Mandibular Arch Length Research Articles

Evaluation of orthodontic treatment after 1 year of retention--a randomized controlled trial

The aim of this study was to use a randomized controlled trial methodology to evaluate and compare three different retention methods. The capacity of the retention methods to retain orthodontic treatment results was in this first phase analysed on a short-term basis, i.e. after 1 year of retention. The subjects were recruited from adolescents undergoing fixed appliance treatment at an orthodontic clinic in the National Health Service (NHS) in Sweden between 2001 and 2007. Seventy-five patients (45 girls and 30 boys with a mean age of 14.4 years at the start of retention) were randomized into three retention systems; a vacuum-formed retainer in the maxilla and bonded canine-to-canine retainer in the mandible (group V-CTC), a vacuum-formed retainer in the maxilla combined with stripping of the 10 proximal surfaces of the lower mandibular anterior teeth (group V-S), and a prefabricated positioner covering the teeth in the maxilla and mandible (group P). The main outcome measures were: Little's irregularity index (LII), intercanine and intermolar width, arch length, overjet, and overbite. Registrations were made before orthodontic treatment, when the fixed orthodontic appliance was removed, and after 12 months in retention. Differences in means between groups were tested by one-way analysis of variance. After 1 year of retention, no clinically significant difference in retention capacity was found between the three retention methods. Small but significant differences (P < 0.05) were observed between the V-CTC and V-S groups regarding mandibular canine width, mandibular arch length, and overbite. In group P, two patients failed to co-operate.

Clinical challenges in treating a patient with deviated dental midlines and delayed root development of the mandibular left second premolar

This case report describes the treatment of a girl, aged 13.3 years, with a concave face and a Class I relationship on the right and Class III on the left at the first molars. The left canines were in full Class II occlusion. The overbite was excessive at 80%. The maxillary dental midline was deviated 2.0 mm to the right, and the mandibular dental midline was 3.0 mm on the opposite side with a 5-mm deviation between both arches. There was a -2-mm mandibular arch length discrepancy, but, considering the intrabony left second premolar with root-delayed development, the arch length discrepancy was -8 mm. The incisors were also retroclined. A nonextraction treatment plan was used. When space to the left of the intrabony second premolar was opened, the tooth erupted passively, and the root had a normal length. The esthetic results were functional and pleasing.

Dental arch dimensions in oligodontia patients

The aim of this study was to compare the dental arch sizes in a group of oligodontia patients with a group of age- and sex-matched untreated Class I control patients. Dental casts were measured of 50 oligodontia patients and 50 untreated Class I patients between 8 and 16 years old. For each subject, dental arch length and width parameters were measured in the maxillary and mandibular arches with digital calipers. The various dental arch parameters were compared between the 2 groups by using ANOVA. The mean age of the subjects was 12.5 years. The average number of teeth missing in the oligodontia group was 11. The mean maxillary and mandibular arch lengths in the oligodontia group were reduced by 4.40 and 2.80 mm, respectively, when compared with the control group. The intercanine widths were reduced in the oligodontia group compared with the control group by 2.82 mm in the maxillary arch and 2.70 mm in the mandibular arch. The intermolar widths of the maxillary and mandibular arches in the oligodontia group were reduced by 3.40 and 1.80 mm, respectively, compared with the control group. These differences were statistically significant (P<0.01). The findings indicate that arch length and width are reduced in oligodontia patients; this should be taken into account when planning orthodontic treatment.

Long-term effectiveness of the continuous and the sectional archwire techniques in leveling the curve of Spee

The correction of a deep overbite with the subsequent achievement of long-term stability is a difficult problem for orthodontists. The role of leveling the curve of Spee (COS) in bite opening and the success of orthodontic treatment has been well documented in the literature. The aim of this study was to investigate whether leveling the COS, by using 2 orthodontic treatment techniques, produces stable results on a long-term basis. We compared the long-term stability of leveling the COS with the straight-wire Alexander technique and the bioprogressive sectional-arch technique. The randomly selected subjects for this retrospective study were obtained from the private practices of Drs R. G. Alexander and Ruel Bench. Study models taken 2 months before treatment (T1), 2 months after treatment (T2), and after retention (T3) were evaluated. Measurements of the COS were made on the mandibular casts with a commercially available palatometer. Mandibular intercanine width, overbite, overjet, mandibular incisor irregularity, and mandibular arch length were also recorded. Both techniques produced highly significant reductions in the COS (T1 to T2). Statistically significant, but clinically insignificant, postretention relapse of the COS occurred (T2 to T3). For both techniques, a statistically significant difference was seen in the incidence of the relapse of the COS between patients who were completely leveled posttreatment and those who were not. We did not find a correlation between pretreatment COS and relapse in any of the other occlusal traits studied.

