Change in the width of the mandibular body from 6 to 23 years of age: an implant study

After the ossification of the mandibular symphysis, shortly after birth, ch anges in mandibular width would be expected to occur only by surface apposi tion or resorption on the buccal surfaces of the left and right mandibular halves. However, evidence for an opening hinge movement of the two mandibul ar halves around a vertical axis located in the region of the mandibular sy mphysis was recently found in longitudinal studies of 29 subjects with unil ateral implant markers in the mandible. These subjects were followed from 8 .5 to 15.5 years of age (Korn and Baumrind, 1990; Baumrind and Kern, 1992). The aim of the present investigation was to examine whether the presence o f such an age-related increase in mandibular body width could be confirmed in a sample with bilateral implant markers in the mandible. The sample comp rised 10 subjects (3F, 7M) from the files of another longitudinal study wit h implant markers (Bjork, 1968). A total of 122 pairs of annual lateral and postero-anterior (p-a) cephalometric records were available, covering long itudinal observation periods ranging from 8 to 16 years within an age inter val of between 6 and 23 years. The width between a right and left side mand ibular implant marker was measured with digital callipers on all p-a radiog raphs. Each measurement was corrected mathematically for various sources of radiographic enlargement. A small, but statistically significant increase in the distance between the right and left implant markers, i.e. in the bilateral width of the mandibu lar body, was observed in all subjects. The total increase in width in each subject ranged from 0.7 to 1.7 mm for the various periods of observation ( P less than or equal to 0.01). For the 12-year period from 6 to 18 years, t he average total increase was 1.6 mm (P less than or equal to 0.001, SD = 0 .42), i.e. 0.13 mm/year. After this age there was no systematic trend. The mechanism for this increase in width is unknown. It is suggested that durin g postnatal growth, an increasing load from the masticatory occlusal forces might influence endosteal bone remodelling in the mandibular body, thus pr oducing or allowing a gradual permanent outward bending of the right and le ft mandibular halves.