Rapid maxillary expansion: Review of literature

Riyyad Al-Battikki, BDS, MDSc
 The Jordan University of Science & Technology and The Jordan University; President, Jordanian Orthodontic Society

Rapid maxillary expansion (RME) can be used to correct unilateral or bilateral posterior cross-bites and it occurs when the forces applied to the teeth and the maxillary alveolar process exceeds the limit needed for orthodontic tooth movement and the applied pressure acts as an orthopedic force that opens the midpalatal suture. The purpose of this  article is to review the literature concerning the etiology of posterior cross-bites, early treatment of maxillary constriction and its effect on the developing dentition, skeletal  changes produced by rapid maxillary expansion, controversy over its effect on respiratory problems, its surgical assistancce in adults and the types of expansion appliances and their management.

Rapid maxillary expansion (RME) had been proposed since the 19th century by Angell1 to correct maxillary constriction. He reported the first case of maxillary constriction  treated by RME. As early as 1920, Mesnard2 demonstrated radio-graphically that the midpalatal suture could be separated using fixed appliance and that the space would be filled with bone around 4-6 weeks. Isaacason and Murphy,3 Graber4 and Nicholson and Plint5 advocated RME  for the treatment of cleft lip and palate patients who suffered from maxillary constriction. In the 1950's and 1960's, Debbane,6 Krebs,7-9 Thorne and Hugo,10 Thorne,11 Haas,12-14 Cleall et al,15 Davis and Kronman,16 Isaacson et al,17  Isaacson and Ingram,18,19 Skieller,20 Starnbach and Cleall,21,22 Zimring and Isaacson23 and Wertz24 advocated using RME to split the midpalatal suture for widening the narrow maxillary arches. In the 1970's  and 1980's, RME went through periods of popularity and decline.

Recently, clinicians have increasingly included the RME appliance in the treatment planning of the maxillary constriction to ensure normal transverse relationship between the maxilla and the mandible25,26 and also to relieve crowding in mild cases. They found that the enhanced skeletal response that accompanies RME in young patients redirected the developing posterior teeth into normal occlusion, corrected asymmetries of condylar position and eliminated both functional shifts and T.M.J dysfunction.27-38

Etiology of the posterior cross-bite

The causes of buccolingual discrepancies (posterior cross-bite) could be either genetic4 or environmental39 in origin. Genetic factors account for narrow developed maxilla and/or wide mandible and maxillary deficiencies in cases of cleft lip and palate patients. Environmental factors involve mouth breathing associated with posterior nasal blockage and oral habits. Harvold et al39 in his experimental work created narrow maxillary arches in Rhesus monkeys by converting them from nasal to obligatory oral respiration.

Mechanism of rapid maxillary expansion

RME occurs when the forces applied to the teeth and the maxillary alveolar process exceed the limit needed for orthodontic tooth movement.17-19 The applied pressure acts as an orthopedic force that opens the mid palatal suture. The appliance compresses the periodontal ligament, bends the alveolar process, tips the anchor teeth then gradually opens the mid palatal suture and separates the maxillary bones. The result is usually an increase in the upper arch transverse dimensions, mainly by skeletal alteration associated with dental change. This depends on the sutural resistance, which increases as the individual matures.  The forces delivered by activation of RME appliance usually exceed the sutural limit and split not only the midpalatal suture but also all other maxillary sutures. Due to this sutural splitting the maxilla is incited to displace itself downward and forward with rotation of the components in both horizontal and frontal planes.

The maxilla articulates with ten other bones of the face and the cranium. The sphenoid bone lies just posterior to the maxilla, the pterygoid plates of the sphenoid, although bilaterally positioned but lacking midsagittal suture that allows them to be displaced laterally and the pyramidal processe of the palatine bone which interlock with the pterygoid plate. This conferring effect of the pterygoid plates of the sphenoid minimizes the ability of the palatine bones to separate at the midsagittal plane. This explains the non-parallel opening of the mid palatal suture in anterio-posterior direction.6,13,16,40-47

Rapid maxillary expansion appliances

Two groups of metallic RME appliances have been used to expand  the dental arches: Appl-iances of the first group were used particularly in the deciduous and early mixed dentition, e.g., the Arnold expander,48 the Coffin palatal arch49 and the quad helix appliances.50,51 The second group of appliances were used in late mixed dentition and permanent dentition for a more controlled expansion and more assured palatal splitting, e.g., the Hyrax appliance, the Cap splint and the Minne expander.52

The Arnold expander, the Coffin palatal arch and the quad helix have been shown to produce the best physiological changes48-51 (Orthopedic and orthodontic movement).  When these devices were used in the deciduous and  early mixed dentition, they delivered uniform maxillary sutural separation   at low force level.

