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, changes in mandibular width would be expected to occur only by surface apposition or resorption on the buccal surfaces of the left and right mandibular halves. However, evidence for an opening hinge movement of the two mandibular halves around a vertical axis located in the region of the mandibular symphysis was recently found in longitudinal studies of 29 subjects with unilateral implant markers in the mandible. These subjects were followed from 8.5 to 15.5 years of age (Korn and Baumrind, 1990; Baumrind and Korn, 1992). The aim of the present investigation was to examine whether the presence of such an age-related increase in mandibular body width could be confirmed in a sample with bilateral implant markers in the mandible. The sample comprised 10 subjects (3F, 7M) from the files of another longitudinal study with implant markers (Björk, 1968). A total of 122 pairs of annual lateral and postero-anterior (p-a) cephalometric records were available, covering longitudinal observation periods ranging from 8 to 16 years within an age interval of between 6 and 23 years. The width between a right and left side mandibular implant marker was measured with digital callipers on all p-a radiographs. 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 mandibular 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 < or = 0.01). For the 12-year period from 6 to 18 years, the average total increase was 1.6 mm (P < or = 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 during postnatal growth, an increasing load from the masticatory occlusal forces might influence endosteal bone remodelling in the mandibular body, thus producing or allowing a gradual permanent outward bending of the right and left mandibular halves.