Dental Arch Growth and Development

• It is necessary to distinguish changes induced by appliance therapy from those that occur from natural growth.
• Moorrees1 has pointed out that considerable individual variation in arch form will occur with normal growth, with a general tendency toward an increase in the intermolar width during the changeover from the deciduous to the permanent dentition. It is difficult therefore to predict the growth potential in individual patients, but information is available on the average changes in arch dimension in untreated samples.
• It is apparent that changes in arch width vary between males and females and that more growth in width occurs in the upper than the lower arch; this growth occurs mainly between the ages of 7 and 12 years of age and is approximately 2 mm in the lower arch and 3 mm in the upper. After the age of 12, growth in arch width is seen only in males.
• Sillman2 looked at 65 normal white children from birth to over 25 years of age, including those with malocclusions, and found a progressive increase in arch width especially in males. Sinclair et al have confirmed that the increase in molar width after the age of 12 is statistically different in males and females, without an increase in arch length or perimeter. Arch width continues to increase to a lesser extent in the third and fourth decades, but this is associated with arch length shortening.


• It seems therefore that:
• Male arches grow wider than female arches.
• The lower intercanine width increases significantly in the changeover dentition but does not increase in the permanent dentition after 12 years of age.
• The upper and lower intermolar widths increase spontaneously to a considerable extent between ages of 7 and 18 especially in males.
• Little change in arch width occurs in the premolar region after the age of 12.
• Changes in arch width may not be accompanied by changes in arch length; there is a tendency toward a decrease in arch depth in the third and fourth decades.

Richardson and Gormley , EJO 19983 stated that changes in lower dental arch alignment and dimensions in untreated subjects during third decade of life are very small and in most cases undetectable clinically. There is not enough evidence to incriminate third molars as being the only or even major etiologic factor in the late lower dental arch crowding. (Sidalauskas et al, 2006)4

Olav Bondevik et al, EJO 20075 found that there was no significant difference between high and low angle subjects except for lower arch length in the females, which means that it is not necessary to differentiate between low- and high-angle males concerning the length of the retention period if treated orthodontically between 23 and 33 years of age. For females, there is a greater necessity for retention in high-angle subjects, if there is a risk of crowding.

APPLIANCE-INDUCED GROWTH

• McNamara and Brudon6 stated that “It seems logical to consider increasing arch size at a young age so that skeletal, dental alveolar, and muscular adaptations can occur before the eruption of the permanent dentition.” If an appliance is inserted in an actively growing patient, a favorable response can be expected. However this response may have occurred in the absence of treatment, the relative contribution of the appliance being difficult to determine.
• Spillane and McNamara7 examined the records of those patients in the Michigan study who presented with narrow arch forms and compared them with the average in the sample.
• In the untreated sample of 74 cases aged 7 to 15 years, the average increase in transpalatal width between the upper first molars was 2.6 mm; in the narrow arch group, growth was 3.3 mm. The implication is that those with initially narrow arch forms tend to become more average, and appliance therapy is therefore more likely to achieve a stable change.
• The routine expansion of both arches in the permanent dentition has not been shown to produce stable change in arch dimension that is significantly greater than that which would have been achieved through normal growth.

CORRECTION OF CROSSBITES

• Selwyn-Barnett8 hypothesized that the lower dentition can be induced to occupy space previously occupied by the upper incisors in Class II Division 2 cases without encroaching on the lips. It could be postulated similarly that correction of a crossbite with expansion of one arch and contraction of the other could be achieved without encroaching on the lips and cheeks.
• An occlusion with a scissor bite will occupy a wider bucco-lingual space than a normal one, and a potential exists for the space of the upper dentition to be transferred to the lower dentition. Such a transfer would be more likely to occur in a growing individual where the vertical tooth movements required can be more readily achieved.

