Orthodontic/restorative interface part 2

The treatment planning process

A large number of factors will influence the treatment planning process and final treatment decision. These include patient factors, dental factors, orthodontic factors and soft tissue factors; these factors are summarized in Table 1.

In order that a thorough and accurate assessment and diagnosis is possible, full records should be taken, including impressions for study models with a wax registration, intra-oral and extra-oral photographs, any clinically justified radiographs, as well as an extra- and intra-oral orthodontic assessment. The records should then be assessed jointly within a multidisciplinary team.

Diagnostic aids, such as Kesling set-ups, are useful for considering different treatment options, for both the patient and the clinician, even though they are limited in terms of showing colour changes. Figure 1 presents a Kesling set-up to demonstrate a potential treatment outcome in a patient with several missing teeth. Treatment options should be discussed with the patient, including a comprehensive and informed discussion of the risks and benefits as well as any alternative treatment options. Patients should be fully informed of the visual outcome, functional outcome, the financial cost to the patient (short- and long-term), the time-scales involved, and the appearance during treatment and retention. Emphasis should be placed on any long-term maintenance implications, therefore treatment planning should be focused on this restorative ‘burden’ and whether the final outcome be space opening or closure.

Figure 2 shows a hypodontia case on which the Kesling set-up shown in Figure 1 was based. This 14-year-old patient had Class II division 1 incisors on a Class I skeletal base with average vertical proportions. The malocclusion was complicated by missing maxillary lateral incisors, upper right first and second premolars, upper left second premolar, mandibular central incisors and second premolars, and a palatally impacted upper right canine (Figures 2a and b). The impacted upper right canine erupted after space relief was provided by extraction of the deciduous teeth. Upper and lower fixed appliances were then used (Figure 2c) to redistribute the space in order to replace the absent maxillary lateral incisors and an upper right premolar, with the remaining deciduous teeth left in situ until the patient's growth has completed (Figure 2d). Upper and lower Hawley retainers were used on a full-time basis (Figure 2e) in order to reduce the risk of drifting of the adjacent teeth into the space, until resin-bonded bridges are placed to restore the upper lateral and upper second premolar spaces.

The main treatment options in cases where teeth are absent are to accept the malocclusion, close the space and camouflage adjacent teeth, or open the space and place an implant or a tooth-supported restoration. These will be discussed throughout this article.

Space closure

Closing space where a tooth is missing is a common treatment option, both in the anterior and posterior dentition. Factors to be taken into account include the amount of space present, the amount of alveolar bone (width and height) present, the anchorage demands, patient compliance, and cost implications; in addition to the usual factors to be taken into consideration (Table 1). If the space is in the anterior segments, additional factors require consideration such as tooth colour, tooth width and bulkiness, gingival margin height and smile line. Essentially, one must ask how successfully can the canines be camouflaged as incisors? If the decision to close space has been made, particularly to close missing maxillary lateral incisor space, communication between the orthodontic and restorative team is important to ensure the optimal outcome for the patient.

Mechanics of closing maxillary lateral incisor space

When the decision has been made to close space, defining who within the clinical team (and in which setting) will be responsible for the orthodontic treatment and for the restorative modifications to the teeth, should be established and discussed with the patient prior to the start of treatment. This is particularly important if the treatment is being done between primary and secondary care; good communication throughout the process is vital.

Table 2 summarizes the advantages and disadvantages associated with space closure.⁶ Several studies in the literature have looked at how both dentists and lay people rate the attractiveness of a smile when space has been opened compared to when space has been closed.^7,8 Armbruster et al looked at how lay people, dentists and orthodontists judged the relative attractiveness of smiles in patients with missing maxillary lateral incisors and were treated with bridges, implants, or orthodontic space closure.⁷ Patients with no missing teeth acted as the control group. The lay population ranked photos where space closure had been carried out as the most attractive out of all the groups. Orthodontists ranked the control group with no missing teeth as the most attractive followed by the space closure group. Interestingly, within all groups the photographs of the subjects treated with implant-supported restorations were rated the least attractive.⁷

Figure 4 demonstrates a case where the maxillary lateral incisors and the mandibular second premolars were congenitally absent (Figure 4a). Space was closed using orthodontic fixed appliances (Figure 4b), and reduction of the cusp tips of the upper canines was undertaken progressively, in order to camouflage the canines as lateral incisors. In this case, the brackets bonded on the canines were inverted, to change the buccal root torque to palatal root torque with a view to reducing the canine eminence. The outcome shows closure of all the space with Class I buccal segments and incisors (Figure 4c). A bonded retainer was placed in the upper arch in addition to upper and lower vacuum-formed retainers, to reduce the risk of the space reopening.

