12-phase flow (12)

1. 1FRAME
   Knee pain is a SYNDROME, not a diagnosis. Two-gate frame: GATE 1 acute trauma → Ottawa Knee Rule decides film vs no film (pooled sens ~98%, spec ~43%, LR− ~0.12 — Kazemi PMID 37215241); GATE 2 phenotype the knee (OA / meniscus / ACL / MCL-PCL / PFP / crystal / septic / referred hip / bursitis-tendinopathy). Population adult; settings ED + outpatient
   inputs: age, trauma_mechanism
   advance: Acute-vs-non-acute and trauma-vs-atraumatic frame established
2. 2ENTRY
   Acute traumatic injury (twist/fall/blow); mechanical locking/giving-way; acute hot swollen knee ± fever; insidious anterior loading-related pain (PFP); chronic activity-related pain (OA); cannot bear weight / cannot extend (Ottawa-positive / extensor rupture) (Stiell JAMA 1996; JOSPT 2019 PMID 31475628)
   inputs: age, trauma_mechanism
   advance: Engine entered via a recognised knee-pain trigger
3. 3CONTEXT
   Capture age, sex, trauma mechanism (pivot/valgus/posterior-force/hyperextension/twist), effusion timing (immediate haemarthrosis vs delayed), activity/occupational demand, AND the comorbidity inventory that GATES pharmacotherapy: eGFR (CKD-EPI 2021 race-neutral), CVD/HF/HTN, PUD/GI-bleed, anticoagulant/antiplatelet (which also raises haemarthrosis risk) (Frobell NEJM 2010 PMID 20660401)
   inputs: age, trauma_mechanism, immediate_effusion_haemarthrosis, ckd_stage, cardiovascular_disease, anticoagulant_or_antiplatelet
   actions: calc.ckd_epi_2021
   advance: Mechanism prior + comorbidity gating matrix inputs captured
4. 4RED_FLAGS
   Cannot-miss screen FIRST: (1) septic arthritis — fever + hot monoarticular knee → STAT arthrocentesis + empiric cover. ED monoarthritis pre-test prior ~27% (95% CI 17–38, Carpenter Acad Emerg Med 2011 PMID 21843213). Synovial WBC LR ladder: <25K LR 0.32; ≥25K LR 2.9; >50K LR 7.7; >100K LR 28 (Margaretten JAMA 2007 PMID 17405973); Carpenter interval LRs corroborate (0–25×10⁹/L LR 0.33; 25–50 LR 1.06; 50–100 LR 3.59; >100×10⁹/L LR ∞ — the strongest wired LR+). CONDITIONAL DEPENDENCE #4 — the synovial-WBC LR is CONDITIONAL on the host: in a prosthetic joint or immunosuppressed/recent-surgery knee the WBC band alone underperforms, while overlying cellulitis on a prosthetic joint carries LR+ 15.0 and prior joint surgery LR+ 6.9 INDEPENDENT of cell count (Carpenter PMID 21843213) — escalate to id.osteomyelitis-septic-arthritis.v1; Gram/culture/16S remain mandatory and septic CO-EXISTS with crystal disease. Decision thresholds from named literature: arthrocentesis test-threshold ~5%, empiric-treatment threshold ~39% (Carpenter PMID 21843213). (2) Ottawa-positive trauma (age ≥55, isolated patellar tenderness, fibular-head tenderness, cannot flex 90°, cannot bear weight 4 steps) → radiograph (sens ~98% — Kazemi PMID 37215241). CONDITIONAL DEPENDENCE #3 — Ottawa-knee APPLICABILITY CONDITIONS: the rule (and its LR− 0.12) is only valid when applied to its derivation context — it is NOT validated for isolated superficial knee injury, age <18 (engine population is adult; paediatric validation is separate — Bulloch PMID 12827123), gross intoxication, distracting injury, decreased sensation (paraplegia), or presentation >7 days; outside these the negative rule does NOT carry its stated LR− and clinical judgement governs imaging. (3) inability to actively extend / SLR = quadriceps or patellar tendon rupture; (4) knee dislocation / multiligament → mandatory pulse + ABI ± CT angiography (popliteal artery injury); (5) compartment syndrome (tense leg, pain out of proportion, paraesthesia)
   inputs: fever_or_systemic_illness, weight_bearing_4_steps, active_knee_flexion_90, extensor_mechanism_intact
   actions: calc.ottawa_knee_rule, workup.fracture_triage, panel.synovial, panel.inflammation
   advance: Septic / fracture / extensor-rupture / dislocation-vascular / compartment screened and escalated if present
5. 5INITIAL_WORKUP