A descriptive study of mandibular incisor alignment in untreated subjects

The development of mandibular incisor crowding appears to be a continuous process throughout life, but more evidence is needed to understand why changes occur. In this study, we describe the longitudinal dental changes in untreated children (n = 15) who had records at 3 times and in an untreated adult group (n = 18) (parents) who had records for 2 times. The mean numbers of years between initial and final observations were 29.8 years for the children and 33.7 years for the parents. All subjects were participants in the original Burlington Growth Research Project at the University of Toronto. The following variables were measured to an accuracy of 0.01 mm: overjet, overbite, mandibular intercanine width, mandibular interfirst premolar width, mandibular intermolar width, mandibular arch length, Little's incisor irregularity index, mandibular anterior space analysis, and Carey's space analysis. There were no statistically significant (P >.01) differences between the sexes for the variables measured. Little's irregularity index continued to increase in all groups (P <.01), although this rate appeared to be lower in the parent group. Intercanine and interfirst premolar widths and arch lengths continued to decrease with age. These results underline the importance of studies showing that untreated dentitions change over time. Orthodontic patient education is imperative about retention protocols and late developmental crowding.

Consideration of the Mechanisms Involved in Dental Crowding-Comparison of Dentition Growth Changes in Children of Two Primary Schools during Two Different Eras-

The aim of this study was to elucidate the mechanisms involved in dental crowding by comparing dentition growth changes in children of two primary schools during two different eras. Dental casts obtained from primary school students aged 6-7 and 11-12 year-olds for the periods of 1975-1982 [Asahi Primary School (APS), Ito City, Shizuoka] and 1997-2002 [Kogasaki Primary School (KPS), Matsudo City, Chiba] were studied. The materials were classified into a group in which mild dental crowding observed (arch length discrepancy (ALD) less than -4.0 mm) at the age of 11-12 year-olds (N-group), and a group in which dental crowding was severe (ALD more than -4.0 mm) (E-group). Dental casts were scanned with a contact type 3D scanner. Arch sizes and inclination of the first molars were measured. Maxillary and mandibular arch lengths of the 6-7 year-olds in KPS were significantly longer than those in APS. There was no significant difference in palatal width and arch width between APS and KPS. Comparison of tooth sizes between the N-group and E-group showed no significant differences in size of all permanent teeth. In APS, maxillary arch length was significantly longer in N-group than in E-group. On the contrary, there were no significant differences in arch width and B-L inclination of mandibular molar between N-group and E-group. In KPS, arch widths of maxilla and mandible in 11-12 year-olds were significantly larger in N-group than in E-group. B-L inclination of mandibular molar in 11-12 year-olds was significantly smaller in N-group than in E-group. In N-group, maxillary arch length of 6-7 year-olds was significantly shorter in APS than in KPS, whereas maxillary arch width of 11-12 year-olds was significantly larger in APS than in KPS. In E-group, both maxillary and mandibular arch lengths of 6-7 year-olds were significantly shorter in APS than in KPS. Palatal width and maxillary arch width of 11-12 year-olds were significantly larger in APS than in KPS. Bucco-lingual inclination of mandibular molars of the 11-12 year-olds in APS was significantly smaller than that in KPS. The dmft rate of 6-7 year-olds was higher in APS than in KPS. The highly progressed caries tooth rate was also higher in APS than in KPS. In conclusion, the background of dental crowding is different between the two eras. In 1980 (APS), the cause was mesial movement of the first molar due to highly progressed caries. On the contrary, in 2000 (KPA), the cause was narrowing of the dental arch due to reduction of palatal growth and lingual inclination of mandibular first molar.

