The cervical alignment parameters discussed in the earlier text are critical during the evaluation and surgical planning for cervical deformity correction. Therefore, CBVA, T1 slope, C2 SVA and regional CL should all be considered in preoperative planning strategies, and consideration should be given to obtaining preoperative 3-ft standing radiographs that provide visualization from the external auditory canal (approximation of the center of mass of the head) to the femoral heads.
Cervical kyphosis is the most prevalent cervical spinal deformity and may develop secondary to multilevel laminectomies, advanced degenerative disease, systemic arthritides, trauma, and neoplastic etiologies.41–46 However, cervical sagittal alignment (e.g., C2 SVA) is closely related to the cervical sagittal Cobb angle (e.g., C2–C7 Cobb angle) as described in the earlier text; however, cervical sagittal alignment also takes into account the alignment of subjacent segments, including the thoracolumbar spine and pelvis (Figure 7A–C). Cervical spine deformities affect, and are affected by, other parameters of the spine in preserving global sagittal alignment. Sagittal alignment factors into maintenance of posture, and patients with poor sagittal alignment often develop potentially painful compensatory mechanisms that affect the cervical spine, including hyperlordosis of subaxial segments.47–51
The current literature reports changes in radiographical parameters of lordosis and kyphosis after surgical correction,52–56 but there lacks a clear indication of an optimal amount postoperative CL to be achieved. It has become an accepted general rule to correct cervical kyphosis to be as close to neutral as possible.43 Current research is adopting a trend toward defining cervical sagittal alignment parameters similar to the accepted C7 SVA (Figure 2)57 traditionally used to measure sagittal alignment of the thoracolumbar spine. Specifically, as noted in the earlier text, the C2 plumb line and CBVA are increasingly being used.25,26,35,58,59 To evaluate the effect of cervical alignment properly in relation to the overall sagittal alignment of the spine, standing 3-ft spine radiographs are needed.
Few studies, report the relationship between radiographical parameters in the cervical spine and HRQOL.25 The effects of the cervical radiographical measurements on outcome scores are not nearly as well-defined as global and pelvic parameters are in thoracolumbar deformity.60–63 The majority of the literature focuses on regional measurements of kyphosis. A common finding throughout the studies is the increase in neck pain in patients with greater kyphosis, whether after cervical spine trauma64 or operative procedures such as anterior cervical spine fusion65 or single-level anterior cervical discectomy and fusion.66 Naderi et al67 concluded that the presence of abnormal cervical curvature predicts less postoperative neurological improvement.
More recent studies of cervical alignment parameters, mostly lordosis between C2 and C7, in relation to postoperative clinical outcomes are weak in suggesting significant correlations. A double-blind, randomized controlled trial evaluating the relationship between lordotic alignment, both cervical and segmental, and clinical outcomes using normal and lordotically-shaped allografts for anterior cervical discectomy and fusion was conducted by Villavicencio et al.68 They found that improved cervical Cobb angle alignment did not correlate significantly with clinical outcomes, but that maintaining or improving segmental sagittal alignment had significant implications for a higher degree of improvement in outcome scores. Guerin et al69 also noted that only segmental sagittal alignment correlated with clinical outcomes after cervical disc arthroplasty, whereas overall cervical lordotic alignment did not. Jagannathan et al70 found no significant relationship between the change in segmental kyphosis and postoperative functional status. CBVA is recognized to be the most objective measure of horizontal gaze; however, it had no significant correlation to overall clinical outcome (Modified Arthritis Impact Measurement Scale) in cervical kyphosis in patients with ankylosing spondylitis.26 Despite this result, it has proven to be a very reliable and useful tool in assessing pre- and postoperative horizontal gaze, and correction of the CBVA does lead to positive postoperative outcomes regarding patient's satisfaction of horizontal gaze improvement.26–31
These studies have primarily focused on lordosis between C2 and C7 and not the aforementioned SVA parameters, with the exception of one study.25 When assessing thoracolumbar deformity, SVA values are standard measurements. It is well known that both Glassman et al71 and Mac-Thiong et al21 concluded that positive sagittal malalignment, defined as a C7 plumb line greater than 50 mm anterior to the posterior-superior aspect of the sacrum, is associated with a deterioration of quality of life in patients with adult spinal deformity. Mac-Thiong et al21 even extended this association to involve the global balance as defined by the gravity line. Currently, only one study has broadened these observations between SVA measurements and HRQOL scores (Neck Disability Index and SF-36 PCS) to include the cervical spine.25 It suggests that increasing C2 SVA is a cause for clinical concern of cervical malalignment indicated by poor HRQOL scores and C2 SVA more than 40 mm is correlated to worse outcomes assessed by the NDI.25 In addition, the authors found significant correlations between T1 slope and C2–C7 lordosis, T1 slope and C2–C7 SVA, as well as C2–C7 SVA and the difference between T1 slope and C2–C7 lordosis (T1 slope − C2–C7 lordosis); Table 5.
Despite the progressive findings linking radiographical parameters and clinical outcomes, study limitations still exist because most are retrospective analyses. Furthermore, the contribution of the overall improvement in postoperative status that may be attributed to spinal cord decompression in many of these procedures is especially overlooked in many of these studies. There is a clear need for future prospective studies to further isolate the effect of cervical alignment on outcome measures and eliminate confounding variables. Analysis of the cervical regional alignment with respect to overall spinal pelvic alignment will be critical as well as evaluation of inter- and intrarater reliability for the cervical parameters.
