SECTION 1
Basic Considerations
- ➔ Clinical Anatomy of Subaxial Cervical Spine
- R Selvan
- ➔ Applied Anatomy of Cervical Spine
- Antonio Figueiredo
- ➔ Clinical Biomechanics Related to Cervical Disc Degeneration and Herniation
- Komal Prasad
- ➔ Genetic Susceptibility of Cervical Disc Herniation
- SV Rege
- ➔ Basic Knowledge of Screws and Plates Used in Anterior Cervical Stabilization Following Excision of Herniated Cervical Disc
INTRODUCTION
The human spine as a whole is also known as vertebral column. It appears straight and upright when viewed from the front or from the back. The vertebrae are stacked like wooden blocks in between the intervertebral discs. When viewed from the side it shows two gentle curves (Fig. 1.1) in the cervical and lumbar spine, lordotic in nature to provide maximum flexibility while providing all the strength that is required to transmit the weight of the body.
CERVICAL SPINE
The cervical portion of the human spine has seven cervical vertebrae (Fig. 1.1). The first-two vertebrae, the atlas and the axis have atypical features while the remaining five have typical features. Herniation of intervertebral disc is prevalent in the lower five or the subaxial cervical vertebra.
The five cervical vertebrae below C2 are similar in shape. The vertebral bodies are small and the canal large in comparison with the thoracic and lumbar spines. The spinous processes are small and bifid except C7. Bifid spinous processes allow more extension without interfering with each other. The intervertebral joints are horizontal and transmit the weight of the head. Each transverse process is composed of two tubercles. The anterior tubercle is the representative of rib and at times a cervical rib is actually seen. The posterolateral corner of the upper surface of lower vertebra is elevated and is called the uncus or uncinate process. The uncus is not found in quadrupeds. It is only found in those who have to support their head. The uncus forms a joint with the lower surface of upper vertebral body called the neurocentral joint of Luschka.1 The uncus actually is a part of the arch and there is no disc2 tissue in that joint. The spinal canal is relatively broad.3,4 The average value is 17 to 18 mm in normal adults but is under 15 mm in cases of myelopathy.5,6 The smallest canal is found in Japanese people being at times less than 13 mm.7
Fig. 1.1: The spine is gently curved to provide maximum flexibility while providing all the strength. It has seven vertebrae in the cervical region
The articular processes of facet joints do not have uniform direction. At times the superior articular process is positioned more anteriorly then the vertebral foramen and the canal at this place becomes narrow causing radiculopathy (Fig. 1.2).8 The position can easily be observed on lateral X-ray of the cervical spine.
INTERVERTEBRAL DISC
The intervertebral disc is thicker in infants than in adults. At birth the discs occupies half the length of the cervical spine. In adults the length is one-third of the cervical spine and after the age of 50 years it is reduced further. The nucleus changes its shape to accommodate the changes due to motion. It bears loads during movements of the spine. The dense collagen fibers of the annulus are running vertically in the front and are strong. Posteriorly they run horizontally and are prone to be fissured.9
LIGAMENTS
There are two longitudinal ligaments, anterior and posterior. The anterior longitudinal ligament covers the anterior surface of the vertebral body and its lateral margins spread under the longus colli muscles. The posterior longitudinal ligament runs along the posterior surface of the vertebral bodies. The posterior longitudinal ligament is composed of two layers—a superficial and a deep layer. Both layers unite firmly in the central part and laterally they separate and the superficial layer invests the dura and the nerve roots.10 The periosteum is lying under the ligaments.
The ligamentum flavum varies slightly in its attachment in comparison to lumbar spine. It covers the anterior one-third of upper lamina and posterior one-third of lower lamina. With this arrangement the ligament buckles in during extension of the neck.
The interspinous ligament in the cervical spine is less well developed and weak. The supraspinous ligament is extremely well developed and forms the ligamentum nuchae.
BONY CERVICAL SPINAL CANAL
The conus medullaris ends at the lower border of L1 vertebra. Beyond that the dural sheath contains only the cauda equina. The shape of the cervical canal varies significantly from C1 to C7. The canal is almost round at C1 and slowly transforms into trifoliate pattern at C7 (Fig. 1.3). The sagittal diameter of the canal is always measured to given an indication of the size of the canal. It is measured from the posterior surface of the vertebral body to the junctional point between the lamina and the spinous process. This point is not always easy to define and one has to resort at times to tomography to define this point. A canal of 20 mm is capacious at C1. Measurements from 17 to 14 mm are normal. A sagittal diameter of 13 mm at C5 is on the borderline (Table 1.1). A canal of less than 13 mm diameter is definitely narrow. The cervical spinal canal is many times known to be narrow in patients with cervical spondylotic myelopathy.
