverse and one spinous process. The vertebral arch together with the body of the vertebra forms the vertebral foramen that transmits the spinal cord, the nerve roots and their coverings. The arch on each side is composed of an anterior part, the pedicle, which is attached to the upper lateral part of the back of the body, and the lamina, which projects backwards from the pedicle to meet its fellow in the midline to form the vertebral arch.
The shape of the pedicle is such that it has a groove on its superior as well as inferior surface. These two half grooves of two vertebrae together form a circular gap, known as intervertebral foramen that transmits segmental nerves and vessels. Each intervertebral foramen is bounded by a facet joint behind, intervertebral disc in front and the pedicles constitute the superior and inferior boundaries of the foramen (Fig. 1.8).8
zoom view
Fig. 1.8: Intervertebral foramen
Processes: Seven processes arise from the vertebral arch (Fig. 1.9). Two transverse processes project laterally on either side from the point where lamina and pedicle meet. A single spinous process (spine) projects postero- inferiorly in the midline from the junction of laminae. The transverse processes and spinous processes provide attachment to the ligaments and muscles. They also provide lever arm to the muscular activity.
zoom view
Fig. 1.9: Vertebral processes
The two superior articular processes and two inferior articular processes arise at the junction of pedicle and lamina. The superior articular processes articulate with the inferior articular processes of the vertebra above and inferior articular processes with the superior articular process of the vertebrae below. The articular surfaces are known as facets.
Superior facets are directed posteriorly, superiorly and medially (cervical and lumbar region), and laterally (thoracic region). Inferior articular processes which oppose the superior articular processes assume the opposing shape for articulation.
9The part of the arch, which separates these articular processes is of clinical importance and is called pars interarticularis (Fig. 1.10).
zoom view
Fig. 1.10: Pars interarticularis
The articulations formed between the vertebral bodies and articular facets of successive vertebrae are called intervertebral joints.
 
Regional Characteristics
There are five regions of vertebral column which are different from each other. However, the transition from one region to the next is gradual.
Cervical region (C1-C7): Bodies of cervical vertebrae are smaller than all other vertebrae, except the coccyx. Cervical vertebrae have three foramina, one vertebral and two transverse foramina. The vertebral foramen is the largest since the spinal cord at its origin has bigger circumference. The transverse foramina are in the transverse processes, through which the vertebral artery, vein and nerve fibers traverse. Spinous processes of C2 to C6 are bifid. C1 and C2 differ considerably from other vertebrae. Superior and lateral views of a typical cervical vertebra are shown in Figure 1.11.
The Atlas (C1): This vertebra is unique. It has no body and spinous process but has the most prominent transverse processes. It is a ring of bone with anterior and posterior arches and large lateral masses as shown in Figure 1.12.
Superior surface of the lateral mass has superior articular facet, which is concave and articulates with the occipital condyles of occipital bone of the skull, forming the Atlanto-occipital joints. It permits nodding movement by flexion and extension of head signifying ‘yes’.
The Axis (C2): It has a body and a peg like process called dens or odontoid process, which projects superiorly, and is the representation of the body 10of the Atlas.
zoom view
Fig. 1.11: A typical cervical vertebra
zoom view
Fig. 1.12: Atlas vertebra (Superior view)
The odontoid process makes a pivot on which Atlas and head rotate to signify the gesture ‘no’. The articulation between the anterior arch of the Atlas and dens is called Atlanto-axial joint. During hanging through neck the Atlanto-axial joint gives way and the dens crushes spinal cord leading to respiratory failure and death. Axis vertebra is shown in Figure 1.13.
The seventh cervical vertebra (C7): It is called vertebra prominens and is different from other cervical vertebrae (Fig. 1.14). It has a large spinous process directed posteroinferiorly and can be felt easily at the base of the neck.11
zoom view
Fig. 1.13: Axis vertebra
zoom view
Fig. 1.14: 7th cervical vertebra
12Thoracic region (T1-T12) also known as dorsal region (D1-D12): Thoracic vertebrae are larger and stronger than cervical vertebrae. Spinous process of T1 and T2 are long and directed posteroinferiorly, whereas spinous process of T11 and T12 are short, broader and directed posteriorly. Thoracic vertebrae articulate with ribs therefore have articular facets in the bodies and the transverse processes. Movements of the thoracic region are restricted by the attachment of ribs to the sternum. A typical thoracic vertebra is shown in Figure 1.15.
zoom view
Fig. 1.15: A typical thoracic (dorsal) vertebra
Lumbar region (L1-L5): The vertebrae in this region are strongest and largest because the body weight to be supported by a vertebra increases towards the lower end of the vertebral column. The spinous processes are quadrilateral in shape, thick and broad, projecting straight posteriorly as shown in Figure 1.16.
The superior articular facets are directed medially and inferior articular facets are directed laterally.
Sacrum: It is a triangular bone formed by the fusion of S1 to S5 as shown in Figure 1.17. The sacrum forms the posterior portion of the pelvic cavity and gives attachment to the hip bones on either side. It forms a strong foundation of the pelvic girdle. The sacrum of female is shorter and wider than that of the male.
Anteriorly, the sacrum is smooth and concave. On posterior side is the median sacral crest, which is formed by the fusion of sacral 13spinous processes.
zoom view
Fig. 1.16: A typical lumbar vertebra
zoom view
Fig. 1.17: Sacrum
Fused transverse processes form lateral sacral crest. Sacral canal is the continuation of vertebral foramen. Narrow inferior portion of sacrum is known as apex and broad superior portion is called base, anterior projecting border of the base is sacral promontory.
On both lateral surfaces, the sacrum has ear shaped articular surface, which articulates with the ilium of the hip bone to form sacroiliac joint.
Coccyx: Like sacrum, it is also triangular and is formed by the fusion of four coccygeal vertebrae (Fig. 1.18). Coccyx articulates superiorly with the apex of the sacrum.14
zoom view
Fig. 1.18: Coccyx
 
