**3. Results**

The study population consisted of 60 consecutive patients with cervical spine disorders other than cervical spine trauma who visited our hospital between 2015 and 2016; of these, 30 (50.0%) were female, and the average age was 44.5 years (range: 34–81 years).

Among the 60 patients, 53.3% (32 patients; 10 men and 22 women) were included in the V group, and 46.7% (28 patients; 20 men and eight women) were included in the I group (Table 1, Figure 1).


**Table 1.** T1 slope visibility.

**Figure 1.** T1 slope visibility.

Among the 60 patients, there were spinal degenerative diseases (*n* = 28), spinal tumors (*n* = 9), ossification of the posterior longitudinal ligament (*n* = 14), and other conditions (*n* = 9). The V group included 16 patients with spinal degenerative diseases, 6 with spinal tumors, 10 with ossification of the posterior longitudinal ligament, and 5 with other conditions. The I group included 10 patients with spinal degenerative diseases, 4 with spinal tumors, 6 with ossification of the posterior longitudinal ligament, and 5 with other conditions. There was no significant difference between the V and I groups (*p* = 0.943) (Table 2).


**Table 2.** Cervical disorders in patients (*n* = 60).

OPLL = ossification of the posterior longitudinal ligament. There was no significant difference between the V and I groups (*p* = 0.943).

Of the 60 patients, 23 and 37 received surgical and conservative treatment, respectively. In the V group, 12 patients received surgical treatment while 20 received conservative treatment. The I group included 11 patients who received surgical treatment and 17 who received conservative treatment. There was no significant difference between the V and I groups (*p* = 0.887) (Table 3).



Surgical treatment: decompression, posterior fusion, anterior fusion, resection of tumors. Conservative treatment: medication, rehabilitation, observation. There was no significant difference between the V and I groups (*p* = 0.887).

There were significantly more males in the I group (*p* < 0.05) (Table 3). No significant differences were observed between the groups regarding age or BMI (Table 4). The mean age was 44.4 years (range: 35–78 years) in the V group, and 44.6 years (range: 34–81) in the I group. The mean BMI was 21.2 (range: 19.6–24.3) and 22.1 (range: 19.9–26.1) in the V and I groups, respectively.


**Table 4.** Baseline demographic characteristics of patients.

Values presented as mean ± SD; n.s., not significant; BMI, body mass index.

We compared the C2-7 SVA and C2-7 angles between the V and I groups for each sex. The mean C2-7 SVA was 16.0 mm in the V group and 28.9 mm in the I group for males. The I group had significantly greater C2-7 SVA than the V group (*p* < 0.05) (Figure 2). For females, the mean C2-7 SVA in the V group was 19.9 mm and 24.4 mm in the I group. Similar to males, there was a higher C2-7 SVA in the I group, but there was no significant difference (*p* = 0.362) (Figure 2).

**Figure 2.** Comparison of the C2-7 SVA between the I and V groups in males and females. \*: *p* < 0.05; V group: T1 slope visible; I group: T1 slope invisible; C2-7 SVA, C2-7 sagittal vertical axis.

In the neutral position, the mean C2-7 angle in males was 11.8◦ in the V group, and 12.1◦ in the I group, and the mean C2-7 angle in females was 11.0◦ and 10.4◦ in the V and I groups, respectively. There was no difference in the C2-7 angle in the neutral position for both males and females between the V and I groups (Figure 3).

Similar to the neutral position, there was no significant difference in the flexion position. In the flexion position, the mean C2-7 angle in males was −14.8◦ in the V group, and −14.0◦ in the I group, and the mean C2-7 angle in females was −9.4◦ in the V group and −8.7◦ in the I group. There was no significant difference in the flexion position in females: 39.2◦ in the V group and 30.7◦ in the I group (*p* = 0.147) (Figure 3). In contrast, in the extension position, the mean C2-7 angle in males was greater in the V group: 37.6◦ and 24.4◦ in the V and I groups, respectively (*p* < 0.05) (Figure 3).

Thus, male patients with greater C2-7 SVA had an invisible T1 slope, as shown in the representative cases in Figure 4A,B.

**Figure 3.** Comparison of the C2-7 angles between the I and V groups in neutral, flexion, and extension in males and females. \*: *p* < 0.05; V group, T1 slope visible; I group, T1 slope invisible.

**Figure 4.** Comparison between invisible and visible T1 slopes of patients. (**A**) Patient with invisible T1 slope: 62 years old, male, ossification of the posterior longitudinal ligament, C2-7 SVA 58 mm, C2-7 angle 4◦. (**B**) Patient with visible T1 slope: 63 years old, female, ossification of the posterior longitudinal ligament, C2-7 SVA 19 mm, C2-7 angle 22◦.

In this study, some surgical cases were included. There was no clear difference in the imaging changes in surgical cases between both groups before and one year after surgery (Figure 5).

**Figure 5.** *Cont.*

**Figure 5.** Pre and postoperative radiographs of patients with visible and invisible T1 slope. (**A**,**B**) Patient with invisible T1 slope: 50 years old, female, ossification of the posterior longitudinal ligament, C2-7 laminoplasty A; the preoperative radiograph, C2-7 SVA 39 mm, C2-7 angle 1◦, C2 slope 27◦, C7 slope 22◦, B; the radiograph 1 year after surgery, C2-7 SVA 37 mm, C2-7 angle 11◦, C2 slope 27◦, C7 slope 17◦. (**C**,**D**) Patient with invisible T1 slope: 51 years old, male, cervical spondylotic myelopathy, C3-7 laminoplasty A; the preoperative radiograph, C2-7 SVA 21 mm, C2-7 angle 15◦, C2 slope 11◦, C7 slope 22◦, B; the radiograph 1 year after surgery, C2-7 SVA 18 mm, C2-7 angle 25◦, C2 slope 5◦, C7 slope 24. (**E**,**F**) Patient with visible T1 slope: 62 years old, female, ossification of the posterior longitudinal ligament, C5-7 laminoplasty A; the preoperative radiograph, C2-7 SVA 4 mm, C2-7 angle 7◦, C2 slope 6◦, C7 slope 7◦, T1 slope 6◦, B; the radiograph 1 year after surgery, C2-7 SVA 14 mm, C2-7 angle 2◦, C2 slope 14◦, C7 slope 16◦, T1 slope 14◦. (**G**,**H**) Patient with visible T1 slope: 44 years old, male, cervical spondylotic myelopathy, C3-7 laminoplasty A; the preoperative radiograph, C2-7 SVA 8 mm, C2-7 angle 12◦, C2 slope 11◦, C7 slope 17◦, T1 slope 24◦, B; the radiograph 1 year after surgery, C2-7 SVA 21 mm, C2-7 angle 1◦, C2 slope 23◦, C7 slope 18◦, T1 slope 20◦.