Predicting first molar width using virtual models of dental arches

In dentistry, large regions of dentition often are restored with minimal information about the original anatomy. The ability to predict missing anatomy from existing anatomy would aid such restorations. This study investigated the relationship between first molar mesial-distal width and arch shape using newly defined reference points and three-dimensional (3D) digital methods. Full-mouth dental stone casts from 167 dental students were scanned and rendered as 3D virtual models. Maxillary and mandibular arch lengths and widths and first molar mesial-distal widths were measured on the virtual models using new definitions incorporating virtual planes. A linear mixed model of the first molar width regressed on the other measurements was done. Intraobserver reproducibility was evaluated by means of intraclass correlation (ICC) and standard deviation of measurement error (SDME). All measured distances were averaged as a combined group and as gender groups. The correlation coefficients between the maxillary and mandibular first molar widths were over 0.70 (P < 0.01). Intraobserver error was small. ICCs were over 0.92 and SDMEs were from 0.11 to 0.21 mm. Arch dimensions and first molar widths were defined and measured. Regression equations were calculated for predicting first molar width. The prediction of first molar width using arch dimension is essential for virtual designing of missing first molars. This approach also provides reliable reference point definitions for the virtual dental model which was impossible with traditional measurement methods. Therefore, this study would be helpful for understanding the 3D anatomy of dental arch and fabrication of automatic prosthodontic restorations.

Long-term stability after orthodontic treatment remains inconclusive

PubMed and Cochrane Library electronic databases were searched from 1966 to January 2005. Reference lists of the relevant studies were searched by hand. For inclusion, articles had to satisfy the following criteria: have a follow-up period of at least 5 years post-retention; report on randomised clinical trials (RCT), prospective or retrospective clinical controlled studies or on cohort studies; and investigate orthodontic treatment including fixed or removable appliances, selective grinding or extractions. Screening of eligible studies, data extraction and assessment of methodological quality was conducted independently and in duplicate. The level of evidence of the included studies was graded as high, moderate or low according to predetermined criteria. The search strategy resulted in 1004 abstracts or full-text articles, of which 38 met the inclusion criteria. Treatment of crowding resulted in successful dental alignment but the mandibular arch length and intercanine width gradually decreased, and crowding of the lower anterior teeth reoccurred post-retention. This condition was unpredictable at the individual level. Treatment of Angle class-II division-1 malocclusion with a Herbst appliance normalised the occlusion. Relapse occurred but could not be predicted at the individual level. The scientific evidence was insufficient to draw conclusions about treatment of cross-bite, angle class III, open-bite and various other malocclusions, or about long-term patient satisfaction. Despite a large number of studies on long-term stability after orthodontic treatment, this systematic review shows that evidence-based conclusions are few. This is mostly because of inherent problems with retrospective and inferior study design. There is a great need for well-designed prospective studies with untreated control groups; sufficient sample sizes; and sample selection according to type of malocclusion, age and growth pattern.

Differences between high- and low-angle subjects in arch form and anterior crowding from 23 to 33 years of age

The aim of this study was to compare changes in arch width and length in high- and low-angle subjects. The material comprised the cephalograms and study models of 17 males and 15 females with a high-angle and 12 males and 16 low-angle females. The age of the subjects at the first examination (T1) was 22 years 10 months and 22 years 8 months in the high-angle groups, 22 years 5 months and 22 years 8 months in the low-angle subjects, respectively, for males and females. The second assessment (T2) was 10 years and 9 years 10 months later in the high-angle and, 10 years 3 months and 9 years 10 months in the low-angle groups, respectively. Statistical analysis of changes for each group during the observation period T1-T2 was undertaken using a paired t-test, and the differences between the groups with two-sample t-tests. The findings showed a general reduction in upper and lower arch length, an increase of intermolar width, and a decrease of intercanine width in both genders. Anterior perimeter and anterior space in both angles decreased in both males and females. The mean changes were small, but with large individual differences. There were no significant differences between the genders or the high- and low-angle groups in the changes, except that the female high-angle group had a statistically significant (P = 0.05) greater decrease in mandibular arch length than the low-angle female group.

