Changes of nasolabial angle from rest to smile in relation to cephalometric parameters.

Introduction: The nasolabial angle (NLA) plays an important role in evaluating a patient's profile and smile appearance. The purpose of this study was to evaluate the changes of the NLA during smile and to correlate those changes with other photographic and cephalometric parameters.

Methods: Fifty young adults ranging from 20 to 30 years of age who had been referred to a private dental clinic for orthodontic treatment for normal maxillary position were enrolled. For the purpose of this study, the angular measurements of the NLA and its compartments as well as changes in the vertical position of the nasal tip were collected from photographs taken at rest and during smile, while other parameters were measured from the lateral cephalograms taken with the head in a natural position. Changes in the NLA were then correlated with the cephalometric parameters in order to observe any potential relationships.

Results: The NLA and its upper compartment decreased during smile (with a mean of 5.42 and 0.77 degrees, respectively). The NLA and its lower compartment increased, with a mean of 3.97 degrees. The nasal tip significantly dropped down from rest to smile (mean = 1.6 mm, P = 0.002). Significant correlations were found between changes of the NLA and the maxilla projection as well as between changes of the lower part of the NLA and the upper lip curvature.

Conclusions: Decreasing the NLA during smile is mainly related to the inherent properties of the soft tissues of the nasolabial area and not to the dental parameters under the control of the orthodontist. Patients should therefore be made aware of this from the beginning of the treatment.