This person was born with a triphalangeal thumb, a rare congenital difference in which the thumb contains three phalanges instead of the usual two. The extra bone can be tiny or large enough to make the thumb resemble another finger.
Most human thumbs contain two phalanges, while the other fingers contain three. In triphalangeal thumb, an additional phalanx develops between the usual proximal and distal bones.The extra phalanx can vary considerably in size and shape, so the appearance and function of the thumb can differ from person to person. Some individuals have little functional impairment, while others may undergo surgery to improve thumb alignment or hand function.
u/ChiRko3 — 7 days ago