Award Date

5-15-2026

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Dental Medicine

First Committee Member

James Mah

Second Committee Member

Brian Chrzan

Third Committee Member

Charles Hill

Fourth Committee Member

Maxim Gakh

Number of Pages

65

Abstract

Background: Since their introduction into the field, clear aligners have significantly grown in popularity in orthodontics as they offer enhanced esthetics, improved oral hygiene, and patient comfort. Clear aligners have allowed clinicians to simulate treatment outcomes, streamline treatment with reduced office visits, control force and movement per tray, and provide more precise control of focal tooth movements. This can enhance efficiency in achieving specific and very finite orthodontic goals. Since their inception, clear aligners have evolved from only accomplishing simple tooth movements to achieving more complex and comprehensive mechanics. While they have significantly advanced, there are still limitations in efficacy and predictability of certain tooth movements, leading to undesired reciprocal movements and outcomes. One such commonly encountered unwanted outcome of clear aligner therapy is a posterior open bite. It is encountered frequently enough, some practitioners believe it is a side effect of clear aligners, citing molar intrusion due to a “bite block effect” from occluding on the tray’s material thickness. While the posterior open bite seen with clear aligner therapy has been shown in literature to be due to premature anterior contacts, there is limited research on the actual effects of clear aligners on vertical measurements and how or if they differ from traditional fixed bracket therapy. If there is a significant difference in the effects of clear aligners on vertical tooth movements compared to fixed therapy this could greatly influence a clinician’s choice of treatment modality based on vertical treatment objectives.

Objective: The purpose of this study is to determine if there is a difference in vertical cephalometric changes produced by Invisalign and fixed orthodontic treatment and exploring the influence of the parameters that could give credence and plausibility to the bite block theory.

Materials and Methods: Pre- and post-treatment cephalometric radiographs of patients who completed comprehensive orthodontics were anonymized and analyzed for 51 patients treated with Invisalign and 82 treated with fixed braces at the UNLV School of Dental Medicine. A series of vertical measurements were recorded on each cephalometric radiograph including overbite (OB) (mm), maxillary first molar height (U6-PP) (mm), mandibular first molar height (L6-MP) (mm), mandibular plane angle (MP-SN) (o ), Frankfort-mandibular plane angle FMA (MP-FH) (o ), and lower anterior face height (ANS-Me) (mm). Each patient record was reviewed to record duration of treatment, use of elastics, and interproximal reduction. Data was subsequently categorized by treatment modality, anteroposterior (A-P) skeletal classification, starting mandibular plane (FMA) classification, and treatment duration then analyzed for comparison and statistical significance.

Results: Invisalign and fixed differed significantly (p< 0.05) in their effects on FMA and OB. FMA increased 0.536o and OB decreased 0.673 mm in fixed treatment. FMA decreased 0.131o and OB increased 0.573mm in Invisalign. Among A-P skeletal classes there were differences in which cephalometric variables demonstrated significant changes and this differed between fixed and Invisalign. Fixed class I had significant changes in OB (-0.812 mm); class II had significant changes in MP-SN (+0.628o ) and OB (- 0.890 mm); and class III had significant changes in OB (+0.863 mm). In Invisalign, class I demonstrated significant changes in SN-MP (+0.965o ), ANS-Me (+1.390 mm), and OB (+0.565 mm); class II had significant changes in L6-MP (+0.511 mm); class III had no significant changes. There was no interaction between starting FMA and vertical cephalometric changes in Invisalign. With regards to treatment duration, the only interaction was with OB in the fixed treatment group. This interaction was due to short treatment producing significantly different OB changes when compared to average and long. Short treatment increased overbite 0.738 mm, while average and long reduced it by 0.698 mm and 0.945mm respectively.

Conclusions: Based on the results of this investigation comparing fixed and Invisalign treatment, within the significant limitations of this study, the following conclusions were made:

1. Invisalign utilizes IPR significantly more than fixed (OR = 55.3). Fixed uses elastics significantly more than Invisalign (OR = 44). This difference likely influences cephalometric changes.

2. Overall, fixed and Invisalign produce different changes in FMA and OB. Invisalign decreases the vertical with reduction of FMA (-0.131o ) and increases OB (+0.573 mm). Fixed therapy increased the vertical by increasing FMA (0.536o ) and reducing OB (-0.673 mm).

3. Skeletal A-P classification interacts with vertical cephalometric changes and this differs between fixed and Invisalign. This likely reflects a difference in mechanics used to ameliorate the A-P discrepancy.

4. Vertical skeletal classification (starting FMA) does not influence vertical cephalometric changes in Invisalign treatment.

5. Overbite changes are different in short treatment compared to average and long duration fixed therapy.

The results of this study do not fully support the bite block theory. The magnitude of differences found between Invisalign and fixed are likely of little clinical consequence outside cases of extreme open or deep bites. The results are likely reflective of different mechanics utilized and not of Invisalign’s tray thickness alone. More thorough follow-up research is needed.

Controlled Subject

Orthodontic appliances; Orthodontic appliances, Removable; Orthodontics, Corrective

Disciplines

Dentistry | Medicine and Health Sciences | Orthodontics and Orthodontology

File Format

PDF

File Size

1151 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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