Occlusal contact changes in patients treated with clear aligners: A retrospective evaluation using digital dental models

Introduction

This study used digital intraoral scans to evaluate how clear aligner treatment affects occlusal contacts and to determine the influence of sex and age on contact changes. Results were compared with contact changes that occur during fixed appliance therapy.

Methods

Patients included in this study were treated in a University setting and private practice. Inclusion criteria were a Class I malocclusion treated nonextraction with clear aligners and the presence of pretreatment and posttreatment digital intraoral scans. Scans were imported into specialized software, and occlusal contacts were analyzed. The effects of age and sex on contact changes during clear aligner treatment were determined. Changes in occlusal contacts were compared with changes that occur during nonextraction treatment of patients with a Class I relationship using fixed edgewise appliances.

Results

A total of 45 clear aligner patients fit the eligibility criteria. Clear aligner treatment reduced the percentage of tight, near, and approximating contacts, whereas the percentage of open and no contacts increased. These changes in occlusal contacts were greater for the older age group studied. Genderinfluenced occlusal contact changes in the anterior dentition only where the decrease in near contacts and increase in open contacts were greater for males. These results for patients treated with clear aligners were similar to those for patients treated with fixed appliances; both treatment modalities reduced close occlusal contacts at the time active treatment was completed.

Conclusions

These results indicated that when clear aligners or fixed appliances are used to treat a Class I malocclusion, the resulting occlusion immediately after debonding is not as “tight” as it was at pretreatment.

Highlights

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Digital intraoral scans were used to determine occlusal contact changes during clear aligner treatment.
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Clear aligners reduced near contacts and increased open contacts.
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Occlusal contact changes during clear aligner therapy were greater in older patients.
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Clear aligners and fixed appliances have similar effects on occlusal contacts.

A malocclusion is a condition of “imperfect positioning of the teeth when the jaws are closed.” A malocclusion may influence esthetics, which is often the primary reason for patients to seek orthodontic treatment. However, the lack of a good occlusion can also compromise the ability to chew properly. Having a normal posterior and anterior occlusion is important because studies have shown that masticatory efficiency is significantly reduced when a patient has a malocclusion. By analyzing masticated particles, these studies showed that patients with an open bite or Angle Class II or III malocclusion are unable to break down particles to the same degree as patients with Class I (ideal) occlusion. In addition, these patients report a decreased ability to chew hard or firm foods, such as celery, carrots, and steak.

Although improved masticatory efficiency is one of the primary goals of patients undergoing orthodontic treatment, there is limited existing evidence that quantitatively evaluates how occlusal contacts change after this treatment. Occlusal contacts are an especially important factor to examine because masticatory efficiency is directly related to the number and sizes of contacts. Lepley et al found that masticatory performance is dependent on a number of variables, such as bite force and chewing cycle kinematics, but occlusal contact area had the largest influence; better masticatory performance was found in patients with larger contact areas.

Quantitative methods that have been used to measure occlusal contacts in the existing literature include (1) dental prescale system, in which the patient occludes on a digital sensor that then measures occlusal contact area, (2) Blu-Mousse impression of posterior dentition followed by manual tracing of the occlusal contact area, and (3) black silicone or dental wax impressions analyzed using specialized devices to quantify the contact area by measuring the amount of light transmitted through the material. ^, Because digital intraoral scanning has become a widely accepted method of obtaining dental models, using these scans to assess occlusal contacts is an accurate and reproducible method of occlusal analysis that uses preexisting patient data. In addition, this method allows contact analysis to be separated in the anterior and posterior regions of the dentition to better understand how each of these segments is affected by orthodontic treatment.

Although orthodontic treatment traditionally uses fixed appliances, clear aligners have become a popular treatment alternative. The technology of aligners has evolved in recent decades, and the appliances have gone through several iterations since their initial development. Although clear aligner technology has significantly improved, the occlusal results obtained with clear aligners are still being investigated. Djeu et al used the Objective Grading System to evaluate posttreatment (T2) records of orthodontic patients who underwent comprehensive, nonextraction treatment with clear aligners or fixed appliances; the group found that occlusal contact scores for patients treated with clear aligners were significantly lower than those of patients treated with fixed appliances. However, Borda et al used the American Board of Orthodontics Cast-Radiograph Evaluation to analyze T2 records of orthodontic patients who presented with mild malocclusions and found no significant difference in occlusal contacts between patients treated with clear aligners and fixed appliances. These conflicting results indicate that a more accurate, reproducible, and quantifiable method of analyzing occlusal contacts after clear aligner treatment is needed.

To better understand how occlusal contacts change during clear aligner treatment, this retrospective study used digital intraoral scans as an accurate and reproducible method to (1) determine how occlusal contacts change during treatment with clear aligners, (2) assess the influence of age and sex on occlusal contact changes during clear aligner treatment, and (3) compare occlusal contact changes between patients treated with clear aligners and full fixed appliances.

