Rare diseases · Sign or symptom
Synostosis involving the 1st metacarpal
Fusion involving 1st long bone of hand
HP:0009703
What it means
Fusion of the 1st metacarpal with another bone. In contrast to the proximal phalanges of the digits 2 to 5, the proximal phalanx of the thumb is embryologically equivalent to the middle phalanges of the other digits, whereas the first metacarpal is embryologically of phalangeal origin and as such equivalent to the proximal phalanges of the other digits.
Rare diseases that can present with this1
The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions. A sign on this page is a conversation to have with a clinician, not a conclusion.
Part of the broader category
Also called: First metacarpophalangeal joint synostosis · Symphalangism affecting the 1st metacarpal
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Start free with EleplanThis page uses the Human Phenotype Ontology (2026-06-23) — Human Phenotype Ontology Consortium, hpo.jax.org. Disease associations from Orphanet (INSERM), CC BY 4.0. Content is educational and is not medical advice.