Answer:
In the Tessier classification, the orbit, nose, and mouth are key landmarks through which the craniofacial clefts follow constant meridians. The clefts are numbered 0 to 14 with cleft number 8 forming the equator. Hence, clefts numbered 0 to 7 of the lower hemisphere represent the facial clefts, and the upper hemisphere of clefts numbered 9 to 14, their cranial prolongations. When a malformation traverses both hemispheres, a craniofacial cleft is produced that generally follows a set “time zone.” Examples of these combinations include 0–14, 1–13, 2–12, 3–11, and 4–10 patterns. Although the craniofacial clefts tend to conform with these vertical precincts across the orbit, the embryonic processes and vascular territories do not necessarily coincide with the “time zone.” Thus, it is difficult to explain the embryopathogenesis of some craniofacial clefts. The concept of “time zones” is important, as it serves to heighten the awareness. A conscious effort must be made to look along the tract in search of related deformities. Findings of subtle faults are indicative of greater involvement. Multiple and bilateral clefts of varying severity from a forme fruste to a complete representation also can occur in the same patient.