Space creation

The amount of space created for an absent tooth is determined by aesthetics and the occlusion. For an anterior tooth, from an aesthetic standpoint, the size is based on the contralateral tooth to maintain symmetry and the size of the central incisors. Often in cases with a missing maxillary lateral incisor, the contra-lateral lateral incisor is peg-shaped and, as such, the space creation for its build-up should be considered in the overall treatment plan. An aesthetic relationship also exists (known as the ‘golden proportion’) between the size of the central incisors and the lateral incisors. In general, the lateral incisors should be two-thirds the size of the central incisors.⁹

Opening space and prosthetic replacement

Resin-bonded bridges (RBBs)

Bridge options include resinbonded bridges, or conventional full coronal coverage bridges. Both can be cantilever or fixed-fixed designs but, in the era of minimally invasive dentistry, the ambition should be a single tooth cantilever.

RBBs are minimally invasive fixed prostheses, which rely on composite resin cements for retention.¹⁰ RBBs can be metal-based (NiCr), zirconia-based, or fibre-reinforced. The pontics are usually ceramic (porcelain) or composite. There is a dearth of evidence about the predictability of allceramic or fibre-reinforced bridges, therefore this discussion will focus on metal and ceramic RBBs.

RBBs should be regarded as the first line choice as they are conservative and minimize tooth preparation.¹¹ By using a RBB it is possible to provide a fixed replacement for missing teeth that does not compromise the abutment tooth. This is particularly important for young patients who may be more likely to experience loss of tooth vitality and endodontic complications as a result of extensive tooth preparation. In this regard, Saunders and Saunders reported the risk of loss of vitality due to preparation prior to a conventional indirect restoration as 19%¹²

When planning treatment, many space factors must be considered, both in the vertical and horizontal planes. To determine the width of a space required for a pontic, the contralateral tooth can be used as a guide, assuming that it is present. If this is peg-shaped, it is important to plan spacing around the tooth in preparation for restorative build-up and duplicate the total planned on the contralateral side. As well as the width proportion of the tooth, vertical space planning is also vital. Bridgework connector thickness is usually 3 mm in vertical height, thus care should be taken during orthodontic treatment to ensure that the opposing tooth does not over erupt into the pontic space. Thus it is important to plan suitable overjet and overbite relations during the orthodontic phase.

The ambition of orthodontic treatment should be:

Sufficient mesio-distal space to allow a pontic that can be placed in golden proportion to the central incisors. This ratio of pontic width to central incisor should be around 1:1.6 for uppers and 1:1 for lowers.

Sufficient interocclusal space for the pontic: minimum 3 mm.

Achieve Class 1 incisor relationship to minimize steep incisal guidance that may result in higher shear forces upon the bridge.

Achieve canine guidance or group function protecting the incisors during excursive movements.

Long-term retention to prevent rotation of the canines. This is a fundamental aspect as, without adequate retention, if a bridge is lost, the space can be lost rapidly leading to multiple issues, including inability to rebond the bridge, asymmetric spaces and possible need for orthodontic retreatment to reopen the space.

The ambition of the restorative dentist should be:

Minimal or no-preparation of the abutment tooth.

Minimum retainer thickness 0.75 mm CoCr (this can be cemented high and allowed to ‘Dahl-in’, ie although cemented high, causing an initial temporary posterior open contact, subtle eruption of the teeth would occur shortly after, allowing a more stable intercuspal relationship).

3 mm connector thickness with cantilever design. The metal framework should be placed palatally to allow sufficient porcelain buccally.

