The influence of lip thickness and strain on upper lip response to incisor retraction.

Placement of teeth according to accepted cephalometric criteria does not necessarily ensure that overlying soft tissue will drape in a harmonious manner. The purpose of this research was to investigate the influence of maxillary lip thickness and lip strain on the relationship between dental and integumental tissue changes in orthodontically treated patients. Data were derived from pretreatment and retention lateral cephalometric head films of forty Caucasian subjects with Class II, Division 1 malocclusions. All patients were treated with edgewise appliances. Percentile groups were created for males and females according to the magnitude of maxillary lip thickness and lip strain. It was found that there was significant correlation between osseous changes and soft-tissue changes in both males (r= 0.83, p less than 0.01) and females (r = 0.85, p less than 0.01). Strong correlations were found between osseous changes and soft-tissue changes in subjects with thin lips (males, r equal to 0.92, p less than 0.01; females, r = 0.98, p less than 0.01), whereas no significant correlations were found in subjects with thick lips. Significant correlations were found between incisor changes and vermillion border changes in males (r = 0.61, p less than 0.01) and females (r = 0.51, p less than 0.05). Correlations were strong between incisor changes and vermilion border changes in subjects with high lip strain (males, r equal 0.92, p less than 0.01; females, r = 0.82, p less than 0.05) but were significant in subjects with low lip strain.