The prosthodontic pathway for patients with anomalies affecting tooth structure

Amelogenesis imperfecta

Amelogenesis imperfecta (AI) is a hereditary genetic disorder which affects enamel formation and is not associated with systemic abnormality or disease.¹ The condition affects the formation of the enamel matrix or the enamel mineralization process. The changes to enamel formation are normally due to single gene mutations that affect enamel matrix proteins (amelogenin, enamelin and amelobastin), which are used to produce enamel during amelogenesis.^2,3 The inheritance pattern can be autosomal dominant, autosomal recessive or X-linked.⁴

In general, AI affects both the primary and permanent dentitions. The estimated frequency of AI ranges from 1:700 to 1:14,000, depending on the population studied.⁵

There has been a number of different classifications of AI that have included genetic pattern, as well as clinical and radiological features. The popular Witkop classification has 15 different variants that have been put together into four groups based on the inheritance pattern (Table 2).⁴

In Type I hypoplastic AI, there is a defect in the enamel matrix formation but mineralization of the enamel matrix is normal. This creates a normal enamel layer, but of a reduced thickness.

In Type II hypomaturation AI and Type III hypocalcified AI, there is a defect in the maturation of the enamel but the enamel is of normal thickness and is of a softer consistency.

In Type IV hypomaturationhypoplastic with taurodontism, this appears similar to Types II and III but there is evidence of taurodontism.

The clinical presentation is variable and can range from minimal pitting, discoloration and minimal breakdown of enamel to severe pitting, discoloration and rapid breakdown of enamel. Figure 1 shows the discoloration and form of a patient's dentition suffering from a severe form of AI. AI has been associated with other dental anomalies, these include:^3,4,6,7,8

This range of features poses a number of specific challenges for the restorative dentist. Table 3 summarizes the restorative challenges associated with AI.

Resin bonding and AI and DI

All adhesive techniques are complicated by a patient's poor oral hygiene. This is compounded by lack of motivation to clean what are seen as ‘ugly’ teeth and, in the case of AI, rough surfaces that are plaque retentive. In patients with AI, teeth are often very sensitive, which sometimes means that local analgesia is needed even for very simple additive composite resin techniques.

The difficultly in patients with AI is being able to attain a predictable adhesive bond to the tooth structure and this can be compromised,²⁸ but there is evidence to show that bonding to teeth with AI can be successfully carried out.^29,30,31 Predictable bonding is also affected by the microdontia noted in both conditions, which reduces the total surface area.

There is some evidence suggesting that teeth affected by AI have an atypical etch pattern and thus have reduced bond strength from enamel to the composite resin. This study also showed that adequate bonding can be achieved with the pre-treatment of hypocalcified AI teeth with sodium hypochlorite prior to adhesive bonding. This improved the bond strengths attained with resin-based materials.³²

There are two concerns with bonding in patients with DI. First, bonding to enamel may not result in a predictable restoration since the enamel may itself detach from underlying dentine, although this varies from case to case. Secondly, the altered structure of dentine itself may affect the formation of a hybrid layer that is required in conventional dentine bonding techniques. A study by Gallusi et al showed that a hybrid layer could not be created in dentine in DI Type II affected teeth with an adhesive system and therefore seemed to be less effective than on normal substrates.³³

However, adhesive bonding and adhesive restorations in patients with DI have been proven to be successful.^34,35 Some clinicians have recommended the use of resin-modified GIC-based bonding agents rather than conventional resin-based adhesives as they produce true chemical bonds to metal ions rather than the hybrid layer. There is some evidence to show glass ionomer cements are as effective as resin-based cements in cementation of cast restorations in both DI and AI.³⁴

In general, adhesive restorations are commonly used to treat patient with AI and DI and have been shown to be successful.^27,31,36 In practical terms, however, in severe forms of AI and DI and where repeated debonding or loss of restorations occur, full coverage restoration may need to be considered. The severity of AI and DI will affect the outcome of restorative treatment and, in particular, the quality of the dentine substrate and ability to form a hybrid layer. Enamel loss due to weakness at the ADJ will affect restoration longevity where restoration margins are within enamel only.

Veneers

Veneers are commonly used on anterior teeth in AI and DI patients. They allow good aesthetics whilst being relatively conservative of tooth tissue in comparison to crowns.³⁷ Even so, preparation is required and can be destructive of reasonably sound tooth tissue.³⁸

Possible indications for the use of veneers in AI and DI patients are to restore areas of wear and to improve aesthetics, particularly if direct composite has been tried and proved unsuccessful.^23,37 They are also generally indicated for hypoplastic and discoloured teeth, when other techniques have failed to create a satisfactory clinical result.³⁹

Veneers tend to be more useful in AI than DI patients due to the issues with bonding and the underlying discoloration of the dentine in DI patients.

