Type 2 diabetes, metabolic health, and the development of frozen shoulder: a cohort study in UK electronic health records

ObjectiveEstimate the effect of type 2 diabetes on the development of frozen shoulder and investigate whether the effect is mediated by other metabolic factors.MethodsPrimary care medical record-based cohort study containing 43,977 people newly diagnosed with type 2 diabetes and 43,977 without diabetes. Variables were identified using established Read codes. A weighting approach with Cox regression was used to decompose the total effect into the direct effect and indirect effect, mediated by metabolic health (which was defined as the number of metabolic factors developed during follow-up). Estimates were expressed as hazard ratios (HR). Confounders were identified using a DAG. Sensitivity to unmeasured confounding, extreme weights, and missing data were tested.ResultsThe total effect of type 2 diabetes on the development of frozen shoulder was HR = 4.38 (95% CI: 3.70-5.21), the natural indirect effect (mediated through metabolic health) was HR = 0.98 (95% CI: 0.93-1.03) and the natural direct effect was HR = 4.46 (95% CI: 3.68-5.41). Results were robust to unmeasured confounding, extreme weights, and missing data.ConclusionsThis study suggests that type 2 diabetes may be a cause of frozen shoulder but does not support the hypothesis that the effect is mediated by metabolic health. Clinicians should remain alert that shoulder pain in people with diabetes could be indicative of a frozen shoulder. This study should raise awareness that, despite often being overlooked, musculoskeletal conditions can be complications of diabetes and should be considered during clinical conversations with patients.ISAC protocol registration number19_219R.