The New Science of Strong Teeth: Class II Preps

INTRODUCTION
At some point in their career, most restorative dentists have questioned themselves in regards to the way they prepare Class II restorations. I have certainly done so as my 57-carbide mowed through mostly healthy tooth structure, knowing that I was seriously weakening the tooth.
Research has now shown that composite is no better than amalgam for reducing subsequent tooth fracturing (Figure 1).1 That research was the final straw for me. At that point, I stepped away and started to design a new cavity preparation from scratch, based on a rational hierarchy of needs (Figures 2 and 3). That hierarchy is as follows: tooth needs, composite needs, and now modern engineering needs for brittle materials. (Note: Enamel and dentin are technically brittle materials.) During my teaching activities, countless dentists have asked me, “Why do we cut such a large hole in a molar to access a tiny spot of dentinal caries?” When I showed the modern Clark-design Class II cavity preps to John Khademi (an endodontist and coauthor of parts 1 and 2 of this article series), he replied, “Yes, that makes sense. The G. V. Black prep made no sense to me, from an engineering standpoint; so I refused to cut them in school [24 years ago].”

Why Do Teeth Break? Better Yet, Why Do They Not Break?
In our first article, “Fracture Resistant Endodontic and Restorative Preparations,”2 we discussed the formula for crack initiation in brittle materials (such as dentin and enamel). The best formula for determining stress concentration in the occlusal of a tooth due to a cavity preparation is (1 + 2√L/R) , where L is length of the cut and R is the radius of the cut. We want L to be as small as possible. R should be as large as it can be, within reason. In simple terms, the longer the cut, the worse the cut. A cylinder-shaped hole (typical G. V. Black Class I) is worse than a v-shaped hole. In the case of a the occlusal of a molar, several separate shallow fissurotomy preps to eliminate stain and caries are far better than one continuous slot cut with a square-ended 556 bur.
In this article, I will combine that strategy along with other pertinent issues, making recommendations on how to cut a Class II composite that is consistent with today’s knowledge and science. Back-to-back Class II cases will be presented in order to demonstrate and teach several modern principles.

CASE REPORT
Note: The brevity of the article does not allow a full discussion of all Class II preparations. Instead, I will fully explain the decisions, materials, and sequencing for this case (Figures 2 to 19).

Clinical Findings
An 18-year-old female presented with interproximal caries on the distal of her mandibular right first molar (tooth No. 19), and in the mesial of her mandibular left second molar (tooth No. 18). Unfortunately, despite repeated nutrition counseling, oral hygiene instruction, office fluoride varnishes, and home fluoride treatments, the distal of tooth No. 19 continued to worsen on radiographs and a small cavity was discovered under 16x magnification with a microscope (Global Surgical).
Images start with a Clark Class II Saucer preparation on the distal of No. 19. Once the distal of No. 19 was cut away, advanced magnification allowed a unique “opportunistic” view of the mesial of tooth No. 18. At that time, a typical crescent-shaped decalcification was observed on the mesial of No. 18 with a microscopic cavitation (not pictured). Because of the patient’s history of noncompliance, I opted to prepare opportunistic access2 on No. 18 rather than take a wait-and-watch approach. Tooth No. 18 mesial was cut from the side via the prep on the distal of No. 19 (Original Fissurotomy Bur [SS White Burs]), and then a calla lily-style enamel shape with infinity edge margins was cut and sanded (round-ended course diamond (Piranha 856-018C [SS White Burs]) followed by a thin flame-shaped fine diamond (Shofu Dental 848F); followed by coarse ContacEZ and or lightning strip; followed with blasting with high power air-water abrasive slurry (Prophy Plus [Bioclear Matrix]) to remove final remnants of biofilm. Again, in this case, the concept opportunistic access for the preparation of the mesial of tooth No. 18 was afforded once the distal of tooth No. 19 was opened up with the saucer preparation. The marginal ridge of No. 18 was spared (Table 1).

Figure 1. Bonded composite restoration (right) leaves tooth just as weakened as an amalgam restoration (left). A better prep, not adhesive dentistry, is what allows the tooth to survive over time. Figure 2. Oblique view of the featured case with opportunistic access on No. 18 and saucer/Clark Class II on distal of tooth No. 19.
Figure 3. Blasted, abraded, and etched tooth No. 18 with blue tinted Bioclear Matrix in position. The mesial preparation is separate from the occlusal fissurotomy preparations.

