An index for the measurement of normal maximum mouth opening.

PURPOSE The aim of this study was to evaluate the relationship between the width of 3 or 4 fingers of one hand and maximum mouth opening (MMO) in healthy subjects. METHODS One hundred and forty dental students (age 21 to 42 years, mean 27.4 years) participated in the study. The ability of each subject to position 3 or 4 fingers, vertically aligned, between the upper and lower central incisors up to the first distal interphalangeal folds, was documented. Measurements of MMO and the width of 3 fingers (index, middle and ring fingers) and 4 fingers (index, middle, ring and little fingers) were recorded. RESULTS All subjects were able to position 3 fingers (of both the right and left hands) between the upper and lower central incisors. Only 12 subjects were able to position 4 fingers (both right and left) in this way. There were no significant differences among the measurements of MMO (mean 48.8 mm), 3 fingers of the right hand (mean 47.3 mm) and 3 fingers of the left hand (mean 47.0 mm) (p > 0.05). However, MMO was significantly different from the width of 4 fingers of the right hand (mean 58.1 mm) and 4 fingers of the left hand (mean 57.5 mm) (p < 0.001). Moreover, there was a strong positive correlation between MMO and the 3-finger measurements (p < 0.0001). CONCLUSIONS These findings strongly suggest that the ability to position 3 fingers in the mouth during dental examination is a convenient index for assessing normal MMO.