Model Analysis of Anatomical Morphology Changes of Palatal Rugae Before and After Orthodontic Treatment

Model analysis was performed to identify palatal rugae anatomical morphology patterns, evaluate their individual-specific properties and stability before and after orthodontic treatments, and investigate their reliability in the use fo r individual identification from the perspective of forensic dentistry. Maxillary models of 70 patients were collected before and after orth odontic treatments, palatine images were taken under standard conditions. Pattern-based individual identification was performed through easurin some indices including changes in number, orientation, shape, and length of palatal rugae, as well as mesial and distal endpoin t displacement of palatal rugae in the anteroposterior (AP) and mesiodistal (MD) direction. At the same time, maxillary models of 10 patients were randomly selected for individuality confirmation and the average rate of correct matches was calculated. palatal rugae anatomic al morphology patterns were highly individual-specific. Palatine of the same pattern were not found before or after orthodontic tr eatment. Model-based analysis confirmed the high individual-specificity properties of the palatal rugae anatomical morphology patterns, and also showed that the incidence of any morphological change in the patterns after orthodontic treatment was 45.7 % for anteroposterior displacement of the distal endpoint of palatal rugae, 40 % for mesiodistal displacement of the distal endpoint of palatine, 32. 9 % for anteroposterior displacement of the mesial endpoint of palatal rugae, and 17.1 % for mesiodistal displacement of the mesial end point of palatal rugae. The incidence of change in length, orientation and shape of palatine after orthodontic treatment was 28.6 %, 1.4 % and 1.4 % respectively; and the incidence of palatal segmentation and unification after orthodontic treatment was 4.3 % and 2.9 % respe ctively. The mean percentage of correct matches of the palatine patterns after orthodontic treatment to their duplicates was 99.05 % and the mean percentage of correct matches of the palatine patterns between before and after orthodontic treatment was 92.19 %. Palatal ruga e anatomical morphology patterns are unique to each individual. After orthodontic treatment, palatal rugae have diverse presentation pattern s. Although the diverse palatine patterns to a certain degree, influence individual identification in the field of forensic identification, pal tal rugae patterns can be used as a novel method for individual identification in forensic dentistry.


INTRODUCTION
Individual identification in forensic dentistry refers to an event to identify unknown individuals according to the features of teeth, jaw bones, cheilogramma, and cheeks based on stomatological knowledge (Bailey et al., 1996;Bing et al., 2014;Wazir et al., 2015;Ali et al., 2016 ).The palatal rugae may remain intact during major disasters because of their position in the oral cavity.They are also of great research value, and are therefore, an increasing area of interest in the field of forensic dentistry all over the world (Gondivkar et al., 2011;Adisa et al., 2014;Patil et al., 2016;Wu et al., 2016).Clinical orthodontic treatment can cause morphological change in palatal rugae patterns, which make palatal rugae-based forensic identification more complex Shukla et al., 2011;Deepak et al., 2014;Selvamani et al., 2015;Mustafa et al., 2015).In China, there are scant reports about palatal rugae pattern stability after orthodontic treatment and the accuracy of taking palatal rouge pattern as a reference in forensic dentistry.In this study, we used model-based analysis to identify palatal rugae patterns, evaluate their individual-specific properties and their stability before and after orthodontic treatments, and investigate their reliability in use for individual identification from the perspective of forensic dentistry.

SUBJECT AND METHOD
Object of study.Seventy patients, consisting of 35 males and 35 females, who received treatment in the Department of Orthodontics, Shanxi Medical University Stomatological Hospital, China between June 2014 and December 2014, were included in this study after screening against the inclusion and exclusion criteria.Inclusion criteria: (1) age ≥18 years; (2) having no history of skull and jaw trauma; (3) having permanent dentition before orthodontic treatment; (4) subjected to orthodontic treatment with straight wire for 12-24 months.Exclusion criteria: (1) having congenital anomalies/malformations; (2) bone and soft tissue protrusion, active lesions; (3) palatal deformity, scars or trauma; (4) mouth opening limited; (5) having a history of wearing a maxillary denture; (6) smoking.
Instruments and software.Silicone rubber impression material (DMG, Hamburg, Germany); type 3 dental super hard plaster (Heraeus Kulzer, Germany); perforated stainless steel instrument trays (Heraeus Kulzer); 2B pencil; magnifier; light source; Vernier caliper.Preparation of palatal rugae models.Maxillary models of 70 patients were collected before and after orthodontic treatments.A perforated stainless steel instrument tray that had been filled with a silicone rubber impression material was pressed over the teeth.Type III dental stone casts were prepared.Each palatal rugae model should have complete clear surface without defects or air bubble.Palatal rugae models prepared before orthodontic treatment were included in group A and those prepared after orthodontic treatment in group B.
Analysis of palatal rugae models.According to Thomas and Kapali classification, under appropriate illumination and application conditions, midpalatal suture, palatal rugae, and incisive papilla were marked and the edge of the palatal rugae was depicted using a pencil Fig. 1.)The length of each palatal rugae, the vertical dimension between the mediodistal end of the palatal rugae and midpalatal suture, and the vertical dimension between mediodistal end of the palatal rugae and incisive papilla were measured using a Vernier caliper.Palatal rugae models of each patient were numbered, and those taken before and after orthodontic treatment were tagged as 'a and b' respectively, so these palatal rugae models were numbered as 1a, 1b, 2a, 2b, 3a, 3b, 70a, 70b.The morphology of palatal rugae patterns was categorized according to Thomas & Kapali classification and detailed as follows according to the method of Mustafa et al.: (1) the number of changes; (2) the direction of the change; (4) the length of the change; (5) far from the midpoint of the distal to the displacement; (6) near the midterm (7) the displacement of the proximal end of the center in the front and rear; (8) the displacement of the distal midpoint in the forward and backward directions.

