Clear matrix use for composite resin core fabrication
Carlos Alberto Jurado, Jose Villalobos Tinoco, Akimasa Tsujimoto, Wayne Barkmeier, Nicholas Fischer, Mark Markham

Abstract  

Post and core fabrication is important in the restoration of endodontically treated teeth. Significant coronal and radicular dentin destruction commonly occurs in endodontically treated teeth. As a result, preserving all the remaining tooth structure is imperative in order to im-prove the prognosis of the restorative treatment. Core height and width need to be carefully designed and built in order to receive the final restoration. Direct free-hand composite resin core buildup in increments may be a challenging and time-consuming procedure in the dental chair. The case report presented in this article describes a clinical technique using a clear matrix to receive composite resin injections in order to achieve an ideal shape and core size.

Introduction

There is a wide variety of treatment options for restoring endodontically treated teeth, which is often a challenging process for the clinician. The need for a post is dictated by the amount of remaining endodontically treated tooth structure.1 It is very common for this structure to be insufficient to pro-vide appropriate coronal support and retention for crown restorations, in which case a post and core fabrication is required.2 Conventional cast post and cores were the standard of care for many years; however, high esthetic demands and the benefits of uncomplicated fabrication led to the development of prefabricated posts, currently made from fiber-reinforced composite res-ins.3 Moreover, it has been reported that the shape and rigidity of cast post and cores and the rigidity of prefabricated metal posts may pose a risk of root fracture.4 Dietschi et al5,6 have suggested that the rigidity of the post should be equal or almost equal to that of dentin so as to distribute the functional forces evenly along the length of the root. Fiber-reinforced composite resin post and core systems have been reported to have similar biomechanics to dentin, and clinical studies have shown convincing re-sults.7,8 The ferrule tooth structure is the amount of tooth located 1.5 to 2.0 mm superior to the projected ferrule margin.9 The ferrule is thought to improve the structural integrity of an endodontically treated tooth by resisting functional forces and the wedging effect of tapered dowels, including the associated lateral forces.10,11 It is re-commended that clinicians preserve intact coronal and radicular tooth structure to create a ferrule effect, as this is crucial to optimize the biomechanical behavior of a restored tooth.12 Moreover, it is thought that the quantity of natural tooth structure within the ferrule is more important than the post in determining whether the abutment will eventually fracture away from the root.13

The gold standard for the cementation of traditional cast post and cores is at a depth of 2⁄3 of the total length of the root canal.14 However, the properties of the pre-fabricated fiber-reinforced composite resin post and cores are different from that of cast metal ones; thus, the cementation depth is controversial.15 Balkaya and Birdal16 propose that striving for a depth of 2⁄3 of the working length causes more tissue to be re-moved, resulting in the root structure be-coming weaker. In addition, the deeper the fiber post is cemented, the more difficult it is to achieve quality resin cement bonding between the fiber post and the tooth struc-ture.17 Ferrari et al18 suggest that a shorter fiber-reinforced composite resin post and core compromises its retention and increases the stress force on the dentin. The gold standard for fiber-reinforced composite res-in post and core systems should be root canal preparation that is sufficient to withstand the pressures of mastication with a minimal reduction of dentin.19

Composite resins are becoming very popular as core build-up materials for fiber posts because they can be reliably bonded to tooth structures.20 Composite resin is recognized as most suitable for use as a direct core build-up material when substantial coronal tooth structure remains for bond-ing.21 However, due to the multiple steps of bonding and required incremental placement, it is considered to be technique sensitive. With the introduction of new generations of composite resins (especially low viscosity bulk-fill composite resins that show reduced polymerization shrinkage and stress associated with polymerization), the use of this kind of composite resin as a core build-up material may be one of the better choices for fiber-reinforced composite resin fabrication. Thus, the aim of this case report is to describe a technique that allows the remaining tooth structure to be preserved by injecting a composite resin through a clear matrix in order to build up a core without additional tooth preparation.