Intervertebral Disc
There are 23 intervertebral discs and form 20% of the height of the vertebral column. There is no disc between C1 and C2 and between sacrum and coccyx. It is a fibro-cartilaginous complex forming a very strong bond between the bodies of the vertebrae (Fig. 1.19). Shape of the disc contributes to the cervical and lumbar lordosis. The disc is biconvex and consists of outer fibrous ring called annulus fibrosus and central gelatin like nucleus pulposus; and has cartilaginous end plate above and below the disc. The cartilaginous end plate is strongly bound with the bodies of the vertebrae.
zoom view
Fig. 1.19: Intervertebral disc
Annulus fibrosus: It consists of concentric rings of collagen, elastic tissue and inner layer of fibro cartilage where it blends with nucleus pulposus. Its inner fibers pass and merge with the nucleus at an angle of 30° to the disc, intersecting at about 120° to each other. This provides additional strength to the disc and also resists torsional stress (Fig. 1.20).15
zoom view
Fig. 1.20: Annulus fibrosus—intersecting fibers
Annulus fibrosus is firmly attached to the outer ring of the bone of the vertebral bodies below and above as well as to the anterior and posterior longitudinal ligaments. Main functions of the annulus fibrosus are (i) to withstand horizontally directed tension produced by the nucleus, (ii) to resist twisting strain of the column, and (iii) to resist separation of the vertebral bodies.
End-plate: It is a thin layer of cartilage adherent to the trabeculae of the bone of the bodies of the vertebrae. It is surrounded by 2 mm thick rim of bone, which is often incomplete posteriorly thus forming a weak area. Through this gap the intervertebral disc can prolapse (Fig. 1.21).
zoom view
Fig. 1.21: End-plate and intervertebral disc
Nucleus pulposus: It is a sphere of hydrophilic gelatinous tissue. It occupies central position in cervical and thoracic region and is closer to the posterior margin in the lumbar region. It has no blood supply, but takes its nutrition through the cartilage. Its main functions are shock absorption 16and allowing movements of spine (Fig. 1.22). It gets compressed in erect position due to loss of some fluid, and re-expands in recumbent position when pressure reduces.
zoom view
Fig. 1.22: Shock absorbing in intervertebral disc
Changing shape of the disc contributes towards the range of movements in the spine. During forward flexion the intervertebral disc bulges posteriorly. However, in degenerative disc disorder it can compress the spinal cord or nerve roots (Fig. 1.23), leading to the symptoms like pain, numbness, muscular weakness or tingling over the area of distribution of the nerve root.
zoom view
Fig. 1.23: Changing shape of disc (Flexion)
17During extension, the intervertebral disc bulges anteriorly as shown in Figure 1.24. Thus, the extension helps undoing the posterior bulge.
zoom view
Fig. 1.24: Changing shape of disc (Extension)
Without the discs, spine will take the shape of one curve with posterior convexity.
 
Curves of Spine
When viewed from behind, the vertebral column presents as a single straight line, which bisects the body. When viewed from the sides there are a number of curves that vary with age.
In the fetus, the vertebral column shows one long primary curve, convex posteriorly, from head to the sacral region. It is due to the position of fetus inside the uterus, which is also present at birth (Fig. 1.25).
At about the fourth month after birth, when the infant begins to raise its head, a forward convexity appears in the cervical region. When the child sits up, stands and begins to walk at the age of about one year, a forward convexity develops in the lumbar region also. These curves continue to develop till the age of 17 years or so.
The developments of these secondary curves help bring the weight of the body over the lower limbs, therefore they are considered as adaptations to the upright posture.
In adults four distinct anteroposterior curves are seen (Fig. 1.26), they are:18
zoom view
Fig. 1.25: Primary curve at birth and at 4 months
  • Two primary curves, which retain their original posterior convexity and are found in the thoracic and sacral region, and
  • Two secondary curves, which are convex anteriorly and are found in cervical and lumbar regions.
zoom view
Fig. 1.26: Curvatures of spine in adults
19The primary curve is mainly due to the shape of the bodies of the vertebrae, which are shorter anteriorly than posteriorly.
The secondary curves are mainly due to greater anterior height and wedging of the intervertebral discs in the cervical and lumbar region.
If the vertebral column is articulated without the discs, it assumes its infantile appearance of long gradual curve, convex posteriorly. However, on many occasions the secondary curves may progressively reduce or lost in old age. This ultimately leads to forward bending of the spine in old age.
These curves increase the strength of the vertebral column, help in maintaining balance in upright position, absorb shock during walking and running and thus help protect vertebrae and the spinal cord from injury. Alteration in the normal curvature of one part of the column must be followed by compensatory change elsewhere, which is essential to maintain the erect posture and to keep the center of gravity over the center of support.
 
Ligaments of Spine
The ligaments contribute towards the stability of the spinal segments by supporting the vertebrae and resisting undue movements. There are two ligament systems.
 
