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Lets say a person is born with both thumbs unable to "bend" more than 5 degrees at the MCP joint (center or middle joint). This picture is of a thumb bending well over 45 degrees at the same joint. enter image description here

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I think you are referring to thumb hypoplasia. Or more specifically thumb hypoplasia Type II. It restricts the movement of the thumb. It is a congenital disorder. The cause is still unknown, maybe some genetic deformalities.

Type II is characterized by a tight web space between the thumb and index finger which restricts movement, poor thenar muscles and an unstable middle joint of the thumb metacarpophalangeal joint. This unstable thumb is best treated with reconstruction of the mentioned structures. [From: Wikipedia]

Further reading:

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