AdultPediatric
Pediatric knee pain ranges from benign overuse to limb- and life-threatening pathology. Always consider referred hip pain (SCFE, septic hip) — knee pain may be the only symptom. Use age, trauma history, fever, and weight-bearing status to drive workup.
🚩 Red-flag clues (must not miss)
- •Fever + refusal to bear weight + effusion = septic arthritis until proven otherwise (Kocher criteria)
- •Obese adolescent or peripubertal child with knee/thigh pain + limp = SCFE (image BOTH hips, frog-leg lateral)
- •Night pain, rest pain, systemic symptoms, palpable mass = bone tumor (osteosarcoma, Ewing)
- •Atraumatic effusion + fever + ill-appearing = septic joint / osteomyelitis
- •Pain disproportionate to exam, tense compartment after trauma = compartment syndrome
- •Locked knee (cannot fully extend) = displaced meniscal tear or osteochondral fragment
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Labs
Imaging
Meds
History
- Age (drives DDx — see age-stratified buckets below), sport, mechanism (twisting, contact, overuse), pop/click
- Locking, giving way, effusion timing (immediate = hemarthrosis: ACL/fracture)
- Fever, weight loss, night sweats, recent illness (post-viral, Lyme exposure)
- Limp, refusal to bear weight, hip or groin pain (referred from SCFE / septic hip)
- Morning stiffness > 30 min in multiple joints = JIA
- AGE-STRATIFIED DDx — 0–4 y: septic arthritis, osteomyelitis, toddler's fx (proximal tibia), JIA, leukemia, non-accidental trauma
- AGE-STRATIFIED DDx — 4–10 y: transient synovitis (referred), Perthes (referred), discoid meniscus, JIA, Lyme, leukemia, osteomyelitis
- AGE-STRATIFIED DDx — 10–14 y: Osgood-Schlatter, Sinding-Larsen-Johansson, OCD, SCFE (referred), patellar instability, stress fx, bone tumor
- AGE-STRATIFIED DDx — 14–18 y: ACL/meniscal tears, patellofemoral pain, patellar dislocation, tibial spine fx, late SCFE, osteosarcoma peak
Exam
- Gait, stance, alignment (genu varum/valgum), leg-length discrepancy
- Effusion (ballottement, bulge sign), warmth, erythema
- ROM active & passive; tenderness — joint line, patella, tibial tubercle, physes
- Lachman, anterior/posterior drawer, McMurray, valgus/varus stress
- ALWAYS examine the hip (log-roll, internal rotation) — referred pain from SCFE/septic hip
- Skin: rash (HSP, Lyme erythema migrans), psoriasis (JIA)
Labs
- If febrile / atraumatic effusion: CBC, CRP, ESR, blood culture
- Joint aspiration (cell count, Gram stain, culture, crystals) if septic joint suspected
- Lyme serology in endemic areas; ASO if post-strep reactive arthritis
- Kocher criteria (hip): fever > 38.5°C, non-weight-bearing, ESR > 40, WBC > 12k — 3/4 = 93% septic
Imaging
- Plain films AP + lateral knee — first line for trauma, effusion, suspected tumor
- Frog-leg lateral of BOTH hips if SCFE suspected (knee or thigh pain in 10–15 y/o)
- MRI knee (without contrast) for suspected ACL/meniscus, osteochondritis dissecans, occult fracture
- MRI with & without contrast if osteomyelitis or tumor suspected
- US for effusion / guided aspiration
Differential & next step
| Diagnosis | Clue | Next step |
|---|---|---|
| Septic arthritis | Fever, refusal to bear weight, hot effusion | Aspiration, IV abx (vanc + ceftriaxone), ortho consult |
| Osteomyelitis (distal femur / proximal tibia) | Fever, focal bony tenderness, ↑ CRP/ESR | MRI with & without contrast, blood cx, IV abx |
| SCFE (referred from hip) | Obese 10–15 y/o, limp, ↓ internal rotation of hip | NON-weight-bearing, frog-leg pelvis XR, urgent ortho |
| Legg-Calvé-Perthes (referred) | 4–8 y/o boy, painless limp, ↓ hip ROM | Pelvis XR, ortho referral |
| Transient synovitis of hip (referred) | Recent viral illness, afebrile or low-grade, willing to bear weight | Rule out septic hip (Kocher); NSAIDs, follow-up |
| Osgood-Schlatter disease | 10–15 y/o athlete, tibial tubercle tenderness/swelling | Activity modification, ice, NSAIDs, quad stretching |
