The Mandible as a Tubular Bone

The mandible is basically a tubular long bone, the tooth-bearing portion of which is bent into a paraboloid (horseshoe-like) configuration.1 The tooth-bearing portion is commonly referred to as the “anterior segment,” and that more distal as the “posterior segment” ( Fig. 5.1A,B ).

Beyond the dentition of the anterior segment, the tubular bone angulates, then ascends and flares outwardly to provide bilateral (condylar) contact with the glenoid fossa of each temporal bone at the lateral cranial base ( Fig. 5.2A,B ).

The alveolar process of the mandible is more robust than the alveolar process of the maxilla.2 The mandible lacks a horizontal platform (such as the palate), but strength is conferred instead by its substantial inner and outer cortex and inferior margin.

The comparative smoothness of the outer surface (cortex) of the mandible varies by region, depending on physiologic and mechanical demand. The angle of the mandible, for example, is grooved (because of the attachment and function of the masseter and the medial pterygoid muscles), the coronoid process is roughened (by the action of the temporalis muscles), and the upper outer surface of the lower jaw is thickened (alveolar bone) to accommodate the developing teeth and their roots after eruption2 ^– 4 ( Fig. 5.3 ).

Studies using a Zeiss Videoplan digital analyzer (Carl Zeiss, Oberkochen, Germany) to assess cross-sectional area support the concept of the mandible as a modified tubular bone, albeit with distinct regional features and distinct load-sharing characteristics. These studies suggest more total similarity than difference in the bone of the tooth-bearing mandible. From angle to angle, for example, in the “anterior segment,” there is little difference in total bone surface area, cortical bone surface area, or spongiosa.5 Further, the cross-sectional area of mandibular bone around the third molar is not statistically different than that in, say, the body, parasymphysis, or symphysis. Similarly, the cross-sectional area on either side of the canine is no different than that of the symphysis or the body, despite outward regional appearances ( Fig. 5.4A,B ).

The mandible continues upwardly beyond the dentition as the “posterior segment,” where the total cross-sectional measures progressively fall from the distal angle and ramus en route to the coronoid process and condyle.

The total cross-sectional area of each mandibular section is depicted and expressed as a relative percentage of the total cross-sectional area at the symphysis 6 ( Fig. 5.5A–C ).

Contrasting the Upper and Lower Margins

The cortical bone is thick along the lower border of the mandible, in the areas of the symphysis, parasymphysis, and anterior body ( Fig. 5.6A ). In the area of the posterior body, the cortical bone begins to thicken in the upper border to house the molar teeth ( Fig. 5.6B ). At the distal angle, the cortical thickening reverses, such that the cortical bone at the lower border is relatively thin, just distal to the last molar. There, the maximum thickness is at the upper, rather than the lower, border ( Fig. 5.6C ).2 ^, 5 At the junction of the angle and ramus and in the subcondylar area, the relative dominance of cortex at the anterior border remains ( Fig. 5.6D,E ).