Pediatrics (System-Integrated)

Pediatric limp: age-stratified differential

Clinical Overview and When to Suspect Pediatric Limp

— Toddler (1–3 yr): toddler's fracture, transient synovitis, septic arthritis/osteomyelitis, developmental dysplasia of the hip (DDH), NAT, foreign body in foot, discitis.

— Child (4–10 yr): transient synovitis (peak 5–6 yr), septic arthritis, LCP disease (4–8 yr, boys), JIA, leukemia, growing pains (nocturnal, bilateral, no limp by day).

— Adolescent (11–16 yr): SCFE (obese male, 12–13 yr; obese female, 11–12 yr), Osgood-Schlatter, sports overuse, osteochondritis dissecans, gonococcal arthritis, stress fracture, bone tumor (osteosarcoma, Ewing).

Board pearl: A febrile, non-weight-bearing child with an irritable hip is septic arthritis until proven otherwise — apply Kocher criteria immediately rather than anchoring on "transient synovitis." Missing this diagnosis causes femoral head necrosis within 24–48 hours and is among the most litigated pediatric ED misses.

Definition: Any abnormal gait in a child — antalgic (short stance phase from pain), Trendelenburg (hip abductor weakness), circumduction (stiff/spastic), or refusal to bear weight. A limp in a child is never normal and demands a structured workup.

Epidemiology: Annual incidence ~1.8 per 1,000 children; ~80% have a benign cause (transient synovitis, minor trauma) but the remaining 20% include septic arthritis, osteomyelitis, malignancy, slipped capital femoral epiphysis (SCFE), Legg-Calvé-Perthes (LCP), juvenile idiopathic arthritis (JIA), and non-accidental trauma (NAT).

Why age-stratification matters: The differential is dramatically reorganized by age because growth plate biology, vascular supply to the femoral head, and immune/infectious exposures change predictably.

Red flags demanding urgent workup: fever, refusal to bear weight, night pain, weight loss, systemic illness, hip held in flexion-abduction-external rotation, bruising in unusual locations, or hemodynamic compromise.

Presentation Patterns and Key History

— Acute (<72 h) with fever → septic arthritis, osteomyelitis, discitis, fracture.

— Subacute (days–weeks) → LCP, JIA, leukemia, stress fracture, SCFE (chronic or stable).

— Acute-on-chronic with sudden inability to bear weight → unstable SCFE (urgent — high AVN risk).

— Hip pathology classically refers to the groin, anterior thigh, or knee. A child presenting with isolated knee pain and a normal knee exam → always image the hip (SCFE, LCP).

— Back pain with refusal to flex spine → discitis or vertebral osteomyelitis.

— Bilateral, symmetric leg pain waking the child at night, relieved by massage, normal exam → growing pains (diagnosis of exclusion, ages 3–12).

Key distinction: Transient synovitis improves over 24–48 h with NSAIDs and rest; septic arthritis worsens. Reassess in 24 hours — a child not improving is not transient synovitis.

Onset and tempo:

Pain character and location:

Systemic symptoms: Fever, weight loss, night sweats, bruising, pallor → leukemia (ALL can present as limp + bone pain + cytopenias). JIA: morning stiffness >30 min, gelling, rash, uveitis.

Trauma history: Toddler's fracture (spiral tibial) often follows minor trip — caregivers may not recall trauma. Inconsistent or implausible mechanism, delayed presentation, or pattern injuries → screen for NAT.

Birth and developmental history: Breech delivery, family hx DDH, late walking → undiagnosed DDH presenting as Trendelenburg limp.

Sick contacts and exposures: Recent viral URI (1–2 weeks prior) → transient synovitis; tick exposure → Lyme arthritis (knee, intermittent, large effusion, minimal pain); unpasteurized dairy → Brucella; sexually active teen → gonococcal/reactive arthritis.

Medications and PMH: Sickle cell (osteomyelitis, often Salmonella; AVN), steroids (AVN), hemophilia (hemarthrosis), recent antibiotics (fluoroquinolone tendinopathy in adolescents).

Physical Exam Findings and Gait Assessment

— Antalgic: shortened stance phase on painful side → infection, fracture, tumor.

— Trendelenburg: pelvis drops to contralateral side during stance → ipsilateral hip abductor weakness (DDH, LCP, SCFE, neuromuscular).

— Equinus/toe-walking: unilateral → leg-length discrepancy, cerebral palsy hemiplegia; bilateral persistent past age 3 → idiopathic vs neuromuscular (check CK if persistent → Duchenne).

— Stiff-knee/circumduction: spasticity.

— Hip: loss of internal rotation is the most sensitive sign of hip pathology. SCFE classically shows obligate external rotation with hip flexion (Drehmann sign). Held in flexion-abduction-ER → effusion (septic, transient synovitis).

— Knee: effusion, warmth, ballottement; popliteal cyst; tenderness over tibial tubercle (Osgood-Schlatter).

