Unilateral cleft lip makes up nine out of 10 of all patients with cleft lip. Unilateral cleft lip is twice as common on the left side as it is on the right. Bilateral cleft lip is present in only one out of 10 patients with cleft lip. There is a spectrum of severity of clefts of the lip.
In the least severe form, the cleft involves only a minor irregularity of the lip such as a scar or ridge along the philtral column or a notch in or above the red portion of the lip – microform cleft lip.
Incomplete cleft lips involve a cleft of the lip though the skin, muscle and intraoral part (mucosa) extending up to but not involving the floor of the nostril.
Complete cleft lip involves an extension of the cleft through the entire height of the lip and floor of the nostril, up to or through the gums (alveolus) and palate. Patients with cleft lip and palate have the most severe changes in the appearance of the lip and nose because the separation in the bones of the gums and palate creates a wide gap that separates the lip segments and drastically changes the shape of the nose.