Veneers can be either composite or porcelain, and both have proved to be aesthetic, with high patient satisfaction.^40,41

Obtaining good aesthetics with veneers can be difficult in patients, particularly when the tooth structure is dark. In a minority of cases the veneers are unable to mask the discoloration and lead to compromised aesthetics. Techniques to alleviate this are for the porcelain veneers to have a layer of opaque porcelain incorporated into the restoration, or the use of an opaque resin cement. They can be used separately or together to help disguise this discoloration but lead to a loss of translucency which can detract from the final appearance.³⁷ Another option, which is yet to be validated with long-term clinical data, is to use zirconia for the coping of the veneer, which is overlaid with feldspathic porcelain. Zirconia can be opaque which enables the blocking out of the underlying tooth colour, however, due to zirconia being inert and lacking silica, bonding to zirconia is difficult.⁴²Figure 3 shows the comprehensive restoration of a patient with AI following previous failure of interim composite restorations. Treatment included surgical crown lengthening with osseous recontouring in the maxilla to improve the gingival contour. This was followed by the placement of zirconia-reinforced veneers anteriorly and full coverage restorations posteriorly.

The same issue with bonding applies as with direct placement of composite. Direct composite placement can be considered as the treatment is reversible, reasonably quick and allows preservation of tooth tissue. It is useful in young growing patients to allow gingival maturation and avoid the risk of pulpal pathology in patients with larger pulps.

Crowns/onlays in patients with AI and DI

Onlays and crowns can be used to protect the underlying tooth structure. They can be placed conforming to the existing occlusion or placed to compensate for loss of occlusal vertical dimension.

Conventional crowns have been shown to be successful in patients with AI and have also been shown to be predictable and durable, but have the disadvantage of being destructive of tooth tissue.²⁵ There are risks associated with placing crowns in AI patients that include cementation failure, material fracture, caries and endodontic treatment,^29,43 all of which are common complications of crowning teeth in the general population.

Where gingival asymmetry, excess gingival display or microdontia is an issue, then crown lengthening surgery may be considered (Figure 3). This is complicated by the close proximity of the roots which are often spindly and the inevitable increased sensitivity following surgery. However, this may be contra-indicated in teeth with short roots, common in DI patients.¹⁶

In posterior teeth where protection of the tooth structure is required in AI and DI patients, gold onlays bonded with resin composite or full coverage gold veneer crowns can be provided. They have been proven to be successful in these patients and help minimize toothwear and maintain the occlusal vertical dimension.^12,44

Adhesive restorations, such as dentine-bonded crowns and gold onlays cemented with resin composite, rely more on the adhesion than the mechanical retentive features of tooth preparation,⁴⁵ and with this have reduced tooth preparation.

In anterior and posterior teeth for patients with aesthetic concerns, metal-ceramic crowns may be considered. Metal-ceramic crowns should be designed to restrict porcelain to aesthetic areas and metal in non-aesthetic areas to preserve tooth tissue.³⁷ Other options include all-ceramic dentine-bonded crowns, which can be a more conservative aesthetic treatment option for the anterior region.^12,37 High strength all-ceramic crowns with zirconia and alumina have been used in patients with AI,⁴³ but require heavier preparations, which could lead to pulpal pathology.³⁷ Zirconia crowns have the advantage of an opaque substructure which can be useful to block out underlying orange/brown discoloration, more translucent ceramics such as alumina, or lithium disilicate which may be inadequate in regards to this.

Crowns are not recommended in DI-affected teeth with short thin roots and marked cervical constrictions.⁴⁴ They may also not be recommended in teeth severely affected by the condition. Prior to the placement of crowns it is essential that an assessment of tooth vitality be made, since DI-affected teeth often become necrotic with asymptomatic apical periodontitis. These teeth should not be root treated but planned for extraction as a canal location is often impossible.

It is the recommendation of the authors that, wherever possible, minimal preparation be carried out to preserve as much tooth tissue as possible and, although adhesion to AI- and DI-affected teeth isn't as predictable, it has been shown to be successful and allows tooth preparation to be more conservative. Timing of treatment in relation to the patient's age needs to be considered. Where possible, extra-coronal restorations should only be placed in patients at least 18 years old and, where feasible, as late as possible. This is due to the higher risk of loss of pulpal vitality in AI patients, that the tooth will be put into the restorative cycle, and the high likelihood of requiring multiple replacements in patients of a young age. Also, the gingival tissues do not fully mature until the end of the second decade.