| Sinding-Larsen-Johansson | Inferior pole patella tenderness, jumper | Rest, NSAIDs, PT |
| Patellofemoral pain syndrome | Anterior knee pain ↑ with stairs/squatting, adolescent female | PT (VMO, hip strengthening), NSAIDs |
| Patellar dislocation/subluxation | Lateral dislocation, positive apprehension | Reduce (extend knee), immobilize, ortho follow-up |
| Osteochondritis dissecans | Vague pain, locking, medial femoral condyle | MRI knee (without contrast), ortho referral |
| ACL tear | Pop + immediate effusion (hemarthrosis), pivot injury | Lachman, MRI knee (without contrast), ortho |
| Meniscal tear | Twisting injury, locking, joint-line tenderness, + McMurray | MRI knee (without contrast), ortho |
| Tibial spine / physeal fracture | Skeletally immature, hemarthrosis post-trauma | XR (Salter-Harris), ortho |
| Juvenile idiopathic arthritis (JIA) | Morning stiffness, > 6 wk, multiple joints | Rheumatology, ANA, RF, slit-lamp (uveitis) |
| Lyme arthritis | Endemic exposure, large monoarticular effusion, often painless | Lyme serology, doxycycline (≥ 8 y) or amoxicillin |
| Post-strep reactive arthritis / ARF | Recent GAS pharyngitis, migratory polyarthritis | ASO, throat cx, ECG (carditis), Jones criteria |
| HSP | Palpable purpura lower extremities, abdominal pain, hematuria | UA, BP, supportive |
| Bone tumor (osteosarcoma, Ewing) | Night pain, rest pain, mass, distal femur > proximal tibia | XR, MRI with & without contrast, oncology |
| Leukemia | Bone pain, pallor, bruising, lymphadenopathy | CBC w/ diff, smear, LDH, hem/onc |
Medications & dosing
| Drug | Dose | Notes |
|---|---|---|
| Ibuprofen | 10 mg/kg PO q6–8h (max 600 mg/dose) | First-line for overuse/inflammatory pain |
| Acetaminophen | 15 mg/kg PO q4–6h (max 1 g/dose) | |
| Ceftriaxone (empiric septic joint) | 50–75 mg/kg IV daily (max 2 g) | |
| Vancomycin (add for MRSA coverage) | 15 mg/kg IV q6h |
Management / next steps
- Always examine the hip — knee pain may be SCFE or septic hip
- Atraumatic effusion + fever → aspirate; do not delay for imaging
- Suspected SCFE → strict non-weight-bearing (wheelchair), urgent ortho for in situ pinning; image both hips
- Septic arthritis → urgent arthrotomy/washout + IV vanc + ceftriaxone (cefazolin if MRSA risk low); narrow per cx
- Osteomyelitis → IV abx 4–6 wk total (transition to PO once afebrile + ↓ CRP); ortho for source control
- Locked knee or hemarthrosis after trauma → knee immobilizer, NWB, MRI, ortho referral within 1–2 wk
- Patellar dislocation → reduce by extending knee, knee immobilizer 3 wk, PT, ortho follow-up
- ACL/meniscus tear → hinged brace, crutches, MRI, ortho — surgical repair preferred in skeletally immature to preserve menisci
- Osgood-Schlatter / Sinding-Larsen-Johansson → activity modification (not full rest), ice post-activity, NSAIDs, quad/hamstring stretching; resolves at physeal closure
- Patellofemoral pain syndrome → PT (VMO + hip abductor strengthening), NSAIDs, taping; avoid prolonged immobilization
- Lyme arthritis → doxycycline 4 mg/kg/day div BID × 28 d (any age now per IDSA 2020); amoxicillin 50 mg/kg/day div TID alternative
- JIA → rheumatology referral, NSAIDs first-line, slit-lamp screening for uveitis
- Discharge return precautions: fever, ↑ pain, inability to bear weight, new neuro deficit, calf swelling, night pain that wakes the child
Pearls
- Hip pathology presents as knee pain in 15–50% of SCFE — image the hip when in doubt.
- Effusion within 2 hours of trauma = hemarthrosis (ACL, fracture, patellar dislocation).
- Night pain that wakes the child is a tumor red flag until proven otherwise.
- In skeletally immature patients, ligaments are stronger than physes — suspect Salter-Harris before sprain.
Source: https://fprmed.com/fprmedcom/Pages/Pedi/Knee_Pain.html
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