Migration patterns of the humeral component of the stemless Simpliciti Shoulder System in stemless anatomic total shoulder arthroplasty A radiostereometric and clinical study with 60-month follow-up.

BACKGROUND: The role of continuous migration in predicting long-term outcomes in total shoulder arthroplasty is yet unknown. We aimed to investigate migration patterns of the stemless humeral component of the Simpliciti Shoulder System using model-based radiostereometric analysis (MB-RSA) at five years postoperatively. METHODS: In this prospective longitudinal cohort study, MB-RSA was used to assess implant migration of the stemless Simpliciti Shoulder system. Primary outcomes were fixation and migration patterns of the humeral component. Secondary outcomes were the Constant-Murley Score, Oxford Shoulder Score (OSS), Disability of the Arm, Shoulder and Hand (DASH) score, the visual analogue scale (VAS), and complications. Linear mixed models were performed to analyze the primary and secondary outcomes. A Mann Whitney-U test was performed to explore differences between the groups with continuous migration and non-continuous migration of the humeral component at the 5-year follow-up. Alfa inflation was prevented with the Bonferroni correction. RESULTS: Twenty-three patients were included and had 5 years of follow-up. Median of the total translation between 24 months and 60 months (IQR) along the x-, y-, z-axis was -0.11 mm (IQR -0.47 to 0.02), -0.19 mm (IQR -0.28 to 0.03), and 0.15 mm (IQR -0.24 to 0.32), respectively. Median of the total rotation between 24 and 60 months around these axes was -0.37° (IQR -0.67 to 0.28), -0.10° (IQR -1.52 to 1.84), and -0.22° (IQR -0.85 to 0.43), respectively. Three patients showed continuous implant migration between 24 and 60 months. All clinical outcomes improved and stabilized in the first 12 months postoperatively and remained stable between 24 and 60 months. CONCLUSION: The current study demonstrated that stemless Simpliciti Shoulder systems remained stable between 24 and 60 months, when already stabilized before within the first 12 months. Furthermore, in three implants continuous migration persisted up to 60 months postoperatively, without impacting clinical outcomes or implant survival. We further demonstrated that implant stabilization is still possible after 24 months of follow-up. LEVEL OF EVIDENCE: Level IV, Case Series, Treatment Study.