The presence of persistent synovial inflammation after “Eradication” unmasks the “Unseen” dormant state of infection allowing the prediction of infection free survival in total joint replacements

Introduction
The conventional clinical criteria for diagnosing a periprosthetic joint infection (PJI) rely on acute inflammatory readouts such as erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and synovial white blood cell counts. These metrics only detect actively septic joint replacements and may miss detecting biofilm-embedded bacteria that suppress neutrophil signaling and persist as a “hidden” subset of implants with a dormant infection. We hypothesize that previously infected joint replacements have a high prevalence of dormant infection that can be distinguished from aseptic revision joint replacements (replaced for instability, loosening, wear, and fracture) by the persistent inflammatory response within synovial fluid and/or circulating plasma, and that detecting a dormant infection indicates an increased risk of infection relapse.

Methods
This is an observational cohort study using synovial fluid and plasma proteomics of 96 immuno-oncology mediators (Olink Proteomics, Sweden) with three-year clinical follow-up from a single academic medical center (Stanford University, USA). Thirty patients undergoing revision joint replacement: culture-positive actively septic joint replacements (n = 7), aseptic revision joint replacements (n = 12), and re-implantations of joint replacements previously classified as infection-free by 2018 Musculoskeletal Infection Society (MSIS) criteria (n = 11). Differential expression, unsupervised clustering, Euclidean distance mapping, principal-component analysis, and gene-set variation analysis were used to define the inflammatory signature of dormant infection present in joint replacements with a prior infection. The identified biomarkers of dormant infection were correlated with the three-year incidence of infection relapse.

Results
Eight of eleven MSIS-cleared joint replacements (73%) clustered with culture-positive active infections despite normal ESR, CRP, and scant synovial neutrophils revealing the synovial inflammatory signature of dormant infection. A nine-analyte synovial panel consisting of PDGF-B, CXCL5, CXCL11, MCP-2 (CCL8), ANGPT1, TIE2, EGF, NOS3, and Gal-1 distinguished dormant infection from truly aseptic cases with 100% specificity and positive predictive value (sensitivity 22%, negative predictive value 74%). Synovial CXCL5 over-expression was a universal hallmark of both active and dormant infection, whereas matched plasma profiles showed no discriminatory power for all immuno-oncology mediators tested. Dormant infections exhibited downregulation of granulocyte activation and T-cell proliferation pathways (FDR < 0.001), mirroring immune evasion programs seen in cancer microenvironments. After a mean of 3 years follow-up, infection relapse occurred in 22% of the biomarker-positive dormant infections, but relapse did not occur in any of the biomarker-negative aseptic cases.DiscussionProfiling of the persistent inflammatory response within the synovial fluid of two-stage re-implantations classified as “infection-free” by the MSIS criteria unmasked a clinically silent reservoir of biofilm-embedded bacteria that suppress clinical diagnostic criteria of active infection and define the novel clinical state of dormant infection. We used that profile to identify a novel culture-free panel of rule-out biomarkers for determining which re-implantations were safe from infection relapse. These findings challenge the use of conventional clinical diagnostic criteria, which are over-reliant on acute phase reactants and neutrophil recruitment, and creates a new prognostic clinical paradigm that now includes a future with precision, immune-guided management of dormant infections likely present in many implant-associated infections.