The Importance of Prep Design in Material Selection

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While advances in materials have led to much more predictable outcomes over the years, they are no substitute for precision and accuracy. Since user error is the most common cause of restoration failure, it’s important to make sure your technique and prep design are as accurate as possible.

“Prep design is intrinsic to clinical success, says John Flucke, DDS, a private practitioner in Lee’s Summit, Missouri. “Retentive failures normally begin with some type of compromise in prep design.”

The goal of all prep design should be to ensure the most esthetic—and functional—result with minimal removal of tooth structure. This means understanding the prep requirements for each material or adhesive used in the process.

Prep

In a perfect world, every prep would be a simple undertaking, with maximum tooth structure remaining and a clean, easily accessible field. However, it’s wishful thinking.

“Unfortunately, not every case allows for a textbook prep,” Dr Flucke says. “In cases where the operator has no choice but to create a compromised prep a strong adhesive can help compensate for that.”

Before adhesive selection, the first step is the evaluate the height and taper of the prep, as this will define a prep as either retentive or non-retentive. A prep’s status as retentive or non-retentive is based on the taper of the prepared tooth’s axial wall in comparison to the longitudinal axis of the tooth. A taper greater than 8 degrees is considered to be non-retentive, as are preps under 4 mm, which are considered short preparations.

“Prep design will have an impact on the need to bond to the tooth structure with a dental adhesive,” says Rolando Nuñez, DDS, manager of clinical research at BISCO. “Preparation height and taper are the most important variables.”

Dr Nuñez cites a 2014 study that found that preparation taper, height and marginal design play a big part in restoration success when it comes to dealing with occlusal loads. It found that a smaller preparation taper of 10 degrees was more effective than a 30-degree taper. The study also concluded that it’s more ideal to decrease the taper as the prep height increases to avoid increased stress, as these stresses could result in increased risk of microfracture or cement failure.¹

An adhesive doesn’t just need to adhere to the tooth, however; it also needs to form a strong bond with the restoration itself. This means that during prep, the intaglio surfaces have to be effectively prepared, or the restoration runs the risk of debonding.

Cements and adhesives

While mechanical retention is important, many adhesives today also provide the added benefit of chemical retention, giving clinicians greater options in adhesive features to enhance bond strength and increase esthetics. Ideal retentive reps allow for both adhesive protocols and traditional cementing strategies, while shorter preps or ones with more drastic tapers usually require adhesives bonding systems.

When selecting an adhesive, clinicians must consider the prep’s retention and resistance form, as well as the ratio of dentin to enamel, margin placement, feasibility of light curing, intaglio surface prep and restorative material—as well as the adhesive features. With so many strong, esthetic materials available, it is the dentist’s responsibility to know and understand the requirements for tooth reduction for each option. This will help ensure restoration success by maximizing both strength and esthetics.

A clinician should first decide between conventional cementation and adhesive cementation. The conventional approach uses the retention/resistance form of the preparation design to lute restorations to the underlying tooth structure by forming a hard cement layer between the tooth structure and restoration. Luting forces of these cements are enough to cement retentive prep design with good mechanical retention, which means they are best indicated for retentive preps. Since conventional cements cannot chemically bond to the tooth and restorations, they are contraindicated for non-retentive preps.

Adhesive cementation produces both a chemical and micromechanical bond between the restoration and the tooth structure. These adhesives display superior chemical properties, but can be tricky to execute properly when proper isolation is difficult.

Adhesive systems come in one- and two-bottle systems, as well as universals. One-bottle systems combine the primer and bonding agent into one, which is convenient, but their acidity level isn’t high enough for etching. As such, they don’t have the strongest retention with enamel, as it requires etching. In contrast, two-bottle systems can be self-etch or etch-and-rinse, and provide a stronger bond with enamel. Universal adhesives allow the clinician more flexibility to choose which bonding method they want to pursue. However, the practitioner still needs to take into account whether a light-, dual- or self-cure cement is more appropriate.

This means that the restoration’s restorative material also plays a part in selection. Light-cure and dual-cure resin cements are contraindicated for metal or zirconia restorations since these are too thick for light to penetrate. With these materials, a self-cure cement should be used. For all-ceramic restorations, adhesive resin cements are the best choice. However, in situations where proper isolation is difficult, they can be problematic.

Advancements in materials and prep for direct restorations

Different materials call for different prep. Even when it comes to direct restorations, the evolution from amalgam to composite restorations has affected how clinicians need to think about prep.

“The introduction of dentin bonding agents and composite resin has obviously made a big impact on the way we practice dentistry,” says Nathanial Lawson, DMD, PhD and director of biomaterials at the University of Alabama at Birmingham School of Dentistry. “When my relatives completed dental school in the 1980s, it was considered taboo to place bonded composites for posterior restorations.”

Now, Dr Lawson notes, many current graduates of U.S. dental schools may have never even placed an amalgam restoration, as dental schools place a strong emphasis on composite materials. He himself has not placed an amalgam restoration in over 5 years, but has seen some that have lasted decades.

“Many of us practicing dentists have seen our own composite restorations experience marginal discoloration and degradation within only a few years. The biggest advantage of composite restorations (other than esthetics) is their ability to bond to tooth structure and allow more conservative preparations,” Dr Lawson says. “Their biggest downfalls are premature failure from sloppy placement and eventual leakage from secondary caries.”

These conservative composite preparations have the benefits of not requiring the undercuts necessary in amalgam preps. In amalgam preps, undercuts are used to keep the material in the filling, which means clinicians must remove tooth structure in addition to decay. Since composite restorations don’t require undercuts, more tooth structure can be retained. However, Dr Lawson cautions that while it’s important to retain as much tooth structure as possible, clinicians shouldn’t skimp, particularly along the margins of a restoration, as it can lead to restoration failure.

Prep for success

Dentists can maximize the chance of restoration success by thoroughly assessing a prep to evaluate which adhesive system is most appropriate. By taking the time to ensure they understand the indications of the case—and the manufacturer’s instructions—clinicians can successfully prep for a long-lasting, esthetic restoration.

References

1. Tripathi S, Amarnath GS, Muddugangadhar BC, Sharma A, Choudhary S. Effect of Preparation Taper, Height and Marginal Design Under Varying Occlusal Loading Conditions on Cement Lute Stress: A Three Dimensional Finite Element Analysis. J Indian Prosthodont Soc. 2014;14(Suppl 1):110-118. doi:10.1007/s13191-014-0378-7