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

Abstract

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 (p > 0.19). Additionally, the force required by the deltoid for active abduction was not affected significantly by cup constraint (p = 0.144). Interestingly, active abduction range of motion was also not affected significantly by changes in cup constraint (p > 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.

Keywords: complication; constraint; instability; polyethylene; reverse total shoulder arthroplasty; rotator cuff tear arthropathy.

Figure 1.

Custom modular instrumented reverse total shoulder arthroplasty implant (left) with load sensing device (A), glenoid base plate (B), glenosphere (C), adjustable humeral insert constraint level (D) and humeral component (E); as well as the three humeral cup constraints investigated (right) including low-constraint, normal, and high-constraint. The implant centre of rotation is constant for all constraints tested, and shown as a blue cross at the centre of the glenosphere.

Figure 2.

Lines of action of the three heads of the deltoid (red), infraspinatus (green), and subscapularis (orange) muscles.

Figure 3.

Mean (SD) reverse shoulder arthroplasty joint load versus abduction angle for all three polyethylene insert constraints investigated (low-, standard- and high-constraint). BW, body weight.

Figure 4.

Mean (SD) reverse shoulder arthroplasty joint load angle as a function of abduction angle for all polyethylene constraints investigated (low-, standard- and high-constraint).

Figure 5.

Mean (SD) deltoid force required to achieve active shoulder abduction versus abduction angle for all three polyethylene insert constraints investigated (low-, standard- and high-constraint). BW, body weight.

Figure 6.

Mean (SD) abduction arc of motion for all three polyethylene insert constraints investigated (low-, standard- and high-constraint). ROM, range of motion.

Figure 7.

Mean (SD) passive (top) and active (bottom) range of motion in internal (left) and external (right) range of motion for all three polyethylene insert constraints investigated (low-, standard- and high-constraint). ROM, range of motion.

Figure 8.

Mean (SD) angles between joint load and polyethylene cup edge throughout abduction for all three insert constraints investigated (low-, standard- and high-constraint).