Detection of individuality of the palatal rugae patterns.
The individuality of the palatal rugae patterns was confirmed when the teeth of each model and the assigned number for each model were covered.In each group, one palatal rugae pattern model was matched to the remaining 69 models, thus there were 2415 matches in each group.Matches were made based on the following 17 questions.(1) Are they the same to the naked eye (2) Are the right palatal rugae the same (3) Are the left palatal rugae the same?(4) Is the total number of palatal rugae the same?(5) Is the number of primary palatal rugae the same?(6) Is the number of secondary palatal rugae the same?(7) Is the total number of the broken palatal rugae the same?(8) Is the total number of transversal palatal rugae the same?(9) Is the total number of forward palatal rugae the same?(10) Is the total number of backward palatal rugae the same?(11) Is the total number of irregular palatal rugae the same?(12) Is the total number of straight palatal rugae the same?(13) Is the total number of arc-shaped palatal rugae the same?( 14) Is the total number of wave-shaped palatal rugae the same?(15) Is the total number of ringshaped palatal rugae the same?( 16) Is the total number of assembled palatal rugae the same?(17) Is the total number of scattered palatal rugae the same?
Matching test of palatal rugae patterns.The matching test of palatal rugae patterns was performed by 10 professional dentists.According to the morphological patterns of the palatal rugae, the models from group B were matched to those in group A (the teeth of each model and the assigned number were not exposed to the dentists

DISCUSSION
Palatal rugae patterns like fingerprints have genetic gene-matched individual-specific stability, diversity, and universality properties.From the point of view of forensic medicine, palatal rugae patterns contribute to forensic identification (Santos et al., 2011Rath & Reginald, 2014;Poojya et al., 2015;Thabitha et al., 2015).There are controversies about whether diverse morphological changes of palatal rugae patterns after clinical orthodontic treatment can influence forensic identification.
Our results showed that after orthodontic treatment, the number of palatal rugae changed in 7.2 % of included patients, including segmentation in 3 (4.3%) patients, and unification in 2 (2.9 %) patients.This differs from a previous  Table I.The mean percentage (%) of correct matches of the palatal rugae patterns among different subjects between groups conclusion that the number of palatal rugae remained unchanged after orthodontic treatment.Our results occurred possibly because the methods and special procedures used in orthodontic treatment led to widened or narrowed palatal rugae.The moving teeth resulted in movement of the palatal rugae by drawing the soft tissue of the palatal rugae, which possibly cause unification or segmentation of the palatal rugae.Maxillary arch expansion treatment was not involved in previous reports.
No clinically significant changes in shape and orientation of the palatal rugae were previously reported.Our study showed that there were very limited changes in the shape and orientation of palatal rugae in 2.8 % of patients after orthodontic treatment.These findings suggest that the shape and orientation of the palatal rugae are likely to be the most stable morphological characteristics.
Our results also showed that the length of the palatal rugae changed greatly in 28.6 % of patients after orthodontic treatment, which is related to maxillary expansion treatment and front tooth restoring.After orthodontic treatment, the distal endpoint displacement of palatal rugae, in particular, the first and second rugae displaced greatly, in 85.7 % of the included patients.This is in agreement with a standpoint, proposed by Peavy & Kendrick that "the closer the rugae are to the teeth, the more prone they are to stretch in the direction that their associated teeth move" This is also consistent with the point of view of van der Linden & Almeida that change in shape of maxillary arch (for example expanding arch) and extraction of maxillary anterior teeth can lead to displacement of distal endpoint of the palatal rugae.The distal endpoint of the third palatal rugae is little influenced because of its position, so it is relatively stable.
The results in this study showed that the mesial endpoint of the palatal rugae was relatively unstable.The mesial endpoint of the palatal rugae displaced in 50 % patients (anteroposterior displacement in 32.9 % of patients and mesiodistal displacement in 17.1 % of patients).These results are contradictory with some previous reports that the mesial endpoint of the palatal rugae are stable and are therefore taken as the reference point for tooth movement in orthodontic treatment.Damstra considered that a rapid maxillary arch expansion in orthodontic treatment led to change in transversal size of the medioproximal end of the palatal rugae but it did not influence the anterioposterior size.This occurs possibly because the soft tissue of the palatal rugae stretched to different degrees during orthodontic treatment but recovered its original position.
The diverse morphological changes of the palatal rugae patterns hardly influence the matches before and after orthodontic treatment, with the average percentage of correct matches being 88.57-97.41% (median 92.38 %), which is basically the same as previous reports.The average percentage of correct matches between duplicates and the palatal rugae following orthodontic treatment was 99.05 %, and it was 92.19 % between before and after orthodontic treatment.The difference between 99.05 % and 92.19 % was statistically significant.

Fig. 3 .
Fig. 3. Box plot of the mean percentage of correct matches of the palatal rugae patterns between groups A and B.

Fig. 5 .
Fig. 5.The mean percentage (%) of correct matches of the palatal rugae patterns between groups B and B' and between groups A and B.