Figure 7.94 ONO angle, both sexes, 5 to 11 years. From Patton (1987), by permission.
Pitfalls Depending on the prominence of the nose, it may be very difficult to draw a straight line between the outer canthus and the base of the nose in the midline. In this situation, photographic estimation is advised.
The mandible, or lower jaw, is the first part of the face to form, as the medial ends of the two mandibular prominences merge during the fourth week of gestation.
Agnathia, or absence of the mandible, is a developmental defect of the first branchial arch. The mandible may be missing on one side only, a condition sometimes associated with microtia or absence of the external or internal ear, or with unilateral congenital macrostomia. Micrognathia, or underdevelopment of the jaw may be severe enough to lead to cleft palate in the Pierre-Robin sequence. This can be seen as an isolated anomaly or in conjuction with other features as part of a syndrome diagnosis. Macrognathia, or a large jaw, is otherwise termed prognathism and is best appreciated in profile. The jaw is abnormally large or jutting forward. Various measurements of the jaw can be made from radiographs. The main clinical measurement, for which a graph is provided, is the mandibular width, or bigonial distance. The shape of the jaw can vary markedly depending on the relative proportions of the ramus and body of the mandible. The jaw articulates with the skull at the temporomandibular joint. Restricted movement at that joint may produce limited mouth opening or trismus.
A median cleft of the mandible is a deep cleft resulting from failure of the mesenchymal masses of the mandibular prominences of the first branchial arch to merge completely with each other. It is extremely rare. More superficial clefts of the lower lip may also occur. Clefts of the mandible are sometimes associated with cleft lip, cleft palate, or oblique facial clefts.
A simple dimple on the chin is a common feature inherited with an autosomal dominant pattern. A more complicated H-shaped groove of the chin is associated with Freeman-Sheldon syndrome. Mental spurs are bony spurs over the most prominent part of the jaw in the midline.
The shape and size of the chin contribute to the overall facial shape. A wide chin will tend to produce a round or square face, while a narrow, pointed chin promotes the appearance of an inverted triangular face because of the relative width of the forehead and cranial vault.
Effective Mandibular Length: Cephalometric
Definition Effective length and prominence of the mandible.
Landmarks Measure from the condylion (the most posterosuperior point of the condylar outline) to the anatomic gnathion (determined by the intersection of the facial and mandibular planes) (Fig. 7.95). Typical values are shown in Fig. 7.96.
Instruments Spreading calipers are used to measure this distance on a radiograph taken with "cephalostat."
Position Standard cephalometry—see introduction to Chapter 7.
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