Hinge axis translation from retruded contact position to intercuspal position in dentulous subjects in treatment

Abstract

The hinge axis translation between the RCP and the IC position in the sagittal plane of a dental population being treated for periodontitis has been presented. The method incorporated the use of a Whip Mix articulator, an approximate hinge axis face-bow transfer, and a Buhnergraph. Data were gathered with the Frankfort horizontal plane as a reference for analysis. Asymmetry between right and left condylar recording was the general observation with 8.16% having an equality in only one axis. The midintercondylar values of the hinge axis in the IC position were located in three quadrants in relationship to horizontal and vertical axes through the RCP: anterior-inferior, 63.3%; posterior-inferior, 22.4%; and anterior-superior, 14.3%. The mean IC position was 0.4 mm anterior and 0.56 mm inferior to RCP. This inferior position accounted for 85.7% of the population. These dimensions and their larger distribution differed from those of the population comprised entirely of young men in good dental health studied by Hoffman et al. 2 An analysis of the data for each of the three quadrants yields the narrowest SD when compared to other groupings of the sample. In all subjects examined the midintercondylar location of the terminal hinge axis in the RCP was posterior and/or superior to the hinge axis in the IC position. The pattern and distribution of values suggest a null line that passes through the terminal hinge axis in the RCP at approximately 34 degrees relative to a perpendicular from the Frankfort horizontal plane. In the null line significant translation of the hinge axis can occur due to rotation of the mandible without obvious movement in the anterior region of the dentition. The straight line, least squares analysis angularly measured to the vertical axis was 37.4 degrees, which approaches the null line value of 34 degrees. This analysis took into consideration the errors in both the horizontal and vertical axes.