The purpose of this study was to evaluate the occlusal contact area and cast-radiograph evaluation (CRE) score changes in patients with 3 different retention protocols after a 1-year retention period.
A total of 90 patients with acceptable final occlusion according to American Board of Orthodontics CRE were equally divided into 3 groups according to the retention protocol: upper bonded retainer and lower Essix or bonded retainer (Essix group); upper bonded retainer and Hawley or lower bonded retainer (Hawley group); and upper bonded retainer and lower bonded retainer (bonded retainer group). Digital models were used to assess occlusal contact area changes after a 1-year retention period. The follow-up occlusion models were assessed with the American Board of Orthodontics CRE.
Occlusal contact areas increased significantly in the Hawley and bonded retainer groups for all teeth except incisors. The bonded retainer group showed the greatest amount of settling, but differences with the Hawley group were not statistically significant. However, statistically significant decreases in occlusal contact areas were recorded for the Essix group except for incisors. A decrease of CRE scores was seen in the Hawley group, whereas a statistically significant increase was recorded in the Essix group.
Both Hawley and bonded retainers allowed settling of the occlusion during the retention phase, whereas the Essix retainer did not allow relative vertical movement of the posterior teeth. The Hawley group showed improvement in the total CRE score, whereas the Essix group showed worsening.
A significant decrease in contact areas was observed with the Essix appliance.
A significant increase in contact areas was observed with the use of Hawley and bonded retainer.
Hawley appliance showed a significant decrease in cast-radiograph evaluation scores.
Cast-radiograph evaluation scores increased with the Essix, indicating a worsening of the occlusion.
Orthodontic treatment goals can be expressed as achieving ideal tooth alignment, esthetic, functional occlusion, and stability at the end of active treatment. One of the most important challenges in orthodontics is maintaining the occlusal stability achieved at the end of active treatment. Long-term studies have shown that various occlusal changes occur after the active phase of orthodontic treatment. , Some of these changes are unwanted changes and are considered as relapse. Others may be beneficial and desirable, such as the improvement of the interdigitation of the teeth.
The retention appliances are used to maintain the arch dimensions and alignment of the teeth after completion of the orthodontic treatment. Settling is the natural vertical, and horizontal movement of the teeth after orthodontic treatment toward the functionally stable interocclusal contacts. Occlusal stability affects the health of the stomatognathic system and may play a role in preventing relapse after active treatment. , Surveys have shown that the number of occlusal contacts may increase during retention period and the selection of retention appliance may affect settling.
There are 2 types of retention appliances, fixed and removable. The use of both types of retention appliances is widely accepted to avoid possible posttreatment changes. The removable retention appliances allow the teeth to perform their normal functions and are expected to provide settling. However, this obliges clinicians to rely on patient cooperation for stability. Essix retainer is a removable, thermoplastic retention appliance that is well tolerated by patients. It is often preferred because of its hardness, size, esthetic appearance, and low-cost characteristics. However, Essix appliances are less effective in long-term stabilization than other retention appliances and may not allow for the settling of the occlusion. , Fixed retainers bonded to anterior teeth are esthetically more pleasing than removable retainers and are less problematic in a matter of patient cooperation. In contrast, accidental debonding occurs in 6%-20% of patients, depending on the bonding technique and retention duration; moreover, fixed retainers can affect oral hygiene negatively.
The cast-radiograph evaluation (CRE), formerly called the objective grading system, created by the American Board of Orthodontics (ABO) and used for the evaluation of treatment results since 1999, is a detailed and reliable index. A total of 8 criteria, including alignment and rotation, marginal ridges, buccolingual inclination, overjet, occlusal contacts, occlusal relationship, interproximal contacts, and root angulation, assess the quality of the final occlusion. The first 7 criteria are measured on the dental models with a special instrument, ABO measuring gauge, and the final criterion, which is the root angulation, is measured on panoramic radiographs. The lower CRE score indicates the better final occlusion. According to the rules set by ABO, 1 of the criteria that a case must meet in order to be successful in the board examination is that the total CRE score be 27 or below.
