Comparison of Reproducibility of Nasolabial Angle Constructed by Anatomic point method and Tangent line method

Introduction: Investigators have drawn nasolabial angle using different soft tissue landmarks. This has created confusion among the orthodontic students and clinicians regarding its construction. Objective: To evaluate two commonly used methods of constructing the nasolabial angle in order to establish a single reproducible method. Materials & Method: Lateral cephalograms of 120 patients undergoing orthodontic treatment were obtained from the records of the patients. Soft tissue profile on the lateral cephalograms were traced manually by the principal investigator. All the tracings were photocopied and 6 copies of each tracing were made. Nasolabial angles were constructed and measured on photocopied copies of the tracings first using the anatomic point method and then using the tangent line method by the principal investigator and another investigator independently. Result: The average nasolabial angle values for anatomic point method and tangent line method were found to be 94.32° ± 14.05° and 92.4° ± 14.59° respectively. The intra-class correlation coefficient demonstrated excellent intra-observer and interobserver agreement among the two methods of nasolabial angle construction. Conclusion: Both anatomic point method and tangent line method of nasolabial angle construction have excellent reproducibility in terms of intra-observer and inter-observer agreement. Key-words: anatomic point method, lateral cephalogram, nasolabial angle, tangent line method extraction or non-extraction during orthodontic treatment is also influenced by nasolabial angle.5 Lack of universal definition of nasolabial angle has led to variability in construction of nasolabial angle. Over the years, various investigators6-9 have drawn the nasolabial angle using different soft tissue landmarks. This has created confusion among the orthodontic students and clinicians alike regarding the construction of nasolabial angle. An improperly constructed nasolabial angle can adversely affect orthodontic diagnosis and treatment planning.10 Till date, there are no studies in orthodontic literature comparing the reproducibility of different methods of construction of nasolabial angle. Hence, the primary objective of this study was to evaluate two commonly used methods of construction of the nasolabial angle, namely anatomic point method and tangent line method, in order to establish a single reproducible method. The secondary objective was to assess whether the two methods of construction would lead to significant difference in the values of the angle.


MATERIALS AND METHOD
Pre-treatment lateral cephalograms of one hundred and twenty patients (55 males and 65 females) who were undergoing orthodontic treatment in the Department of Orthodontics, BP Koirala Institute of Health Sciences (BPKIHS), Dharan were obtained from the records of the patients. Lateral cephalograms of the patients with lip and nose deformities were excluded from the study. Ethical clearance was obtained from the Institutional Review Committee of BPKIHS before conducting the study.
Soft tissue profile on the lateral cephalogram was traced manually on 0.003 inch matte acetate paper using sharp 3H drawing pencil by the principal investigator (JG) and verified by another investigator (PRP) for all lateral cephalograms. All the tracings were photocopied and six copies of each tracing were made: two each for anatomic point method and tangent line method of nasolabial angle construction for the principal investigator and one each for anatomic point method and tangent line method of nasolabial angle construction for another investigator (RG).
Nasolabial angles were constructed on photocopied tracing first using the anatomic point method and then using the tangent line method separately by the principal investigator. Another investigator (RG) also constructed these angles using both methods independently ( Figure   1). Landmarks given by Park and Burstone 7 were used for the construction of nasolabial angle in anatomic point method ( Figure 2). Similarly, Scheideman and coworkers' method 6 was used to construct nasolabial angle using tangent lines (Figure 3). Then the angles were measured and the values were recorded by both the investigators independently. Additionally, the nasolabial angles were reconstructed and measured by the principal investigator after 2 weeks of initial measurement. SPSS software version 11 was used for data analysis. Intraclass correlation coefficient (ICC) was calculated to assess the intra-observer and inter-observer reproducibility.
Independent samples T-test was used to compare the mean values of nasolabial angle obtained by two construction methods. Cephalometric analysis which is used for orthodontic diagnosis and treatment planning is not foolproof. Human errors can creep which may jeopardise the accuracy, reproducibility and even validity of the cephalometric measurements. 14 So in an attempt to minimize errors, 6 copies of original tracing were made using photocopy machine instead of tracing each of them. Also, hand tracing was preferred over digital tracing even though studies have shown no clinically significant difference between the two methods because hand tracing is all-time gold standard. 15,16 In reviewing the literature, no study was found which compared the reproducibility of nasolabial angle construction by anatomic point method and tangent line method alone.
However, two studies were found which assessed the reproducibility of angular measurements of soft tissue profile with nasolabial angle as one of the components using the two methods. 12,17 This study found excellent inter-observer reproducibilities of   The mean difference between the nasolabial angle values constructed using anatomic point method and tangent line method was 1.92°. This was found to be statistically insignificant.
Furthermore, any cephalometric measurement difference less than 2° is considered clinically insignificant. 19 It can thus be suggested that either of the two methods could be used for cephalometric analysis. However, it would be prudent to specify the method of nasolabial angle construction along with its value in orthodontic practice.

CONCLUSION
Both anatomic point method and tangent line method of nasolabial angle construction have excellent reproducibility in terms of intra-observer and inter-observer agreement. Also, there is no significant difference between the average values of the nasolabial angle constructed by the two methods.
Hence, either of the two methods can be used for nasolabial angle construction during cephalometric soft tissue analysis.