Standard-length stems require lower distal filling ratios than short stems to achieve neutral alignment in reverse total shoulder arthroplasty.

Background: Short humeral stems in reverse total shoulder arthroplasty have been introduced to reduce stress shielding by decreasing distal filling ratios (DFRs). However, reduced diaphyseal engagement may compromise coronal alignment. This study compared coronal alignment between short and standard-length stems and identified implant-related and bone-related factors associated with malalignment. Methods: Consecutive patients undergoing primary reverse total shoulder arthroplasty with short or standard-length (long) Perform humeral stems (Stryker) were included from 2 institutional registries from 2023-2025. Standardized radiographs were analyzed retrospectively. Coronal alignment was measured on true anteroposterior radiographs at a minimum 6-month follow-up. Malalignment was defined as > 5° deviation. Pre-operative computed tomography assessed metaphyseal cancellous bone density and cortical thickness (Tingart method). Post-operative radiographs were used to measure DFR and metaphyseal collar filling ratio. Between-group comparisons were performed using the Wilcoxon rank-sum test. Multivariable linear regression evaluated the independent effect of stem type on alignment, adjusting for age, sex, metaphyseal cancellous bone density, and metaphyseal collar filling ratio. A sensitivity model incorporating DFR and stem length assessed the contribution of endosteal contact. Results: A total of 114 patients were included (57 short stems and 57 standard-length stems). Short stems demonstrated greater varus-valgus deviation than standard-length stems (median 3.50° [interquartile range: 1.90°-5.10°] vs. 1.90° [interquartile range: 1.00°-3.90°]; Conclusion: This study demonstrates that standard-length humeral stems are associated with more consistent coronal alignment than short stems, although the clinical significance of this radiographic difference remains unknown. Standard-length stems demonstrated consistent alignment above an observed DFR of 48%, whereas short stems did so above 59%. Stem length independently influenced coronal alignment and allowed stable positioning despite lower DFRs. These findings suggest that greater stem length may permit reduced distal cortical engagement without compromising coronal alignment.