— Ankle/foot: point tenderness on tibia → toddler's fracture; tender navicular → Köhler; tender metatarsal head → Freiberg.

Step 3 management: Document gait pattern, weight-bearing status, ROM in degrees, and a temperature — these four data points drive the Kocher decision tree and your imaging choice.

Observation first: Watch the child walk a hallway before touching. Identify gait pattern:

Systematic exam: Always expose both legs fully, examine the unaffected side first, and palpate from foot upward — a tack in the shoe or a hair tourniquet on a toe is easily missed.

Joint-by-joint:

Skin and systemic: Salmon-pink evanescent rash (systemic JIA), petechiae/bruising (leukemia, ITP, NAT), café-au-lait (NF, McCune-Albright fibrous dysplasia), psoriatic plaques, erythema migrans.

Vitals: Fever >38.5°C plus inability to bear weight is a Kocher criterion. Tachycardia out of proportion to fever → sepsis.

Neurologic: DTRs, tone, Gowers sign (proximal weakness → Duchenne, dermatomyositis).

Diagnostic Workup — Initial Labs and Imaging

— Kocher criteria (hip septic arthritis, ages 2–12): (1) non-weight-bearing, (2) fever >38.5°C, (3) ESR >40, (4) WBC >12,000. CRP >2 mg/dL is the modern fifth criterion. Probability of septic arthritis: 0 criteria <0.2%, 1 = 3%, 2 = 40%, 3 = 93%, 4 = 99%.

— Hip: AP pelvis plus frog-leg lateral (essential — SCFE may be invisible on AP alone). Look for Klein's line (should intersect femoral head on AP); widened/irregular physis; crescent sign (LCP); joint space asymmetry.

— Knee: AP, lateral; sunrise if patellar pain.

— Tibia (toddler): AP + lateral + internal oblique to catch spiral toddler's fracture.

CCS pearl: Order CBC, CRP, ESR, blood culture, hip US, AP+frog-leg pelvis, then orthopedics consult simultaneously, not sequentially — clock advances during sequential ordering and septic hip outcomes worsen hourly.

Lab triad for suspected infection/inflammation: CBC with differential, CRP (rises in 6–8 h, most useful acute marker), and ESR (peaks 48 h). Add blood culture before antibiotics if febrile or toxic.

When leukemia is on the differential (bone pain + limp + systemic symptoms): peripheral smear, LDH, uric acid, reticulocyte count. Bicytopenia or pancytopenia plus bone pain → leukemia until marrow proven otherwise. Never start steroids before excluding leukemia.

Imaging — start with plain radiographs:

Ultrasound: Highly sensitive for hip joint effusion — first-line if hip effusion suspected and exam is equivocal. Cannot distinguish septic from sterile effusion but guides aspiration.

Joint aspiration: Definitive for septic arthritis. Synovial WBC >50,000 with >75% PMNs, low glucose, positive Gram stain/culture. Do not delay aspiration for MRI if Kocher score ≥3.

Diagnostic Workup — Advanced and Confirmatory Studies

— Sedation often required <6 yr; coordinate with anesthesia early.

— Septic arthritis: arthrocentesis with cell count, Gram stain, culture, crystal analysis (rare in kids). PCR for Kingella kingae (most common organism age 6–48 months; often falsely culture-negative — request oropharyngeal PCR as well).

— Osteomyelitis: MRI + blood culture (positive ~50%); bone biopsy if cultures negative and not improving.

— JIA: ANA, RF, anti-CCP, HLA-B27, slit-lamp exam for uveitis (ANA+ oligoarticular JIA = highest uveitis risk).

— Lyme arthritis: ELISA → Western blot reflex. Synovial fluid PCR if needed.

— Leukemia: bone marrow biopsy.

— CRMO (chronic recurrent multifocal osteomyelitis): whole-body MRI; biopsy to exclude malignancy/infection.

— Tumor: alkaline phosphatase, LDH; biopsy by orthopedic oncology (not the local ED — improper biopsy tract worsens limb-salvage outcomes).

Board pearl: In a child <4 yr with limp, low-grade fever, mildly elevated CRP, and a normal hip US → think Kingella osteomyelitis or septic arthritis; obtain MRI and add oropharyngeal Kingella PCR because blood/joint culture sensitivity is poor.

MRI: Gold standard for osteomyelitis, discitis, pyomyositis, stress fracture, early LCP, bone tumor, and soft tissue abscess. Order with and without contrast. In a child with limp, fever, normal X-ray, and elevated CRP but no joint effusion → MRI to find osteomyelitis or psoas/pyomyositis.

Bone scan (Tc-99m): Useful when location of pain is unclear (multifocal osteomyelitis, occult fracture, CRMO). Largely supplanted by MRI but still helpful for whole-body screen.

CT: Best for cortical bone detail — suspected occult fracture not seen on X-ray, cortical destruction of tumor, planning surgical drainage. Higher radiation; use selectively.

