Changes in joint architecture and muscle loading resulting from total shoulder
arthroplasty (
TSA) and reverse total shoulder
arthroplasty (RSA) are known to influence joint stability and
prosthesis survivorship. This study aimed to measure changes in muscle moment arms, muscle lines of action, as well as muscle and joint loading following
TSA and RSA using a
metal-backed uncemented modular
shoulder prosthesis. Eight cadaveric upper extremities were assessed using a customized testing rig. Abduction, flexion, and axial rotation muscle moment arms were quantified using the tendon-excursion method, and muscle line-of-force directions evaluated radiographically pre-operatively, and after
TSA and revision RSA. Specimen-specific musculoskeletal models were used to estimate muscle and joint loading pre- and post-operatively.
TSA lateralized the glenohumeral joint center by 4.3 ± 3.2 mm, resulting in small but significant increases in middle deltoid force (2.0%BW) and joint compression during flexion (2.1%BW) (p < 0.05). Revision RSA significantly increased the moment arms of the major abductors, flexors, adductors, and extensors, and reduced their peak forces (p < 0.05). The superior inclination of the deltoid significantly increased while the inferior inclination of the rotator cuff muscles decreased (p < 0.05).
TSA using an uncemented
metal-backed modular
shoulder prosthesis effectively restores native joint function; however, lateralization of the glenoid component should be minimized intra-operatively to mitigate increased glenohumeral joint loading and
polyethylene liner contact stresses. Revision RSA reduces muscle forces required during shoulder function but produces greater superior joint shear force and less joint compression. The findings may help to guide component selection and placement to mitigate
joint instability after
arthroplasty. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1988-2003, 2019.