The association of body mass index with patient outcomes after shoulder replacement surgery: Population-based cohort study using linked national data from the United Kingdom and Denmark.

BACKGROUND: There is growing evidence that access to joint replacement surgery is being restricted based on body mass index (BMI) despite any formal recommendations. Our aim was to investigate the association between BMI and patient outcomes after elective primary shoulder replacement surgery to inform future commissioning and national guidance. METHODS AND FINDINGS: In this population-based cohort study, patients aged 18-100 years having elective primary shoulder replacement surgery were identified using linked national joint registry and hospital data from public and private hospitals in the United Kingdom (2018-22) and Denmark (2006-21). The main outcome measure was mortality within 365 days of surgery. Secondary outcome measures included mortality within 90 days, serious adverse events within 90 days, and revision surgery within 4.5 years of surgery. The association between BMI and patient outcomes was assessed using flexible parametric survival models and logistic regression models, adjusting for age, sex, deprivation, main surgical indication and American Society of Anaesthesiologists (ASA) score. 15,320 and 5,446 shoulder replacement procedures from within the United Kingdom and Denmark, respectively, met the inclusion criteria. In the United Kingdom, the average age was 72.2 years, 68.3% were female and the average BMI was 29.4 kg/m2. In Denmark, the average age was 70.5 years, 65.3% were female and the average BMI was 28.0 kg/m2. There was a decreased risk of 365-day mortality in obese (BMI 40 kg/m2) patients (hazard ratio (HR) 0.40 [95%CI 0.21, 0.73]) and an increased risk in underweight (BMI < 18.5 kg/m2) patients (HR 1.18 [95%CI 1.06, 1.32]), compared to patients with BMI 21.75 kg/m2. Underweight patients had an increased risk of 90-day mortality (HR 1.69 [95%CI 1.14, 2.52]), 90-day serious adverse events (odds ratio 1.36 [95%CI 1.05, 1.77]) and revision surgery (HR 1.70 [95%CI 1.25, 2.33]). Increasing BMI was not associated with a significantly increased risk of any secondary outcome. The main limitation of this study was the high proportion of missing BMI data and the small case numbers for the underweight study population (n = 131[UK], 70[Denmark]). CONCLUSIONS: Increasing BMI was associated with lower 365-day mortality, and no poorer outcomes after elective primary shoulder replacement surgery. This surgery is safe and effective in obese patients and access to shoulder replacements should not be restricted based on BMI alone. Clinicians and hospitals should be aware that underweight patients appear more at risk of mortality, serious adverse events and revision surgery after shoulder replacement.