20.4 Musculoskeletal & Rheumatologic Disorders

Musculoskeletal and Rheumatologic Disorders

Why This Matters for the Exam

The limping child is one of the highest-yield vignette formats in pediatric musculoskeletal exam content, because the differential is sharply age-dependent and the most dangerous mimic, septic arthritis, must be distinguished quickly from a much more common benign condition. Juvenile idiopathic arthritis questions focus on matching a clinical pattern to the correct classification subtype.

The Limping Child: An Age-Based Approach

A practical framework is to think about likely diagnoses by age band, while always keeping septic arthritis and trauma or non-accidental injury on the differential at any age.

Developmental Dysplasia of the Hip (DDH)

Developmental dysplasia of the hip (DDH) is abnormal development of the hip joint, ranging from mild instability to frank dislocation.

• Risk factors: female sex, breech presentation, first-born status, a positive family history, and oligohydramnios or otherwise tight intrauterine positioning.
• Newborn screening exam: the Barlow maneuver (adducting and pushing posteriorly, which attempts to dislocate an unstable hip) and the Ortolani maneuver (abducting and lifting anteriorly, which attempts to reduce, or relocate, a dislocated hip, felt or heard as a clunk). Both maneuvers become less reliable after about 3 months as the muscles tighten around a persistently dislocated hip.
• Later findings, after the newborn period: asymmetric thigh or gluteal skin folds, a limb-length discrepancy shown by a positive Galeazzi sign (unequal knee heights with the hips and knees flexed), and eventually a limp or waddling gait once walking begins.
• Screening and diagnosis: a clinical hip exam at every well-child visit in infancy; ultrasound is the imaging modality of choice before the femoral head ossifies, typically usable from about 4–6 weeks up to about 4–6 months; plain radiography becomes more useful after the femoral head ossifies, around 4–6 months.
• Management: a Pavlik harness is first-line for infants under about 6 months, maintaining hip flexion and abduction to allow the hip to stabilize; older children or harness failures may require closed or open reduction with casting.

Transient Synovitis vs Septic Arthritis

This distinction is the single highest-yield musculoskeletal topic on the exam, because the two conditions can look similar early on but require completely different management, since septic arthritis is a surgical emergency.

Kocher Criteria (Concept)

The Kocher criteria are a validated clinical decision tool combining four predictors to estimate the probability of septic arthritis, as opposed to transient synovitis, in a child with an irritable hip:

1. Fever greater than 38.5°C
2. Inability to bear weight
3. Erythrocyte sedimentation rate (ESR) greater than 40 mm/hr
4. Serum white blood cell (WBC) count greater than 12,000/mm³

The predicted probability of septic arthritis rises steeply with the number of criteria present — roughly under 3% with one criterion, about 40% with two, about 93% with three, and about 99% with all four (negligible with zero criteria). Memorize this as a rising curve, not four isolated numbers. The concept, rather than debating exact percentages on every vignette, is the testable point: more positive criteria mean a higher likelihood of septic arthritis, and the diagnosis cannot be excluded by clinical impression alone. C-reactive protein (CRP) is increasingly incorporated alongside the original four criteria in modern practice as an additional, faster-turnaround marker. Definitive diagnosis requires joint aspiration with synovial fluid analysis, showing a markedly elevated WBC count and a positive Gram stain or culture in septic arthritis.

Legg-Calve-Perthes Disease

Legg-Calve-Perthes disease (LCP) is idiopathic avascular necrosis of the femoral head in a growing child.

• Typical patient: a boy, aged 4–10 years, with a peak around 5–7 years, often described as short for age or with delayed bone age.
• Presentation: an insidious, painless-to-mildly-painful limp, often with pain referred to the groin, thigh, or knee — a classic exam trap, since knee pain in a child may actually reflect hip pathology.
• Exam: limited internal rotation and abduction of the hip.
• Imaging: plain radiographs show femoral head flattening, fragmentation, and sclerosis; MRI is more sensitive early in the disease, before radiographic changes appear.
• Management: the goal is to keep the femoral head contained within the acetabulum to preserve a round head as it reossifies. Options range from observation and activity modification in younger children with mild disease, to bracing or surgical containment procedures in older children or those with more severe disease. Overall prognosis is better the younger the child is at onset.

Slipped Capital Femoral Epiphysis (SCFE)

Slipped capital femoral epiphysis (SCFE) is displacement of the proximal femoral epiphysis, or growth plate, posteriorly and inferiorly relative to the femoral neck.

• Classic patient: an obese adolescent, typically male, around 10–16 years old, sometimes with an underlying endocrinopathy such as hypothyroidism, growth hormone deficiency or therapy, or renal osteodystrophy, especially if the patient is younger or thinner than typical.
• Presentation: hip, groin, thigh, or knee pain, again a classic trap for referred pain, together with a limp. The affected leg may rest in obligate external rotation, and hip flexion produces obligate external rotation of the leg.
• Can be acute (sudden displacement, often after minor trauma), chronic (gradual, more common), or acute-on-chronic.
• Imaging: anteroposterior and frog-leg lateral radiographs of the pelvis; a line drawn along the superior femoral neck should normally intersect a portion of the femoral head, and failure to do so suggests slippage.
• Management: urgent orthopedic referral; the child should be made non-weight-bearing immediately to reduce the risk of further slippage and avascular necrosis. Treatment is surgical, with in-situ screw fixation. SCFE is bilateral in a meaningful minority of cases, so the contralateral hip must also be assessed.

Juvenile Idiopathic Arthritis (JIA)

Juvenile idiopathic arthritis (JIA) is the umbrella term for chronic arthritis of unknown cause beginning before age 16 and persisting for at least 6 weeks, after other causes are excluded. The classification system used to divide JIA into subtypes is based on the number of joints involved, the presence of systemic features, and serology.

Exam-relevant distinctions:

• The subtype with the highest risk of uveitis that is classically asymptomatic, making routine screening essential, is oligoarticular JIA, especially in young, ANA-positive girls.
• Systemic-onset JIA is distinguished from the other subtypes by its prominent systemic inflammatory features, including the fever pattern, rash, and serositis, and is now understood as more of an autoinflammatory disease than a classic autoimmune one.
• Unlike adult rheumatoid arthritis, most JIA, apart from RF-positive polyarticular disease, is not primarily driven by rheumatoid factor.