Clinical report  

A 55-year-old female patient presented to the clinic with the chief complaint of dissat-isfaction with the esthetics and experience of her provisional crown. She presented for a second dental opinion on a provisional crown on tooth 21. At the initial evaluation, the patient was diagnosed with a diastema between teeth 11 and 21, uneven incisal edges of teeth 11 and 21, composite veneer on tooth 11, endodontic treatment on tooth 21, and a provisional restoration on tooth 21 (Figs 1 and 2). The patient was informed of the need to remove the temporary restoration on tooth 21, and for the remaining tooth structure to be reevaluated (Fig 3). After reevaluation without the provisional res-toration and radiographic evaluation, the patient was informed of the need for a post and core on tooth 21. Due to esthetic concerns, it was recommended that a ceramic veneer (feldspathic porcelain) be placed on tooth 11, and a ceramic core (lithium disilicate) and ceramic veneer (feldspathic porcelain) placed on tooth 21.

The provisional restoration on tooth 21 was removed, and a polyvinyl siloxane (PVS) impression (Virtual; Ivoclar Vivadent) was secured in order to fabricate a Type IV stone diagnostic cast (Fujirock; GC) (Fig 4). A diagnostic wax-up (Geo Classic Wax; Remfert) for the planned future core was completed, and then a clear matrix of transparent PVS (Exaclear; GC) was fabricated based on the core diagnostic wax-up using room-temperature water in a pressure pot (Aquapres; Lang Dental) under 30 psi for 5 min (Fig 5). The diagnostic wax-up was completed, providing ideal contours for future restor-ations on teeth 11 and 21 (Fig 6). The provisional restoration was removed and blue rubber dam (Dental Dam; Nic Tone) was placed from teeth 15 to 25 and retained with clamps (Clamp No. 00; Hu-Friedy) to achieve good isolation. A clamp (Hygenic Brinker Clamps, Tissue Retractors B4; Coltene) was also placed along the gingival contours on tooth 21. Gutta-percha was re-moved to the preplanned depth with drills (TaperLux Post System Drills; Coltene) following the manufacturer’s instructions until the preplanned diameter and depth was achieved (Fig 7). A radiograph was taken to verify the desired length. The root canal was rinsed with water, and the excess water was dried with air and paper points (HyFlex Paper Points; Coltene). A chemically cured adhesive (ParaBond; Coltene) was used for the core buildup. A conditioner was placed in the canal using a micro brush for 30 s, excess was removed with paper points and a gentle air stream for 2 s, and then the adhesive was applied with a micro brush. The excess was gently removed with air for 2 s. A dual-cure bulk-fill composite resin (Fill Up!; Coltene) was injected directly in the canal with a narrow tip and also applied around the fiber post (ParaPost Fiber Lux Post; Coltene). The post was then inserted slowly, allowing for excess composite resin to vent, and pressure was applied for 60 s (Fig 8). The excess composite resin was re-moved and the restoration light cured (Valo; Ultradent) for 60 s. The rubber dam was then removed and retraction cord No. 00 (Ultrapak; Ultradent) was placed on tooth 21 (Fig 9). A clear matrix was inserted in the mouth, verifying correct placement (Fig 10). The composite resin (Fill Up!) was inserted through the incisal access hole where it was light cured without removing the clear matrix for 20 s on the facial, 20 s on the incisal, and 20 s on the palatal surfaces. Light curing was performed with Valo on a setting of 1000 mWcm-2. The clear matrix was re-moved and another 20 s of light curing on each surface was completed (Fig 11). After the desired core was obtained (Fig 12), veneer preparation was performed on tooth 11 (Fig 13). A final impression was made with PVS in heavy-body and light-body consistencies (Virtual 380; Ivoclar Vivadent). The final master cast was made in Type IV stone (Fujirock). A ceramic core was made of lithium disilicate using the pressed technique (IPS e.max Press MO  0; Ivoclar Vivadent), and subsequently refractory feldspathic porcelain veneers were fabricated (IPS e.max Ceram; Ivoclar Vivadent) (Figs 14 to 16). A black rubber dam (Dental Dam) was placed from tooth 15 to 25 and retained with clamps (Clamp No. 00) to achieve good isolation. Clamps were also placed on teeth 11 and 21 (Clamp No. 212) (Fig 17). The lithium disilicate core was treated with hydrofluoric acid (IPS Ceramic Etching Gel; Ivoclar Vivadent) for 20 s, and then placed in alcohol (Alcohol Isopropyl 70% Solution; Henry Schein) for 5 min. Finally, silane (Si-lane; Ultradent) was applied for 1 min. The porcelain veneers were etched with hydro-fluoric acid (IPS Ceramic Etching Gel) for 60 s, rinsed with water for 30 s, and placed in an ultrasonic bath (Maxisweep S3100

