The boundary between microbial detection and implant-related infection: a structured narrative review of analytical credibility, biological persistence, host response, and clinical causality

BACKGROUND: Unexpected microbial signals are frequently encountered in patients with orthopaedic implants who do not satisfy validated criteria for confirmed infection. The central interpretive problem is not limited to periprosthetic joint infection (PJI); it also arises in fracture-fixation and spinal instrumentation, and it involves conventional culture, sonication, histopathology, broad-range polymerase chain reaction (PCR), 16 S rRNA sequencing, and metagenomic next-generation sequencing (mNGS). METHODS: We performed a structured narrative review of literature identified in PubMed, Embase, and Web of Science, updated through 27 June 2026. Eligible publications addressed microbial or biofilm-associated signals in orthopaedic implants without unequivocal infection at the time of sampling, or reported outcomes after unexpected positive cultures. Evidence was organized by implant type, indication for implant removal, detection method, host-response evidence, and longitudinal outcome. Particular attention was paid to the distinction between routinely removed fracture-fixation devices and failure-related arthroplasty or spinal revisions. RESULTS: Routine removal of fracture-fixation devices provides the least confounded clinical window and demonstrates that microbial signals may be recovered from implants in the absence of recognized failure or overt infection. By contrast, arthroplasty and spinal cohorts labelled "presumed aseptic" are intrinsically confounded because pain, loosening, osteolysis, or pseudarthrosis may represent occult infection. Molecular positivity is further limited by low-biomass contamination and inability to distinguish viable organisms from relic DNA. Favorable outcomes after revision do not prove that infection was never present, because revision surgery itself may eradicate or substantially reduce an occult microbial burden. CONCLUSIONS: Microbial detection should be interpreted through three sequential domains: analytical credibility, biological activity or persistence, and clinical causality supported by host response or implant-related pathology. "Stable coexistence" is best treated as a longitudinal clinical phenotype or testable mechanistic hypothesis, not as an established diagnostic category. This framework complements, but does not replace, validated PJI, fracture-related infection (FRI), or other implant-specific definitions.