Implants

There is no evidence to suggest the need for any special consideration on implant provision in patients with AI. Provision of implants in patients with AI will usually occur in patients with advanced cases where teeth are deemed unrestorable through caries or severe microdontia. Teeth which remain unerupted and cannot be extruded orthodontically may need removal.

Implant treatment can only be considered in patients who have stopped growing,⁴⁶ as it can lead to infraocclusion of the implant restoration. It has been suggested by Patel et al that implant treatment may not be the ideal choice in the first instance for young adult patients with AI.³⁷ This is due to the fact that further treatment will be required in later life and the possibility of improved outcomes in implant treatment in the future due to the constant improvements in technology.

Specific challenges in AI may relate to the occlusion and limited interproximal space for implant placement due to narrow spindly roots and narrow clinical crowns. In DI patients with Type II or III DI, the bone quantity and quality is the same as in an unaffected individual,⁴⁷ but there are a limited number of studies on dental implant provision in DI patients.⁴⁸

Patients with Type I DI have OI but, despite this, implant placement has been shown to be successful in OI patients^49,50 with similar success rates to non-OI patients,⁵¹ but there is limited evidence available. Type I DI patients may also be on bisphosphonate (BP) medication due to OI. The use of BP medication comes with the risk of BP-induced osteonecrosis of the jaw (BRONJ).

In DI patients, the bone volume is often reduced, particularly in terms of width. It is the authors' opinion that this is related to short roots and, therefore, a lack of alveolar development. It is often necessary to perform a guided bone regeneration procedure at the time of implant placement.

Overall, careful planning of treatment is required to ensure optimal results are attained. Figure 4 shows the management of a patient with DI following repeated failure of anterior veneers and fracture of premolars on the right side. Initial management included increasing the vertical dimension with direct composite veneers placed on the mandibular incisors and canines. Later two implant-supported crowns were placed and restored in the first premolar regions on the right side followed by the placement of six porcelain-bonded crowns to restore the maxillary anterior teeth. Note the discoloration of the prepared maxillary incisors.

Removable prosthesis

In the past, treatment of AI and DI patients often involved the extraction of teeth and provision of a partial or complete denture.³⁷ With advances in dental materials and techniques, treatment can be more conservative with benefit to the patient.⁵²

Even in advanced cases, and when teeth are deemed unrestorable, they can be retained with provision of an overdenture or onlay denture.²³

Dentures are particularly useful in cases of extensive toothwear,⁵³ as they may be used to protect remaining tooth tissue, and maintain occlusal face height.^21,23

They may also be used to help retain teeth and be of particular importance in young patients who are not of a suitable age or are in the planning stages of treatment for more invasive treatment, such as implants. The retention of teeth helps to preserve alveolar bone.⁵⁴ Until growth is complete, the treatment of choice for the replacement of missing teeth in DI²⁰ and AI may be dentures.

A removable partial denture has been shown to be a good temporary or permanent solution, particularly in patients with low aesthetic concerns or medical limitations.⁵⁵ This may include Type I DI patients who have OI, where restorative treatment options are restricted to implants or a removable prosthesis for tooth replacement. These patients with OI are at risk of bone fractures and BRONJ (if taking bisphosphonates) if implants are placed. The risk and benefits of providing an implant-retained prosthesis and a removable prosthesis need to be weighed up. The disadvantages of dentures include: increased plaque retention with the potential consequences, including gingivitis; periodontal disease or caries; trauma from denture components and decreased patient tolerance.⁵⁶

However, dentures are not usually the first choice of treatment in AI and DI patients, but do serve a role, and can provide temporary or definitive treatment in some patients.

Dens invaginatus

Dens invaginatus is a development abnormality resulting in malformation of teeth due to invagination of the enamel organ into the dental papilla during tooth development. It has also been called dens-in-dente and an invaginated odontome. The exact aetiology of dens invaginatus is unknown but it has been proposed that there is likely to be a genetic cause.⁵⁷

The prevalence of dens invaginatus ranges from 0.04–10%.⁵⁸ The permanent maxillary lateral incisor is the most frequently affected tooth,⁵⁹ with posterior teeth less likely to be affected.⁶⁰ Bilateral occurrences are common, accounting for 43% of cases,⁶¹ so contralateral teeth should be routinely assessed.

Teeth affected can present with an infolding of enamel and dentine with variable degrees of severity. This ranges from the invagination affecting the amelo-cemental junction only, to extension into the pulp cavity and extending into the root, which can sometimes form a second apical foramen.

The most commonly used classification is proposed by Oehlers⁶² and is categorized into three types (Table 6 and Figure 5).

