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PROSTHODONTICS: Anatomical Post Design Applied to Quartz Fiber/Epoxy Technology: A Conservative Approach

November 1, 2001
by Pierre Boudrias, DMD, MSD; Salam Sakkal, D.M.D., MSD, and Yulian


ABSTRACT

A wide variety of prefabricated post systems are manufactured with different materials and are offered with different shapes. Post-and-core adaptation represents an important element in the biomechanical performance of the prosthetic restoration. The double taper post system was designed with the purpose of providing close canal adaptation with minimal tooth structure removal. The association of a quartz fiber/epoxy material with a more anatomical double taper shape provides a conservative and esthetic approach for the restoration of endodontically-treated teeth.

The post and core is a frequent procedure in dentistry. It is used as a foundation to support the coronal restoration built on an endodontically-treated tooth. However, it has been suggested that clinical and restorative factors may be associated with the failures of endodontically-treated teeth.1 Remaining tooth structure,2,3 circumferential ferrule4-7 post shape,8-12 post material and cement type,13-18 together participate in a delicate restorative equilibrium.

The ideal post shape should embrace the following concepts: a conservative canal preparation to preserve tooth integrity, a shape that is retentive and resistant to dislodgment, minimal stress distribution to the remaining tooth structure and good post design to approximate the canal walls.

The dental literature reveals multiple articles describing methods to improve post retention by combining different variables of parallel or tapered shape with smooth, threaded, grooved, or slotted sides.8-12 In addition to canal etching, bonding to dentin and sandblasting of the post are effective means of increasing retention values. Passive (cemented) parallel-sided posts have shown good retention and lower stress concentration than active (threaded) posts.8,10 As a result the parallel-sided post gained wide popularity.

For the most part, all of these retention studies, using mainly laboratory conditions, failed to assess their clinical performance in-vivo, and overlooked a major and obvious problem associated with the placement of parallel-sided post in a tapered canal. The post does not conform to the shape of an endodontically-treated canal.19 The preparation of the canal with cylindrical instruments may result in an over-preparation of the apical third to achieve post/canal adaptation and a void filled with cement is created in the coronal portion of the canal. Therefore in most cases, it requires excessive removal of tooth structure in already weakened teeth. Furthermore, a major canal modification is needed to adapt the canal walls to the post shape, rather than selecting a post system that has a shape and taper closer to the space left after a conventional root canal treatment is completed (Fig. 1).

Given that it is the nature of today’s endodontic treatment to create an increasing gradual taper, it seems only logical that while restoring these teeth, a tapered post taking advantage of the entire available canal space with minimal modification would be the treatment of choice.

For many years, in-vitro studies reported high stress concentrations and a wedging effect associated with tapered posts resulting in root fractures. Still, the in-vivo applications of these studies may be limited. Weine et al.20 reported in a retrospective study of tapered, smooth posts in place for 10 years or more, 9 failures in 138 teeth (6.5%)–three restorative, two endodontic, two periodontal and two root fractures. They concluded that when tapered smooth posts are used properly, root fractures are minimal and retentive problems do not occur.

Reports on carbon fiber/epoxy posts revealed high fatigue strength16-18 and good clinical performance.21 Advancement in technology has led to the recent development of quartz fiber/epoxy post with physical properties similar to the carbon fiber material.18,22,23

This article reviews the advantages proposed by a quartz fiber/epoxy post with an anatomical double taper shape, the DT Post (RTD, St-grve 38120, France), in relation to its new shape and clinical procedures.

Improved Design: The Double Taper Post

When the authors proposed a system with a 0.02 taper (Endocomposipost UM: RTD, St-grve 38120, France), they believed that this small taper was less likely to have a negative impact on post retention and would create a natural continuation to the endodontic treatment.

Observations made on extracted endodontically-treated human teeth that had received a post with a 0.02 taper, led to these conclusions:24

1. The apical 4 mm of the post is generally well adapted to the canal after endodontic therapy.

2. The coronal 1/2 portion of the canal is generally too wide (after endodontic therapy) to provide good post adaptation resulting in an excessive adhesive cement thickness. This increase in the canal taper at the coronal level is mainly related to the initial tooth anatomy and to the taper orifice opening caused by the various canal preparation techniques during root canal therapy.

In view of these findings, it was concluded that a post with a minimal taper of 0.02 was too narrow to adapt to the coronal part of many canals treated with endodontic therapy.

A radiographic evaluation on extracted teeth was performed to assess post endodontic radicular canal dimensions.25 This study included 967 teeth from 11 different categories of teeth with root canal treatments performed by different practitioners with variable levels of clinical skills during continuing dental education courses at the Universit de Montral, using different canal preparation techniques, including step-back, crown-down and various rotatory systems.

The canal width at 5mm, 10mm, and at the cemento-enamel junction or bifurcation for multi-rooted teeth, was measured buccolingually and mesiodistally using a digital radiography system.

