The Metallic Versus the Nonmetallic Restored Dentition

An increasingly discernible trend occurring in dentistry is the movement away from metallic components in the restored dentition. The motivation for this trend is the increasing importance of aesthetics in dentistry and the new equipment and materials that are broadening its potential. These new materials and equipment include rapid curing lights, air abrasion, lasers that optimize the dentinal surface for bonding, better bonding agents, fiber-reinforced composites, and stronger ceramics.

The advantages of pure ceramics and fiber-reinforced composites include the following:

• They have superior aesthetics to traditional PFM.

• Tooth-colored composite-fiber posts cast no shadow through ceramic restorations.

• The use of porcelain alone prevents the gray line around the necks of PFM-restored teeth (Figure 1), and increases the translucency associated with natural dentition.

Figure 1. Note the gray areas around the necks of the central incisors, a common sight when porcelain is fused to cast gray metals.

The advantages of pure ceramics and fiber-reinforced composites, however, must be weighed against their disadvantages, which include:

Figure 2. Note the knife edge relationship of the enamel to the underlying supportive dentin. Butt-joint porcelain restorations do not support the external surfaces of the root as does a ferrule of 1 to 2 mm.

• Having margins that are limited to either a butt joint or minimum chamfer.1 Porcelain does not have adequate edge strength for a knife-edge margin. Interestingly, pure porcelain restorations are said to mimic the aesthetics of natural teeth most closely. However, the enamel on the intact natural tooth has a thin ferrule relationship to the underlying dentin at the dentino-enamel junction, not a butt joint (Figure 2). This relationship is vital to the natural strength of a tooth. When the enamel must be replaced, a ferrule margin of 1 to 2 mm in metal comes much closer to mimicking nature’s architecture than a butt joint.

• The potential leakage because of inadequate bonding when the ceramic restoration is cemented in a moist environment.2

• The inability to remove a bonded restoration when secondary decay occurs.3

Figure 3. A schematic drawing showing the margins between the composite core material and the coronal surface of the root opening up when the core is supported by a flexible post.

• Fiber-reinforced composite posts bending far more than the tooth they are in, thus putting stress on the composite core and opening margins under prolonged function4 (Figure 3).

• Fiber-reinforced composite and ceramic posts have low retention.5,6

BUTT-JOINT RESTORATIONS AND FIBER-REINFORCED COMPOSITE POSTS

Figure 4. Schematic drawings demonstrating the importance of coronal dentin. If sufficient dentin exists, any post will work; if minimal coronal dentin exists, a flexible nonmetallic post offers insufficient resistance to prevent gap formation between the core and the root.

Butt-joint restorations and fiber-reinforced composite posts have potential weaknesses. If substantial tooth structure exists, the prepared tooth should have enough bulk to support a ceramic restoration. Even in these situations, a common occurrence is the fracture of maxillary anterior teeth at the gingiva after several years when they have been restored with full ceramic crowns. Ceramic is a nonresilient material that transmits all the forces of occlusion to the underlying dentition. In effect, the underlying prepared tooth suffers from dentin fatigue and may eventually fracture. This phenomenon is far more likely to occur if the tooth had endodontics, had little coronal dentin, and required a post and core buildup (Figure 4). The axial walls that bear the brunt of all lateral forces are now composed of man-made materials, producing margins between the core material, the post, and the crown.

Figure 5. A schematic drawing demonstrating that the cross-sectional area of a post is often about 1/20 that of the root it is in. Figure 6. A schematic drawing demonstrating how a nonmetallic post with the same modulus of elasticity as dentin will bend far more than the root it is in when under function, thereby creating gaps between the core and the root.

A fiber-reinforced composite post that has the same modulus of elasticity as dentin will bend significantly more than the root it is in because its cross-sectional area is approximately 1/20 that of the root (Figures 5 and 6). To bend the same as the root, the modulus of elasticity would have to be approximately 20 times greater than the root, a material that suggests stainless steel or titanium alloy, and not fiber-reinforced composites. Research has shown that the use of a metal ferrule margin intimately fitted to an external bevel on the outer surface of the root is the most efficient design feature in reducing the chances of fracture when the tooth requires a post.7 If a PFM restoration is the desired restoration from a functional point of view, the need for an aesthetic tooth-colored post becomes a secondary consideration.

Figure 7. A drawing of the split-shank Flexi-Flange.

Further reinforcing this viewpoint is the weakness of the fiber-reinforced post and its poor retention. It is far better to use a post that combines excellent retention with minimal insertional stresses and an even distribution of functional stresses8,9 (Figure 7). A post with a much higher modulus of elasticity than a fiber-reinforced composite post means that the post will bend similarly to the root when under function.

