A study of occlusion as related to the Frankfort-mandibular plane angle.
The relation of the Frankfort-mandibular plane angle (FMA) to naturally occurring occlusion groups was studied in 112 subjects. Occlusion types were categorized into four groups depending on the magnitude of first-molar contact on the working side. Percentage (frequency) of occurrence of occlusion groups were: disclusion, 17.9 per cent; delayed disclusion, 19.6 per cent; progressive disclusion, 8.9 per cent; and group function, 53.6 per cent. The mean FMA for the study sample was 25.68 degrees with a range of 13 to 40 degrees. Mean FMA's for the occlusion groups showed a progressive increase from disclusion (19.6 degrees) to group function (28.9 degrees). A statistical test, extension of the median, indicated a highly significant relation between occlusion groups and the FMA. Considering these results, an FMA determination provides a structure-function basis to consider in selecting a scheme for occlusal reconstruction and should be incorporated in the treatment planning procedure as a major adjunctive diagnostic tool. However, further research, focused upon the many variables that determine a type of occlusion, must precede the formulation of a set of "rules" for selection of occlusion.