Ex vivo shoulder motion simulators are commonly used to study shoulder biomechanics but are often limited to performing simple planar motions at quasi-static speeds using control architectures that do not allow muscles to be deactivated. The purpose of this study was to develop an open-loop tendon excursion controller with iterative learning and independent muscle control to simulate complex multiplanar motion at functional speeds and allow for muscle deactivation. The simulator performed abduction/adduction, faceted circumduction, and abduction/adduction (subscapularis deactivation) using a cadaveric shoulder with an implanted reverse total shoulder prosthesis. Kinematic tracking accuracy and repeatability were assessed using maximum absolute error (MAE), root mean square error (RMSE), and average standard deviation (ASD). During abduction/adduction and faceted circumduction, the RMSE did not exceed 0.3, 0.7, and 0.8 degrees for elevation, plane of elevation, and axial rotation, respectively. During abduction/adduction, the ASD did not exceed 0.2 degrees. Abduction/adduction (subscapularis deactivation) resulted in a loss of internal rotation, which could not be restored at low elevation angles. This study presents a novel control architecture, which can accurately simulate complex glenohumeral motion. This simulator will be used as a testing platform to examine the effect of shoulder pathology, treatment, and rehabilitation on joint biomechanics during functional shoulder movements.

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.

A muscle's moment arm represents its mechanical advantage and indicates its role in joint actuation and rotational stability. The objective of this study was to use an ex-vivo simulator to map the moment arms of eight major shoulder muscles across a continuous range of motion. The three-dimensional moment arms for the deltoid (anterior, lateral, and posterior), subscapularis (inferior and superior), supraspinatus, infraspinatus, and teres minor were measured in eight specimens (57 ± 6 years) using the tendon excursion method. The anterior deltoid had a significantly larger elevation moment arm in anterior planes of elevation (p < 0.001) while the lateral deltoid had a significantly larger elevation moment arm in posterior planes (p < 0.001). The posterior deltoid was an antagonist to elevation with anterior arm orientations (p < 0.001). The supraspinatus had biphasic function; in anterior elevation planes it was a horizontal extensor and internal rotator but was a horizontal flexor and external rotator in posterior planes (p < 0.001). The infraspinatus and superior subscapularis were both arm elevators, but the infraspinatus was an external rotator and horizontal extensor while the superior subscapularis was an internal rotator and horizontal flexor. The inferior subscapularis was a horizontal flexor and internal rotator while the teres minor was an antagonist to elevation, horizontal extensor, and external rotator. Each muscle had a multifaceted function which changed significantly with arm orientation for all muscles except the inferior subscapularis. The muscle moment arms maps created in this study improve current understandings of the three-dimensional function of eight major muscles in the shoulder.