First System
This consists of three ligaments, which run the full length of the spinal column adjacent to the vertebral bodies. They are (i) Anterior longitudinal, (ii) Posterior longitudinal, and (iii) Supraspinous ligaments (Figs 1.27 and 1.28).
Anterior longitudinal ligament: It stretches on the anterior aspect of the vertebral column from basiocciput (part of the occipital bone that lies anterior to the foramen magnum) to the sacrum and is attached to the vertebral bodies and the intervertebral discs. It is a strong ligament and consists of short and long fibers. The short fibers are deep and run between two adjacent vertebrae and blend with disc whereas superficial fibers bridge several vertebrae. It is wider in lower thoracic and lumbar region and thicker and narrower in the cervical region. It supports the spine and resists hyperextension of the spine.20
zoom view
Fig. 1.27: Ligaments of spine
zoom view
Fig. 1.28: Superior view of lumbar vertebra
Posterior longitudinal ligament: It stretches from basiocciput to sacrum and lies within the vertebral canal on the posterior aspect of the vertebral bodies. It is wider over the disc and narrower over the bodies (Fig. 1.28) and is attached to them. It is wider and thicker in the cervical and upper thoracic region. It resists hyperflexion of the spine.
Supraspinous ligament: It is a strong fibrous cord connecting the tips of spinous processes from C 7 to sacrum. From C 7 upwards it is thickened and forms ligamentum nuchae which is attached to occipital 21protuberance. It blends with the surrounding fascia, which resists hyper- flexion of the spine.
 
Second System
Ligaments of the second system are shown in Figures 1.27 to 1.29.
zoom view
Fig. 1.29: Oblique view of spine
They run in between the vertebrae in a motion segments and comprise of four ligaments, namely (i) Ligamentum flavum, (ii) Interspinous, (iii) Intertransversus, and (iv) Facet capsular ligaments.
Ligamenta flava (singular-ligamentum flavum, Latin for yellow ligament) are ligaments which connect the laminae of adjacent vertebra, all the way from the axis to the first segment of the sacrum (C2 to S1). They are best seen from the interior of the vertebral canal, when looked at from the outer surface they appear short, being overlapped by the laminae. They are thin in the cervical region and thickest in the lumbar region. Their marked elasticity serves to preserve the upright posture, and to assist the vertebral column in resuming it after flexion. The elastin prevents buckling of the ligament into the spinal canal during extension, and thus avoids cord compression. Hypertrophy of this ligament may cause spinal canal stenosis, since it lies in the posterior portion of the vertebral canal.
Interspinous ligaments: These ligaments run between adjacent spinous processes of the vertebrae and blend with supraspinous ligaments and ligamentum flavum. They are thickest in lumbar region.
22Intertransverse ligaments: These ligaments extend between transverse processes of the vertebrae and are well developed in lumbar region.
Capsular ligaments: They are attached to the peripheral margins of the articular facets. They allow sliding movements at the facet joints and resist segmental flexion.
 
Muscles of Spine
The spinal muscles are functionally arranged as a continuous sheet of muscles from base of the skull to the sacrum. They attach themselves to many different vertebrae and their parts as well as to the arms and legs, the head, the rib cage, and the pelvis. They are capable of four kinds of activities, which are: (i) Maintain the postural tone, (ii) Support the torso, (iii) Provide the freedom of motion, within physiological limits, and at the same time, (iv) Protect the spine during various movements as well as from trauma.
The muscles of the spine are classified: (i) According to the regions they act upon like cervical, dorsal and lumbar, (ii) According to their position in relation to the vertebral column like prevertebral and postvertebral, and (iii) According to their actions like flexors, extensors, lateral flexors and rotators.
Movements of the spine include flexion and extension (forward and backward bending), lateral flexion (left and right side bending), and rotation (twisting) as shown in Figure 1.30. The muscles of the spine act behind the spine to extend it, in front of the spine to flex it, and in combination of different muscles on each side to create side bending and twisting.
Extensors of spine: The spinal extensor muscles are postvertebral muscles, lie behind the vertebral column and span its entire length. They may be divided in three groups superficial, intermediate and deep.
The superficial postvertebral muscles, collectively called erector spinae (extensor spinae), also known as sacrospinalis in older texts, is not just one muscle, but a group of muscles and tendons. It extends throughout the lumbar, thoracic and cervical regions and gives an appearance of one common back muscle.
In the sacral region, it is narrow and pointed and at its origin from the iliac crest, it is mainly tendinous. It also takes origin from most of 23the lumbar vertebrae, and several of the lower thoracic vertebrae. In the lumbar region, it is larger, and forms a thick fleshy mass. In the upper lumbar region, it splits into three smaller groups, a lateral, the Iliocostalis, an intermediate, the Longissimus, and a medial, the Spinalis as shown in Figure 1.31.
zoom view
Fig. 1.30: Movements of spine
zoom view
Fig. 1.31: Erector spinae muscles
Each of these consists from below upward of three parts depending upon the area they cover. Iliocostalis forms (i) Iliocostalis lumborum, (ii) Iliocostalis dorsi, and (iii) Iliocostalis 24cervicis. Longissimus forms (i) Longissimus dorsi, (ii) Longissimus cervicis, and (iii) Longissimus capitis; and Spinalis form (i) Spinalis dorsi, (ii) Spinalis cervicis, and (iii) Spinalis capitis.
The intermediate muscles are more defuse. They originate from the transverse process of each vertebra and attach to the spinous process of the vertebra above. According to the region they are (i) Semispinalis lumbosacral (Multifidus), (ii) Semispinalis thoracic, (iii) Semispinalis cervicis, and (iv) Semispinalis capitis (Fig. 1.32).
zoom view
Fig. 1.32: Deep extensor muscles
The deep extensor muscles are short muscles. They are: (i) Interspinales that connect adjacent spinous processes, (ii) Intertransversarii—between adjacent transverse processes, (iii) Transversospinales (musculi rotators) connecting transverse process below to the spinous process above, and in thoracic region (iv) Musculi levatores costarum running between transverse process and ribs. They have the shortest fibers and span only one or two vertebrae. The extensor muscles located more superficially span many vertebrae and have multiple attachments. Many spinal muscles overlap in different planes, and provide stability during various movements.
Extensors of cervical spine are: (i) Iliocostalis cervicis, (ii) Longissimus cervicis, and (iii) Spinalis cervicis, which are part of the medial portion of erector spinae, and (iv) Semispinalis cervicis from the intermediate 25postvertebral muscle group. When muscles of both sides act together they extend the spine. Individually along with prevertebral muscles they contribute to lateral bending.
Flexors of spine: The spinal flexor muscles are prevertebral muscles and are in front of the vertebral column in the cervical and lumbar regions. There are no flexor muscles in the thoracic region.
The main lumbar spine flexors are the four abdominal muscles. They are (i) External oblique, (ii) Internal oblique, (iii) Transversus abdominis, which encircle the abdomen, and (iv) Rectus abdominis which is located anteriorly towards the midline (Fig. 1.33). The abdominal muscles are not directly attached to the spine, but contribute to the spinal movements by contracting leading to the flexion of the spine.
zoom view
Fig. 1.33: Abdominal muscles (left) and trunk muscles (right) for flexion of spine
Other trunk muscles are: (i) Quadratus lumborum, (ii) Trapezius, and (iii) Latissimus dorsi, which participate in flexion of the vertebral column. These muscles, unlike the abdominals are attached to the spine. The (iv) Psoas muscle (from the lumbar vertebrae to the thigh bone) flexes the trunk at the hip and has little effect on flexing the spine itself.
Sternocleidomastoid, right and left acting together, are strong flexors of the cervical spine; other flexors of the cervical spine are scalenus anterior and inferior oblique portion of longus colli cervicis.
26Lateral flexors of spine: When the extensors and flexors of only one side contract, lateral bending of the spine occurs. The quadratus lumborum is a muscle that contributes mainly to lateral bending of the lumbar spine.
Rotators of spine: Rotation occurs only in the neck and thoracic region of the spine. Little or no rotation occurs in the lower back, as this motion is blocked by the position and orientation of the facet joints. The most powerful rotator muscles are the abdominal internal and external oblique muscles (Fig. 1.33) whose fibers run obliquely to the long axis of the body. These prevertebral muscles along with multifidus, which is a long muscle (Fig. 1.32) and most of its fibers extend from the vertebral process to the spinous process of the vertebrae several levels above, also contribute to the extension in addition to the rotation of the spine.
The erector spinae muscles along with prevertebral muscles contribute to lateral bending, rotation of the spine as well as the movement of the head.
 