Specific confirmatory tests:

Echocardiogram: If gonococcal or Staph bacteremia with prolonged positive cultures → rule out endocarditis.

Risk Stratification and First-Line Management Logic

— Toxic-appearing, febrile, refusing weight-bearing → IV access, blood cultures, empiric antibiotics after joint aspiration, ortho STAT, admit.

— Well-appearing, afebrile, mildly antalgic, normal labs → outpatient NSAID trial with 24–48 h reassessment by PCP.

— Equivocal (some Kocher features) → MRI and/or arthrocentesis; admit for observation.

— Score 0–1: observe, NSAIDs, recheck in 24 h.

— Score 2: hip US ± aspiration; admit if effusion.

— Score 3–4: arthrocentesis + surgical washout + empiric IV antibiotics.

Step 3 management: When Kocher score = 2, the test answer is MRI or arthrocentesis, not "trial of NSAIDs" — the cost of missing a septic hip dwarfs the cost of an MRI. In outpatient setting, refer to ED rather than schedule next-day follow-up.

Decision tree by acuity:

Kocher-driven management (suspected septic hip):

SCFE pathway: Any suspected SCFE → immediate non-weight-bearing (wheelchair/crutches), NPO, urgent orthopedic consult for in-situ percutaneous screw fixation. Do not attempt reduction (increases AVN). Evaluate contralateral hip — bilateral in 20–40%; prophylactic pinning considered if endocrinopathy or young age.

LCP pathway: Containment principle — keep femoral head within acetabulum. Activity restriction, NSAIDs, PT for ROM (especially abduction and internal rotation). Surgery (femoral or pelvic osteotomy) if age >8 or >50% head involvement.

DDH: <6 months → Pavlik harness; 6–18 months → closed reduction + spica; >18 months → open reduction.

Transient synovitis: rest, NSAIDs (ibuprofen 10 mg/kg q6–8h), reassurance, 48-h reassessment. Resolves in 1–2 weeks.

Pharmacotherapy — First-Line Regimens

— Neonate (<3 mo): cefotaxime + nafcillin (or vancomycin if MRSA prevalence high) — covers GBS, gram-negatives, S. aureus.

— 3 mo – 5 yr: cefazolin or clindamycin; add vancomycin if MRSA prevalence >10% or severe sepsis. Covers S. aureus, Kingella, strep. Cefazolin preferred for Kingella (clindamycin-resistant).

— >5 yr: cefazolin or nafcillin; vancomycin if MRSA suspected.

— Sickle cell: add ceftriaxone for Salmonella coverage.

— Sexually active adolescent: ceftriaxone + azithromycin for gonococcal arthritis.

— Puncture wound through shoe (Pseudomonas osteo): ceftazidime or cefepime + clindamycin.

Board pearl: Never give empiric steroids for a limping child with bone pain until leukemia is excluded — partial treatment of ALL delays diagnosis and worsens prognosis.

Septic arthritis / osteomyelitis empiric IV antibiotics (after cultures):

Duration: Transition to oral antibiotics after 3–5 days of IV therapy if CRP trending down, afebrile, and joint clinically improving (modern data support early oral switch). Total duration 3–4 weeks for osteomyelitis, 2–3 weeks for septic arthritis. Follow CRP weekly.

Transient synovitis: Ibuprofen 10 mg/kg q6–8h × 3–7 days; activity as tolerated.

JIA: NSAIDs first; intra-articular triamcinolone for oligoarticular; methotrexate ± biologics (etanercept, adalimumab) for polyarticular/refractory. Slit-lamp screening every 3–12 months.

Lyme arthritis: Amoxicillin × 28 days (age <8) or doxycycline × 28 days (age ≥8; now considered safe short-course at any age per AAP).

Pain control: Acetaminophen 15 mg/kg q4–6h adjunct. Avoid opioids unless post-op.

Procedures and Surgical Management

— Hip aspiration requires ultrasound or fluoroscopic guidance and is typically performed by orthopedics or interventional radiology under sedation. Knee, ankle, elbow aspirations can be done at bedside.

— Send fluid for cell count, Gram stain, aerobic + anaerobic culture, glucose, and Kingella PCR. Crystal analysis only if clinically indicated.

CCS pearl: For septic hip on CCS, the order set is: NPO, IV fluids, blood cultures × 2, CBC/CRP/ESR, hip US, orthopedics STAT consult, OR for I&D, then empiric vancomycin + cefazolin. Antibiotics after cultures and joint fluid drawn unless septic shock.

Joint aspiration (arthrocentesis):

Surgical irrigation and debridement: Septic arthritis of the hip is a surgical emergency — open arthrotomy or arthroscopic washout within hours. Knee septic arthritis often managed with serial arthroscopic washouts. Delay >4 days dramatically increases chondrolysis and AVN.

