Until puberty, narrow maxillary arches can be widened by either fixed or removable expansion, after this age the most common approach is to use appliances fixed to the teeth and/or bone, often assisted by surgery. Claims have also been made that the maxilla can be widened in adults using removable appliances but this is generally rejected because of the degree of tipping involved. The objective of thisresearch wasto test the null hypothesisthat “it is not possible to expand adults with removable appliances without excessive tilting”. Material. 18 adults with first molarslessthan 36.5 millimetres apart were selected and expanded with a removable Stage 1 Biobloc appliance at the ‘semi-rapid’ rate. Results. The intermolar width was increased an average of 7.03 millimetres and the first molars tilted an average of 3.89 degrees.


The advantage of lateral maxillary expansion has been recognised since the arches were first widened by Pierre Fauchard in the eighteenth century. Towards the end of the nineteenth century, dentists were concerned with the increasing number of children with narrow upper jaws (Chapman 1927). In 1960 Angell Emerson appears to be the first person to publish a paper recommending Rapid Palatal Expansion (RME), but it was not until the 1900s that it became more widely used after it was found that a rate of three to four millimetres a week would force open the central suture and avoid much of the tilting of the teeth.
Early Orthodontistssuch as Babcock (1911) designed removable appliancesto widen the dental arch with a screw in the palate pushing on the teeth either side. For comfort the rate of opening was usually one quarter of a full rotation every two or three days (about one half a millimetre a week). This widened the arch but tended to tilt the teeth at the same time.
In the 1930s Schwartz created a removable expansion appliance intended for younger patients. This waslater altered by Mew (figure 1) who in the 1960s first described ‘semi-rapid expansion’(1977). He recommended one eighth of a millimetre per day until the age of 20 when this should be changed to an opening of one sixteenth of a mm morning and evening (one thirty-second of a millimetres each side). Currently there are many similar removable appliances, along with others deigned for adults using fixed techniques some of which are assisted bone screws and/or surgery. However, there has been little research to confirm if removable appliances can widen adults.

Previous research

Teeth have evolved to deliver very high bite forces and animals such as the Saltwater Crocodiles can deliver a vertical force of around 3700 PSI but they do not respond well to  ateral forces. Despite thisrapid expansion isstill widely used despite signs of damage noted by Moss & Timms (1971). Currently many orthodontists use fixed appliances to widen the dental arch by around 4 millimetres(Proffit 1999). However Kurol (1996) and many others have shown that “root resorption is an early and frequent iatrogenic consequence” of most lateral tooth movement with fixed appliances, especially in older patients.
There is no doubt that the periodontal membrane, roots, and bone are frequently damaged by this, with little evidence to suggest that this is balanced by long-term benefits. Kurol (1998) also showed that even light lateral forces (under 40 grams) caused hyalinization which was likely to restrict further tooth and bone movement.
Is it possible to widen the palate without subsequent relapse? In 1964 Skieller 5 expanded at around half a millimetre a week and used metal implants to measure the precise widening of the midline suture. He found that the expansion widened both the teeth and suture but that afterwards the teeth tended to relapse while the widening of the suture itself was largely maintained.
Storey suggested (1973) that the most physiologically appropriate rate for the movement of bone was about 1mm per week. The teeth and bone are rigidly set in the alveolus giving the jawsthe ability to crush tough food, however in the long-term the lateral position of the teeth can be influenced by very light forces. Notsurprisingly most research has concentrated on the ‘ideal’or perhaps‘fastest’way to move teeth and there has been little research into the minimum force required. However, Weinstein’s classical research (1967) suggested that teeth will move with a force of under 2 gm, the force of a feather.
More recent research (Theodorou et al 2019) on the ‘optimal’ force to produce bodily tooth movement suggests that it should be between 50 and 100 grams. Clearly this‘optimal’force may not equate with the minimal force.Also, bodily movement requires more force than tilting, so in reality we have little idea of the minimum force required to move a tooth.
Our own rather primitive research, using blobs of composite resin on the lingual or buccal surfaces of unopposed teeth, has led us to believe that the tongue can move a tooth.The fact that both bone and teeth are so firm leads many orthodontists to use quite heavy forces to move them, however the periodontal membrane is little more than the width of a hair and a force of more than a few grams will push a tooth to one side of the rigid socket occluding the blood vessels.
If this lateral force continues for more than a few hours many thousands of cementoblasts and periodontal cells are likely to die from lack of oxygen. This may result in hyalinization (Kurol et al 1998) which will restrict further tooth movement.
This probably also accounts for the pain when fixed appliances are fitted and adjusted as well as the damage noted by Kurol and others. This evidence suggeststhat neithersutures nor teeth should be moved much more than about 1/16th of a millimetre a day. This distance avoidsreducing the periodontal thickness by more than halfso allowing vitality to be maintained.The frequently recommended expansion rate of one quarter of a turn undoubtedly causes occlusion.There is much evidence to suggest thatslow intermittent opening avoids much of the damage caused by continuous forces. Kumasako-Haga and her colleagues (2009) found “an 8-hour intermittent force, efficiently recruits osteoclasts while causing minimal root resorption”.
At puberty the bone of the mid-palatal suture becomes progressively interlocked and Fernanda Angelieri (et al) proposed that there are five stages of maturation, in the last two, they include ‘late adolescents’ and ‘young adults’ for whom “RME is unpredictable”. They suggest that after this age “surgically assisted RME would be necessary”. This is the statement we wish to test by the null hypothesis, asin our experience the palate can be widened by semi-rapid expansion (Fig. 1) withoutsurgery or trauma at almost any age.

