Rapid Maxillary Expansion RME, What Is It All About?

Expansion of the upper arch is a common method of transverse correction, numerous expansion appliances have been described with varying protocols, force levels, techniques and duration of treatment. Broadly speaking expansion can be divided into dento-alveolar expansion or skeletal expansion.

In this blog we will focus on a common method of skeletal correction, Rapid Maxillary Expansion. How it works, indications, and risks.

How does it work?

The principle behind RME is not part of conventional orthodontic tooth movement. Conventional orthodontic tooth movement entails a light force of 12-150gm being transmitted to the periodontal ligaments, resulting in the dento-alveolar bone re-modelling in the direction of the force, and moving the tooth with it. RME on the other hand involves much larger forces of 900 to 4500 gm, this higher force separates the mid-palatal suture. As the palatal shelves separate, the host response is to heal the bony separation, a process with is driven by the tension across the gap, resulting in new bone between the separated palatal shelves (distraction osteogenesis).

Palatal shelves are different to other craniofacial bones, as they are not considered to fuse until the last decade of life, instead the sutures ‘close’ (Kokich 1975). The difference in the two seemingly similar terms, is that with closure there remains a lack of continuous bone cross the suture site, RME can therefore separate the suture, seemingly without the need of surgery, as it is not a continuous bone. However ‘closure’ is a progressive term and results in an increase in interlocking of bone, which results in greater resistance to separation. From an appliance perspective this increase in interlocking, as well as increasing resistance from adjacent bones of the palate, renders RME ineffective after a period of development, indicating the need for surgery. RME is considered to work effectively and reliably in patients below 14 years of age (Sokucu 2009). It is worth mentioning at this point expansion of the screw is not a 1:1 ratio of expansion of the maxilla, this is due to the resistance experienced in successful cases. The ratio of screw expansion to maxillary expansion is around 3:1, i.e. 9mm of screw expansion results in 3mm of maxillary skeletal expansion Bazargani 2013.

Bell, 1982 described the biomechanical response of RME in his paper. He described that upon application of transverse force the following reactions occurs:

  1. Orthopedic separation: If the applied transverse forces are very high (enough to overcome the palatal bone closure and adjacent bone resistance), orthopedic separation of the palatal shelves occurs.
  1. Reorganization and remodeling: Across the newly formed gap between the palatal shelves, the soft tissue reorganizes to produce bone, which stabilizes the expansion of the arch.
  1. lateral tipping: When a transverse force is applied to the posterior teeth, the periodontal ligaments and palatal soft tissues are compressed and stretched, after 1 week bodily translation occurs as the compressed buccal alveolar wall resorbs at the root-periodontal interface from continued force application.

Effects of RME

Hugo De Clerck made an incredibly simple and enlightened statement at the AAO meeting in 2021, stating orthopedic changes of the craniofacial region are rotational, NOT linear (covered as a podcast previously). This is due to the differing number of sutures and interactions each bone surface has, resulting in a varying resistance and therefore movement. Consequently all orthopedic changes must have an effect (albeit varying) in 3 planes of space. The effects with RME on the maxilla in 3 different planes:

  1. Transverse: RME results in triangular expansion, with greater expansion anteriorly, and less posteriorly. The reason for this is due to the increased resistance of the posterior maxilla, mainly due to the zygomatic buttress. Average expansion (intermolar) with RME is 4.09-6.24mm (Zhou 2014, Mohan 2016).
  2. Vertical:
    1. Anterior changes at A point: A point moves inferiorly by 2.99mm Woller 2014
    2. Posterior changes at the molars: there is a vertical rotational effect of the palatal shelves resulting in a superior movement of the posterior bone by 6o Bishara 1978.
    3. Posterior changes at the mid-palate: The mid-palate moved inferiorly, although it is a small amount, 0.02mm Woller 2014
  3. Antero-posterior: A point is advanced. This seems to be an unexpected effect, and occurs as a consequence of the palatal shelves rotating whilst expanding the transverse plane. There is a high resistance at the back of the maxilla where the palate meets the pterygopalatine sutures, this results in a ‘springboard effect’, moving the palate forwards. However, this movement is small and not of clinical significance at 0.5mm Pereira 2017.

Effects to the mandible have been reported as follows

  1. Intermolar expansion: The premise of maxillary expansion resulting in expansion in the mandible is based on two theories:
    1. A change to the functional environment allows the mandibular molars to upright, as the restriction of the occlusion with the maxilla has been removed Kucukkeles 2003
    2. Transverse forces from the maxilla are transferred to the mandibular teeth through cusp interaction

However the quantity of expansion in the mandible is small at 0.96-1.08mm, which is not of clinical significance, and mandibular expansion should not be expected in a clinically relevant way with the use of RME Lo Giudice 2020.