The Hyrax appliance  is essentially a non-spring loaded jackscrew with an all-frame  that is soldered to the bands on the abutment teeth. This type of appliance causes the least irritation to the palatal mucosa and it  is easier to maintain good oral hygiene with it.

The Minne expander52 is a heavy caliber coil spring that  expands by turning a nut to compress the coil and has two metal flanges perpendicular to the coil are soldered to the bands on the abutment teeth. The Minne expander may continue to exert expansion forces after completion of the expansion phase unless it is partially deactivated.

Haas introduced an appliance which is a tissue-borne fixed appliance.13 He believed that his appliance  can cause  more parallel expansion forces on the two maxillary halves and that the forces are more evenly distributed on the teeth and the alveolar processes. The appliance is attached to the teeth, with bands on the first molars and first premolars, and to the palate, by acrylic pads between the first premolar and first molar.

 The Bonded rapid palatal expansion appliance has been described by Cohen and Silverman,53 Spolyar54 and also by Howe.55 It was used as  a workable alternative to the traditional Haas banded expansion appliance. This appliance can be bonded directly to the teeth using any of the orthodontic bonding agents available today. The increase in the vertical dimension often seen with banded rapid palatal expansion appliance, may be minimized or negated with bonded RME appliance.56,57

Wendell58 developed a tandom loop-nickel titanium temperature activated palatal expander (RME) with the ability to produce high continuous pressure on the mid-palatal suture in treating young patients.

Ivanovski59 advocated the use of  upper removable acrylic rapid palatal expander. This appliance  was fabricated without bands or clasps and  can be adapted perfectly to the patient's mouth. Irritation is almost entirely eliminated and the patient can easily remove the appliance for cleaning and activation. In addition, using cold cure acrylic to build up the appliance from the lingual occlusal side of the palate with the attention touching the lower posterior teeth can use it for treating maxillary and mandibular constrictions

Histological changes following  rapid maxillary expansion

Many articles6,16,44,,60,61 have been published concerning structural and histological changes of the sutures with RME. Ekstrm et al60 showed that the mineral content within the suture rose rapidly during the first month after the completion of  suture opening. The mineral content in the bone beside the suture decreased rapidly in the first month but returned to its initial level within 3 months. The RME is usually  performed in two phases: the first phase is an active expansion of the maxilla by sutural expansion, the second phase is a retention phase; that allows (for) re-organization and calcification of the midpalatal suture.     

Skeletal effects of rapid maxillary expansion

Timms,61 Inoue62 and Bell63 noticed that the palatine processes of the maxilla were separated in a non-parallel manner in antero-posterior direction in 75% to 80% of the RME cases observed, that is, in a wedge-shaped manner. On the other hand, Haas64 found that the midpalatal suture was opened in a parallel manner in antero-posterior direction. Haas,64 Bell63 and Wertz41 found that the maxillary halves were separated in superio-inferior direction in a non-parallel manner. This separation was pyramidal in shape with the base of the pyramid located at the oral side and the apex located at nasal cavity.

The magnitude of the opening varies greatly in different individuals and at different parts of the midpalatal suture. In general, the opening was greater in young patients than in adult patients.7-9,64 The actual measurement ranges from practically no separation (ossified mid-palatal suture in adult)  to 10 mm or more in young patients. Krebs7-9 noted that sutural opening was equal to or less than one half the amount of dental arch expansion. He also found that sutural opening, on average, was more than twice as large between the incisors as it was between the molars.

Krebs,9 Haas13 and Wertz41 showed that the maxilla displaced downward (coincided with inferior movement of the palatal processes) and forward during RME. Haas14 observed that the maxilla partially returns to its  original position whereas Wertz42 and Turbyfill65 observed that the maxilla  completely returns to its original position. Zimring and Isaacson66 concluded that RME therapy with the Haas type expander has little long-term effect (more than 6 years  after treatment)  on either the vertical dimensions or the antero-posterior dimensions of the face.

