RME has been used extensively.^3,4 Some limitations associated with it have been reported, including bite opening,⁵ relapse,⁶ microtrauma of the temporomandibular joint and the midpalatal suture,^7,8 root resorption,^7,8 tissue impingement and pain,⁹ and excessive tipping of anchorage teeth.⁹

SME procedures produce less tissue resistance around the circummaxillary structures and, therefore, improve bone formation in the intermaxillary suture, which theoretically should eliminate or reduce the limitations of RME.^2,10,11 For SME, only 10 to 20 newtons^10,12 of force should be applied to the maxillary region, depending on the age of the patient, compared with 15 to 50 N for RME.^1,2,13 The most frequently used SME appliances are minne-expanders^3,8,10,14 and quad-helixes.^15–17

Although the objective of both appliances is to achieve physiological intermaxillary sutural expansion, the design and activation are different. Clinicians should not assume that the skeletal and dental effects of the two appliances are equivalent, and they should consider the two treatment modalities separately.

We identified in the dental literature a systematic review¹⁸ and two meta-analyses^3,19 that reported dental arch changes after RME, but we could not find any systematic reviews or meta-analyses on the dental or skeletal changes associated with SME treatment. We conducted a study to evaluate skeletal and dental changes after fixed appliance SME in orthodontic patients with constricted arches, using all available published scientific literature.

	MATERIALS AND METHODS

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We selected two literature search terms—"slow palatal expansion" and "slow maxillary expansion"—with the help of a librarian who had experience in searching health sciences databases. We conducted computerized searches using the following databases: MEDLINE from 1966 to week three of July 2004; MEDLINE In-Process and Other Non-Indexed Citations July 23rd, 2004; LILACS, a literature database of Latin American and Caribbean health research, from 1982 to July 2004; PubMed from 1966 to week three of July 2004; EMBASE Excerpta Medica from 1988 to week 30 of 2004; Thomsen ISI’s Web of Science from 1975 to week three of July 2004; and all databases in the Evidence Based Medicine (EMB) Reviews database—Cochrane Database of Systematic Reviews (DSR), American College of Physicians (ACP) Journal Club, Database of Abstracts of Reviews of Effects (DARE) and Cochrane Central Register of Controlled Trials (CCRCT)—to the second quarter of 2004.

All of the eight studies the authors finally selected lacked control groups.

We used the following inclusion criteria to select the appropriate articles: clinical trials, measurements made from dental casts or facial radiographs, and no surgical or other simultaneous treatment that could affect SME during the evaluation period.

We read the articles’ abstracts to determine the eligibility of articles. Two of us (M.O.L., C.F.-M.) independently completed the selection process. If a discrepancy arose, a third researcher (P.W.M.) helped make the final decision.

We obtained articles for which the abstracts did not present enough relevant information to help us make a final decision regarding their inclusion. We also searched the reference lists of the selected articles for additional relevant publications that we may have missed in the database searches.

All three of us obtained and independently evaluated all of the articles from the selected abstracts. Then we reached a consensus regarding which articles should be included in the systematic review. For articles in cases in which relevant data were not available in the publication, we contacted the authors to obtain the required extra information.

	RESULTS

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We observed that MEDLINE and PubMed had the greatest diversity of abstracts, but these databases did not include all of the abstracts included in other databases (Table 1). All except one of the selected abstracts from PubMed were included in MEDLINE and vice versa. All of the abstracts selected by Thomsen ISI’s Web of Science, EMBASE and all EBM reviews—Cochrane DSR, ACP Journal Club, DARE and CCRCT—were included in MEDLINE. After we reviewed the selected abstracts’ reference lists, we included only one more article²⁰ that did not appear in any of the database’s searches (Table 1).

	DISCUSSION

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In the absence of the highest level of evidence, clinicians have to make decisions based on lower levels of evidence. Because lower levels of evidence are prone to confounding and selection bias,²⁴ clinicians should carefully analyze the studies’ limitations.

Scientific evidence alone does not dictate the selection of the treatment. When making health care decisions, clinicians also should consider a combination of values from patients and from professionals (clinical, personal and social) that determine if the intervention benefits are worth the costs.²⁵ Therefore, the application of evidence into clinical practice has to be related to professional expertise and the needs of the patient.

Since SME involves an active expansion period of up to four months, clinicians cannot eliminate a patient’s normal growth as a confounding factor for the changes found. Because none of the reviewed studies reported the use of a control group for comparison at the end of treatment, clinicians should interpret carefully the clinical significance of the findings. No strong conclusion can be made on dental or skeletal changes after SME. When evaluating studies on SME treatments, we noticed some trends in the studies’ results. For example, the greatest changes reported were on the transverse plane, especially interdental widths, compared with those on the sagittal and vertical planes.

The studies we selected that used quad-helix and minne-expander appliances reported active treatment times that varied from one to 3.6 months.^{8,10,14–17,20,21} These studies reported dental and skeletal changes through the use of dental casts and cephalometric radiographs. Both of Akkaya and colleagues’ studies^8,14 reported the same sample and expansion protocol, differing only on the auxiliary examination used for measuring the changes; thus, clinicians could combine their results and consider them one whole study.