Intervertebral Joints
The basic anatomic and spinal unit in the vertebral column is called functional spinal unit (FSU), spinal segment or motion segment, which consists of two adjacent vertebrae connected by intervertebral disc, facet joint, ligaments and muscles (Fig. 1.34).
Movements are permitted by three types of articulations. The disc forming symphysis; the facet joints, which are diarthrodial and the ligaments forming syndesmosis articulations.
Joint between vertebral bodies: The joint is formed by the adjacent vertebral bodies and intervertebral disc in-between them. The disc has cartilaginous end plate above and below the disc. It is a thin layer of cartilage adherent to the trabeculae of the bone of the bodies of the vertebrae. In addition to the adherence with the vertebrae it also has an important function of shock absorption.
Symphysis—A fibro-cartilaginous fusion between two bones with distinct line of fusion like articulation between two pubic bones. Diarthrosis—A joint having mobility in any direction. Syndesmosis—It is a movable articulation where the opposing bone surfaces are united by a layer of connective tissue as in the inferior tibio-fibular articulation and radio-ulnar joint.27
zoom view
Fig. 1.34: Motion segment
Facet joints: The articular facets of the adjacent vertebrae form the synovial joints. The direction of the joint surfaces determines the direction of movements between the adjacent vertebrae. Greatest range of movement occurs where one type of vertebra changes to other and the vertebrae at these levels are most vulnerable to injury (Fig. 1.35).
zoom view
Fig. 1.35: Facet joints
Occipito-atlanto-axial articulation: This articulation differs from the rest of the spinal column. It controls movements of head and has two components (Fig. 1.36).
Atlanto-occipital joint: Occipital condyles of the skull are convex and fit into the concavity of the lateral masses of Atlas vertebra forming the Atlanto occipital joint. It is supported by the posterior longitudinal ligament and anterior Atlanto-occipital membrane, connecting the anterior rim of the foramen magnum to the anterior aspect of Atlas. Flexion and extension occurs at this joint.28
zoom view
Fig. 1.36: Atlanto-occipital joints
Atlanto-axial joint: There are two Atlanto-axial joints, median and lateral (Fig. 1.37).
The median Atlanto-axial joint is between anterior tubercle of the Atlas with its transverse ligament and the odontoid process of axis. The lateral Atlanto-axial joint is formed between the lateral mass of Atlas and superior articular process of axis. It is an arthrodial or gliding joint. 50% of rotation of spine takes place at this level.
zoom view
Fig. 1.37: Atlanto-axial joints
29
 
Movements of Spine (Kinematics)
The study of range and pattern of motion of spinal segments under normal or pathological conditions, without any consideration of forces is known as kinematics of the spine. Vertebral column consists of several separate vertebrae accurately positioned one on the other and separated by inter vertebral discs. The vertebrae are held in position by strong ligaments, which limit the degree of movements between adjacent vertebrae. The summation of all these movements gives vertebral column as a whole a remarkable degree of mobility. The movements depend upon the thickness of inter vertebral discs and the shape and direction of the articular processes.
Movements of the spine include flexion and extension (forward and backward bending), lateral flexion (left and right side bending), and rotation (twisting) as shown in Figure 1.38.
zoom view
Fig. 1.38: Movements of spine
Flexion and extension: Flexion is forward and extension is backward bending. They are extensive in cervical and lumbar region but restricted in thoracic region. A forward tilt of hip by hip flexion gives another 25° of flexion. However, it should be noted that forward bending can be achieved by hip flexion alone without flexion of spine. Hip extensors control hip flexion and provide counter balance to support the trunk against gravity.
30Lateral flexion: It is bending of the body to the right or left side. It is more in cervical and lumbar region but restricted in thoracic region.
Rotation: It is twisting of the vertebral column, most in the cervical and least in lumbar region.
Circumduction: It is combination of all the above movements.
Gliding movements: These are most common in cervical spine. They are floating movements of a vertebra over the other, which can be forward, backwards or side to side, as can be seen in dancers.
The measurement of joint motion is of considerable importance in determining the degree of deformity, prescribing an orthosis and measuring the progress during treatment. This applies to the spine as well. The data on Indian subjects are not specifically available, the average range of motion available from various studies are described below.
 
