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Normal Cervical Lordosis
To understand cervical spine alignment and deformity treatment properly, several basic concepts must be understood:
- 1.
The significant mass of the head is supported by the cervical spine, and significant deviation from normal alignment increases cantilever loads and muscular activity.
- 2.
The flexible, mobile cervical segment is connected to the relatively fixed thoracic spine.
- 3.
The T1 inclination determines the amount of subaxial lordosis required to maintain the center of gravity of the head in a balanced position.
- 4.
The T1 inclination varies depending on global spinal alignment as measured by the sagittal vertical axis (SVA) and by inherent upper thoracic kyphosis.
- 5.
The radiographic parameters that affect health-related quality of life scores are not well defined compared with global and pelvic parameters in thoracolumbar deformity. Chin-brow to vertical angle (CBVA), cervical SVA (C2 SVA), and regional cervical lordosis should all be considered in preoperative planning strategies involving standing 36-inch radiographs in which the external auditory canal (approximation of head center of mass) to the femoral head is visible.
In asymptomatic normal volunteers, cervical standing lordosis is greatest at C1 to C2, and little lordosis exists in the lower cervical levels ( Table 15-1 ). Approximately 75% of total cervical lordosis is taken at C1 to C2. Mean total cervical lordosis is approximately −40 degrees, with, on average, the occiput-C1 segment being kyphotic ( Fig. 15-1 ). Only 6 degrees (15%) occurs at the lowest three cervical levels (C4 to C7) (see Table 15-1 ). Furthermore, no difference is noted between asymptomatic men and women in total cervical lordosis, and a positive correlation exists with cervical lordosis and increasing age. The average odontoid-C7 plumb line distance ranges from 15 to 17 ± 11.2 mm (see Table 15-1 ).
| Segmental Cervical Angles | C2-C7 Lordosis | |||
|---|---|---|---|---|
| Level | Angle (Degree) | Age Group (yr) | Men (Degree) | Women (Degree) |
| C0-C1 | 2.1 ± 5.0 | 20-25 | 16 ± 16 | 15 ± 10 |
| C1-C2 | −32.2 ± 7.0 | 30-35 | 21 ± 14 | 16 ± 16 |
| C2-C3 | −1.9 ± 5.2 | 40-45 | 27 ± 14 | 23 ± 17 |
| C3-C4 | −1.5 ± 5.0 | 50-55 | 22 ± 15 | 25 ± 11 |
| C4-C5 | −0.6 ± 4.4 | 60-65 | 22 ± 13 | 25 ± 16 |
| C5-C6 | −1.1 ± 5.1 | |||
| C6-C7 | −4.5 ± 4.3 | |||
| C2-C7 | −9.6 | |||
| Total (C1-C7) | −41.8 | |||
| Cervical Sagittal Vertical Axis | |
|---|---|
| Odontoid marker at C7 | 15.6 ± 11.2 mm |
| Odontoid marker at sacrum | 13.2 ± 29.5 mm |
∗ Values presented as the means ± SD and the negative sign indicates lordosis in the segmental values.
Anatomically and biomechanically, the cranium and cervical spine are placed over the thoracic inlet, a fixed bony circle that is composed of the T1 vertebral body, the first ribs on both sides, and the upper part of the sternum. The sagittal balance of the cranium and cervical spine may possibly be influenced by the shape and orientation of the thoracic inlet to obtain a balanced, upright posture and horizontal gaze, similar to the pelvic incidence in the pelvis. The authors have found linkage of significant correlation from the thoracic inlet angle to the cranial offset and craniocervical alignment. The ratio of the C0-C2 to C2-C7 angles to total cervical lordosis was 77%:23%, and the ratio of cervical to cranial tilting to T1 slope was 70%:30% in asymptomatic individuals.
Neck tilting was maintained at approximately 45 degrees to minimize energy expenditure of the neck muscles. These results indicate that a small thoracic inlet angle makes the small T1 slope to maintain physiologic neck tilting and makes the small cervical spine lordotic angle, and vice versa. According to the study, the thoracic inlet angle and the T1 slope may be used as parameters to evaluate sagittal balance, predict physiologic alignment, and guide deformity correction of the cervical spine.
Pathophysiology (Epidemiology, Pathophysiology, Natural History, and Differential Diagnoses)
Cervical spine lordosis is required for sagittal plane alignment because it counteracts thoracic kyphosis, aids in maintaining a neutral global SVA, and keeps the center of gravity of the head over the spine. Any disruption of cervical lordosis leads to a loss of horizontal gaze and severe mechanical neck pain, which can be very debilitating both physically and mentally to the patient.
Deformities of the cervical spine present many challenges to the surgeon, one of which is determining the ideal treatment option. The most common type of cervical spine deformity occurs in the sagittal plane as a kyphotic deformity, whereas malalignment in the coronal plane is much less common. Furthermore, the most common type of cervical kyphotic deformity is iatrogenic, specifically after multiple-level laminectomy, with an incidence of 20%. The primary goals of the various treatment options are to restore cervical sagittal alignment and thus improve horizontal gaze, reduce neck pain, and, if the deformity is severe enough, improve swallowing and respiration. This chapter focuses on the different causes of cervical deformity and presents various treatment options for the proper management of these debilitating conditions.
