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 vertebrae have important common features: an anterior, weight-bearing element, called the vertebral body, and the posterior elements, including the neural arch and facet joints. The vertebrae and vertebral bodies increase progressively in size from C2 to S1 and decrease from S2 to the fourth coccygeal vertebra.
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 thoracic vertebrae have unique, long, posterolaterally directed transverse processes. A facet near the end of each transverse process articulates with the neck of the corresponding rib (costotransverse joints) , and notches in the posterolateral aspect of adjacent vertebrae form articulations for the head of each rib (costovertebral joints) . Movements of the thoracic spine are limited by the buttressing effects of the rib cage, the small size of the intervertebral disks, and the frontal direction of the facet joints. The superior thoracic facet joints face upward, backward, and slightly laterally.
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.
The wedge-shaped sacrum provides the inferior anchor for the spinal column, where it articulates with the posterior bony pelvis at the sacroiliac (SI) joints on each side. The coccyx consists of four small, fused vertebrae at the inferior end of the spinal column.
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.
The facet (apophyseal) joints between the superior and inferior articular processes allow movement of the spine. The articulating facets are coated with hyaline articular cartilage, lined with synovium, and joined by a thin articular capsule.
The SI joints are irregular, narrow articulations that join the spinal column to the pelvis on each side and lend stability to the posterior pelvic circle. The anterior (ventral) and inferior part of each SI joint is a synovium-lined articulation, whereas the posterior (dorsal) and superior part is a fibrous joint supported by strong ligaments. There is little or no movement at the SI joints.
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.
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.
The vertebral column, a modified segmented rod, is stabilized by both intrinsic (static) and extrinsic (dynamic) factors. Intrinsic (static) stabilizers include the intervertebral disks, the capsule of the facet joints, and various ligaments, particularly the anterior longitudinal and supraspinous ligaments and the ligamentum flavum. Extrinsic (dynamic) stabilizers include the paraspinal muscles (erector spinae, trunk, and abdominal muscles). Contraction of the trunk and abdominal muscles produces a rigid-walled cylinder in front of the spine. This transmits to the pelvis part of the forces generated on loading the spine and also acts as a lever that assists in reducing the load on the spine. The center of gravity falls either through or 1 cm anterior to the L4 vertebra. Shock absorption and vertical loads on the spine are attenuated by the balanced spinal curves, intervertebral disks, and paravertebral and trunk muscles.
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
An accurate and focused history is the essential first step in the diagnosis of patients with spinal pain. The information obtained from the history, which should include an assessment of possible risk factors for potentially serious underlying pathology, directs the subsequent physical examination and decision making regarding the need for additional tests and the choice of therapeutic measures to reduce the patient’s pain and restore function.
Categorizing patients with spinal pain into one of three broad groups can be particularly useful both diagnostically and therapeutically. The groups comprise the three kinds of pain:
Mechanical, spine-predominant (neck or low back) pain
Neurological, extremity-predominant radicular (arm or leg) pain
Spinal pain associated with another specific etiology
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 .
The overwhelming clinical probability that there is a purely structural pain generator involved in the patient’s neck or back pain makes it logical to begin the history by attempting to recognize a clearly defined mechanical pattern. Although spinal pain can arise from a myriad sources, the patient’s presenting symptoms are critical to understanding the clinical problem and should be obtained in a systematic and consistent manner.
ESSENTIAL INITIAL QUESTIONS
“Where exactly is your pain the worst?” The interrogation is precise, because the patient’s response must separate axial pain, along the spine, from pain primarily in the extremities. Although the pain may be referred into the arms or legs, mechanical pain usually remains most intense around the axial skeleton. In contrast, pain felt more strongly in the limbs than in the neck or back is usually radicular, representing direct nerve root irritation and typically traveling along the course of the involved roots.
Making identification even more problematic is the fact that mechanical neck or back pain is not necessarily felt mainly in the neck or back. Referred pain that originates in the neck can be most intense along the trapezius ridge; the interscapular region; at the occiput, sometimes with headache extending to the retroorbital area; and along the jaw line or the anterior chest (cervical angina). Referred pain that originates in the back may be perceived as most excruciating in the buttocks, groin, flanks, or trochanters.
