Short stem humeral components in reverse shoulder arthroplasty: stem alignment influences the neck-shaft angle

Introduction Shorter humeral reverse total shoulder arthroplasty (RTSA) stems may reduce stress shielding, however, potentially carry the risk of varus/valgus malalignment. This radiographic study's purpose was to measure the incidence of stem malalignment and thus the realized neck-shaft angle (NSA). The hypothesis was that malalignment of the stem is a frequent postoperative radiographic finding. Methods Radiographs of an uncemented curved short stem RTSA with a 145 degrees NSA were reviewed. The study group included 124 cases at a mean age of 74 (range 48-91) years. The humeral stem axis was measured and defined as neutral if the value fell within +/- 5 degrees of the longitudinal humeral axis. Angular values > 5 degrees were defined as malaligned in valgus or varus. The filling ratio of the implant within the humeral shaft was measured at the level of the metaphysis (FRmet) and diaphysis (FRdia). Results The average humeral stem axis angle was 4 +/- 3 degrees valgus, corresponding to a true mean NSA of 149 +/- 3 degrees. Stem axis was neutral in 73% (n = 90) of implants. Of the 34 malaligned implants, 82% (n = 28) were in valgus (NSA = 153 +/- 2 degrees) and 18% (n = 6) in varus (NSA = 139 +/- 1 degrees). The average FRmet and FRdia were 0.68 +/- 0.11 and 0.72 +/- 0.11, respectively. A low positive association was found between stem diameter and filling ratios (r = 0.39; p < 0.001); indicating smaller stem sizes were more likely to be misaligned. Conclusion Uncemented short stem implants may decrease stress shielding; however, approximately one quarter were implanted > 5 degrees malaligned. The majority of malaligned components (86%) were implanted in valgus, corresponding to an NSA of > 150 degrees. As such, surgeons must be aware that shorter and smaller stems may lead to axial malalignment influencing the true SA.