Understanding the cervical sagittal balance has always been very important to spine surgeons, due to its great influence on the clinical outcomes in cervical spine surgery. Actually, cervical balance also plays an important role in determining surgical strategy (surgical approach or instrumentation level) and in predicting clinical prognosis (adjacent segmental disease) after surgery [6,7,17,24]. In line with the perception of the importance of cervical balance itself, many different radiologic factors have been proposed and evaluated for their usefulness [16].

Recently, after the concepts of the thoracic inlet angle (similar to pelvic incidence in the thoracolumbar spine) and T1 slope (T1s) had been established, another derived factor (T1s minus cervical lordosis [CL]) has also been viewed as an essential parameter [15,25,26]. Although the usefulness of the above radiological parameters has been proven in many previous reports, the accurate measurement of T1s is not usually easy for many patients, due to their short necks or obstruction of shoulder shading [22]. Therefore, there is a fundamental limit to the general application of these good cervical parameters to all patients. The reliable way to identify the change of spine balance and to evaluate the usefulness of a specific radiologic parameter can be determined through the cervical spine operation. Generally, anterior cervical discectomy and fusion (ACDF) is the most widely performed procedure in cervical spondylosis patient. Therefore, we aimed to define the clinical significance of T1s minus CL (T1s-CL) in the patients who underwent ACDF.

This study was approved by the Institutional Review Board of St. Vincent Hospital, The Catholic University of Korea (VC19RISI0232).

The ideal approach for the evaluation of cervical sagittal alignment may be through major cervical deformity patients; however, these cases are very rare compared to thoracolumbar deformity cases [4,19,21,23]. Therefore, we assumed that the next best thing is to evaluate sagittal alignment with the most widely used procedure and disease. As we know, ACDF is the most common and widely performed procedure for cervical spondylosis. Recently, some previous reports have been published for the same purposes as ours, and the results showed that ACDF can influence the change of spine alignment regionally (cervical spine) or globally (whole spine) [8,9]. Moreover, as described in our previous report (n=61, conducted at a different institution), the cervical alignment change after ACDF was variable and dependent on whether or not pre-operative segmental kyphosis existed [10].

Previous studies also revealed some important factors about sagittal balance analysis. First, the use of a plate construct may induce better sagittal alignment change than using a cagealone technique. Second, more obvious sagittal alignment changes occur in two-level ACDF than in one-level ACDF [3,11]. In an effort to leverage the above phenomenon, we selectively conducted our study with two-level ACDF and only with procedures using a plate (allobone spacer) system to maintain consistency of biomechanical effects [20].

In the present study, two-level ACDF induced statistically significant changes of many radiological factors (C7s, T1s, O-C2A, C2-7A, USS, LSS, SA, and CVA). However, other factors (Os, C2s, T1s-CL, O-C7A, and C2-7SVA) did not show significant changes after ACDF. Although correlation analysis may be more precise to estimate the relationships among many radiological factors, the grasp of directional or numerical changes of each parameter can also give us some clues to estimate relationships between specific parameters. From this point of view, the directions and numerical changes between C7s and T1s were very similar (Δ-1.8° vs. Δ-1.9°), and C2s and T1s-CL also showed the same absolute change value (Δ|0.8|°). Like our previous report, there was a close association in directional nature between T1s and C7s and these two parameters did not significantly differ in some previous reports [1,14].

T1s-CL has been recognized as an important factor, which might affect cervical balance and clinical symptoms [12]. Some previous papers insisted not to exceed a certain T1s-CL value (14.5°-26.5°) to prevent malalignment and the worsening of clinical results. Moreover, some other reports showed the normal value of T1s-CL from 15.8° to 22.2° [2,5,18,21]. Therefore, we selected 16° as the reference value of T1s-CL for dividing patients into two groups (mismatch group : 23.0° vs. balance group : 7.6°) in this study, as most patients did not show any major deformity [21].

When we compared the two groups (mismatch vs. balance), there were statistically significant differences in most factors, excepting three radiological factors (C7s, T1s, and LSS). Anatomically, LSS was measured in the lower cervical vertebrae (C6 or C7), so the difference in lower cervical slopes may not be a decisive element of mismatch. Rather, the factor with the large difference value between the two groups may be the definite one in determining a mismatch. Therefore, we came to the conclusion that C2 (Δ13.3°) and C2-7A (Δ14.7°) should be the most decisive factors for representing a mismatch. Moreover, since the value of C2-7A is obtained by the difference value between C2s (significantly different between the two groups) and C7s (insignificantly different between the two groups), C2s will ultimately be the most important factor.