The observations were recorded, and the means, standard deviations and ranges were calculated, with the minimum and maximum post size at the apical level, and the closest taper possible (Tables 1 & 2). From all these measurements, an optimally-sized and tapered post was suggested for each root. Based on these findings, a new post modification was needed to achieve a system that adapts well to the entire available canal space of endodontically-treated teeth with an increased taper only in the coronal part of the canal while maintaining the same apical morphology (taper 0.02). Twenty-two prototypes of double taper posts were fabricated to be assessed. After a pilot study, only seven of these posts were retained to be tested.

Three-hundred-and-forty-six extracted human teeth were restored with seven post prototypes to evaluate post/canal adaptation. All these prototypes were machined with a double taper along the longitudinal axis of the post. Only the middle taper of the post varied to achieve a better adaptation to the canal walls in order to reduce the excessive amount of the weaker adhesive cement (versus greater mechanical properties of the quartz fiber/epoxy post) and to minimize its potential polymerization shrinkage.

All extracted monoradicular teeth with a cemented double taper post prototype were radiographed sagittally and buccally, and divided equally in two groups (Fig. 2). In group one, 50% percent of the samples were cut buccally and the other 50% sagittally (Fig. 3). Teeth in group two were sectioned transversally (Fig. 4).

All extracted teeth with multiple canals treated with a post prototype were examined with transversal sections only at and below the furcation (Fig. 5).

Post prototypes with small middle taper (02 and 04) were discarded because of their lack of adaptation to the dentinal walls and the excessive amount of adhesive cement at the interface dentin/ post. Based on this investigation, three double taper post modifications were retained. The size at the apex of the post corresponds to ISO 90, 100 and 120. The apical portion (~5mm in length) of the posts maintains a 02 taper. However, the middle taper of the post was modified and increased from post #1 to #2 to #3. The dimensions of this new DT Post system are displayed in Figure 6.

These three posts with a double taper configuration improved post adaptation in 85% of the extracted endodontically-treated teeth, and therefore decreased the amount of adhesive cement surrounding the post. The recommended post for each category of teeth are summarized in Table 3.

The smallest post #1 (90-06), has a middle taper convenient to use in smaller canals such as in premolars, molars, and lower incisors. Post #2 (100-08) has a middle taper of 08 and fits properly in most maxillary lateral incisors, maxillary and mandibular canines. Post #3 (120-10) has the greatest middle taper and is mostly used in larger canals such as the palatal roots of maxillary molars, distal canals of mandibular molars, and maxillary centrals and canines.

Despite this attempt to recommend a post for specific groups of teeth, clinical judgement will dictate the most adequate post size and technique to be used in each restorative situation.

CLINICAL TRIALS

A longitudinal clinical study is currently in progress to evaluate the long-term performance of the quartz fiber/epoxy double taper post. Undergraduate students at the Universit de Montral are using the DT Post according to an established protocol and follow-up.

All teeth restored with a DT Post in this project had approximately 30% of the coronal structure remaining after tooth preparation. The size of the post is chosen to achieve maximum post-to-radicular canal approximation and minimal adhesive cement space.

Between January 1999 and May 2001, a total of 157 teeth (142 patients) were restored with 194 DT Post: 91 posterior teeth (58%) and 66 anterior teeth (42%) (Tables 4-6).

As anticipated in our previous in vitro investigation, a greater number of posterior teeth were restored mainly with post #1 (68%). Eight posts #3 were used in molars: three in the palatal canal of maxillary molars and five in the distal canal of mandibular molars.

Maxillary central incisors received the highest percentage of the larger post #3 (10 teeth – 41.7%).

Patients were advised to inform the head of fixed prosthodontics or endodontics of any type of failure that may occurred. A final recall of all patients who have participated in this project will take place after a period of five years in function for each tooth restored with a DT Post. At this early stage of our evaluation, no failure has been reported.

CLINICAL PROCEDURE WITH THE LOGIPOST QUARTZ 2

A 43-year-old man presented with an endodontically-treated and discolored tooth #11 (Fig. 7). This tooth had been previously restored with composite resin mesially and lingually. After initial tooth preparation, sufficient tooth structure was present to insure a good circumferential ferrule.

1. Canal instrumentation:

Gutta-percha is removed by softening it with a heat carrier followed by drilling with the universal post drill. Approximately 5 to 6mm of gutta-percha is left to preserve the apical seal.

The DT Post drills are applied successively on a post-endodontic periapical x-ray of tooth #11 to indicate the most adequate drill size that will remove all the remaining gutta-percha on the canal walls (Fig. 8).

The canal preparation is performed by sequential use of drills #1 to #3: initially drill #1 (90-06) is used followed by drill #2 (100-08) and completed with drill #3 (120-10). A periapical radiograph is taken at this point to insure total gutta-percha removal from the canal walls.