Etiologic Factors
The causes of common cervical spine deformities may be characterized in terms of four broad categories: primary, inflammatory, degenerative, and iatrogenic. Primary deformities of the cervical spine include congenital scoliosis, skeletal dysplasias, and neurofibromatosis. The exact origin of congenital scoliosis remains unclear; however, many factors have been implicated, such as genetics, drugs, chemicals, vitamin deficiency, and environmental factors. These factors cause physiologic injury during the early embryonic period before the development of cartilage and bone. This injury leads either to failure of the vertebrae to form completely or to failure of segmentation (nonfusions of phenotypically normal-appearing vertebrae). These malformations in the vertebrae result in curvature of the spine and may continue to be progressive during the growth of the child.
Cervical spinal deformities caused by systemic inflammatory arthritic conditions tend to be a result of rheumatoid arthritis (RA) or ankylosing spondylitis. RA is a common autoimmune disorder; approximately 2 million people in the United States are affected, and it is the most common inflammatory disorder of the cervical spine. The current theory of the etiology of RA is an immune response against synovial cells. The immune response causes destruction of cartilage, ligaments, tendons, and bone, and it leads to ligamentous laxity and bone erosion. The upper cervical spine is the most commonly affected because the occipital-C1 and C1-C2 joints are primarily synovial. Three types of deformities result: atlantoaxial instability or subluxation; superior migration of the odontoid; and subaxial subluxation, usually at multiple levels. Atlantoaxial instability is caused by erosion of the C1-C2 joint and may be classified as reducible, partially reducible, or fixed; all classes of instability affect treatment. The superior migration of the odontoid is also the result of erosion of the C1-C2 joint and the occipital-C1 joint that causes vertical height reduction between the brainstem and C2. The multiple-level subaxial subluxations all combine to produce significant cervical kyphosis, which may need surgical correction.
The pathophysiology of ankylosing spondylitis (AS) is similar to that of RA in that AS is a chronic inflammatory disease; however, AS is characterized by ossification of the joints and ligaments and typically affects the axial spine. The initial onset is localized to the sacroiliac joint, from which it progresses superiorly to the lumbar spine and then to the cervical spine. Inflammation of the annulus fibrosus causes squaring of the vertebral bodies and formation of bridging osteophytes. Furthermore, inflammation of the apophysial joints with ossification of the adjacent ligaments leads to complete fusion of the spinal column. During this process, lumbar and cervical lordosis is lost, and significant cervical kyphosis may develop as the disease progresses. Eventually, the patient may develop a chin-on-chest deformity with a severe loss of horizontal gaze. Despite the increased bony fusion and appearance on radiographs, the spine may be osteoporotic as a result of stress shielding, and this possibility must be considered during surgical planning.
Cervical spondylosis is a form of degenerative etiology resulting in cervical deformity. Three main symptomatic complexes associated with cervical spondylosis are neck pain, radiculopathy, and myelopathy. The primary pathogenesis of cervical spondylosis leading to cervical deformity lies within intervertebral disk desiccation. This process leads to biochemical changes within the disk that cause the nucleus pulposus to lose elasticity and to become smaller and more fibrous. This change shifts the primary weight-bearing mechanism to the annulus fibrosus, causes it to bulge posteriorly into the spinal canal, and thus reduces intervertebral disk height. The initial loss of height occurs anteriorly, which ultimately produces a positive feedback loop of increased anterior weight bearing leading to cervical kyphotic deformity. Changes in cervical spine biomechanics cause the peripheral fibers of the annulus fibrosus and Sharpey fibers to be dissected away from the vertebral body edges and the posterior longitudinal ligament to buckle and peel off the vertebral bodies. The annular disk herniation, ligamentous laxity, and degenerative changes all lead to progressive cervical kyphosis, abnormal cervical spine movement, and neck pain.
The most common cause of cervical spine deformity is iatrogenic, of which the most common type is postlaminectomy kyphosis. The natural biomechanics of the spine relies on a lordotic curvature in which the posterior columns withstand approximately 65% of the load and the anterior columns 35%. Thus, the posterior neural arch is responsible for most of the load transmission down the cervical spine, and removal of this structure causes a significant loss of stability. Initially, performing extensive multiple-level laminectomies does not immediately destabilize an intact spine. However, the added instability with losing the posterior arch–facet complex causes a shift in load bearing from the posterior column to the anterior column. Over time, this shift places added stress on the cervical musculature that requires constant contraction to maintain an upright head posture. This results in fatigue and pain. Cervical kyphosis occurs as the load is shifted anteriorly, and as the disks and vertebral bodies become wedged, it progresses to greater imbalance ( Fig. 15-2 , A ).
