Positional information on the shoulder girdle (the clavicle and scapula) is important for a better understanding of the function of the upper limb in the locomotive system as well as its associated disease pathogenesis. However, such data are limited except for information on the axial position of the scapula. Here, we describe a three-dimensional reconstruction of the shoulder girdle including the clavicle and scapula, and its relationship to different landmarks in the body. Thirty-six human fetal specimens (crown-rump length range: 7.6-225 mm) from the Kyoto Collection were used for this study. The morphogenesis and three-dimensional position of the shoulder girdle were analyzed with phase-contrast X-ray computed tomography and magnetic resonance imaging. We first detected the scapula body along with the coracoid and humeral head at Carnegie stage 18; however, the connection between the body and coracoid was not confirmed at this stage. During development, all landmarks on the shoulder girdle remained at the same axial position except for the inferior angle, which implies that the scapula enlarged in the caudal direction and reached the adult axial position in the fetal period. The scapula body was rotated internally and in the upward direction at the initiation of morphogenesis, but in the fetal period the scapula body was different than that in the adult position. The shoulder girdle was located at the ventral side of the vertebrae at the time of initial morphogenesis, but changed its position to the lateral side of the vertebrae in the late embryonic and fetal periods. Such a unique position of the shoulder girdle may contribute to the stage-specific posture of the upper limb. Adequate internal and upward rotation of the scapula could help in reducing the shoulder width, thereby facilitating childbirth. The data presented in this study can be used as normal morphometric references for shoulder girdle evaluations in the embryonic and fetal periods.

The upper extremity posture is characteristic of each Carnegie stage (CS), particularly between CS18 and CS23. Morphogenesis of the shoulder joint complex largely contributes to posture, although the exact position of the shoulder joints has not been described. In the present study, the position of the upper arm was first quantitatively measured, and the contribution of the position of the shoulder girdle, including the scapula and glenohumeral (GH) joint, was then evaluated.Twenty-nine human fetal specimens from the Kyoto Collection were used in this study. The morphogenesis and three-dimensional position of the shoulder girdle and humerus were analyzed using phase-contrast X-ray computed tomography and magnetic resonance imaging. Both abduction and flexion of the upper arm displayed a local maximum at CS20. Abduction gradually decreased until the middle fetal period, which was a prominent feature. Flexion was less than 90 at the local maximum, which was discrepant between appearance and measurement value in our study. The scapular body exhibited a unique position, being oriented internally and in the upward direction, with the glenoid cavity oriented cranially and ventrally. However, this unique scapular position had little effect on the upper arm posture because the angle of the scapula on the thorax was canceled as the angle of the GH joint had changed to a mirror image of that angle. Our present study suggested that measuring the angle of the scapula on the thorax and that of the GH joint using sonography leads to improved staging of the human embryo.