Long-term Stability of Orthodontic Treatment and Patient Satisfaction

To evaluate morphologic stability and patient satisfaction at least 5 years after orthodontic treatment. Published literature was searched through the PubMed and Cochrane Library electronic databases from 1966 to January 2005. The search was performed by an information specialist at the Swedish Council on Technology Assessment in Health Care. The inclusion criteria consisted of a follow-up period of at least 5 years postretention; randomized clinical trials, prospective or retrospective clinical controlled studies, and cohort studies; and orthodontic treatment including fixed or removable appliances, selective grinding, or extractions. Two reviewers extracted the data independently and also assessed the quality of the studies. The search strategy resulted in 1004 abstracts or full-text articles, of which 38 met the inclusion criteria. Treatment of crowding resulted in successful dental alignment. However, the mandibular arch length and width gradually decreased, and crowding of the lower anterior teeth reoccurred postretention. This condition was unpredictable at the individual level (limited evidence). Treatment of Angle Class II division 1 malocclusion with Herbst appliance normalized the occlusion. Relapse occurred but could not be predicted at the individual level (limited evidence). The scientific evidence was insufficient for conclusions on treatment of cross-bite, Angle Class III, open bite, and various other malocclusions as well as on patient satisfaction in a long-term perspective. This review has exposed the difficulties in drawing meaningful evidence-based conclusions often because of the inherent problems of retrospective and uncontrolled study design.

Mandibular Incisor Extraction Treatment of a Class I Malocclusion with Bolton Discrepancy: A Case Report

MANY APPROACHES FOR CROWDED MANDIBULAR ANTERIOR TEETH ARE CURRENTLY EMPLOYED: distal movement of posterior teeth, lateral movement of canines, labial movement of incisors, interproximal enamel reduction, removal of premolars, removal of one or two incisors, and various combinations of the above. Selecting the best treatment is often difficult, and all guidelines do not apply to every case. Treatment by extraction of one single mandibular incisor is not popular in the orthodontic profession despite the apparent advantages of the extraction in the region of crowding. A case report is presented one mandibular incisor extraction treatment of a 16 year-old female with a Class I malocclusion that shows a significant mandibular arch length deficiency and mandibular tooth-size excess. In this case, the degree of mandibular anterior dental crowding, existing mandibular tooth-size excess, and the dental midline discrepancy were indicated the extraction of one mandibular incisor.

Estabilidade pós-contenção das alterações da forma do arco inferior na má oclusão de Classe II de Angle tratada com e sem a extração de pré-molares

OBJETIVO: o objetivo desse estudo foi comparar o comportamento da forma do arco inferior durante as fases de tratamento e pós-contenção, em pacientes portadores de má oclusão Classe II de Angle, que haviam se submetido ao tratamento ortodôntico com extrações de dois e de quatro pré-molares, portanto com e sem extrações no arco inferior. METODOLOGIA: foram selecionados 66 pacientes que apresentavam inicialmente uma má oclusão de no mínimo meia Classe II, e que foram tratados com uma das combinações de extrações propostas. Esses pacientes foram divididos em dois grupos, sendo o grupo 1 composto de 19 pacientes tratados com extração de dois pré-molares superiores, com idade média de 14,04 anos; e o grupo 2 composto de 47 pacientes com extração de quatro pré-molares, com idade média de 13,03 anos. Para verificação das diferenças entre os grupos, mediu-se os modelos referentes às fases pré-tratamento, pós-tratamento e, em média, cinco anos após o final do tratamento ativo. As alterações das dimensões do arco inferior (distância intercaninos, intermolares e comprimento de arco), entre as três fases, também foram comparadas. RESULTADOS: os resultados demonstraram que diferenças estatisticamente significantes durante o tratamento se devem à opção por extrações no grupo 2. Não houve diferença entre os dois grupos para as mudanças pós-contenção no comprimento e na largura do arco inferior. CONCLUSÕES: pode-se esperar o mesmo grau de recidiva pós-contenção da forma do arco inferior em pacientes com má oclusão de Classe II, quando o tratamento é conduzido com extrações de dois pré-molares superiores, ou quando realizado com extrações de quatro pré-molares.