The post #3 is tried in the tooth to verify complete seating (Fig. 9). Post length is adjusted before cementation by cutting the coronal extension of the post with a diamond disk (diamond instruments only).

2. Post cementation:

The post is cleaned with an alcohol swab. To insure proper wetting of the surface, a primer/adhesive resin is painted on the post, dried with a gentle air stream and light-cured for 15 seconds.

The canal and surrounding tooth structure are etched for 15 seconds with phosphoric acid (H3 PO4 37%) and rinsed thoroughly. The excess of water is removed with absorbing points only, to avoid drying the dentin. Moist dentin is essential for proper wetting with the primer/adhesive resin.

The dentin is coated twice with primer/adhesive resin using a saturated microbrush. Excess resin is removed with a dry absorbing point to prevent the rapid setting of the cement. The coated dentin is sprayed with a gentle air stream to evaporate the solvents and light-cured for 15 seconds.

The components (base and catalyst) of a self-curing cement are mixed together and the canal is filled with a Centrix syringe and an Accudose needle tip (Centrix Inc., Shelton, CT 06484-5458, USA). A post #3 is inserted in the tooth and held in position for 30 seconds.

Voids may be introduced in the cement material during post placement.26 Injecting the cement with a syringe provides a rapid and clean approach to fill the canal and it prevents the incorporation of air bubbles in the cement material.

Light-curing cements are prohibited since the light cannot be transmitted through the white quartz fiber/epoxy post. Dual-cure cements may be used to fixate the post. However, it has been shown that these cements have different polymerizing potential when light-curing is not optimal.27 These findings suggest that we should be cautious when using a dual cure cement. Their potential to fully cure without light varies and it should be verified before luting a post.

The core build-up is fabricated with a light-cured composite resin followed by conventional tooth preparation with diamond burs (Fig. 10). To create a solid prefabricated post and core unit, sufficient bulk of build-up material needs to be built around the post. Whenever the post cannot adequately support the build-up reconstruction material, there is a higher risk of dislocation and cohesive fracture of the core build-up. For these clinical situations, a more homogeneous post such as a cast post and core is preferable.

Post cementation, core build-up, impression and temporization of tooth #11 are achieved at the same appointment.

An IPS-Empress (Ivoclar Vivadent Inc., Schaan, Liechtenstein) crown is cemented at a latter appointment (Fig. 11). The periapical radiograph confirms good approximation of the post to the canal walls (Fig. 12).

Vichi et al28 have shown that a minimum thickness of 2mm of IPS-Empress glass-ceramic was necessary to mask and avoid a change in color when cemented over carbon fiber post material. This quartz fiber/epoxy material prevents undesirable low value appearance of the coronal restoration frequently observed with metal post systems and carbon fiber/epoxy posts.

DISCUSSION

The restoration of an endodontically-treated tooth with a post should be conservative to decrease the risk of tooth fracture. The post-and-core should also provide good anchorage for the final restoration and it should not interfere with its final esthetic qualities.

The wide variety of restorative techniques, materials and shapes offer a wide range of clinical possibilities. Two in vitro studies have indicated less favorable sites of fracture with cast posts.14,15 However, both studies reported inferior strength properties and lower threshold fracture values with the carbon fiber/epoxy post. These findings suggest that a greater amount of ferrule is needed to maintain fiber post integrity and led us to use the double taper quartz post when 30% of the tooth structure remained after tooth preparation. Cormier et al. has shown that different fiber post systems were readily retrievable after failure, whereas cast post-and-core, zirconia and titanium alloy posts were non retrievable.22

The practitioner must also determine whether to use a double tapered post system that adapts well to the canal walls in most clinical situations, or use a parallel-sided post and modify the existing shape of the canal to adapt the post or fill the remaining space with cement. The double taper post imitates closely the post-endodontic shape of a radicular canal and leaves a thin and uniform thickness of cement at the post/canal interface. This improved adaptation of the post promotes the mechanical properties of the quartz fiber/epoxy post material instead of the weaker composite resin cement.

The double taper white post fabricated with quartz fiber and epoxy allows one to rebuild the missing tooth structure using adhesive technology without obstructing the esthetics of the all-ceramic restorative systems.

Observations on extracted endodontically-treated teeth have shown good post/canal adaptation. Early clinical follow-up is encouraging. In as much as the in vitro and early clinical follow-up are promising with this double taper quartz fiber/epoxy post system, long-term clinical study is still needed to evaluate the behavior of this post and the restorative prognosis of teeth with extensive coronal destruction. OH

Dr. Boudrias is Professor, Universit de Montral, Facult de mdecine dentaire, Head of section in Fixed Prosthodontics. Dr. Salam Sakkal is Associate Professor, Universit de Montral Facult de mdecine dentaire, Head of section in Endodontics. Dr. Yuliana Petrova has a private practice in Montral.

Oral Health welcomes this original article. Complete references available upon request.

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