There are a wide range of materials on the market, each having specific properties that make the material favourable for use as a post (Fig. 1). Metal posts have the longest history of use and are extremely popular for cast posts because metals can be utilised for lost wax casting. More recently, techniques to mill posts from various materials using CADCAM programmes have become available. Aside from metal posts, prefabricated carbon fibre, glass fibre reinforced composite posts and ceramic materials are now also popular choices for post crowns.
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Figure 1a) and b) examples of post and core systems
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A retrospective study of up to ten years comparing cobalt chromium cast, titanium prefabricated, and stainless steel prefabricated posts found no significant difference in survival between metal types and a mean survival estimate of 83%.2 The relative strengths and weaknesses of metal posts when compared to other post materials are discussed in more detail in the following sections.
Fibre posts although not as rigid as metal, have a number of key advantages. Firstly, the use of a fibre reinforced composite post allows bonding of the post to resin cement compared with traditional mechanical retention with non-adhesive cements. This is particularly advantageous where there is minimal ferrule or perhaps a shorter post is required. Fibre posts are also aesthetic, as they are typically closer to a natural shade, and therefore have less of an effect on the appearance of translucent crowns. Lastly, these fibre posts are more flexible than metal or ceramic posts, producing a more homogenous stress distribution on the canal.3 However, a systematic review found no conclusive evidence that this decreases the chance of root fracture.4 The ability of glass fibre posts to survive over long time periods has been demonstrated. A retrospective study of 109 custom fibre posts found a mean overall survival estimate of approximately seven and a half years.5
There are two main ways to make a fibre post. The first is the use of a prefabricated fibre post and the second is a custom made fibre post at the chairside. A randomised controlled trial of six years found that prefabricated fibre posts tend to survive longer than custom fibre posts.6 One explanation for this is that operator error from making a fibre post chairside may introduce weaknesses or design flaws into a post.
Ceramic posts are another aesthetic post choice as they are available in light shades. Zirconia is a popular ceramic and may be milled to create a custom indirect post or used prefabricated. Two in vitro studies comparing zirconia and metal posts under loads found that zirconia posts would tend to fracture due to brittleness, rather than distort and bend like the metal post.7,8 This may be advantageous because ceramic posts may fracture before the tooth does which is a serious but potentially less detrimental outcome for a tooth. However, retrieval of a fractured zirconia post can prove to be extremely challenging.
Similar to fibre posts, using finite element analysis, carbon posts showed a more homogenous stress distribution along the root canal surface when compared to metals.9 However, the disadvantage of carbon posts is that they are dark, non-tooth coloured materials which are hard to mask, so will often shine through translucent restorations or the root (Fig. 2).
Figure 2: a) Photograph showing a failed composite core and b) carbon fibre post in situ.Note the black colour of the post showing through the composite. This tooth was part of a failed six-unit bridge which was dismantled
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As with the wide choice of materials, there are many designs of posts and often the design is linked to the properties of the material.
Passive posts sit in the prepared canal space without engaging with the dentine directly for retention, whereas active posts are those which engage the root dentine via threads and gain retention from this, typically acting like screws (Fig. 3). An active post engages with the dentine and therefore exerts a force upon it which may cause microcracks that propagate. A systematic review found that active screw posts decrease tooth survival over that of passive posts due to the resultant stresses from placement of an active post.10 Since then, a randomised controlled trial found that 93.5% of passive glass fibre posts and only 75.6% of metal screw posts survived one year.11 Furthermore, the vast majority of the active post failures were root fracture. For this reason, active (threaded) posts are no longer recommended.
Figure 3: Radiographs and photographs of a threaded post being removed and replaced.a) Pre-treatment radiograph showing rlatively short threaded post in situ. b) The post following removal of the surrounding core material and after a number of revolutions to unscrew the post in a coronal direction. c) The threaded post following removal with a millimetre scale below. d) Post-treatment radiograph showing endodontic retreatment and a new composite fibre post in situ with a provisional crown
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A longer post is more retentive than a shorter post because of the increased surface area contact between the post, cement and tooth.12 However, teeth naturally taper and as they do so, the risk of perforating laterally increases. An in vitro study exerting forces on post crowned teeth found that longer posts also increase the chance of root fracture as they are thought to concentrate stress on the thinner apical portion under load.13 Despite the findings in this study, other studies have found shorter fibre posts increase the chance of tooth fracture due to production of higher lateral forces.14 It is important for the post preparation to extend well beyond the level of the alveolar bone, as this provides 'bracing' against these lateral forces. Furthermore, dowel preparation disturbs the apical seal of gutta-percha. This must be weighed against the negatives of shorter posts and so it is generally accepted that the apical 3-6 mm of gutta-percha should remain undisturbed to preserve the apical seal.15,16,17
Width of post is another compromise between the properties of the material and preservation of tooth structure. Too thin and the material risks fracture under load; too thick and excess tooth structure will be removed resulting in weakened root structure. In vitro studies have shown that wider posts increase the risk of tooth fracture.18 If a wider post is required, a material with a higher Young's modulus of elasticity such as glass fibre is favoured as the stiffness of a thick metal post could induce significant stress on the canal wall.19
Another disputed area of post crowns is whether to place a tapered or a parallel post. A taper will reduce apical preparation of the dowel space but retention of the post will also decrease.20 Tapered posts may also act as a wedge to concentrate lateral forces on the root. Cast posts are often naturally tapered as the coronal portion of the root canal and the pulp chamber may be significantly tapered. Despite in vitro studies showing differences in retention of parallel and tapered posts, a retrospective study found that 98.6% of parallel fibre posts compared to 96.7% of tapered fibre posts survived a mean observation period of 5.3 years, which was not statistically significant.21
Posts with a roughened surface, as opposed to a smooth surface, can provide micromechanical retention to increase retention of the post when cemented.22 This can commonly be produced through air abrasion techniques. Macromechanical features such as grooves also increase retention and aid venting of cement for accurate seating of posts.22 Surface design is not thought to influence stress distribution.3
In the past, zinc phosphate-based cements have been a popular choice for cementation of post crowns as these cements have been found to provide good retention.23 These cements rely on mechanical retention by filling the gap between the post and the root canal space. However, although some authorities still advocate their use for the cementation of metal posts, zinc phosphate cements have now largely been succeeded by cements which can adhere to the tooth, post or both. These include glass ionomer cements (GIC), resin-modified glass ionomer cements (RMGIC), and composite resin cements.
