triangular face triangular face a triangular face, in the simplest sense, is a human face shape with a lower half that becomes relatively thin, approaching an appearance of a triangle with a tip facing downwards. it is not necessarily caused by any disease, but is common in individuals with osteogenesis imperfecta. in a broader sense, triangular face encompasses a constellation of a hypoplastic face with prominent zygomatic arches, orbital hypertelorism, sunken cheeks, down-turned mouth, and occasionally brownish facial discolouration. this constellation is characteristic of mulibrey nanism, but has also been described in russell-silver syndrome and turner syndrome.