Third molars and incisor crowding: when removal is unwarranted.

Recent evidence indicates that extracting unerupted mandibular third molars neither decreases interdental force measurably nor prevents mandibular incisor crowding. There are valid reasons for extracting third molars. But extraction for the exclusive purpose of relieving interdental pressure and thereby preventing incisor crowding is unwarranted.     

The use of the lingual arch in the mixed dentition to resolve incisor crowding.

In the mixed dentition, arch length preservation, maintaining the leeway space, can often provide adequate space to resolve lower incisor crowding. Yet the frequency of this occurrence is not known. To obtain this information, lingual arches were placed in the mandibular arches of 107 consecutive mixed dentition patients with incisor crowding to preserve arch length and make the leeway space available to resolve the crowding. Arch length decreased only 0.44 mm whereas the intercanine, interpremolar, and intermolar dimensions increased between 0.72 and 2.27 mm. There was adequate space to resolve the crowding in 65 (60%) of the 107 patients. If perfect arch length preservation had occurred, there would have been adequate space to resolve the crowding in 73 (68%) of the 107 patients. The correlation between leeway space and tooth size-arch size discrepancy was only 0.44.     

Incisor shape and incisor crowding: a re-evaluation of the Peck and Peck ratio

Although lower incisor tooth shapes, defined as mesiodistal length divided by labiolingual width, have bee proposed as important factors in lower incisor crowding, the question of whether or not these ratios are more useful than simple measurements of incisor mesiodistal length has not been addressed. In order to test this question, we measured mesiodistal and labiolingual incisor dimensions and lower incisor crowding (defined as the crowding index proposed by Little) on dental casts from two groups: 100 pretreatment orthodontic patients and 100 Hutterites from a religious isolate in Canada. The orthodontic patients are our primary interest. The Hutterites serve mainly to test whether or not results are consistent in another population of different ethnicity, age distribution, and occlusal status. In each population, incisor crowding is correlated with the tooth shape ratios, confirming the general observations of Peck and Peck. However, mesiodistal incisor lengths have slightly higher correlations with crowding than the shape ratios. In multiple regression equations to predict crowding in each population, incisor mesiodistal lengths are the most important variable, and neither the tooth shape ratios nor labiolingual widths significantly improve the equations. Although statistically significant, none of the correlations is higher than 0.30, and they are thus of little clinical value. The use of tooth size measurements or ratios as a guide to clinical procedures is an oversimplification of a complex problem.     

Prediction of mandibular incisor and canine crowding changes in the mixed dentition

To test the hypothesis that preeruptive tooth positions and dental arch parameters might forecast crowding changes, a sample of 47 aboriginal children (26 males, 21 females) was selected stringently on the basis of Class 1 characteristics and an untreated dentition unaffected by caries or attrition. Crowding scores, radiographic relationships of 765E, and dental arch dimensions were evaluated for the mixed dentition (Stage 1, 8.91 +/- 1.05 years) and earliest emergence of the permanent dentition (Stage 2, 12.48 +/- 0.97 years). Changes between the stages were calculated and the sample was divided into two groups, according to an increase (Group 1) or decrease (Group 2) in incisor and canine crowding. Multivariate and multiple regression analyses were used to identify predictors (Stage 1 radiographic and dental arch parameters) of incisor and canine crowding behavior. Dental arch form and tooth size were important factors in measuring the amount of incisor or canine crowding at Stage 1 and Stage 2. Neither the radiographic nor the dental arch predictors proved useful in forecasting crowding changes. Group 2 dental arches tended to be initially narrower, shallower, and more crowded; however, they showed greater molar and canine width expansion and lessened arch depth reduction than Group 1 cases. Many cases showed a reduction in canine crowding from mixed to permanent dentition. This appeared to be largely independent of the observed incisor crowding. Sexual, racial, and individual variations in dentofacial pattern reinforce the need to carefully consider interceptive extraction or space-regaining therapy for each patient because of the unpredictability of crowding behavior during the transition from mixed to permanent dentition.     

