The Complex Relationship Between Body Mass Index and Risk for Adverse Outcomes Following Pyrocarbon Metacarpophalangeal Arthroplasty for Rheumatoid Arthritis.

BACKGROUND Use of pyrocarbon metacarpophalangeal (MCP) implants in rheumatoid arthritis (RA) patients has been associated with high rates of complications and reoperations. To date, few patient-specific prognostic factors have been identified. This study investigated body mass index (BMI) as a risk factor for RA patients undergoing pyrocarbon MCP arthroplasty.

METHODS We included 148 joints (51 patients) that underwent pyrocarbon MCP implantation at our institution from 2000 to 2022. We analyzed survival free from revision, reoperation, and development of postoperative complications using cluster-robust Cox proportional hazard models with restricted cubic splines to plot non linear relationships. Secondarily, we compared survival above and below the World Health Organization cutoff for obesity of ≥ 30 kg/m2, adjusting for patient age and sex.

RESULTS The relationship between BMI and risk for complications and revisions was J-shaped, with an increase in risk beyond 29 kg/m2. The relationship between BMI and late reoperations was U-shaped, with patients above 27.3 kg/m2 and below 23.2 kg/m2 at elevated risk. A BMI ≥ 30 kg/m2 was associated with a 7.9-times increased risk for revision, 8.3-times increased risk for late reoperations, and 3.7-times increased risk for postoperative complications. Body mass index did not significantly influence risk for early reoperation.

CONCLUSIONS Body mass index was associated with pyrocarbon MCP implant failure in a non linear fashion, with overweight patients at risk for complications leading to revisions and late reoperations. Underweight patients may be at increased risk for late reoperations. These findings should inform patient and implant selection and help guide counseling for postoperative expectations.