= HYPERPLASTIC ACHONDROPLASIA = METATROpIC DWARFISM

metatropic = “changeable” (change in proportions of trunk to limbs over time ← developing kyphoscoliosis in childhood)

• longitudinal double skin fold overlying coccyx
• long bones short with dumbbell-like / trumpet-shaped configuration (= exaggerated metaphyseal flaring)
• “hourglass” phalanges (= short phalanges with widened ends)
• wide separation of major joint spaces (thick articular cartilage)
• delayed ossification of flat irregular epiphyses
• Chest
  • cylindrical narrowed elongated thorax
  • short + wide ribs
  • pectus carinatum
• Vertebrae
  • odontoid hypoplasia with atlantoaxial instability
  • progressive kyphoscoliosis
  • platyspondyly + very wide intervertebral spaces
  • wedge- / keel-shaped vertebral bodies
• Pelvis
  • coccygeal appendage similar to a tail (rare but CHARACTERISTIC)
  • short squared iliac bones + irregular acetabula
  • narrowed greater sciatic notch

Prognosis: compatible with life; increasing disability from kyphoscoliosis

DDx: achondroplasia, mucopolysaccharidoses