In a previous article (“Choosing an Endodontic Post System,” Inside Dentistry®, October 2013, Volume 9, Issue 10), the authors reviewed the different choices clinicians have in restoring severely broken-down teeth with posts. In an attempt to come to a scientifically based decision, the cited literature focused on protecting and preserving dentin, working with tapered and ovoid canals, managing C-factor polymerization contraction stresses, and examining threaded post factors. In this article, however, the authors use independent research to address additional questions that should be asked when assessing the relative merits, drawbacks and challenges of using currently available endodontic post systems:
- Is there any difference in failure mode when comparing metal and fiber posts?
- Does modulus of elasticity really play a factor in long-term clinical success?
- In highly esthetic cases, do metal posts of any type have a role in the restoration process?
- In cyclic fatigue, is there a difference in longevity between metal and fiber posts?
- What is the nature of the relationship between round posts and anti-rotational effects?
- Does the penetration of the light down the canal change the conversion factor of dual-cured resin cements and dual-cured self-adhesive cements?
In an extensive 2011 review of post systems, Goracci noted several recurring findings in the literature regarding failure modes. These include the fact that the risk of vertical root fractures was reduced by the biomimetic behavior of fiber-reinforced composite posts; the risk of vertical root fracture was increased with rigid posts because of the internal transmittance of stress toward the apical (Figs. 1, 2); and root fractures are very rare with less rigid fiber posts.1 If fractures do occur, fiber posts produce favorable tooth fractures, which are retrievable and restorable.2 In one study, the two-year survival rate was 93.5 percent for glass fiber-reinforced posts and 75.6 percent for metal screw posts, which were also associated with more unfavorable complications — e.g., root fractures.3
Figure 1: Radiograph of typical oblique root fracture with metal post.
Figure 2: Remaining gutta-percha compromises success.
Stainless steel is stronger than titanium alloy but has the potential for an adverse reaction to the nickel it contains. This fact, combined with concerns about corrosion, contributed to the shift to titanium.4 It is also well-documented that non-precious metal posts corrode in the presence of bimetallism or moisture, which leads to decementation and creates unrestorable failures and fractures.5,6 Dissimilar metals used in the fabrication of post and cores create electrolytic action, resulting in 72 percent of the failures reported in a study of 468 teeth that failed with oblique or vertical fractures.7
Modulus of Elasticity
The elastic modulus, or modulus of elasticity, is the mathematical description of the tendency of an object or substance to be deformed elastically (i.e., nonpermanently) when force is applied. When a post has a higher modulus of elasticity than its anchoring material (i.e., the tooth), the stress concentration is at the bottom of the post. When the post and tooth have similar moduli, however, the stress is concentrated at the top of the post, where it can be dissipated by the ferrule.8
The moduli of elasticity of stainless steel and titanium are 20 and 10 times that of dentin, respectively. Stainless steel and titanium posts with a high modulus of elasticity do not flex with teeth under loading and are believed to cause root fracture.2 In discussing low versus high modulus posts, Peutzfeldt says that high-modulus posts are associated with a higher incidence of root fractures when they finally fail; that is, they cause more damage to the remaining tooth structure and often result in extraction of the tooth involved.9 If the clinician uses a parallel metal post (threaded or not) and over-prepares the canal for a large (wide) post, forces are transmitted and concentrated at the end of the post due to the high elastic modulus, where there has been needless removal of dentin to create a weakened root structure.4 Horizontal loading of a stainless steel post-restored tooth causes a stress that is three times as high as that caused by vertical loading.10 Fiber posts have an elastic modulus that closely mimics the elastic modulus of dentin at 18 GPa, which has been proven to protect teeth from fractures.10
Dietschi states that metal and ceramic isotropic posts prove less effective than fiber posts at stabilizing post-and-cores.11 Metal posts are isotropic in that their elastic modulus is the same at every angle of incidence. Fiber posts are anisotropic and differ from metal in that the modulus of elasticity varies from angle to angle. Their elastic modulus is highest at vertical load and matches dentin when stressed at an angle of 30 to 40 degrees, which is more representative of the forces of mastication.12 Fiber-reinforced posts absorb and dissipate stresses due to their parallel fiber content, and therefore induce a stress field quite similar to that of the natural tooth.6,13
Endodontics in the anterior is often accompanied by darkening of the tooth structure. In many cases, gingival staining caused by some endodontic sealers and the use of gutta-percha in the canal and pulp affect the ability of the tooth to reflect and transmit light, creating a loss of translucence.14 Using a metallic post further accentuates the gingival shadowing of the tissues, negatively affecting the esthetic results of all anterior restorations (Figs. 3, 4).15 Fiber posts, on the other hand, diffuse light through the restoration, creating natural translucence and improving the esthetic result of anterior composites and ceramics (Figs. 5, 6).16
Figure 3: Example of the gingival shadowing that can occur when using a metallic post.
Figure 4: Root darkening due to endodontic treatment with metallic post.
Figure 5: Preoperative view of patient with gingival darkening with metallic post.
Figure 6: Postoperative view shows improved esthetics with use of fiber post.
