From Volume 6, Issue 1, January 2013 | Pages 17-20
A tooth size discrepancy (TSD) indicates disproportion in tooth sizes. TSDs should be considered in treatment planning for the orthodontic patient to ensure an optimal occlusal outcome. The method of calculation of a TSD is explained. The prevalence in orthodontic patients, their aetiology and management are outlined.
A tooth size discrepancy (TSD) is defined as a disproportion among the sizes of individual teeth.1 Anterior TSDs relate to the six anterior teeth, whereas overall TSDs involve all teeth excluding second permanent and third molars. TSDs are an important consideration in orthodontic diagnosis and treatment planning, especially in the maxillary anterior segment.
An anterior or overall TSD may influence dental aesthetics and the occlusion. This may arise from spacing or crowding in the dental arch, centre-line discrepancies of greater than 2 mm2 and difficulty in achieving good inter-arch relationships. Smile aesthetics may also be compromised owing to the interplay between the smile arc, buccal corridor width and the Golden Proportion.3 This proportion describes the relative width of teeth in a harmonious smile. When the maxillary permanent teeth are in Golden Proportion, the relative widths of the central incisor, lateral incisor and canine are 1.618:1.0:0.618, respectively.4
Where a TSD is suspected, initial comparison of the size of upper and lower lateral incisors and second premolars should be undertaken. An anterior TSD will exist if the upper lateral incisors are either narrower or wider than the lower lateral incisors. A posterior tooth size discrepancy is highly probable if the upper and lower second premolars are not of equal size.1 For a more comprehensive assessment of a TSD, the mesiodistal widths of all teeth, excluding second and third permanent molars, must be measured. The most straightforward way involves the use of conventional fine-pointed callipers.5 The measurement is transferred to a stainless steel ruler and recorded. The use of digital callipers is more accurate.6 Software programs, such as the Hamilton Arch Tooth System (TOC Dental, Bristol, UK), allow automatic calculation of a TSD from measurements made on plaster models. Digital models can also be used to measure mesiodistal tooth widths accurately, with automatic calculation of tooth size ratios with proprietary software programs.7,8
The best known study of the significance of mesiodistal tooth widths was carried out by Bolton.5 Two ratios (overall and anterior) were calculated (Figures 1, 2).
The Overall Ratio (OR) is calculated by dividing the sum of the mesiodistal widths of the mandibular (X’) first molar to first molar by the mesiodistal widths of the maxillary (X) first molar to first molar (Figure 1).
The Anterior Ratio (AR) is calculated by dividing the sum of the mesiodistal widths of the mandibular (Y’) six anterior teeth (right canine to left canine) by the mesiodistal widths of the maxillary (Y) six anterior teeth (canine to canine) (Figure 2).
Bolton's OR was 91.3 ± 1.91% and AR was 77.2 ± 1.65%. Values lower than these ranges signify that the discrepancy is in the maxillary arch, whereas values greater than these ranges indicate that the discrepancy is in the mandibular arch. Bolton defined a clinically significant TSD as one in which either the OR or AR was greater than two standard deviations away from the relevant mean. Others have considered that discrepancies of 1.5 mm or greater should be regarded as clinically relevant.1 The determination of Bolton tooth size ratios provides useful diagnostic information which may become more apparent with a diagnostic wax set-up.
Nonetheless, Bolton's analysis is not without limitations. Bolton's ‘ideal’ sample was drawn mainly from white American females, most of whom had received orthodontic treatment. Consequently, the use of Bolton ratios with other ethnic groups and with males may not be valid.9 It has also been suggested that the predominance of perfect Class I occlusions within the sample indicates that Bolton's standard deviations may be underestimates.10
In epidemiological surveys of orthodontic patients, overall TSDs vary from 4–11%11,12,13,14 whilst anterior TSDs have a higher prevalence of 17–31%.11,12,13,14,15,16 The prevalence of TSDs varies with malocclusion type, gender and ethnic group.14 TSDs are more commonly identified in Class II division 1 and Class III malocclusions, manifesting, respectively, as relative maxillary17,18,19,20 and mandibular,21,22 tooth excess. Clinically significant differences in tooth size ratios between males and females have not been found.14 Negroid populations tend to have greater mean overall and anterior tooth size ratios when compared to Mongoloids and Caucasians.23
Variations in tooth sizes result from disturbances during the histo-differentiation and morpho-differentiation stages of tooth development.1 The teeth most commonly affected are the lateral incisors and second premolars (Figures 3, 4). Microdontia can be generalized, in association with hypodontia, or localized, most frequently affecting the maxillary permanent lateral incisor.24 Macrodontia is much less common but can affect the maxillary permanent central incisors (Figure 5) or mandibular second premolars and is often symmetrical.25 TSDs have a polygenetic mode of inheritance and may be expected in siblings of the same, but not of different, gender.26 Environmental factors such as inadequate nutrition during tooth formation may also contribute to tooth size disturbances,27 although the relative input of genetic and environmental factors is unknown.
