Posts – when it all goes wrong! part 1: case assessment and management options

Active or passive?

There is a multitude of post systems available on the dental market and Figure 3 illustrates just a small range of post types. Posts can generally be divided into two separate subgroups, depending on whether they are active or passive and hence on how retention is achieved. Active posts are usually prefabricated and threaded in design, and can be either self-threading or pre-tapped. When placing a self-threading post, the thread of the post itself cuts the counter thread into the walls of the dentine. Strains of a greater magnitude are generated if a self-threading post is threaded rapidly compared to slowly as there is less time for strain relaxation to occur between turns.7 Manufacturers of self-threading posts recommend threading the post space until the post is firmly and completely seated prior to cementation, as this reduces the strain generated at the time of cementation.7 Pre-tapped post systems use a pre-tapping device to cut the counter thread into the walls of the dentine prior to post cementation.

Whilst active posts are more retentive than passive posts of comparable dimensions, the latter are the preferred choice, as less strain is introduced into the root and the risk of irretrievable vertical root fracture is reduced. Passive posts can either be custom-made (usually as a cast post and core) or prefabricated. Their surface is usually either smooth or serrated (negative recesses cut into the post surface) and their shape can be either tapered or parallel. In general, parallel-sided, serrated posts are the most retentive design of passive post.

Metal or non-metal?

Posts can be fabricated from a range of materials which can be divided into metallic or non-metallic compositions. Custom-made cast posts and cores (indirect, laboratory made) are most commonly formed from gold alloys, which have a lower gold content and are harder compared to pure gold, whilst prefabricated metallic posts are mostly made out of stainless steel, titanium, titanium alloy, nickel-chrome or other non-precious metal alloys. Nickel-chrome has the advantage of being strong but, more recently, there has been a trend towards the use of titanium alloy and titanium posts owing to their strength and biocompatibility. Titanium alloys are much stronger than pure titanium due to the addition of small amounts of aluminium and vanadium and, whilst both are easy to use, they do have the disadvantage that they are slightly more radiolucent than nickel-chrome and tend to spark when cut with a high-speed bur. A potential disadvantage of using non-precious metal alloy posts is that they tend to undergo corrosion in situ.8

The newer, non-metallic posts are made out of either carbon or quartz fibres embedded longitudinally in an epoxy resin, or zirconia, and are all substantially more radiolucent than the metallic alternatives. Fibre posts have the advantage that their modulus of elasticity is closer to that of dentine, which results in microscopic flexure. This means that occlusal forces are not directly transmitted to the root dentine, as with a rigid metallic post, which may result in a reduced risk of vertical root fracture. A retrospective study which evaluated the treatment outcome of fibre-reinforced epoxy resin posts and cast post and cores, over a four-year period, found that 9% of the teeth in the group restored with cast post and cores demonstrated root fracture compared to none in the group restored with fibre-reinforced epoxy resin posts.9 Quartz fibre and zirconia posts have the advantage of being translucent, white or tooth-coloured, which has an added aesthetic benefit when used in conjunction with composite cores and all-ceramic crowns or bridges.

Decementation

Physiological micro-movement of natural teeth occurs throughout the day due to normal masticatory forces, which can lead to partial breakdown of the cement lute around post-retained restorations.14 Disruption of the cement lute around the most coronal aspect of the post results in the fulcrum of movement transferring apically, which amplifies the lever action. This magnifies the stress, which is transmitted to the remaining cement lute, resulting in failure of the restoration due to complete decementation of the post or, at worst, root fracture.^15–17 This pattern of failure is often accelerated with passive, prefabricated parallel-sided posts as there is often poor adaptation of the post to the coronal third of the post space.^17–19 Active, threaded posts pose a specific problem because significant coronal leakage and caries can occur around the post prior to the threads disengaging, at which stage the tooth may be unrestorable.

Root fracture

Root canal treatment has been implicated as a major aetiological cause of tooth fracture, the reasons for which are due to a variety of factors including:

It was at one stage thought that endodontic treatment resulted in a decrease in the moisture content of the dentine, which resulted in the tooth becoming more brittle and therefore more prone to fracture. A study by Papa et al, however, contradicts this theory as they demonstrated, using matched pairs of freshly extracted human teeth, that there was no statistical difference in the moisture content between root canal treated teeth and vital teeth.21 This having been said, it is generally accepted that root canal treated teeth have decreased strength, but this is more likely to be due to loss of coronal tooth structure rather than any changes in the biochemical properties of the dentine which may have occurred following removal of the pulp.

