Horizontal expansion is an orthodontic procedure that widens the upper jaw (maxilla) to correct a mismatch between the width of the upper and lower dental arches. It works by separating or reshaping the palate, the bony roof of the mouth, so the upper teeth align properly with the lower teeth. The procedure is most common in children and adolescents, though newer techniques have made it possible for adults as well.
Why the Upper Jaw Needs Widening
A narrow upper jaw creates what orthodontists call a transverse discrepancy, meaning the top arch is too narrow relative to the bottom. This shows up in several ways. The most obvious is a posterior crossbite, where some or all of the upper back teeth sit inside the lower back teeth instead of slightly outside them. Crowding is another common sign: when the arch is too narrow, there simply isn’t enough room for all the permanent teeth to come in straight.
A narrow palate also affects breathing. The roof of your mouth is the floor of your nasal cavity, so a constricted palate can reduce the volume of the nasal airway. Widening the palate has been shown to significantly increase nasal airway volume, which is why horizontal expansion is now used as a treatment for children with obstructive sleep apnea. In one systematic review, children who underwent palatal expansion saw their apnea-hypopnea index (a measure of how often breathing is disrupted during sleep) drop by 73 to 77 percent, with improvements lasting beyond three years.
Skeletal Expansion vs. Dental Tipping
Not all widening is the same. True skeletal expansion physically separates the two halves of the palate at the midpalatal suture, the seam of connective tissue running down the center of the roof of the mouth. New bone fills in the gap over time, creating a permanently wider jaw. Dental expansion, by contrast, just tilts the teeth outward on the existing bone without changing the bone itself. Most expansion procedures produce some combination of both, but the ratio matters: skeletal change is more stable long-term, while dental tipping is more prone to relapse and can stress the roots and surrounding bone.
The midpalatal suture gradually fuses as you age. In children and young adolescents, it’s still flexible enough to separate with moderate force. During later adolescence, the suture begins to ossify, making non-surgical expansion progressively harder. By adulthood, the suture is typically fused, which is why adult patients often need either miniscrew-assisted devices or surgery to achieve true skeletal widening.
Rapid vs. Slow Expansion
There are two main speeds of expansion, and both produce comparable results in terms of how much the jaw widens. The difference is primarily in comfort and how the bone responds.
Rapid maxillary expansion (RME) uses heavy, intermittent forces applied over a short period, typically one to three weeks. The device, usually a metal framework cemented to the back teeth with a central screw, is turned once or twice daily. This approach is effective at splitting the midpalatal suture in growing patients, maximizing skeletal change. The trade-off is that pain intensity during the first week is significantly higher compared to the slower approach.
Slow maxillary expansion (SME) uses lighter, continuous forces over a longer treatment window. It can be achieved with spring-based appliances like a quad helix or W-arch, or simply by turning the same type of screw expander less frequently. Research shows that SME produces less tissue resistance in the structures surrounding the upper jaw, better bone formation at the suture, and less discomfort for the patient. It also produces slightly less molar tipping than rapid expansion, meaning a greater proportion of the widening is true skeletal change. After the first week, pain levels are similar between the two approaches. Both methods cause temporary difficulty swallowing in about 80 percent of patients during the first week.
Options for Adults
Because the midpalatal suture fuses with age, adults have historically needed surgically assisted rapid palatal expansion (SARPE), where a surgeon cuts through the fused suture and surrounding bone before activating an expander. This remains an effective option, but a newer alternative called miniscrew-assisted rapid palatal expansion (MARPE) has changed the landscape. MARPE uses small titanium screws anchored directly into the palatal bone to deliver force to the skeleton rather than through the teeth.
Comparing the two, MARPE produces greater skeletal changes in the midface and at the base of the upper jaw, with a more parallel, even opening pattern. SARPE tends to produce a V-shaped opening, wider at the front than the back, and results in greater outward tipping of the teeth and the bone surrounding them. Neither approach showed a significant difference in how much the roots of the teeth spread apart. For adults with obstructive sleep apnea, MARPE has shown particularly promising results: one study found a 65 percent reduction in the apnea-hypopnea index along with significant increases in both nasal and throat airway volume.
What to Expect During Treatment
Regardless of the method, the active expansion phase is relatively short. For rapid expansion, you or your child will turn the device’s screw at home, typically for two to three weeks. Slow expansion takes longer, often two to three months. After the target width is reached, the expander stays in place as a retainer for several months while new bone fills in the gap at the suture. This retention phase is critical: removing the device too early allows the expansion to collapse.
Pressure across the bridge of the nose and under the eyes is common during the first few days of activation, especially with rapid expansion. Some patients notice a temporary gap forming between the two front teeth, which is actually a positive sign that the suture is separating as intended. The gap typically closes on its own or is corrected with braces afterward.
Long-Term Stability and Relapse
Some degree of relapse is expected with any expansion. A long-term study following patients for five years after treatment found that the width gained between the back molars held up well, with only about 17 percent relapse. The premolar region showed similar stability at 19 percent relapse. The canine area was less stable, with 37 percent of the gained width lost over time. In practical terms, a patient who gained about 4 mm of molar width kept roughly 3.3 mm of it after five years. One study tracking patients 8 to 10 years post-treatment found a net increase of 5.5 mm still maintained at the molar level.
These numbers highlight why orthodontists typically over-expand slightly beyond the ideal width, anticipating that some narrowing will occur once the retainer is removed.
Potential Risks
Horizontal expansion is generally safe, particularly in growing patients, but complications can occur. The most commonly reported issues include root resorption (shortening of tooth roots from the forces applied), outward tipping of the teeth beyond the supporting bone, and gingival recession on the side of the teeth facing the cheek. In rare cases, damage to blood vessels or nerves near the palate can occur, and uneven expansion may lead to facial asymmetry. These risks increase when expansion is attempted in patients whose sutures have already fused, which is one reason imaging with cone beam CT scans has become more common before treatment to assess suture maturity.