The adult orthodontic–restorative interface. Part 2: treatment strategies for the management of developmental conditions

Sites in the anterior dentition (aesthetic zone)

The upper lateral incisor is the second most commonly absent tooth (24% of total), with bilateral absence being more frequent than unilateral.² This can present specific challenges owing to the high impact of spacing in the aesthetic zone.

A common approach to orthodontic–restorative space closure is the alignment of maxillary canines within the site of developmentally missing lateral incisors. Orthodontic space closure is often preferrable because it reduces the requirement for prosthetic units, lowers the long-term maintenance burden and has been shown to achieve aesthetic results that are viewed more favourably by patients and lay people.³ However, in general, canines differ from lateral incisors by having a more convex labial form, a cuspal tip rather than incisal edge, narrower gingival emergence, and lower shade values. These factors should be considered when planning whether a canine can be successfully disguised as a lateral incisor. Orthodontically, the appearance of the canine can be amended by adding negative root torque to improve the labio-palatal angle, and limited extrusion to align the gingival margins with the central incisors, accepting that excessive movement can create an occlusal interference because the canine tooth is likely to be thicker and more convex. Restorative camouflage may include bleaching, enameloplasty to reduce the convexity or cuspal tip, and composite resin additions to create mesial/distal marginal ridges and an incisal edge.⁴

If canines are used to replace the lateral incisors, there usually follows a requirement to move and align the maxillary first premolars into the usual canine position, prior to camouflaging them as canine teeth. This is not always viable if the alveolar bone width is insufficient to accommodate the more mesial position of a two-rooted tooth without perforation of the cortical plate. The bicuspid anatomy of maxillary premolars may also ‘trap’ them within the occlusion, and achieving the required mesial movement may require the use of bite raising devices or composite resin occlusal stops at increased vertical dimension. Premolars are smaller than canines, with a higher and less convex emergence profile. This size discrepancy may be managed orthodontically via rotation of the tooth, to create increased mesio-distal width and intrusion (with or without crown lengthening) to appropriately align the zenith of the gingival margin level. Subsequently, a canine-like tooth can normally be created by the reduction of the premolar palatal cusp, along with the addition of composite resin to widen the tooth and create a more convex labial surface and cusp tip. In the highly aesthetically driven patient, with high lip line and gingival show, crown lengthening surgery may also be required to align emergence profiles with golden proportions.

If primary canines are present and stable, composite resin may be added to restore lost tooth structure, or to disguise as an adult successor.⁵ However, consideration must be given to whether composite additions will reduce the prognosis of the primary tooth, particularly if increased functional occlusal forces lead to increased rates of root resorption, and earlier tooth loss. Adult patients may often present with primary canines of poor prognosis, where retention is no longer viable.

Options to restore the spaces left by developmentally absent teeth are described in Table 1. When movement of teeth is required to redistribute space, the restorative objectives will normally be to provide a fixed prostheses that is aesthetic and conservative to the remaining dentition (Figure 1). Whether the patient is having a resin-bonded bridge (RBB) or implant, the orthodontist should aim to create a minimum 6.5–7 mm of inter-tooth/radicular space within lateral incisor sites. In this way, the patient could still choose to have an implant at a later date. In cases with crowding or increased overjet, it may also be necessary to create additional space within the arch, with extractions or molar distalization. While implants with narrower platform diameters (3 mm) may be employed when this inter-tooth space cannot be achieved,^6,7 less is known about the longer-term survival of narrow implants exposed to high levels of occlusal force, for instance in patients with parafunctional habits.⁸ Narrower platform implants may also make it more difficult for the dental technician to achieve an aesthetic emergence profile, without over-contouring the restoration and compromising the cleansibility of the abutment/implant interface,⁹ which may result in an increased risk of peri-implantitis.¹⁰

The success of RBBs can be dependent on the quality of the tooth surface that is available to bond to, which is often poorer in adult patients owing to previous damage (caries/trauma/tooth wear) and the presence of existing restorations. Full coverage retainer wings with opaque luting cements and pre-cementation media blasting is recommended to maximize bond strength. RBBs can be cemented high in occlusion so long as the excursive movements are controlled. Cantilever designs can be prone to rotational relapse if orthodontic retention is inadequate.