Osteomyelitis drainage: Subperiosteal abscess, sequestrum, or failure of medical therapy → surgical debridement. Image-guided aspiration may suffice for small abscess.

SCFE — in-situ percutaneous screw fixation: Single cannulated screw across physis. Stable SCFE: elective within 24–48 h. Unstable SCFE (cannot bear weight): urgent fixation; AVN risk 20–50%. Avoid forceful reduction.

LCP surgical options: Femoral varus osteotomy or pelvic (Salter) osteotomy for containment in older children or severe involvement.

DDH closed/open reduction: Performed under anesthesia with arthrography; spica cast 3–4 months.

Toddler's fracture: Long-leg cast 3–4 weeks; non-displaced spiral tibial fractures heal reliably without surgery.

Tumor biopsy: Performed at the definitive treating center by orthopedic oncology — biopsy tract must be excisable with the tumor. Improper biopsy is a Step 3 ethics/safety stem.

Special Populations — Renal, Hepatic, and Immunocompromised

— Vancomycin requires trough monitoring (target AUC 400–600 mg·h/L); reduce dose for eGFR <60. Cefazolin renally dosed. NSAIDs contraindicated in significant renal impairment — use acetaminophen.

— Children with nephrotic syndrome on chronic steroids have ↑ AVN risk and ↑ infection risk; lower threshold for imaging.

— Methotrexate, NSAIDs, and acetaminophen all require dose adjustment. Check baseline LFTs before starting JIA therapy.

— Clindamycin metabolized hepatically — reduce dose in severe dysfunction.

— Broader empiric coverage: vancomycin + cefepime ± antifungal if persistent fever. Consider atypical organisms — Aspergillus, Candida, mycobacteria.

— Fungal osteomyelitis in chronic granulomatous disease (CGD) → Aspergillus, Serratia.

— Limp/bone pain differential: vaso-occlusive crisis vs. osteomyelitis vs. AVN of femoral head. CRP and ESR are unreliably elevated in VOC.

— Salmonella is the most common osteomyelitis organism in sickle cell — empirically cover with ceftriaxone + vancomycin.

— MRI distinguishes infarction from infection (infection: cortical breakthrough, periosteal abscess).

Key distinction: In sickle cell with bone pain + fever, do not assume VOC — image and culture. Missed osteomyelitis causes chronic deformity and amputation.

Renal impairment:

Hepatic impairment:

Immunocompromised (oncology patients, primary immunodeficiency, post-transplant):

Sickle cell disease:

Cystic fibrosis / chronic Pseudomonas exposure: anti-pseudomonal coverage (cefepime, piperacillin-tazobactam) for osteomyelitis.

Hemophilia: Recurrent hemarthrosis → target joint (knee, ankle, elbow). Treat with factor replacement (factor VIII or IX) to 80–100% activity; ice, rest, no NSAIDs (use COX-2 or acetaminophen). Prophylactic factor infusions prevent arthropathy.

Special Populations — Neonates, Adolescents, and Athletes

— A neonate who "doesn't move a limb" is septic arthritis/osteomyelitis until proven otherwise — transphyseal vessels permit easy spread from metaphysis to joint. Pathogens: GBS, S. aureus, E. coli, gonococcus (consider maternal STI exposure).

— Workup: CBC, CRP, blood culture, urine culture, LP (consider full sepsis workup), MRI, joint aspiration. Empirics: cefotaxime + nafcillin/vancomycin.

— Other neonatal causes: birth trauma (clavicle, humerus fracture from shoulder dystocia), congenital syphilis (pseudoparalysis of Parrot), brachial plexus injury.

— Stress fractures of tibia, metatarsals, femoral neck (high-risk — non-weight-bearing, surgical fixation may be needed). Female athlete triad: low energy availability, menstrual dysfunction, low bone density.

— Apophysitis: Osgood-Schlatter (tibial tubercle), Sever (calcaneus), Sinding-Larsen-Johansson (inferior patella). All managed conservatively — rest, stretching, ice, return to play as tolerated; resolve at physeal closure.

— Osteochondritis dissecans (OCD) of medial femoral condyle or talus — MRI; non-operative if stable, surgery if loose body or unstable.

— Slipped capital femoral epiphysis in obese adolescent — always evaluate hip in adolescent with knee pain.

Step 3 management: Adolescent with knee pain and no knee findings → AP pelvis + frog-leg lateral. Missing SCFE in this scenario is a classic Step 3 vignette and a medico-legal pitfall.

Neonate (<3 months) with pseudoparalysis:

Adolescent athlete:

Adolescent with hip pain + obesity + short stature: screen for hypothyroidism, growth hormone deficiency, panhypopituitarism, renal osteodystrophy — endocrinopathies cause atypical/bilateral SCFE.

Pregnant adolescent: Avoid radiation when possible; ultrasound and MRI without gadolinium are safe.