Dental effects of rapid maxillary expansion

It was estimated that, during active midpalatal suture opening, the incisors separate approximately half the distance the expansion screw has been opened.13 Following this separation, the incisors crown converge due to the trans-septal fibers, which leads to divergence of incisors roots, once the crowns in contact. The continued pull of these fibers causes the roots to converge toward their original axial inclination. This cycle generally takes about 4 months.

Sandstrom et al67 and Gryson68 reported that RME  could lead to a concurrent expansion of the lower arch  as much as 4 mm in inter-canine width and 6 mm in inter-molar width. Sandstrom et al67 studied the effect of RME on the mandibular inter-canine and  inter-molar width in relation to patient's age. They found no correlation between the amount of  increase in arch width and the facial type and age of the subjects.

Langford and Sims,69 Langford,70 Barber and Sims71 and Kittel and Sampson72 reported that RME was associated with marked resorptive damage to the roots of anchor teeth, on going repair gradually restores the defects. The application of a fixed device is recommended in light of accelerated apposition of repair cemen-tum during the retention period.73

Control of vertical dimension during rapid maxillary expansion

Bonded acrylic RME57 appliance was recommended in cases of constricted maxilla where inferior and anterior movements of maxilla were restricted. Bonded RME appliances are designed to cover  the posterior occlusal buccal segments  so that the appliance not only serves as an expansion device but also intrudes on the freeway space through its vertical thickness. It acts as a functional appliance with small range of action. Theoretically, by infringing on the freeway space with the displacement of the mandible (2-3mm)  below the inter-cuspal position. A constant passive force is exerted on the maxilla and the mandible.

It was postulated that a full coverage of the  occlusal surfaces by acrylic would prevent interference during the lateral displacement of the two maxillary bones  and would lessen the resistance. RME should be cautiously performed on persons with steep mandibular plane and/or open bite tendencies because with RME there was a downward and backward movement of the mandible.74 However, there was  disagreement regarding the magnitude and the permanency of the change.41,75

Chin cup appliance76 or posterior high pull headgear77,78 was recommended to be used with RME in treating cases of constricted maxilla associated with open bite and with long face syndrome and also in high mandibular plane angle. Majourau and Nanda74 advocated using vertical or oblique pull chin cups during RME and immediately after RME to prevent the side effects and to maintain control of the vertical dimension of the lower facial height. This movement of the mandible during RME could be explained by disruption of the occlusion caused by the extrusion and tipping of the maxillary posterior teeth along with alveolar bending.

Effects of rapid maxillary expansion on nasal airways

Immediately following RME there is an increase in the width of the nasal cavity, particularly at the floor of the nose adjacent to the mid-palatal suture. As the two halves of the maxilla separate, the anterior walls of the nasal cavity move laterally, so the total effect was an increase in the intra nasal  space.13,24,79-85 Chate81 noticed that the mean cross-sectional of the nasal cavity enlargement associated with RME was between 1.4 mm - 4 mm.  Haas13 and  Montgomery et al79 reported that the effects of RME  on the nasal cavity were not uniform and the changes in the nasal dimensions were progressively less toward the posterior part of the nasal cavity.

There has been a long-standing controversy over the efficiency of RME in relieving nasal obstruction and improving respiration; Hershey et al82  and Linder Aronson & Aschan84  reported that there was no correlation between nasal resistance and inter-molar width or enlargement of nasal cavity. Warren et al85 observed that RME proce-dure remains an unpredictable way of improving the nasal airway. Warren et al80 concluded that RME in improving nasal airways alone was not justified. Wollen et al86 and McDonald87 advo-cated the use of  RME technique in diagnosis and treatment of children suffering from mouth breathing. Graber4 believed that the claim of improved nasal breathing  as a result of RME was most likely a temporary result.

Wertz41 concluded that, opening the midpalatal suture for the purpose of increasing nasal  airflow could not  be justified unless the obstruc-tion was shown to be in the lower anterior portion of the nasal cavity and accompanied by a relative maxillary arch width deficiency.

Timms40,47 reported in his study that the correlation between trans-alar increase and trans-palatal expansion and the trans-alar increased and the reductions in the nasal airway resistance a weak correlation (r=0.115,  r=0.30,  respectively). In the meanwhile Timms88 observed some success in the treatment of nocturnal enuresis  when he used the RM expander. Laptook89 showed that RME may aid in improving hearing due to normal functioning of the pharyngeal ostea of the Eustachian tube as a result of the effect of RME on the palatal and naso-pharyngeal tissues especially in cleft palate patients.

