8 THE SPINE
Applied Anatomy and Biomechanics of the Spine
The vertebral column consists of 33 vertebrae—7 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 4 coccygeal vertebrae—and 23 intervertebral disks. Its structure provides a remarkable combination of rigidity, stability, and flexibility. Rigidity provides an essential vertical bony axis, stability provides strong scaffolding for cavities and extremities, and flexibility permits complex movements of the neck and low back. The spinal column is composed of four balanced curves: a cervical lordosis, a thoracic kyphosis, a lumbar lordosis, and a sacrococcygeal kyphosis (Figure 8-1). The compensatory nature of the balanced spinal curves allows the normal resting, erect posture to be maintained with minimal muscular effort.
The neural arch is made up of two pedicles attached to the vertebral body and two laminae, which fuse in the midline to form the spinous process. Three pairs of bony processes project from the arch close to the junction of the pedicles and laminae: two transverse processes, two superior articular processes, and two inferior articular processes. The paired articular processes at each level form the facet (apophyseal) joints (Figure 8-2). In the cervical spine, these joints bear about half of the weight of the head. In the lumbar spine, they accept less than a fifth of the load. This fact accounts for the relative difference in size between the facet joints and the vertebral bodies in the neck and low back. In the cervical spine, the joints are flat and slide easily. In the lumbar spine, they are curved to lock together and provide stability. In both areas the superior joints face backward.
Several vertebrae deserve special comment, because they have unique features. The first cervical vertebra, C1, also called the atlas, lacks a vertebral body and consists of anterior and posterior arches and two cup-shaped lateral masses (Figure 8-3). Just as Atlas in Greek mythology was forced to bear the world on his shoulders, so the cervical atlas (C1) bears the skull on its “shoulders” (lateral masses), each articulating with the occipital condyles on either side of the foramen magnum at the atlantooccipital joints. These joints allow nearly 40° of flexion–extension, for nodding the head, and 10° of lateral flexion. The second cervical vertebra, C2 or the axis, has a vertebral body anteriorly; from it a fingerlike peg projects superiorly. This bony process, called the odontoid or dens (den and dont from the Latin “tooth”), fits snugly against the anterior arch of the atlas, forming the atlantoaxial joint. The two are held together by the fibrous transverse ligament, which runs behind the odontoid process (see Figure 8-3). Rotation of the cervical spine, such as when shaking the head “no,” occurs mainly at the atlantoaxial joints (about 50°). There are no intervertebral disks between the atlas and occiput or between the atlas and the axis.
The third through the seventh cervical vertebrae possess more typical vertebral bodies and posterior elements, as well as intervertebral disks, and a foramen in each transverse process for the vertebral arteries. In addition, C3 through C7 vertebrae frequently form bony projections posteriorly and laterally from the superior end plate of each vertebra, which articulate with the beveled inferolateral surface of the vertebra above to form the uncovertebral joints or joints of Luschka. They also provide lateral stability to the discovertebral complex and form a barrier to extrusion of disk material posterolaterally (Figure 8-4). The C3 through C7 vertebrae allow cervical spine flexion, extension, lateral inclination, and rotation.
The lumbar vertebrae are remarkable for their size. The larger cross-sectional area of the lumbar vertebral end plates facilitates load bearing by the intervertebral disks. The larger surface area of the lumbar facet joints provides increased torsional and sheer stability to these spinal segments, limiting rotation but allowing side bending. The superior facet joints face medially and backward (see Figure 8-2). The lumbar spine allows a greater range of motion (ROM) than the thoracic spine, including flexion, extension, lateral flexion, and rotation.
SPINAL JOINTS
Discovertebral Joints
Each vertebral end plate is coated with a layer of hyaline articular cartilage (cartilaginous end plate). Adjacent vertebrae are united by a fibrocartilaginous intervertebral disk. Concentric, crossing layers of tough fibrous tissue, the annulus fibrosus, make up the outer circumference of the disk, enclosing a central, shock-absorbing gelatinous core, the nucleus pulposus (Figure 8-5). The intervertebral disks account for about one fourth of the height of the vertebral column above the sacrum. The sacrococcygeal vertebrae are fused and have no intervertebral disks. The disks are thickest in the lumbar spine and thinnest in the thoracic spine, and the cervical disks are intermediate in size. The intervertebral disks distribute the weight over the surface of the vertebral body and act as shock absorbers during loading, converting vertical load into horizontal thrust, which is absorbed by the elastic mechanism of the annulus. The disks provide a strong tie between the vertebrae yet allow a greater range of spinal movements compared with a solid column.
