A pre-specified surgical-complexity index is associated with a graded increase in periprosthetic joint infection after total knee arthroplasty: a competing-risks analysis using coronal alignment as a negative control

Prediction of periprosthetic joint infection (PJI) after total knee arthroplasty (TKA) relies almost wholly on host factors, and whether the operation itself carries infection risk independent of the patient is unknown. We analyzed 2920 primary TKAs in 2350 patients from a single institution with national joint registry linkage, using death as a competing risk and coronal alignment as a negative-control exposure. A pre-specified surgical-complexity index counted five construct markers at the index operation: constraint, a tibial stem, a femoral stem, a previous implant and a pre-existing extra-articular deformity or fracture. Septic revision was the primary outcome and aseptic revision the control. Fine–Gray subdistribution hazard models with patient-level clustering estimated complexity and, in the paired-radiograph subset, neutral-alignment effects on both outcomes. There were 51 septic and 104 aseptic revisions and 525 deaths over a median follow-up of 7.4 years. Each additional complexity marker raised the subdistribution hazard of PJI (sHR 1.61, 95% CI 1.27–2.04) but not of aseptic revision (sHR 1.06, 95% CI 0.84–1.33); 5-year PJI incidence rose from 1.5% with zero or one marker to 5.9% with two or more. Neutral coronal alignment was protective against aseptic revision (sHR 0.42, 95% CI 0.26–0.67) but null for PJI (sHR 0.87, 95% CI 0.41–1.87). Adjustment for skin-to-skin operative time, available for 93% of the cohort, moved the complexity estimate in the same knees only from 1.58 to 1.55 and attributed 5 % of the effect to operative time. These are observational associations and not causal effects, but surgical exposure merits consideration alongside host factors in infection-risk assessment.