Most studies investigating the effect of retainers on the number of occlusal contacts during the retention period indicate that the number of contacts increases and the occlusion improves over time. , However, some studies reported no improvements or worsening of the occlusion. , , , There is no definite conclusion about the effects of various retention devices on settling, and it is still to be discussed as to which appliance is more successful.
Sauget et al reported that at the end of the 3-month-retention period, more settling was seen with Hawley compared with clear overlay retainers, whereas Basciftci et al found no statistically significant difference in the posterior occlusal contacts between Hawley and Jensen plate at the end of the 1-year retention period. Durbin and Sadowsky reported significantly more settling with the active positioners than with passive Hawley retainers; in contrast, Haydar et al found no significant difference in occlusal contacts using the same appliances. Method differences used to determine occlusal contacts and biased case selection may explain these conflicting results because most studies did not randomly assign their patients.
The purpose of this study is to compare the effects of Essix, Hawley, and fixed retainers on occlusal contact areas and CRE scores after a 1-year retention period, and to reveal, if possible, their superiority to each other.
Material and methods
Ninety patients treated by postgraduate students at the Department of Orthodontics, Faculty of Dentistry, Bezmialem Vakif University, Istanbul, Turkey, between 2014 and 2017 were included in the study. All patients received nonextraction fixed orthodontic treatment with a straight wire technique. Patients were selected based on the availability of the records such as dental models, bite records and panoramic radiographs taken after the treatment (T1) and 1-year posttreatment (T2).
The patient selection was based upon the following criteria: (1) treatment with fixed orthodontic appliances without extraction with or without auxiliary appliances; (2) optimum occlusion at the end of treatment with Class I molar and canine relationship (CRE score ≤27); (3) good compliance regarding retainer wear; (4) good oral hygiene and no periodontal disease; (5) normal growth pattern (SN/GoMe: 32° ± 7°); (6) no missing teeth except third molars; and (7) no presence of prosthetically restored tooth.
The exclusion criteria for this study included patients with a dental and skeletal open bite or deep bite before treatment, patients with incomplete records, final occlusion scores more than 27 according to CRE, large restorations on posterior teeth, and symptoms related to temporomandibular joint disorder.
Thirty patients were provided for each retention groups: maxillary bonded retainer and Essix or mandibular bonded retainer (Essix group; n = 30), maxillary bonded retainer and Hawley or mandibular bonded retainer (Hawley group; n = 30), and maxillary and mandibular bonded retainer (bonded retainer group; n = 30). Patients with removable appliances were instructed to wear their retainers full time, except during meals, for 6 months and only at nights for the next 6 months.
The sample included 58 females (64.4%) and 32 males (35.6%). The average age was 18.2 ± 7.3 years, with a range of 10.41-41.0 years. The average length of the treatment time was 15.8 ± 3.4 months and the average length of retention time was 12.1 ± 0.5 months. Fifty-three patients had Class I malocclusion, 29 had Class II Division 1 malocclusion, and 8 had Class III malocclusion before treatment. The group characteristics of different retention protocol groups are described in Tables I and II .
All dental models and panoramic radiographs were measured according to ABO’s CRE including all 8 criteria: alignment and rotation, marginal ridges, buccolingual inclination, overjet, occlusal contacts, occlusal relationship, interproximal contacts, and root angulation as described by Casko et al.
Dental casts and occlusal records were gathered from all patients at 2 time points: after the removal of orthodontic appliances (T1), and following the retention period 12 months later (T2). The dental casts were digitized separately and together with maximum intercuspation relationship by using 3Shape (Copenhagen, Denmark) R900 3-dimensional laser scanner ( Fig 1 ). After obtaining a pair of digital occluded models, the Ortho Analyzer program was used to assess the occlusal contacts. This program includes the “occlusion map” tool, which reveals occlusion contacts with a color scale ( Fig 2 ). In order to measure the occlusal contact areas which are defined as red areas on digital models, the image analysis software ImageJ (version 1.52a for Macintosh, National Institutes of Health, Bethesda, Md) was used to trace the red areas manually. The images were taken from 3Shape separately for all teeth and calibrated to ImageJ with the distance between 2 points in mm dimensions. The outlines of the occlusal contact areas were traced using the freehand preselection tool with the computer mouse, and the ImageJ program automatically calculated the traced area in mm 2 dimensions ( Fig 3 ).
