An Investigation of the Effects of Incisal Overjet on Mandibular Movement During Speech

Abstract

The objective of this study was to explore the possible existence of mandibular protrusion during speech for those subjects possessing at least three millimeters of incisal overjet. The most important clinical consequence would be the question of whether or not temporomandibular joint symptomatology becomes manifest with advancing age in these subjects. If proof of a “speech protrusion” were to be found in the course of this research, a future investigation would be in order using an older group of subjects with incisal overjet to answer the above question. A Class I control group was gathered, consisting of ten female subjects, all of whom had zero incisal overjet. Ten female subjects also made up a Class II group, with the criterion being a minimum of three millimeters of incisal overjet. The age range of all the subjects, with one exception, was 17 to 30, none of the subjects had any perceptible speech defects. In both groups, a wide range of overbite was sought and obtained. Models as well as lateral headplates were taken for all twenty subjects. The test instrument was the Mandibular Kinesiograph located in the Complete Denture Department of the Indiana University School of Dentistry. The test parameter components included “s,” a consonant, and the three vowels representing the extremes of the vowel diagram, “ee,” “oo,” and “ah.” These were combined to yield three consonant-vowel combinations, namely “ee-see,” “oo-soo,” and “ah-sah.” After making a reference scribe which consisted of habitual occlusion, a protrusive slide to the end-to-end incisal position, and a retrusive slide, if present, back to centric occlusion, a given subject was asked to repeat each of the consonant-vowel combinations five times. Thus for each subject there were three tracings, each with five trials and a reference scribe. The data were collected by a tape recorder attached to the Kinesiograph, and the subsequent tape sent to Pharmadynamics Research, Inc. in West Lafayette, Indiana, for computer analysis. The computer’s first step was to take each set of five trials for a given consonant-vowel combination and produce one averaged curve. As a result, each subject was left with three curves. Using the end-to-end incisal position, habitual occlusion, and when present, centric occlusion, as reference points, for all subjects anteroposterior distance measurements were made for “s,” and both anteroposterior and vertical distance measurements were made for “ee,” “oo,” and “ah.” These distances were then compared using F-tests and t-tests both between and among the classes (Class I and Class II groups). Statistically significant differences, or lack of, were searched for in evaluating the t-test results. Although the basic purpose of the research was to seek proof of a Class II mandibular protrusion, during “s” to a typical Class I anteroposterior “s” posture, other secondary findings centered around the anteroposterior and vertical positions of “ee,” “oo,” and “ah” relative to habitual occlusion and centric occlusion. The results of the research were as follows: (1) Class II subjects demonstrated complete anterior translation of the mandible during “s” production, validifying the main hypothesis of the study. There was no statistical significance between Class I and Class II “s” position. (2) For the Class I group only, the “oo-soo” “s” mandibular position was statistically posterior to the “ee-see” “s,” with the “ah-sah” “s” appearing to be in an intermediate anteroposterior position. (3) The Class II “ah” mandibular position was more inferior than that of the class I “ah.” (4) For both class I and Class II, the order of mandibular position from superior to inferior was “oo,” “ee,” and then “ah.” (5) The class I “ee” mandibular position was posterior to that of the Class II “ee.” (6) For both Class I and Class II, then mandibular position of “ah” was posterior to that of both “ee” and “oo.”