“Is your pain constant or intermittent?” Over time, individual episodes of pain can blend into an unbroken, painful continuum. Some patients may be reluctant to admit that their pain is intermittent for fear of minimizing its importance, so how this question is framed is critical. The most effective way to introduce the question is to set the parameters before the patient is given the opportunity to respond. Acknowledge that you understand that the pain is severe and likely to recur before asking if there is ever a time when it is gone. Inquire specifically about such things as “the best time of day” or “the activity most likely to give you relief.”
“When your pain goes away, does it disappear completely?” The patient’s description of intermittent pain should be verified with this follow-up question, because patients may qualify their first response by saying that although the pain is much reduced, they are never totally pain free. Uncomplicated mechanical pain is typically intermittent. Without periods of total freedom from pain, decreased symptoms do not qualify as intermittent pain; the symptoms must be regarded as constant.
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.
Information about the onset, quality, and radiation of the symptoms may offer clues regarding the source of the pain. Identifying activities that heighten or diminish the pain may offer insight into the nature or location of a structural problem and may point to a possible treatment strategy. Because neck and back pain are frequently recurrent complaints, much can be learned from a detailed account of any previous episodes. These may forecast the characteristics of the present attack. Ascertain the effect of prior treatment. If the current situation is similar to the last one, what worked before may well work again. The reported intensity of the pain is typically of little diagnostic significance but does give a measure of the patient’s anguish and can serve as a baseline against which to measure future progress. Inquiries concerning changes in bowel or bladder function are mandatory with all patients. The history may uncover alterations in neurological status that must be investigated in the physical examination.
Finally, the history should establish the patient’s level of disability. The question, “Because of your pain, what can’t you do now that you were able to do before?” can elicit the degree of functional limitation. The intensity of treatment, indeed the need to treat at all, depends upon the answer.
NOTE: The lack of a defined mechanical response combined with a complaint of constant pain shifts the thrust of the questioning toward other nonstructural, potentially more serious etiologies. Questions should be directed at features associated with malignancy, infection, underlying visceral or systemic disease, psychosocial distress, or major injury. A group of important risk factors must be reviewed (see History and Physical Examination: Nonmechanical and Nonradicular Spinal Pain).
The physical examination of the neck or back should not be an isolated exercise. Its focus and, to some extent, its composition are determined by the history. Its principal purpose is to confirm or refute the hypotheses generated by listening to the patient’s story.
Observation: The patient’s general appearance, posture, and level of distress should be noted at the start of the interview. Watching the patient’s gait, ease of movement, ability to sit without obvious discomfort, and facility in shifting from standing to sitting to a recumbent position provides valuable information.
With the patient suitably draped and the back exposed, inspect the skin over the spine for anatomic abnormalities, discoloration, superficial masses, and scars. Deformity or asymmetries can be recorded but should be considered significant only if they are unequivocal. Subtle alterations in alignment are rarely clinically important.
Palpation: Attempting to feel the bony prominences of the spine through the overlying skin, fat, fascia, and muscle is of limited usefulness. Depending on the size of the patient, it may be possible to approximate the locations of some of the vertebral spinous processes, particularly over the thoracic spine, but no other deep structures can be reliably identified. In the resting neutral position, the spinous processes of C7 and/or T1 are readily palpable in the midline at the base of the neck. Rarely in the lower lumbar spine it is possible to detect a step defect due to a high-grade spondylolisthesis (forward slip of one vertebra over the vertebra below), or an absent spinous process (spina bifida). Palpation can elicit areas of increased temperature, swelling, tenderness, or regions of painful, localized muscle tension. The relevance of these findings is usually determined by the history.