We tried to prove the above phenomenon analytically and interpreted the T1s-CL formula as below. Because usually T1s and C7s are represented as negative values in this study, the T1s-CL formula can be interpreted as |T1s|-(C2s-C7s) or |T1s|- C2s+C7s. As a result, T1s-CL value is determined by two factors (C2s and C7-T1 angle [C7-T1A]). At this stage, it should be necessary to understand the aspect of the C7-T1A. The mean difference value of C7-T1A was less than 2.6° and was maintained constantly pre- and postoperatively in this study. Moreover, only seven patients (7/117, 5.9%) had a C7-T1A greater than 6° before surgery. If we consider that C7-T1A was a constant value (about 3°) in most patients, the most influential factor in determining sagittal mismatch will be C2 itself (Fig. 2).

The patterns of change between the two groups after the ACDF procedure were very different. ACDF induced upward movement of C2s (Δ3.2°) and downward movement of C7s (Δ3.1°); thus, CL was much improved in the mismatch group. Two segmental factors (USS : Δ2.1° and LSS : Δ-3.1°) also showed similar change patterns. However, in contrast to the mismatch group, upward movement of C2s or USS did not occur in the balance group. We can also observe the smaller downward movement of C7s and LSS (Δ-1.9° and Δ-2.2°) in the balance group. Although the ACDF procedure itself has enough ability to correct regional or overall cervical deformity, the alignment change after ACDF was not very influential to well-balanced patients. The above results are in line with our previous study [10].

In addition, looking at the change between the pre- and post-operative differences between the two groups, only four factors (C2s, T1s-CL, O-C2A, and C2-7A) showed statistical significance. If we think about C2s again, the other three factors (T1s-CL, O-C2A, and C2-7A) are closely related to C2s. As we said earlier, T1s-CL can be represented as C2s, and the other cervical angles (O-C2A and C2-7A) were fundamentally measured with C2s. As a result, all changes of sagittal alignment may be related to C2s and represented as changes in C2s.

The close relationship between T1s-CL and C2s can be also confirmed by Pearson correlation analysis (r=-0.96 before surgery; r=0.88 after surgery). Moreover, these two factors showed very similar relationship patterns with other radiological factors. First, C2 and T1s-CL were not statistically related to the lower cervical slopes (C7s, T1s, and LSS). Some previous reports have also demonstrated that there is no significant correlation between T1s and T1s-CL [21]. This is because, as explained earlier, T1s-CL is just a representation of C2s. Second, C2 and T1s-CL (r=|0.60-0.71|) are more closely related to the SVA than are the other three factors (C2-C7A, C7s, and T1s, r=|0.28-0.31|). Generally, C2-7SVA is recognized as an important indication of clinical and radiological malalignment; therefore, it is very important to identify the most relevant factors to the SVA. Kwon et al. [11] showed that SVA after two-level ACDF was affected more significantly by C2-7A than T1s. The above result is very similar to our present result. Third, these two factors (C2 and T1s-CL) did not statistically correlate to the lower cervical slopes (C7s and T1s). In other words, the lower cervical slopes (C7s and T1s) were independent of the upper cervical slopes (C2s or Os) but were closely related to each other (r=0.91-0.94).

Through the correlation analysis, we found that C2-7A is most widely associated with many other radiological factors (Fig. 3). This is because the C2s reflects the upper cervical segment (Os and T1s-CL) and the C7s reflects the lower cervical segment (LSS and T1s). Therefore, measuring two absolute parameters (C2s and C7s) separately will help us to understand the overall cervical sagittal balance, rather than measuring C2-7A (relative parameter) using Cobb’s method.

Our study has three limitations. First, the present study did not include clinical data that can be used to estimate the influence of sagittal balance on clinical symptoms. We assumed that the clinical outcome of ACDF may depend on the well nerve decompression rather than the cervical balance itself, as described in other reports [11,13,24]. Second, we did not include cases involving C7-T1 level, which might significantly change C7-T1A. At this time, there were very few cases involving C7-T1; however, further analysis of the C6-7-T1 ACDF will be meaningful. Third, we also need to see if the present findings will be consistent with the posterior cervical operation.

Generally, the incidence of major deformity pathology is very low; therefore, it may be unreasonable to apply ambiguous (T1-related) parameters to minor or relatively well-balanced cervical pathology routinely. Through the actual clinical analysis and arithmetical interpretation of formulas, we can conclude that the T1s-CL may just represent the C2s. However, C2s has little correlation with lower cervical parameters (C7s and T1s). Therefore, if we can recognize C7s as a substitute for T1s, just measuring C2s and C7s will give us a good guideline for understanding the overall cervical balance in ACDF patients. These two parameters (C2s and C7s) should be more easily measurable than T1-related parameters for estimating cervical sagittal balance.