12-phase flow (12)

1. 1FRAME
   Bone & joint infection spectrum: acute osteomyelitis (hematogenous — peds metaphyseal AHO + adult vertebral osteomyelitis/discitis; contiguous — post-trauma/surgical/diabetic-foot/decubitus/dental; direct-inoculation — open fracture/plantar puncture) + native septic arthritis (closed-space orthopaedic EMERGENCY — cartilage destruction within hours) + prosthetic-joint infection (biofilm-mediated) × acute (< 2 wk, no sequestrum) vs subacute/chronic (sequestrum, involucrum, sinus tract, Brodie abscess) × pathogen-host band (S. aureus incl MRSA default; Kingella < 4 yr; GBS neonate; gonococcal sexually active; Salmonella sickle-cell; Pseudomonas IVDU/plantar-puncture; Streptococcus) (Berbari IDSA 2015; IDSA/PIDS 2021; Osmon IDSA PJI 2013)
   inputs: age, symptom_duration_and_route
   advance: BJI phenotype framed (syndrome × route × acuity × pathogen-host band × native/prosthetic)
2. 2ENTRY
   Recognise via: acute monoarthritis + fever; refusal to bear weight / limp / atraumatic hip pain in a child (apply Kocher); pseudoparalysis in a neonate; focal bone pain + fever; subacute back pain + fever ± neuro deficit; synovial WBC > 50,000 PMN-predominant; elevated CRP/ESR with MSK focus; MRI marrow oedema; painful arthroplasty / sinus tract; sepsis criteria with bone/joint focus
   inputs: monoarthritis_features, weight_bearing_status
   actions: workup.septic_arthritis
   advance: BJI hypothesis (osteomyelitis / native septic arthritis / PJI) framed
3. 3CONTEXT
   Age + host risk factors (RA, prosthetic joint, recent intra-articular procedure, immunosuppression, diabetes, IVDU, sickle-cell, advanced age) + sexual history (gonococcal) + prosthetic/hardware status + temperature + symptom duration/route — these set the pathogen prior and the empiric regimen
   inputs: age, temperature, host_risk_factors, sexual_history_for_gonococcal, prosthetic_or_hardware_status
   advance: Host + pathogen-prior context captured
4. 4RED_FLAGS
   Time-critical features: native septic arthritis (cartilage destruction within hours — STAT arthrocentesis + urgent drainage; hip/shoulder/paediatric hip → surgical); neonatal pseudoparalysis; vertebral osteomyelitis with neuro deficit / epidural abscess / cord compression (neurosurgical emergency — do NOT delay antibiotics for biopsy); sepsis / septic shock with bone-joint focus → SSC Hour-1 bundle + cross-route id.sepsis.core.v1; source control within 6-12 h if bacteraemic (SSC 2026; IDSA/PIDS 2021; Berbari IDSA 2015)
   inputs: neuro_deficit_or_spinal_red_flags, weight_bearing_status
   actions: workup.septic_arthritis, panel.cbc, panel.inflammation
   advance: Red flags actioned; arthrocentesis / urgent imaging done; empiric therapy + source-control plan initiated; sepsis routed if criteria met
5. 5INITIAL_WORKUP
   STAT arthrocentesis before antibiotics where feasible — synovial WBC + differential (> 50,000 /µL PMN-predominant suggestive; LR+ 7.7), Gram stain, culture, crystal microscopy (crystals do NOT exclude co-existent sepsis); two sets of blood cultures (positive in ~ 30-50 %); CRP + ESR baseline (CRP is the monitoring workhorse; CRP > 20 mg/L strong paediatric predictor); CBC with differential (WBC > 12,000 Kocher predictor); creatinine baseline for vancomycin AUC dosing. Empiric antibiotics immediately AFTER cultures in suspected septic arthritis / sepsis / Kocher-positive septic hip (treatment benefit > test-delay cost) (Margaretten JAMA 2007 PMID 17405973; Kocher 1999 PMID 10608376; IDSA/PIDS 2021)
   inputs: synovial_fluid_analysis, blood_cultures, crp_esr_baseline_and_trend, cbc_with_differential, creatinine_and_renal_function
   actions: workup.septic_arthritis, panel.cbc, panel.renal, panel.inflammation