It is necessary to etch and bond the tooth surface receiving a ceramic restoration. If all margins are supragingival, it is routine to dry, etch, and bond the prepared dentinal surface prior to placing the composite cement. However, many margins are not supragingival, cannot be rendered dry enough, and have a compromised seal when cemented into place, leaving the margins vulnerable as future sights of fluid ingress and subsequent decay.1 The compromised bonding occurs between the ceramic and the dentin, an interface gap that, as the resin cement polymerizes, becomes wider than that between a razor-thin metal bevel of a PFM and the prepared tooth. Subgingival margins are difficult if not impossible to keep dry. Bonding agents are not effectively placed in moist environments. In these situations, the natural thermocycling that occurs in the mouth leads to a further diminished bond strength between the crown and the prepared tooth.10

Cast porcelain has a less accurate fit to the dentin than cast metal. Consequently, the potential for leakage is greater. If and when decay does occur, a bonded ceramic restoration cannot be removed intact. It must be drilled off, subjecting the tooth to further trauma.

THE SIGNIFICANCE OF A POST WITH LOW RETENTION

Figure 8. A schematic drawing showing how the force needed to fracture a previously fractured root was determined.

One point of view is that retention is not a significant factor in post placement because the pure tension needed to remove a post in a coronal direction is not generated in the mouth. However, Friedman et al11 demonstrated that a vertical force of 220 pounds was needed to refracture a natural tooth that had been previously fractured and reinforced with an adhesive restorative material, dramatically demonstrating that the 220 pounds needed is truly a worst case scenario (Figure 8). This figure is lower than that needed to detach a natural crown from a natural root in pure tension. Assuming mother nature designs things for a reason, it would be reasonable to assume that the minimum retention a post should have would be 220 pounds without causing any significant insertional stresses.

The cement used with the post can affect retention values. For example, research has demonstrated that a No. 2 Flexi-Post cemented with Flexi-Flow Cement (Essential Dental Systems) has a retention value of 336 pounds. This is a statistically superior result compared with a No. 2 Flexi-Post cemented with zinc phosphate cement that produced retention of 266 pounds.12 Flexi-Flow composite resin cement also has the additional advantage of proven 5 years of fluoride release.13

This is not to say that a solid core and a well-fitting overlying restoration cannot compensate for a poorly retentive post. It is, however, structurally more sound to optimize each component of the restoration to maximize its longevity. Restorations are built up from the inside out, but they fail from the outside in through an ongoing process of decementation and micromovement that accelerates with time.

DISCUSSION

The promise and importance of aesthetic materials in dentistry is undeniable. Patients demand excellent aesthetics, and the rewards for providing them are great and growing. The dentist, however, must balance the desire for maximum aesthetics with the long-term best interests of the patient. A thorough explanation of the possibilities may not alter the original treatment plan, but the patient should have the right to make a fully informed decision. Pure porcelain restorations are very tempting. The aesthetics are superb and the requirements for an accurate impression are far less severe than for a PFM with knife-edge margins. Any inaccuracies that exist are filled in with the bonded composite resin cement.

Figure 9. A schematic drawing of a butt-joint porcelain restoration over a flexible nonmetallic post distributing stress internally and leading to early failure versus the way a PFM restoration with a bevel of 1 to 2 mm coupled to a highly retentive split shank threaded post directs stresses externally and optimizes the longevity of the entire restoration.

However, a key question is what underlying support do these restorations need to avoid long-term breakdown? Butt-joint restorations having no metal collar may be further weakened by the use of fiber-reinforced composite or epoxy resin posts. This article suggests that the combination of a butt-joint ceramic restoration coupled to a post of low retention and a low modulus of elasticity is contraindicated for long-term clinical success (Figure 9).

At present, research has established the following facts:

(1) Fiber-reinforced posts bend significantly more than metal ones.4,14

(2) Fiber-reinforced posts do not adequately support an overlying composite core, leaving open the possibility of gap formation through a combination of both compressive and tensile forces, degrading the bonded interface between the composite core and the root.15

(3) Bonded composite posts have poor retention, never exceeding 90 pounds and usually within the range of 30 to 40 pounds,15 far less than nature provides for intact teeth.

Figure 10. A slide of a Captek crown exhibiting a ferrule made of metal. Figure 11. A slide demonstrating the excellent results that can be achieved with Captek technology optimizing aesthetics and based on solid restorative principles.

(4) Captek (Captek, Precision Metals Inc), a new PFM technology, provides a strong metal ferrule effect16 while supplying excellent aesthetics (Figures 10 and 11).

(5) Captek eliminates the need for aesthetic but structurally weaker nonmetallic posts.17,18

(6) Ceramic posts that are as stiff as metal provide minimum retention within the root and minimum retention of a core,18 two factors that are central reasons for placing a post in the first place.

