MRI Is Inadequate to Assess Cervical Sagittal Alignment Parameters

Assessment of cervical sagittal alignment is important when treating patients with cervical spondylotic myelopathy, especially when posterior-based decompressive surgery is being considered.

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Upright lateral cervical radiographs are the standard modality used for that assessment, but positional variability and limited visibility of lower cervical vertebral bodies often hamper measurement. Since MRI is almost always used to characterize central spinal cord and nerve root compression, some surgeons and even payors have questioned the utility of additionally obtaining radiographs.

However, Massachusetts General Hospital researchers recently found some cervical sagittal measurements differ in upright radiographs compared with supine MR images in patients undergoing posterior cervical surgery for spondylotic cervical myelopathy.

Brian C. Goh, MD, PhD, a senior resident in the Harvard Combined Orthopaedic Residency Program (HCORP), Christopher M. Bono, MD, executive vice chair of the Department of Orthopaedic Surgery and director of HCORP, Stuart H. Hershman, MD, acting chief of the Orthopaedic Spine Service, and colleagues report in Clinical Spine Surgery.

Methods

The team identified 117 adults (56% male, average age: 66) who underwent laminectomy and fusion or laminoplasty between 2017 and 2019. They did not have constructs cranial to C2 or caudal to T2, and they underwent MRI within six months of radiography.

On both radiographs and MR images, two clinicians independently measured the:

• C2–C7 sagittal angle—The angle subtended from lines drawn along the posterior vertebral bodies of C2 and C7
• C2–C7 sagittal vertical axis (SVA)—The distance from a vertical plumb line from the center of C2 to the posterosuperior corner of the C7 vertebral body
• T1 tilt—On radiographs, the angle formed between the superior endplate of T1 and a horizontal reference; MR images were rotated counterclockwise 90° to appear upright and the measurement was obtained in the same way

Reliability

Intraobserver reliability (intraclass correlation, 0.86–0.98) and interobserver reliability (0.88–0.98) were excellent across all three parameters for both radiography and MRI.

Correlation

The mean imaging measurements were:

• C2–C7 angle—15.4° for radiography vs.16.0° for MRI (P=0.46; r=0.76)
• C2–C7 SVA—3.3 vs. 1.8 cm (P<0.01; r=0.48), respectively
• T1 tilt—34° vs. 28° (P<0.01; r=0.62), respectively

Guidance for Patient Care

These findings support the conclusions that, although supine MRI alone may be sufficient to characterize the C2–C7 sagittal angle, extrapolation of C2–C7 SVA and T1 tilt from MRI is unreliable.

While not always necessary for preoperative planning, C2–C7 SVA and T1 tilt provide more context for evaluating the overall alignment of the cervical spine. Lateral upright radiographs should be used in conjunction with MRI to thoroughly assess global cervical sagittal alignment when planning posterior-based surgery in patients with cervical spondylotic myelopathy.

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