Optimal time for  rapid maxillary expansion

The midpalatal suture can  ossify as early as age 15 years90  or as late as age 27 years,91 but the optimal period for performing RME procedure was between 8-15 years.64

Surgically assisted  rapid maxillary expansion

An occlusal radiograph  should be taken on the fourth or fifth day after treatment initiation, in order to evaluate the separation of the palatal shelves, which usually occurs between the third and the eighth day. If separation does not occur fifteen days within active treatment, the appliance is discontinued.

Surgically assisted RME is applicable  during the permanent dentition in order to overcome the strong resistance of the opening of the  mid-palatal suture.92,93 In overcoming the strong resistance of opening the ossified midpalatal suture in an adult, surgical approach was applied by a midline cut about 3 mm deep but not reaching the foramen incisive (the mucosal and bony cuts of the palate should not overlap)  and 2 bony cuts 4 mm long each were then made on each side of the lateral maxillary buttress above the root apices and parallel to the occlusal plane .

In some cases more extensive surgery involving the separation of the maxilla from the pterygoid plates is needed because it is important to remember that the main resistance to mid palatal suture opening in an adult is probably not in the suture itself but in the surrounding structures particularly the sphenoid and zygomatic bones.Pogrel at al,92 Bell and Turvey,93 Lehman & Hass,94,95  Mossaz et al,96  Kennedy97 and Susami et al98 reported that a conservative osteotomy of the zygomatico-maxillary buttress in combination with RME was indicated for the treatment of horizontal maxillary deficiency in adults. Mossaz et al96 demonstrated that unilateral cross bites associated without mandibular displacement in adults can be corrected with unilateral corticotomy and RME, using contra-lateral non-operated side as anchorage.

Retention

Skeletal tissues offer the immediate resistance to expansion force, but other equally important factors are the muscles of mastication, the facial muscles and the investing fascia. Pogrel  et al92 observed that the resistance to deformation from circum-maxillary sutures and surrounding soft tissue matrix was the main cause of relapse of the rapid maxillary expansion.

The duration of the post expansion retention period was controversial.7-9,23,47,54,70,73,85,98,99 Spolyar,54 McNamara and Howe100 advocated early treatment of maxillary constriction with bonded RME in early mixed dentition stage. They reported that the majority of increased arch dimension in patients produced by early orthopedic RME were maintained at the end of the mixed dentition. Porgel et al92 stated  that the changes obtained by short-term simple mechanical interference with a complex biological system tend to reverse spontaneously.

Timms47 suggested that the cases treated with RME should be over expanded and kept in retention for a minimum of two years to overcome the resistance of the tensile forces produced by stretching the soft tissue during expansion (allow the soft tissue to reorganize in their new position).

Vardimon et al73 recommended the application of a fixed retainer immediately and subsequent to rapid maxillary expansion, then followed by an intermittent removable retention appliance. Zimring and Isaacson23 noticed that six weeks of retention were sufficient to establish an equilibrium between contiguous sutural articulation, but Barber and Sims71 noted that the bony base continue to relapse for at least nine months after expansion. Kerbs7-9 was able to show latent instability, four to five years after treatment when he used the metallic implants.

The early correction of posterior cross-bites may offer the advantages of redirecting the developing teeth into more normal positions, correcting asymmetries of condylar position, and allowing normal vertical closure of the mandible without functional shifts to avoid occlusal  interference.

Bonded RME should be used in correcting the maxillary constriction cases associated with anterior open bite, long  lower face, and Class II  patients.

Opening the midpalatal suture for the purpose of increasing nasal patency cannot be justified unless the obstruction was in the lower anterior portion of the nasal cavity and  accompanied  by a maxillary constriction.

In overcoming the strong resistance of opening the ossified midpalatal suture in adult, surgical intervention was applied either by palatal midline cut, unilateral or bilateral corticotomies and by more extensive surgery involving the separation of the maxilla from the pterygoid plates.

Cases treated with RME should be over-expanded and kept in retention for a minimum of two years to overcome a relapse.

Address reprint requests to:

Dr. Riyyad Al-Battikki

Director of Dental Corps
Royal Medical Services  Jordan Armed Forces
P.O. Box 43
Amman 1118, Jordan
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