Photogrammetric analysis of postextraction space closure

the Material and Methods The total sample in study consisted of 38 post-extraction spaces in 19 patients who were indicated for first premolars extraction and use of fixed orthodontic appliances in order to achieve treatment goal. Elastic chains were used to close post-extraction spaces. Post-extraction sites were monitored for 6 months with follow-up examinations every 4 weeks. Mechanisms were activated at controlled examinations, and measurements of the width of post-extraction spaces were performed by photogrammetry and digital caliper. Results The results showed that average reduction of post-extraction space width was 1.00 mm per month by photogrammetric measurement, while the average values of measurements with a digital caliper were 1.02 mm. The average values were measured in six time intervals and a statistically significant change in average intervals of post-extraction spaces during 6 months was determined. Conclusion The elastic chain has proven to be a very effective mechanism for closing post-extraction space, and photogrammetry as a simple and precise method for monitoring results of treatment. Since the difference in relation to measurements with a digital caliper is minimal, photogrammetry can be routinely applied in everyday practice.


INTRODUCTION
When planning orthodontic treatment, it is sometimes necessary to apply extraction of individual teeth, in order to achieve proper occlusion and an acceptable aesthetic appearance. The first premolar is usually the tooth of choice in extraction treatment with fixed orthodontic appliances. Closing the post-extraction space is performed by different methods, and two basic ones are sliding mechanism and methods without friction using loops [1]. The use of a sliding mechanism for post-extraction space closure is very common in clinical practice due to its simplicity. Studies show that clinicians give the greatest advantage to elastic chains as a method for this type of treatment [2,3].
Although they were introduced in practice in the 1960s, elastic chains remained a traditional method for tooth retraction and space closure in general. Even today, they are one of the most commonly used techniques for closing space, due to easy application, patient and therapist comfort [4,5]. They are primarily composed of polyesters or polyethers formed by the polymerization of rubbers with multiple molecular structures linked by a series of urethane bonds [6]. The advantage of elastic chains is that they are not expensive, easy to use and can be applied to various clinical cases. However, they must be changed every 4 to 6 weeks due to plaque retention, difficult hygiene and expected decrease in strength [7]. Therefore, it is necessary to monitor the closure of post-extraction space at control examinations in order to summarize achieved results and correctly direct treatment plan.
By introducing dental photography as an integral part of dental treatment documentation, it is possible to show condition at the beginning and the end, the process and its phases. This way, photographs can be used for various purposes, such as communication with patient, case presentation, diagnosis and treatment plan [8]. Along with development of digital photography, there was an innovation in the field of dentistry called photogrammetry. This method is using geometric properties of objects that can be determined from photographs and this proved its usefulness in orthodontics for studying three-dimensional occlusion of dental arches, teeth and their dimensions. Chadwick defines photogrammetry as the art, science, and technology of obtaining reliable information about physical objects through the processes of capturing, measuring, and interpreting photographic images [9].
Due to its simplicity, economy and without the danger of radiation, photogrammetry on standardized extra oral and intraoral photographs is used in everyday orthodontic practice [10,11,12].
The aim of this study was to analyze using a photogrammetric method the efficacy of post-extraction space closure with elastic chains during orthodontic treatment with fixed orthodontic appliances.