ANTEROPOSTERIOR MOVEMENTS

• Arch width changes must not be considered in isolation, and the interdependence of the arches has significant implications in arch width. Arch width modification is most frequently seen in distal movement of the upper arch or forward movement of the lower arch; the upper arch is expanded in either case to accommodate the lower arch.
• A long-term study by Sadowsky9 et al of 22 cases over 6.3 years after nonextraction treatment reported a mean expansion of 2.7 mm between the upper first premolars and 4.5 mm between the upper molars. This was accompanied by a lesser expansion in the lower arch of the premolars and molars by 1.6 mm and 2.9 mm, respectively.
• An article by Glenn et al10 contrasted the effects of nonextraction treatment in 14 Class I and 14 Class II cases at least 3 years out of retention. In both groups, the intercanine width relapsed to its pretreatment value; only the Class II sample showed a stable expansion in the intermolar width—effectively a 1 mm expansion of the lower first molars .
• Moussa11 et al considered the effects of rapid maxillary expansion and non extraction edgewise therapy in a mixed sample of malocclusions of 55 patients on average 6 years out of retention. An average expansion of 2.7 mm and 4.6 mm for the upper canines and molars was maintained, with 0.4 mm and 2.5 mm maintained for the lower canines and molars.
• It would seem that in the correction of Class II malocclusion, approximately 3 mm stable upper molar expansion can be achieved; the lower molar equivalent would be around 1 mm beyond that which might have been expected with growth. The degree of expansion achieved will be affected by the type of malocclusion.

SAYINSU ET AL , EJO 200512

• Compared pre- and post-treatment width changes in the canine, premolar and molar regions in subjects treated with extraction of four first premolars, non-extraction, and non-extraction with rapid maxillary expansion (RME). The results revealed that the distance between the upper canines was not affected by the treatment modality. Upper premolar and molar arch widths increased more in the non-extraction subjects when compared with those with extractions, with the greatest increase in patients with RME. In the lower canine area the extraction group showed the widest arch width at the end of treatment. There was also a 0.60 mm decrease in the lower canine width in the non-extraction group. A decrease was found in lower inter-premolar and molar distances due to consolidation of the extraction spaces.

MUTINELLI ET AL , EJO 200813

• Expansion of the maxilla using a Haas appliance anchored to the primary teeth resulted in a significant increase in transverse width. The most stable result was in younger patients (first transitional period) with a lateral crossbite.

