WHAT ARE THE CRITERIA THAT CAN BEST GUIDE US IN CHOOSING THE TYPE OF POSTS AND CEMENT TO USE FOR OUR SPECIFIC CASE TO BE EFFECTIVE?
The primary function of a dowel post is to provide retention for a core. The core replaces the lost tooth structure and houses the final restoration while preserving an apical seal of the endodontic filling.
Base on its function, it is important, then, to select a post system that provides maximum retention, yet should conserve more of the viable tooth structure. There are several dowel systems available in the market that we can choose from for the final restoration of our endodontically treated teeth cases, unfortunately, there's limited knowledge on how effective these dowels really seal the RCT-treated tooth/teeth as posts whatever it maybe encounter so many challenging factors wherein these posts are subjected to different forces such as tension, compression and torquing. And, most of these forces clinically manifest themselves as tensile forces or shear stress on post-cement-dentin interfaces that can affect the success of our treatment.
Let's face it, with so many choices of endodontic dowel systems available in the market with their own marketing strategies to make us choose their system, as practitioners how can we really decide which one will be the best choice for us in general in our practice when it comes to treating our endodontically treated / mutilated teeth cases. On this post, kindly allow DMD cEnter to provide you a criteria that may be able to make this decision easier with a more scientific and evidence-base considerations.
By rule of thumb, it is clinically shown that as a length of the post increases, the retention also increases without jeopardizing the apical seal and still be able to conserve the remaining root and tooth structure. This is as stated based on the studies done by Stockton LW and Kurer et al . According to the study, a post that is too short is more or less bound to fail. It has been shown that inadequate length of post is one of the leading cause of failure of restorations on endodontically treated teeth. However, on the other hand, if a post is too long, it may also result the damage the seal of root canal or risk perforation in the end.
Thus, the advice is it is always best that the length of post to use, it must be at length wherein we can still leave ideally 5mm of gutta percha. Why?
➢To avoid disturbing the apical seal
➢Curves and lateral canals occur in the apical 5mm
Base on the clinical studies published by Rosentiel, Contemporary Fixed Prosthodontics, 4th Edition on 2006, if 5mm can't be achieved, the absolute minimum should be at 3mm.
➢ Retention seems adequate with 2/3 root length.
➢ Resistance to deflection is greatest at 3/4 the root length.
If we can't get that post that long:
As per mentioned above, if 5mm gutta percha cannot be retained with a post 3/4 the length of the root (or even 2/3 length of the root):
➢ The solution might be make the post as long as possible, while still leaving 3mm of gutta percha.
➢ The post should be at least the same size as the height of the crown.
➢ Others say 1/2 the length of the root in bone is enough.
➢ Molar posts should not exceed 7mm in length
(a) On distal canal of lower molars
(b) Palatal canal of upper molars
➢ If placing a second post in a molar is needed, try to avoid doing this to the:
(a) the MB canal on upper (root curvature and concavities)
(b) the ML canal on lower molars (not uniform)
➢ Try to avoid:
(a) the Distal surfaces of the Mesial canals in mandibular molars (thin walls)
(b) the M & D surfaces of the Buccal canals in maxillary molars (easy to strip)
➢ Many studies recommend removing the gutta percha with heat and not rotary instruments. Rotary instruments can be used to clean the walls and to prepare a post space. This not only preserves dentin, but, also helps avoid perforations.
The increase in post diameter does not mean that its increases its retention as stated by Standlee and Kurer et al. It is because of that every tooth has a different variations in canal morphology.
The diameter of post should not be increased at the expense of the dentin, as the dentin provides a the structural strength of the tooth depends on the quantity and inherent strength of the dentin.
➢ Post diameter should not be greater than 1/3 of the root diameter. Root concavities are not always easy to see on the radiograph.
➢ Peter et al found that if fitted correctly, longer thicker posts were a little less likely to fracture as long as parameters were not exceeded.
➢ Larger diameter posts require higher tensile strengths to break the cement seal and pull them out of the roots of extracted teeth.
➢Trabert et al found that loner narrow stainless steel posts absorbed less energy.
➢ A wide post generates more stress when it is short.
➢ Post length is more important than width in generating stresses. Medium to wide posts should be avoided if the post is short (causes too much stress)
➢ A thicker post actually stresses a tooth more, since it is sitting in a thinner tooth. Since length is more important than width, going for a wider post is not worth the risk of fracture.
A parallel-sided post is more retentive followed by the parallel tapered combination design, whereas tapered post is the least retentive as stated by Johnson et al. The lower retention obtained with the tapered-end post is attributed to the lack of parallelism in the apical portions.