   Trauma path: apply calc.ottawa_knee_rule — if ALL five criteria negative, no radiograph (LR− ~0.12; ~49% film reduction — Emparanza PMID 11574791); if any positive, knee radiograph (AP/lateral ± skyline; lipohaemarthrosis = occult fracture; Segond fragment = ACL). Atraumatic hot/effused knee: arthrocentesis for synovial WBC/%PMN, crystals (negatively birefringent MSU vs positively birefringent CPP), Gram/culture; inflammatory panel (CRP/ESR) + CBC; eGFR (CKD-EPI 2021) before any NSAID/colchicine (Margaretten JAMA 2007 PMID 17405973; Stiell JAMA 1996)
   inputs: weight_bearing_4_steps, active_knee_flexion_90, tense_effusion_for_aspiration, synovial_fluid_analysis, knee_radiograph
   actions: calc.ottawa_knee_rule, workup.knee_pain, workup.acute_monoarthritis, panel.synovial, panel.inflammation, panel.cbc, panel.renal, calc.ckd_epi_2021
   advance: Ottawa gate applied + films/aspirate obtained where indicated; septic excluded or escalated
6. 6BRANCHING_WORKUP
   Non-fracture internal derangement: targeted exam-test clusters → MRI only when it would change management. ACL: Lachman (bivariate pooled sens 81% / spec 85%, LR+ ~5.4, LR− ~0.22 — Sokal KSSTA 2022 PMID 35150292; legacy Benjaminse sens 85% / spec 94%, LR+ ~14 PMID 16715828; Huang Lachman LR+ 5.65 PMID 35945782), pivot-shift (spec 94–97%, LR+ ~14 [Huang 13.99], confirmatory rule-IN), Lever sign (LR+ 9.56 — Huang PMID 35945782), anterior-drawer (office LR+ ~1.9–3.6). CONDITIONAL DEPENDENCE #1 — Lachman/pivot-shift/anterior-drawer awake-vs-EXAM-UNDER-ANAESTHESIA (EUA): the awake and EUA exams are NOT independent observations — guarding/pain attenuate the awake exam, so EUA materially raises sensitivity (Lachman 0.81→0.91; anterior-drawer 0.38→0.63; pivot-shift 0.28→0.73 — van Eck PMID 23085822); do NOT chain awake + EUA LRs as if independent, and a negative awake exam in a guarded patient does NOT carry its stated LR−. CONDITIONAL DEPENDENCE #2 — meniscus test-cluster NON-INDEPENDENCE: joint-line tenderness (sens 83% / spec 83%, LR+ ~4.9), McMurray (sens 61% / spec 84%, LR+ ~3.8), Thessaly 20° (sens 75% / spec 87%, LR+ ~5.8) (Smith Evid Based Med 2015 PMID 25724195) all interrogate the SAME lesion and co-vary — do NOT multiply their LRs as independent; a concordant cluster raises probability but by less than the naive product. True locked knee / displaced bucket-handle → urgent surgical referral (AAOS meniscal CPG 2024 PMID 40674138). ALWAYS examine the ipsilateral HIP (referred pain) and assess patellofemoral signs (JOSPT 2019 PMID 31475628)
   inputs: mechanical_block_to_extension, knee_mri
   actions: workup.knee_pain, panel.synovial
   advance: Phenotype localised; MRI obtained only where it crosses a management threshold
7. 7DIFFERENTIAL
   MECE terminal differential by pivot: OSTEOARTHRITIS (chronic, activity-related, age ≥45, KL osteophytes — route msk.osteoarthritis.core.v1) vs MENISCAL TEAR (twist, joint-line tenderness, +McMurray/Thessaly) vs ACL RUPTURE (pivot trauma, immediate haemarthrosis, +Lachman/pivot-shift) vs MCL/PCL/MULTILIGAMENT (valgus / posterior-force / dislocation — vascular screen) vs PATELLOFEMORAL PAIN (insidious anterior pain, loading-aggravated, JOSPT 2019 PMID 31475628) vs CRYSTAL (acute hot joint, MSU/CPP crystals — route msk.gout-acute.core.v1) vs SEPTIC ARTHRITIS (fever, synovial WBC ladder — Margaretten PMID 17405973) vs REFERRED HIP / L3 radicular (hip exam reproduces pain) vs BURSITIS / TENDINOPATHY (prepatellar/pes anserine/patellar tendon). Named pivots: trauma mechanism, effusion timing, joint-line vs anterior vs diffuse pain, synovial analysis, hip exam
   inputs: trauma_mechanism, immediate_effusion_haemarthrosis, synovial_fluid_analysis
   advance: Terminal phenotype assigned (named pivots + selective imaging/aspirate)
8. 8RISK_STRATIFICATION
   Septic arthritis = highest acuity (synovial WBC >50K LR 7.7, >100K LR 28 + Gram → emergent washout — Margaretten PMID 17405973). Ottawa-positive fracture → ortho. ACL rupture: stratify by activity demand + functional instability for the shared decision (KANON: early reconstruction NOT superior to rehab-first at 2 yr [ΔKOOS4 0.2, 95% CI −6.5 to 6.8] or 5 yr; ~51% rehab-first avoided surgery — Frobell NEJM 2010 PMID 20660401 / BMJ 2013 PMID 23349407). Locked/displaced meniscus → time-sensitive surgery within ~6 mo (AAOS CPG 2024 PMID 40674138). OA severity → WOMAC; eGFR tier gates oral NSAID
