A Prosthetically Guided Technique for Cast Post-and-Core Fabrication

The restoration of endodontically treated teeth with limited remaining coronal tooth structure often requires the use of post-and-core restorations. A post serves to retain the core material, which serves to replace the missing tooth structure to provide adequate resistance and retention form for a crown.^1,2 When coronal tooth structure is compromised, the use of a post and core may reduce the risk of coronal fractures.^3,4

Many different techniques and materials have been advocated for the placement of a post-and-core restoration. Direct restorative procedures utilizing prefabricated stainless steel, titanium, carbon-fiber, glass-fiber, and zirconia posts, usually in conjunction with a resin composite core build-up, have been described in the literature.¹ The advantages of a direct restorative technique include reduced treatment time, lower costs, and improved esthetics when a composite core build-up is used. However, based on the available literature, this technique should be used in situations where adequate coronal tooth structure remains to support the crown; when significant amounts of tooth structure are missing, cast metal posts may be better indicated.⁵

The custom-fabricated cast post-and-core approach has long been advocated as the gold standard because of its high success rate and favorable long-term prognosis.^6,7 The advantages of using a cast post and core include custom fit to the root configuration, minimal alteration of canal anatomy, and adaptation to large, irregularly shaped canals and orifices.^8,9 An argument often made against the use of cast post-and-core restorations is that they are prone to promote incidence of catastrophic root fracture due to their high modulus of elasticity. Although this result has been demonstrated in several in vitro studies,¹⁰ multiple meta-analyses comparing the success of cast and direct posts have found no difference in either the survival rate or the fracture behavior between the two modalities.^5,11,12

CAD/CAM-generated zirconia post and cores have been more recently introduced as an alternative approach for a customized post and core.^13,14 Bittner and colleagues showed in an in vitro study that the load-bearing capacity of CAD/CAM zirconia posts was comparable to that of conventional cast metal posts.¹⁵ However, the literature is sparse regarding the long-term success of this technique, and no definitive workflow has been established. In addition, ceramic posts have shown to be difficult to be fabricated when the post is thin and nearly impossible to remove in the case of failure when retreatment is needed.¹⁶ As such, until further research brings clarity to these limitations, the classic metal cast post and core may still be considered the best option for the restoration of severely deteriorated dentition.

Traditionally, two main techniques have been described for the fabrication of a cast post-and-core pattern: indirect and direct techniques. The indirect method involves taking an elastomeric impression of the post space and surrounding tooth structure. The laboratory technician pours this impression in gypsum, creates a wax pattern, and casts the post and core in a gold alloy. In the direct method, the clinician uses an autopolymerizing acrylic resin to create an acrylic pattern, which is then sent to the laboratory for burnout and casting. This technique can provide better predictability for the clinician, because the cast post-and-core pattern is physically tried in and confirmed for fit on the actual tooth prior to casting.

One of the challenges of the direct method stems from the need to create an accurate core design according to the desired dimensions of the abutment. Because the pattern resin must be added incrementally using a "salt-and-pepper" method, voids are often present at margins in difficult-to-reach areas. The physical dimension of the core must be estimated based on the surrounding and opposing tooth anatomies, or by the use of a putty matrix. Oftentimes, when multiple adjacent post and cores are needed or adjacent teeth are missing, these anatomical references are no longer present, introducing considerable guesswork and potential inefficiencies. Several techniques have been described to facilitate the core pattern design process, but they involve additional laboratory procedures, including the preparation of teeth on a stone model.^17,18

In this clinical technique article, the authors propose the use of supplementary materials and a novel workflow to facilitate the creation of an anatomically guided cast post-and-core pattern.

The clinical scenario presented here describes the fabrication of cast post-and-core restorations on four adjacent maxillary anterior teeth that were previously endodontically treated and required pre-prosthetic functional crown lengthening. The teeth are shown in Figure 1 prior to crown lengthening.