Sites in the posterior dentition

Excluding the third permanent molar, the lower second premolar is the most common developmentally missing tooth. This is an anomaly that is often unilateral¹¹ and may result in centreline shift and localized crowding. Where the aesthetic requirements are reduced, the maintenance of primary teeth is a viable medium to long-term option¹² because many teeth are still present in patients’ fourth decades and beyond.¹³ Retained primary molars commonly show some form of infra-occlusion, attributed to ankylosis,¹⁴ which increases the risk of over-eruption of the opposing tooth and mesial/distal tipping of the adjacent permanent teeth.¹⁵

Where loss of a primary molar is necessary, or desirable, orthodontic space closure may require the extraction of other, potentially healthy teeth to maintain centrelines, preserve the occlusion and reduce the mechanical interference from opposing cusps. Space closure in the upper arch is more suited to Class II occlusions when overjet reduction is required, and Class III relationships favour space closure in the lower jaw. It should also be considered that retained lower second primary molars are larger than their adult successors, therefore it can be difficult to fully close these large spaces in adult patients, especially if bone defects are present. The use of bone-supported mini-screws to provide anterior anchorage support can be used to minimize undesirable movements, and are often more predictable in adults owing to an increased bone density.

Ectopic canines

It is uncommon for the presence of ectopic canines not to be identified and managed in childhood. However, adults with ectopic canines may seek treatment when either concerns emerge regarding the risk of the ectopic tooth to the adjacent roots (e.g. pressureinduced root resorption), or if the primary tooth is no longer viable. Within the adult patient, interceptive treatment via extraction of the primary canine (and spontaneous space closure) is not possible, and treatment options are limited to accepting the ectopic canine in situ, orthodontic alignment (potentially requiring surgical exposure) or extraction.²²

Ectopic canines that are most favourable for alignment will normally have mesial inclination and lie within 14 mm proximity of the canine site occlusal plane.²³ Even if the tooth is favourably positioned, the success of alignment is reduced by the increased incidences of ankylosis and slower rates of tooth movement found in adult patients.²⁴ In addition, ectopic canines can also be subject to malformations (Figure 2) or discolouration, which may pose unknown challenges to disguise within the smile line. Subsequently, patients over 30 years of age should be carefully consented before attempting to align an ectopic tooth, in all but the most favourable positions.

In cases with generalized crowding, extraction of the ectopic canine and orthodontic space closure can produce aesthetic results with a lower maintenance requirement. A small amount of residual space often remains following alignment owing to the mismatch in mesio-distal widths between the upper and lower arch, and restorative disguise of first premolar and/or the lateral incisors is often necessary (Figure 3). The extraction of other teeth within the arch should be considered for centreline correction and management of occlusal relationships. Correction of the vertical position of teeth in the opposing arch following over-eruption is often required. It should also be considered that surgical extraction of ectopic canines in adult patients may leave a significant bony defect, which complicates future dental implant placement, and less frequently presents issues with orthodontic space closure (Figure 4).

If the ectopic tooth is to be left in situ, even though continued movement in adulthood is uncommon,²² it is still important to confirm the ectopic tooth's three-dimensional position, so that the risk of root resorption to adjacent roots can be assessed. The presence of an unerupted canine will also often preclude other orthodontic tooth movement in the affected arch, or dental implant placement.

Canines are also the teeth most affected by transposition (interchange in the position of two teeth within the same quadrant of the dental arch) with an incidence of 0.1–0.5%.²⁵ Such transpositions can normally be disguised using the enamel-shaping and composite additions discussed above.