Thesis abstract: Spontaneous mandibular arch response following rapid palatal expansion: a long-term study on Class I malocclusion

The purpose of this study was to investigate the spontaneous mandibular arch response to rapid palatal expansion (Haas type) as the sole orthodontic intervention. Particular attention was paid to clinically significant effects and long-term (x = 11.3 years) posttreatment outcome stability. This study differs from other published reports on the effects of rapid palatal expansion on the mandible in that many of them were of shorter duration, or the associated treatment protocols also involved the use of orthopedic and orthodontic fixed appliances. Data were obtained from orthodontic records including dental casts taken at each of 4 assessment stages for 30 patients (18 female, 12 male). All were treated without extractions and had Class I malocclusions and transverse maxillary/mandibular skeletal discrepancies treated during the early or mid mixed dentition. Posterior and anterior mandibular arch width, arch length, and arch perimeter were measured directly on study casts obtained at pre-expansion (x = 8.2 years), short-term follow-up (x = 9.4 years), progress (= 13.2 years), and long-term follow-up (x = 20.7 years). Descriptive and inferential analyses of the 6 variables at the 4 assessment stages for all patients were performed. Differences between pre-expansion and short-term follow-up resulted in statistically significant (P < .001) increases for mandibular intermolar widths (lingual and occlusal values). During the transition to the permanent dentition, expected significant (P < .001) decreases occurred in arch length and arch perimeter. Also in this period, there was a decrease of 25% from the initial net gain for intermolar width (occlusal value), but the lingual values remained unchanged. Further differences between the short-term and the long-term follow-ups resulted in a mandibular intermolar width (occlusal value) net gain of 50% and the expected significant (P < .001) decrease for the mandibular arch length and arch perimeter. Finally, there was remarkable stability in the mandibular intermolar width (lingual value) and intercanine width (occlusal value), indicating that the increase in the width dimension in the mandibular arch was in response to the orthopedic effects of rapid palatal expansion in the early or mid mixed dentition and that the stability was maintained until adulthood. Based on the findings of this study, several statistically significant changes were noted in the mandible that could be attributed to rapid palatal expansion. No clinically adverse effects were observed. The long-term effects associated with this form of maxillary intervention were positive, especially for the mandibular arch-width dimensions.

Long-term stability of class i premolar extraction treatment

This study evaluates Class I, 4-premolar-extraction patients who were treated with the edgewise appliance by 1 practitioner, according to the philosophy of Tweed, and who had been out of retention a minimum of 5 years. The sample includes 32 patients, who started treatment at an average age of 12.8 years and who were examined a mean of 15 years posttreatment (11.7 years postretention). Cephalometric and model analyses were conducted to evaluate treatment and posttreatment tooth movements. The results showed that irregularity, as measured by the irregularity index, decreased 5.3 mm during treatment and increased 0.7 mm (SD 1.1 mm) during the posttreatment period. Eighty percent of the patients had satisfactory (<3.5 mm) mandibular incisor alignment over 10 years postretention, and none was in the severe category (>6.5 mm). Mandibular intercanine width increased (1.7 mm) during treatment, whereas intermolar width decreased (−2.1 mm). Maxillary molar widths remained unchanged posttreatment, and mandibular intercanine width decreased 1.4 mm from immediately posttreatment to postretention. Arch lengths decreased during treatment because of molar protraction and incisor retraction. Mandibular arch length continued to decrease posttreatment (−1.4 mm) because of mesial molar movement rather than distal incisor movement. Satisfactory long-term results can be achieved for most Class I, 4-premolar-extraction patients for whom evidence-based treatment objectives—including minimal alteration of the mandibular arch form and the retraction and uprighting or maintenance of mandibular incisors in their original position—have been met.