It appears that approximately 1 millimeter per week is the maximum rate at which the tissue of the midpalatal suture can adapt so that tearing and hemorrhaging are minimized compared with rapid expansion protocols.¹⁰ The overall result of rapid versus slow expansion is similar; however, with slower expansion, a more physiological sutural response should be obtained.¹²

To produce expansion at this rate, 10 to 20 N of force appear optimal, depending on the age of the patient.^10,12 When comparing the forces applied by the different appliances used in SME, the minne-expander appliance spring applies forces of up to 10 N (2 pounds),^10,21 while the quad-helix appliance exerts an average of 5 N (1 pound).¹⁶ In contrast, RME has a cumulative force of approximately 100 N.^1,2,13

Since a direct relationship has been reported between increased resistance to skeletal expansion and increasing patient age, the use of SME could be questioned for older patients.² Thus, the degree of orthopedic change during SME is considered to be related inversely to the patient’s age.¹³

When reviewing the appliances used in SME treatments, we found that minne-expander^8,10,14,21 and quad-helix^15–17 appliances were used most frequently. Although both types of appliances obtain maxillary expansion, their approaches are different. The minne-expander appliance uses a coil spring, and the quad-helix appliance uses wire bending with helixes to exert the expansion force.

Concerning maxillary expansion with a quad-helix appliance, Boysen and colleagues¹⁶ reported that a small percentage of expansion was caused by skeletal separation. The greatest percentage was caused by expansion of the teeth and tipping. Sandikcioglu and Hazar¹⁷ reported similar findings; they compared expansion tooth gain with RME hyrax appliance, semirapid expansion with a removable plate and SME with a quad-helix appliance. All three treatments presented the greatest change on the transverse planes. Expansion with a quad-helix appliance was primarily dentoalveolar, with more tipping of teeth than the other groups. Malagola and coleagues¹⁵ reported similar results.

Chaconas and Caputo²⁶ reported that the effects produced by a quad-helix appliance are dependent on the patient’s age. When midfacial sutures are patent, as seen in children seven to nine years old, activation of the quad-helix appliance meets with little resistance, meaning that more skeletal expansion can be obtained than for older patients. Conversely, during treatment of adults, the intermaxillary and surrounding sutures are less patent and, in some cases, fused, which makes orthopedic results difficult to obtain. In most cases, an interdental width increase along with often severe tipping of the posterior teeth is observed in adults with expansion produced by a quad-helix appliance. The effect of palate separation in older patients is minimal even with increased activation of the quad-helix appliance. The result of more aggressive activation is increased tipping of the posterior teeth, especially those used for anchorage.

Different studies reported maxillary expansion obtainment using minne-expander appliances, but the dental to skeletal ratios varied. Hicks¹⁰ attributed 28 percent of the expansion gained to skeletal response or separation of the suture, whereas Mossaz-Joelson and Mossaz²¹ attributed 50 percent to skeletal expansion. Although both studies involved adolescents, the patients in Hicks’ sample were older than those in Mossaz-Joelson and Mossaz’s study. Also, the appliance used in Mossaz-Joelson and Mossaz’s study was of a much more rigid design than the one used in Hicks’ study, even when both were based on the use of similar coil springs.

Mossaz-Joelson and Mossaz²¹ found no significant differences regarding the amount of maxillary expansion, percentage of skeletal response and degree of dental tipping between banded or bonded minne-expander appliances for SME.

Another type of appliance used for SME treatments is a nitanium maxillary expander.²⁰ This appliance is similar to the quad-helix appliance in that it applies approximately 5 N of force on the dental structures. Although this appliance does not need activation appointments, it produces results similar to those of the quad-helix appliance, especially concerning the ratio of dental to skeletal expansion. Karaman²⁰ also reported that there was significant buccal crown tipping after the expansion.

Subjects who had undergone SME had less relapse compared with subjects who had undergone RME.^3,8,14 This can be explained because of the lower forces applied on the suture over a longer period, which could permit a continuous adaptation of the tissues to the skeletal and dental changes.

	CONCLUSIONS

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Although several studies concerning SME treatment outcomes have been published, none of them included a control group that did not receive treatment to factor out changes in the dental arch and skeletal structures associated with normal growth. Even though SME treatments involve an average treatment time of three to four months, normal growth patterns can produce some dental and skeletal changes during that time. When the reported treatment changes were small—with significant standard deviations and measurement errors—the importance of the use of a control group increased. Thus, we recommend that further research on the effects of SME treatments on patients should include a control group.

Since the research studies on SME treatment we reviewed failed to give a higher level of scientific evidence, we could not make strong conclusions regarding dental or skeletal changes after SME. We recommend that clinicians use the results of the studies we reviewed with caution. As in any clinical environment, clinicians should rely on their clinical experiences, the opinion of experts and the limited evidence concerning SME treatments.