Variation in size of the mouth is well documented; detailed graphs are provided in the measurement section. It is far more difficult to evaluate the lips objectively. However, with the aid of calipers and a cooperative, relaxed patient, the visible vermilion of the upper lip, in Caucasians, when measured from the middle of the lip to the aperture, ranges from 3-3.5 mm in the newborn, to 4.7-4.8 mm in the one-year-old, to about 6 mm in the adult. Corresponding lower lip measurements are 3.6-4.7, 5.9-6.3, and 9-10 mm, respectively. Further variation in lip shape is associated with differences in philtral development. A well-formed, deeply grooved philtrum is usually associated with wide-spaced peaks to the vermilion border of the upper lip. Conversely, a poorly formed, flat philtrum is usually associated not with marked peaks, but with a relatively thin, even surface to the vermilion border. A tented upper lip often results from long-standing, bilateral, facial muscle weakness, such as that seen in congenital myotonic dystrophy.
Congenital pits or recesses of the lips are present in 2-3 percent of neonates. They usually occur in the lower lips, where they may be bilateral or unilateral, showing a well-defined circular depression on the vermilion border. In the upper lip, where they are rare, they are found lateral to the philtrum. They represent the orifices of mucous tracts that extend into the substance of the lip. A double lip is a deformity that consists of redundant tissue in the mucosal portion of the lip just inside its vermilion border. The anomaly, which can occur in the upper or in the lower lip, has been observed in both males and females and is individually rare. It is more conspicuous in the upper lip because the mucosal duplication, which shows a median notch, hangs down and partly covers the incisors. Double lip may be associated with relaxation of the supratarsal fold (blepharochalasis) and with thyroid enlargement.
Width of the mouth is a difficult soft tissue measurement to assess. Macrostomia, or large mouth, may be caused by lateral or transverse facial clefts running from the mouth toward the ear. This abnormality results from failure of the lateral mesenchymal masses of the maxillary and mandibular prominences to merge. Congenital microstomia (small mouth) results from excessive merging of the mesenchymal masses of the maxillary and mandibular prominences of the first branchial arch. In severe cases, it may be associated with mandibular hypoplasia. A small mouth, with pursed lips, may also result from perioral fibrosis.
Frenula are strands of tissue extending between the buccal and alveolar mucosae. Abnormal numbers of frenula may be associated with various craniofacial syndromes. An assessment of alveolar ridge thickness should be included in the evaluation of the mouth. Thickening may be congenital or acquired, for example, following prolonged therapy with antiepileptic medications.
The reader is referred to the references at the end of this chapter for further information.
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