   inputs: activity_demand_and_instability, ckd_stage
   actions: calc.ottawa_knee_rule, calc.womac_oa_index, calc.ckd_epi_2021
   advance: Acuity tier + surgical-candidacy + NSAID-eligibility tier documented
9. 9TREATMENT
   PHENOTYPE-ROUTED, not knee-generic. SEPTIC → emergent arthrocentesis/washout + empiric IV antibiotics + ortho (NOT in this regimen axis — escalate). OA → route msk.osteoarthritis.core.v1 (core non-pharm + topical NSAID + comorbidity-gated oral NSAID). MENISCUS: stable degenerative → exercise therapy first; acutely locked/displaced bucket-handle → early surgical referral (AAOS CPG 2024 PMID 40674138). ACL RUPTURE: activity-based shared decision — structured rehab with optional delayed reconstruction is non-inferior to early reconstruction at 2 & 5 yr and avoids surgery in ~half (Frobell NEJM 2010 PMID 20660401); early reconstruction for high-demand pivoting athletes / persistent instability (AAOS ACL CPG 2022 — reconstruct within 3 months, autograft preferred). PFP → exercise-therapy first (hip+knee strengthening), foot orthoses, taping; NOT injection (JOSPT 2019 PMID 31475628). CRYSTAL → route msk.gout-acute.core.v1. SYMPTOMATIC EFFUSION → aspirate (diagnostic + therapeutic). Analgesia: acetaminophen / topical NSAID / short-course oral NSAID at lowest effective dose, COMORBIDITY-GATED by the encoded eGFR/CVD/PUD/anticoagulant matrix (CKD-EPI 2021)
   inputs: activity_demand_and_instability, ckd_stage, cardiovascular_disease, anticoagulant_or_antiplatelet
   actions: calc.ckd_epi_2021, workup.knee_pain
   advance: Phenotype-specific plan set (route / refer / rehab / aspirate) + comorbidity-appropriate analgesia selected
10. 10DISPOSITION
    Mostly outpatient. ADMIT/ED: septic arthritis (emergent washout), knee dislocation / vascular injury (vascular surgery + ICU), open fracture, compartment syndrome, irreducible dislocation, displaced fracture needing fixation. Outpatient ortho/sports-med referral: ACL rupture (shared decision), locked meniscus, recurrent instability, refractory PFP/OA. Routine follow-up: sprain, contusion, stable degenerative meniscus, controlled OA/PFP
    inputs: fever_or_systemic_illness, extensor_mechanism_intact
    advance: Care level set; emergent surgical/vascular referral placed if threshold met
11. 11MONITORING
    Septic: serial exam, inflammatory markers, repeat aspiration/washout response. ACL rehab-first: track instability episodes, quadriceps strength, KOOS — convert to delayed reconstruction if functional instability persists (Frobell BMJ 2013 PMID 23349407). PFP/OA: pain/function trajectory (WOMAC), exercise adherence. On oral NSAID: surveil eGFR, BP, GI symptoms (CKD-EPI 2021). Post-injection / post-aspiration: re-accumulation, infection signs
    inputs: ckd_stage, activity_demand_and_instability
    actions: calc.womac_oa_index, calc.ckd_epi_2021, panel.renal, panel.inflammation
    advance: Phenotype-specific response + drug-safety surveillance plan running
12. 12FOLLOWUP
    Phenotype-specific: ACL — re-evaluate need for delayed reconstruction by activity/instability; counsel on post-traumatic OA risk (~50% at 10–20 yr after ACL/meniscus injury — Lohmander Am J Sports Med 2007 PMID 17761605) regardless of surgical choice. Meniscus — recovery expectations, OA-risk counselling, preserve meniscal tissue if operated (AAOS CPG 2024 PMID 40674138). PFP — long-term exercise maintenance (relapsing course — JOSPT 2019 PMID 31475628). OA — route msk.osteoarthritis.core.v1 chronic pathway. Return precautions: fever + hot joint, locking, recurrent giving-way, inability to extend
    inputs: activity_demand_and_instability
    actions: workup.knee_pain
    advance: Long-term phenotype plan + OA-risk counselling + return precautions booked