The restoration of structurally compromised endodontically treated teeth: principles and indications of post and core restorations

Tooth position

In most cases, post and core restorations are only considered a suitable treatment option for anterior teeth with insufficient coronal tissue to retain a crown independently. Posts are usually not indicated for molar teeth, as retention can usually be obtained using the pulp chamber and the coronal 2 mm of the root canal system (Nayyar core).⁶ The same does not apply to premolars, however, as they usually lack sufficient pulp chamber space, and coronal tooth tissue to allow reliable restoration. Furthermore, post preparation in posterior teeth is more likely to result in lateral perforations due to the curvature of the roots.⁶ Despite the increased likelihood of iatrogenic root perforation during post preparation, studies have shown the use of post-retained crowns in heavily restored molars, as an alternative to extraction, to be successful, with a large number of these restorations remaining intact 4 years post-treatment.⁷ This highlights the importance of case selection and operator technique. Although treatment outcomes may be generally less favourable, an intimate knowledge of root anatomy, along with appropriate radiographic assessment, can allow successful post and core restorations in posterior teeth.

Quality of the endodontic treatment

Root canal treatment carried out to a high standard is a prerequisite for a post and core restoration. Radiographic examination should demonstrate a root canal filling within 2 mm of the apical terminus, of good radiodensity, with an absence of voids and absence of apical pathology.⁸ A prospective study, carried out by Ng et al, highlighted endodontic prognostic factors, all of which require consideration when assessing teeth prior to provision of a post and core.^9,10 These factors are highlighted in Table 1. If the endodontic treatment is suboptimal, the tooth must be endodontically retreated prior to post and core placement.

Assessment of root anatomy

Careful assessment of the root anatomy is vital, in not only appropriate case selection, but also in the selection of post type and material.

Root length

Studies have shown that the greater the post length, the greater the resistance to vertical displacement and the greater the distribution of stresses within the root canal system.⁸ Ideally, the post length should be greater or equal to the crown length.⁸ Factors such as root canal morphology and ease of preparation should be taken into account, however, it is recommended that the post be as long as possible, leaving a minimum 4 mm of remaining gutta percha to maintain an apical seal.⁸Figure 1 demonstrates the dimensional considerations required for successful post and core restorations in a single-rooted tooth.

Presence of a ferrule

A ferrule can be described as ‘a 360° metal collar of the crown surrounding the parallel walls of the dentine extending coronal to the shoulder of the preparation’¹⁶ (Figure 1). The presence of a minimum of 2 mm of dentine coronal to the preparation margin has been shown to increase resistance to axial rotation and displacement significantly,^16,17 as well as improving fracture resistance in both in vitro studies and in clinical scenarios.¹⁸ In the absence of an adequate quantity and quality of ferrule, one may consider surgical crown lengthening (Figures 2a and b) or orthodontic root extrusion.¹⁶

Post selection

Over recent years, advances in dental materials have led to a variety of different post materials and configurations, as can be demonstrated in Figure 3. Each one has prophesied to confer either superior mechanical, aesthetic or failure properties. An understanding of their advantages and disadvantages is crucial in aiding appropriate selection. Table 2 and Figure 3 summarize the different classifications.

Initial assessment

Clinical examination revealed a temporary restoration on the UR1, replacing the incisal two thirds of the tooth (Figures 4a and b). The temporary restoration was unaesthetic and there was a marked difference in the colour and dimensions of the tooth in comparison to the UL1. Radiographic examination (Figure 5) revealed a post-retained temporary crown. There appeared to be supra-crestal tooth structure evident, however, it was necessary to dismantle the existing restoration to assess restorability fully.

Assessment of restorability

Initially, the UR1 was dismantled under rubber dam. This was done not only to reduce bacterial contamination of the canal, but would also allow assessment of whether the remaining tooth structure beneath the temporary restoration was sufficient to allow adequate isolation for endodontic retreatment and whether subsequent restoration was feasible. The crown was dismantled using a combination of hand instruments, a high-speed handpiece and diamond burs and an ultrasonic scaler. Following this all that remained was the post, which had retained the temporary crown (Figure 6). Full assessment of the remaining tooth structure could then be carried out. Unfortunately, by this point, adequate isolation with rubber dam was not possible due to the minimal palatal coronal tissue available, so a split dam technique was employed.

