The Impact of Shoulder Abduction on the Anatomy of the Interscalene Triangle

Neurogenic thoracic outlet syndrome (TOS) occurs in three major anatomical locations, including the interscalene triangle. Because symptoms of TOS are reproduced on upper limb abduction, this study aimed to examine the impact of glenohumeral abduction on the triangle's dimensions and whether this may contribute to compression of the structures which traverse it. Ten interscalene triangles were dissected from five body donor specimens bilaterally to measure the length of the anterior and middle scalene muscles, and inferior border of the scalene triangle when the upper limb was abducted to 0°, 90°, and at maximal abduction. Both the anterior and middle scalene lengths decreased as the angle of abduction increased, with the greatest decrease being between 0° and 90° (p = 0.0003). No significant decrease in length was shown between 90° and > 90° abduction (p = 0.48) nor was there a significant change in the overall area of the triangle throughout abduction (p = 0.58). This suggests that TOS symptoms may not correlate with the degree of upper limb abduction as previously thought. Additionally, there were significant differences between parameters of the superficial triangle measured by most previous studies, and a deeper triangle within the same space at all levels of abduction. This study therefore recommends further exploration into the dynamic nature of the interscalene triangle.