Cervical spondylotic myelopathy (CSM) has been reported as the most common cause of spinal cord dysfunction in patients older than 55 years.73 Traditionally, the etiology of CSM has been described as a result of multilevel spondylosis in which osteophyte formation occurs as a result of degenerative changes in the discs.74,75 However, less attention has been made to the fact that progressive cervical kyphosis has also been associated with myelopathy. The mechanism behind the development of the myelopathy is a result of the kyphosis forcing the spinal cord against the vertebral bodies inducing anterior cord pathology as well as increasing the longitudinal cord tension due to the cord being tethered by the dentate ligaments and cervical nerve roots41,44 (Figure 8A–C). Over time, as the curve becomes more pronounced, the anterior and posterior margins of the cord compress and the lateral margins expand.5 Cord tethering has been shown to cause an increase in intramedullary pressure leading to neuronal loss and demyelination.1–3,5 Furthermore, the small feeder blood vessels on the cord become flattened, leading to reduced blood supply.5 As the curve magnitude increases these pathological changes become more pronounced, especially on the anterior side that is directly exposed to the mechanical compression.5 It has been shown in animal models that greater cord tension increases intramedullary cord pressure1–4 and leads to neuronal apoptosis.5 Shimizu et al5 quantitatively analyzed that the severity of demyelination and neuronal loss in histological sections of spinal cords after induction of cervical kyphosis was small game fowls. They found a significant correlation between the degree of kyphosis and the amount of cord flattening.5 Analysis with angiography demonstrated that the vascular supply to the anterior portion of the cords was decreased.5 Furthermore, neuronal loss and atrophy of the anterior horn as well as demyelination of the anterior fasciculus was observed with the extent of demyelination progressing as the kyphosis became greater.5 The pattern of demyelination began with the anterior fasciculus, but then progressed to the lateral and posterior fasciculi.5 Thus, sagittal alignment of the cervical spine may play a large role in the development of cervical myelopathy.
Dynamic radiographical studies have found cervical cord changes upon flexion-extension, which may contribute to possible mechanisms of cervical myelopathy. Muhle et al76 measured the sagittal cord diameter in 40 healthy subjects and found that, upon flexion, the cord diameter significantly reduces compared with the neutral position. Yanase et al77 measured the cervical cord volume in healthy patients and found that the cord volume varies with sex, age, height, and body weight.77 However, when calculating a volume ratio (foramen magnum to inferior C2 volume to foramen magnum to inferior C7), there were no variations noted.77 When radiographically measuring cervical cord volumes in patients with cervical myelopathy as a result of cord atrophy, they recommend using the ratio, given it's free of individual variation. Yu et al11 investigated the relationship of high intensity lesions on T2-weighted MR images and dynamic changes in the cervical spine for patients with CSM. On a T2-weighted MR image, they found that segmental hyperextension and range of motion were risk factors for high intensity lesions.11 These results are similar to the results derived by Zhang et al,12 in which they also investigated flexion-extension MRI in patients with spondylotic CSM. They showed that the cervical cord is significantly longer in flexion than in the neutral or extension positions.12 The cord available space was found to be the greatest in the neutral position with it being the least in extension.12 Furthermore, patients were more likely to have cord impingement on extension than flexion.12 In patients with spondylotic cervical myelopathy, flexion may be a larger contributor to the etiology as cord impingement is exacerbated. However, in patients with primary cervical malalignment, in which flexion/kyphosis is the predominant position of the spine, the myelopathy may be due to cord lengthening, flattening, and vascular compromise. Further flexion-extension MRI studies in patients with primary cervical deformity are required to expand on this potential difference in etiology.
There exist a great deal of controversy surrounding the optimal surgical approach to correct cervical myelopathy.73 Surgical considerations and options for cervical myelopathy must take into account the sagittal alignment of the cervical spine as it affects the approach as well as myelopathy etiology and progression. Decompression alone, even ventral decompression, which does not decrease cord tension induced by kyphosis may therefore not result in optimal outcomes.5 Cervical myelopathy correction without sagittal malalignment may develop postlaminectomy kyphosis, the most common etiology of cervical spinal deformity,41,44,78 and should be considered preoperatively. The posterior neural arch is responsible for the majority of the load transmission through the cervical spine as the natural biomechanics of the spine rely on a lordotic curve to distribute most of the load posteriorly. Removal of it causes a significant loss of stability. However, the spine may not become destabilized initially. Over time, the added instability with losing the posterior arch-facet complex tends to cause a shift in load bearing from the posterior column to the anterior column. This shift leads to cervical kyphosis as the discs and vertebral bodies become wedged with greater sagittal malalignment during the course of months to years. Cervical myelopathy may develop from the change in sagittal alignment to a kyphotic predominance and draping of the cord as discussed in the earlier text. In the end, the postlaminectomy kyphosis created a worsened myelopathy from a surgical treatment that was intended to treat myelopathy. With the recent work of Kim et al72 specifically investigating postlaminoplasty, the preoperative T1 slope should be measured as the likelihood of developing kyphotic changes at 2 years increases with a higher preoperative T1 slope.
Posterior approaches alone may not be sufficient to correct cervical lordotic alignment in the subaxial spine above C7. Reconstruction using lordotic interbody spacers through an anterior approach may be needed to restore the natural lordotic curve of the cervical spine. If posterior decompression alone is undertaken or the cervical spine is fused in a kyphotic position, future myelopathy may develop because of the reasons discussed in the earlier text. Studies have shown that patients who underwent 1- or 2-level corpectomies for CSM had maintenance of CL and positive long-term HRQOL (modified Japanese Orthopaedic Association myelopathy scale) outcomes.79
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