A biomechanical investigation of the surface strains on the acromion and scapular spine during simulated ex-vivo arm motion

While several biomechanical investigations have measured acromion and scapular spine strains for various pathological conditions to better understand the risk factors for fracture, no study has measured strains in the native shoulder. The objective of this study was to use an ex-vivo shoulder motion simulator to measure principal strain during continuous, unconstrained, muscle-driven motion of the native shoulder. Eight cadaveric specimens (57 ± 6 years) were used to simulate scapular plane abduction (27.5 to 80° of humerothoracic elevation), forward flexion (27.5 to 72.5° of humerothoracic elevation), external rotation (0 to 40° of external rotation), and circumduction (elliptical path) with glenohumeral rotation speeds of 10°/s. Principal strain was measured throughout motion in four clinically relevant regions of the scapular spine and acromion according to the Levy classification using tri-axial strain gauge rosettes. Increases in humeral elevation during scapular plane abduction and forward flexion were associated with increases in deltoid force and scapula strain. However, above approximately 60° of humerothoracic elevation, strains plateaued while deltoid forces continued to increase indicating that scapula strain patterns are influenced by deltoid force magnitude and direction. Scapula strain was higher during scapular plane abduction than forward flexion in all regions but was only significantly higher in Levy 3B (p = 0.038). The highest strains were observed in Levy regions 2 and 3A (p ≤ 0.01) which correspond to regions with the highest clinically observed fracture rates demonstrating that the shape of the acromion and scapular spine may influence strain distribution irrespective of the joint condition.