Cervical Spine
Cervical spine is the most mobile part of the spine. It has flexion, extension (Fig. 1.39), lateral flexion (Fig. 1.40), rotation (Fig. 1.41), and gliding (translatory) movements. Table 1.1 lists flexion, extension, lateral flexion and rotation in cervical spine at various levels.
It is worth noting that females show slightly greater movements than males especially gliding (translatory) movements.
zoom view
Fig. 1.39: Flexion and extension in cervical spine
31
zoom view
Fig. 1.40: Lateral flexion in cervical spine
zoom view
Fig. 1.41: Rotation in cervical spine
Table 1.1   Motion in cervical spine (in degrees)
Movements
Occiput-Atlas
Atlas-Axis
C2-C7
Total
Flexion
10
5
45
60
Extension
25
10
45
80
Lateral flexion right
5
10
30
45
Lateral flexion left
5
10
30
45
Rotation right
0
45
30
75
Rotation left
0
45
30
75
32
 
Thoracic and Lumbar Spines
Thoracic and lumbar spines have similar movements as that of cervical spine like flexion, extension, lateral flexion, rotation as shown in Figure 1.42. Lateral flexion of the thoracic spine is accompanied by rotation. Maximum flexion extension takes place between L4 and L5. There are no significant differences between males and females in the movement of thoracolumbar spine.
zoom view
Fig. 1.42: Motion of thoracic and lumbar spine
Table 1.2 shows flexion, extension, lateral flexion and rotation in cervical, thoracic and lumbar spine. The lateral flexion and the rotation take place on either sides (left and right).
Above the age of 50 years the range of motion gradually starts reducing and by the age of 80 years the range reduces by 10°.
Table 1.2   Motion at cervical, thoracic and lumbar spine (in degrees)
Movements
Cervical
Thoracic
Lumbar
Total
Flexion
60
15
40
115
Extension
80
15
25
120
Lateral flexion
45
15
20
80
Rotation right
75
40
5
120
33
 
