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General information for researchers, lab and clinic staff. It is not medical advice and does not describe how to fit, activate or adjust any appliance.
Short answer
Orthodontic expanders are usually grouped by how they anchor: to the teeth (for example the Hyrax), to the teeth with a spring mechanism (the leaf expander and memory screw), or to the teeth and bone together (the Hybrid Hyrax and miniscrew-assisted designs such as MARPE). A 2023 scoping review of 15 studies found that all the devices it examined produce meaningful skeletal widening of the palate, and that skeletally anchored devices were reported to give larger and more successful growth in adolescents. In the United States, the expansion screw retainer component is a Class I device under 21 CFR 872.5410.
Definition and scope
A palatal or maxillary expander is an orthodontic appliance used to widen the upper jaw when it is too narrow relative to the lower jaw, a condition the literature calls a transverse maxillary deficit. The 2023 scoping review in Children (Inchingolo et al.) reports that this transverse discrepancy is found in 8 to 22 percent of orthodontic patients and is often accompanied by dental crowding, crossbite or other malocclusions. It describes rapid maxillary expansion (RME) as typically carried out with a Hyrax appliance, a leaf expander or a MARPE appliance, and notes that these open the midpalatal suture under different activation regimens. Slow maxillary expansion (SME) is the other main category in the same review.
This page covers the appliance families, their anchorage, what published comparisons show and how the component is regulated. It does not cover patient selection, treatment ages or activation schedules, which belong to the treating orthodontist.
How expanders are used in practice
Expanders are prescribed and supervised by an orthodontist. The review compares them on dental effects, skeletal effects and patient discomfort. For supply and lab staff the practical point is that “expander” is a family of appliances, not one product.
Orthodontic expander types compared by anchorage
| Type | Anchorage and mechanism | What the published study reports |
|---|---|---|
| Hyrax (conventional RME) | Dental-anchored rapid expander, the conventional comparator in the studies. | The review lists buccal tipping, reduced buccal bone and marginal bone levels, thinning of anchoring teeth and root resorption among the undesirable effects reported for conventional RME. |
| Haas-type and Hyrax-type | Two rapid expander designs compared in a randomised trial of 33 children (mean age 10.7 years; 18 Haas-type, 15 Hyrax-type). | Hyrax showed greater orthopaedic effects and less first-molar tipping, but the difference was under 0.5 mm per side and the trial authors conclude the effect might not be clinically significant. |
| Leaf expander | Fixed slow expander using nickel-titanium spring activation. | The review concludes outcomes are clinically and radiographically similar to RME and SME. It also reports less pain than a standard Hyrax in the studies it included. |
| Memory screw | Screw with nickel-titanium springs. | Described as applying continuous force rather than the intermittent force of a conventional screw. |
| Hybrid Hyrax | Hyrax supported by both teeth and bone (skeletal anchorage). | In a 42-patient study (ages 11 to 14, cone beam CT) it showed more skeletal changes and fewer dental side effects than a conventional Hyrax. |
| MARPE | Miniscrew-assisted rapid palatal expansion, listed with Hyrax and leaf expanders as an RME appliance. | The review concludes that skeletally anchored devices promote larger and more successful growth in adolescent patients. |
Where it matters for labs, clinics and supply decisions
How FDA classifies the expansion screw
21 CFR 872.5410 defines an orthodontic appliance and accessories as a device intended for use in orthodontic treatment that is affixed to a tooth so that pressure can be exerted on the teeth. The regulation’s list of devices includes the “orthodontic expansion screw retainer”. It is Class I (general controls) and exempt from 510(k) premarket notification, subject to the limitations in 21 CFR 872.9.
The openFDA classification database records this component under product code DYJ, “Retainer, Screw Expansion, Orthodontic”, class 1, regulation number 872.5410, in the Dental panel. The database has no product code specific to a complete palatal expander, so the Class I status above attaches to the screw retainer as classified.
For other orthodontic components see our guides to orthodontic retainer types, orthodontic brackets, orthodontic bands and orthodontic separators.
Common misunderstandings
- “All expanders give the same result.” The 2023 review found similar outcomes for the leaf expander versus RME and SME, but reported larger skeletal growth for skeletally anchored devices in adolescents. Within the Haas versus Hyrax trial the gap was under 0.5 mm per side.
- “Class I means unregulated.” Class I devices are subject to general controls; the 510(k) exemption has limits set in 21 CFR 872.9.
- “A newer design is automatically better.” The review reports that outcomes with the leaf expander were similar to those with RME and SME, not superior.
FAQ
What are the main orthodontic expander types?
Rapid and slow expanders, each either dental-anchored or skeletally anchored; named designs include the Hyrax, Haas-type, leaf expander, memory screw, Hybrid Hyrax and MARPE.
Is an orthodontic expander a regulated medical device in the US?
The expansion screw retainer is listed in 21 CFR 872.5410 as a Class I device, exempt from 510(k) subject to 21 CFR 872.9. Check the regulation for any other component.
Does the published evidence rank one expander above the others?
No. The studies cited here compare devices on specific measures, such as skeletal change and tipping, and none names an overall winner. Choices about an individual patient belong to the treating orthodontist.
Sources
- 21 CFR 872.5410, Orthodontic appliance and accessories, https://www.law.cornell.edu/cfr/text/21/872.5410, checked 2026-10-31.
- openFDA device classification, product code DYJ, https://api.fda.gov/device/classification.json?search=product_code:DYJ, checked 2026-10-31.
- Inchingolo AM et al., Comparison of Different Types of Palatal Expanders: Scoping Review, Children 2023;10(7):1258, https://doi.org/10.3390/children10071258, checked 2026-10-31.
- Weissheimer A et al., Immediate effects of rapid maxillary expansion with Haas-type and hyrax-type expanders: a randomized clinical trial, Am J Orthod Dentofacial Orthop 2011;140(3):366-76, https://pubmed.ncbi.nlm.nih.gov/21889081/, checked 2026-10-31.
