Introduction
We aimed to determine whether appliance type affects changes in maximum voluntary bite force (MVBF) and the number of occlusal contacts (NOC) during retention, controlling for sex, age, and body mass index.
Methods
The sample comprised 176 examinees (70 male, 106 female) aged 14 to 20 years: 30 had maxillary and mandibular Essix retainers, 30 had wrap-around retainers, and 30 had a combination of fixed mandibular canine-to-canine retainers bonded on each tooth separately (double twisted, 0.254 mm in diameter, stainless steel ligature wire) and Essix retainer in the maxillary arch; 86 with normal occlusion were not treated. MVBF and the NOC were measured immediately after removal of preadjusted edgewise appliances (Roth prescription), 6 weeks after that, and after the next 4 weeks.
Results
Increases in MVBF and the NOC were demonstrated, but subjects with 2 Essix retainers showed lower values than did the others. Changes were related to type of appliance, sex, and age ( P <0.05) but not to body mass index. The increase in NOC occurred faster than the increase of MVBF, more and sooner with the wrap-around retainer and in male subjects than with the Essix and in female subjects. MVBF and NOC nearly reached the values of the control subjects.
Conclusions
Settling of the occlusion depends on appliance type: it takes longer in female patients and with Essix in both dental arches than with the other tested appliances.
Highlights
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Maximum voluntary bite force and number of occlusal contacts increase during the first 10 weeks of retention.
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Settling occurs sooner with the wrap-around retainer and in males than with the Essix and in females.
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Body mass index is not related to changes in maximum voluntary bite force and number of occlusal contacts.
A phase of retention after orthodontic treatment is imperative to allow the reorganization of periodontal fibers, minimizing changes in the achieved tooth positions due to growth, and to enable neuromuscular adaptation to the new occlusal situation.
There is no consensus on the duration of the retention period. Although it has been shown that on average 232 days are needed for the reorganization of periodontal and gingival fibers, sometimes a long-term relapse can occur even after following this recommendation. Thus, some clinicians prolong the retention period and sometimes opt for permanent retention. In addition to these undesirable effects, positive movement of teeth during retention also occurs in the form of settling of the occlusion, which leads to better intercuspation and masticatory function. Research has shown that the number of occlusal contacts may increase during the retention period and that the choice of retention appliance may affect settling. The wrap-around retainer is advantageous for settling due to the absence of retentive elements that reach across the occlusal surfaces or contact points; its downsides are proneness of its wire to deformation as well as discomfort. The Essix retainer is a removable thermoplastic appliance that is well accepted by patients, and it is used more often because of its hardness, esthetics, size, and affordability. However, it may not allow for settling of the occlusion, and it lacks the durability of acrylic retainers. Fixed retainers bonded to anterior teeth are esthetically more pleasing than removable retainers, and patient compliance is a lesser issue. Accidental debonding occurs in 6% to 20% of patients, depending on the bonding technique and retention duration.
During active treatment, the orthodontic appliance does not completely allow for functional occlusion, since groups of teeth may be tied together. After removal of the appliance and setting disconnected teeth into function, slight movements of teeth occur. This process happens to achieve a balanced position in the oral musculoskeletal environment. Changes are more pronounced in posterior teeth; this improves masticatory performance and efficiency in terms of capacity to grind food. One indicator of the functional state and health of the masticatory system is maximum voluntary bite force (MVBF). Its values vary in accordance to the location of measurement (highest at the first molar, lower at the incisors). Various physiologic and morphologic factors affect MVBF. It reaches its peak around the age of 12 years, stabilizing after 14, and then decreasing after age 25 in women and after age 45 in men. In general, men have higher MVBF than do women. There is contradictory evidence on the effect of periodontal health on bite force; some studies have shown that periodontal status has little influence, whereas others found that patients with chronic periodontitis have decreased biting ability and that attachment loss results in reduced control of bite force. The number of teeth and number of occlusal contacts are important factors influencing MVBF. People with pronounced horizontal craniofacial growth have somewhat higher values of MVBF, and those with vertical growth have lower values than do those with an average growth pattern. On the other hand, no difference was found in MVBF between children with long faces and those with “normal” patterns.