MATERIAL AND METHODS
The research was conducted at the Faculty of Medicine -study program of Dentistry in Banja Luka, with the consent of the Ethics Committee of the Department of Dentistry. The total sample consisted of 38 post-extraction spaces in 19 patients who were indicated for the extraction of the first premolars in order to conduct orthodontic treatment. The age of patients at the beginning of treatment was 12-20 years, who did not have contraindications for orthodontic treatment and who had no other extractions (except for the first premolars). Subjects who had poor oral hygiene, who came for check-ups irregularly and did not follow the instructions given at the beginning of treatment, were excluded from the study.
After extraction of first premolars, fixed orthodontic appliance was applied to the subjects (Dentaurum, Discovery, Roth prescription, slot 0.022 in), and then, initial leveling with round and rectangular NiTi arches was performed, before the placement of steel rectangular arches. A rectangular steel arch wire of 0.019×0.025 in was left in brackets slots for at least 4 weeks, to become passive, and then elastic chains were applied. The elastic chain was placed to connect the hook on the tube of the first molar and the hook of the bracket on the canine, whereby it is stretched to approximately twice the initial length, and on control examinations it was replaced with a new one ( Figure 1).
Post-extraction spaces were monitored for 6 months from the beginning of the application of elastic chains at intervals of one month (T 0 -T 6 ). Control examinations were performed every 4 weeks and it was checked whether there was damage to applied mechanisms and their activation.
For the photogrammetric method, a Canon camera (EOS 750D Body) with macro lens (EF 100 mm / 2.8 IS USM) and a ring-shaped flash (macro Ring Lite MR-14 EXII), a flat occlusal mirror and a retractor for occlusal photography with millimeter scale were used. During regular check-ups (T 0 -T 6 ), standardized occlusal photographs of the upper and lower dental arch were obtained. When photographing, the mirror rested on the opposite dental The millimeter scale on the retractor enabled the measurement results to be recognized by software with the computer program Digimizer for photo analysis and processing. Using this program, linear measurements in pixels were obtained. Then, the number of pixels in one centimeter that was on the retractor with a millimeter scale was read. This way, one centimeter of a segment of a photograph, measured in pixels, was used as a parameter to convert the number of pixels in a real photograph into centimeters. All measurements in pixels were converted to millimeters through a computer program. The shortest distance from the distal surface of the canine to the mesial surface of the second premolar was measured. Measurements were performed 3 times alternately, and the reference value represented the mean value of these 3 measurements (Figure 2).
In order to establish the accuracy of this method, measurements were also performed using a digital caliper with an accuracy of 0.01 mm. The direct distance from the distal surface of the canine to the mesial surface of the second premolar was measured by direct method. These measurements were performed for each post-extraction space 3 times alternately. The reference value was the mean of these 3 measurements.

RESULTS
For measuring the efficiency of the elastic chain, the photogrammetry method was used. The Tukey test was used to test whether there was a difference between all time intervals measuring the space between teeth by this method. This test showed that a statistically significant difference exists between all time intervals. The average distance between the teeth decreased over time, and the average distance of post-extraction space at the beginning of the measurement (T 1 ) was 4.03 ± 1.44 mm, while at the final measurement (T 6 ) it was 0.44 ± 0.57 mm (Table 1).
Using one-factor analysis of Repeated Measure ANOVA, it was examined whether there was a statistically significant change in the average values of post-extraction spaces when using the elastic chain method measured by photogrammetry. Mean values were measured at six time intervals and a statistically significant change in the average intervals of post-extraction spaces over 6 months was found (Wilks' lambda = 0.019, F = 103.98, p = 0.000). The influence of the elastic chain as a mechanism of post-extraction spaces closure was also examined. The value of the Wilks lambda of 0.01-small influence, 0.06-moderate influence and 0.14-big influence, and the obtained η² of 0.98, indicates a very large influence of the elastic chain as a mechanism of post-extraction space closure ( Table  2). The graph shows the reduction of the average values of post-extraction spaces measured by the method of photogrammetry, under the action of an elastic chain ( Figure 3). Control measurements were performed by a direct method using digital caliper. The average distance of the post-extraction space at the beginning of the measurement (T 1 ) was 4.14 ± 1.47 mm, and at the end (T 6 ) 0.46 ± 0.59 mm ( Table 3).
A statistically significant difference was found in the average intervals of post-extraction spaces during 6 months (Wilks' lambda = 0.006, F = 322.06, p = 0.000). The influence of the elastic chain, as a mechanism of space closure between the teeth, was also examined. In this case, the obtained η² is 0.99, which indicates a very large influence of the elastic chain as a mechanism of reducing space between the teeth (Table 4). Figure 4 graphically shows the average values of post-extraction space width, that is, the maximum distance between the teeth when using an elastic chain measured with a digital caliper.