Biomechanics of Spine
Biomechanics is the science of the internal forces (muscular activity and gravity) and the external forces acting on the human body and the effects produced by these forces. Biomechanics can also be defined as the study of the mechanics of a living body, especially of the forces exerted by muscles and gravity on the skeletal structure.
Spine biomechanics is the study of combination of forces and resistance generated against these forces over the spine. Four types of force acting on the spine are (i) Compression—forces, like gravity, which compress vertebrae and discs, (ii) Distraction—force that tends to elongate the vertebrae and discs, (iii) Torque—forces perpendicular to the axis of the spine, not in the plane of spine, that try to rotate the vertebrae and discs around the axis, and (iv) Shear—forces perpendicular to the axis of the spine that try to slide the vertebrae and discs away from the axis.
The fundamental equation that describes physical actions and reactions in the spine biomechanics is that which describes the relationship between bending moment, force, and moment arm length. It is M = F × D, where M is the bending moment, F is the force applied, and D is the distance from the point of force application to the axis of rotation (moment arm length).
Spine can be considered mechanically as a series of semi rigid bodies (vertebrae) separated by viscoelastic linkages (discs and ligaments), providing movements as well as protecting the spinal cord.
Balanced horizontal forces: Horizontal forces provide efficient bending movement for correction of lateral curvature and immobilization of spine. This can be expressed as three point loading system, where arrows represent the magnitude and direction of the forces.
These forces are applied along the length of spine, two in one direction and one in opposite direction. Since the system is in equilibrium, the sum of forces at A and C has to be equal to the force at B (A + C = B) as shown in Figure 1.43.
In order to have same skin pressure, the size of skin pad should be proportional to the pressure applied through it. By placing the forces B at the apex of curvature, the correctional efficiency becomes optimal.34
zoom view
Fig. 1.43: Balanced horizontal forces
Traction: By applying traction alone it is possible to achieve certain amount of correction, stability and immobilization of spine even if there is lateral instability (Fig. 1.44).
zoom view
Fig. 1.44: Traction of spine
Fluid compression: Pascal’s law states that fluids in a close chamber behave like solids and the pressure applied at any point is transmitted equally in all directions. This principle is utilized in supporting the spine by compressing the abdominal cavity by tightly applied brace, corset or abdominal support. This increases the intra-abdominal cavity pressure and produces a distracting force thereby effectively distracting the lumbar spine (Fig. 1.45).35
zoom view
Fig. 1.45: Pascal’s law
Skeletal fixation: This is the most effective method of applying reliable control on the spine. Halo traction and halo pelvic fixation devices are the examples (Fig. 1.46). After the diagnosis is made one has to decide the specific goals to be achieved, whether to support, immobilize or correct the deformity of the spine and what degree of freedom is to be controlled, to what extent and in which manner.
zoom view
Fig. 1.46: Halo traction of cervical spine
Sleeve principle: It is caging the patient between two semicircular fixation points, one above and other below. Between these two semicircular fixation points there are various uprights. The uprights may be in front, 36at the sides, posterior or in the paraspinous region of the patient. These uprights serve as a sleeve, a splint or as a distracter.
Balancing of spine: In a balanced vertical posture, there is minimal activity of the spinal muscles. Under normal circumstances the center of gravity (CoG) is located about 1 cm anterior to S1. Since the CoG is anterior to the spine, the posterior muscles remain in contracted state to achieve erect posture as shown in Figure 1.47.
zoom view
Fig. 1.47: Center of gravity
The posterior muscles of the spine further contract to maintain the balance when the center of gravity moves anteriorly. Similarly the anterior muscles of the spine contract, when the center of gravity moves posteriorly. Thus muscular activity occurs to maintain a balanced position.
Abdominal muscles initiate flexion, the weight of the upper part of the body further increases flexion due to gravity. Posterior spinal muscles control this spinal movement by resisting forward bending. As the flexion further increases, the activity of the posterior spinal muscles increases with the aim to counter-balance the overhanging weight of the upper body anteriorly. At full flexion the muscular activity of the back muscles becomes minimal and the ligaments assume major role of restricting the motion as well as provide counterbalancing force.
37In each motion of flexion, extension and lateral bending the movement is initiated by muscles. Gravity then increases the movement, and the balance is provided by the antagonist muscles. Further motion is restricted by the ligaments.
These movements increase the load on the spine. Intracavity (abdominal) pressure is a significant factor in reducing the load applied to the spine, whether the load is due to the movements or otherwise. Contraction of the abdominal muscles creates nearly a rigid walled cylinder of thoracic and abdominal cavities and increases the intracavity pressure, which produces a distracting force on the spine, which results in reducing the load on spine. The force on the lumbosacral disc is reduced by 30% and that on lower thoracic is reduced by 50% as a result of the anterior support provided by the contracting trunk muscles.
Weakening of the abdominal muscles due to any reason like physical inactivity, trauma, elongation due to pregnancy or obesity, does not help off-loading the spine and can lead to low backache.
Load-bearing characteristics of the spinal segments: Vertebral bodies, the disc as well as the facets carry the compressive load. The load is transmitted from the superior end plate of the vertebra to the inferior end plate through the cortical shell and the cancellus core of the vertebral body. In early years cortical shell and cancellus core carry equal body weight, as the age advances the weight carried by the cancellus core reduces to half as compared to cortical shell. The cancellus bone of the vertebra undergoes compressive deformation up to 10% before it fails, whereas the corresponding deformation in the cortical bone is less than 2%. Therefore, in vertical loading, pain due to the injury is more likely due to the cortical plate fracture rather than the cancellus bone.
Compression of the disc (nondegenerated) produces high pressure within the nucleus that results compression in the middle of the end-plate and some pressure at the periphery. This pushes the annulus and two end-plates outwards and produces deflection of the end-plate resulting in the high stress in the center (Figs 1.48A and 1.22) and this may lead to Schmorl’s nodes.
In a degenerated disc the compressive load is transmitted from one end-plate to other mainly through the annulus as shown in Figure 1.48B 38and the nucleus does not contribute much. The end-plate is loaded at its periphery and failure in load bearing results in the fracture of vertebral body.
zoom view
Figs 1.48A and B: Compression of (A) Nondegenerated; (B) Degenerated disc
It is worth remembering that the nucleus pulposus has highest compressive stress bearing property whereas the annulus fibrosus has highest tensile stress bearing property. Disc pressure gradually increases in the lower discs due to increased weight over them, whereas it is less in cervical spinal vertebrae. This pressure is lowest in lying position. The disc pressure also decreases when an object is carried close to the body as shown in Figure 1.49.
zoom view
Fig. 1.49: Load bearing on spine
39Spinal stability: The spine is considered stable when, under physiologic loading, there is neither strain nor excessive motion in the functional spinal unit (FSU). The stability is maintained by FSU, muscular tension, intra-abdominal and thoracic pressure, and rib support.
Sagittal stability and balance is also maintained by cervical lordosis, dorsal kyphosis, lumbar lordosis and sacral kyphosis.
Torsional stability in lumbar spine is contributed from facets (40%), disc (40%) and resistance to torsional load (20%).
 
Blood Supply of Vertebrae
The vertebral, intercostal and lumbar arteries, also known as segmental arteries, are closely related to the vertebrae. They give rise to anterior and posterior spinal branches. The anterior spinal branches travel anteriorly close to the vertebral bodies as shown in Figure 1.50, and give rise to minute vessels which pierce the bony cortex of the vertebral bodies to which they are associated.
zoom view
Fig. 1.50: Blood supply of the vertebrae
The posterior spinal arteries enter spinal canal and give rise to the ascending and descending branches which are related to the back of the vertebral bodies and anastomose with the similar branches that lie both above and below (Fig. 1.51).
40
zoom view
Fig. 1.51: Blood supply of the vertebrae
They also have connections with the corresponding vessels of the opposite side. It is from this arterial plexus three or four vessels arise and pass forward into each vertebral body through the large vascular foramen on the posterior surface of the vertebral body.
 
