Bilateral extra tarsal bones in Rubinstein-Taybi syndrome: the fourth cuneiform bones.

Rubinstein-Taybi syndrome is a syndrome of mental retardation associated with digital changes, consisting mainly of broad thumbs and large toes. This paper deals with pedal changes in a patient with this syndrome in whom bilateral, large, extra tarsal bones occurred between the medial cuneiform and intermediate cuneiform bones. They articulated with these bones, as well as with the first and the second metatarsals. It is suggested that these extra tarsal bones be labelled as the fourth cuneiform bones (os cuneiformis IV). To the best of the author's knowledge, such an extra tarsal bone has never been reported in humans or other mammalians. One could speculate that they originated from bilateral extra ossification centers or from an isolated, accelerated ossification and extreme enlargement of the so-called os intercuneiforme, a supernumerary ossicle, in this patient with Rubinstein-Taybi syndrome.