Effects of Different Humeral Stem Length on Stem Alignment and Proximal Stress Shielding in Reverse Total Shoulder Arthroplasty

Background: Few comparative studies on the correlation between stem length, stem alignment, and/or stress shielding have been conducted in reverse total shoulder arthroplasty (rTSA). This study aimed to investigate the effects of different humeral stem lengths on stem alignment and proximal stress shielding after rTSA.

Methods: A total of 320 patients who underwent primary rTSA from October 2010 to May 2020 with at least 2 years of follow-up (mean follow-up: 32.6 months) were retrospectively reviewed. The participants were classified into three groups according to the humeral stem length of different prostheses types: group A (short stem, range: < 80 mm, n = 88), group B (medium stem, range: 80-100 mm, n = 155), and group C (standard stem, range: ≥ 100 mm, n = 77). The parameters, including stem alignment, stem distal tip decentering, and canal filling ratio (CFR) were determined and recorded on the radiographs at 4 weeks after surgery. Proximal humeral stress shielding and stem subsidence were evaluated using radiographs at 4 weeks after rTSA were compared with those at the final follow-up. Subgroup analyses were conducted according to the presence of stress shielding to determine correlations among stem length, malalignment, and stress shielding.

Results: Humeral stem malalignment was significantly higher in group A (21.6%) than in groups B (11.6%) and C (9.1%) (P = 0.018). However, stress shielding at the lateral metaphyses (36.4%) was more frequently observed in group C (P = 0.004). Longer stem, stem malalignment, and higher diaphyseal CFR were independent risk factors for stress shielding occurrence, with stem malalignment showing the highest odds ratio (14.82, P < .001).

Conclusion: Although shorter stems are beneficial for bone preservation, they could lead to stem malalignment, resulting in increased proximal humeral stress shielding.