A well adapted, passively cemented, parallel post is considered most retentive with least stress. On the basis of retention and stress distribution, parallel-sided posts should be favoured for clinical use over parallel posts with tapered apical ends.
➢ Categories of Posts:
3. Threaded (Screw)
(a) The most retentive post is the threaded (screw) post.
(b) The least retentive post is a tapered post. A parallel post is more retentive than a tapered post.
(c) In a certain study, on average, it took about 4 1/2 times the tensile strength to pull out a paralle post than it did to remove a tapered post from tooth of similar diameter.
➢On Resistance to Fracture:
(a) Screw posts are the most likely to fracture a root.
(b) In a study, teeth extracted for vertical root fractures shows 67% of those teeth had screw posts.
ON SURFACE CONFIGURATION
Surface configuration plays an important role in retention. The posts with the rough surfaces produce a higher strength of the bond to the cement than do the untreated posts, because of increase in surface area for bonding. The threaded posts are more retentive than the serrated posts, whereas the smooth posts have the least retention. The retentive values for sandblasted round parallel sided dowels are significantly higher than the values of non sandblasted dowels of similar forms as stated by Balbosh A et al and Nergiz et al.
While D’Arcangelo C et al, has shown by treating the surface of the fiber posts with silanization, acid etching with hydrofluoric acid and sand blasting through SEM analysis that presense of microretentive morphological changes, which certainly increase post-retentive properties without decreasing their flexural properties.
According to Monticelli has shown the adverse effects of using Hydrofluoric acid for etching the fiber posts as it can cause the extensive damage to the fiber posts by giving rise to the micro-cracks and longitudinal fractures of the fiber layers.
➢ Serrations, roughening and grooves increase retention
➢ Serrated posts are 30%-40% more retentive than smooth posts.
➢ Smooth-sided parallel posts generate the greates apical stresses.
➢ Serrated parallel posts distribute forces on both sides of the post.
➢ Serrated parallel posts place the root under less stress than other posts.
But, this all depends on having:
(a) Adequate length
(b) A round canal:
i. maxillary central
ii. maxillary 1st molar
iii. mandibular 2nd premolar
iv. disto-buccal canals of maxillary molars
ADAPTATION OF THE POST
A well adapted post has a better retention within the root canal. When the resin adhesive in the well fitting canals provided very good retention. The well adapted post will most likely resists torsion forces.
ON POST SPACE PREPARATION
The mechanical removal of the sealer-impregnated dentin from the canal walls during post-space preparation is a critical step in achieving optimum post retention when resin cement is used.
When sealers are introduced into the canal and obturation forces are applied, it is likely that sealer constituents penetrate into the dentinal tubules.
Therefore, when using resin cements a critical factor is the post-space preparation of the walls of the canal so that a “freshened” surface is produced into which resin cement could adequately flow and bond.
ON CHELATING AGENT
In order to enhance retention, it is advised that before the cementation of a post, the canal space should be cleaned by application of a chelating agent to remove the smear layer. When smear layer is removed, it allows the cement to enter the dentinal tubules and provide micro- mechanical retention.
Commonly used agents for irrigation include EDTA or 5-5.25% NaOCl that denatures protein and removes collagen, and Acids (such as 50% citric acid and 37% phosphoric acid) that remove the smear layer and demineralise dentin as stated by Keles A et al.
ON TYPE OF CEMENT
All posts, to a greater or lesser extent, gain their final retention by cementation into the tapered root canal. One criterion for selection of the cementing medium would be that the one chosen has the greatest retention. Many dental cements do not exhibit any real adhesion to the dentin or enamel surfaces. Their retention is essentially a locking of thin cement layer into irregularities of both the post and the canal.
The ability of a cement to retain a post influences the prognosis of the restoration. Cement that yields high retentive values will allow the use of shorter posts, thus maintaining the apical seal and preserving sound root structure. Various types of cements are used for post retention.
According to Assif D and Ferber A studies has shown that when resin-based cements were used, a significant increase in dowel retention was shown compared with conventional cements.
Adhesive resin cements are being used for cementation of the post because of their stronger bond to tooth structure. Current dentin bonding agents have hydrophilic and hydrophobic components that react with both the tooth structure and the resin.
These agents achieve chemical adhesion as well as micromechanical bonding to dentin collagen and tubules, leading to reduced microleakage between the tooth and the restorations, thus reducing recurrent caries around restorations and minimizing coronal and apical contamination of the root canal system .
ON CEMENT APPLICATION
The most reliable method of placing the cement into the prepared space is with the help of a lentulo spiral as described by D’Arcangelo C et al .