   advance: Synovial fluid + blood cultures obtained; CRP/ESR/CBC/creatinine baseline established; empiric antibiotics started per phenotype
6. 6BRANCHING_WORKUP
   Phenotype-directed: MRI of target region (marrow oedema, abscess, sequestrum; vertebral — disc/endplate destruction + paravertebral/epidural abscess); image-guided or open bone biopsy / deep operative cultures (≥ 3-5 samples; sinus-tract swabs unreliable) — for stable vertebral osteomyelitis HOLD antibiotics until biopsy (Berbari IDSA 2015); in child < 4 yr inoculate synovial/bone aspirate into blood-culture bottles + Kingella / 16S PCR (IDSA/PIDS 2021; Yagupsky); TEE if S. aureus bacteraemia with vertebral osteo (cross-route id.endocarditis.core.v1); probe-to-bone + IDSA DFI grading if diabetic foot (cross-reference workup.diabetic_foot_ulcer)
   inputs: mri_target_region, bone_or_operative_culture, kingella_pcr_or_blood_culture_bottle_inoculation
   actions: workup.diabetic_foot_ulcer, workup.crbsi
   advance: Imaging + microbiologic source obtained; organism identified or empirically covered; surgical strategy defined
7. 7DIFFERENTIAL
   Septic arthritis vs crystal arthropathy (gout/CPPD — crystal microscopy pivot; crystals + infection can co-exist) vs reactive/post-infectious arthritis (sterile culture + antecedent GI/GU infection + HLA-B27) vs rheumatoid/inflammatory flare; septic hip vs transient synovitis in children (Kocher predictor count + CRP > 20 mg/L pivot — transient synovitis afebrile, weight-bearing-tolerant, low markers, self-limited); osteomyelitis vs bone infarction in sickle-cell (MRI + aspiration pivot) vs Charcot neuroarthropathy in diabetic foot (probe-to-bone + biopsy pivot); vertebral osteomyelitis vs metastatic disease / compression fracture (disc-space + endplate destruction + paravertebral abscess favour infection); PJI vs aseptic loosening (ESR/CRP, synovial PJI thresholds, alpha-defensin, ≥ 2 deep cultures, sinus tract); S. aureus vertebral osteo ↔ infective endocarditis (persistent bacteraemia + new murmur + TEE — bidirectional) (Margaretten JAMA 2007 PMID 17405973; Berbari IDSA 2015)
   advance: Non-infectious + alternative-infectious look-alikes evaluated; synovial/bone/blood cultures + MRI discriminate
8. 8RISK_STRATIFICATION
   Stratify by syndrome + host + pathogen: (1) native septic arthritis any pathogen = orthopaedic emergency, urgent drainage + empiric vancomycin + ceftriaxone/cefepime; (2) paediatric septic hip Kocher-positive = US-guided aspiration + urgent arthrotomy/arthroscopic washout (hip not reliably decompressed by aspiration); (3) child < 4 yr Kingella = β-lactam (cefazolin/ceftriaxone — vanc/clinda-resistant); (4) neonate pseudoparalysis = broad empiric (anti-staph + GBS + Gram-negative) + multifocal survey; (5) adult vertebral osteo = MRI + biopsy-before-antibiotics if stable, ≥ 6 wk therapy, endocarditis screen if S. aureus bacteraemia; (6) disseminated gonococcal = ceftriaxone + chlamydia co-treat; (7) sickle-cell = cover Salmonella + S. aureus; (8) IVDU/plantar-puncture = add anti-pseudomonal; (9) PJI = source-control strategy (DAIR vs 1-/2-stage exchange) + rifampin combination for staph; (10) BJI + bacteraemia/shock = cross-route id.sepsis.core.v1 with carryover (Margaretten JAMA 2007 PMID 17405973; IDSA/PIDS 2021; Osmon IDSA PJI 2013)
   inputs: age, host_risk_factors, prosthetic_or_hardware_status
   advance: Severity tier + empiric regimen + source-control strategy + setting assigned
9. 9TREATMENT