Effect of Aging on Spine
The aging process causes changes to the spine, which may lead to a variety of conditions. All biomechanical parameters decrease with age and affect each structure of spine like discs, vertebrae and ligaments. Progressive changes are shown in Figure 1.52.
zoom view
Fig. 1.52: Effect of aging on spine
41Effect of aging on the disc: As the age advances the nucleus loses water and elasticity and thus loses its volume and cushioning effect. In the degenerated disc the compressive load is transmitted directly from one vertebra to the other mostly through the annulus, more at the periphery and the nucleus carries only minimal load as shown in Figure 1.53.
zoom view
Fig. 1.53: Load bearing in degenerated disc
This is unlike normal disc where the load is mainly transferred through the nucleus that compresses middle of the end plate and transmits little load on the periphery. This results high pressure stress in the center as shown in Figure 1.54. This may cause central fracture of the end plate and Schmorl’s nodes.
zoom view
Fig. 1.54: Load bearing in normal disc
42Gradually the intervertebral discs become fibrous and they shrink, which accounts for the decreased height of the individual and kyphosis with aging. Later on the fissures appear in the disc leading to the disc bulge and at a later stage prolapse, through the thinner and weakest area of the annulus, which is on its posterior aspect towards the spinal canal and posterolaterally towards the intervertebral foramen (Fig. 1.55).
zoom view
Fig. 1.55: Prolapse intervertebral disc
Effect of aging on the vertebrae: Bone mineral density starts declining after the age of 30, this also affects the vertebral bodies. Over the age of 50, the numbers as well as thickness of the trabeculae, particularly the horizontal, reduce resulting in the reduction of the bone density called osteoporosis. This leads to collapse of some of the trabeculae under compressive load of the body weight, leading to change in the shape like vertebral wedging and collapse of the body leading to forward bending as well as loss of the height.
There is a fivefold decrease in the vertebral strength with aging. 30% increase in vertebral cross-sectional area occurs from the age of 20 to 80 years. The weight bearing capacity of the vertebrae reduces, the bone shifts from center to periphery in an effort to support normal loads.
Vertebral widening also results in formation of osteophytes, in an effort to bear more weight as shown in Figure 1.52. Widening of the vertebral body leads to spinal canal stenosis especially in the lumbar spine, osteophytes further aggravate the stenosis. The osteophytes compress spinal cord, blood vessels and nerve tissue. Though these 43changes happen to all of us, not everyone will experience symptoms of lumbar canal stenosis.
Due to the changes in the vertebral bodies, the axis of rotation migrates to the facet joints. The facet joints then degenerate with increase of the pedicle’s outer diameter with aging. This further compresses the spinal cord and nerve roots.
Most of the total height loss is due to disc degeneration, but vertebral wedging and collapse also contribute to this.
Effect on the ligaments: With increasing age, the ligaments lose their elasticity and they also become lax. Ligamentous laxity leads to vertebral subluxation, which would add to the spinal stenosis. On many occasions the ligaments, which are lax invaginate into the spinal canal and compress the spinal cord and nerve roots leading to symptoms.
Overall effect of aging of discs, vertebrae, ligaments and various vertebral joints lead to osteoarthritis, osteoporosis, compression fracture of vertebral bodies, spinal deformities like kyphosis and scoliosis, spinal canal stenosis, degenerative disc disease, reduced height, spondylolisthesis, facet joint arthritis, and spinal cord compression.
Effect on the movements: With advancing age, various changes takes place in the spine. They are increasing rigidity in the rib cage due to bony changes, kyphosis due to degeneration of the intervertebral discs, disuse atrophy of the muscles, degeneration of elastic tissue in the ligaments and osteoarthritic changes in the joints of the spine. Overall effect of these changes effect the normal flexible architecture of the spine. Above the age of 50 years, the range of motion gradually starts reducing and by the age of 80, the range of motion reduces by 10°.
 
SPINAL CORD
The spinal cord is a thick whitish cylinder of nervous tissue running down the central canal of the vertebral column. It is an extension of brain, and together with the brain it forms the central nervous system.
The spinal cord is continuation of the medulla oblongata at the bottom of the brainstem, and leaves the skull through a large opening called foramen magnum and extends in the spinal canal up to the level of first lumbar vertebra (Fig. 1.56).44
zoom view
Fig. 1.56: Spinal cord as continuation of medulla
The spinal cord is about 45 cm (18″) long, equal to the length of the femur and roughly as thick as a finger. Figure 1.57 shows position of spinal cord in the vertebral column.
zoom view
Fig. 1.57: Spinal cord and vertebral column
There are two enlargements, the cervical and the lumbar. The cervical enlargement is at the cord segments from C3 to T1 and innervates the upper limbs via the brachial plexus. The lumbar enlargement arises from segments L1 to S3 and innervates the lower limbs via the lumbar and sacral plexuses as shown in Figure 1.58.
The spinal cord proper ends at the level of L1 vertebra. At its lower end it tapers into the conus medullaris from which a thread like connective tissue, filum terminale continues and get attached to the dorsal surface of the first coccygeal vertebra (Fig. 1.59).45
zoom view
Fig. 1.58: Spinal cord showing enlargements
zoom view
Fig. 1.59: Conus medullaris and filum terminale
46The relations of the cord to the vertebral column differ greatly in fetal, infant and adult life. The vertebrae grow faster than the cord, in a newborn; the cord terminates at the lower border of the 3rd lumbar vertebra. In adults, on an average, the cord terminates at the level of the disc between the 1st and the 2nd lumbar vertebral bodies. This differential growth results in the elongation of lumbar and sacral nerve roots, to reach the corresponding intervertebral foramina, thus forming cauda equina.
The cervical roots pass almost horizontally in their intraspinal course and the upper dorsal roots are inclined a little. As a rough guide, there is a difference of one segment in the cervical region, two in upper and three in lower dorsal, four to five in the lumbar and the sacral region as shown in the following table.
Vertebral body
Cord segment
C7
C8
T4
T6
T9
T12
T12
L5
L1
Sacral
 
