Cervical Spinal Deformity

Etiology

Most cervical kyphotic deformity is iatrogenic. ^, Surgical disruption of the stabilizing posterior elements leads to loss of the posterior tension band, which has a destabilizing effect on the spine as the force vectors are transferred to the anterior vertebral bodies. One of the most important risk factors is the presence of preoperative kyphosis. ^, Although spinal alignment may be initially maintained by the posterior musculature, fatigue over time may cause the transfer of stress forces anteriorly, which leads to a cervical kyphosis. ^, The reported incidence of postlaminectomy kyphosis ranges from 21% to 53% and may approach 95% when coupled with a suboccipital decompression. ^{, ,} Postlaminectomy kyphosis deformity is less likely when less than 50% of the facet is resected and when the facet capsule is left intact. ^,

Progressive degenerative changes are another significant cause of kyphosis. As individuals age, the disc spaces and vertebral bodies may erode and are no longer able to tolerate the weight-bearing and translation load they once did, which leads to vertebral body flattening and disc space narrowing. ^, Osteophytes form to increase the surface area of the vertebral body endplates to compensate for the decreased load-bearing capacity of the cervical spine. This, along with the increased weight load now transferred to the facets and ligamentous complexes, can result in hypertrophy calcification and ossification, all of which can result in narrowing of the spinal canal with eventual spinal cord compression. These degenerative cervical processes ultimately result in myelopathy.

Ankylosing spondylitis is a unique cause of kyphotic cervical deformity, because it is associated with the HLA-B27 antigen. Ankylosing spondylitis is an autoimmune disorder characterized by chronic lymphocytic infiltration of the joints of the axial skeleton. The anterior and posterior segments of the entire spine can be affected, with inflammation causing erosion and fusion of the disk spaces, uncovertebral joints, and facets. The kyphotic deformity is a result of progressive flexion of the spine, secondary to a compensatory stance adopted by patients to relieve stress on painful facet joints. Although the cervical spine is often the last part of the spine to be involved in this disease process, the erosion and inflammation can become so severe that patients may present with chin-on-chest deformity.

Additional causes of deformities include disease processes that induce structural abnormalities that prevent the cervical spine from withstanding normal compressive forces. These conditions include Klippel-Feil syndrome, Goldenhar syndrome, Down syndrome, Morquio syndrome, skeletal dysplasias, neurofibromatosis 1, ^, osteogenesis imperfecta, and other type I collagen disorders. Congenital scoliosis also may affect kyphosis of the cervical spine in addition to causing coronal plane abnormalities, including hemivertebrae or absent pedicles. In addition, cervical deformity can develop secondary to trauma, infection, or oncologic disease.

Radiographic Cervical Alignment

An understanding of cervical alignment begins by evaluating the forces that act upon the cervical spine. The center of gravity of the head is found 1 cm anterior to the external auditory canal. The head transmits its weight from the occipital condyles to the lateral masses of C1 and then to the C1–C2 facets. Louis et al. used a three-column system to describe the cervical spine, in which one anterior column consists of the vertebral body and intervertebral disc, and two posterior columns are made up of the two facet joints. In this model, 34% of the axial weight-bearing load is transferred to the anterior column while 64% is transferred to the posterior columns. Thus, the force vector is directed to the posterior elements, which helps to maintain cervical lordosis ( Fig. 171.1A–C ). Most individuals have an average cervical lordosis of −40 ± 9.7 degrees, with the most lordosis between C1 and C2 (an average of −32 ± 7.0 degrees). When the axial and translational forces that act on the cervical spine are in malalignment, cervical lordosis is lost and the anterior column begins to bear most of the weight of the head. Musculature fatigue and degeneration of the intervertebral disc space and vertebral body result in cervical kyphosis.

