A triangular face, in the simplest sense, is a face

Face

The face is a central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can, depending on the definition in the human case, include the hair, forehead, eyebrow, eyelashes, eyes, nose, ears, cheeks, mouth, lips, philtrum, temple, teeth, skin, and...

 whose lower half 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

Osteogenesis imperfecta

Osteogenesis imperfecta is a genetic bone disorder. People with OI are born with defective connective tissue, or without the ability to make it, usually because of a deficiency of Type-I collagen...

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In a broader sense, triangular face encompasses a constellation of a hypoplastic face with prominent zygomatic arch

Zygomatic arch

The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...

es, orbital hypertelorism

Hypertelorism

Hypertelorism is an abnormally increased distance between two organs or bodily parts, usually referring to an increased distance between the orbits . In this condition the distance between the inner eye corners as well as the distance between the pupils is greater than normal...

, sunken cheeks, down-turned mouth, and occasionally brownish facial discolouration. This constellation is characteristic of Mulibrey nanism

Mulibrey nanism

Mulibrey nanism , also called Perheentupa syndrome and pericardial constriction with growth failure, is a rare autosomal recessive congenital disorder. It causes severe growth failure along with abnormalities of the heart, muscle, liver, brain and eye...

, but has also been described in Russell-Silver syndrome and Turner syndrome

Turner syndrome

Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...

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