A rare case of an anatomical variant of the carpal bones, an os paratrapezium, is reported. Since Vesalius described the first wrist supernumerary bone in the sixteenth century, around 20 different accessory carpal bones have been identified; global references to them can be found in the literature, but previous references to the os paratrapezium are extremely scarce.
The case reported here is an incidental radiological finding, in a 69-year-old woman. After some clinical sessions and an intensive study of the available literature, any clinical, pathological, or other anatomical conditions were discarded, and the structure was identified as a paratrapezium, an accessory carpal ossicle.
A few more than twenty accessory carpal bones have been described in the literature, and its development and presence have been associated to a non-fused primary ossification centre. Occurrence of a paratrapezium within the accessory carpal bones has been reported as exceptional; however, as happens with all the carpal supernumerary ossicles, it is relevant to be aware of its existence, and to get an accurate analysis of wrist radiologic image findings to avoid both anatomical and clinical misdiagnosis.