Restoring Severely Compromised Teeth with a Core Build-Up
The advantages of using a direct, light-cured only composite core material.
Jeff T. Blank, DMD
Teeth that have become severely compromised by decay, large failing restorations, or fracture often require full-coverage crowns to restore them to their original form and function. In cases where the destruction has led to pulpal necrosis, root canal therapy is necessary. Clinicians must then evaluate whether there is enough structural integrity remaining to support a definitive crown by a core build-up alone, or use an endodontically retained post to secure the core foundation to the root.
The primary purpose of a core build- up is to replace enough missing tooth structure to permit the creation of ideal retention and resistance form in the crown preparation. While many modern ceramics can be adequately bonded directly to the defective area, it is often advantageous to create an idealized crown preparation using a core material. Crown preparations exhibiting parallel walls with proper taper and height, coupled with adequate ferrule 1 mm to 2 mm beyond the core material itself without violation of biological width, have historically proven to be clinically successful regardless of the material chosen for the definitive crown.1-3
Core Preparation Material
Clinicians have several choices of core materials available, including amalgam, glass ionomer, and composite filling materials. While amalgam remains very useful as a core material when metal or metal-based crowns are employed, it must be retained with slots, grooves, or undercuts that require more invasive preparations compared to adhesive alternatives. Additionally, pins are often necessary to secure the amalgam, which can lead to crack propagation and iatrogenic penetration into vital pulp spaces.4 With the rise in popularity and recent advancements of adhesive all-ceramic crown materials, amalgam core materials are no longer the most ideal substrates for resin bonding, and the silver color may negatively influence the overall appearance of the more translucent ceramic materials. But most significantly, the setting reaction of amalgam can take up to 24 hours depending on the formulation, thus making it unsuitable for crown preparation at the same visit. Historically, glass-ionomer materials have been widely used as core materials because of their ability to bond to tooth structure, the low incidence of postoperative sensitivity, and their ability to release fluoride. However, when compared to modern adhesive composite core materials, glass ionomers possess inferior physical properties, are difficult to manipulate, and cannot be prepared until the chemical curing reaction is completed.5
Advantages of Dedicated Composite Core Materials
While the concept of using composite resin as a core material is not new, recent advances in composite core materials have advanced in recent years and offer many distinct advantages over amalgam and glass-ionomer materials. Coupled with either a total-etch or self-etch adhesive, composite resin can be bonded to both enamel and dentin with shear bond strengths significantly greater than glass-ionomer materials without the need for undercuts, grooves, slots, or pins. This permits more conservative core preparation.6 Depending on the formulation, the physical properties of some composite materials include high compressive strength, high fracture resistance and tensile strength, and the ability to "cure-on-demand" for immediate preparation. Composite materials offer the greatest range of shades to match the residual tooth structure and are ideal for use under even the most translucent ceramic system without negatively impacting the desired shade of the final restoration.
Composite resins specifically designed for core build-ups are available in both dual-cure and light-cure formulations. Dual-cure composite core materials are dispensed in either a dual-barreled bulk cartridge gun similar to most impression materials, or in a smaller, more compact dual-barreled syringe. Both are fitted with a spiral mixing tip to which a cannula can be attached for precise delivery to the cavity form. These systems use a photocatalyst coupled with a catalyst/base redox reaction that permits "dark-cure" auto-polymerization in deep preparations where access for adequate curing light intensity is questionable. Practitioners need to be aware that some total-etch, single-bottle primer/adhesive systems (often called "fifth-generation" adhesives) are highly acidic and are not compatible with many dual-cure core materials without using a dual-cure or self-cure additive. These additives temporarily neutralize the acidic monomers of these adhesives that otherwise interfere with the redox catalyst/base reaction of the auto-polymerizing component of the dual-cure core resin. Failure to use these additives when indicated can lead to incomplete polymerization of the core material, resulting in low bond strengths and adhesive failure.7
The flowable consistency of the dual-cure composite core systems allow for easy adaptation to irregular preparations and endodontic posts, but they can be difficult to manipulate and contain when crown-forms or band retainers are not used. Other potential negative issues with dual-cure core composite materials are the potential of trapping air while dispensing, which creates voids and limited working time. For these reasons, some practitioners prefer more viscous, non-slumping light-cure-only core materials that can be condensed and sculpted to the desired form with nearly unlimited working time, which can reduce crown preparation time.
The primary concern with light-cure-only core materials is the depth of cure. While appropriate for small to moderate cavity preparations, traditional composite formulations used in restorative dentistry are not designed to be used in increments greater than 1 mm to 2 mm and are not ideal for bulk placement in large core preparations. For this reason, dedicated light-cure-only composite core materials specifically formulated for placement in largeincrements with sufficient depth of cure and ideal physical and handling properties have been created.