Similar to sagittal balance parameters in the lumbar spine, cervical sagittal radiographic parameters are correlated with pain and disability. Moreover, patients experience better clinical outcomes when such parameters are taken into account during surgery. ^{, ,} Cervical kyphosis is defined by a C2–C7 Cobb angle greater than 0 degrees. The amount of kyphosis found in cervical deformity can vary wildly; the greater the degree of kyphosis, the more severe the presenting symptoms. ^, Although most deformity of the adult cervical spine exists in the sagittal plane, if coronal imbalance occurs, cervical scoliosis is measured by the coronal C2–C7 Cobb angle of greater than 10 degrees in either direction. The cervical sagittal vertical axis (cSVA)—which measures the translation of the cervical spine in the sagittal plane—is defined by the measurement of the distance between the posterior-superior corner of C7 and the C2 plumb line (a vertical line drawn from the middle of the C2 vertebral body down). ^, This parameter measures how well the head is situated over the cervical spine, with malalignment defined as any cSVA greater than 4 mm.

Overall sagittal alignment is another key parameter for measuring the forces on the spine. The T1 slope is defined as the angle between the horizontal plane and a line drawn parallel to the superior end plate of the T1 vertebrae. The T1 slope can also be measured by the addition of the thoracic inlet angle and cervical tilt ( Fig. 171.2 ). ^, The thoracic inlet angle is the angle between lines drawn perpendicular to, and originating from, the center of the T1 endplate body and a line drawn from the center of the T1 endplate and the upper end of the sternum. ^, The thoracic inlet angle is a considered a fixed, nonmobile parameter because it is made up of the T1 vertebra with its associated ribs and articulation with the sternum and is unique to each individual. Because of its location at the cervicothoracic junction, the thoracic inlet angle is a transition point between the kyphotic thoracic spine and the beginning of the lordotic cervical spine. Cervical tilt is defined as the angle between two lines originating from the center of the superior endplate of T1, with one being perpendicular to the upper endplate and the other passing through the tip of the dens. Cervical tilt is a compensatory process by which the kyphosis of the thoracic spine is corrected in the cervical segment to bring the head to its upright position and keep it above the pelvis. T1 slope is therefore a measurement of overall sagittal alignment, which correlates with global sagittal alignment as measured by sagittal vertical alignment (SVA) measured from the superior posterior corner of the sacrum and the C2 plumb line (see Fig. 171.1D and E ). Cervical deformity can be measured by mismatch between the T1 slope and cervical lordosis. Mismatch between these two parameters shows an inability of the cervical spine to correct for thoracic kyphosis; normal T1 slope minus cervical lordosis (TS-CL) is defined as less than 15 degrees, with greater than 20 degrees being severe. ^,

One of the most significant parameters of cervical deformity is the chin-brow vertical angle (CBVA), measured between the vertical plane and a line drawn from the chin to the brow (see Fig. 171.1F ). ^, The CBVA is a measurement of horizontal gaze and is one of the most important parameters in cervical deformity because correction to normal strongly correlates with improved quality of life. The normal CBVA is between −10 and 10 degrees.

Although cervical parameters can be used to measure deformity isolated to the cervical spine, they should also be taken into context within the global spinal alignment, as each segment cannot be considered independent from one another. Studies have correlated cervical lordosis with thoracic kyphosis, lumbar lordosis, and pelvic incidence. The global alignment of the spine should not be neglected in deformity correction nor in the workup and initial evaluation of these patients. This global spinal approach has been mirrored in the classification system proposed for cervical spinal deformities, as demonstrated in Table 171.1 .

