Median hare-lip is extremely rare. It occurs in two forms: one in which there is a simple cleft in the middle of the lip, the result of non-union of the two globular processes; another in which there is a wide gap due to entire absence of the parts developed from the mesial nasal process—the central portion of the lip, the mesial segment of the os incisivum, and the septum of the nose. The second form is usually associated with cleft palate.
Lateral hare-lip is much more common. It is due to imperfect fusion of the globular process with the labial plates of the maxillary process. There may be a cleft only on one side of the lip, or the condition may be bilateral. In some cases the cleft merely extends into the soft parts of the lip—simple hare-lip ([Fig. 232]) forming a notch with rounded margins on which the red edge of the lip shows almost to the apex. In other cases the cleft passes into the alveolus of the jaw—alveolar hare-lip—partly or completely separating the mesial and lateral segments of the premaxillary bone ([Fig. 233]). These cases are usually combined with cleft palate ([Fig. 236]).
Fig. 232.—Simple Hare-lip.
Fig. 233.—Unilateral Hare-lip with Cleft Alveolus.
When the hare-lip is bilateral, the two clefts may be unequal, one forming a simple notch in the lip, the other passing into the nostril. In most cases, however, both clefts are complete, and the mesial portion of the lip is entirely separated from the lateral portions. The central portion or prolabium is usually smaller than normal, and is closely adherent to the os incisivum. This bone may retain its normal position in line with the alveolar processes of the maxilla ([Fig. 234]), or it may be tilted forward so that the incisor teeth, when present, project beyond the level of the prolabium ([Fig. 235]). In aggravated cases, the os incisivum and prolabium are adherent to the end of the nose. In these cases there is a Y-shaped cleft in the palate.