Structure of the Spinal Cord
The spinal cord presents an anterior median fissure and a shallow posterior median sulcus from where a posterior median septum extends about the half way into the substance of the cord (Fig. 1.60).
zoom view
Fig. 1.60: Structure of spinal cord
47On either side of the posterior sulcus lie the posterolateral sulci, through which posterior nerve roots emerge. There is no sulcus at the emergence of the anterior nerve roots on either side of the anterior fissure.
In transverse section, the cord comprises of a central canal, an H shaped zone of gray matter (nerve cells) and outer zone of white matter (nerve fibers) as shown in Figure 1.61.
zoom view
Fig. 1.61: Transverse section of spinal cord
The white matter consists of the bundles of specialized tracts that conduct impulses triggered by pressure, pain, heat, and other sensory stimuli from the periphery to brain or conduct motor impulses from the brain, activating muscles and glands.
The inner layer, or gray matter, has a butterfly-shaped (H shaped) cross-section and is mainly composed of nerve cell bodies. Within the gray matter runs the central canal, which is continuation of 4th ventricle as a narrow tube and traverses through the whole length of the cord. It is lined with ciliated epithelium and contains cerebrospinal fluid (CSF).
The lateral limb of gray matter consists of short and broad anterior column and a thinner, pointed posterior column, usually referred as anterior and posterior horns. The cross limb of H is termed as transverse commissure.
There are 31 pairs of spinal nerves. They transmit sensory impulses from skin, muscle, joints and viscera through afferent fibers via 48ascending tracts. The motor impulses generated in the brain are relayed by the spinal cord to the spinal nerves through the descending tracts that pass the impulses to muscles or glands through the efferent fibers in the nerves (Fig. 1.62). Most of the ascending fibers cross over to the other side in medulla oblongata and descending fibers in the spinal cord.
zoom view
Fig. 1.62: Spinal tracts shown in thoracic region
The ascending tracts that are: (i) Column of Goll or Gall ( fasciculus gracilis), (ii) Column of Burdach ( fasciculus cuneatus), (iii) Spino- cerebellar tracts, and (iv) Spinothalamic tracts.
The descending tracts mainly are (i) Lateral cerebrospinal, or pyramidal tract, also known as crossed motor tract, (ii) Anterior cerebrospinal, direct pyramidal or uncrossed motor tract.
The spinal cord also mediates the reflex response which is an automatic response to a given stimulus directly, without recourse to the brain, like knee jerk reflex, when a person’s leg is tapped at the patellar tendon. This is known as reflex arc.
Three protective membranes, known as the meninges, wrap the spinal cord and cover the brain. They are the pia mater, the innermost layer, the arachnoid lies in the middle, and the dura mater is the outer most layer, to which the spinal nerves are attached as shown in Figure 1.63. They protect the cord during various movements of spine.49
zoom view
Fig. 1.63: Spinal cord showing meninges
 
Blood Supply of Spinal Cord
Blood supply of spinal cord is from an anterior and two, right and left, posterior spinal arteries that run along its length and descend down from the level of the foramen magnum. These travel in the subarachnoid space and send branches into the spinal cord (Figs 1.64 and 1.50).
zoom view
Fig. 1.64: Blood supply of spinal cord
50Anterior spinal artery is in the mid-line and runs down within the anterior median fissure. It is formed at the foramen magnum by the union of a branch from each vertebral artery. In lower region it is formed by the branches of segmental arteries from either side. It supplies the whole of the cord anteriorly.
Posterior spinal arteries are two in number, on either side of the posterior median sulcus. They are fed from the posterior radicular artery, branch of segmental spinal arteries and run down the sides of spinal cord close to the attachment of posterior spinal nerve roots. They supply the posterior columns on either side.
These arteries are connected with each other (see inset) and are reinforced by other arteries, which enter the vertebral canal through inter vertebral foramen from vertebral, ascending cervical, posterior intercostals, lumbar and lateral sacral arteries.
 
Venous Drainage of Spinal Cord
Around the vertebral column and within the vertebral canal there are dense plexus of veins called external and internal venous plexus; the two plexuses together is called Batson’s plexus. The external venous plexus surrounds the vertebrae and the internal plexus lies within the vertebral canal outside the dura mater and frequently termed as epidural plexus. The venous drainage of spinal cord and spinal column is through these venous plexuses.
 
SURFACE MARKING OF SPINE AND BACK
In order to identify the level of a particular vertebral body or spinous process, it is customary to match their levels with various easily identifiable land marks on the torso. This can also be done by counting from the spinous process of the seventh cervical vertebra (C7), which is prominent and easily identified. The spines of the vertebra slope down so that the palpable tips of the spines are lower than their corresponding bodies.
In the back the scapula can be palpated easily. Spine of scapula and the lower angle of scapula can easily be identified and so is the iliac crest. Figure 1.65 shows the vertebral levels of these identifiable landmarks as seen from the back.51
zoom view
Fig. 1.65: Surface marking on the back
In the front, sternum can easily be felt that can be used to identify the vertebral levels. Sternal notch is at the level of T2, sternal angle between T4-5, xiphisternum at the level of the body of T9 and lowest portion of the rib cage is at the level of L2 as shown in Figure 1.66.
zoom view
Fig. 1.66: Surface marking on the rib cage
52The umbilicus is at the level of disc between L3-4. Surface marking from the front is shown in Figure 1.67.
zoom view
Fig. 1.67: Surface marking on the front
One should keep it in mind that surface marking and corresponding vertebral level are for general guidance, and are subjected to minor variations from person to person.
The following table summarizes the levels of various vertebrae in relation to identifiable structures on the body surface.
Vertebral level
Identifiable level
Body of C6
Cricoid cartilage
Spinous process C7
Prominent
Spinous process T1
More prominent
Disc between T2
Sternal notch
Spinous process of T3
Spine of scapula
Disc between T4-5
Sternal angle
Spinous process of T7
Inferior angle scapula
Body of T9
Xiphisternum
Body of L2
Rib cage lowest portion
Disc between L3-4
Umbilicus
Spinous process of L4
Highest point iliac crest
53