Abstract

Introduction

Proper evaluation of sagittal balance is of paramount importance during the management of cervical myelopathy. While the clinical diagnosis is based on history and physical examination, plain radiographs (X-rays) and Magnetic Resonance Imaging (MRI) represent an important part of evaluation and surgical planning. Surgical decompression and stabilization remains the mainstay of treatment for patients with severe clinical symptoms. During pre-operative planning, the surgeon must make important decisions regarding surgical approach, stabilization, and the restoration of sagittal balance. The combination of these factors has profound implications for the improvement of neurological recovery following decompression.^1–3

It has been well-established that patients who present with severe kyphosis deformity pre-operatively have suboptimal outcomes when anterior pathology is not appropriately decompressed. Many authors have suggested that a kyphosis of more than 10 degrees necessitates an anterior approach (or combined approach) to adequately reduce cord tensioning.^4–7 Similarly, the development of post-laminectomy kyphosis can become accelerated when decompression is performed without fusion in the presence of pre-existing deformity.

Upright plain radiographs are a reliable tool to assess bony alignment and are considered by most authors the gold standard for evaluating cervical lordosis or kyphosis. However, with the increasing use of MRI, it may not be a part of certain practitioner’s standard pre-operative evaluation. Furthermore, in an increasingly restricted payer environment where radiographs and MRI are often being limited, knowing the value of each in the surgical planning process is especially relevant at the present time.

We therefore determined it would be clinically prudent to perform a review comparing how cervical curvature angle (lordosis or kyphosis)⁷ and cervical curvature index (Ishihara)⁸ varied between plain radiograph and MRI. The purpose of this study was to compare various measures of sagittal balance on plain radiograph versus MRI. We hypothesized that the magnitude of sagittal plane deformity would be greater on plain radiograph when compared to MRI.

Materials and Methods

Following Institutional Review Board approval, a series of 151 patients whose history, physical exam, and imaging were consistent with the diagnosis of cervical myelopathy were retrospectively reviewed.^{9, 10} All patients were from the practice of a single, fellowship-trained spine surgeon and were evaluated between May 2009 and October 2013. Patients were included if they had high-quality plain radiographs of the lateral cervical spine in a standing, upright position, and T1 and T2-weighted magnetic resonance imaging (MRI) of the cervical spine available for review. Patients were excluded for having incomplete radiographic records or incomplete visualization of C7 on plain radiograph (20), prior spine surgery or trauma (18), more than 3 months duration between plain film and MRI measurements (10), or severe coronal plane deformities (2). In total, 50 patients were excluded, and 101 unique patients were enrolled for primary analysis. Patient demographic information was also available for review. Odom’s criteria were recorded postoperatively.

Results

The mean and standard deviation of patient age and was 59± 14 years. There were 49 males (49%) and 52 females. Inter-observer reliabilities for measurements of curvature angle were 0.84 and 0.92 on plain radiograph and MRI, respectively. Intra-observer reliabilities were 0.88 and 0.92 on plain radiograph and MRI, respectively. Inter-observer reliabilities for measurements of curvature index were 0.77 and 0.84 on plain radiograph and MRI, respectively, and intra-observer reliabilities were 0.80 and 0.90, on plain radiograph and MRI, respectively. All measurements showed excellent reliability (> 0.75) as defined by Fleiss et al. for agreement of like measurements of continuous variables.²¹

The average curvature angle was 9.8± 13.8 degrees on plain radiograph, and 6.2± 12.7 degrees on MRI. There were 25 patients with kyphosis deformity using this technique (curvature angle less than 0 degrees on plain radiograph. The average curvature index (Ishihara) was 5.4± 13.6 degrees on plain radiograph, and 4.0± 12.8 degrees on MRI. There were 30 patients with kyphosis deformity using this technique (curvature index less than 0 degree on plain radiograph). Individual correlations between measurements techniques were calculated (Table 1), and were above 0.8 for comparisons between like measurements and like techniques. One-sample statistics showed an average difference of 3.5± 7.2 degrees between plain radiograph and MRI measurements for curvature angle (p< 0.001), and an average difference of 1.5± 5.9 degrees between plain radiograph and MRI measurements for curvature index (p= 0.015).

There were no differences in post-operative Odom’s criteria based on curvature angle, measured on plain radiograph or MRI, or based on curvature index measured on plain radiograph or MRI (p >0.05 for all).

Bland-Altman plots were generated for comparisons between curvature angle measurements on plain radiograph and MRI (Figure 5) and comparisons between curvature index measurements on plain radiograph and MRI (Figure 6). Average (solid lines) and 95% confidence intervals (dashed) of the differences in measuring technique were created.

Discussion

Cervical myelopathy is the most common cause of spinal cord dysfunction among adults and usually results from degenerative spondylosis.²² Brain et al. first described a progressive clinical syndrome of spinal cord compression that can present with a wide spectrum of signs and symptoms.²³ Classic literature described a characteristic presentation of paresthesias, neck pain, weakness, subtle changes in gait and locomotion, and overall clumsiness.^24–26 Cervical myelopathy is a progressive disorder and typically does not improve; most patients experience a slow deterioration over time. The diagnosis is based on history and physical examination, however, Magnetic Resonance Imaging (MRI) has an important role in diagnostic workup and is useful for determining the degree of compression of the spinal cord and nerve roots.⁹ Some authors have suggested using it as a means to quantify sagittal balance of the cervical spine, which has traditionally been performed on plain radiograph.²⁷ However, to our knowledge, there have been no reports in the literature comparing cervical sagittal alignment measured using standing, upright plain radiographs versus recumbently positioned MRI. Therefore, we designed a radiographic study designed to evaluate the differences in sagittal spine alignment in a patient population with cervical myelopathy.