Figure 4. Bioclear M406 Matrix in straight and lateral view. Blue tint allows better visualization for
posterior teeth and has 99% light transmission.

Figure 5. Application of disclosing solution is part of the modern composite preparation. Even when using a microscope, the biofilm (plaque) can be nearly impossible to see without using dyes. Phosphoric acid alone does a poor job of cleaning. Figure 6. “Blasting” of biofilm with high power air-water abrasive slurry. A cup and coarse pumice is futile but remains the current accepted method to clean teeth before sealants or composites.
Figure 7. After blasting, disclosing solution is applied a second time because the disclosing solution does not stain deeply. At this point, cleaning/abrading of the fissure will require the use of a fissurotomy bur. Figure 8. Low- and high-magnification views of the Original Fissurotomy Bur (SS White Burs). The bur will gently abrade and clean this defect without significant weakening of the tooth. In this application, it would not require anesthesia.

Filling a Minimally Invasive Preparation Using an Injection Molding Technique
In reality the bur, prep design, biofilm removal at the margins, adhesive, composite, and polishing are all interdependent. When you change one element, it will usually affect all the others. The preparation on tooth No. 18 presents unique challenges and wonderful conservation. Figures 10 and 11 demonstrate the constricted access for etching, bonding, and filling; impossible with ordinary metal matrices and cold paste composite in a normal “fat”-tipped paste composite syringe. A new cavity prep design demands new filling instruments. Instead, the sectional translucent Bioclear matrix allows itself to be temporarily teased away from the tooth for access for etching. Later the space was injected with bonding resin, and then chased with a low-stress bulk-fill flowable composite (Filtek Supreme [3M ESPE]); then chased with heated Filtek paste composite in special “step-down micro-flow tips” (Bioclear Matrix). These 3 resins are injected, in sequence, without curing individually using an injection molding composite technique. It is similar to using light-body and heavy body impression materials. At present, it is the optimal method to achieve a monolithic (unibody or nonlayered) Class II composite. The goal is for at least 80% to 90% of the overall mass of composite to be paste composite (with less than 20% being flowable composite).3 An anatomic translucent matrix allows buccal-lingual curing, but more importantly, it allows the composite and the light to wrap around the tooth where a metal matrix cannot reach, even with bulk-fill composites. The 0.7-mm (Figure 17) clearance at the buccal and lingual aspects allows for 5 key properties. First, the use of a lightning strip (Integra Miltex) abrades away biofilm near the margins and removes undermined enamel rods. (Tragically, today’s composites are designed to fill “pothole preps” with a definitive margin and a metal matrix.) The second advantage is that the long margins allow the composite to wrap around the tooth to potentially strengthen the tooth; not through adhesion per se, but from an engineering design perspective. A third advantage of the infinity edge interproximal margin is better aesthetics from an invisible margin and resistance to stain. A fourth advantage is better wear resistance than a sharp—or beveled—wall margin, which invariably begins to ditch when in function. The fifth advantage is that the neighboring tooth can be spared from the almost inevitable iatrogenic gouging; an abrasive strip is used to prep the enamel near where the teeth touch, not a bur.


The goal is to avoid the over-use of flowable composites or reliance on grainy, ugly bulk-fill paste composite. Although bulk-fill shows promise, it is no substitute for cutting a better prep. (Watch videos at dentistrytoday.com.)

Figure 9. Diagram of the same fissurotomy bur used to initiate the saucer Class II preparation (Clark Class II). Figure 10. Saucer preparation (Clark Class II) on right, distal of tooth No. 19. Opportunistic Class II on left (mesial of tooth No. 18). Opportunistic access is loosely defined as maximally conservative tooth preparations based on the unique situation.

In Enamel We Trust
One of today’s biggest confusions is the term adhesive dentistry. When I see symposiums on adhesive dentistry, I scratch my head. That’s a term that should be discarded or clarified. Composite/enamel adhesion is essentially unrelated to dentin bonding in real practice. We have 20-year outcome studies showing that porcelain veneers will stay bonded to enamel, but they may just fall off the dentin after a few years. Ninety-nine percent of the dentists I talk to refuse to risk themselves and their patients by relying on dentin bonding for certain restorative tasks, and for good reason. When outcome studies show long-term retention of bonding to dentin substrates in the everyday practices, we can talk. Until then, dentin bonding is something we do to avoid sensitivity, and to avoid significant microleakge and stain. No one wants to “hang their hat” on just dentin bonding. Let someone else deal with dentistry that falls off at 3 to 5 years. Not me!
Today’s careful clinician will maximize enamel engagement, and carefully seal dentin but not rely on dentin’s slowly weakening bond to retain the restoration. Table 2 features an overview of current Class II cavity preparation styles and relevance.

