Participants

This study was approved by the Institutional Review Board of Keio University School of Medicine (reference study number 20160384), and written, informed consent was obtained from all participants. The inclusion criteria for this study were healthy volunteers without any past medical history, age ranging from 30 to 60 years, and who showed complete understanding of the details of their involvement and provided their informed consent to participate in this study. The exclusion criteria were shoulders with obvious degenerative changes in the sternoclavicular, acromioclavicular, and glenohumeral joints, and spinal scoliosis with Cobb’s angle greater than 10° [9] on the CT scans obtained. A total of 106 healthy volunteers were prospectively recruited by a volunteer recruitment company. Of these, 6 participants were excluded because CT showed asymptomatic spinal scoliosis. Thus, 200 shoulders from 100 healthy volunteers (60 females and 40 males) were included in the analysis. The participants’ mean (± standard deviation) age, height, and weight were 43.2 ± 8.0 years (range, 30-59 years), 171.4 ± 6.4 cm (range, 159.1-187.5 cm), and 68.1 ± 9.6 kg (range, 47.4-88.2 kg) in males, and 44.9 ± 8.5 years (range, 30-60 years), 157.0 ± 5.2 cm (range, 147.7-170.7 cm), and 53.9 ± 8.0 kg (range, 37.8-77.5 kg) in females, respectively.

CT scans of both shoulders of the volunteers were taken in the supine position using a conventional 320-detector row CT scanner (Aquilion ONE; Canon Medical Systems Corporation, Otawara, Japan) and in the standing position using an upright 320-detector row CT scanner (prototype TSX-401R; Canon Medical Systems Corporation) on the same day. During CT scanning in the supine position, the volunteers were placed on the floor of the CT scanner with their arms at their sides (Fig. 1a). During upright CT scanning, the volunteers stood in the transverse gantry with the shoulders adducted and the arms held in the neutral position, and a support pole was placed behind them to stabilize the back (Fig. 1b). CT scans of both shoulders in the standing position were acquired during up-and-down movements of the transverse gantry [7]. The total effective dose of radiation exposure was tracked during all CT scanning and was controlled to not exceed 30 mSv. The image data were extracted in the obtained Digital Imaging and Communication in Medicine data format.

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Measurements of shoulder girdle alignment

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Using OsiriX MD 11.0.0 software (Pixmeo, Geneva, Switzerland), the specific bony landmarks were identified on both supine and standing CT scans of 200 shoulders. According to the International Society of Biomechanics standardization proposal for the upper extremity, the 7th cervical vertebra (C7), 8th thoracic vertebra (T8), sternal notch (incisura jugularis; IJ), and xiphoid process (processus xiphoideus; PX) were digitized as the thoracic landmarks [10]. The sternoclavicular joint, which was defined as the most ventral point on the proximal end of the clavicle (SC), and the acromioclavicular joint, which was defined as the most dorsal point on the distal end of the clavicle (AC), were used for the clavicle, whereas the root of the scapular spine (trigonum spinae scapulae; TS), inferior angle (angulus inferior; AI), and posterolateral edge of the acromion (angulus acromialis; AA) were used for the scapula.

The local 3-dimensional coordinate systems of the thorax, clavicle, and scapula were defined from the specific bony landmarks on both supine and standing CT scans. In the thoracic coordinate system, the Y-axis was defined as the line connecting the midpoint between PX and T8 and the midpoint between IJ and C7, pointing upward; the Z-axis was the line perpendicular to the plane formed by IJ, C7, and the midpoint between PX and T8, pointing lateral; and the X-axis was the common line perpendicular to the Z- and Y-axes, pointing forward. The origin of the thoracic coordinate system was coincident with the sternal notch. In the clavicular coordinate system, the Z-axis was defined as the line connecting SC and AC, pointing lateral; the X-axis was the line perpendicular to the clavicular Z-axis and the thoracic Y-axis, pointing forward; and the Y-axis was the common line perpendicular to the X- and Z-axes, pointing upward (Fig. 2a). In the scapular coordinate system, the Z-axis was defined as the line connecting TS and AA, pointing lateral; the X-axis was the line perpendicular to the plane formed by AI, AA, and TS, pointing forward; and the Y-axis was the common line perpendicular to the X- and Z-axes, pointing upward (Fig. 2b). The angular rotation of the clavicle and scapula were calculated using Cardan or Euler angles relative to the thorax following recommendations of the International Society of Biomechanics [10]. Clavicular rotation with respect to the thorax was described as clavicular elevation/depression and retraction/protraction, and scapular rotation with respect to the thorax was described as scapular upward/downward rotation, anterior/posterior tilting, and internal/external rotation [5]. Clavicular axial rotation is usually defined as 0° on the thoracic coordinate systems and could not be evaluated in this study. To assess the movements of the clavicle and the scapula, the 3-dimensional position of the clavicle center, which was defined as the midpoint between the acromioclavicular joint and the sternoclavicular joint, and that of the scapula center, which was defined as the center of gravity of the triangle consisting of the 3 bony landmarks of the scapula, were also calculated in superior/inferior, anterior/posterior, and medial/lateral directions on the thorax coordinate system.

Statistical analysis

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Statistical analyses were performed using IBM SPSS Statistics 25.0.0.0 software (IBM Corp., Armonk, NY, USA). Intrarater and interrater reliabilities were first evaluated in 20 randomly selected cases using intraclass correlation coefficients (ICCs). Repeated measurements by 1 observer with a 3-month interval (ICC model 1, 1) and blinded measurements by 2 observers (ICC model 2, 1) were performed. After determining intra- and interrater reliabilities, analyses were performed for all shoulders by 1 observer.

The values of the angular rotations of the clavicle and the scapula and of the positions of the clavicle center and the scapula center in standing CT scans were compared with those in supine CT scans using Wilcoxon signed-rank tests. Sex differences in angular rotations and positions of the clavicle and the scapula were also assessed using Mann-Whitney U tests. The significance level was set at 0.05 for all analyses.

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