**1. Introduction**

In the past years, the prevalence and clinical use of reverse total shoulder arthroplasty (rTSA) in the USA has dramatically increased by 40.8%, with 30,850 procedures being performed in 2013 compared to 21,916 in 2011 [1]. First designed by Paul Grammont in 1985 for the treatment of arthritic shoulders with severe cuff insufficiency [2,3], the rationale of rTSA was to medialize the center of rotation, and to distalize the humerus relative to the acromion, resulting in increased deltoid muscle tension in an attempt to facilitate active forward elevation (AFE) [4]. The initial design with a humeral inclination of 155◦ showed promising long-term functional outcomes, however it failed to restore active external rotation (AER) and led to significant scapular notching, which has been reported to occur in 74% to 88% of cases [2,3,5–8].

Thus, recent studies have focused on significant design modifications to improve active range of motion (ROM) by increasing lateralization on the glenoid side, implementing a more anatomic humeral inclination of 135◦, and decreasing distalization of the humeral shaft [5,6,9–12]. As a result, Boileau et al. demonstrated that lateralization of the glenoid improved postoperative AER, and subsequently decreased the risk of scapular notching [5]. However, the increased use of rTSAs still elicits high rates of postoperative complications, occurring in 39% to 59% of cases [13,14]. However, of interest, Mahendraraj et al. recently showed that the distalization shoulder angle (DSA) and lateralization shoulder angle (LSA) may be reproducible measures, but seem to have only marginal correlation with postoperative clinical outcomes. As such, further investigations into the prognostic utility of minimally cumbersome rTSA measurement methodologies are warranted [15].

As intraoperative implant positioning has been shown to influence complication rates, attempts have been made to correlate pre- and postoperative radiographic measurements to clinical and functional outcomes [16,17]. However, these measurements and their correlation to outcomes in patients following rTSA are controversial among shoulder surgeons, while current evidence on the importance of these measurements is still lacking. Previous attempts to measure distalization of the humerus as well as medialization of the center of rotation have been considered to be too demanding for daily clinical practice [18,19], which has led Boutsiadis et al. to introduce more reproducible measurements and to evaluate their impact on postoperative clinical outcomes [20]. The authors showed that a lateralization shoulder angle (LSA) of 75◦ to 95◦ was correlated wirh increased AER, whereas a distalization shoulder angle (DSA) of 40◦ to 65◦ was correlated with increased AFE. However, the reported findings were obtained from a heterogeneous rTSA cohort using 145◦ and 155◦ designs and were limited in external validity and due to small sample sizes. Further, Jeon et al. found insufficient AFE in patients with increased postoperative lateral humeral offset (LHO) [21]. However, this was only observed when rTSAs were performed using medialized implants, to increase the force on the anterior deltoid (in patients with severe cuff tear arthropathy). As such, data on patients undergoing rTSA using a 135◦ prosthesis design remain limited.

The purpose of this study was to determine prognostic radiographic factors affecting clinical and functional outcomes in patients undergoing primary rTSA using a design with a humeral inclination of 135◦. The authors hypothesized that there would be no significant correlation between radiographic measurements and clinical and functional outcomes following primary rTSA.
