Can the C7 slope substitute the T1 slope?

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Can the C7 slope substitute the T1 slope?The T1 slope was first reported in 2010 by Knott and is considered one of the key players in cervical balance. Many studies have verified the strong relationship between the Health-Related Quality of Life or surgical outcomes and T1 slope. However, there are difficulties in identifying T1 vertebral body on radiographs in certain cases. Several groups have tried to evaluate cervical balance using C7 slope instead of T1 slope, but it is still unclear whether this offers similarity, write Zorica Buser, Jeffrey C Wang and Koji Tamai.

We retrospectively reviewed 45 consecutive radiographs and 120 magnetic resonance images (MRIs) taken in weight-bearing neutral position. The endplate visibility of weight-bearing radiography was determined as “visible” if the endplate could be observed clearly, “unclear” if the shape of vertebra could be observed but the endplate could not be observed clearly, or “invisible” if the shape of vertebra could not be observed.

Subsequently, the C7 slope of upper endplate, C7 slope of lower endplate, T1 slope, C1 inclination, C2 slope, atlas-dens interval (ADI), C2–C7 Cobb angle, cervical sagittal vertical axis (cSVA), cervical tilt, cranial tilt, neck tilt and thoracic inlet angle (TIA) were measured. A chi-squared test and residual analysis were used for analysis of the endplate visibility, as well as linear regression analysis and Pearson correlation coefficient were used to verify the correlation of parameters on the MRIs.

The results showed that 82% of the upper endplate of C7, 51% of the lower endplate of C7 and 18% of upper endplate of T1 were clearly visible. In contrast, 18% of the upper C7, 31% of the lower C7 and 62% of upper T1 endplate were invisible. The upper C7 endplate was significantly visible, whereas T1 endplate was significantly invisible (residual analysis, p<0.01 respectively). In terms of the relationship between the C7 and T1 slope, linear regression analysis showed high correlation between the upper C7 slope and T1 slope (r2=0.818, p<0.01) and, lower C7 slope and T1 slope (r2=0.840, p<0.01). In addition, T1 slope significantly correlated with neck tilt, TIA, C2–C7 angle, cSVA, cervical and cranial tilt, but not with the C1 inclination, C2 slope, ADI. Upper and lower C7 slopes showed close resemblance with the T1 slope in terms of correlation with those parameters (Table 1).

Thoracic parameters Cervical parameters Upper cervical parameters
Neck tilt TIA Cervical tilt Cranial tilt cSVA C2-7 angle C2 slope Clinclination ADI
uC7s -0.341** 0.380** 0.506** 0.470** 0.407** 0.599** 0.015 -0.094 -0.028
1C7s -0.358** 0.374** 0.501** 0.466** 0.402** 0.583** 0.027 -0.083 -0.049
T1s -0.350** 0.469** 0.421** 0.545** 0.316** 0.545** 0.03 -0.053 -0.094

Table 1

These results indicated that C7 slope itself can be a strong parameter for cervical sagittal balance. In addition, the C7 slope was highly correlated with T1 slope and showed very similar characteristics to T1 slope in relation with other cervical and thoracic parameters. This suggests that the C7 slope could potentially substitute the T1 slope, and that it is possible to apply knowledge of T1 slope to C7 slope. Although the C7 slopes of upper endplate or of lower endplate showed similar correlation with other parameters, C7 slope of the upper endplate was significantly more visible than of the lower endplate on cervical radiographs. Therefore, the C7 slope—in particular the slope of the upper C7 endplate—has a potential to substitute the T1 slope.

Zorica Buser is assistant professor of Research Orthopaedic Surgery at the Department of Orthopaedic Surgery, Keck School of Medicine, University of Southern California, Los Angeles, USA.

Koji Tamai is a visiting research scholar at the same institution.

Jeffrey C Wang is professor of Orthopaedic Surgery and Neurosurgery at the same institution, and co-director of the University of Southern California Spine Center.

This article was originally published in issue 44 of Spinal News International.

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