Thesis
Advancing the assessment of forces in rapid maxillary expansion: a continued clinical investigation
University of Iowa
Master of Science (MS), University of Iowa
Spring 2025
DOI: 10.25820/etd.007984
Abstract
Maxillary transverse hypoplasia, also known by maxillary constriction, is a common condition of the upper jaw that often can result in crowding, blocked out teeth, functional shifts, asymmetric jaw growth, and posterior crossbites. The rate at which this transverse discrepancy occurs is estimated to be anywhere from 13% to 30% of the population. The standard of orthodontic care is treatment with a rapid maxillary expander (RME) with the goal of separating the midpalatal suture. An RME uses a jackscrew to translate a force to the dentition and maxillary bone with the goal of separating the midpalatal suture. Few studies have attempted to measure the forces applied by RMEs during treatment. The objective of this study is to pilot a novel method for quantifying these forces.
Materials and Methods
Forty-eight patients from 8 to 15 years of age, with a treatment recommendation of rapid maxillary expansion using either a 2 or 4-banded Hyrax-style RME, were recruited for this study. Each expander underwent pre-clinical calibration before patient delivery utilizing a force gauge, an expander key that turns the expander using the force gauge, a vice apparatus to hold the expander, and a tray to place weights on to the expander. Using multiple weights, a calibration curve for each expander was generated. The expander was modified for delivery by removing the extensions for the bench test. The expander was delivered, and patients were scheduled for 2 turns of the expander jackscrew per day over a period of 11-17 days. Turning of the expander ceased once clinically sufficient expansion was deemed to be achieved, with the maxillary molars and premolars nearing a buccal crossbite with the mandibular molars and premolars. For the daily turns, the force was recorded using a force gauge that pushes on an expander key. Each visit also included maxillary occlusal and intraoral frontal photos, diastema width measurements, and intraoral scans. Maxillary occlusal radiographs were taken after the final day of turning. The forces of maxillary resistance to expansion were then estimated using turn force data in comparison to the expander-specific calibration curves.
Results
The maximum force range measurements of this study resemble other studies that examined the forces of expansion, being about 6-20 pounds of force as the maximum load. Older patients (greater than or equal to 13 y.o.) exhibited higher overall expansion loads compared with younger patients (less than 13 y.o.), which became clearer comparatively in the later turns of the expanders. Frankfort-mandibular plane angle (FMA), as assessed in the lateral cephalogram from the patient’s initial records, did demonstrate a significant influence on forces required for expansion, with patients that have a FMA 30 degrees or greater requiring more force. In a plot of measured force vs turns of the expander, the slope, that is the change in force with time, showed a decrease after midpalatal sutural separation in all patients. The magnitude of slope change was not significantly different between groups of patients based on RME band number, age, or FMA (all groups decreased approximately 295 g/day). A midline diastema emerged after an average of 11.5 activations with a standard deviation of approximately 3 activations (one activation equates to 0.2 mm of jackscrew expansion), with no substantial influence from the number of bands, age group, or MPA. There was no significant change in diastema between 2-banded and 4-banded RMEs, with diastema size being approximately 41% of total expansion with a standard deviation of about 12%.
Conclusions
Older individuals demonstrated greater overall expansion forces compared to younger individuals, with this difference becoming statistically significant in the later stages of treatment towards the maximum load. Patients with a higher Frankfort-mandibular plane angle (FMA) demonstrated a statistically significant greater force for expansion to occur. There was no significant decrease in predicted load after midpalatal suture separation across any group, suggesting that the suture is not the only source of skeletal resistance to expansion. The midpalatal suture likely makes a notable contribution to skeletal resistance as there is a substantial slope decrease to all groups (by 55-65%) following suture separation. The extent of this reduction in slope was consistent across age groups, indicating that the resistance from the midpalatal suture does not significantly increase with age. The increase in expansion forces and RME failure rates with age is likely driven by the maturation of the maxillary bony articulations, particularly those involving the zygomaticomaxillary and pterygomaxillary sutures.
Details
- Title: Subtitle
- Advancing the assessment of forces in rapid maxillary expansion: a continued clinical investigation
- Creators
- Benjamin Donald Bouchard
- Contributors
- Kyungsup Shin (Advisor)Steven D. Marshall (Committee Member)Michael A. Callan (Committee Member)Lina Moreno Uribe (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Orthodontics
- Date degree season
- Spring 2025
- DOI
- 10.25820/etd.007984
- Publisher
- University of Iowa
- Number of pages
- x, 89 pages
- Copyright
- Copyright 2025 Benjamin Donald Bouchard
- Language
- English
- Date submitted
- 04/23/2025
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (page 83-89).
- Public Abstract (ETD)
Narrowing of the upper jaw, known as maxillary constriction, is a common condition that can lead to crowded teeth, jaw misalignment, and bite problems. A widely used treatment is the rapid maxillary expander (RME), a device that gradually widens the upper jaw by applying force. Little is known about the exact forces involved during this treatment.
This study measured the forces used by RMEs in 48 children and teens aged 8 to 15. Each RME was tested and calibrated before being placed in the mouth. During treatment, patients had the devices turned twice daily, and the forces applied were recorded. Photos and scans were taken at each visit to monitor progress, and a final X-ray was used to confirm that space had developed between the two halves of the upper jaw.
The study found that older patients required higher forces to achieve the same amount of expansion as younger ones, especially toward the end of treatment. Patients with certain jaw shapes also needed more force. After the midline of the upper jaw separated, resistance to expansion dropped significantly in all patients. However, resistance did not vary much by age, suggesting other facial bones also play a major role in how much force is needed for expansion.
These findings help explain how jaw structure and age affect RME treatment and introduce a promising new way to measure expansion forces more accurately. This improved understanding can help orthodontists tailor treatment plans to individual patients better, potentially increasing the effectiveness and patient comfort.
- Academic Unit
- Orthodontics; Craniofacial Anomalies Research Center
- Record Identifier
- 9984831022902771
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