Arm positions with increased risk of subscapularis external impingement at the subcoracoid arch

Abstract

Background: A cadaveric biomechanical study was used to analyze arm positions that could lead to increased risk of subscapularis tears due to subcoracoid impingement. Methods: Six cadaveric shoulders (two male and four female; mean age, 68.4±2.3 years) were evaluated for subcoracoid external impingement using a custom shoulder testing system with a pressure-mapping sensor. The contact area and the mean and peak contact pressures between the subcoracoid arch and the subscapularis complex were measured. Eight arm positions were assessed, including 20° and 60° forward flexion (FF) and abduction (ABD) with maximal internal rotation (IR) and external rotation (ER). Results: The overall incidence of subcoracoid impingement was 52.1% across all tests, with no contact observed at 20° ABD in the maximal IR position. Except for 20° ABD with maximal IR, the mean contact area significantly differed across the seven arm positions (P=0.009). However, mean and peak contact pressures did not show significant differences (P=0.188 and P=0.065, respectively). The highest mean contact pressure was recorded at 60° ABD with maximal ER (25.7±17.4 kPa), followed by 20° FF with maximal IR (23.2±12.5 kPa), 60° FF with maximal IR (18.2±8.3 kPa), and 60° ABD with maximal IR (18.3±12.0 kPa). The contact area and peak contact pressure exhibited similar trends to mean contact pressure. Conclusions: This cadaveric study demonstrated increased subcoracoid arch contact when shoulders were at 20° and 60° FF with IR and at 60° ABD with both ER and IR. These findings suggest potential external subscapularis impingement in these positions, although not all comparisons were statistically significant. Level of evidence: Cadaveric biomechanical study.