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Adhesive bonding allows an improved marginal seal between the post and the tooth and improves the mechanical retention of the post core unit when compared to zinc phosphate.24,25 This can be beneficial, especially when using thinner or shorter posts. For composite resin bonding to have its maximum benefit, the resin must be allowed to enter the dentinal tubules of the root canal. After mechanical preparation of root canals a smear layer is left which impedes entry of resin cements into the tubules. A study using chlorhexidine, ethylenediaminetetraacetic acid (EDTA) and then phosphoric acid to prepare the root canal surface showed an increase in bond strength of fibre posts.26 However, studies have also shown that chemicals such as EDTA used before resin cementation of fibre posts could decrease the retention by interfering with the resin bond27 and may in fact increase microleakage when using glass ionomer cements.28 The use of resin cements with dentine bonding agents and silane has been found to be advantageous. A systematic review of in vitro studies concluded that the use of self-adhesive resin cements may improve retention of glass fibre posts.29 A further systematic review and meta-analysis of in vitro fibre post silanisation found that silanisation can improve the retention of glass fibre posts but only when a fibre post surface is pre-treated and then silanated.30 Pre-treatment included a wide range of non-specific approaches such as air abrasion, etching with phosphoric acid, tribochemical coating and the use of chemicals such as hydrogen peroxide. Excluding the use of self-adhesive resin cement, another systematic review of in vitro studies found a 43.4% increase in retention of glass fibre posts pre-treated before silanisation.31 It may therefore be worthwhile pre-treating/cleaning a fibre post prior to silanisation, however, the results are from low quality studies of which the majority have a high risk of bias. Metals and certain ceramics are hard to bond, but certain proprietary cements for example, Panavia 21 (Kuraray, Japan) are formulated to provide chemical adhesion to metal.
Though removal of cemented posts is often challenging, a study looking at retrievability of posts found that zinc phosphate cemented posts can be retrieved reliably with ultrasonics with a low (0.06%) root fracture rate.32 Even with magnification and ultrasonic techniques the removal of resin-cemented posts has been found to take significantly longer than both GIC and zinc phosphate cemented posts.33
The main issue with RMGIC as a post cement is hygroscopic expansion, which, when compared with other cements may cause higher stresses to develop in the intraradicular dentine and decrease fracture strength of the post-crowned tooth. Studies have shown the degree to which it expands varies on the specific RMGIC chosen and how it is placed, but the impact of its effect on cementing post and cores is not well demonstrated.34
Of the many diverse materials a dentist uses to improve your smile and your bite, one of the more common is the dental post.
A dental post is a small, metal shaft implanted in a tooth. This is often the last option if the root of the tooth isn&#;t healthy enough to support a tooth or dental appliance.
When a tooth becomes decayed, chipped, or broken, the result can be intense pain. This is because the nerves inside the tooth become exposed or irritated. A root canal treatment can save the remains of a tooth. But the procedure includes drilling into the tooth to clean out the infected pulp. Your dentist will then seal the area off to protect it from further damage. The procedure can often be intensive and isn&#;t for everyone.
A root canal can sometimes leave behind a missing or misshapen tooth. So a prosthodontist goes to work on a crown to complete and fortify the tooth.
But the remaining tooth must be structurally sound before creating and placing a cosmetic replacement. That&#;s where the dental post comes in.
Your dentist will advise you on whether your root canal requires a post. In some cases, if sufficient tooth remains after a root canal, the crown won&#;t need a post.
When performed by an experienced professional, the post-and-crown treatment can save your tooth while improving your health and appearance.
For some patients, a dental post and crown may be just the answer for a tooth that&#;s been through root canal treatment.
But for others, the procedure has taken too much of the tooth to restore. In such cases, your dentist will recommend a dental implant (dental implants in one day may be an option). Unlike a post, a dental implant is a completely new, whole tooth that replaces the one lost to root canal treatment.
A titanium root is surgically implanted into the jawbone and allowed to heal. Once it integrates, the root becomes the anchor for a bridge, crown, or set of dentures that can look and feel just like the rest of your teeth.
As an implant expert, John C. Stone, DDS (Doctor of Dental Surgery) has more than 30 years of experience in prosthodontics&#;also known as cosmetic dentistry.
Additionally, Dr. Stone not only performs successful cosmetic procedures, but he also teaches implant dentistry to other dentists at two local continuing dental education programs. Generations of family members trust Dr. Stone to replace or restore teeth and to provide ongoing dental care.
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