It is possible that some types of endodontic sealer may affect the physical properties of root canal treated teeth. For example, eugenol has been shown to increase the microhardness of dentine and is often a constituent in root canal sealers.22 Some types of endodontic sealer may also influence the retention of posts, as a study by Bergeron et al demonstrated that the use of a resin-based sealer, AH26 (Dentsply/Maillefer, Tulsa Dental, OK), significantly increased the retention of posts, compared to groups obturated with a zinc oxide and eugenol sealer, Roth's 801 Elite Grade cement (Roth International, Chicago, IL).23

Preparation of a root for a post further weakens the root dentine, however, careful removal of gutta-percha root filling material and post space preparation with gradually increasing sized instruments (Gates Gliddens and post space drills/reemers) to no greater than a third of the diameter of the root, can potentially reduce the risk of root fracture. This having been said, the risk can never be completely eliminated as occlusal forces are transmitted to the weakened root.

There are a number of signs that may indicate the presence of a vertical root fracture. The classic signs include mobility of the post and restoration, repeated decementation of the restoration and the presence of an isolated, deep and narrow periodontal pocket. Other signs and symptoms may include sinus formation, tenderness to biting, tenderness to percussion and increased mobility of the tooth, however, these signs could also be associated with peri-radicular periodontitis.24 Radiographs may aid in the identification of a vertical root fracture by demonstrating the presence of a complete or incomplete fracture line, or by a general widening of the periodontal ligament space.24 The classic radiographic sign of a long-standing vertical root fracture is a halo radiolucency (Figure 4). However, the use of a radiograph for the diagnosis of a vertical root fracture in the absence of any clinical signs or symptoms should be interpreted with care as, for a fracture line to be visible radiographically, the x-ray beam has to pass along the line of an undisplaced fracture, which would be a rare event. Horizontal and oblique root fractures that occur in relation to posts are usually as a result of excessive occlusal forces, poor post design or a lack of ferrule. The importance of the ferrule effect in increasing the fracture resistance of a post restored tooth has been well documented in the literature.25 The ferrule is the collar of tooth tissue that is encompassed and braced by the crown that is cemented onto the core and tooth preparation. In Figure 5a, the teeth have approximately 4 mm of coronal tooth tissue to brace and create a ferrule effect with the cemented crown, whilst in Figure 5b the teeth have no coronal tooth tissue for a ferrule and, as such, are more susceptible to root fracture.

Perforation

Root perforation during post space preparation is usually the result of poor pre-operative assessment (narrow, curved or tilted root) and/or poor operator technique (use of post drills that are too wide and end-cutting). Most perforations, whilst usually avoidable, significantly reduce the prognosis of the tooth. The success of perforation repair is largely dependent on the material used, timing of the repair and location of the perforation. MTA (Mineral Trioxide Aggregate, Tulsa Dental, OK) is the material of choice for perforations created below bone level, whilst glass ionomers or resin composite-based materials can be used if the perforation is supra-crestal. Perforation repairs should ideally be placed as soon as possible after the perforation has occurred. Generally, cervical third perforations have a poorer prognosis, especially if they are involved with the epithelial or connective tissue attachment of the tooth compared to those occurring in the mid or apical third. Perforations are often easier to repair internally with the aid of an operating microscope and surgical repair (external repair) of a perforation is easier if the perforation is accessible from the labial/buccal aspect of the root. A newer material, Biodentine™ (Septodont, France), a bioactive, calcium silicate cement, was launched in 2010 and shows great promise for applications such as perforation repair and other indications advocated for MTA.

Endodontic failure

Distinguishing between healing of an endodontic lesion and failure can be difficult. It is recommended that endodontically treated teeth are followed up for at least four years to assess the treatment result and therefore serial radiographs are required to determine radiographic failure.^26–29 One study reported little deterioration in teeth with small periapical areas, sometimes over an extended period of time.30 Therefore, careful case selection may allow monitoring of small, clinically asymptomatic periapical areas, however, large, expanding periapical radiolucencies require either repeat orthograde root canal treatment, peri-radicular surgery or extraction and replacement if indicated.

Once it has been established that endodontic failure has occurred, it needs to be addressed. Inadequate root canal treatment is usually the cause and may be due to:

Microleakage may also contribute to endodontic failure, and the importance of the coronal seal has been well documented.31 The chances of endodontic failure can be reduced by preparing the post space immediately after obturation and placing the definitive post crown as soon as possible, providing the tooth remains asymptomatic.^32,33 Temporary post crowns leak considerably more along their entire length compared to permanently cemented posts and their prolonged use should be avoided.34