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Can C2-6 Cobb Angle Replace C2-7 Cobb Angle?

An Analysis of Cervical Kinetic Magnetic Resonance Images and X-rays

Zhang, Jiandang, MD; Buser, Zorica, PhD; Abedi, Aidin, MD; Dong, Xiangyu, MD; Wang, Jeffrey C., MD

doi: 10.1097/BRS.0000000000002795
CERVICAL SPINE
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Study Design. Retrospective study of consecutive 113 cervical kinetic magnetic resonance images (kMRIs) and 57 radiographs.

Objective. To elucidate the relationship between C2-6 or C2-7 Cobb angle and cervical sagittal alignment parameters using kMRI, and evaluate the visibility of C6 and C7 inferior endplates on cervical radiographs.

Summary of Background Data. Several studies have used C2-6 Cobb angle instead of C2-7 Cobb angle as C7 inferior endplate is not always visible because of overlying shadows. However, the relationship between C2-6 or C2-7 Cobb angle and cervical sagittal alignment parameters remains unclear. Moreover, visibility of C6 inferior endplate remains unknown.

Methods. C2-6 Cobb angle, C2-7 Cobb angle, occiput-C2 angle, Atlas-dens interval (ADI), narrowest oropharyngeal airway space (nPAS), cervical sagittal vertical axis (cSVA), cervical tilt, cranial tilt, neck tilt, thoracic inlet angle (TIA), and T1 slope were measured using kMRI, and analyzed for their relationship with C2-6 or C2-7 Cobb angle. Visibility rates of C6 or C7 inferior endplates were evaluated using cervical radiographs.

Results. Linear regression analysis showed high association between C2-6 and C2-7 Cobb angle (R2 = 0.696, P < 0.01). C2-6 Cobb angle was significantly correlated with occiput-C2 angle, TIA, T1 slope, neck tilt, cSVA, and cervical tilt; but not with nPAS, ADI, and cranial tilt. C2-7 Cobb angle resembled C2-6 Cobb angle regarding the relationships with other parameters. In our study, 94.7% C6 and 50.9% C7 inferior endplate were clearly visible; 1.8% C6 and 24.6% C7 inferior endplate were invisible. Chi-square test and residual analysis showed significant difference between the two groups (P < 0.01).

Conclusion. C2-6 Cobb angle highly resembled C2-7 Cobb angle regarding its relationships with parameters of craniovertebral, cervical and thoracic inlet alignment. C2-6 Cobb angle could be an alternative to C2-7 Cobb angle because of its significantly higher visibility rate.

Level of Evidence: 3

Several studies used C2-6 instead of C2-7 Cobb angle due to the invisibility of C7 inferior endplate. In the current study C2-6 Cobb angle highly resembled C2-7 Cobb angle regarding its relationships with cervical sagittal parameters. C2-6 Cobb angle could replace C2-7 Cobb angle due to good visibility.

Department of Orthopedic Surgery, Keck School of Medicine, University of Southern California, Los Angeles, CA.

Address correspondence and reprint requests to Zorica Buser, PhD, Department of Orthopedic Surgery, Keck School of Medicine, University of Southern California, Los Angeles, CA; E-mail: zbuser@usc.edu

Received 29 May, 2018

Accepted 28 June, 2018

The submitted manuscript does not contain information about medical device(s)/drug(s).

No funds were received in support of this work.

Relevant financial activities outside the submitted work: board membership, consultancy, royalties, stocks, and grants.

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