SPINAL CORD
The spinal cord in the cervical region is thick with well developed gray matter and is oval in shape. Its blood supply comes from one anterior and two posterior spinal arteries. Branches of these vessels which form the coronal artery surround the cord. The central artery, a branch of anterior spinal artery, enters the cord from the anterior fissure.11 Additional radicular arteries coming from the vertebral artery supply blood to this network.
The function of the cord is compromised either by direct mechanical pressure or vascular insufficiency. Compression anteriorly comes from prolapsed disc, osteophytes or ossification in the posterior longitudinal ligament. Posteriorly the cord can be compressed by the ligamentum flavum. Fibrosis around the root and pathological anchoring of the denticulate ligaments can cause further compressive damage to the cord.12–14
Fig. 1.3: Bony cervical spinal canal tends to be round at C1 and transforms into trifoliate pattern at C7
When the canal is developmentally narrow and the space around the cord is compromised any friction occurring during daily activities can cause damage to the cord. The shape of the cord is oval and the shape of the canal is triangular resulting in crowding of nervous tissue posterolaterally in the canal. This explains early appearance of pyramidal signs (pyramidal tracts are located posterolaterally) in compressive myelopathy.
NERVE ROOTS
Of the 31 pairs of nerve roots the first and the last nerve roots are not available on the surface for examination. The first cervical nerve root is entirely motor without sensory branches and it serves the suboccipital muscles. The posterior branch of second nerve is thick and is called the great occipital nerve. The first-two cervical nerves (C1 and C2) do not come out through the intervertebral foramina like the rest but they come out through a narrow fissure between occiput and C1 posterior arch and C1/ C2 posterior arches. They are frequently compressed in hyperextension injuries.
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In the lower cervical spine sometimes the discrepancy can be found when the 7th cervical root leaves through the foramina between 5th and 6th cervical vertebra.15 Pathology in this region can cause radiculopathy in both 6th and 7th cervical roots. Osteophytes without disc prolapse are known to produce pure motor weakness with atrophy without pain and without sensory disturbance16 (Table 1.2) and needs to be differentiated from progressive spinal muscular atrophy.
VERTEBRAL ARTERY
The vertebral artery enters the transverse foramina of the sixth cervical vertebra. Presence of osteophytes on the joint of Luschka can compromise the canal width. Being in the foramen at this level the vessel cannot slide laterally resulting in stricture in the vessel. In elderly with established spondylotic changes the cervical spine is shortened and the vertebral artery is forced to persue a tortuous course and it can then cave into the vertebral body17 and needs care during anterior cervical fusion surgery or corpectomy surgery. Two or three radicular arteries supply the spinal cord. Usually they enter the spinal cord at the level of 6th cervical vertebra.18 The artery usually runs on the ventral side of the nerve root and it can be compressed much earlier by the osteophyte than the nerve root.
MUSCULATURE
The muscles of the neck are broadly divided into three groups. The three groups are: (1) muscles involved in the movements of head and neck; (2) muscles involved in the movements and suspension of arms and (3) muscles involved in the movements and suspension of thoracic cage. When a load is applied to the arms say, e.g. while lifting a weight to be placed on the head the weight of the load is transferred to the cervical spine 6through the muscles of the arms and hence this group of muscles deserve special attention. Raising something by the hand means raising it by the cervical spine. This explains why cervical spine degenerates early in workers doing heavy manual work.
To hold the head in proper position it is essential to have a delicate balance of contraction and relaxation among neck muscles. The free nerve endings in the cervical spinal musculature are disproportionately large and their discharges control not only the head position but also control the posture of the whole body. The small suboccipital muscles play a vital role in this function and the concentration of spindle density in these muscles is much higher than the density in the lumbrical muscles of the hand.
