The effect of humeral polyethylene insert constraint on reverse shoulder arthroplasty biomechanics.

Background: There is little information on the effects of altering reverse shoulder arthroplasty (RSA) polyethylene constraint on joint load, load angle and deltoid force. The present biomechanical study aimed to investigate the effects of changing RSA polyethylene constraint on joint load, load angle, deltoid force and range of motion.

Methods: A custom RSA implant capable of measuring forces across the joint with varying polyethylene constraint was tested in six cadaveric shoulders. Standard-, low- and high-constraint (retentive) polyethylene liners were tested, and joint kinematics, loads and muscle forces were recorded.

Results: When polyethylene constraint was altered, joint load and load angle during active abduction were not affected significantly ( > 0.19). Additionally, the force required by the deltoid for active abduction was not affected significantly by cup constraint ( = 0.144). Interestingly, active abduction range of motion was also not affected significantly by changes in cup constraint ( > 0.45).

Conclusions: Altering polyethylene cup constraint in RSA to enhance stability does not significantly alter resultant joint loads and deltoid forces. Surprisingly, terminal abduction range of motion was also not significantly different with varying cup constraint, indicating that terminal impingement may be tuberosity related rather than polyethylene.