One factor that is usually investigated in studies of bite force is body mass index (BMI), calculated according to this formula: BMI = mass/height 2 (kg/m 2 ). Body weight and height are considered possible factors that could influence the value of MVBF. In obese persons, a proposed underlying mechanism that reduces muscle strength is the accumulation of free fatty acids in skeletal muscular tissues. Also, a recent study found reduced bite force in obese adolescent boys. Although Linderholm and Wennström 26 found a positive correlation between MVBF and body height and weight, some studies have shown low “correlations” or the absence of correlation. Since some of the aforementioned studies demonstrated that BMI can influence MVBF, it would be useful to determine whether BMI also affects the number of occlusal contacts, the other parameter that can be measured during settling.
Modern electronic instruments measure MVBF based on electric resistance of the material under tension. Bilateral measurement results in higher values of bite force than does unilateral measurement. It is believed that a protective mechanism causes forces produced and recorded by masticatory muscles not to be of the same magnitude during unilateral and bilateral measurement due to inhibition caused by receptors in the periodontium and temporomandibular joints.
On the other hand, quantitative assessment of occlusal contacts can provide useful information about the distribution of bite forces in the dental arches. Several methods of occlusal registrations are available. The use of plastic foil is the simplest, yet it requires multiple opening and biting by the subject to obtain data for the entire dental arch. Certain modern systems, such as T-Scan and the Dental Prescale System, can be used to determine both occlusal contacts and bite forces; however, they may interfere with the occlusion, and the results obtained are 2-dimensional. Addition silicones cause minimal interference, but the quantification of data remains problematic. Although translucent segments can be considered “contacts,” the results are subject to the orientation of the registration toward the light source.
The aim of this research was to determine the extent to which MVBF and occlusal contacts are related to the type of appliance in the 10 weeks of retention after removal of edgewise appliances, controlling for the effects of sex, age, and BMI.
Material and methods
This was a controlled clinical trial that included 176 white subjects (70 male, 106 female) aged 14 to 20 years (median, 16 years; interquartile range, 15-18 years). Subjects in test groups (n = 90) were patients of the dental clinic at Zagreb University in Croatia assigned to 1 of the 3 test groups using quota sampling for this experimental prospective study. The untreated control group (n = 86) comprised subjects of similar age and sex distribution with normal occlusion who had not previously had orthodontic treatment. Controls were recruited during the epidemiologic survey in the area of Zagreb. All subjects were recruited between 2007 and 2009. Exclusion criteria were extractions, hypodontia, extensive carious lesions, prosthodontic restorations, and large fillings on the permanent first molars, as well as systemic muscular and joint diseases to prevent possible impacts of the aforementioned factors on the stability of the occlusion and MVBF. The minimum sample size was calculated in the following manner: assuming that the mean difference in changes of MVBF (T2-T1) between 2 appliances could be 0.050 kN, and that the standard deviations of changes of MVBF in both groups could be 0.050 kN, a minimum of 17 subjects were needed in each appliance group to obtain a detectable difference. Assuming that the difference in mean values of MVBF in the same appliance group between 2 time periods could be 0.100 kN with a standard deviation of 0.130 in both time periods, to obtain a detectable difference, 28 subjects were needed (with the level of significance at P = 0.05 and power beta = 0.8). Since 3 appliances were tested, it was decided to triple the minimal number of subjects for the control (28 × 3 = 84). Calculations were performed with software (version 14.8.1; MedCalc Software, Ostend, Belgium).
Subjects from the test groups had their preadjusted edgewise appliance, Roth 0.018 in, removed immediately before the first measurement. They were randomly assigned to 1 of 3 test groups: the first comprised 30 subjects with Essix retainers in both dental arches, the second had 30 subjects with wrap-around retainers in both arches, and the third included 30 subjects with a combination of a directly bonded multistranded retainer in the intercanine region of the mandibular arch and an Essix retainer in the maxillary arch. The Essix retainers were made from 1-mm thick Essix ACE plastic foil (Dentsply Raintree Essix, Sarasota, Fla) and were extended to full crown coverage of all erupted teeth to half occlusal coverage of the second molars. The wrap-around retainers had the acrylate base placed orally, and the labial archwire was made from round, 0.8-mm diameter, stainless steel wire (Dentaurum, Ispringen, Germany). The labial arches surrounded all erupted teeth including the second molars and formed a U-shaped loop between the canines and first premolars. The bonded retainer was made from double twisted, 0.254-mm diameter, stainless steel ligature wire (Rocky Mountain Orthodontics, Denver, Colo); it was the standard material used at the dental clinic at Zagreb University when applying bonded retainers. The retainer was bonded to oral surfaces of individual teeth with Adper Single Bond Plus bonding agent (3M ESPE, Monrovia, Calif) and Filtek Supreme Ultra flowable composite resin (3M ESPE).