LOCAL FACTORS

• Although the lower intercanine width in general will not withstand expansion, inevitably individual cases will exist where expansion can be maintained. In extraction cases, there is generally associated crowding with the canines frequently being buccally displaced. Retraction of these canines into extraction sites is not generally associated with stable expansion; their buccal position is likely to have been induced by crowding and their retraction into the space of the first premolars does not seem to allow for stable expansion.
• Shapiro14 noticed a difference in the varying malocclusions, his Class II Division 2 sample showed potential for slight increase in the lower intercanine width. This factor is likely to be related to the intrusion of the upper incisors and canines described by Selwyn-Barnett,8 the space occupied by 3|3 is vacated and made available for accommodation of expanded lower canines. Conversely, extraction will reduce arch width. Thus, any expansion carried out in association with dental extractions is not generally likely to be a stable change.It is therefore essential to discriminate between extraction and non extraction cases when assessing arch width and stability.
• A stable expansion of the molars might be expected in the Class II correction cases where an anteroposterior movement was likely to have occurred (Gardner & Chaconas)15
• A prospective study was carried out to consider changes achieved from treatment. The sample of 63 cases were all treated with identically shaped and sized arch wires regardless of the presenting arch form or extraction pattern. Twenty of these cases were treated without extraction, 26 with extraction of lower first premolars, and 17 with extraction of lower second premolars.The study found that the arch wires achieved full expression in arch form in the lower anterior region and almost full expression in the posterior region. Forty-six of these cases were subsequently examined 1 year or more after removal of all retention appliances. The mean changes were compared between extraction and non extraction samples. In the nonextraction sample, the arch wires were found to have little effect in the intercanine or intermolar width that were effectively unchanged and stable. Some expansion of the lower inter first premolar width was maintained, a mean of 1.21 mm, and the lower second premolar width was increased to a lesser extent, 0.93 mm.In the extraction sample, the effect of the arch wires was to produce a small mean expansion in the intercanine width, which relapsed, and a contraction of the lower intermolar width, which remained stable. It would seem therefore that expansion of the lower arch is likely to be stable only in the absence of extraction. Expansion of the lower arch is also more likely to be achieved when anteroposterior movement of the arches has been undertaken, with a potential for even greater expansion in the upper arch.
• Bishara16 et al examined 30 first premolar extraction cases that were on average 1.2 years out of retention. They reported that 71.4% of any expansion produced in the lower intercanine width resulted in relapse, with less likelihood of relapse of the upper. Subsequently, Little et al reported on the long-term follow-up of 65 cases with extraction of first premolars. The lower intercanine width had been increased by more than 1 mm during treatment in 60% of the cases but, after treatment, constriction in the intercanine width occurred in 60 of 65 cases, usually more than 2 mm.
• Kahl Nieke et al (AJO 1996)18 in a long-term follow-up study of orthodontically treated patients analyzed post retention changes in arch width dimension and evaluated factors that may serve as predictors of long-term prognosis.
• The findings indicate that post retention arch width relapse occurred more frequently in the upper intermolar (25.8%) and lower intercanine region (23.9%) than in the lower intermolar (19.0%) and upper intercanine (13.8%) region. Pretreatment and post treatment alignment as well as the kind of treatment and the amount of expansion were found to be influencing factors.
• In AO 1998, Burke et al19 used meta-analysis to review 26 previous studies of mandibular inter-canine width. They concluded that regardless of patient-diagnostic and treatment modalities, mandibular inter-canine width tends to expand during treatment by about 1 or 2 millimeters, and to contract post-retention to approximately the same dimension.

Hirano miyake et al , AO 200820

• Twenty-six patients with Class I crowding who attained a favourable occlusion after treatment with preadjusted appliance were divided into non extraction and extraction groups and found that the upper dental arch might become tapered after treatment used concomitantly with premolar extraction, as a result of the increase in anterior length while maintaining intercanine width for dealing with crowding of the anterior teeth.


• Heiser et al , AJODO 200421 compared 25 non extraction and 24 extraction subjects. All casts were digitized and were divided into 4 areas as A, B, C, D. Coordinates were defined.


• In the maxilla, arch length between distobuccal cusp of maxillary molars (UL3) increased in nonextraction patients; all other lengths in both arches and both groups decreased.
• Area A increased in the maxillary arches in both groups, whereas in the mandibular arch, it decreased. From bracket removal to follow-up, all areas in both groups decreased. The relapse tendency was less in the maxillary arch than in the mandibular.
• The extraction group showed similar relapse tendencies in both arches. Wellens et al , EJO 20022 A significant increase in intercanine distance (0.6 mm) was found when distalizing the canines into a wider part of the mandibular alveolar process during the correction of crowding using the leeway space.

BUCCAL CORRIDOR SPACE

Dustin Roden-Johnson et al, AJODO 200523

• An attractive, well-balanced smile is a paramount treatment objective of modern orthodontic therapy.
• Photographs of 20 women treated by 2 orthodontists were collected: 1 group had narrow tapered or tapered arch forms, and the other had normal to broad arch forms. Photographs of 10 untreated women served as a control sample.
• All photographs showed the subjects smiling. The photographs were digitized and evaluated for Buccal Corridor Space. Then, photographs with Buccal Corridor Space were altered to eliminate the dark triangular areas, and those without Buccal Corridor Space were altered by the addition of dark triangular areas at the lateral aspects of the smile. The altered photographs were randomized into a survey with the 30 original photographs .Three groups of raters (dentists, orthodontists, and laypeople) used a visual analogue scale to rate the photographs. There was no significant difference in smile scores related to Buccal Corridor Space for all samples and for all viewers. Dentists rated broader arch forms as more esthetic than untreated arch forms. Orthodontists rated broader arch forms as more esthetic than narrow tapered arch forms and untreated arch forms. Lay people showed no preference of arch form.
• In a study done by Gandhi and Valiathan24, on arch width changes in canine and premolar region after extraction and non extraction treatment , it was concluded that the notion that extraction treatment will result in narrower dental arches is not justified.