Figure 11. High-magnification view (4x) of the opportunistic access (left, mesial tooth No. 18) with Bioclear Matrix in position. Figure 12. Higher magnification view (8x) of the occlusal access of the Opportunistic prep (highlighted in green). The matrix can be teased open for insertion of the etch tips, dispensing resin flowable and then paste composites. Then the matrix is teased back into position.
Figure 13. The 27% blue phosphoric acid etchant was placed. Note how the Bioclear Matrix allows an interesting “aquarium” view of the procedure. This allows the operator to evaluate the completeness of etching and delivery of resins to the cavity. Figure 14. Clear matrix allows direct light penetration from curing light from many angles. In this case, no layering is needed even with conventional (nonbulk-fill) composites.
Figure 15. Once the smaller prep is filled (left), another clear molar matrix is placed, soft wooden edge replaced, and Bioclear Tetra ring is placed. Note how the 4 patent pending independently hinging engagers of the clear ring apply pressure to fully adapt the matrix, eliminating overhangs. Figure 16. High-magnification view of the Tetra ring’s 4 independent suspension engagers on teeth Nos. 18 and 19.
Figure 17. Green arrows show optimal clearance of 0.75 mm, which strikes a perfect balance, allowing for 5 key properties. Figure 18. Heating of paste composite allows “Step Down” Tips that are self-threading and are screwed onto traditional Filtek Supreme (3M ESPE) compules. Regular (top), 2x (middle), and 4x (bottom) Step- Down Tips (Bioclear Matrix) are shown.
Figure 19. High-magnification immediate postoperative view of contacts. Note the invisible “infinity edge” margins, mirror smooth finish, broad contacts, and lack of white lines. The tooth, composite, and marginal ridges should deliver decades of excellent service to the patient without retreatments. Figure 20. Illustration of the evolution of the modern Class II: Clark Class II (left), slot prep (middle), and traditional G. V. Black preparation (right).

Here We Go Again! Partial and No Dentinal Caries Removal
G. V. Black mandated that all dentinal caries must be removed. That “rule” is contradicted by modern science, yet remains the standard. As dental schools slowly but surely change, the cavity preparations will go through another painful, chaotic, and exciting transformation as the rulemakers decide how much and where we will practice strategic selective dentinal caries removal. As I prepare our textbook on modern cavity preparations, these changes will make a profound effect on cavity design, materials and the approach to the pulp/predentin/dentin/enamel/restoration continuum (Figure 20).

IN SUMMARY
Our most common restorative procedures, Class I and Class II restorations, are in a state of chaos as the manufacturers design adhesives and composites for a cavity prep that is 120 years old (Table 2). Many of the current preparations are tweener preps, somewhere in between a G. V. Black and a slot prep, neither of which are well suited for composite. What the schools teach is even more curious. We also see a resurgence of direct pulp-capping materials, when the research tells us to never expose the pulp. Instead, in teeth with an intact pulp, we should leave a clean margin and 1.0 mm of carious dentin over the pulp! As mentioned above, the tunnel prep was designed to preserve enamel while accessing the dentinal caries, but we are currently questioning the need to remove carious dentin. Can we all begin to appreciate how cataclysmic these changes will be?
It is my hope that the principles advanced in this article will aid clinicians in their journey toward ideal conservative restorative dentistry.


References

  1. Wahl MJ, Schmitt MM, Overton DA, et al. Prevalence of cusp fractures in teeth restored with amalgam and with resin-based composite. J Am Dent Assoc. 2004;135:1127-1132.
  2. Clark DJ, Khademi J, Herbransen E. Fracture Resistant Endodontic and Restoratve Preparations. Dent Today. February 2013;32:118-123.
  3. Clark DJ. The injection-molded technique for strong, esthetic class II restorations. Inside Dentistry. 2010;6:68-76.

Dr. Clark founded the Academy of Microscope Enhanced Dentistry, which is an international academy formed in 2002 to advance the art and science of microdentistry, microendodontics, microperiodontics, and dental microsurgery. He has also developed the Bioclear Matrix System, a comprehensive, tooth specific, clear anatomic matrix and interproximal restorative system. He can be reached at drclark@microscopedentistry.com.

Disclosure: Dr. Clark is the owner of Bioclear Matrix Systems, and is a consultant for SS White Burs.