Characteristics and Outcomes of Occult Infections in Presumed Aseptic Nonunions: A Retrospective Cohort Study

Objectives: To determine (1) the rate of positive cultures in presumed aseptic nonunions, (2) the rate and microbial spectrum of positive cultures that represented occult infection, and (3) rates of nonunion healing. Design: Retrospective cohort study. Setting: Tertiary referral center. Patients Selection Criteria: Adult patients with a presumed aseptic nonunion treated with single-stage revision between 2002 and 2022. Outcome Measures and Comparisons: The rate of positive cultures compared for two protocols: old: 1 – 2 samples cultured 7 days vs. new: 5 samples cultured 14 days. The rate of positive cultures meeting occult infection criteria with the new protocol (≥ 2 samples with phenotypically indistinguishable microorganisms, or ≥1 sample with a high virulent microorganism). Nonunion healing rates between protocols and between groups based on culture results with the new protocol. Results: 179 patients were included. The rate of positive cultures was 14% (n = 15/105) with the old protocol, and 51% (n = 38/74) with the new protocol (p < 0.001). With the new protocol, the rate of positive cultures meeting occult infection criteria was 19% (n = 14/74), and Coagulase-negative staphylococci (48%) and Cutibacterium acnes (38%) were the most common microorganisms. Nonunion healing rates after the primary revision did not differ between protocols (old: 82% vs. new: 86%, p = 0.41) and groups based on culture result (sterile: 86% vs. occultly infected: 93%, p = 0.66). The final overall nonunion healing rate was 97%. Conclusion: Occult infections were identified in 1 in 5 presumed aseptic nonunions using a standardized protocol with 5 intraoperative samples cultured 14 days, and were predominantly caused by slow growing, gram-positive microorganisms. The local spectrum and antimicrobial sensitivity of occult infections should be considered when developing empiric antimicrobial protocols. Patients with presumed aseptic nonunions can expect high healing rates regardless of the culture result. Level of evidence: Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.