The Influence of Reverse Total Shoulder Arthroplasty Implant Design on Biomechanics

Abstract

Purpose of review: As reverse total shoulder arthroplasty indications have expanded and the incidence of its use has increased, developments in implant design have been a critical component of its success. The purpose of this review is to highlight the recent literature regarding the effect of implant design on reverse total shoulder arthroplasty biomechanics.

Recent findings: Implant design for reverse total shoulder arthroplasty has evolved considerably from the modern design developed by Paul Grammont. The Grammont design had a medialized center of rotation and distalized humerus resulting from a 155° humeral neck shaft angle. These changes intended to decrease the forces on the glenoid component, thereby decreasing the risk for implant loosening and improving the deltoid moment arm. However, these features also led to scapular notching. The Grammont design has been modified over the last 20 years to increase the lateral offset of the glenosphere and decrease the prosthetic humeral neck shaft angle to 135°. These changes were made to optimize functional range of motion while minimizing scapular notching and improving active external rotation strength. Lastly, the introduction of preoperative planning and patient-specific instrumentation has improved surgeon ability to accurately place implants and optimize impingement-free range of motion. Success and durability of the reverse total shoulder arthroplasty has been contingent upon changes in implant design, starting with the Grammont-style prosthesis. Current humeral and glenoid implant designs vary in parameters such as humeral and glenoid offset, humeral tray design, liner thickness, and neck-shaft angle. A better understanding of the biomechanical implications of these design parameters will allow us to optimize shoulder function and minimize implant-related complications after reverse total shoulder arthroplasty.

Keywords: Biomechanics; Glenoid offset, Humeral offset; Reverse total shoulder arthroplasty.

Fig. 1

The medialization in center of rotation (COR) of the reverse total shoulder arthroplasty as compared to a native shoulder joint. The increase in deltoid moment arm and resultant force is demonstrated with medialization of the COR. With permission from Roche CP. Reverse shoulder arthroplasty biomechanics. JFMK. 2022;7:1–17 [4]

Fig. 2

Demonstrates a Grammont design immediately postoperatively (A) as well as an 8-year follow-up radiograph where scapular notching is demonstrated (white arrow) (B)

Fig. 3

Reverse total shoulder arthroplasty design classification scheme. With permission from Roche CP. Reverse shoulder arthroplasty biomechanics. JFMK. 2022;7:1–17

Fig. 4

Reverse total shoulder arthroplasty with inlay humeral component (A) compared to an onlay humeral component (B). Increased humeral lateralization can be seen in to the onlay prosthesis, and both constructs have equal glenosphere size/offset and neck shaft angle