Editorial Type:
Article Category: Letter
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Online Publication Date: 15 Apr 2025

Letters From Our Readers

Page Range: 350 – 351
DOI: 10.2319/ANGL_Response_Letter
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To Editor, The Angle Orthodontist

Re: Response to: Vertical and transverse treatment effects of Invisalign First system compared to Hyrax maxillary expanders with fixed appliances in mixed dentition patients. Parisa Moravedje Torbaty; Heeyeon Suh; Sandra Khong Tai; Marta Baird; Robert L. Boyd; Heesoo Oh. The Angle Orthodontist. 2024;94:496–503.

Thank you for your thoughtful comments and for your interest in our study, Vertical and transverse treatment effects of Invisalign first system (IFS) compared to hyrax maxillary expanders with fixed appliances in mixed dentition patients (Angle Orthod. 2024;94:496–503).

We appreciate the opportunity to clarify the indications for expansion in our patient groups. As noted, bilateral posterior crossbite was an exclusion criterion, and expansion was primarily performed for patients with unilateral crossbite and/or arch length deficiency. The rationale for excluding bilateral posterior crossbites was that clinicians using the Invisalign First System (IFS) also employ the fixed Hyrax expander for cases requiring significant skeletal maxillary expansion. Since IFS is generally limited to providing less than 5 mm of transverse expansion, it is not typically used in cases where substantial skeletal expansion is necessary, such as in the correction of bilateral posterior crossbites. As a result, nine cases with bilateral posterior crossbites from the Hyrax group were excluded, while patients with unilateral crossbites were included in the study. Specifically, there were 14 cases of unilateral crossbite in the Hyrax group and nine cases in the IFS group.

The majority of patients in both the Invisalign First and Hyrax groups underwent expansion due to a narrow and constricted maxillary arch characterized by a high palatal vault, a V-shaped maxillary arch, and retroclined lower posterior teeth. Expansion was performed to alleviate crowding and create sufficient space for the proper eruption of the permanent dentition. This determination was based on clinical evaluations and study cast assessments indicating a constricted maxillary arch.

Regarding your suggestion to include pre- and post-treatment images, while these were not included in the original publication due to space constraints, we recognize that such visual representation can enhance the understanding of treatment effects. To address this, we have included two representative cases from each group (Figures 1 through 4).

Figure 1.Figure 1.Figure 1.
Figure 1.Hyrax group—unilateral crossbite. Case 245. An 8.4-year-old female patient with a left unilateral crossbite at initial evaluation. The inter-molar (U6-6) width increased by 5.2 mm.

Citation: The Angle Orthodontist 95, 3; 10.2319/ANGL_Response_Letter

Figure 2.Figure 2.Figure 2.
Figure 2.Hyrax group—no crossbite. Case 205: An 11.3-year-old female patient with no crossbite at initial evaluation. The inter-molar (U6-6) width increased by 5.7 mm.

Citation: The Angle Orthodontist 95, 3; 10.2319/ANGL_Response_Letter

Figure 3.Figure 3.Figure 3.
Figure 3.IFS group—unilateral crossbite. Case 543. An 10.6-year-old female patient with a left unilateral crossbite at initial evaluation. The inter-molar (U6-6) width increased by 2.0 mm.

Citation: The Angle Orthodontist 95, 3; 10.2319/ANGL_Response_Letter

Figure 4.Figure 4.Figure 4.
Figure 4.IFS group—no crossbite. Case 510. An 9.6-year-old female patient with no crossbite but a narrow V-shaped maxillary arch at initial evaluation. The inter-molar (U6-6) width increased by 2.5 mm.

Citation: The Angle Orthodontist 95, 3; 10.2319/ANGL_Response_Letter

We appreciate your engagement in this topic and your insightful questions. If you have any further inquiries or require additional clarification, please feel free to reach out.

Copyright: © 2025 by The EH Angle Education and Research Foundation, Inc.
Figure 1.
Figure 1.

Hyrax group—unilateral crossbite. Case 245. An 8.4-year-old female patient with a left unilateral crossbite at initial evaluation. The inter-molar (U6-6) width increased by 5.2 mm.

Figure 2.
Figure 2.

Hyrax group—no crossbite. Case 205: An 11.3-year-old female patient with no crossbite at initial evaluation. The inter-molar (U6-6) width increased by 5.7 mm.

Figure 3.
Figure 3.

IFS group—unilateral crossbite. Case 543. An 10.6-year-old female patient with a left unilateral crossbite at initial evaluation. The inter-molar (U6-6) width increased by 2.0 mm.

Figure 4.
Figure 4.

IFS group—no crossbite. Case 510. An 9.6-year-old female patient with no crossbite but a narrow V-shaped maxillary arch at initial evaluation. The inter-molar (U6-6) width increased by 2.5 mm.

Contributor Notes

Department of Orthodontics, University of Pacific, Arthur A. Dugoni School of Dentistry, San Francisco, CA

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