Complications and Adverse Outcomes

— Cartilage destruction (chondrolysis) within 8 hours of bacterial enzyme exposure.

— Avascular necrosis of femoral head — femoral head vessels traverse joint capsule in children; intra-articular pus compresses vessels.

— Growth arrest (physeal damage), limb-length discrepancy, chronic dislocation, sepsis, death.

— Avascular necrosis (20–50% in unstable SCFE, 0–10% stable) — worst with attempted reduction.

— Chondrolysis (especially with pin penetration into joint).

— Femoroacetabular impingement and early osteoarthritis (cam deformity).

— Contralateral slip in 20–40%.

Board pearl: A child with JIA who develops fever, hepatosplenomegaly, falling ESR with rising ferritin = macrophage activation syndrome — admit ICU, start high-dose steroids ± anakinra/cyclosporine; mortality 8–20%.

Septic arthritis untreated or delayed:

Osteomyelitis complications: Chronic osteomyelitis with sequestrum and involucrum, pathologic fracture, growth disturbance, sinus tract formation, secondary squamous cell carcinoma (Marjolin ulcer, late).

SCFE complications:

LCP complications: Permanent femoral head deformity (coxa magna, coxa plana), early osteoarthritis by age 40–60, leg-length discrepancy.

DDH complications: Persistent dysplasia → early arthritis; Pavlik harness complications include femoral nerve palsy and Pavlik disease (AVN from over-abduction).

JIA complications: Uveitis (silent — slit-lamp screening mandatory), growth failure, macrophage activation syndrome (life-threatening — fever, cytopenias, ferritin >10,000, falling ESR).

Iatrogenic: Improper tumor biopsy tract, missed leukemia from steroid trial, NSAID gastritis/nephrotoxicity, radiation exposure from excessive CT.

When to Escalate Care

— Suspected septic arthritis (Kocher ≥3, or any toxic-appearing febrile child with joint effusion).

— Suspected SCFE — must be non-weight-bearing en route.

— Open fracture, neurovascular compromise, compartment syndrome (5 Ps: pain out of proportion, pallor, paresthesia, pulselessness, paralysis).

— Pseudoparalysis in neonate.

— Suspected NAT — child welfare hold.

— IV antibiotics for osteomyelitis/septic arthritis until oral switch criteria met (afebrile 48 h, CRP trending down, clinical improvement).

— Diagnostic uncertainty with concerning labs.

— Inability of family to ensure outpatient follow-up.

— Septic shock from disseminated S. aureus (especially MRSA with PVL toxin — high risk of septic pulmonary emboli, DVT, multifocal disease).

— Macrophage activation syndrome.

— Necrotizing fasciitis or pyomyositis with hemodynamic instability.

— Orthopedics: all surgical conditions.

— Rheumatology: suspected JIA, vasculitis, autoinflammatory disease, CRMO.

— Hematology-oncology: suspected leukemia, bone tumor (osteosarcoma metaphyseal "sunburst," Ewing diaphyseal "onion-skin").

— Infectious disease: atypical organisms, prolonged therapy planning, immunocompromised.

— Child protection team: NAT.

— Endocrinology: atypical/bilateral SCFE, suspected hypothyroidism.

CCS pearl: On CCS, escalate care at the moment Kocher score reaches 3 — order admission, NPO, IV access, orthopedics consult, and OR booking simultaneously. Watch the clock; delays score against you.

Immediate ED transfer / call orthopedics STAT:

Admit (inpatient pediatrics):

ICU/PICU criteria:

Subspecialty consults:

Key Differentials — Same-Category Musculoskeletal/Infectious

— Oligoarticular (≤4 joints, ANA+, uveitis risk) — most common.

— Polyarticular (≥5 joints, RF can be +/−).

— Systemic (Still's) — quotidian fever, salmon rash, hepatosplenomegaly, serositis, very high ferritin.

— Enthesitis-related (HLA-B27, older boys, axial involvement).

— Psoriatic JIA.

Key distinction: Septic arthritis worsens motion passively and actively; transient synovitis allows some passive motion. Logrolling-preserved hip rotation with fever → think pyomyositis or psoas abscess, not septic hip.

Transient (toxic) synovitis: Post-viral, ages 3–10, mild fever, ambulatory or mildly antalgic, CRP normal or mildly up, resolves 24–48 h with NSAIDs. #1 cause of acute hip pain in children.

Septic arthritis: Toxic, fever >38.5°C, non-weight-bearing, elevated WBC/CRP/ESR, joint held in FABER. S. aureus most common overall; Kingella in 6–48 mo; gonococcus in adolescents.

Osteomyelitis: Acute hematogenous, metaphyseal location (vascular loops), S. aureus > Kingella > GBS (neonate) > Salmonella (sickle cell). Point bone tenderness, normal joint exam, MRI diagnostic.

Pyomyositis: Primary muscle abscess, often obturator internus or iliopsoas — mimics septic hip but joint motion preserved with logrolling. MRI diagnostic.

