The normal formation of the skull occurs due to the presence in it of centers of primary growth and bone remodeling - craniofacial synarthroses (joints), which, in the process of development of the head skeleton , close at a certain time, providing bone fusion. 
The frontal bone (os frontale) of the skull of a newborn consists of two halves, between which there is a vertical fibrous connection - the median frontal or metopic suture (from the Greek metopon - forehead), running from the top of the back of the nose up the midline of the forehead to the anterior fontanel. This is the only fibrous cranial suture that heals during infancy: from 3-4 months to 8-18. 
See also - Changes in the skull after birth
The causes of trigonocephaly are metopic craniosynostosis (craniostenosis) or metopic synostosis (from the Greek syn - together and osteon - bone), that is, premature (up to the third month) immobile fusion of the bones of the cranial vault together along the median frontal suture. Thus, craniosynostosis and trigonocephaly are related as cause and effect, or as a pathological process and its result. 
In most cases, trigonocephaly in a child is the result of primary (isolated) craniosynostosis, the exact cause of which is unknown. Isolated craniosynostosis occurs sporadically, probably due to exposure to a combination of genetic and environmental factors. 
But trigonocephaly can be part of congenital syndromes resulting from chromosomal abnormalities and mutations in various genes. These include: Opitz trigonocephaly syndrome (Bohring-Opitz syndrome), Apert syndrome, Lois-Dietz syndrome, Pfeiffer syndrome, Jackson-Weiss syndrome, craniofacial dysostosis or Crouzon syndrome , Jacobsen, Setre-Chotzen, Müncke syndromes. In such cases, trigonocephaly is called syndromic. 
At birth, the brain is typically 25% of its adult size, reaching about 75% of an adult's brain by the end of the first year of life. But with the primary growth retardation of the brain, the so-called secondary craniosynostosis is possible. The etiology of the delay is associated with metabolic disorders, some hematological diseases, teratogenic effects on the fetus of chemicals (including those contained in pharmaceuticals). 
According to experts, trigonocephaly in adults who were not treated in childhood, as a result of isolated craniosynostosis or a congenital syndrome, persists throughout life.