TSDs often make it difficult to achieve an optimal aesthetic and occlusal outcome in orthodontic patients. Therefore, in many cases, management of TSDs should involve close liaison with a restorative colleague or primary care dentist at treatment planning stage and at appropriate stages during treatment. Patients with a clinically significant TSD should be treated according to the mean value for AR and OR of their ethnic group, where data are available. The identification of a TSD, particularly in the anterior region, should be discussed with the patient and parent before treatment planning. A diagnostic wax set-up (Figure 6) or morphed digital images28 are useful adjuncts to treatment planning for TSDs. Various treatment options can be demonstrated to inform the consent process.
Strategies in the management of TSDs are as follows.
Where a minor TSD has been identified owing to a slightly narrow permanent maxillary lateral incisor or second premolar, acceptance may be considered to avoid orthodontic treatment and long-term maintenance of restorative build-ups. Where orthodontic treatment is undertaken, unless compensatory reduction of tooth tissue is carried out in the opposing arch, the occlusion will be compromised29 and relapse is likely.30
Maxillary incisor angulation (tip) and inclination (torque) may be modified during fixed appliance treatment to mask an anterior TSD. By increasing maxillary incisor angulation, the crowns occupy more space in the line of the arch concealing small insufficiencies in tooth size. This is most effective when the incisor is rectangular. By reducing palatal root torque, the upper incisors occupy less space in the line of the arch and partially compensate for smaller upper incisors.31
Reduction of mesiodistal tooth dimension can be used in the management of wide teeth, which most frequently affects the mandibular incisors and second premolars. When carried out for triangular-shaped mandibular incisors, this broadens the contact areas and enhances stability of the occlusion following treatment. Care should be taken to avoid excess enamel removal. Abrasive strips (Figures 7, 8), discs or thin diamond burs can be used for tooth size reduction. Most of the enamel reduction should be done following initial alignment with a fixed appliance and further adjustment during the finishing stage, if required. This allows direct observation of the occlusal relationships before appliance removal.1 Topical fluoride should be applied to the enamel surface immediately following reduction.
Extraction of a lower incisor may be considered in cases with mild-moderate mandibular tooth excess (1.3–4.6 mm).32 Dental and periodontal factors, along with incisor angulations, rotations, midlines and the extent of the TSD, dictate the choice of incisor for extraction. A diagnostic wax set-up should be undertaken to confirm the likely occlusal outcome. Extraction of a lower central incisor, particularly if triangular, may lead to an open gingival embrasure on space closure with a fixed appliance.32 This can be eliminated by second order (tip) wire bends during finishing and detailing to ensure paralleling of the other lower incisor teeth. Interproximal reduction of the anterior maxillary teeth is usually necessary to resolve any residual TSD.32
TSDs are often caused by diminutive maxillary lateral incisors.33 Before restorative build-up of the lateral incisor a fixed appliance should be used34 (Figure 9) to ensure equal distribution of space mesially and distally for optimal contouring and emergence profile.33 Composite resin is the most conservative restoration and is best performed during the finishing stage of orthodontic treatment (Figure 10). To facilitate this, the orthodontic bracket should be temporarily removed. Alternatively, the build-up could be provided once the patient is in retention, necessitating an interim retainer and a new retainer once the restoration is complete.1 The appropriate timing for the build-up should be agreed with the restorative dentist at the treatment planning stage.
Extraction of a diminutive upper lateral incisor is indicated if the root is short or narrow. This may provide space for relief of crowding and/or overjet reduction. Modification of the crown shape of the maxillary canine is then required, often by a composite addition to the mesial aspect and recontouring of the cusp tip. The first premolar cusps should be adjusted to facilitate lateral excursions. Alternatively, it has been recommended that the first premolar should be intruded and the occlusal surface built up for an optimal gingival and occlusal result.35
Extraction of four premolars can reduce the OR from 91.3% to 88%.5,36 Therefore, the first and second premolar widths should be measured before finalizing any extraction decision, as large mandibular, or small maxillary second premolars may be the major contributor to a TSD. Where a centre-line discrepancy exists, asymmetrical premolar extractions may be appropriate.
In cases where a combination of the above treatment options is required, input from several dental disciplines will be needed.
TSDs should be recognized at diagnosis and incorporated in treatment planning for the orthodontic patient. A diagnostic wax set-up or morphed digital image is useful to help decide the most appropriate treatment option. Liaison with a restorative dentist may be required to finalize treatment planning and to achieve an optimal outcome.
