Optimizing Superimposition of Lateral Cephalograms on the Mandibular Symphysis

Objective: To identify the most stable cephalometric landmarks in the mandibular symphysis for use in cephalometric registration.

Method: Registration of lateral cephalograms on the mandibular symphysis is an important component of the standard technique for estimating displacement of the mandibular dentition during growth and orthodontic treatment.  The structural method originally developed by Bjork and based on previous experience on implants, identified “the inner cortical structure of the inferior border of the symphysis” as being the most stable through time.  But there has been some uncertainty as to which parts of the symphysis change the least and hence should be weighted most heavily in the superimposition process.  We report the mean displacement of selected landmarks of the inner and outer surfaces of the symphysis between the ages of 8.5 and 15.5 years for 14 subjects with mandibular implants of the Bjork type. (See Table) Data are derived from paired cephalograms superimposed on mandibular implants and are reported in the Sella-Nasion frame of reference.

Results:  Mean changes in the horizontal direction (x) were generally smaller than those in the vertical direction (y).  The smallest mean changes were observed on the posterior inner table but the vertical variability there was relatively high. Displacements of representative landmarks are tabulated below.

Conclusion: In general, the results corroborate Bjork's finding concerning the stability of the inner table.  Note however, that the variability in the vertical direction tends to be larger than that in the horizontal direction.