Tooth size-arch length relationships in the deciduous dentition: a comparison between contemporary and historical samples

In a recent study, it was reported that maxillary and mandibular arch lengths were significantly shorter in a sample of contemporary children in the deciduous dentition compared with a historical sample from about 50 years earlier. The purpose of this study was to describe secular changes that might have occurred in tooth sizes and tooth size-arch length relationships in the same cohorts of contemporary and historical North American white children in the deciduous dentition. The 2 samples were similar in terms of geographic location, racial and ethnic backgrounds, and socioeconomic status. Both samples were restricted to white children with a normal overjet (<4 mm) and a normal anteroposterior molar relationship, no anterior open bite, and no crossbite. In addition, subjects were excluded if any permanent teeth were erupted. Measurements of mesiodistal tooth sizes and arch lengths of maxillary and mandibular arches were made, and tooth size-arch length discrepancies (TSALD) were determined. The results indicated that tooth sizes were generally similar in the 2 cohorts but slightly larger in contemporary children. Crowding, as measured by TSALD, was found to be common in the mandibular arch for contemporary children in the deciduous dentition of both boys and girls. Moreover, crowding was much more common and severe in contemporary children compared with children in the historical cohort. Further research is needed to determine whether the increase in mandibular crowding in the deciduous dentition will continue to be observed in the mixed and permanent dentitions and to further establish these possible secular trends.

Natural craniofacial changes in the third decade of life: A longitudinal study

The aim of this study was to evaluate longitudinally the natural craniofacial and dentoalveolar changes that occur in the third decade of life. Natural head position lateral cephalometric films and dental casts of 30 people (14 women and 16 men) were evaluated. The mean age at the beginning of the observation period was 22.35 years for the women and 22.19 years for the men, and the observation period was approximately 10 years. Cephalometric films were superimposed by the structural method, and the measurements of the dental casts were made with a digital caliper. All tracings were digitized, and changes in the 65 cephalometric and 10 dental cast measurements were evaluated statistically. In this early adult period, small changes were found in the craniofacial and craniocervical parameters; the changes were more significant in the women. The most significant changes were found in the vertical dimension. The total anterior face height increased in both genders, while the lower anterior face height increased significantly in the female group. Soft tissue measurements reflected the vertical skeletal changes. The retrusion of the upper lip was significant in the women, and the upper lip thickness decreased in both genders. In the dentoalveolar region, the main movement was eruption of the teeth. The overbite amount increased significantly only in the female group. All dental arch measurements decreased in both sexes. The decrease in the mandibular arch length discrepancy was significant in the men. These findings have important clinical implications regarding the long-term stability and retention of orthodontic and orthognathic surgery treatment results. (Am J Orthod Dentofacial Orthop 2002;122:512-22)

A new method of evaluating posterior occlusion and its relation to posttreatment occlusal changes.

The purpose of this study was to assess the relationship between posterior occlusion and posttreatment changes in other occlusal variables. Pretreatment (T1), posttreatment (T2), and postretention (T3) records were obtained for 49 Class I (n = 23) and Class II (n = 26) extraction cases. Overbite, overjet, mandibular incisor irregularity, right and left molar deviations, midline deviation, and mandibular arch length were measured, and occlusal registrations were made of each set of dental casts. The proximity of posterior occlusal surfaces was measured as the contact or near-contact areas at or below 300 microm thickness, based on the optical densities of scanned images of the posterior occlusal registrations. The treatment changes for overbite, overjet, right and left molar deviation, and arch length were significantly greater in the Class II group, and the reductions in incisor irregularity were greater in the Class I group. No other class differences were found. Overbite, overjet, and incisor irregularity increased after treatment, and arch length continued to decrease, with no significant class differences. Contact and near-contact areas at or below 300 microm constituted 7% of the functional occlusal table for both classes at the end of treatment, having decreased significantly in both groups (21% in Class I and 29% in Class II) during treatment. Because contact and near-contact areas increased in some patients and decreased in others, there were no significant posttreatment changes. Negative correlations were found between contact and near-contact areas at T2 and changes in overjet from T2 to T3, and between contact and near-contact areas at T3 and changes in overbite from T2 to T3. No relationships were found between posterior contact and near-contact area and incisor irregularity. We concluded that (1) the area of actual and near contacts at or below 300 microm decreased significantly with treatment, indicating that, despite excellent treatment results by conventional standards, the proximity of posterior occlusal surfaces lessened; (2) the proximity of the posterior occlusal surfaces should not be expected to increase posttreatment; (3) posttreatment contact and near-contact areas may be factors in overbite and overjet stability; and (4) posttreatment contact and near-contact areas are not related to incisor irregularity.