Discitis / vertebral osteomyelitis: Toddler refusing to sit, lordotic posture, normal limb exam, point tenderness over spine. MRI; antibiotics 4–6 weeks.

Reactive arthritis: Post-streptococcal (acute rheumatic fever — Jones criteria, migratory polyarthritis); post-enteric (Yersinia, Salmonella, Campylobacter); post-genitourinary (Chlamydia). HLA-B27.

Lyme arthritis: Late manifestation, large painless effusion (knee), intermittent, history of tick or travel to endemic area. Doxycycline or amoxicillin × 28 days.

JIA subtypes:

Key Differentials — Other-Category (Non-MSK)

— Acute lymphoblastic leukemia (ALL): bone pain (metaphyseal infiltration), limp, night pain, cytopenias, lymphadenopathy, hepatosplenomegaly. Smear and marrow diagnostic.

— Neuroblastoma: <5 yr, abdominal mass, bone metastases causing limp, raccoon eyes, opsoclonus-myoclonus. Urine catecholamines (HVA, VMA).

— Osteosarcoma: adolescent, metaphysis of distal femur/proximal tibia, "sunburst" periosteal reaction, Codman triangle.

— Ewing sarcoma: diaphyseal, "onion-skin" periosteum, may have fever and elevated inflammatory markers — mimics osteomyelitis.

— Langerhans cell histiocytosis: lytic skull/long bone lesions, "punched-out."

— Legg-Calvé-Perthes (idiopathic AVN of femoral head, ages 4–8, boys 4:1): insidious painless limp, decreased internal rotation and abduction, hip flexion contracture.

— AVN secondary to sickle cell, steroids, SCFE, post-traumatic.

— Cerebral palsy (toe-walking, asymmetric spasticity).

— Duchenne muscular dystrophy (boys, Gowers sign, calf pseudohypertrophy, elevated CK).

— Spinal cord tumor / tethered cord (back pain, bowel/bladder dysfunction, abnormal neuro exam).

— Guillain-Barré (ascending weakness, areflexia).

— Henoch-Schönlein purpura (IgA vasculitis): palpable purpura on legs/buttocks, arthritis, abdominal pain, hematuria.

— Acute rheumatic fever: post-strep, migratory polyarthritis, carditis.

— Kawasaki disease: rare cause of arthritis; fever >5 days plus mucocutaneous criteria.

Board pearl: Bilateral bone pain + fatigue + cytopenia + LDH↑ + uric acid↑ → ALL — get a smear and bone marrow before any steroid.

Hematologic-oncologic:

Vascular/ischemic:

Neurologic:

Dermatologic/Infectious systemic:

Functional/psychogenic: conversion gait — diagnosis of exclusion; consistent with stress, inconsistent exam.

Secondary Prevention and Discharge Planning

— Transition to oral antibiotics (typically cephalexin, clindamycin, or amoxicillin-clavulanate based on susceptibility) when afebrile 48 h, CRP declining by ~50%, and clinical improvement. Complete 3–4 weeks osteo / 2–3 weeks septic arthritis.

— Outpatient parenteral antibiotic therapy (OPAT) reserved for select cases — most pediatric uncomplicated infections complete therapy orally.

— Weekly CBC, CRP, LFTs while on antibiotics.

— Avoid contact sports until cleared by ortho (typically 6–8 weeks).

— Non-weight-bearing 4–6 weeks post-op, then gradual return.

— Monitor contralateral hip every 3–6 months until physeal closure (or prophylactic pin if endocrinopathy/high-risk).

— Screen for underlying endocrinopathy if atypical (short stature, age <10 or >16, bilateral).

— Lifelong osteoarthritis risk → weight management counseling.

— Slit-lamp ophthalmology every 3 months (ANA+ oligoarticular) for uveitis.

— Methotrexate monitoring: CBC, LFTs every 4–8 weeks; folate supplementation; pregnancy counseling for teens.

— Vaccinate (avoid live vaccines on biologics).

— Bone health: vitamin D, calcium, DEXA if chronic steroids.

Step 3 management: Vaccination status review at discharge — children on or starting biologics should complete inactivated vaccines (including annual flu, pneumococcal) and live vaccines (MMR, varicella) at least 4 weeks before immunosuppression.

Post-septic arthritis / osteomyelitis discharge:

Post-SCFE:

Post-LCP: Long-term orthopedic follow-up; PT for ROM; activity modification (avoid impact during fragmentation stage).

Post-DDH (Pavlik): Serial ultrasounds q2 weeks; harness 22–23 h/day for 6–12 weeks; switch to abduction brace.

JIA chronic care:

Lyme arthritis: If persistent after 28-day course → retreat once with IV ceftriaxone; if still persistent → "post-antibiotic Lyme arthritis," manage with DMARDs.