MOBILITY OF THE CERVICAL SPINE
The cervical spine is the most mobile segment of the whole spine. Maximum range of motion is possible in this portion of the spine. Therefore, it is also subject to significant injury being extremely mobile. The range of motion and its reduction with increasing age has been described in the section on age-related changes in the cervical spine. The spine as a whole and particularly the cervical spine is made of several segments. Thus, there are eight motion segments related to the cervical spine. Any motion simply cannot occur in one given motion segment. When a movement has to occur all motion segments cooperate to produce a smooth gliding coordinated motion.
MOVEMENTS IN THE CERVICAL SPINE
- Flexion and extension
- Lateral bending
- Rotation.
It is important to understand the difference between one motion and range of motion. In range of motion two parameters are involved and hence range of motion is different from motion in one movement. All motions have multiplanar coupling, e.g. ratio of rotation in lateral bending varies at different levels depending on orientation of facets. Inclination of facet joints at 45 to 80 degrees with respect to the horizontal plane of intervertebral disc causes simultaneous sliding and rotation. The orientation of facet joints is partly responsible for this multiplanar coupling. It has been shown19 that at the level of C3 and C4 the superior articular facets are displayed posteromedially. At C7/T1 level the superior facet is displayed posterolaterally and it correlate well with the pattern of cervical movements.
FLEXION/EXTENSION
Total extension is relatively less than total flexion. Total flexion possible is 53 degrees and total extension is 38 degrees with range of motion in flexion/extension in normal adults below the age of 50 years being 130 degrees in males and slightly more in females.
LATERAL BENDING
There is very little lateral bending in the upper cervical spine. All the lateral bending is done in the lower cervical spine. The range of motion in males is 88 degrees and in females about five degrees more than males in normal adults.
ANATOMICAL CONSIDERATIONS OF UNCO-VERTEBRAL JOINTS
A study of 54 dry cervical spines from C3 to C7 in 270 cervical vertebrae20 showed that the uncinate process of C4 to C6 vertebrae was significantly higher and the anteroposterior diameter of the intervertebral foramina is small at C4 and C5 and C6 levels. At C3 and C7 levels the uncinate process is not as high and the anteroposterior diameter of the foramina is not small (Fig. 1.4). The length of the nerve root between the lateral border of dural tube and the medial border of the vertebral artery gradually increased from C3 to C7.
A combination of high uncinate process, small AP diameter of foramina and long course of the nerve root in close proximity of neurocentral joint from C4 to C6 levels explains the predilection of the nerve root for compression by neurocentral osteophytes.
REFERENCES
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- Payne EE, JD Spillane. The cervical spine. An anatomicopathological study of 70 specimens (using a special technique) with particular reference to the problem of cervical spondylosis. Brain 1957;80:571–96.
- Hinck VC, Sachdev NS. Developmental stenosis of the cervical spinal canal. Brain 1966;89:27–36.
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- Hayashi K. The anterior and the posterior longitudinal ligaments of the lower cervical spine. J Anat 1977;124: 633–6.
- Schneider RC, Crosby EC, et al. Vascular insufficiency of brainstem and spinal cord in spinal trauma. Neurology 1959;9:643–56.
- Scovill WB. Cervical spondylosis treated by bilateral facetectomy and laminectomy. J Neurosurg 1961;18:423–8.
- Frykholm R. Deformities of dural pouches and strictures of dural sheaths in the cervical region producing nerve root constriction. A contribution to the etiology and operative treatment of brachial neuralgia. J Neurosurg 1947;4:403–13.
- Kahn EA. The role of the dentate ligaments in spinal cord compression and the syndrome of lateral sclerosis. J Neurosurg 1947;4:191–9.
- Hayashi K. Topographic anatomy of the cervical spine, spinal cord and its neighboring tissues. Injuries of the spine (1) (in Japanese), Nankodo Tokyo; 1986. pp. 1–5.
- Keegan JJ. The cause of dissociated motor loss in the upper extremity with cervical spondylosis. A case report, J Neurosurg 1965;23:528–36.
- Hayashi K. Clinical anatomy for treating cervical spondylotic patients. (in Japanese), Orthopedic Mook 1979;6:1–12.
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- Pal GP, Routal RV, Saggu SK. The orientation of the articular facets of the zygapophyseal joints at the cervical and upper thoracic region. J Ant 2001;198(4):431–41.
- Ebraheim NA, Lu J, Biyani A, et al. Anatomical considerations for uncovertebral involvement in cervical spondylosis. Clin Orthop 1997;334:200–6.