Comparison of dental arch measurements in the primary dentition between contemporary and historic samples

The purpose of this study was to investigate secular changes that may have occurred in dental arch dimensions by comparing a sample of contemporary North American white children (born between 1992 and 1995) in the primary dentition to those children found in an earlier study of North American white children (born between 1946 and 1948). The sample of contemporary children has been followed prospectively since birth and was assessed at 4½ to 5 years of age. Data for the historic sample were obtained from 5-year-old children who were enrolled in the Iowa Growth Study. The 2 samples were similar in terms of geographic location, racial and ethnic backgrounds, and socioeconomic status. To further enhance the comparison and to match the 2 groups, the 2 samples were restricted to white children with a normal overjet (<4 mm) and normal anteroposterior molar relationship, no anterior open bite, and no crossbite. In addition, individuals were excluded if any permanent teeth were erupted. Measurements of maxillary and mandibular arch lengths and intercanine and intermolar arch widths were made, with measurements of overjet and overbite. The results indicated that maxillary and mandibular arch lengths in both sexes were significantly shorter in the contemporary sample; all arch widths were significantly smaller in contemporary boys, but not in girls. These findings suggest that average arch dimensions may be smaller in contemporary children than in past generations. Further research is needed to determine whether smaller arch dimensions are associated with more crowding in the primary, mixed, and permanent dentitions. (Am J Orthod Dentofacial Orthop 2001;119:211-5)

A mathematic-geometric model to calculate variation in mandibular arch form.

A mathematic-geometric model was used to evaluate the variation of mandibular dental arch length with respect to the incisor inclination, but without modifying the intercanine width. In analytical terms, the equations of the curves representing the lower dental arch, before and after incisor inclination of 1 mm and of 1 degree, with controlled and uncontrolled tipping, were studied. The length of the mandibular dental arch changed in the parabolic arch form by 1.51 mm for each millimetre of incisor inclination with respect to the occlusal functional plane, by 0.54 mm for each degree of controlled tipping and by 0.43 mm for each degree of uncontrolled tipping. In the elliptical arch form (e = 0.78), it changed by 1.21, 0.43, and 0.34 mm, respectively, in the hyperbolic form by 1.61, 0.57, and 0.46 mm, in the circular form by 1.21, 0.43, and 0.34 mm, and in the catenary form by 2.07, 0.74, and 0.59 mm. The results show that by changing the arch form without modifying the dimension of the dental arch, different arch lengths can be gained for each millimetre of proclination. In addition, by controlled tipping an inter-incisive arch one-fifth longer than by uncontrolled tipping can be obtained. It would be advisable in orthodontic treatment planning to evaluate the type of dental arch, since the space available or the space required changes depending on the arch form and on the orthodontic tooth movement.

Longitudinal post-eruptive mandibular tooth movements of males and females.

Unbiased estimates of post-eruptive eruption and migration of the mandibular teeth for large representative samples are presently unavailable. The purpose of this study was to evaluate pure tooth movements of untreated children and adolescents longitudinally. Lateral cephalograms of 214 French-Canadians, followed bi-annually between 8 and 15 years of age, were traced, and the positions of the mandibular permanent central incisors and first molars were digitized. Temporal changes in tooth position were evaluated relative to naturally stable mandibular reference structures, using the mandibular reference line for orientation. The statistical analyses included t-tests to assess gender differences and Pearson product-moment correlations to evaluate associations. The results showed that the incisors proclined significantly more for males (6 degrees) than females (3 degrees). The incisor tips displayed early mesial movements that were countered by later distal movements. The incisor apex showed a consistent pattern of distal migration between 8 and 15 years. Mandibular arch length decreased over the 7-year observation period. Rates of mesial molar migration accelerated until 11 years of age and then decelerated. There was no significant change in the mandibular occlusal plane angle between 8 and 15 years of age. Incisor eruption showed the greatest rates during adolescence, attaining peaks at approximately 12 years for females and 14 years for males. The molars erupted approximately 5 mm between 8 and 15 years of age. The greatest gender differences occurred at the older ages, with males showing greater eruption potential than females. It was concluded that the mandibular teeth show significant migration and eruption during childhood and adolescence, with gender differences in the amount, direction, and timing of movement.