Follow-Up, Monitoring, and Rehabilitation

— Septic arthritis/osteomyelitis: ortho + ID at 1 week, 2 weeks, 4 weeks, then 3 and 6 months. CRP/ESR until normalized. Radiographs at 6 weeks and 3–6 months to assess physeal integrity and growth.

— SCFE: post-op week 2, week 6, 3 months, then every 6 months until physeal closure to monitor contralateral hip and AVN.

— LCP: every 3–4 months with serial radiographs through fragmentation, reossification, and remodeling stages (~2–4 years total).

— JIA: rheumatology every 3 months; ophthalmology every 3 months if uveitis risk.

— DDH: AP pelvis at 6 months, 1 year, then yearly until skeletal maturity.

— Restore ROM (especially hip internal rotation and abduction).

— Strengthen hip abductors (counter Trendelenburg).

— Gait training, proprioception, gradual return to play.

— Aquatic therapy useful for non-weight-bearing strengthening.

— Septic joint: light activity at 4 weeks, full return at 8–12 weeks with normal exam and labs.

— Stress fracture: protected weight-bearing 4–6 weeks; address female athlete triad.

— Osgood-Schlatter: activity as tolerated; resolves at physeal closure.

— Obesity in SCFE — nutrition referral, motivational interviewing.

— Sickle cell — hydroxyurea adherence reduces VOC and may reduce osteonecrosis.

— Sun and tick avoidance for Lyme prevention in endemic areas.

— Bone health: vitamin D 600 IU/day, calcium 1000–1300 mg/day for adolescents.

Board pearl: Monitor growth charts — limb-length discrepancy >2 cm at skeletal maturity may warrant epiphysiodesis of the longer leg; refer to ortho early when discrepancy is documented.

Routine follow-up cadence:

Physical therapy goals:

Activity restrictions and return to play:

Counseling:

Ethical, Legal, and Patient Safety Considerations

— Physicians are mandated reporters in all 50 states; reasonable suspicion, not proof, triggers report to Child Protective Services. Failure to report carries criminal and civil liability.

— Red flags: implausible mechanism, delayed care, multiple fractures at different healing stages, posterior rib fractures, metaphyseal corner ("bucket-handle") fractures, femur fracture in non-ambulatory infant, retinal hemorrhages.

— Workup: skeletal survey (children <2), head CT/MRI, ophthalmology for retinal exam, social work, child protection team. Admit for safety while investigation proceeds — do not discharge to a potentially unsafe environment.

— Most procedures (joint aspiration, surgery) require parental consent. Adolescents may consent to STI testing/treatment (e.g., gonococcal arthritis workup) without parental consent in most states — preserve confidentiality.

— Emancipated minors, married minors, or pregnant adolescents may consent for themselves (state-dependent).

— Limp is a classic "bounce-back" diagnosis — ensure explicit return precautions (fever, worsening pain, refusal to bear weight) and 24-hour reassessment plan in writing. Document this clearly.

— Handoffs from ED to ward: SBAR including Kocher score, pending cultures, antibiotic timing.

— Avoid unnecessary CT in children (radiation); prefer MRI/US.

— Avoid premature steroid trials in undifferentiated bone pain (mask leukemia).

Step 3 management: When NAT is suspected, admit the child and notify CPS before discharge — never discharge first, report later. The hospital is the safe holding environment.

Non-accidental trauma (NAT) — mandatory reporting:

Consent and adolescent confidentiality:

Transition of care risks:

Diagnostic stewardship:

Tumor biopsy ethics: Refer suspected bone tumor to a sarcoma center before biopsy — improper biopsy tract may convert limb-salvage to amputation. This is a recognized standard-of-care issue.

Health equity: Delayed diagnosis of SCFE, DDH, and JIA disproportionately affects uninsured and rural children — advocate for timely referral.

High-Yield Associations and Rapid-Fire Clinical Facts

— 0–3 yr: toddler's fracture, DDH, NAT, septic arthritis (Kingella).

— 4–8 yr: transient synovitis, LCP, JIA-oligoarticular.

— 9–16 yr: SCFE, Osgood-Schlatter, stress fracture, osteosarcoma, Ewing.

— Neonate: GBS, S. aureus, gram-negatives.

— 6 mo–4 yr: Kingella kingae (often from URI).

— Older child: S. aureus.

— Adolescent sexually active: Neisseria gonorrhoeae.

— Sickle cell osteomyelitis: Salmonella.

— Puncture through sneaker: Pseudomonas.

Board pearl: Frog-leg lateral view is the single most important radiograph in adolescent hip/knee pain — without it, you will miss SCFE.

Age peak pearls:

Knee pain in a child = hip exam. Always.

Loss of internal rotation of the hip = most sensitive sign of hip pathology.

Klein line = drawn along superior femoral neck on AP pelvis; should intersect femoral head epiphysis. Failure to intersect = SCFE.

Kocher: NW-bearing, fever >38.5, ESR >40, WBC >12k. Add CRP >2.