Our findings suggest that despite strong correlations between all measurements used in these analyses, there still existed considerable difference among measurement modalities. Some have suggested that a correlation above 0.6 among measurement techniques constitutes “good” agreement for clinical practice,²⁸ although most modern authors agree that a more thorough comparative analysis is required to see subtle differences.²⁹The Bland-Altman plots used in this paper highlight this concept and show that despite strong correlations between radiographic and MRI measurements of curvature angle (Cobb), there was a statistically significant difference between them for patients with both kyphotic and lordotic deformities. Moreover, the relatively large standard deviations suggest that these differences are not consistent among individuals. The average discrepancy between modalities has the potential to represent a clinically significant effect. Moreover, the dispersion of data in these plots suggests there is no fixed bias between the two measurements.^{13, 16}

In other words, one cannot simply assume that radiographic measurements are a constant number of degrees different than those found on MRI. This is likely due to the development of rigid versus flexible kyphosis, age-related changes of the disc space and posterior elements, and patient-specific parameters.^30–32 Similarly, smaller, but still significant differences in curvature index (Ishihara) between plain radiograph and MRI existed making their interchangeable use impossible. Given that even small changes in sagittal balance have important implications in cervical myelopathy, the accurate assessment of such is critical. Lateral radiographs have the strongest literature precedent for such, and the results of this study would suggest that measuring sagittal balance on MRI is inadequate for quantifying cervical lordosis or kyphosis as traditionally defined.^33–35 It is possible, however, that there exist future applications for these measurements.

The results of this study are consistent with the anecdotal observations of spine surgeons at our institution and elsewhere. The increasing use of MRI has caused some non-spine care providers and third party payers to question the role of traditional radiography, although this study suggests that proper evaluation of sagittal balance—of critical importance during the evaluation of cervical myelopathy—can only be performed on plain radiograph.

It is likely that major differences between radiographic and MRI measurements are related to patient-positioning. Traditionally, sagittal balance has been evaluated using lateral plain radiographs. This places the cervical spine in a weight-bearing and load-sharing position. Radiographs performed in the upright position cause gravity to stress the anterior and posterior ligaments, which may be crucial for establishing alignment.^36–38 While no differences in postoperative outcomes were notes based on any imaging parameter, this result is likely confounded by the fact that surgical planning was dictated by sagittal balance. However, post-operatively, there were no differences in clinical outcomes, suggesting that most data is needed to determine the appropriate application going forward.

Curvature index (Ishihara) has been proposed as an alternative means to quantify sagittal alignment of the cervical spine. While computing its individual measurements may be too time-consuming for routine clinical application, proponents of its use have suggested that it is less influenced by small changes in the endplates of C2 and C7, provides a more accurate global assessment of sagittal balance, with equal emphasis on each cervical unit. It may also offer advantages in quantifying sagittal balance in spines with coronal plane deformities.^{8, 39} A previous mathematical model by Takeshita et al. showed extremely high correlations between curvature index and cervical curvature angle on MRI.⁸ This study confirms this relationship and validates those results, although the correlations in this experiment were slightly lower than reported in conceptual modeling. This is to be expected as inherent measurement errors are introduced during clinical application and were not accounted for in mathematical modeling. While comparisons of curvature index (Ishihara) between plain radiograph and MRI had much smaller differences than those of curvature angle (Cobb), these differences were still significant and with non-fixed variability, as shown on Bland-Altman plots. This makes them too dissimilar to be considered congruent.

It has been suggested that the Cobb angle method for determining cervical lordosis underrepresents cervical lordosis when measured from C2-C7 and overestimates lordosis when measured from C1-C7. Alternative methods to assess sagittal balance include the Harrison posterior tangent method,⁴⁰ and Jackson physiological stress lines.⁷ The Cobb method was chosen for use in this study given its good inter- and intra-relator reliability in previous studies, and is simple, effective application in clinical use.^{41, 42} Measurements of curvature index (Ishihara)⁸ have been proposed as an alternative technique, although there are only limited other studies available validating its use.^{39, 43–45}

This study has important limitations. Functional outcomes before or after surgery were not included in these analyses. This introduces a similar problem found in other radiographic studies. However, previous authors have well-established the relationship between kyphosis deformity and cervical myelopathy, and all patients with significant deformity in this series were managed with either a combined anterior/posterior approach or an anterior-only approach. Additionally, this study used only two measurement techniques to quantify cervical kyphosis. As discussed above, previous authors have shown certain differences exist between techniques, although the C2 Cobb method may be the most readily applicable in clinical practice.⁷ This study assumes that differences in measurement values were due to differences in recumbent patient positioning and imaging output, although other factors such as rotation, lateral bending, magnification, and resolution may have contributed. Additionally, this study only included patients with cervical myelopathy to assess sagittal balance; however, this cohort was chosen given the important emphasis this has during their treatment course. Nonetheless, it is possible that patients with other pathologies, such as radiculopathy, or those with normal spines, may have produced different results. While this experiment did not evaluate upright MRI, we think this represents an important and fertile area for future research.

In conclusion, our radiographic review of 101 patients with cervical myelopathy showed that important differences exist in the measurement of sagittal balance on plain radiograph and MRI when using a variety of techniques. Standing, upright radiographs of the lateral cervical spine has been the gold-standard for evaluating cervical lordosis or kyphosis. These measurements cannot be reproduced or emulated using other modalities despite the strong correlations between techniques and modalities.

References