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Magnetic Resonance Imaging of the Cervical Spine Under-Represents Sagittal Plane Deformity in Degenerative Myelopathy Patients

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Magnetic Resonance Imaging of the Cervical Spine Under-Represents Sagittal Plane Deformity in Degenerative Myelopathy Patients

Douglas S Weinberg et al. Int J Spine Surg.

Abstract

Background: In treating patients with cervical myelopathy, surgical approach may be dictated by sagittal balance, highlighting the need for accurate pre-operative assessment. Magnetic Resonance Imaging (MRI) is widely-recognized for its utility in the diagnosis and surgical planning of cervical myelopathy. Plain radiographs (X-rays) are a reliable tool to assess bony alignment. However, they may not always be included in standard pre-operative evaluation, especially in an era of restricted payer-environments. Failure to appropriately acknowledge a patients' preoperative kyphotic deformity may cause the surgeon to choose a posterior-only approach, which would provide suboptimal sagittal plane correction and decompression of anterior pathology.

Methods: 101 patients with cervical myelopathy with MRI and plain radiographs were identified. Cervical lordosis and kyphosis were measured using the Cobb method on standing lateral x-ray and sagittal T2-weighted MRI. CI (Ishihara) was also measured on standing lateral x-ray, and sagittal T2-weighted MRI. Bland-Altman plots were generated and used to compare subtle differences in measurement techniques and modalities. Odom's criteria were recorded.

Results: The average difference between plain radiograph and MRI measurements for curvature angle was 3.5± 7.2 degrees (p< 0.001), and the average difference between plain radiograph and MRI measurements for curvature index was 1.5± 5.9 degrees (p= 0.015).

Conclusions: MRI may under-represent the respective sagittal plane deformity in patients with degenerative cervical myelopathy.

Clinical relevance: We would recommend the use of standing x-rays when considering surgical planning in all myelopathy patients. This manuscript was reviewed and approved by an institutional review board. Informed consent was not obtained because patient specific identifying information was not used. It was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.

Keywords: cervical myelopathy; curvature index; kyphosis; sagittal balance; x-ray.

Figures

Fig. 1
Fig. 1
Curvature angle measured using the Cobb technique on plain radiograph. A line was placed along the inferior aspect of C2 (yellow solid line), and a perpendicular line (yellow dashed) was drawn. A line was placed along the inferior aspect of C7 (red line), and a perpendicular line (yellow dashed) was then drawn. The acute angle subtended between the two crossing lines is the curvature angle. In this example the measurement was 12.2 degrees of lordosis.
Fig. 2
Fig. 2
Curvature angle measured using the Cobb technique on MR, on the same individual seen in Figure 1. A line was placed along the inferior aspect of C2 (solid line), and a perpendicular line (yellow dashed) was drawn. A line was placed along the inferior aspect of C7 (red line), and a perpendicular line (yellow dashed) was then drawn. The acute angle subtended between the two crossing lines is the curvature angle. In this example the measurement was 1.6 degrees of kyphosis, notably different than the measurement performed on plain radiograph.
Fig. 3
Fig. 3
Curvature index (Ishihara) measured on plain radiograph. A line is connected between the most posterior-inferior portion of C2 (A) and the most posterior-inferior portion of C7 (B). The distance of line (AB) was recorded. The distances between the posterior-inferior aspects of C3, C4, C5, and C6 to the orthogonal intersection of the line from C2 to C7 was calculated (d1-4). The curvature index was calculated: d1+d2+d3+d4AB×100=CurvatureIndex In this example the measurement was 31.1.
Fig. 4
Fig. 4
Curvature index (Ishihara) measured on MRI, on the same individual seen in Figure 3. A line is connected between the most posterior-inferior portion of C2 (A) and the most posterior-inferior portion of C7 (B). The distance of line (AB) was recorded. The distances between the posterior-inferior aspects of C3, C4, C5, and C6 to the orthogonal intersection of the line from C2 to C7 was calculated (d1-4). The curvature index was calculated: d1+d2+d3+d4AB×100=CurvatureIndex In this example the measurement was 30.9.
Fig. 5
Fig. 5
Bland-Altman plots of comparisons of curvature angle in lordotic (a) and kyphotic (b) patients. The average between pairs of measurements are plotted against their difference. The solid line indicates mean bias (average difference), and the dotted lines are limits of agreement (95% confidence limit lines).
Fig. 6
Fig. 6
Bland-Altman plots of comparisons of curvature index in lordotic (a) and kyphotic (b) patients. The average between pairs of measurements are plotted against their difference. The solid line indicates mean bias (average difference), and the dotted lines are limits of agreement (95% confidence limit lines).

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