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]; p = 0.034).

Conclusions: Humeral lateralization was the only parameter that improved joint and muscle loading, whereas glenosphere lateralization resulted in increased loads. Humeral lateralization may be a useful implant parameter in countering some of the negative effects of glenosphere lateralization, but this should not be considered the sole solution for the negative effects of glenosphere lateralization. Overstuffing the articulation with progressively thicker humeral polyethylene inserts produced some adverse effects on deltoid muscle and joint loading.

Clinical relevance: This systematic evaluation has determined that glenosphere lateralization produces marked negative effects on loading outcomes; however, the importance of avoiding scapular notching may outweigh these effects. Humeral lateralization's ability to decrease the effects of glenosphere lateralization was promising but further investigations are required to determine the effects of combined lateralization on functional outcomes including range of motion.

Fig. 1A–C

(A) An isometric computer rendering of the custom modular humeral implant is shown. (B) The middle images show an isometric exploded view of the humeral implant with the modular components separated: a +3-mm polyethylene cup (Delta XTEND^TM, DePuy) (1), a humeral spacer which produces changes in humeral cup thickness in increments of 3 mm (2), a 155° head-neck angle component (3), and a humeral stem and baseplate component which facilitates cup adjustability through the use of retroversion dowel holes spaced at 5° (0°–20°), and threaded holes for lateralization of the humeral shaft spaced at 5 mm (−5 to +15 mm) (4). (C) A side view of the component shows its 155° head-neck angle.

Fig. 2A–C

A computer rendering of the custom modular glenoid implant shows (A) an isometric view of the assembled glenosphere components with coordinate frame indicating the load measurement directions; (B) an isometric exploded view of the four glenoid components separated: a custom 38-mm diameter hemispheric glenosphere component with hollowed-out back (1), a 5-mm (also 0 and 10 mm) glenosphere lateralization spacer which nests in the hollow of 1 (2), a six-axis load sensor designed to nest in the hollow of 1 and in the reamed glenoid fossa (3), and a glenosphere baseplate for fixation of the glenoid implant to the scapula (4); and (C) the reverse angle of the isometric exploded view.

Fig. 3

A computer rendering of the in vitro muscle loading-driven active motion simulator with a right shoulder mounted shows: a scapula and humerus implanted with the custom adjustable, instrumented RTSA prosthesis (1); rotating scapula pot (2); motor and linkage mechanism to drive scapula pot rotation (3), low-friction deltoid and rotator cuff cable guide system which routes sutures from the muscle attachment to low friction pneumatic actuators (out of frame to the right) (4), optical trackers used to provide real-time kinematic feedback to the control system (5), and weight used to replace the mass of the resected distal arm (6). Soft tissues are omitted for clarity.

Fig. 4

Implant parameters whose effects on the deltoid force varied across abduction are shown. The means (SDs omitted for clarity) of deltoid force averaged across all levels of the geometric parameters (Abduction Main Effect) and for differing levels of humeral and glenosphere lateralization (0, 5, 10 mm) are shown. SDs range from 4% to 16% body weight.

Fig. 5

The deltoid force interaction between humeral and glenosphere lateralizations is shown. The data represent means (SDs omitted for clarity) of deltoid force averaged across abduction and humeral cup thickness for changes in humeral and glenosphere lateralization. SDs range from 6% to 12% body weight.

Fig. 6

The implant parameters whose effects on joint load varied across abduction are shown. The data represent the mean (SDs omitted for clarity) joint load averaged across all levels of the geometric parameters (Abduction Main Effect) and for differing levels of humeral and glenosphere lateralization (0, 5, 10 mm). SDs range from 7% to 12% body weight.

Fig. 7

The changes in joint load angle across abduction are shown. The data represent the means (SDs omitted for clarity) of joint load angle averaged across all levels of the geometric parameters (Abduction Main Effect) and for differing levels of glenosphere lateralization (0, 5, 10 mm), which produced a significant interaction with abduction whereby lateralization decreased load angle early during abduction but had no effect at the end of motion. SDs range from 10° to 13°.