The contacts recorded on the maxillary and mandibular dental models at each of the 2 time points were divided into 2 to obtain the contact areas of molars, premolars, canines, and incisors. All measurements were made by a single examiner (BK) at each time point, repeated twice and averaged.
All statistical analyses were performed using SPSS (version 21.0; Chicago, Ill) software package. Kolmogorov-Smirnov test was performed to assess the normal distribution of the studied samples. Paired sample t tests were used to assess differences between the mean at T1 and T2 for occlusal contacts, and the Wilcoxon test was used for CRE scores. One-way analysis of variance test was used to compare the intergroup means for occlusal contacts and the Kruskal-Wallis test was used for CRE scores.
Comparisons of the posttreatment patient characteristics between groups showed significant differences for age ( P = 0.002; Table I ).
Table III shows the changes of mean contact areas of the 3 groups on the anterior and posterior segments and total in mm 2 . The contact area differences recorded during the retention period for different retention appliances were statistically significant. In the Essix group, the total occlusal contact areas decreased from 34.41 mm 2 to 32.21 mm 2 , which was statistically significant ( P = 0.003). The mean contact areas of anterior ( P = 0.018) and posterior ( P = 0.007) segments decreased from 7.86 mm 2 to 7.32 mm 2 and 26.55 mm 2 to 24.89 mm 2 , respectively.
There was a statistically significant increase in the anterior segment ( P = 0.002; P = 0.004), posterior segment ( P <0.001; P <0.001), and total contact areas ( P <0.001; P <0.001) in Hawley and bonded retainer groups. The total occlusal contact areas increased from 33.47 mm 2 to 35.41 mm 2 in the Hawley group and increased from 34.21 mm 2 to 37.02 mm 2 in the bonded retainer group.
The mean occlusal contact areas recorded in canines, premolars, and molars decreased in the Essix group and increased in the Hawley and bonded retainer groups, with all changes being significant ( P < 0.05). The change of the contact area recorded in incisors was statistically insignificant in any group ( P > 0.05; Table III ).
Table IV shows the comparison of occlusal contact area changes among three retention groups. The results of the analysis of variance demonstrated statistically significant differences in 3 groups for all investigated teeth ( P <0.05). The Tukey test showed that occlusal contacts for all segments increased significantly in Hawley and bonded retainer groups compared with the Essix group ( P < 0.05). No statistically significant changes were determined between the Hawley and bonded retainer groups ( P >0.05).
|Variable||Essix||Hawley||Bonded retainer||P||Intergroup comparison|
|Incisor||−0.35||0.97||0.27||0.7||0.35||1.11||0.008*||2 > 1; P = 0.003*|
|3 > 1; P = 0.013*|
|Canine||−0.19||0.87||0.17||0.62||0.19||0.59||<0.001**||2 > 1; P = 0.001**|
|3 > 1; P < 0.001**|
|Premolar||−0.82||2.14||0.66||0.82||1.01||0.94||<0.001**||2 > 1; P < 0.001**|
|3 > 1; P < 0.001**|
|Molar||−0.95||2.16||0.82||1.59||1.25||1||<0.001**||2 > 1; P < 0.001**|
|3 > 1; P < 0.001**|
|Anterior||−0.54||1.63||0.45||0.99||0.54||1.3||<0.001**||2 > 1; P = 0.001**|
|3 > 1; P = 0.001**|
|Posterior||−1.65||3.12||1.48||1.93||2.27||1.46||<0.001**||2 > 1; P < 0.001**|
|3 > 1; P < 0.001**|
|Total||−2.2||4.13||1.94||2.36||2.81||1.94||<0.001**||2 > 1; P < 0.001**|