Assessment of restorability involved reviewing multiple elements. Firstly, the quantity and quality of remaining tooth structure was assessed. This entailed ensuring that there was sufficient supra-gingival, caries-free tooth structure circumferentially to ensure preparation of a ferrule was possible. Clinical examination revealed that there was sufficient labial tooth structure, however, the palatal fracture of the tooth extended subgingivally. Despite this, assessment of the pre-operative radiograph revealed that the fracture remained supra-crestal therefore, despite the clinical appearance, it appeared that this tooth was restorable, although removal of palatal gingival tissue would be required.

Another element of restorability involved assessing the root morphology, to determine if endodontic treatment and a post- and core-retained crown was possible. This involved assessing the root curvature, length, diameter and periodontal support. In this case, a slight curve was seen in the apical third of the root, however, the tooth appeared to be amenable to endodontic treatment. Further assessment also revealed that there appeared to be sufficient root length to allow 4 mm of apical gutta percha, and a post of equal length to the crown, however, in its present state, there was insufficient coronal tooth tissue to allow for 2 mm of a ferrule (Figure 1). In terms of periodontal support, radiographically the tooth had adequate bone support, with only minimal crestal bone loss. From this assessment, it was determined that restorability was possible, if removal of palatal gingiva revealed sound tooth tissue to allow for a ferrule.

Gingivectomy to create a ferrule

At the following appointment the hyperplastic palatal gingiva was removed to ensure enough palatal tissue for a ferrule (Figure 7). This now meant that endodontic treatment could be commenced.

Endodontic treatment

Endodontic treatment was carried out on the UR1 using rotary Protaper NiTi endodontic instruments under rubber dam isolation using ‘Wedgets’ for dam stabilization. A master cone radiograph revealed a gutta percha cone within 2 mm of the radiographic apex (Figure 8a). The tooth was then obturated with gutta percha, using a warm vertical condensation technique, and an overlying layer of glass ionomer cement was placed. The tooth was then temporized with a temporary post crown.

Post space preparation

In order to determine the preparation needed, further assessment of the post-operative radiograph was necessary (Figure 8b). The radiograph was used to gauge the widest diameter at the post depth required, in order to determine the final Parapost drill size and to ensure no lateral perforations occurred during preparation. It is also important to consider root canal form and ease of preparation when determining post thickness, as it is important to ensure that no undue forces are generated during the preparation. Once this had been determined, the root canal was prepared using a Parapost bur, starting with the narrowest bur, until the final bur size could be taken down to the planned post length. As a substantial amount of coronal tooth tissue was now present following the gingivectomy, the post length was kept the same length as the crown height, to avoid removing further tooth tissue. An overlying layer of glass ionomer cement was left sealing the remaining gutta percha. This ensured that the endodontic treatment was not compromised during any point of the post space preparation or during the inter-appointment time. To reduce the risk of perforation, care must be taken whilst preparing the canal system, ensuring a low speed but high torque with the handpiece to facilitate good handpiece control. When preparing a circular canal, anti-rotation features should be considered to reduce the likelihood of the post rotating during function, thus applying undue stress on the luting agent. Small grooves or notches may be prepared into the coronal aspect of the canal to allow for this.

Following completion of the preparation and cleaning of the canal, a master impression was recorded using heavy- and light-bodied, addition-cured silicone impression material, using the smooth Parapost impression post (Figure 9a). A prescription for a cast metal post and core was issued, and the tooth was provisionalized (Figure 9b). An indirect precious metal post was the post design and material of choice, in this case, as the irregular anatomy of the root canal space meant that a prefabricated post would not intimately fit the internal anatomy, and thus a cast post would be more retentive.

Cementation of the post and core

At the following appointment, the provisional crown was removed and the post space was cleaned using Chlorhexidine and an interdental brush, in order to eliminate any debris of remnant temporary cement. Following this, the definitive post and core was cemented in place using a universal resin cement (Figure 10). Following this, the tooth margins were refined to create an idealized preparation for receipt of a crown.

An addition-cured silicone was used to record a master impression for the construction of a definitive crown (Figure 11).

Final restoration

At this visit, the provisional crown was sectioned and removed, and the preparation was sandblasted prior to cementation of the definitive crown (Figure 12a). This removed any temporary cement debris and micromechanically roughened the surface of the tooth to improve cementation effectiveness. Following this the definitive crown was cemented in place using a universal resin cement (Figure 12b). A post-operative periapical radiograph was taken following treatment completion (Figure 13).