The original design, however, had its shortcomings, which included limited rotational range of motion (ROM) due to the reduced tension of the rotator cuff muscles as well as notching due to increased contact between the humeral component PE liner and the inferior scapular neck.
2,
22,
23,
24 As such, changes were proposed with the goal of repositioning the COR closer to that of a normal shoulder and increasing the clearance distance between the humerus and the scapular neck.
25,
26,
27,
28,
29 This was achieved by (1) lateralizing the glenosphere’s COR with metal
30 and bone graft augmented (BIO-RSA) constructs
31 and (2) lateralizing the humerus using an onlay stem design.
32 In addition, the latter was also achieved by decreasing the neck-shaft angle (NSA) from 155° to 135°.
28,
32 This allowed for different implant combinations and has resulted in more than 29 implant designs that are available in the marketplace.
5 This multitude of designs have led researchers to attempt to understand which construct most closely replicates the normal shoulder and leads to optimal outcomes using classification systems based on glenosphere and stem lateralization.
24,
33 However, the majority of the research comparing these configurations has been based on biomechanical and computer simulation studies,
24,
25,
26,
27,
28,
29 whereas patient outcomes remain less well understood across designs
6,
7,
8 as they are influenced by several factors not assessed in biomechanical models, such as surgical technique, amount of bone resection and loss, and soft-tissue tension.