Ultrasonic Cleaner; Henry Schein) for 5 min. The veneers were then treated with a silane application for 1 min, and the ceramic core was bonded to tooth 21. The surface of the composite core was sandblasted with water and 30-μm aluminum oxide particles (AquaCare Aluminum Oxide Air Abrasion Powder; Ve-lopex), and then cleaned with 37% phosphoric acid (Total Etch; Ivoclar Vivadent). A resin cement was ap-plied (Variolink Esthetic LC; Ivoclar Vivadent) for cementing the ceramic core on tooth 21 (Fig 18). Tooth 11 was sandblasted with water and 29-μm aluminum oxide particles (AquaCare Aluminum Oxide Air Abra-sion Powder), followed by a total etch procedure on enamel with a 37% phosphoric acid (Total Etch) for 15 s, then gentle air drying. A universal adhesive (Adhese Universal; Ivoclar Vivadent) was applied on the ceramic core. Light shade cement (Variolink Esthetic LC) was then applied and the ceramic restorations were bonded. Any excess of adhesive and cement was removed. Each of the final restorations was light cured on the facial surface for 20 s. Floss was used to clean the interproximal surfaces, followed by another 20 s of light curing on the facial, 20 s on the mesial, and 20 s on the distal surfaces of each ceramic restoration (Figs 19 and 20). An oxygen inhibition treatment was provided, with glycerine-based gel (DeOx; Ultra-dent) applied on all the surfaces of the restoration and light cured for 40 s.

The patient was very pleased with the contours, shape, and shade of the final feldspathic veneer restorations. She presented for subsequent follow-ups, the most recent being at 6 months (Fig 21).

Discussion  

Although current techniques for fiber-reinforced composite resin post and core fabrication have been previously described in the literature,22 clinicians still require knowledge of the technique and specific skills to per-form it. Building up a composite resin core to a specified height and width may be time consuming. Free-hand composite resin core buildup with a fiber post may also need to be prepared again to remove any composite resin excess. Unfortunately, this may cause additional tooth structure removal.

The clinical technique presented in this article allows the clinician to prepare a more predictable core shape, since the clear matrix is fabricated during a diagnostic wax-up. The final fiber-reinforced composite resin post and core will follow the shape of the diagnostic wax-up. Fabricating a core with a low viscosity bulk-fill composite resin using a clear matrix provides a faster and more predictable result than conventional composite core placed in increments. This is due to the lower polymerization shrinkage and stress as well as greater depth of cure of bulk-fill composite resins.23-25 However, it is necessary to ensure that the depth of the composite resin does not exceed 4 mm at any point, as this is the maximum working depth for dual-cure bulk-fill composite resins. In addition, light irradiation appears to be advisable even for dual-cure resins, as it is necessary to attain a high level of polymerization before the matrix is removed. The core resin has no external support and must itself support the veneer, so full polymerization should be achieved as soon as possible during the procedure. Chemical cure is slower and thus should not be relied on in this case. On the other hand, polymerization of a composite resin core with a clear matrix eliminates the influence of oxygen inhibition of the material, increasing the strength of a fiber-rein-forced composite resin post and core. The mechanical strength of low-viscosity bulk-fill composite resins has improved over time, and recent study results25 show that these materials have comparable flexural strength to established core buildup resins.26

One issue with low viscosity bulk-fill composite resins is that they are more translucent than conventional flowable composite res-ins, leading to a clear difference in color, as can be seen in Figures 12, 13, and 17. For es-thetic reasons, this must be masked in the final restoration. There are two main options for this: a monolithic ceramic crown, or a ceramic core with a ceramic veneer. In the case presented here, the latter option was selected to ensure a good color match with the adjacent veneer restoration.

Conclusion  

A clear matrix fabricated from a diagnostic cast with an ideal wax buildup is a very con-servative way to fabricate a composite resin buildup. The clear matrix allows the clinician to verify the seating before injecting composite resin through the access hole on the incisal surface. Since an inject-able composite resin will precisely follow the designed length and width, there is typically little need for additional tooth preparation after fabrication. This novel and conservative technique will help clinicians in the restoration of endodontically treated teeth.