Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load

Abstract

Background Reverse total shoulder arthroplasty (RTSA) is widely used; however, the effects of RTSA geometric parameters on joint and muscle loading, which strongly influence implant survivorship and long-term function, are not well understood. By investigating these parameters, it should be possible to objectively optimize RTSA design and implantation technique. Questions/purposes The purposes of this study were to evaluate the effect of RTSA implant design parameters on (1) the deltoid muscle forces required to produce abduction, and (2) the magnitude of joint load and (3) the loading angle throughout this motion. We also sought to determine how these parameters interacted.Methods Seven cadaveric shoulders were tested using a muscle load-driven in vitro simulator to achieve repeatable motions. The effects of three implant parameters-humeral lateralization (0, 5, 10 mm), polyethylene thickness (3, 6, 9 mm), and glenosphere lateralization (0, 5, 10 mm)-were assessed for the three outcomes: deltoid muscle force required to produce abduction, magnitude of joint load, and joint loading angle throughout abduction. Results Increasing humeral lateralization decreased deltoid forces required for active abduction (0 mm: 68% ± 8% [95% CI, 60%-76% body weight (BW)]; 10 mm: 65% ± 8% [95% CI, 58%-72 % BW]; p = 0.022). Increasing glenosphere lateralization increased deltoid force (0 mm: 61% ± 8% [95% CI, 55%-68% BW]; 10 mm: 70% ± 11% [95% CI, 60%-81% BW]; p = 0.007) and joint loads (0 mm: 53% ± 8% [95% CI, 46%-61% BW]; 10 mm: 70% ± 10% [95% CI, 61%-79% BW]; p \ 0.001). Increasing polyethylene cup thickness increased deltoid force (3 mm: 65% ± 8% [95% CI, 56%-73% BW]; 9 mm: 68% ± 8% [95% CI, 61%-75% BW]; p = 0.03) and joint load (3 mm: 60% ± 8% [95% CI, 53%-67% BW]; 9 mm: 64% ± 10% [95% CI, 56%-72% BW

A Critical Review on Prosthetic Features Available for Reversed Total Shoulder Arthroplasty

“…However, a glenosphere of more than half a sphere will allow a lateralization of the center of rotation. This in turn facilitates the function of the remaining rotator cuff [23, 26, 41]. This morphology of the glenosphere determines the lateralization of the center of rotation of the joint.…”

“…The thickness of the insert can also differ to help achieve optimal stability and lateralization but one should be careful not to overstuff the joint creating negative effects on deltoid force and joint loading [26]. There is interadaptibility available so that most sizes of humeral components can be combined with most sizes of glenospheres.…”

“…This is why a prosthesis with a built-up lateralized center of rotation exhibits a less weakened external rotation [23, 24]. Another way to overcome this limitation can be found in certain surgical techniques and prosthetic designs creating a more lateral position of the humerus to increase the deltoid wrapping resulting in a better tension in the anterior and posterior fibers [25, 26]. …”

Development and Application of a Novel Metric to Characterize Comprehensive Range of Motion of Reverse Total Shoulder Arthroplasty

“…Several experimental studies have used biomechanical models, with cadavers or bone substitutes, to identify relationships between implant design parameters and ROM . In a study utilizing artificial shoulder bones, Gutierrez et al found that increasing lateral offset of the center of rotation increased impingement‐free abduction ROM.…”

“…In addition, current computational models do not consider the effects of multiple motion‐limiting factors simultaneously. For example, while increasing the center of rotation lateralization decreases impingement, it simultaneously increases the required deltoid force . So, what is the ultimate effect on the comprehensive ROM?…”