**1. Introduction**

The T1 slope is defined as the angle between the horizontal line and superior endplate of the T1 vertebra [1]. It has been used to evaluate the sagittal balance of the cervical spine and has been reported to have a strong correlation with greater sagittal malalignment of the dens [1]. T1 slope angle, neck tilt, and thoracic inlet angle have been reported as significant cervical sagittal parameters, similar to the concept that pelvic incidence, pelvic tilt, and lumbar lordosis are important lumbosacral parameters in patients with adult spinal deformity [2–6]. The relationship between health-related quality of life and surgical outcomes and T1 slope has been examined in several studies [7,8].

T1 slope minus cervical lordosis can predict ideal cervical lordosis, and T1 slope plays an important role in planning cervical surgery: predicting the progression of kyphosis after cervical laminoplasty and the ideal correction angle in posterior cervical instrumentation [2,3,9]. However, the T1 vertical body is not always clearly visible on lateral cervical radiographic images because of interference by the shoulder and thoracic trunk in obese and short-necked patients [10,11]. Qiao et al. reported that the T1 upper endplate had poor visibility in 34% of cases with plain X-ray radiographs [12]. In such cases, appropriate surgical planning and clinical studies excluding cases with invisible T1 slopes have a selection bias and can be challenging.

**Citation:** Ito, S.; Nakashima, H.; Matsumoto, A.; Ando, K.; Machino, M.; Segi, N.; Tomita, H.; Koshimizu, H.; Imagama, S. Differences in Demographic and Radiographic Characteristics between Patients with Visible and Invisible T1 Slopes on Lateral Cervical Radiographic Images. *J. Clin. Med.* **2022**, *11*, 411. https://doi.org/10.3390/jcm11020411

Academic Editor: Victor Valderrabano

Received: 14 December 2021 Accepted: 11 January 2022 Published: 14 January 2022

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However, to the best of our knowledge, the percentage of patients with invisible T1 slopes and the difference in radiological characteristics between patients with visible or invisible T1 slopes remain unknown [13]. Therefore, this pilot study aimed to investigate the differences between these characteristics in outpatients with visible or invisible T1 slopes.

#### **2. Materials and Methods**

#### *2.1. Study Design and Ethics Approval*

Our study retrospectively included adult patients with neck pain who underwent lateral radiography with their necks in neutral, flexion, and extension positions from 2015 to 2016. The reason for radiography included spinal degenerative diseases, spinal tumors, and ossification of the posterior longitudinal ligament. Patients with a history of cervical infection, fractures, or surgery were excluded. This study was approved by the Research Ethics and Conflicts Committee of our University and was performed in accordance with the Declaration of Helsinki.

#### *2.2. Radiological Assessment*

Cervical lateral radiographs were obtained using standard radiographic techniques, in which the tube was centered on the level of the center of the xiphoid process. Lateral radiographic images were obtained with each participant standing and looking straight ahead. Flexion and extension radiographs were obtained with the neck in maximum flexion and extension. T1 slope angles were measured using these images, with the T1 slope defined as the superior endplate of the T1 vertebrae [12]. Three spine surgeons evaluated the visibility of the T1 slope. In cases where the surgeons disagreed regarding the visibility of the T1 slope, a discussion was held to reach a conclusion. Patients were divided into two groups based on whether the T1 slope was visible (V) or invisible (I), as decided by the three surgeons.

In this study, we used dynamic range control processing methods to improve the clarity of the radiographs. Dynamic range control processing can change the density and contrast of only low- and high-density areas [14].

The measured parameters in the radiographs were as follows: the Cobb angle from C2 to C7 (C2-7 angle) was defined as the angle between the inferior endplates of C2 and C7 on standing lateral radiographs. The C2-7 angle was measured in neutral, flexion, and extension positions. The C2-7 sagittal vertical axis (SVA) was defined as the deviation of the C2 plumb line (extending from the centroid of the C2 vertebra) from the superior posterior endplate of C7, with positive sagittal alignment defined as an anterior deviation. All parameters were measured twice by the same researcher independently using the same method.

#### *2.3. Statistical Analyses*

Sex was compared between the V and I groups using the Chi-square test. Patient age, body mass index (BMI), C2-7 SVA, and C2-7 angle measurements on radiographs were compared between the V and I groups using the Student's t-test. Each analysis was performed separately for men and women. For data aggregation and analyses, we used the IBM SPSS Statistics version 24.0 software (IBM Corp., Armonk, NY, USA), and *p* < 0.05 was considered statistically significant.
