Pyknodysostosis, (alternatively spelled pycnodysostosis) also known as osteopetrosis acro-osteolytica or Toulouse-Lautrec syndrome, is a rare autosomal recessive bone dysplasia, characterized by osteosclerosis and short stature.

Clinical presentation

Patients present in early childhood with:

  • short stature, particularly limbs
  • delayed closure of cranial sutures
  • frontal and occipital bossing
  • short broad hands and hypoplasia of nails
  • multiple long bone fractures following minimal trauma


Pyknodysostosis is a lysosomal disorder due to genetic deficiency in cathepsin K which has been mapped to chromosome 1q21. Cathepsin K is essential for normal osteoclast function.

Radiographic features

Osteosclerosis with narrowed medullary cavities is the main generalized imaging finding. Long bone fractures are common. Obtuse angle of the mandible. Lack/abnormal teeth.

Plain radiograph/CT
  • short, stubby fingers
  • partial agenesis/aplasia of terminal phalanges, simulating acro-osteolysis 
  • delayed bone age 
Cranial and maxillofacial
  • marked delay in sutural closure
  • frontoparietal bossing
  • calvarial thickening
  • Wormian bones (lambdoidal region)
  • relative proptosis
  • nasal beaking
  • obtuse mandibular gonial angle often with relative prognathism
  • persistence of primary teeth

History and etymology

Derived from the Ancient Greek terms "pucnos" (dense), "dys" (defective) and "ostosis" (bone condition).

ακρον (acron) in osteopetrosis acro-osteolytica is Ancient Greek for "at the extremity" and refers to resorption at the distal phalanges as observed in this condition.

The condition is also known as Toulouse-Lautrec syndrome, named after the famous French artist Henri Marie Raymond de Toulouse-Lautrec-Monfa (1864-1901) who was thought to be afflicted with the disease.

Differential diagnosis

General imaging differential considerations include:

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