The lentulo spiral gives better spinning and spreading of the cement in post space, due to centrifugal dispersion of the cement. This method of spreading helps in reducing the voids and increases contact of the cement with the walls leading to optimal retention .
Coating only the posts with cement always gives an incomplete lute.
There are other methods of cementation that are used sometimes but have some drawbacks:
I. ENDODONTIC EXPLORER- It incorporates a vertical pumping that channels the cement efficiently to the apical region but not along the lateral walls, possibly because of adherence of the cement to the instrument.
II. DIRECT APPLICATION- Cement is applied to the post and transported to the canal. The voids in the apical region are result the presence of air trapped as the during the seating of the post.
During final seating, the air incorporated into the cement is forced along the shaft of the post to create voids and irregularities.
III. PAPER POINT- It creates the greatest number of voids. During cementation, the paper point becomes saturated by absorbing liquid from the body of the cement. As the cement adheres to the moist porous paper point, the continual pumping action incorporates air into the cementing medium thus creating voids inside the post space.
Base on the information given above from different studies, we can conclude the following:
➢ The ideal post is at least as long as the crown, not wider than 1/3 root diameter, serrated, rough or grooved and parallel.
➢ Cast posts remove a lot of tooth structure. Stainless steel posts can corrode. Titanium posts are weaker than steel. Ceramic posts do not bond well. All of these types of posts can fracture roots.
➢ Carbon fiber posts bond well, but, are black. Glass fiber posts bond well. Quartz fiber posts are stronger than glass. None of the above render a tooth unrestorable after failure.
➢ A post is indicated ONLY when it is needed to retain a core.
➢ A ferrule of at least 2mm is critical to prevent loss of post or fracture of the tooth.
➢For retention and fracture resistance, posts should be long, parallel and rough/serrated.
➢ While metal posts are stronger, fiber posts (carbon, silica, or quartz) generally do not cause catastrophic fractures that cannot be retreated.
➢ Bonding a post gives the best retention and resistance to fracture.
➢ Composite resin cores render endodontically tooth more resistant to fracture. When resin cores fail, the tooth can usually still be restored.
➢ Molar crown preparations should be at least 4mm tall.
➢resistance features can be useful in short crown preps ONLY if total occlusal convergence (taper) is not excessive. Re-preparing an overtapered preparation is the most effective resistance feature.
With these conclusions based on studies, we at least have an evidence-based factors we can consider before we choose and purchase our posts and cements to use. However, at the end of the day, we are the ones clinically can see and assess our cases, therefore, the final decision depends on us of what's best for our specific case.
1. Stockton LW. Factors affecting retention of post systems: a literature review. J Prosthet Dent. 1999; 81: 380-5.
2. Kurer HG, Combe EC, Grant AA. Factors influencing the retention of dowels. J Prosthet Dent. 1977; 38:515-25.
3. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: Effect of cement, dowel length, diameter and design. J Prosthet Dent. 1978; 39: 400-05.
4. Johnson JK, Sakumura JS. Dowel form and tensile force. J Prosthet Dent. 1978; 40: 645-9.
5. Balbosh A, Kern M. Effect of surface treatment on retention of glass-fiber endodontic posts. J Prosthet Dent. 2006; 95: 218-23.
6. Nergiz I, Schmega P, Platzer U, McMullan-Vogel CG. Effect of different surface textures on retentive strength of tapered posts. J Prosthet Dent. 1997; 78: 451-7.
7. D’Arcangelo C, D ́Amario M, Vadini M, De Angelis F, Caputi S. e323 Selecting and Cementing A Post- A review.
Influence of surface treatments on the flexure properties of fiber posts. J. Endodon. 2007; 33: 864-7
8. Monticelli F, Ferrari M, Toledano M. Cement system and surface treatment selection for fiber post luting. Med Oral Patol Oral Cir Bucal 2008; 13(3): E214-21
9. Chan FW, Harcourt JK. The effect of post adaptation in the root canal on retention of posts cemented with various cements. Aust Dent J. 1993; 38: 39-45.
10. Mayhew JT, Windchy AM., Goldsmith LJ., Gettleman L. Effect of root canal sealers and irrigation agents on retention of preformed posts luted with resin cement. J Endod. 2000; 26(6): 341-4.
11. D’Arcangelo C, D ́Amario M, Vadini M, De Angelis F, Caputi S. An evaluation of luting agent application technique effect on fibre post retention. J Dent. 2008; 36: 235-40
12. Grewal PS, Grewal KP, Grover D. A Literature Review of Different Criterias for Selecting and Cementing a post- A review. J Clin Exp Dent. 2011;3(4):e321-4.
Dr. Bryan Anduiza - Writer
Dr. Mary Jean Villanueva - Writer | Editor