(7) The cement interface for butt-joint ceramic restorations is more prone to gap formation than the margins of a PFM restoration.10

(8) Bonding is compromised when the surfaces to be etched are not free of excessive moisture (saliva), a situation that exists in the sulcus of all prepared teeth.19

(9) When a ferrule does not exist, the forces of occlusion are distributed internally to the axial walls. The axial walls of endodontically treated teeth are compromised and should be supported. Natural teeth distribute functional stresses externally, which makes orthodontic movement possible.7

(10) All composite resins have the potential to lose their adhesion when bonded in a moist environment. Because a bis-GMA resin is hydrophobic, it is repelled by dentin, which contains approximately 30% water, encouraging open margins that enhance the degradation process.19 Thermal cycling also increases the degradation process.

CONCLUSION

While the desire to maximize aesthetics with nonmetallic restorations is strong, the list of deficiencies associated with these techniques should give the dentist pause, especially when excellent aesthetics can be achieved with the newer PFM technologies. If aesthetics becomes an end in itself without proper regard for long-term functional success, then the progress made will truly be a step backwards.


References

1.Clark M, Richards M, Meiers J. Seating accuracy and fracture strength of vented and non-vented ceramic crowns luted with three cements. J Prosthet Dent. 1995;74:1-24.

2.White S, Ingles S, Kipnis V. Influence of marginal opening on microleakage of cemented artificial crowns. J Prosthet Dent. 1994;71:257-264.

3.Appeldorn R, Wilwerding T, Barkmeier W. Bond strength of composite resin to porcelain with newer generation porcelain repair kit. J Prosthet Dent. 1993;70:6-11.

4.Sidoli G, King P, Setchell D. An in vitro evaluation of a carbon fiber-based post and core system. J Prosthet Dent. 1997;78:5-9.

5.Cohen BI, Pagnillo MK, Musikant B, et al. Comparison of the retentive and photoelastic properties of two endodontic post systems. J Oral Rehabil. 1999;26:488-494.

6.Cohen BI, Pagnillo MK, Newman I, et al. Retention of four endodontic post designs cemented with composite resin cements. J Den Res. 2000; 79, Abstract No. 3220: 546.

7.Assif D, Bitenski A, Pilo R, et al. Effect of post design on resistance to fracture of endodontically treated teeth with complete crowns. J Prosthet Dent. 1993;69:36-40.

8.Musikant BL, Deutsch AS. A new prefabricated post and core system. J Prosthet Dent. 1984;52:631-634.

9.Greenfeld R, Roydhouse R, Marshall F, et al. A comparison of two post systems under applied compressive shear loads. J Prosthet Dent. 1989;61:17-24.

10. Kato H, Matsumura H, Tanaka T, et al. Bond strength and durability of porcelain bonding systems. J Prosthet Dent. 1996;2:163-168.

11. Friedman S, Moshonov J, Trope M. Resistance to vertical fracture of roots, previously fractured and bonded with glass ionomer cement, composite resin and cyanoacrylate cement. Endod Dent Traumotol. 1993;9:101-105.

12. Cohen BI, Condos S, Musikant BL, et al. Retentive properties of threaded split-shank posts with titanium-reinforced composite cement. J Prosthet Dent. 1992;68:910-912.

13. Cohen BI, Pagnillo MK, Deutsch AS, et al. A five year study: fluoride release of four reinforced composite resins. Oral Health. 1998;April:81-86.

14. Purton D, Love R. Rigidity and retention of carbon fiber versus stainless steel posts. Int Endod J. 1996;29:262-265.

15. Cohen BI, Pagnillo MK, Musikant BL, et al. Comparison of the retentive and photoelastic properties of two prefabricated endodontic post systems. J Oral Rehabil. 1999;26:488-494.

16. Morgano SM, Brackett SE. Foundation restorations in fixed prosthodontics: current knowledge and future needs. J Prosthet Dent. 1999;82:643-657.

17. Hollis RA, Christensen GJ, Christensen W, et al. Comparison of strength for seven different post materials. J Dent Res. Abstract No. 3421 1999;78:533.

18. Cohen BI, Pagnillo MK, Newman I, et al. Retention of four endodontic posts cemented with composite resin. Gen Dent. 2000;48:320-324.

19. Aboush Y. Removing saliva contamination from porcelain veneers before bonding. J Prosthet Dent. 1998;80:649-653.



Disclosure: Dr. Cohen is vice-president of dental research, Essential Dental Laboratories, S. Hackensack, NJ. Drs. Musikant and Deutsch are codirectors of dental research, Essential Dental Laboratories, and practicing endodontists in New York, NY.

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