Most common organism for septic arthritis by age:

Osteosarcoma: "sunburst," Codman triangle, metaphyseal, distal femur > proximal tibia, age 10–20, ↑ alk phos, retinoblastoma and Li-Fraumeni associations.

Ewing sarcoma: "onion-skin" periosteum, diaphyseal, t(11;22) EWS-FLI1, may mimic infection.

Galeazzi sign (knee-height asymmetry supine, hips flexed) and Barlow/Ortolani = DDH.

Trendelenburg gait = weak hip abductors (gluteus medius) on stance side.

Growing pains: bilateral, nocturnal, normal exam, normal labs, ages 3–12 — diagnosis of exclusion.

Pavlik harness: hips flexed, abducted; risk of femoral nerve palsy and AVN if over-flexed/abducted.

Macrophage activation syndrome: ↓ESR with ↑ferritin in systemic JIA = emergency.

Board Question Stem Patterns

Key distinction: When the stem gives fever + non-weight-bearing, the answer is almost always septic arthritis workup, even if some Kocher criteria are borderline.

Stem 1: "A 13-year-old obese boy presents with 3 weeks of right knee pain and a limp. Knee exam is normal. He keeps his hip externally rotated when you flex it." → SCFE; next step: AP + frog-leg pelvis, non-weight-bearing, ortho consult. Treatment: in-situ percutaneous screw fixation.

Stem 2: "A 4-year-old presents with refusal to bear weight, temperature 39°C, ESR 55, WBC 14,500." → Septic arthritis (Kocher 4); next step: hip aspiration + empiric antibiotics + OR washout.

Stem 3: "A 5-year-old with 1 week of mild limp after a URI, afebrile, ambulatory with discomfort, CRP normal." → Transient synovitis; ibuprofen and 48-h reassessment.

Stem 4: "A 6-year-old boy with painless limp for 2 months, decreased hip abduction and internal rotation, AP pelvis shows flattened femoral head with crescent sign." → Legg-Calvé-Perthes.

Stem 5: "A 3-year-old refuses to walk after a fall from couch; tender over tibia, X-ray shows faint spiral lucency." → Toddler's fracture; long-leg cast.

Stem 6: "An 8-year-old girl with morning stiffness, swollen knee for 6 weeks, ANA positive." → Oligoarticular JIA; next step: slit-lamp exam for uveitis.

Stem 7: "A 14-year-old with knee swelling, large painless effusion, recent camping trip in Connecticut." → Lyme arthritis; doxycycline 28 days.

Stem 8: "A 2-year-old with bilateral leg pain, pallor, bruising, hepatosplenomegaly, WBC 2.0, platelets 40k." → ALL; bone marrow biopsy. Avoid steroids.

Stem 9: "A 4-month-old with femur fracture; mother says baby rolled off bed; skeletal survey shows healing rib fractures." → NAT; admit, CPS, skeletal survey, ophtho, head imaging.

Stem 10: "A neonate not moving the left leg, febrile, irritable." → Septic arthritis/osteomyelitis; sepsis workup, MRI, joint aspiration, empiric cefotaxime + vancomycin.

One-Line Recap

A limp in a child is never normal — stratify by age, score with Kocher, image the hip even when the knee hurts, and never miss SCFE, septic arthritis, NAT, or leukemia.

Board pearl: The single most tested concept is adolescent knee pain → image the hip → diagnose SCFE → percutaneous in-situ screw fixation, non-weight-bearing in the interim — memorize this pathway cold.

By age: toddler → toddler's fracture/DDH/NAT; child → transient synovitis/LCP/JIA; adolescent → SCFE/stress fracture/tumor.

By urgency: Kocher ≥3 = septic until proven otherwise → aspirate, image, antibiotics, OR. Unstable SCFE = non-weight-bearing now, screw fixation today.

By red flag: fever + refusal to bear weight = septic workup; night pain + cytopenias = leukemia; inconsistent history + pattern injuries = NAT and CPS report.

By exam: loss of hip internal rotation localizes to hip; knee pain with normal knee = image the hip; Trendelenburg = abductor weakness (DDH, LCP, SCFE).

By imaging: start with AP + frog-leg lateral pelvis and US for effusion; MRI for osteomyelitis, tumor, pyomyositis, early LCP; CT only when cortical detail required.

By treatment: empiric IV antibiotics after cultures (cefazolin ± vancomycin; add ceftriaxone for sickle cell/gonococcus); transition to oral once CRP halves and afebrile 48 h; total 2–4 weeks.

By follow-up: ortho/ID at 1, 2, 4 weeks; rheumatology every 3 months with slit-lamp for JIA; contralateral hip surveillance after SCFE until physeal closure.

By safety net: explicit 24-hour return precautions for any child sent home with a limp; document Kocher score and reassessment plan; mandatory CPS report if NAT suspected before discharge.