Material and methods

Institutional Review Board approval was obtained from the University’s Human Subjects Divisions before starting data collection for this retrospective study. Patients treated in the University’s orthodontic clinic between January 2017 and August 2021 were screened for eligibility in the study, as well as patients treated by a local private practice orthodontist between May 2016 and February 2021. Inclusion criteria were (1) Class I Angle classification of malocclusion, (2) completed clear aligner treatment with the Invisalign system (Align Technology Inc, San Jose, Calif), (3) presence of pretreatment (T1) and T2 digital intraoral scans that were taken using an iTero scanner (Align Technology Inc), (4) T2 scans taken the day of debonding, and (5) the presence of permanent dentition from second molar to second molar in both arches. These inclusion criteria were also applied to comparison data on fixed appliance treatment that was obtained previously. Angle classification was determined using the T1 intraoral scan. Occlusion was considered Class I if the mesiobuccal cusp of the maxillary first molar occluded any location between the mesiobuccal groove and mesiobuccal line angle of the mandibular first molar. Exclusion criteria were (1) craniofacial syndromes, (2) medical or dental conditions that would affect treatment outcome, (3) prosthodontic treatment during orthodontic treatment, or (4) early debonding.

Digital scans were taken in maximum intercuspation. The T1 intraoral scan was the scan used for the initial Invisalign Clincheck software (Align Technology Inc, San Jose, Calif). The T2 scan was the final scan taken on the day of debonding. Scans were exported from www.mycadent.com as an open shell stereolithography file with the maxilla and mandible oriented in occlusion. Files were imported into GOM Inspect software for analysis (Precise Industrial 3D Metrology; GOM, Braunschweig, Germany).

The stereolithography files of the mandibular teeth were imported into the GOM Inspect software as a new part and then a mesh element. The upper model was added as a computer-aided design (CAD) body. All points of the upper model (element) were selected and then inverted. The upper model was then removed from view so the lower model could be visualized. The dentition on the mandibular model was outlined. If present, third molars were not outlined. A CAD comparison was done to measure the distance between the maxillary and mandibular occlusal surfaces; the distance analyzed was from −1.25 mm to 0.25 mm. The legend type was changed to tolerance legend, and statistics were used to obtain contact measurements at each distance analyzed. Measurements were recorded in an Excel spreadsheet (Microsoft, Redmond, Wash). Five distances between the maxillary and mandibular occlusal surfaces were evaluated: tight contacts (0.00-0.25 mm), near contacts (−0.25 to 0), approximating contact (−0.50 to −0.26), open contacts (−1.00 to −0.51), and no contact (−1.25 to −1.00). Three areas of the dentition were analyzed: total (second molar to second molar), anterior (canine to canine), and posterior (first premolar to second molar). Figure 1 provides a visualization of these methods.

Data analysis

To evaluate intrarater and interrater reliability, T1 and T2 measurements for 5 patients were remeasured independently by 2 investigators (E.S.F. and M.M.). To compare the 2 measurements, the following were computed: mean ± standard deviation for each set of measurements (either within a rater or between the 2 raters), the mean of the differences, 95% confidence interval (CI) for the mean difference, the intraclass correlation coefficient (ICC), 95% CI for the ICC, Dahlberg’s error, and minimum and maximum for the absolute value of the difference between the 2 measurements. ^,

For analysis of each contact type, the percentage was determined at T1 and T2, then the change from T1 to T2. Mean ± standard deviation was computed for T1 and T2 and the change from T1 to T2. A 95% CI for the average change between T2 and T1 was also calculated. Multivariate analysis of variance was used to test for any change among the 5 contact types, and the paired t test was used to test for change for each contact type. The Bonferroni method was used to adjust the paired t test P values to maintain a significance level of 0.05 for the 5 separate tests. This analysis was done for the total contact area, anterior area, and posterior areas.

To determine the effect of age on contact changes during clear aligner treatment, the patient sample was divided into 3 approximately equal age groups: 12-16 (n = 15), 19-28 (n = 14), and 32-53 (n = 15) years. Age groups were determined on the basis of the distribution of ages in the sample and on growth; patients in the youngest age group were expected to be growing, the middle age group was mixed growing and nongrowing, and the oldest age group had completed growth. There were no patients aged between 17-18 and 29-31 years. Multivariate analysis of variance and Welch 2 sample t test were used to test for any difference between the 3 age groups among the 5 contact types. The Bonferroni method was used to adjust the 2 sample t test P values. The same methods were used to determine the effect of sex on contact changes during clear aligner treatment and to compare contact changes that occur during treatment with clear aligners and fixed appliances. Because results indicate that age influences contact changes during clear aligner treatment, the clear aligner sample was limited to patients aged <23 years to match the age of patients in the fixed appliance sample. A total of 21 clear aligner patients were used for this analysis.

Results

A total of 45 patients treated with clear aligners and 37 patients treated with fixed appliances met the inclusion criteria for this study. One patient was excluded from the clear aligner sample because of a poor-quality intraoral scan. One patient was excluded from the fixed appliance group because they were aged 64 years, which was significantly older than the rest of the sample. The mean age of patients in the clear aligner sample was 27 years; there was a higher percentage of females (66%) vs males (34%), and the mean treatment time was 15 months. The mean age of patients in the fixed appliance group was 14 years; there were more males (58%) than females (42%), and the mean treatment time was 21 months ( Table ).

Table

Demographics of patients included in the clear aligner and fixed appliance groups

Characteristic	Fixed group (n = 36)	Clear aligner group (n = 44)
Age, y
Mean ± SD	14 ± 3	27 ± 14
Median (IQR)	13 (12-16)	24 (15-34)
Range	12-23	12-56
Sex, n (%)
Female	15 (42%)	29 (66%)
Male	21 (58%)	15 (34%)
Treatment time, mo
Mean ± SD	21 ± 4	15 ± 7
Median (IQR)	21 (19-24)	13 (10-18)
Range	10-29	5-31