   Empiric: vancomycin (MRSA cover, AUC-targeted) + ceftriaxone (Gram-negative + gonococcal + Salmonella) OR cefepime (Pseudomonas risk: IVDU / plantar puncture); in child < 4 yr retain a β-lactam (cefazolin/ceftriaxone — Kingella vanc/clinda-resistant); neonate broaden (anti-staph + GBS + Gram-negative); disseminated gonococcal → ceftriaxone (+ chlamydia co-treat). Narrow on culture: MSSA → cefazolin or nafcillin (β-lactam superior to vancomycin for MSSA); MRSA → vancomycin or daptomycin/linezolid; Streptococcus → penicillin/ceftriaxone. Source control: native septic arthritis → STAT joint drainage (serial aspiration vs arthroscopic vs open; hip/shoulder/paediatric hip → surgical); osteomyelitis → debridement of devitalised bone/sequestrum/abscess; PJI → DAIR (well-fixed + < 3 wk symptoms + susceptible + exchangeable liner) vs 1-/2-stage exchange + RIFAMPIN combination (always with a companion to prevent rifampin resistance — never monotherapy) for staph with retained hardware. Duration / route: IV-to-oral early switch once source controlled + susceptible + reliable absorption (OVIVA Li NEJM 2019 PMID 30699315 — oral non-inferior, failure 13.2 % vs 14.6 %; paediatric short-course per Peltola / IDSA-PIDS 2021); septic arthritis 2-4 wk, osteomyelitis 3-6 wk, native vertebral osteomyelitis ≥ 6 wk (Berbari IDSA 2015), PJI 6 wk IV-active backbone + prolonged oral suppression in retention. Adjunct: analgesia, immobilisation/early ROM per ortho, sepsis bundle if bacteraemic.
   inputs: age, creatinine_and_renal_function, prosthetic_or_hardware_status
   advance: Empiric antibiotics started + source control booked/done; narrowing plan on culture defined; IV-to-oral + duration plan set
10. 10DISPOSITION
    ED: arthrocentesis + empiric antibiotics + orthopaedic consult for drainage + admit. ICU: BJI with septic shock / bacteraemia / multi-organ dysfunction (cross-route id.sepsis.core.v1). Inpatient: source control completed, organism-directed IV backbone, CRP/ESR trajectory monitoring, IV-to-oral early-switch decision (OVIVA), PJI surgical-strategy execution. Outpatient / OPAT: oral continuation per OVIVA / Peltola once stable + source controlled + susceptible + responding; OPAT if IV agent required for duration.
    inputs: weight_bearing_status
    advance: Setting + source-control status + antibiotic route/duration assigned
11. 11MONITORING
    CRP/ESR trajectory (CRP is the workhorse — failure to fall by ~ 50 % by day 3-5 despite adequate drainage → retained focus / wrong organism / resistance → repeat imaging + surgical re-look); clinical exam (joint ROM, pain, weight-bearing, wound/sinus); blood cultures clearance if bacteraemic; vancomycin AUC + creatinine (nephrotoxicity); rifampin LFTs + drug-interaction review (potent CYP3A4 inducer); linezolid CBC if > 14 d (myelosuppression) + serotonin-syndrome screen; daptomycin CPK weekly; fluoroquinolone tendinopathy/QT counsel; paediatric — CRP-guided IV-to-oral switch + total-duration decision (IDSA/PIDS 2021)
    inputs: crp_esr_baseline_and_trend, creatinine_and_renal_function
    actions: panel.inflammation, panel.renal, panel.cbc
    advance: Inflammatory markers trending down; source control confirmed adequate; antibiotics narrowed by culture; oral switch executed if criteria met
12. 12FOLLOWUP
    Orthopaedic + ID follow-up; CRP/ESR surveillance until normalised; functional rehabilitation (joint ROM, weight-bearing progression); relapse / chronic-osteomyelitis return precautions (recurrent pain, swelling, sinus drainage, fever); vertebral osteo — follow-up MRI only if clinical failure (not routine; Berbari IDSA 2015); PJI — chronic oral suppression audit in retention strategies + arthroplasty-team follow-up; address seeding source (endocarditis if S. aureus bacteraemia; line if CRBSI; diabetic-foot offloading + glycaemic control)
    advance: Follow-up + rehabilitation + relapse-precaution plan delivered; seeding source addressed