Treatment of mandibular anterior crowding with incisor extraction using lingual orthodontics: a case report

A 20-year-old woman presented with a labially displaced mandibular right central incisor with severe attrition. Her maxillary dentition was well-aligned, and she had a straight profile. The respective central incisor was extracted. Subsequently, all teeth were aligned and the extraction space was closed with 2D lingual brackets. The result was esthetically pleasing.     

成人下颌第三磨牙与下前牙拥挤的相关性研究

[摘要] 目的 通过讲究成人下颌第三磨牙倾斜角及萌出间隙与下前牙不齐指数间的相关性,探讨下颌第三磨牙是否为导致下前牙拥挤的原因及下颌第三磨牙是否应在正畸治疗中拔除。方法 选择成人骨性Ⅰ类下颌前牙区拥挤的患者34例,在治疗前的下颌记存模型上测量下前牙不齐指数,在曲面断层片上测量下颌第三磨牙倾斜角及萌出间隙,分析下颌第三磨牙与下前牙拥挤的相关性。结果 下颌第三磨牙倾斜角及萌出间隙与下前牙不齐指数间无线性相关关系(P>0.05)。结论 不能证明下颌第三磨牙是下前牙拥挤的主要原因,应尽量避免预防性拔除没有临床症状的第三磨牙。     

Treatment of a Class III malocclusion with a missing mandibular incisor and severe crowding

This case was chosen for publication by the CDABO student case selection committee. (Am J Orthod Dentofacial Orthop 1999;116:661-6)     

Long-term development in the mandible and incisor crowding with and without an orthodontic stabilising appliance

Summary In children with dentoalveolar Class Il malocclusion with proclined upper incisors treated with extraction of the maxillary first premolars and appliance in the upper jaw only has been reported to increase the lower arch crowding when compared with children with untreated normal occlusion. Stabilising orthodontic appliances might therefore be useful in the lower jaw. A comparison was made of Class ll:1 malocclusion with extraction in the upper arch in 35 individuals in whom a fixed orthodontic, appliance was used in the upper arch only and 26 individuals with fixed appliances in both jaws. The mean age at the start of treatment was 12.9 and 12.8 years, respectively. Treatment effects and post-retention changes up to 4 to 5 years out of retention at the age of 20 to 22 years were evaluated from lateral head films and plaster casts. During treatment the orthodontic appliance in the lower arch relieved crowding. The available lower anterior space increased from –0.6 to +0.2 mm, compared, to a decrease from –0.4 to –1.3 mm in the group without mandibular appliances. After 4 to 5 years out of retention the lower arch available space had decreased in both groups, to –1.4 mm in the group where orthodontic appliances had been used in both jaws and to –2.5 mm in the group without an orthodontic appliance in the lower jaw. This difference was significant. But the subjective ranking of the amount of crowding in the lower jaw models showed no significant difference between the 2 groups at the age of 20 to 22 years.     

Short-term treatment effects of quad-helix on maxillomandibular expansion in patients with maxillary incisor crowding

The purpose of this study was to evaluate the effects of the maxillary arch expansion on maxillomandibular arch widths in patients treated with the quad-helix versus untreated controls. The treatment group consisted of 50 consecutive patients treated for maxillary incisor crowding with a quad-helix appliance in the early mixed dentition. Lateral cephalograms and dental casts taken at the start (T0) and end (T1) of the quad-helix treatment were obtained. The control group consisted of 50 untreated patients with the same type of malocclusion. Two consecutive lateral cephalograms and dental casts of each untreated patient were taken at about the same time as T0 and T1. All these study materials were analyzed for comparison between the two groups. The mean ages at T0 and T1 in the two groups were about the same. The maxillary first molars moved and tipped distally in the treatment group and mesially in the control group. The quad-helix treatment actually expanded the mandibular and maxillary arches concurrently. The more the maxillary arch widths were expanded and the less the maxillary first molars were inclined distally, the more the mandibular arch widths were expanded. The quad-helix activation caused lingual tipping and mesiobuccal rotation of the maxillary first molars. The mesiobuccal rotation of the maxillary first molars could turn molar occlusal relationships for the better from Class II to Class I. The quad-helix treatment gives rise to spontaneous expansion of the mandibular arch concurrent with maxillary expansion in the early mixed dentition patients with maxillary incisor crowding.