Modified ridge-lap design of pontic.

Light holding contacts on the pontic in intercuspal position (ICP) but free from lateral guidance.

Either canines or central incisors may be used as abutments? The former are biologically more favourable in terms of root length, available bonding area and thickness of dentine to prevent shine through but are liable to orthodontic relapse and may rotate without retention. Central incisors may offer more stable long-term abutment teeth but may suffer greying and do not offer as big a bonding area. Furthermore, most evidence regarding survival is based upon canine retention. When both lateral incisors are missing, it is possible to prescribe a linked double-wing retention on both central incisors, thus linking both bridges. Though there is minimal evidence to support this, it offers a bigger surface area for retention, effectively acting as fixed retainer for the centrals and mitigating the risk of de-rotation of the canines. This could be considered when the canines are diminutive and offer less area for bonding a wing.

Figure 5 shows a case where a RBB was used to replace a missing lower incisor. This 13-year-old patient presented with Class II division 2 incisors on a mild skeletal II base and reduced vertical proportions (Figure 5a). Her malocclusion was complicated by hypodontia of the mandibular central incisors and the right maxillary second premolar, an increased overbite and small maxillary lateral incisors. A range of treatment options were considered and discussed. The decision was made to accept the second deciduous molar in the upper right quadrant, as it had good root length and crown height and morphology. Despite the presence of some occlusal staining in the upper right second deciduous molar, this was hard to probe and there was no evidence of caries radiographically. The retained deciduous mandibular incisors were extracted and space opened to replace one incisor adjacent to the lower right canine with the use of orthodontic fixed appliances (Figure 5b). This was in order to use the canine as an abutment tooth rather than an incisor.

With regards to the small lateral incisors in the maxillary arch, space was created mesial and distal to them in order that they could be built up with composite restorations. Liaison with the restorative team was required to determine the horizontal and vertical space requirements for the lower incisor replacement, as well as the space requirements for the build-ups of the maxillary lateral incisors. Overbite reduction was carried out with the use of an upper removable appliance with a flat anterior bite plane, in order that the maxillary teeth would not be causing any interference during protrusive excursions. Figure 5d shows the post-treatment intra-oral views after the upper lateral incisors have been built-up and an interim RBB placed until the patient completes facial growth. Although the long-term plan for this patient is for an RBB, the patient was informed that the initial RBB may need to be replaced in case of alveolar changes during facial growth.

Some issues associated with RBBs include grey shine through the abutment tooth due to the pontic wing. The development of opaque cements should hopefully reduce this problem in the future. In addition, there is the risk of debonding of the RBB despite recent advances in bonding technology. The latest failure rates in the literature suggest that we can optimistically expect a bridge to last 11 years.¹³ Furthermore, prosthetic replacement commits patients to a lifetime restorative cycle, which is costly, both financially, emotionally, and time wise. Survival rates are discussed in further detail later in the article.

Implants

In the past, much of the concern over space creation related to concerns over prosthetic replacement, with the failure rate of RBBs or the more destructive fixed-fixed bridges leading to pulp damage and deteriorating periodontal heath. The replacement of missing teeth with implants offers fixed replacement without collateral damage to adjacent teeth. Furthermore, it allows optimal positioning and shape of the prosthesis not restricted by the abutment tooth. Prosthetic replacement with dental implants is, however, a more involved process requiring not only correct interdental space management but interradicular space and bone levels. In addition, these patients may require bone grafting. Patients must be consented to lengthy treatment and planning must be meticulous to prevent unwanted delays.¹⁴

Orthodontics and implant site development

The labio-lingual dimensions of the alveolar ridge must be adequate in order to place the implant in its correct transverse position relative to the opposing arch. This can be an issue particularly when teeth are congenitally absent. Without the development and eruption of a tooth, the osseous ridge in the agenesis area is typically deficient. In the case of missing lateral incisors, this may be avoided should the canine erupt adjacent to the central incisor.⁵ As the canine is moved distally, bone is laid down along the periodontal fibre tracts. There is conflicting evidence in the literature with regards to how stable this newly created bone is.¹⁵ Some advocate that, in contrast to extraction sites, the bone laid down in this fashion is relatively stable, even if implant placement is delayed until the end of facial growth.¹⁶ If this width is inadequate, bone must be created and frequently a bone graft is required after completion of orthodontic treatment.