DISCUSSION
It has been shown that among clinicians, sliding mechanics is the most commonly used method for the post-extraction space closure [13]. The possibility of quick and easy reactivation with minimal bending of the wire and saving time are the advantages that make sliding mechanism the method of choice during treatment of extraction spaces closure [14]. The results of research conducted in Brazil by Monnini et al. confirmed that most specialists (63.81%) prefer a technique based on sliding mechanics, while elastic chains are among the most used methods [2].
In a similar study in Great Britain, they came to the conclusion that loops are practically not used for space closure, and the sliding technique was used in 98% of cases [3]. Chitra and Prakash analyzed the use of photogrammetry in orthodontic diagnosis and treatment planning. They found that occlusal photogrammetric analysis could be used as an adjunct to the analysis of study models and to assess treatment progress. In cases where appliances for expansion were used, photogrammetric measurement of intercanine distance, anterior and posterior widths could be used periodically for quantitative monitoring to determine the change during treatment [12]. They can also be used to measure the mesiodistal dimension of the tooth and to calculate the Bolton mismatch, as well as to plan the correction of deviations from the midline [13].
The results of this study showed that post-extraction spaces were closed by an average of 1.00 mm per month when measurements were performed by photogrammetry. Control measurements with a digital caliper gave average values of 1.02 mm per month. Both methods confirmed the high efficiency of elastic chains in post-extraction space closure.
Numerous authors have analyzed post-extraction space closure in different ways [14,15]. Direct measurement with a digital caliper has proven to be the simplest method for both the patient and the therapist. The biggest advantage is that the measurement results are obtained immediately, which can be of great importance for assessing the success so far and treatment planning. This method was used by Dixon et al., who used elastic chains in 10 patients and found that the space was closed by 0.58 mm per month [4]. A similar study was conducted by Chaudhari and Tarvade observing the closure of postextraction spaces in 20 patients. During 4 months, measurements were performed using a digital caliper from the tip of the canine to the tip of the mesiobuccal cusp on the first permanent molar, and they found that this value decreased by 0.62 mm per month [16]. Direct measurement was used by both Kanuru et al. when examining the amount of space closure by moving the canines using 4 different elastic chains over 6 weeks. They measured the distance from the hook of the bracket on the canine to the hook of the tube on the first molar. After 3 weeks, the measured values for space closure ranged from 0.75 mm to 1.12 mm, while after 6 weeks these values ranged from 1.47 mm to 2.07 mm [17]. However, the biggest disadvantage of this method is the correct positioning of the caliper arms, because the measurement is performed in a limited space in a humid environment, so it can happen that wrong results occur.
Indirect measurements involve taking impressions at check-ups, which allows the orthodontist greater comfort for analysis and measurement. This method was used by Talwar and Bath who also examined the efficiency of elastic chains. Impressions were taken at the beginning and at the end of the observed 12-week period. Measurements were performed on study models from the tip of the canine to the tip of the mesiobuccal cusp on the first permanent molar. The average values were 1.62 ± 0.14 mm per month [18]. Fang et al. conducted a similar study, but the measurement points were located at the tip of the canine and   in the middle of the central fissure of the second premolar, while the space was closed by 0.52 mm per month [19]. Sabrina et al. investigated the closure of post-extraction spaces in self-ligating and conventional locks using an elastic chain in 11 patients over 8 weeks. At the control examinations, the arches were removed, and the prints were taken through brackets that were protected with wax. Measurements were performed indirectly on the obtained study models, from the distal surface of the canine to the mesial surface of the second premolar using a digital caliper. The average value for conventional brackets was 0.75 mm, and for self-ligating 0.48 mm per month [20].
A similar split-mouth study was conducted by Mezomo et al. comparing conventional and self-ligating brackets, and as a sliding mechanism an elastic chain. Impressions were taken at the monthly controls, and measurements were performed by the indirect method with digital caliper. The results showed that the space closure with conventional brackets takes place at a rate of 0.84 mm per month; while for self-ligating locks this value was 0.90 mm per month [21]. The biggest disadvantage of indirect measurement is that the time of the control inspection is extended, and the inadvertent detachment of the brackets can contribute to that even more, which requires additional time planning.
Bokas and Woods observed the closure of 12 post-extraction spaces by sliding mechanics using elastic chains. Control examinations were performed at 28-day intervals and impressions were taken for study models which were then scanned. Measurements were performed on digital photographs obtained by occlusal scanning using the appropriate software. The results showed that elastic chains closed spaces by 1.68 mm per month [22]. Scanning models and analyzing their 3D replicas gives the orthodontist the ability to measure in three dimensions, and software manipulation allows monitoring and prediction of treatment results. However, properly analyzing and measuring scanned models requires practice and a good knowledge of the capabilities provided by the software.
Like previous methods, photogrammetry has its advantages and disadvantages. The advantage is first of all that you avoid taking impressions. The record obtained by this method is in color and faithfully depicts the area of measurement, but also the structures around it. Another advantage of digital photos is that the image can be repeated immediately if it is not satisfactory, and the measurements can be done later without hastle. In addition to a certain inconvenience when taking photos, the biggest drawback of this method is the lack of a third dimension. Therefore, it is necessary that the picture taking conditions be standardized.

CONCLUSION
The elastic chain has proven to be a very effective mechanism for closing the post-extraction space, and photogrammetry is a simple and precise method for monitoring treatment results. Since the difference in relation to measurements with a digital caliper is minimal, photogrammetry can be routinely applied in everyday practice.