Bowing Bones

Christopher G. Anton, MD

DIFFERENTIAL DIAGNOSIS

Common

Physiologic Bowing
Blount Disease

Less Common

Rickets
Fibrous Dysplasia
Neurofibromatosis
Osteogenesis Imperfecta

Rare but Important

Congenital Tibial Dysplasia
Congenital Bowing
Achondroplasia
Camptomelic Dysplasia

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Must determine which causes of lower extremity bowing are physiologic vs. pathologic
Isolated or generalized bowing
Cortical thickening along convex side and thinning along concave side of curve
Neonates and infants have normal varus angulation of lower extremities
- Correction of bowing by 6 months after beginning to walk or aged 1.5-2 years old
- Hint: Considered abnormal if varus angulation of knee in child > 2 years old
Changes to valgus angulation by 1.5-3 years old (11° in 3 year old)
5-6° of valgus angulation by 13 years old

Helpful Clues for Common Diagnoses

Physiologic Bowing
- a.k.a. developmental bowing
- Exaggerated varus angulation when younger than 2 years old
  - If exaggerated during 2nd year of life, probably normal but follow to exclude development of Blount disease
- More common in early walkers, heavier children, and African-American children
- Tibial metaphysis appears prominent, depressed with small beaks of distal medial and posterior tibia and femur
  - Not fragmented, thickened medial tibial cortex
  - Tilted distal tibial growth plate laterally
- Usually resolves without treatment
Blount Disease
- Infantile type: 1-3 years old, bilateral in 60-80%
  - Must differentiate from physiologic bowing
- Adolescent type: 8-14 years old, more commonly unilateral
- Thought to result from abnormal stress on proximal medial tibial physis
  - May reflect normal physiologic bowing that progresses and fails to predictably correct
- Diagnosed by progressive clinical bowing on clinical examination in combination with characteristic radiographic changes
- Predisposed: Early walkers, obese children, and African-Americans
- Medial tibial metaphysis depression and fragmentation, constriction ± bone bridging of proximal medial tibial physis, genu varum
- Enlarged epiphyseal cartilage and medial meniscus on MR
- Metaphyseal-diaphyseal angle
  - Angle between line drawn parallel to proximal tibial metaphysis and another line drawn perpendicular to long axis of tibial diaphysis
  - Abnormal if > 11° on standing radiographs
  - Indeterminate angle (8-11°), should follow clinically ± radiographically

Helpful Clues for Less Common Diagnoses

Rickets
- Generalized bowing, changes at sites of rapid growth
- Deficiency in mineralization of normal osteoid, widening zone of provisional calcification
- Metaphyseal flaring and fraying
Fibrous Dysplasia
- Hamartomatous lesion, replacement of portions of medullary cavity with fibroosseous tissue
- Long bone medullary space widening, endosteal scalloping, coarse or obliterated trabeculation
  - Lytic, ground-glass, or sclerotic
- 70% monostotic
- 90% of polyostotic lesions are unilateral
- Sarcomatous degeneration: 0.5%
Neurofibromatosis
- Anterolateral bowing of tibia, ± hypoplastic fibula, often narrowing or intramedullary sclerosis or cystic change at apex of angulation
  - Hamartomatous fibrous tissue
- Bowing typically at junction of middle and distal 1/3 of tibia
- May develop pathologic fracture, pseudoarthrosis of tibia ± fibula, tapering or penciling ends of bones at fracture site
Osteogenesis Imperfecta
- History of osteogenesis imperfecta
- Bowing results from soft bones
- Generalized bowing of long bones, osteoporosis, and multiple fractures

Helpful Clues for Rare Diagnoses

Congenital Tibial Dysplasia
- a.k.a. congenital pseudoarthrosis
- Rare
- 70% will eventually be diagnosed with neurofibromatosis (NF)
- 1-2% of neurofibromatosis patients
- Anterolateral tibia bowing or fracture
- If fibular bowing is absent, tends to resolve spontaneously
Congenital Bowing
- Abnormal intrauterine or fetal positioning
- Convex posteromedially, rarely laterally
  - Calcaneovalgus deformity of ipsilateral foot
  - ± diaphyseal broadening
- Tends to resolve
- ± protective bracing
Achondroplasia
- Generalized bowing
- Most common form of short-limb dwarfism
- Autosomal dominant or spontaneous mutation
- Small thorax with short ribs
- Short and thick long bones with metaphyseal cupping and flaring, short broad phalanges
- Short rectangular iliac bones (elephant ear-shaped), narrow sacrosciatic notches, flat acetabular roof
- Bullet-shaped vertebral bodies with posterior scalloping, narrowed interpedicular distances in lumbar spine
Camptomelic Dysplasia
- a.k.a. campomelic dysplasia
- Autosomal dominant
- Often fatal in infancy
- Anterolateral bowing of lower extremities > upper extremities
  - Bowed femur with short-bowed tibia
- Pretibial skin dimples
- Large skull with small face, hypoplastic scapula, narrow pelvis, dislocated hips, bell-shaped chest