An indirect technique to fabricate a fibre-reinforced custom post and core restoration

The introduction of carbon or glass fibre post systems may provide an alternative to metal posts for their biomimetic behaviour and favourable aesthetics when all-ceramic crowns are to be placed. However, prefabricated posts with direct cores made of glass ionomer or composite resin are less reliable than one-piece cast post and core, primarily because of delamination at the interface between the post and the core and compromised retention, especially in cases of wide, non-circular, or extremely tapered canals. A new indirect technique for the fabrication of a custom-fabricated post and core restoration made of laboratory composite resin material reinforced with fibres is presented.

Endodontically treated teeth often require post and core restorations for retention purposes because of extensive loss of tooth structure due to caries or fracture. The cast gold post and core has been considered the gold standard because of its excellent success rate, favourable long-term prognosis and good adaptation. Unfortunately, several disadvantages are associated with metal post and core restorations, such as root fractures and a compromised aesthetic result when placed under all-ceramic crowns.1^,2

Carbon or glass fibre post systems may provide an alternative to cast or prefabricated metallic posts. These post systems have been developed with a modulus of elasticity values closer to that of human dentine, distributing the stresses more evenly along the root and causing fewer catastrophic failures.2^,3^,4 Moreover, post and core restorations made of glass or quartz fibre posts and composite resin build-ups allow improved aesthetics when all-ceramic crowns are to be placed. However, fibre post and core restorations have disadvantages and clinical problems. Delamination at the interface between the post and the core and compromised retention in cases of wide, non-circular, or extremely tapered canals, where cylindrical prefabricated posts may not achieve adequate adaptation to the canal, have been reported.5 Post adaptation to the canal walls represents an important element in the biomechanical performance of the prosthetic restoration. Dislocation of the posts and compromised fracture resistance of the teeth due to interfacial gaps between the post and the intracanal dentine have been reported.6^,7

Teeth with wide or flared canals are more susceptible to fracture due to the weakened thin remaining walls. A variety of direct clinical techniques have been introduced as alternative options to restore structurally compromised endodontically treated teeth damaged by caries, trauma or congenital disorders. Experimental dentine posts8^,9 or reshaped fibre posts10 to adapt to the walls of non-circular root canals have been proposed. Prefabricated posts associated with resin reinforcement of the root dentine walls or individually shaped polyethylene woven fibre ribbon reinforced post and core systems have been advocated.11^,12^,13 Increased fracture resistance10^,14^,15 and decreased microleakage16 have been reported when adapted fibre-reinforced custom fabricated dowel systems were used to restore endodontically treated teeth with wide flared canals. It is questionable whether wall adaptation or complete light polymerization of the composite resin material can be achieved at the apical portions. Limited moisture control and geometry factors (c-factor) have been correlated with inferior bonding effectiveness to radicular dentine.14^,17^,18

This article describes an indirect technique for the fabrication of a one piece custom-fabricated post and core restoration made of laboratory light-cured composite resin material and reinforced with fibres.

Procedure

After canal preparation, fabricate a pattern with autopolymerizing resin (Resin Pattern LS, GC Corporation, Tokyo, Japan) directly in the patient's mouth, as for a metal cast post and core restoration (Figure 1). Evaluate resin pattern for passive fit and wall adaptation.

Mix the base and the catalyst of a putty silicon impression material (Temp-Silicon Putty, MICERIUM SpA, Ge, Italy) and fill up the base of a transparent plexiglass flask (Tender Transparent Flask, MICERIUM SpA, Ge, Italy) (Figure 2). Immerse the resin pattern horizontally halfway in the putty material and keep it still while impression material undergoes polymerization (Figure 3). When the impression material sets, remove excess silicon up to the margins of the flask to avoid interference with the perfect seating of the flask cover in the next step.

Spray the polymerized putty material with a silicon separator (Silicon Tender Separator MICERIUM SpA, Ge, Italy). Fill up the cover of the flask with auto-mix transparent silicon material (Temp-Silic Clear, MICERIUM SpA, Ge, Italy) and position it properly on the flask base applying continuous light pressure to extrude excess material. Secure by tightening the enclosed screws and wait until the silicon material is polymerized (Figure 4). Follow manufacturer's recommendations for maximum working time and minimum setting time to avoid distortion.

After the material is fully set, open the flask, remove the resin pattern carefully and inspect the impression in both sides of the flask for bubbles or voids.

Determine the length of the unidirectional pre-impregnated glass fibre reinforcement (EverStick C&B, Stick Tech, Ltd, Turku, Finland) by measuring from the occlusal to the most apical end of the resin pattern using a periodontal probe (Figure 5a). Using sharp scissors, cut the fibre to the appropriate length (Figure 5b). Prevent light activation of the adhesive and hardening by placing the fibres under a dark cover. Fill both created imprints with laboratory resin material (Gradia®, GC America Inc, Alsip, IL, USA) carefully to adapt well to the walls and prevent void formation in the interface between the resin and the impression material. Immerse the continuous bar of multiple layers of the glass unidirectional fibres halfway in one side of the composite resin material. In cases of very wide flared canals, a second bar of fibres should be immersed on the other half of the impression for additional reinforcement and to prevent having the fibres on only one side of the the constructed post and core restoration (Figure 6).

Reposition the flask parts and secure safely by tightening the screws slowly to give enough time for the excess composite material to flow away. Light cure through the transparent mould for 60–90 sec (Steplight SL-1, GC America Inc, Alsip, IL, USA). After light polymerization, open the flask (Figure 7), light cure directly again from all sides the created post and core system for an additional 40 sec. Inspect for voids and place the restoration in a post-curing device for additional polymerization (Labolight LV-III, GC America Inc, Alsip, IL, USA).

Trim away residual flashes of material with finishing diamond burs and disks. Evaluate and adjust the restoration in the mouth for proper seating. Correct any small defects with composite resin and light cure again. To avoid gross excess of material, attach a plastic or wax sprue on the coronal part of resin pattern to create an escape channel for the residual composite resin material.

After verifying the fit at evaluation, cement the post and core restoration in place with a resin luting cement following manufacturer's instructions (Figure 8).