Orthodontic ambitions for implant-based restoration should include:

There are several other issues to consider regarding implant placement. Gingival recession and dark margins along porcelain crowns can occur and darkening of the labial gingival due to resorption of labial bone has recently been reported above more than 50% of single implant crowns at 4-year follow-up.¹⁸ Other issues are summarized in Table 3.

There are many potential complications and considerations that may impact upon success of implant-supported restorations. In the long-term, it is not possible to estimate when, to what degree, or in which patients, changes in the soft and hard tissues which could impair aesthetics will occur around implant-supported crowns.⁶ Faced with this problem, Zachrisson et al advocated that, if the treatment plan for young patients includes space opening, it might be preferable to open the space later and place implants in the premolar areas.⁶

Figure 6 shows a case that has been treated with a combination of fixed appliances and the placement of implants and implant-supported restorations. This 61-year-old patient presented with Class III incisors, in a traumatic edge-to-edge bite. His malocclusion was complicated by the presence of localized severe crowding in the lower left premolar region, edentulous spans in the upper left and lower right quadrants and a heavily restored dentition. For the spaces, the options were to accept them, use tooth-supported restorations or an implant-supported restoration. The patient wanted to restore the missing teeth. As the teeth adjacent to the spaces were endodontically treated and heavily restored, they would have been poor abutment teeth for bridge placement. Thus, in this case, implant-supported restorations were the most appropriate treatment option. Extraction of the buccally crowded lower left first premolar allowed relief of crowding. Upper and lower fixed appliances were used to align the teeth, correct the incisor relationship, and correct the edge-to-edge bite. After debond, composite restorations were placed on the upper and lower incisors to improve their aesthetics. The end of treatment views, prior to restoration of the spaces, are shown in Figure 6c; they show the correction of the incisor relationship, good buccal interdigitation and significantly improved aesthetics. Figure 6d shows the occlusion after restoration of the spaces, with implant-supported crowns.

Survival rates of resin-bonded bridges and implants

Implant success rates for single crowns are around 90% at 5 years and 80% at 10 years,¹³ when success is defined as an absence of complications at follow-up. Although these are good success rates, they are far from perfect; patients must be informed that implant-retained crowns may suffer many of the complications described above. Issues to consider are outlined in Table 4.

A systematic review, looking at the survival rates of 1374 RBBs that had been placed, showed the survival rate to be 87.5% at 5 years.¹³ However, there were no randomized controlled trials available, thus this was based on 17 prospective and retrospective cohort studies. A more recent prospective study by King et al was conducted at Bristol Dental Hospital that examined 1000 RBBs in 805 patients.¹¹ It found the 5-year and 10-year survival rate to be 80.8% and 80.4%, respectively (survival was defined as the time to first debond or porcelain fracture).¹¹ Of the RBBs that failed, 20% of them failed within the first 4 years. It is therefore not unreasonable to inform patients that anterior RBBS should last 8–9 years.¹⁹ More importantly, failure is most likely going to be a debond. Though inconvenient and embarrassing, these RBBs can often be easily cleaned and replaced chairside. Furthermore, although these rates are lower than the 90% success reported for implant-retained single crowns over the same five-year follow-up,²⁰ RBBs have the advantages of being less invasive, requiring a shorter total treatment time and less financial commitment.

Thus decisions should be made on a case-by-case basis, taking into account all the factors at the treatment planning stage and discussing these issues with the patient with both the restorative and orthodontic teams present. This will help to ensure that the optimal outcome can be achieved and that the patient has realistic expectations regarding the long-term prognosis of any treatment provided.

Conclusion

Establishing optimal aesthetics, function and periodontal health in patients with missing teeth is a complex and challenging process, which demands effective multidisciplinary management. There is a plethora of different factors to consider when space closure or space opening is planned, and informed consent is a vital aspect of this process due to the long-term consequences of both options.