During the bite force measurements, all subjects were in habitual occlusion with natural head posture (sitting and looking straight forward). All measurements were performed by the same examiner (S.V.) on 3 occasions: T1, immediately after removal of the edgewise appliance; T2, after 6 weeks of retention; and T3, after 10 weeks of retention. All subjects were instructed to wear their removable appliances for 24 hours a day during the first 2 weeks (removing them only during meals), for 16 hours a day during the next 4 weeks, and only during sleep in the last 4 weeks of the study. This was the standard protocol used in Croatia, based on the retention regimen of the dental clinic. The control subjects had MVBF recorded in 1 session. Weight and height for the calculation of BMI were self-reported at T1. MVFB was determined using the Occlusal Force-Meter GM10 (Nagano Keiki, Tokyo, Japan) on the mandibular permanent first molar unilaterally, according to the standard procedure. The subjects of the control and test groups bit into the instrument on each side of the dental arch 4 times and repeated the process in reverse order after 3 minutes of rest. Measurement reliability of MVBF was statistically processed with a method suggested by Bland and Altman.
Occlusal contacts were determined using plastic foil, 6 mm in width and 0.05 mm in thickness: Hawe Transparent Strips No. 690, straight (Kerrhawer Sa, Bioggo, Switzerland). Contacts were registered in places where the foil could not be drawn out by strong pulling during habitual occlusion. The previously reported method error is 10% of the mean value.
The normality of distribution of the data from each group was tested with the Shapiro-Wilk test, the homogeneity of variance with the Levene test, and the sphericity with the Mauchly test. For the analysis of differences in MVBF and the number of occlusal contacts between retention appliances, mixed analysis of variance (ANOVA) and analysis of covariance (ANCOVA) for repeated measures with Bonferroni post-hoc tests were used in a stepwise manner. MVBF and number of contacts were outcome variables; type of appliance, sex, and assessment period were categorical predictors, and age and BMI were used as covariates measured on a continuous scale. Generally, appliance type and reading were entered first in the statistical model and then supplemented with sex, age, and BMI. Partial eta squared was used for the evaluation of effect size. In asymmetric distributions, Kruskal-Wallis and Mann-Whitney tests were used for analysis. Multivariate discriminant analysis was used for the assessment of changes in MVBF and number of contacts between time periods that discriminated the most between sex and appliance type. For this purpose, new variables were created: ratio of the second and first readings, and ratio of the third and second readings, for both MVBF and number of contacts. Canonical discriminant analysis is a dimension-reduction technique that derives a linear combination of several interval variables (in this case ratios for MVBF and number of contacts) that has the highest possible multiple correlation with the groups (appliance type and sex). Statistical analysis was performed with SPSS software (version 10.0; SPSS, Chicago, Ill) and Statistica software (version 8.0; StatSoft, Tulsa, Okla) at the P <0.05 level of significance.
The ethical committee of the School of Dental Medicine at University of Zagreb approved the research. Informed consents were obtained from all participants or their legal guardians before the study.
Results
The results of measurement reliability of MVBF according to Bland and Altman were as follows: intrasession reliability was excellent, intraclass correlation coefficients were 0.976 for the controls and ranged from 0.953 to 0.986 for the test groups (lowest for wrap-around and highest for bonded-Essix combination). The difference between 2 measurements for the same subject was within limits of agreement for 96.5% of the controls and for 93.3% to 100% of the test groups. Intersession reliability for untreated subjects was assessed by 2 consecutive measurements with a 1-week interval on 10 subjects (with normal occlusion and similar age and sex distribution as the test groups) and were excellent (intraclass correlation coefficient, 0.890; 95% were within the limits of agreement). The biologic variation of MVBF, assessed as the standard deviation, was always larger than the measurement error. To prevent random error, the mean value of MVBF was used for analyses.