Build-It Light Cure Core material
Recently, Pentron Clinical Technologies (http://www.pentron.com) released Build-It® Light Cure Core Material. This is a bisphenol A-free composite resin material specifically formulated to permit a depth of cure of up to 10 mm and ideal handling properties to permit bulk adaptation to both vital core preparations and endodontic posts without stickiness or voids. It comes in a single chameleon-like, radiopaque, translucent shade that optically mimics the surrounding residual tooth structure and does not negatively affect the final shade of even the most translucent ceramic system.
According to the manufacturer, Build- It Light Cure Core Material exhibits a compressive strength of 272 MPa, a flexural strength of 189 MPa, and a fracture toughness of 2.27 MPa. These physical properties are ideally suited for supporting both metal-based and all-ceramic crowns and bridges and closely mimic natural tooth structure to give the clinician the feel of cutting tooth structure without ditching during preparation. Because it is a light-cure-only core material, it is compatible with all "generations" of adhesive systems without the need for self-cure additives. It offers nearly unlimited working time and can be light-cured with both LED and halogen curing lights in 20 seconds. Once cured, Build-It Light Cure Core Material can be prepared immediately without the need for a second appointment.
A female patient presented with a severely decayed maxillary right canine with irreversible pulpitis and carious lesions on teeth Nos. 5 and 7 (Figure 1). The periapical radiograph revealed endodontic pathology on tooth No. 6 and caries extending to the osseous crest. Root canal therapy was performed and the decay was removed. The post space was created using Gates-Glidden drills and the FibreKleer® Post (Pentron Clinical Technologies) parallel wall preparation bur, size 1.5 mm (Figure 2). This latest generation of fiber post offers superior strength and outstanding esthetics with the added feature of light transmission through the post. FibreKleer posts have a flexural strength of 1,423 MPa for durability in high-stress areas; a flexural modulus engineered to minimize root fractures; high radiopacity for easy detection on radiographs; transparency for esthetics and light transmission; is pre-silanated for convenience and enhanced adhesion of resin cements; and comes available with a parallel post with a retentive head, tapered, and serrated. The 1.5-mm FibreKleer Parallel post with a retentive head was chosen for this case and was tried in the post preparation to ensure proper length and fit. Once the fit was established, the post was wet with Bond-1® adhesive (Pentron Clinical Technologies) and air-thinned. Debris from the preparation was removed with a small microbrush and the preparation was rinsed and dried. Breeze® Self-Adhesive Cement (Pentron Clinical Technologies) was then syringed into the post space with a back-filling injection technique (Figure 3) and the FibreKleer post was seated to place. Excess resin cement was wiped away with a microbrush and the curing light was placed directly on the post head (Figure 4) for 20 seconds to transfer light through the entire length of the post to the resin cement to ensure complete polymerization. Figure 5 shows the view of the FibreKleer Post cemented in place.
Build-It Light Cure Core Material was chosen as the core material because of its physical properties, handling, and ability to adapt directly to the post and residual tooth structure with a composite instrument. The remaining tooth structure was etched for 15 seconds, rinsed, dried, and left moist, and Bond 1 adhesive was applied, air-thinned, and light-cured for 10 seconds. Build-It Light Cure Core Material was syringed around the post and adapted with a small composite instrument and light-cured prior to bulk-filling the preparation (Figure 6). The tooth was then filled and sculpted to full contour in a single increment and light-cured for 20 seconds (Figure 7 and Figure 8). The tooth was then grossly prepared for a single crown and the carious lesions on teeth Nos. 5 and 7 were repaired with Artiste® Nano Composite (Pentron Clinical Technologies). A "closed" crown-lengthening procedure was performed to remove an adequate amount of osseous crest to permit the establishment of biologic width after the final preparation for a full-coverage all-ceramic crown. Figure 9 shows the final prepared post-and-core preparation using Build-It Light Cure Core Material and a FibreKleer Post after surgery. A provisional restoration was fabricated and the surgical site was allowed to heal for approximately 8 weeks prior to a final impression being made.
While there are several types of core materials available in dentistry today, light-cured materials such as Build-It Light Cure Core Material offer advantages as demonstrated in this case presentation. When visual access is unencumbered, clinicians may prefer to hand-adapt the core material to the preparation and post, minimizing the potential for voids that can occur with the application of flowable dual-cure composite alternatives. Without concern for limited working time, Build-It Light Cure Core Material can be either bulk-filled up to several millimeters or adapted in smaller increments to ensure intimate adaptation to a serrated post head or hard-to-access areas of a preparation. Because this material does not slump or flow below the sulcus or onto adjacent teeth, the core can be sculpted to full contour quickly and predictably. Build-It Light Cure Core Material possesses exceptional physical properties, cuts like tooth structure, and its "chameleon-like" translucent shade blends in nicely with residual tooth structure to create a vital tooth preparation. When highly esthetic ceramic restorative systems are used, both the patient and dentist will be pleased with the results.
Dr. Blank has received an honorarium from Pentron Clinical Technologies.
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About the Author
Jeff T. Blank, DMD
Fort Mill, South Carolina