What are We Matching On and Why?: A Systematic Review of Matched Study Designs in Shoulder Arthroplasty.

Background: Establishing patient-matched cohorts can be a valuable technique for minimizing selection bias in outcomes research pertaining to shoulder arthroplasty. This systematic review evaluated the variety and inconsistency with which matching techniques are employed in matched studies assessing outcomes following shoulder arthroplasty.

Methods: The PubMed, EMBASE, and Cochrane computerized databases were queried from their inception through December 2023 to identify clinical outcome studies of shoulder arthroplasty that employed a matched study design. Study quality was assessed via the Methodological Index for Non-Randomized Studies criteria. Matching techniques, covariates included, and covariate justification were aggregated.

Results: Among 110 studies encompassing 483,738 shoulder arthroplasties, 82 (74.6%) studies employed direct matching and 28 (25.5%) employed propensity score matching. Seventy-four distinct covariates were used in at least one study, with 86 unique combinations of covariates employed. Studies used a median of 4 covariates (range 1-27). The most common covariates were age (94.5%), sex (89.1%), body mass index (26.4%), smoking (19.1%), and follow-up duration (19.1%). Only 16 (14.6%) studies reported justification for the covariates included.

Conclusions: There are marked methodological discrepancies among studies using covariate matching methods in the shoulder arthroplasty literature. Future matched studies in shoulder arthroplasty should provide justification for included covariates and properly account for matching in their statistical analyses to enhance the validity of study findings. When patient matching is deemed appropriate, key variables to consider for matching may include patient age, sex, comorbidity burden and, when analyzing multiple clinical conditions, surgical indication.