The mean treatment duration in the test groups was 20.9 ± 2.2 months, and pretreatment tooth size-arch length discrepancies were –1.2 ± 3.0 in the maxilla and –1.3 ± 2.5 mm in the mandible. Before orthodontic treatment, the majority were Class I (53%); similarly, both divisions of Class II—Class II Division 1 and Class II Division 2—were 17% and 20%, respectively, and the least were Class III (10%). The majority had a neutral growth pattern (51%), and the least had a horizontal one (14%), as evaluated with the Bjork polygon and y-axis on lateral cephalograms. Distribution was similar in the test groups. There were no significant differences in age between the test groups; however, there were significantly more female subjects in each group ( Table I ; P <0.05). On average, male subjects had higher BMI values than female subjects ( P <0.001). In the sample, 69% had normal weight, and 39% were overweight. Male subjects more often had normal weight (84.14%) compared with female subjects (59.39%). The distributions were similar in all groups, and mean BMI did not differ significantly between groups. Mmale subjects in the control group had significantly higher forces than female subjects, with sex accounting for 15.3% of the variability in MVBF in untreated Class I subjects ( Table II ; P <0.001), yet there were no differences in the number of occlusal contacts between the sexes. The covariates age and BMI did not significantly affect this finding.
| Group | Sex | n | % | Age (y) average ± SD (95% CI) | BMI average ± SD (95% CI) |
|---|---|---|---|---|---|
| Control | Male | 36 | 41.9 | 16.6 ± 1.9 (16.0-17.3) | 22.2 ± 2.5 (21.4-23.0) |
| Female | 50 | 58.1 | 16.8 ± 2.0 (16.2-17.3) | 21.2 ± 2.5 (20.6-21.9) | |
| Total | 86 | 100 | 16.7 ± 2.0 (16.3-17.1) | 21.6 ± 2.5 (21.1-22.2) | |
| Essix | Male | 13 | 43.3 | 16.1 ± 1.5 (15.1-17.2) | 21.6 ± 2.8 (20.3-22.9) |
| Female | 17 | 56.7 | 16.4 ± 2.2 (15.4-17.3) | 19.6 ± 2.0 (18.5-20.8) | |
| Total | 30 | 100 | 16.3 ± 1.9 (15.6-17.0) | 20.5 ± 2.5 (19.5-21.4) | |
| Wrap-around | Male | 11 | 36.7 | 15.9 ± 1.3 (14.8-17.1) | 20.5 ± 2.6 (19.0-21.9) |
| Female | 19 | 63.3 | 16.1 ± 1.9 (15.2-17.0) | 20.5 ± 2.1 (19.4-21.6) | |
| Total | 30 | 100 | 16.0 ± 1.7 (15.4-16.7) | 20.5 ± 2.3 (19.6-21.3) | |
| Bonded + Essix | Male | 10 | 33.3 | 17.7 ± 2.2 (16.5-18.9) | 22.9 ± 2.0 (21.4-24.4) |
| Female | 20 | 66.7 | 16.1 ± 1.9 (15.2-16.9) | 19.9 ± 2.2 (18.9-21.0) | |
| Total | 30 | 100 | 16.6 ± 2.1 (15.8-17.4) | 20.9 ± 2.6 (20.0-21.9) | |
| Total | Male | 70 | 39.8 | 16.6 ± 1.8 (16.1-17.0) | 21.9 ± 2.6 (21.3-22.5) |
| Female | 106 | 60.2 | 16.4 ± 2.0 (16.1-16.8) | 20.6 ± 2.4 (20.1-21.1) | |
| Total | 176 | 100 | 16.5 ± 1.9 (16.2-16.8) | 21.1 ± 2.5 (20.7-21.5) |
| Sex | n | Mean ± SD (95% CI) | P | η 2 | |
|---|---|---|---|---|---|
| MVBF (kN) | Male | 36 | 0.625 ± 0.190 (0.565-0.685) | ||
| Female | 50 | 0.471 ± 0.174 (0.420-0.522) | <0.001 | 0.153 | |
| Total | 86 | 0.535 ± 0.195 (0.494-0.577) | |||
| NOC | Male | 36 | 10.3 ± 2.1 (9.6-11.0) | ||
| Female | 50 | 10.3 ± 2.1 (9.7-10.9) | 0.975 | <0.001 | |
| Total | 86 | 10.3 ± 2.1 (9.9-10.8) |
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