Relationship Between the Axis of the Humeral Heads and the Spinal Sagittal Alignment in Asymptomatic Volunteers

Background Currently, there is no known correlation between relevant parameters of respective non-adjacent spinal segments, which suggests the possibility of a third external factor. To our knowledge, no study has examined the humeral head position in consideration of spinal sagittal alignment.This study aims to establish new parameters related to the humeral head axis and evaluate the relationship between the axis of the humeral head and spinal sagittal alignment in asymptomatic volunteers. Methods Standing posteroanterior and lateral radiographs of the entire spine were obtained from 62 asymptomatic volunteers. We analyzed the relationship of the newly established parameters related to the humeral head axis We analyzed the relationship of the newly established parameters related to the humeral head axis and other established spinal parameters. The new parameters are: shoulder tilt (ST: the angle between a line drawn from the center of the humeral head to the center of the superior endplate of T1 and vertical line through the center of the humeral head), shoulder incidence (SI: the angle between a line perpendicular to the upper endplate of the T1 vertebra and a line joining the center of the upper endplate of the T1 vertebra and the axis of the humeral head), shoulder pelvic angle (SPA: the angle between a line joining the axis of the humeral head and the femoral head and a line joining the axis of the femoral heads and the center of the upper endplate of the S1 vertebra), C2 shoulder angle (C2SA: the angle between a line joining the C2 body center and the axis of the humeral head and a line joining the axis of the humeral head and the center of the upper endplate of the T1 vertebra). Statistical analysis was performed using Pearson's correlation coefficient. Results Mean ST, SI, SPA, and C2SA were 20.0±20.6°, 43.7±21.6°, 7.7±6.6°, and 17.8±13.5° respectively. The SI was found to be correlated with pelvic tilt (r=0.373), lumbar lordosis (r=0.351), and thoracic kyphosis (r=0.469). Pelvic incidence was correlated with SPA (r=0.724), T1 pelvic angle (r=0.691), and C2 pelvic angle (r=0.667). Correlations were observed between SPA and T1PA (r=0.955) and C2PA (r=0.956), and between TIPA and C2PA (r=0.985). Conclusions This report introduced novel parameters related to humeral head position and confirmed their correlation with sagittal alignment from the cervical spine to the pelvis. Humeral head position may play a role in maintaining spinal sagittal balance.