It is well-documented that cyclic fatigue, or repetitive loads below the mechanical resistance limit, is a more common cause of the structural failure of restorations than a single load or force that is over the limit.17 Numerous studies have examined the effect of fatigue on different types of posts. Fiber resin posts resist fatigue better than teeth with cast posts, and fiber-reinforced dowels and bonded composite cores give significantly stronger crown retention than titanium alloy dowels with composite cores under fatigue loading.18,19 After cyclic fatigue testing, the flexural strength of metal posts and fiber posts decreases by 40 percent and 14 percent, respectively.20 Quartz fiber posts are more than twice as fatigue-resistant as stainless steel and titanium alloy posts.21
There is a wide variability in the physical properties and quality of manufacture of fiber posts, however. They can present as dentin-colored, translucent, white or color-changing translucent, and their capacity for light transmission may be excellent, good, fair or poor. Different fibers used include zirconia-enriched glass fiber, quartz fiber, or glass and carbon fiber. Fiber diameters range from 8.2 to 21 μm, with a fiber/matrix ratio ranging from 41 percent to 76 percent.22 In addition, fracture load can range from 60 N to 96 N, and the flexural strength ranges from 565 MPa to 898 MPa. The quality, type and volume of the fibers, the way the fibers are silanated, and the type of resin used all affect the clinical performance of fiber posts, with some failing in cyclic fatigue in a few thousand cycles and others surviving for more than two million cycles.17
Resistance, or the ability to withstand lateral and rotational forces, is affected by many factors, including the amount of remaining tooth structure, a post’s physical characteristics, and the presence of anti-rotational features (Fig. 7).4
Resistance, or the ability to withstand lateral and rotational forces, is affected by many factors, including the amount of remaining tooth structure, a post’s physical characteristics, and the presence of anti-rotational features.
Figure 7: Multiple fiber posts provide antirotational resistance.
If the clinician uses a restorative post system that allows for integration of the natural shape of the access opening, which is typically ovoid, by using the “augmented” approach into the spaces lateral to the “master post,” the addition of multiple posts will provide more anti-rotational stability.
Multiple studies in the dental literature support this restorative approach. Akkayan maintains that when looking at a cross-sectional view of the prepared root canal in relation to the form of the post system used, fracture resistance can be improved by using accessory posts to fill the post space, thereby decreasing the cement layer as well.23 Accessory glass fiber posts have been shown to improve the biomechanical behavior of flared roots, and glass fiber posts associated with accessory posts have been shown to be the method of choice for restoring structurally weakened roots.24,25 Maceri maintains that the multi-post technique increases the bearing capacity and durability of endodontically treated teeth, and that when prefabricated composite posts’ overall cross-section increases, the multi-post solution induces a significant reduction of stress levels into the residual dentin.26 There is a lower risk of catastrophic failure due to better stress distribution when fiber posts are associated with accessory fiber posts.27
Penetration of Light and Conversion Factor
There are many luting or cementation materials available to clinicians, including zinc phosphate cement, polycarboxylate cement, glass ionomer cement, resin-modified glass ionomers, and various composite resins. The current shift has been to use a dual-cured composite resin [e.g., CosmeCore™ (Cosmedent, Inc.; Chicago, Ill.), Zircules™ (Clinician’s Choice Dental Products, Inc.; New Milford, Conn.), CoreCem® (RTD; Saint-Égrève, France)] that can be used not only for bonded cementation of the post, but also for fabrication of the bonded core.28 The potential of immediate polymerization of the composite allows for simultaneous post insertion, fabrication and contouring of the core, and either immediate finishing and polishing of the final restoration, or impressioning, if full coverage is indicated.
A discussion on adhesion using bonding agents or self-adhesive cements would require a separate article, but there are a few issues that do need to be discussed here. The preparation of a canal space for a post creates a “secondary smear layer” (sealer, gutta-percha, dentin debris) that is much different from coronal dentin, and thus the bonding values provided by manufacturers mean little when bonding to radicular dentin.1 Radicular dentin is also different morphologically and physically, making the creation of a traditional “hybrid layer” more difficult. The current literature suggests that the most reliable results for adhesive cementation to radicular dentin are attained with the use of etch-and-rinse adhesives and dual-cured resin cements.29 The use of “simplified” systems, such as single-bottle all-in-one self-adhesives and self-adhesive cements, is appealing because the technique is easier with fewer steps involved. However, these systems offer questionable durability in bonding to radicular dentin, due to their variability of penetrating the smear layer — especially the secondary smear layer — and their hydrophilicity, leading to hydrolytic degradation of the bond.30,31
The best physical properties and conversion of single to double bonds of dual-cure systems are attained with sufficient light exposure, even though it is claimed that they polymerize in the absence of light.32 To achieve good bonding to root canal dentin with increased microtensile bond strength and increased hardness, photo-initiated polymerization of the adhesive resin and dual-cured resin composite is necessary.33
As mentioned previously, fiber posts do vary considerably in their capacity for light transmission — some do not transmit light at all and, of course, any metal will block the light transmission — and curing lights vary considerably in power and their ability to cure at a distance. It is incumbent upon the clinician when using dual-cured composites to find a post that transmits the most light while at the same time combining the best fiber, loading, structure, quality control and research documentation, and to find the curing light that loses the least amount of energy with distance.34
When clinicians treat severely compromised teeth endodontically, the choice of post system has a significant impact on the overall success and esthetic value of the restoration. Using independent research to help understand the physical properties of different posts in terms of their behavior in clinical situations is critically important.