Quantitative evaluation of lip symmetry in functional asymmetry

The objectives of this study were to quantitate lip symmetry/asymmetry from clinical photographs; to demonstrate that asymmetry due to functional side shifts (functional asymmetry) leading to unilateral crossbites including the canines, results from measurable thinning of the upper lip and thickening of the lower lip on the side of the crossbite when viewed in the intercuspal contact position; and to show that orthodontic treatment aimed at eliminating the functional shift and crossbite would achieve lip symmetry, both visually and quantitatively. The study consisted of 26 patients, who were divided into two groups: a study group of 13 patients (eight females, five males, aged 8-17 years) with a functional asymmetry, and a control group of 13 age- and gender-matched subjects with other forms of malocclusion without functional asymmetry. All patients in the study group exhibited unilateral crossbites including the canines in intercuspal contact position. Digitized images of frontal facial photographs were analysed for upper and lower lip symmetry pre- and post-orthodontic treatment. The upper and lower lips were subdivided into four quadrants and the surface area and length of each quadrant were measured and expressed as a percentage of the total surface area/length of the relevant lip. The degree of asymmetry was obtained by calculating the difference in percentage area or length between the two quadrants of each lip. In the study group, the lower lip quadrant on the shift side was enlarged while the contralateral side was reduced (mean area ratio 59.9 to 40.1 per cent, mean length ratio 53.0 to 47.0 per cent). The upper lip demonstrated differences that were smaller and inverse. The controls showed a small difference between the right and left sides (less than 1 per cent). After treatment, both groups displayed visual and quantitative lower and upper lip symmetry, i.e. an area or length of approximately 50 per cent of each quadrant. In absolute values, the control patients had up to 3 per cent asymmetry in area regardless of treatment. The patients in the study group exhibited mean absolute asymmetry of 9.2 per cent in the upper lip and 19.8 per cent in the lower lip. Asymmetry values in the study group were reduced to approximately 3 per cent post-treatment. The absolute values of asymmetry in length of all patients were up to 2 per cent in the control group regardless of treatment. The subjects in the study group exhibited mean absolute asymmetry of 6.3 per cent in the upper lip and 8.6 per cent in the lower lip. Asymmetry values in the study group were reduced post-treatment to approximately 2 per cent. Although asymmetry in the study group could be quantitated using both parameters (lip surface area and lip length), the surface area parameter proved to be a more sensitive tool for measuring lip asymmetry.