ALTERED MUSCLE FUNCTION

• The dentition is established in a position of balance between the muscles of the lips and cheeks and the pressures of the tongue. Although the forces are not equivalent, the general effect of intermittent forces by the tongue and resting forces of the cheeks are likely to result in the final positioning of the teeth.
• The soft perioral tissue stretching hypothesis formulated by Solow and Kreiborg25 can explain how the resting muscular activity depends on the head posture in relation to the vertebral column.
• According to this hypothesis, the soft tissue layer (skin, muscles, and fascia) that covers the head and neck, stretches and relaxes itself in relation to the degree of extension or flexion of the head. In cases of long-term hyperextension of the head posture, these soft tissues stretch, creating a dorsal and caudal force against the teeth and skeleton leading to crowding.

Parkkinen et al, EJO 200926 reported children with diagnosed OSA had a significantly increased overjet, a reduced overbite, and narrower upper and shorter lower dental arches when compared with the controls. There were more subjects with mandibular crowding.

RACIAL DIFFERENCE IN ARCH FORMS

• Thota Gopinath and Valiathan Ashima (JPFA 1992)27 studied 20 Indians and 20 Chinese samples to evaluate differences in intercanine, interpremolar and intermolar width. They found that Chinese have wider dental arches at all the three areas of measurement. This study confirms that ethnicity will influence size of maxillary and mandibular arches.

GENDER DIFFERENCE IN ARCHFORMS

• Valiathan et al (J Int Coll Dent 1999)28 selected 50 Chinese and 50 Indian samples to compare the intercanine and intermolar widths between boys and girls within the same population and also between the two populations. From the results authors concluded that the overall Chinese had a significantly wider arches in upper intermolar region , lower intermolar region and lower intercanine region compared to Indians for both male and females comparisons. Within the same population comparison of arch widths showed that Indian males have significantly wider arches compared to Indian females. In Chinese population arch width comparison did not show any significant sexual dimorphism.
• Heritability estimates of arch form: Kevin M. Cassidy (1996)29 studied pretreatment dental casts of 155 siblings consisting of a total of 320 adolescents. Results disclosed significant sexual dimorphism for all the linear variables studied; boys were larger than girls. Significant sexual dimorphism also was present for arch form, where girls presented with a more square arch form than did the boys. .
• This study agreed with prior research that concluded that bone-based arch size and shape are moderately controlled by genotype, whereas tooth based measurements are almost all environmentally controlled.

CONCLUSIONS

1. The use of expansion can be considered applicable in the growing child. However, it is difficult to predict the degree to which this would have occurred from natural growth in any individual. There is no evidence that appliances can stimulate “growth” beyond that which would occur normally.
2. The presence of crossbites will affect the space available within the arches. The correction of such crossbites is generally achievable by a combination of arch expansion and contraction, intercuspation of teeth, and potential alteration in the vertical resting position of the tongue.
3. Arch expansion is more likely to be stable in the absence of extractions and is most effective in the posterior region. There is unlikely to be stable expansion of the lower intercanine width unless the canines are displaced lingually by the occlusion.
4. Expansion of the arches posteriorly can be achieved more readily where anteroposterior movement of the arches also takes place. An upper arch moved distally is likely to accommodate expansion and, in conjunction with this, some expansion of the lower arch may be achieved to a lesser extent.
5. Each individual should be assessed to identify any factors that might allow expansion either of the whole arch or of individual teeth. However, the space gain may be less than expected, with expansion posteriorly allowing a decrease in the arch depth of approximately one third the amount of expansion achieved.

REFERENCES