Movement: The normal range of spinal movement is a function of age, sex, body mass, and physical condition. Precise measurement is difficult, and wide variations in accepted normal values make the effort of little practical value. Of far greater clinical significance are the rhythm and symmetry of movement and the reproduction of the patient’s usual pain as reported in the history. Uninhibited spinal movement is a smooth segmental progression; interference produces a consistent, unilateral shift or a block to flexion or extension that forces compensatory movements in the adjacent spine or large joints. Rotation and side bending are coupled movements (one cannot be carried out without the other) and should be about equal on each side. Ask every patient who exhibits a reduction in range of movement or an alteration in spinal rhythm whether attempting to move reproduces their typical pain.
CERVICAL SPINE ( Table 8-1 )
Inspection: Assess the position of the head over the shoulders. Most patients with chronic neck complaints have a head-forward posture that is accentuated in sitting and noticeably reduced when the patient stands. In its middle position, the cervical spine should be in lordosis.
|Observe posture, movement, and behaviors|
|Note resting posture and alignment, both sitting and standing|
|Inspect skin anteriorly and posteriorly|
|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|
|Gait: note broad base or unsteadiness, check|
An assessment of the shoulder should be part of every complete neck examination. Neck posture has a significant effect on the range of shoulder movement.
Movement: Assess cervical flexion, extension, rotation, and lateral bending, and ask the patient about any reproduction of the typical pain during movement. Assess shoulder range of motion.
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 ) may be diminishing nerve root irritation by reducing tension on the lower plexus or lower cervical roots.
Conductive tests. Motor testing is the most reliable physical measure of nerve conduction. In order, the most commonly involved nerve roots in the cervical spine are C6, C7, and C5. The examiner should gauge the strength of elbow flexion (C6, biceps), forward elevation of the arm (C6, anterior deltoid), elbow extension (C7, triceps), finger extension at the MCP joints (C7, extensor digitorum), and abduction of the arm (C5, central deltoid).
Reflex changes are identified by side-to-side comparison. The arms should be relaxed and supported. The biceps reflex is C5, C6; the brachioradialis reflex is C6; the triceps reflex is C7. Sensory testing is the least reliable investigation. The C6 dermatome includes the thumb, C7 covers the index and middle fingers, and C5 is best tested over the lateral deltoid.
Upper motor tests. Involvement of the spinal cord can produce upper motor findings in both the upper and lower extremities. In the cervical myelopathic patient, flicking the tip of the middle finger will cause a sudden involuntary flexion of the thumb and index finger (Hoffman sign; analogous to, but not nearly as specific as, the Babinski sign in the lower extremities). In the lower limbs, cervical cord compromise can produce hyperactive reflexes, sustained clonus, and an extensor plantar response with an up-going great toe and spreading of the other digits (Babinski sign).
Seen from behind, the thoracic spine should be straight. In the adolescent patient, a lateral curvature, usually convex to the right, is suggestive of an idiopathic scoliosis. This is frequently associated with a rotational deformity that distorts the symmetry of the rib cage and is best visualized by viewing the thorax from behind with the patient flexed forward at the waist. The curvature may be fixed or flexible and can be assessed by observing any change in the curve with side bending.
Seen from the side, the thoracic spine should exhibit a gentle kyphosis that balances the lordotic curves in the cervical and lumbar spines. Excessive kyphosis over the mid-thoracic spine or short segment angulation (gibbus) may indicate a developmental abnormality or old trauma with vertebral collapse.
Upper motor tests. The only possible neurological impairment that requires routine examination in the thoracic spine is spinal cord impingement. Individual root irritation produces focal pain around the chest that can be recognized on history but for which there is no corresponding physical test. Patients with suspected pathology in the thoracic spine should have their legs examined for increased reflex activity, clonus, and an abnormal plantar response.
LUMBAR SPINE (Table 8-2)
When standing erect, the lumbar spine should be lordotic. The amount of lordosis is reduced with age, in structural abnormalities such as spondylolisthesis, and in ankylosing spondylitis. In the young, fit patient, the lumbar paraspinal muscles are easily visible as well-defined ridges running along the spine. Muscle wasting is common with advancing age and with reduced activity.