When failure of tissues to fuse together that are forming early in pregnancy, defect can happen and jaw is incomplete. For example, cleft lip and palate when one of the jaws embryological processes failed to fuse together or the agenesis of the condylar process of the mandible, which may be seen in hemifacial microsomia. Clinically, cleft palate  presents as opening in the palate that can affect the front palate only or extending from the front to the back palate. Cleft lip is presented clinically with opening of the upper lip which can be a small slit on the upper lip or large opening connected to the nose. The cleft can be unilateral on the upper lip or bilaterally. Cleft lip can happen together with cleft palate.Palatal clefts are one of the most common congenital abnormalities which occur in 1:2500 live births. Embryologically, palate formation takes place in two stages, with the primary palate formed after 6th week followed by secondary palate formation between sixth and eighth weeks. The palate will fuse with the medial nasal process to form the roof of the mouth in order to complete the developmental process. However, if this process is incomplete that is when developmental defect occur. Initially, it is just a common oro-nasal cavity within the embryo with nothing to separate the nose and mouth cavity. This space is occupied with tongue. The primary palate formed when medial nasal prominences fused together to form the intermaxillary segment to demarcate the oral from the nasal components.
One way in which a vertical jaw abnormality can occur is if the condylar growth process is damaged, and thus the intramembranous growth continues along the posterior condylar margin, becoming predominant, thus resulting in a high angle type.Assessing a patient for vertical and horizontal excess
The Maxillary Mandibular Plane Angle (MMPA) is important in assessing a patient for vertical jaw abnormality. The important landmarks are the Frankfurt plane and the Mandibular plane. The Frankfurt plane is a line charted from the upper border of external auditory meatus to the lower border of the orbit. It is similar to the ala-tragus line however should not be confused with it. Whereas the Mandibular angle is a line measured along the natural lower border of the patients mandible. The intersection of the Frankfurt and Mandibular plane is what determines the pattern of vertical growth. If the intersection occurs behind the occiput i.e. decreased angle, then the patient is considered to have horizontal growth. On the other hand, if the intersection occurs in front of the occiput, then the patient is considered to have vertical growth. The normal range is considered to be between 25 and 30 degrees in a typical Caucasian person. Those with excess vertical jaw growth are most often seen having an anterior open bite, as the mandible is growing away from the maxilla. Whereas those with excessive horizontal growth are seen as having an overbite.