Presentation

Cervical deformity typically presents with myelopathy, radiculopathy, or mechanical pain and gross anatomical deformity. Knowledge of these three main types of presenting symptoms is essential for the evaluating physician to order appropriate imaging and make an accurate diagnosis. Myelopathy is a clinical entity that signals damage to the spinal cord whether in a chronic step-wise fashion or acutely. Myelopathy is represented by classic findings on a neurologic physical examination of hyperreflexia including positive Hoffman and Babinski signs, numbness in a glove-like distribution, gait imbalance, ataxia, increased urinary frequency, and weakness in the lower and upper extremities. ^, These findings are often subtle and can be missed by the less experienced practitioner. Radiculopathy is caused by obstruction of the nerve root as it exists the neural foramen, which causes compression of the nerve root resulting in a painful, electric shock–like sensation that travels in the dermatomal distribution of that nerve. Compression of the nerve can also cause weakness in a characteristic myotome. Radiculopathy is a common presentation of cervical deformity as the disease progresses, which create unbalanced forces on the cervical spine, lead to hypertrophy of the lateral and posterior elements such that the exiting root becomes entrapped in its bony foramen. Whereas myelopathy and radiculopathy are two of the most common presentations of cervical deformity, less frequently patients may present with solely mechanical pain from muscular strain or with mechanical deformities, such as chin-on-chest and chin-on-shoulder, which are described by their gross appearance. Often the pain or deformities are combined with presentations of myelopathy and radiculopathy.

Cervical Deformity Classification

The Ames adult cervical deformity (Ames-ACD) classification system is a comprehensive and validated system used to characterize cervical spinal deformity by location, type, and severity. ^, Each case is assigned a cervical deformity descriptor and five alignment modifiers (see Table 171.1 ). The cervical deformity modifiers are based on the apex of the coronal or sagittal deformity: C = apex in the cervical spine, T = apex in the thoracic spine, S = primary coronal deformity, CVJ = craniovertebral junction deformity. The five alignment modifiers are: cSVA; horizontal gaze (as measured by the CBVA); TS-CL; disability as measured by the modified Japanese Orthopedic Association score (mJOA), which is a measure of myelopathy and used to qualify quality of life ; and the Scoliosis Research Society (SRS)-Schwab adult spinal deformity classification (SRS-Schwab ASD). The SRS-Schwab ASD classification system was included in the Ames ACD classification system because it takes global spinal alignment into consideration when considering cervical deformity.

Each modifier has a degree of severity associated with it. cSVA is graded from 0 (<4 cm) to 2 (>8 cm). Horizontal graze is graded as 0 (CBVA = 1 to 10 degrees), 1 (CBVA = −10 to 0 degrees or 11 to 25 degrees), or 2 (CBVA = <−10 degrees or >25 degrees). TS-CL is graded from 0 (<15 degrees) to 2 (>20 degrees). Disability as measured by the mJOA has four grades from 0 (no disability) to 3 (severe disability). The SRS-Schwab classification modifier is subcategorized with the curve type as discussed above, and with modifiers for each sagittal alignment parameter as 0 (<10 degrees), + (10 to 20 degrees), or ++ (>20 degrees). SVA is also subdivided into gradations of 0 (<4 cm), + (4 to 9.5 cm), and ++ (>9.5 cm). Pelvic tilt is similarly divided as 0 = <20 degrees, + = 20 to 30 degrees, and ++ = >30 degrees. ^,

The main purpose of this classification system is to provide a standardized and common nomenclature to facilitate communication and research on patients with cervical spinal deformity. The system is designed to be comprehensive to include the wide variety of deformities that may be encountered. It also incorporates symptom severity and patient disability. ^,

Nonoperative Management

For patients with mild myelopathy, nonoperative treatment may be an option, but patients must weigh this against the possibility of neurological deterioration. A wide range of nonoperative modalities has been described, including physical therapy, nonsteroidal anti-inflammatory drugs, analgesics, muscle relaxants, neuromodulatory drugs, bracing, traction, and steroid injections; however, there are few data to suggest whether certain types of nonoperative treatment are better than observation alone. Patients should be cautious given the reports of neurological worsening after cervical manipulation, chiropractic treatment, and massage. ^, If symptoms show no change or worsen with nonsurgical treatment, surgical intervention should be considered.

For patients with moderate or severe myelopathy, outcomes are generally superior with timely surgical intervention. Patients with more severe disease who are treated nonoperatively tend to have declining ability to perform activities of daily living and worsening neurological symptoms over time. ^, Although most patients included in the surgical arm of these studies have severe disease, these patients tended to experience the most benefit after surgery.