Teeth affected with dens invaginatus are associated with an increased risk of developing pulpal pathology,⁶³ and this may occur without evidence of any obvious caries or history of trauma.⁶⁴ It is assumed that the infection results from increased plaque accumulation within narrow grooves that are impossible to clean. Dens invaginatus has been associated with other dental anomalies and disturbances of dental development⁵⁹ including:

Table 7 summarizes the restorative challenges associated with dens invaginatus.

'Double teeth'

During tooth development, there can be developmental abnormality whereby a ‘double tooth’ is formed. These ‘double teeth’ can be classified as: gemination, fusion or concrescence.^66,67,68

Gemination is caused by an attempt of a tooth germ to divide unsuccessfully during development.^69,70 It may result in the formation of a large tooth with a bifid crown or two crowns and have a single root and one root canal.^71,72 It may also result in varying depths of coronal notches or grooves. In these cases, the normal number of teeth is observed if the affected tooth is counted as one.⁶⁶

Fusion is caused by the combination of two tooth germs during development.⁷⁰ It may result in the formation of either a large tooth with a bifid crown or roots and may involve the entire length of the tooth or the roots only.⁶⁸ In these cases, there are usually a reduced number of teeth observed if the affected tooth is counted as one, unless there is a supernumerary tooth.⁷³

Concrescence is the union of two adjacent teeth by the cementum.⁷⁴

The prevalence of gemination or fusion for the primary dentition is between 0.4–0.9%, and for the permanent dentition averages 0.2%.^75,76

The aetiology of gemination/fusion is unknown, but there is a potential genetic cause.⁷⁰

Gemination is more common in the anterior maxillary dentition and fusion in the anterior mandibular dentition.⁶⁶Table 8 shows the restorative challenges associated with 'double teeth'.

Resin bonding and veneers

Bonding is the same as for a normal tooth as the enamel and dentine are structurally normal. The major challenge is correction of the unusual aesthetic appearance. It is easier to make a microdont appear of normal dimensions than to camouflage a tooth with macrodontia. Therefore, creative techniques are required that include reduction of bulbosities and modification of buccal grooves that create the desired illusion. These cases are best planned using laboratory diagnostic wax-ups and careful technician–dentist interaction to plan the result. Showing the patient the planned aesthetic changes as part of a wax-up is part of the consent process.

Figure 6 shows the management of a patient with dens invaginatus of the maxillary right lateral incisor. The patient presented with pain and swelling due to apical periodontitis secondary to pulpal necrosis. Clinically, the tooth had a deep labial grove, was proclined and was macrodont. Treatment involved disinfection of the root canal by eliminating the invagination to create a single pulp chamber. The root canal was filled with an apical plug of MTA and guttapercha. Following non-vital bleaching, the tooth was camouflaged with a combination of enameloplasty and direct composite placement.

Veneers are generally less invasive and require less preparation than crowns, but teeth affected with dens invaginatus are associated with an increased risk of developing pulpal pathology. They often have enlarged pulp chambers that can be easily exposed during conventional veneer preparation, particularly if the tooth is rotated. Veneers should only really be considered when there is minimal or no preparation required or the tooth has had successful endodontic treatment carried out. Veneers are particularly useful in teeth that have abnormal tooth shape and size. Composite and porcelain veneers have been used to camouflage 'double teeth'.⁸⁰ They have also been used to restore/close the space of 'double teeth' that have been sectioned.^81,82

Novel techniques using double veneers have been described to make a double tooth appear as two separate units.⁸³Figure 7 shows a case report previously published in Dental Update that shows the use of ‘double veneers’ to camouflage ‘double teeth’.⁸³

Crowns

The teeth affected by dens invaginatus that are not endodontically treated are at a high risk of developing pulpal pathology.⁶³ Preparation of vital teeth therefore carry a high risk of pulp death. Crowns should only be considered when there is minimal or no preparation required or the tooth has had successful endodontic treatment carried out.

Crowns are particularly useful in teeth that have abnormal tooth shape and size.

In 'double teeth', it has been suggested post-endodontic and sectioning of the tooth, that a crown may be a consideration in restoring teeth that have been fused.⁸⁴

Metal-ceramic⁸⁵ and all-ceramic crowns⁸⁶ have been used to restore affected anterior teeth.

Removable prosthesis

Dentures can be used to replace dens-in dente teeth that have been extracted or used as overdenture abutments, if applicable. Dentures that are used around dens invaginated teeth, which are vital, should be designed, where possible, to provide clearance around them so that there is a reduced risk of harbouring bacteria that can contaminate or cause dental caries and potential sequelae of pulpal pathology. The changes within the invagination increase the risk of bacterial contamination and therefore increase the risk of developing pulpal pathosis.⁵⁷