To begin the fabrication process, post spaces are irrigated with sodium hypochlorite and dried with paper points. The post spaces are lubricated with a thin layer of Vaseline^® petroleum jelly, and the post-and-core patterns are initiated following the traditional direct method with an autopolymerizing acrylic resin (Pattern Resin^™, GC America, gcamerica.com) on a plastic burnout post (DuraLay, Reliance Dental Manufacturing, reliancedental.net).¹⁹

Once an accurate and passive pattern of each post space is confirmed, additional increments of the acrylic resin are added coronally to minimally capture the pulp chamber and seal any existing voids between the pattern resin and the tooth structure (Figure 2). The plastic burnout post is cut with scissors, leaving about 2 mm extending above the post pattern (Figure 3).

Next, a putty impression (Exaflex^® putty, GC America) of the idealized wax-up for the four maxillary incisors is filled with a bis-acrylic provisional material (Luxatemp^®, DMG America, dmg-america.com) (Figure 4) and seated in the patient's mouth. The bis-acrylic provisional material is allowed to fully set in the mouth, and the putty matrix is then removed. What results is a full-contour "mock-up" of the idealized restorations connected to the post space patterns that were confirmed to be accurate in the previous clinical step (Figure 5).

Depth grooves are then made into the bis-acrylic mock-up with a coarse diamond bur to the desired depths based on the restorative material of choice, and crown preparations are carried out in the conventional manner (Figure 6).

Lastly, the individual patterns are then removed from the mouth and refined outside of the mouth with removal of any flashes of material and/or sharp corners (Figure 7). The patterns are then sent to the laboratory for burnout and casting in a type III gold alloy.

In the present case, once the cast post-and-core restorations were returned from the dental laboratory (Figure 8), they were checked for passive seating and accurate marginal fit before being cemented into the mouth with a glass-ionomer cement (Ketac^™ Cem, 3M Oral Care, 3m.com) (Figure 9). Minimal refinements of the crown preparations were needed following cementation, and the case was ready to proceed to the final impression stage.

The conventional technique for creating a post-and-core pattern with an autopolymerizing resin provides a predictable method to capture the internal anatomy of the post space while minimizing the chances of engagement into an undercut due to the physical properties of the setting autopolymerizing acrylic resin.¹⁹ However, this resin is less than ideal when used for the creation of the core pattern due to its challenging handling, exothermic setting reaction, and difficulty adjusting it with commonly used diamond burs. In addition, teeth requiring cast post-and-core restorations often are missing significant amounts of coronal tooth structure, adding to the challenge of building up an ideally configured core pattern.

The technique proposed in the present article utilizes a bis-acrylic mock-up technique, similar to that used for veneer preparations, in order to add an element of predictability to the process. As compared to traditional polymethyl methacrylate (PMMA) acrylic resin, the authors find that the use of a bis-acrylic provisional material in this workflow allows for better handling, easier adjustment with diamond burs, and less discomfort for the patient.

The presented technique combines the benefits of autopolymerizing resin for capturing the post space with the advantages of a bis-acrylic composite resin for designing the core contours according to the ideal dimensions of the final restorations. This approach also reduces the amount of refinement necessary, which subsequently minimizes the waste of gold material as well as clinical chairtime during insertion of the post and core.

The classic cast post and core is still a useful and essential foundation restoration in the prosthodontist's armamentarium; however, the process of fabrication can be tedious. The technique presented facilitates the cast post-and-core pattern fabrication process by eliminating any guesswork involved in determining the appropriate configuration and dimensions of the core.

Jason D. Lee, DDS, MMSc
Instructor, Department of Restorative Dentistry and Biomaterials Sciences, Harvard School of Dental Medicine, Boston, Massachusetts

Mona Khan, DDS, MMSc
Advanced Graduate Student in Prosthodontics, Department of Restorative Dentistry and Biomaterials Sciences, Harvard School of Dental Medicine, Boston, Massachusetts

Sang J. Lee, DMD, MMSc
Assistant Professor, Department of Restorative Dentistry and Biomaterials Sciences, Harvard School of Dental Medicine, Boston, Massachusetts