Figure 1

Before puberty, ten to fifteen millimetres of semi-rapid expansion is usually possible and there should be little tilting (Fig. 2).As can be seen, a well-shaped palatal vault can be achieved by appropriate grinding of the palatal acrylic during expansion. This is difficult to control without palatal coverage

This gentle widening of the maxilla impinges on all the surrounding mid-face sutures and bones often creating a visible change in appearance (Fig. 3). If force orsurgery is used to assist maxillary expansion there is a 30.3% chance of producing differential craniofacial changes and a risk thatsome sutures will split while others may not, risking distorted facial deviations.(Kyung-A Kim et al 2019).

In 1983, follow up research was published,showing thatsemi-rapid expansion with Biobloc appliances wasrelatively stable in youngsters, although the expansion was less at that time. (Mew 1983).

During rapid expansion the midline suture separatestoo quickly for new bone to be able to fill the space,so a black area ofscar tissue will be seen on X-rays between the right and left maxillae, also the fracture of the interlocking bones risks of damage to the suture itself. Within nine monthsto a year this void should be filled with new bone. Expansion at the semi-rapid rate, isslow enough for new bone to fill in this area so that no black space appears (figure 4) which may account for its increased stability. This X-ray shows that the unerupted teeth have moved apart suggesting that in children expansion semi-rapid expansion separates the suture without harm.


18 adult patients (age range 22 to 49). Selection was from two practices using Stage 1 Biobloc appliances (Fig. 1) and restricted to only two factors, an intermolar distance of lessthan 36.5 millimetres, and an age of over 21. All patients were opened at the semi-rapid rate. There was no control group used. The age and the amount ofsemi-rapid expansion wasrecorded but asthis was a retrospective study, impressions were not taken at all points.

Measurements were taken by three trained observers using callipersto record the minimal width between the first molars at the level of the free gingival margin on stone models before and after expansion and then averaged. Tilting was measured in full degrees at a tangent at the mid-point of the buccal curvature of the first molars on each side (Fig. 6). Many of these measurements were identical between the observers. Tilting was often greater on one side than the other, so measurements were taken by the same three observers to the nearest full degree on both sides and then added together and then halved to give a mean.


The first molars of these 18 adults were widened by an average of 7.03 millimetres. In a narrow arch the shape of the vault is very different from a wide one. Ifsurgery is used to weaken orseparate the sutures or if ‘temporary attachment devices’ are fixed in the bone then the vault is likely to becomes distorted. When using the Biobloc Stage 1 the vault should be recontoured both before (Fig. 5) and during expansion.This will avoid palatalsores and ensure an appropriately shaped vault. (Fig. 2)

Figure 2
Molars hardly tilted after 14.5mm of semi-rapid expansion. Improved shape of palate after removable orthotropic expansion.  
Figure 4
Age 5 Before expansion (left) After four months (right) Nine Months later (center)  
Figure 6
Measuring expansion and tipping. First, place a straight line just touching the tips of the cusps of the teeth. Second, mark halfway between gum margin and cusp tip on each side. Third, use a protractor to measure the angle between the orange and green lines. Add the two angles together and halve.
Figure 3
Ben age 8 ; Eighteen months later  
Figure 5
Recontouring the palate for a patient
with a high arch (right side)
Figure 7
Cary Age 20 ; 18 months later. “My friends don’t recognize me.”


The increased width between the first molarsresulted from a combination of the tilting of the teeth, the tooth movement within the bone and some widening of the mid palatalsuture. The first molarstilted 3.89 degreessome of which was desirable asseveral molars were tilted lingually before treatment (Fig. 3). There was little sign of gingival recession but the roots tilted inwards while the crown tilted outwards, so the four degrees of tilt probably represents less than two millimetres of lateral movement, leaving about five millimetres due to the combined tooth movement and palatal widening. Monson in the late nineteenth century suggested that there should be a natural bilateral curve of the occlusion so that the cusps conform to a segment of a globe. Most of these cases were near vertical before treatment so some tilting might seem beneficial. It is difficult to estimate how much of the estimated 5mm widening was due to either dental orsutural movement. However, it was constantly found that there wassome discomfort and occasional ulceration between the fifth and seventh week which did not occur during the remainder of the expansion. This suggests that the majority of the widening was because central suture separated at this time. In effect the precise ratio is irrelevant although it seems certain that some was sutural.
It would seem that Semi-rapid expansion can widen the centralsuture and dental arch of adults using slow, intermittent light forces. Five millimetres must involve substantial changes to the adjacent and distant sutures which may explain the changes to the appearance of the mid-face (Fig. 7). The Stage 1 Biobloc appliance (Fig. 1) has many other objectives outside the scope of this article, including 1/ a space to allow the tongue to maintain contact with the palatal rugae during expansion, 2/ Catenary wires to create a correct arch form and avoid the need for subsequent fixed appliances. 3/ Proclination of the upper incisors, to encourage forward remodelling of the whole maxilla. 4/ Sloping ‘shelves’to centralise the mandible and encourage it to remodel forward 3 to 5 millimetres.


The null hypothesis is disproved, Semi-rapid expansion with the Stage 1 Biobloc appliance appears able to widen the mid-palatal suture and dental arch of many adults as well as children. We would like to express our gratitude to Dr Naraliya Gryb and Anastasia Yanatieva for their assistance with the measurements.


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