Hypoplasia of the clavicles and bell-shaped rib cage in the patient with CDD
|Classification and external resources|
It is usually autosomal dominant, but in some cases the cause is not known. It occurs due to haploinsufficiency caused by mutations in the CBFA1 gene (also called Runx2), located on the short arm of chromosome 6, which encodes transcription factor required for osteoblast differentiation. It results in delayed ossification of midline structures of the body, particularly membranous bone.
A new article reports that the CCD aetiology is thought to be due to a CBFA1 (core binding factor activity 1) gene defect on the short arm of chromosome 6p21 . CBFA1 is vital for differentiation of stem cells into osteoblasts , so any defect in this gene will cause defects in membranous and endochondral bone formation.
People with the condition usually present with a painless swelling in the area of the clavicles at 2–3 years of age. Common features are:
- Clavicles (collarbones) can be partly missing leaving only the medial part of the bone. In 10% cases, they are completely missing. If the collarbones are completely missing or reduced to small vestiges, this allows hypermobility of the shoulders including ability to touch the shoulders together in front of the chest. The defect is bilateral 80% of the time. Partial collarbones may cause nerve damage symptoms and therefore have to be removed by surgery.
- The mandible is prognathic due to hypoplasia of maxilla (micrognathism) and other facial bones.
- A soft spot or larger soft area in the top of the head where the fontanelle failed to close, or the fontanelle closes late.
- Bones and joints are underdeveloped. People are shorter and their frames are smaller than their siblings who do not have the condition.
- The permanent teeth include supernumerary teeth. Unless these supernumeraries are removed they will crowd the adult teeth in what already may be an underdeveloped jaw. If so, the supernumeraries will probably need to be removed to make space for the adult teeth. Up to 13 supernumarary teeth have been observed. Teeth may also be displaced. Cementum formation may be deficient.
- Failure of eruption of permanent teeth.
- Bossing (bulging) of the forehead.
- Open skull sutures, large fontanelles.
- Delayed ossification of bones forming symphysis pubis, producing a widened symphysis.
- Coxa vara can occur, limiting abduction and causing Trendelenburg gait.
- Short medial fifth phalanges, sometimes causing short and wide fingers.
- Vertebral abnormalities.
- On rare occasions, brachial plexus irritation can occur.
- Scoliosis, spina bifida and syringomyelia have also been described.
Other features are: parietal bossing, basilar invagination (atlantoaxial impaction), persistent metopic suture, abnormal ear structures with hearing loss, supernumerary ribs, hemivertebrae with spondylosis ,small and high scapulae, hypoplasia of illiac bones, absence of the pubic bone, short / absent fibular bones, short / absent radial bones, hypoplastic terminal phalanges.
Clinically, different features of the dysostosis are significant. Radiological imaging helps confirm the diagnosis. During gestation (pregnancy), clavicular size can be calculated using available nomograms. Wormian bones can sometimes be observed in the skull.
Around 5 years of age, surgical correction may be necessary to prevent any worsening of the deformity. If the mother has dysplasia, caesarian delivery may be necessary. Craniofacial surgery may be necessary to correct skull defects  Coxa vara is treated by corrective femoral osteotomies. If there is brachial plexus irritation with pain and numbness, excision of the clavicular fragments can be performed to decompress it. In case of open fontanelle, appropriate headgear may be advised by the orthopedist for protection from injury.
The incidence is estimated at 1:200,000.
The comedian Emmett Furrow has no collarbones and uses the resulting extra shoulder mobility in comedy routines.
At the rescue of Jessica McClure in 1987, Ron Short, a muscular man (a roofing contractor) who was born without collarbones because of cleidocranial dysostosis and so could collapse his shoulders to work in cramped corners, arrived at the site and offered to go down the shaft; they accepted his offer, although did not use it.
Dustin Henderson (fictional character), portrayed by Gaten Matarazzo on the Netflix Original Stranger Things, claims to have this disorder.
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- The National Craniofacial Association
- Medical Imaging on CCD
- GeneReviews/NCBI/NIH/UW entry on Cleidocranial Dysplasia
- Radiology of CCD Images from MedPix
- YouTube video of Shoulder Dance by comedian Emmet Furrow
- YouTube video of male teenager with no collarbones and thus hypermobile shoulders