  1. Clockwise rotation of the mandibular plane 0.85o -1.9o . This is thought to be due to cusp-to-cusp contact during maxillary expansion, followed by maxillary molar extrusion. These vertical changes seem to be transient according to recent systematic review Chhutani 2020.


The following are the main indications for RME:

  1. Correction of crossbite by correcting the transverse maxillary discrepancy Zhou 2014.
  2. Relief of crowding by gaining space as a result of the transverse expansion, usually every 1mm of transverse expansion provides 0.5 to 0.7mm space in the arch McNamara 2006.
  3. Interception of palatally impacted canines Baccetti 2011.
  4. Correction of class 3 malocclusions with facemask Woon 2017.

There are other claims of RME use, which have not been investigated sufficiently to conclude an association with so far, these controversial areas are:

  1. Improve symptoms of obstructive sleep apnoea
  2. Reduce nasal airflow resistance.
  3. Improve hearing in patients with conductive hearing loss
  4. Treating Nocturnal Enuresis

We will be exploring breathing related disorders and orthodontics in a future blog, suffice it is to say a source of debate amongst us.

Timing of RME

Chronological age is a poor measure of skeletal development, so to work out the ideal time to use RME with predictable outcomes cervical vertebral maturation has been proposed Perrson 1977, Baccetti 2001. Early treatment carried out before peak skeletal maturation was defined as CVMS 1-3  (cervical vertebral maturation) and late treatment after peak in skeletal maturation as CVMS 4-6. Baccetti’s landmark paper from 2001 showed early treatment resulted in greater maxillary skeletal expansion of 2.5mm. However a number of recent studies with more robust protocols found no significant difference between early Vs late RME treatment for younger patients (up to 18 years of age), Sayar 2019, Mohan 2016.

What defines ‘late’ treatment is being re-defined due to the advent of mini-screws and a modified RME appliance with Miniscrews (MARPE) with a recent systematic review showing the possibility of separation of the palatal shelves from ages 18-42, although the studies were retrospective Kapetanović 2021.

Risks of RME

RME, like any other orthodontic intervention, comes with some risks. These can be avoided or prevented by careful history taking, clinical and radiographic examination and diagnosis.

  1. Recession: Less than 1mm changes have been found in a systematic review Bastos 2019
  2. Pulpal damage. Due to the high forces during RME there is a disruption to pulpal blood flow, however the changes tend to reverse after the first week Babacan 2010.
  3. Boney defect. RME seems to result in a reduction of both horizontal and buccal bone. There is a loss of 0.78-1.2mm of buccal bone height, as well as loss of buccal bone thickness of 0.14-0.71mm, with a greater effect at the apical area when compared tot he cervical area of bone Baysal 2013. The effects on the buccal bone are also associated with a 47% chance of a buccal bone fenestration of the 1st The mesio-buccal aspect of the molar is the most vulnerable. Interestingly the amount of expansion does not seem to be a predictive factor, but initial buccal bone levels are Sperl 2021.
  4. Root resorption According to the findings of Baysal 2013, the maxillary first molars resorbed an average of 6.87% volume, with first and second premolars resorping a similar amout of 6% and 7% respectively.
  5. Relapse: A long term study (7 years) showed an average relapse rate of 22% Gurel 2010. Both dental and skeletal relapse occur following RME, at a ratio of 4:1 Janakiraman 2014.

RME should therefore be considered in light of the potential risks of bony fenestrations and root resorption Vs the clinical benefit.

Risks of RME


< 1mm

Bone defect

Up to 0.71mm horizontal bone loss. 47% chance of buccal fenestration 1st molar

Root resorption

6-7% posterior teeth




McQuiellen J. Review of the dental literature and art: separation of the superior maxilla in the correction of irregularity of teeth. Dent Cosmos 1860; 2: 170−173.

Bell, R., 1982. A review of maxillary expansion in relation to rate of expansion and patient’s age. American Journal of Orthodontics, 81(1), pp.32-37.

Baccetti T, Franchi L, Cameron CG, McNamara JA Jr. Treatment timing for rapid maxillary expansion. Angle Orthod. 2001

Almuzian, M., Short, L., Isherwood, G., Al-Muzian, L. and McDonald, J., 2016. Rapid maxillary expansion: a review of appliance designs, biomechanics and clinical aspects. Orthodontic Update, 9(3), pp.90-95.

Stockfisch H. Rapid Expansion of the Maxilla – Success and Relapse. Paper presented at Report of the Congress: European Orthodontic Society, 1969.

Baysal, A., Karadede, I., Hekimoglu, S., Ucar, F., Ozer, T., Veli, İ. and Uysal, T., 2012. Evaluation of root resorption following rapid maxillary expansion using cone-beam computed tomography. The Angle Orthodontist, 82(3), pp.488-494.



  • Contents by Kerolos AlHakeem 
  • Contents by James Andrews 
  • Edited and contents by Farooq Ahmed

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 Bazargani; ar