What is the differential diagnosis for conditions affecting true and apparent leg length?

True vs. Apparent Leg Length Discrepancy: Differential Diagnosis

True leg length discrepancy reflects actual anatomical shortening of skeletal structures, while apparent leg length discrepancy results from functional or positional abnormalities without true bone shortening—distinguishing between these is essential because they have fundamentally different causes and management strategies.

Definitions and Measurement

True leg length discrepancy is measured from the anterior superior iliac spine to the medial malleolus with the pelvis level, reflecting actual bone length differences 1, 2. Apparent leg length discrepancy is measured from the umbilicus or xiphoid process to the medial malleolus, and changes with pelvic obliquity or joint contractures without actual bone shortening 1.

Differential Diagnosis for True Leg Length Discrepancy

Congenital/Developmental Causes

• Skeletal dysplasias including achondroplasia, osteogenesis imperfecta, and enchondromatoses (Ollier disease, Maffucci syndrome) cause disproportionate limb shortening with characteristic radiographic findings 3
• Hemihypertrophy/hemiatrophy syndromes produce unilateral limb length differences from asymmetric growth 3
• Congenital femoral deficiency or fibular hemimelia result in significant shortening of affected bones 3
• Enchondromatoses (Ollier disease and Maffucci syndrome) cause bone shortening with leg length discrepancy due to multiple enchondromas affecting long bones, typically presenting in early childhood (median age 4.8-11 years) 3

Acquired Causes

• Trauma including physeal injuries, fractures with malunion, or growth plate damage leads to asymmetric growth 1
• Infection such as osteomyelitis or septic arthritis can damage growth plates and cause premature closure 1
• Tumor or tumor-like lesions affecting bone growth, including transformation of enchondromas to chondrosarcoma (occurs in ~30% of enchondromatosis patients) 3
• Vascular abnormalities including arteriovenous malformations can stimulate overgrowth 1
• Neurological conditions such as poliomyelitis or cerebral palsy may cause undergrowth of affected limbs 3

Metabolic/Systemic Causes

• X-linked hypophosphatemia causes severe leg bowing and growth abnormalities with characteristic biochemical findings (hypophosphatemia, elevated alkaline phosphatase, elevated FGF23) 4
• Vitamin D deficiency and rickets affect bone growth and can contribute to limb deformities 5

Differential Diagnosis for Apparent Leg Length Discrepancy

Pelvic/Spinal Causes

• Pelvic obliquity from scoliosis creates apparent shortening without true bone length difference 3, 1
• Tethered cord syndrome can cause progressive scoliosis and exaggerated lumbosacral lordosis, leading to apparent leg length discrepancy; patients may have leg length discrepancies noted in childhood along with subtle neurological signs 3
• Sacroiliac joint dysfunction or pelvic rotation produces functional shortening 1

Joint Contractures

• Hip flexion contracture causes apparent shortening on the affected side by tilting the pelvis 1
• Knee flexion contracture similarly creates functional length difference 1
• Ankle equinus deformity reduces functional leg length 1

Neuromuscular Causes

• Muscle spasticity in conditions like cerebral palsy leads to joint contractures and apparent shortening 3
• Chronic muscle weakness with compensatory posturing affects apparent length 3

Critical Diagnostic Approach

Physical Examination Specifics

• Measure both true and apparent lengths to differentiate between anatomical and functional discrepancies 1, 2
• Assess with pelvis level using blocks under the short side to determine true discrepancy 1
• Examine for joint contractures by testing hip extension (Thomas test), knee extension, and ankle dorsiflexion 1
• Evaluate spine alignment for scoliosis or lordosis that might cause pelvic obliquity 3
• Check for disproportionate body habitus by measuring upper-to-lower segment ratios and arm span, as skeletal dysplasias often show characteristic proportions 3

Red Flags Requiring Urgent Evaluation

• Progressive neurological symptoms including sensory deficits, muscle weakness, or gait abnormalities suggest tethered cord syndrome or other neurological pathology 3, 5
• Rapid growth of a mass or persistent bone pain raises concern for malignant transformation, particularly in enchondromatosis patients 3
• Severe discrepancy >5 cm warrants comprehensive skeletal survey and consideration of surgical intervention during growth years 1
• Associated systemic symptoms such as fever, weight loss, or constitutional symptoms suggest inflammatory or neoplastic conditions 5

Imaging Strategy

• Standing radiographs (orthoroentgenograms or scanograms) provide accurate measurement of true leg length and are superior to supine films for clinical decision-making 6
• Skeletal survey is mandatory when disproportionate short stature or skeletal dysplasia is suspected, including AP and lateral views of spine, pelvis, and all long bones 3
• Spinal MRI should be obtained when tethered cord syndrome is suspected based on cutaneous markers, neurological findings, or unexplained leg length discrepancy with back pain 3

Laboratory Evaluation

• Metabolic workup including serum phosphate, alkaline phosphatase, vitamin D, PTH, calcium, and magnesium when metabolic bone disease is suspected 4
• FGF23 levels should be measured if X-linked hypophosphatemia is considered 4
• Genetic testing may be indicated for suspected skeletal dysplasias, though many enchondromatoses arise from somatic mosaicism rather than germline mutations 3

Common Pitfalls

• Assuming all discrepancies are true anatomical shortening without measuring apparent length leads to misdiagnosis of functional problems 1
• Measuring femur length linearly in bowed bones rather than actual bone length overestimates shortening in conditions like osteogenesis imperfecta 3
• Missing subtle skeletal dysplasia features in familial short stature cases; skeletal survey should be considered even in seemingly isolated short stature, especially if >3 standard deviations below mean 3
• Overlooking tethered cord syndrome in patients with longstanding "minor" orthopedic issues, late toilet training, or chronic constipation—these may represent early manifestations 3
• Failing to store DNA from fetuses with suspected skeletal dysplasia prevents accurate molecular diagnosis for future genetic counseling 3