LIGAMENTS
Numerous ligaments stabilize the anterior and posterior elements of the spinal column. The anterior longitudinal ligament is a strong, broad fibrous band that runs from the occiput to the sacrum, where it anchors the anterior vertebral surfaces and intervertebral disks (Figure 8-6); it prevents excessive extension of the spine. The posterior longitudinal ligament also runs the length of the spinal column. It is a weak and narrow band but broadens where it attaches to the posterior intervertebral disks. Multiple ligaments also stabilize the posterior elements. The ligamentum flavum interconnects the vertebral laminae at the posterior roof of the spinal canal, and the weak interspinous ligaments and the stronger supraspinous ligaments interconnect the spinous processes. The latter two sets of ligaments partially limit forward and lateral flexion of the spine (see Figure 8-6). The intertransverse ligaments interconnect the transverse processes.
MUSCLES
A number of muscles span multiple spinal segments and provide both mobility and considerable stability to the spine. The cervical and lumbosacral spines are endowed with muscles anteriorly, laterally, and posteriorly, whereas the thoracic spinal muscles are exclusively posterior. Abdominal, trunk, and limb muscles assist the intrinsic muscles of the spine in achieving a wide range of movements.
SURFACE ANATOMY
Several landmarks can facilitate localizing specific spinal segments. The T2 vertebra is at the level of the top of the manubrium sterni. The apex of the spine of the scapula is even with the spine of T3, and the lower pole of the scapula is in line with the spine of T8. Although the position of the umbilicus varies with age, sex, obesity, and posture, it is generally on the same plane as the bottom of L3. A line connecting the upper border of the iliac crests crosses the midline at the level of L4, and a line drawn between the posterior superior iliac spines—beneath the sacral dimples, or “dimples of Venus”—crosses the second sacral segment.
History and Physical Examination
FOCUS OF INITIAL HISTORY IS CRITICAL
It is crucial to focus the initial history on the one problem that accounts for more than 80% of neck and back symptoms: mechanical, spine-predominant (neck or low back) pain (Figure 8-7). Mechanical pain can be defined as symptoms arising from the irritation of a physical element or elements within the spine, predictably aggravated and relieved by specific movements and positions. It is the result of an anatomic malfunction unrelated to infection, neoplastic disease, systemic illness, or major trauma. An additional 10% of patients may present with symptoms of nerve root irritation with neurological, extremity-predominant radicular (arm or leg) pain.
History
NECESSARY INFORMATION
Taking a history of spinal pain should be directed yet comprehensive. Appropriate evaluation requires a careful delineation of pain characteristics and associated features. A helpful mnemonic for characterizing spinal pain is OPQRSTU: O = onset; P = precipitating/ameliorating factors or prior episodes/treatment; Q = quality; R = radiation; S = severity; T = timing; and U = urinary or upper motor neuron symptoms.
Physical Examination
CERVICAL SPINE (Table 8-1)
Basic Examination |
Observation |
Observe posture, movement, and behaviors |
Inspection |
Note resting posture and alignment, both sitting and standing |
Inspect skin anteriorly and posteriorly |
Palpation |
Palpate occiput and spinous processes |
Check for fibromyalgia tender points (suboccipital muscle insertions, medial upper border of trapezius, supraspinatus, and medial scapular borders) |
Range of Motion |
Cervical spine flexion, extension, right and left rotation, and right and left lateral flexion |
Special Testing: Suspected Shoulder Pathology (Neck and Proximal Arm Pain) |
Examination of shoulders |
Special Testing: Suspected Nerve Root Compression |
Reflexes: biceps (C5), brachioradialis (C6), and triceps (C7) |
Muscle strength: deltoid, resisted shoulder abduction (C5); biceps, resisted elbow flexion (C6); triceps, resisted elbow extension (C7); interosseous, resisted finger abduction (C8) |
Sensation: over lateral deltoid (C5), at thumb and index finger (C6), at middle finger (C7), and at ring and little fingers (C8) |
Spurling sign: reproduction of radicular pain by applying gentle, firm pressure to occiput during combined rotation and extension to the affected side |
Abduction relief sign: relief of radicular pain with placing distal forearm/wrist of affected upper extremity on occiput |
Special Testing: Suspected Myelopathy |
Hoffman sign: flick tip of middle finger; note involuntary flexion of thumb and index finger together |
Knee and ankle reflexes and ankle clonus |
Babinski sign |
Gait: note broad base or unsteadiness, check |
Neurologic Examination
Irritative tests. Because of the mobility and multiple branches of the brachial plexus, the validity of irritative tests in the cervical spine to identify nerve root irritation is less certain than for those in the lower back. Rotating the head toward the painful side while forcing the neck into extension (Spurling maneuver, Figure 8-8) may reproduce the patient’s described arm pain, but a negative test does not rule out direct root involvement. Extending, abducting, and externally rotating the arm while extending the wrist and tilting the head to the contralateral side may also reproduce radicular symptoms. The intervening brachial plexus significantly diminishes the test’s sensitivity. Patients who experience a reduction in their arm pain by sitting with the hand on the affected side on top of their head (abduction relief sign, Figure 8-9