27 Orthodontic Journal of Nepal, Vol. 7 No. 1, June 2017 INTRODUCTION Smile is an involuntary action denoting happiness, pleasure, sociability, appreciation or amusement. It comes from inner satisfaction and is a means of nonverbal communication. It evolved from a need to communicate and is contagious. Smile plays an important role in social acceptance, interaction, and development of personality. Frush and Fisher 1 and Jameson 2 discussed the concept of smile line and defned it as ‘the harmony between the curvature of the incisal edges of the maxillary anterior teeth and the upper border of the lower lip’. According to Sarbi; 3 smile line (lip line) is the amount of vertical tooth exposure. On smiling the height of the upper lip relative to the maxillary central incisors and line depends on six factors; upper lip length, lip elevation, vertical maxillary height, crown height, vertical dental height, and incisor inclination. A bit of gingival display is acceptable and is considered a sign of youthful appearance. 4 When the smile line is above the cemento-enamel junction (CEJ); it is regarded as ‘gummy smile’ which is considered undesirable. 5 The initial point of a smile is the lip line at rest, with an average maxillary incisor display of 1.91 mm in male and nearly twice that amount i.e. 3.40 mm in female. 6 Smile line can be used as a valid tool for assessment of the esthetic appearance of a smile and can be applied universally. 7 Smile involves the teeth including its shape, size, color, position and also visible healthy periodontium. Its evaluation provides information about the teeth and surrounding mucosa which is an essential factor for esthetic dentistry. This factor makes the assessment of smile line a quintessential element for many felds in dentistry including orthodontic treatment. Most of the studies had analyzed only a natural smile. However, forced smile should also be taken into consideration during esthetic planning because the patient can exhibit more periodontium during the forced smile. Hence the present study determined the smile line classifcation as per Liebart et al 8 which considered smile, in both the natural and forced smile as well as the amount of periodontium visibility. The objective Dr Binam Sapkota, 1 Dr Shatakshi Srivastava, 2 Dr Sushmit Koju, 3 Dr Ritesh Srii 4 1 Asst Prof, Dept of Prosthodontics, 2 Lecturer, Dept of Community & Public Health Dentistry, 3 Dental Surgeon, 4 Lecturer, Dept of Oral Pathology Kathmandu University School of Medical Sciences, Dhulikhel, Nepal Correspondence: Dr Binam Sapkota; Email: binamsapkota7@hotmail.com Evaluation of Smile Line in Natural and Forced Smile Position: An Institution-based Study Research Article ABSTRACT Introduction: Smile line is a major factor considered in esthetic treatment planning. Objective: To assess the position of the smile line both in natural and forced position in relation to age and gender in Nepali population sample. Materials & Method: A cross sectional study was done in 220 adults. DSLR camera was used to take photograph of the participants. Two different photographs were taken of each participant in natural and forced smile position in standardized format. The smile lines were determined and classifed according to Liebart et al. Data analysis was done by SPSS 20.0 software. Chi square test was performed to analyze statistical signifcance between the gender and age groups Result: Maximum individuals had low type of natural smile (59.1%) and average type of forced smile (40.0%). Very few individuals had very high type of natural smile (1.4%) and very high type of forced smile (15.9%). Among all, 41% had visibility of the periodontium during normal smile while on forced smile 79.1% had visibility of the periodontium. Conclusion: Females have signifcantly more periodontal visibility when compared to males during forced smile in Nepali population sample. This concept of smile line should be utilized for treatment planning to achieve desirable facial esthetics in many felds of dentistry including Orthodontics, Prosthodontics, Periodontics, Restorative Dentistry. Keywords: esthetics, periodontium, smile, smile line