How Do Your Preparations Measure Up?

On one hand, the availability of a wide assortment of dental materials empowers dentists and laboratories with restorative options, whether conservative or aggressive, for every indication. On the other hand, having too many choices can create a quagmire of material- and restoration-specific preparation requirements that when challenged by patients' individual needs, may compete with their goals and expectations. This month, Inside Dentistry assists dentists in determining how their preparations measure up in terms of restorative objectives, material qualities, and most  importantly, individual case characteristics in order to ensure the function and long-term predictability of the restorative treatments they provide.

The principles of tooth preparation include the preservation of tooth structure, retention and resistance form, marginal integrity, structural durability, and esthetic considerations.¹ The majority of the problems associated with adhesively bonded restorations result when preparations are extended into dentin. Research broadly supports the fact that bonds to natural enamel are more predictable and durable than those to dentin.² Regardless of the preparation design, an enamel substrate is a better choice than dentin.

"The goal should always be minimal, conservative dentistry, but there are compromises that have to be made, depending on the clinical situation," says Amanda Seay, DDS, the restorative section editor for Inside Dentistry and a private practitioner in Mount Pleasant, South Carolina. Unfortunately, patient-specific conditions, the region of the mouth being restored, treatment requirements, and the restorative materials chosen present challenges that can limit a dentist's ability to conservatively prepare the remaining natural tooth structure. "For example, one may want to do a minimal veneer preparation that requires very little tooth reduction, but if you are bonding to dentin, then you may need to change your preparation design," says Seay. "If you are trying to close a diastema between two teeth, you may need to lower the interproximal margins to create a natural emergence profile."

Therefore, establishing the patient's goals and objectives, determining the patient's diagnosis and prognosis, and understanding the requirements of the materials that will be used are three critical prerequisites for determining which preparation techniques and protocols are the most appropriate for any given case.

Conservative direct adhesive procedures offer a number of benefits when compared with indirect restorations, including significantly fewer endodontic complications, a reduced risk of wear to opposing teeth, occlusal schemes that can be assessed and corrected immediately, and easier re-intervention because the restorations are more reversible and amenable to repair.³ Both the region in the mouth where the restoration will be placed and the type of restoration being placed (ie, direct or indirect) greatly impacts the design of the preparation itself.

There are guidelines that apply to both anterior and posterior preparations, but where they differ is key, and there are some recommendations and tips to keep in mind. "When It comes to treating a tooth, we have to look at why we're doing a restoration in the first place, identifying and addressing the causative factors to breakdown and considering the future requirements of the tooth based on predicted challenges. Treating a tooth for one mode of failure does not necessarily prevent failure from another source of breakdown," says Betsy Bakeman, DDS, president of the American Academy of Cosmetic Dentistry's board of directors and a private practitioner in Grand Rapids, Michigan.

Anterior

Patients' increased demands for esthetics and clinicians' increased capacity to preserve the dental structure have resulted in the development of different incremental techniques for restoring fractured anterior teeth in a natural-looking way.⁴ There are a variety of factors that will influence the preparation design and the amount of tooth structure that is required to be removed in esthetic anterior restorations. These factors include reproduction of the optical properties of natural teeth, shade matching, and appropriate material selection to achieve seamless integration of restorations among natural teeth within the smile.

The main advantage of direct ad-hesive procedures over indirect restorative procedures is that they require minimal or no tooth preparation to enhance resistance and retention form.⁵ Preparation for anterior direct restorations simply involves removing unwanted, existing restorations, carious tissue, unsupported tooth structure, etc.^6,7 "There are times when very minimal enamel preparation may be needed in order to blend the composite into the tooth structure and control color transition, but it is a nominal amount," says Seay.

The use of certain materials pre-sents inherent requirements for thickness and tooth reduction in order to realize sufficient strength and esthetic characteristics, particularly when treating severely compromised teeth.⁸ Although adhesive bonding protocols and advancements in dental ceramics have enabled esthetic treatments that are more conservative, each restorative material requires different degrees of tooth preparation, which makes it important to select restorative materials on a case-by-case basis.⁸

For example, in cases requiring diastema closure or the correction of malformed anterior teeth, no-preparation or minimal reduction porcelain veneers of different thicknesses could be used to reestablish function and esthetic harmony-depending on specific case characteristics-and provide predictable results.⁹ However, despite being used for more than 30 years, no-preparation veneers can be prone to failure in some situations.^9,10 Furthermore, although advanced materials have enabled dentists to provide patients with more conservative and esthetic smile makeovers, some (eg, lithium disilicate crowns) still present technique-sensitive considerations, such as the need for additional precision when preparing margins and bonding with adhesive resin cement.¹¹

Interestingly, there is a lack of universally accepted preparation classifications outlining the extent of tooth reduction necessary for different veneer treatments, and that could be contributing to the under- and overpreparation of teeth when veneers are indicated.¹⁰ A system has been proposed that acknowledges that veneer preparation criteria should be determined and categorized based on a combination of case considerations, such as the need for reduction (ie, space requirement, working thickness, material room), the volume of remaining enamel, and the percentage of exposed dentin.¹⁰ This type of classification provides an accurate guideline for quantifying the necessary amount of tooth structure to be removed on a case-by-case basis during treatment planning.¹⁰

Posterior

In the posterior region, occlusal tooth surfaces should be contoured and shaped to provide sufficient, evenly distributed restorative space to accommodate restorations of uniform thickness.¹² Unfortunately, achieving even tooth reduction-particularly in lingual and palatal areas-can be challenging, potentially compromising efforts to create adequate occlusal clearance in both static and functional occlusion.¹² Among the factors affecting the simplicity or difficulty of posterior tooth preparation are the tooth's anatomy and morphology.¹³

Similar to veneer restorations for anterior teeth, there exists a range of posterior preparation designs with respective requirements for tooth structure removal, which are largely dependent upon the specific type of restoration (eg, full-coverage crown, inlay, onlay) and the material from which it is fabricated (eg, metal, all-ceramic).^13,14 Likewise, a measurement system for accurately quantifying and classifying preparations for fixed prosthodontics has been introduced to help conserve tooth structure and contribute to a better prognosis for restored teeth.¹³

When restoration-free teeth are prepared for full-coverage ceramic crowns, the procedure is typically invasive despite the fact that the selection of all-ceramic restorations can significantly lessen the amount of tooth structure required for removal.¹⁴ The degree of tooth structure removal required for ceramic crowns varies depending upon the material as well as the location in the mouth.¹⁴ Full-coverage monolithic zirconia crown restorations appear to be the least invasive for either anterior or posterior teeth, and the use of buccally veneered zirconia rather than fully veneered zirconia significantly reduces preparation invasiveness.¹⁴ Of course, these are guidelines, and every scenario should be considered on a case-by-case basis.

It has been widely accepted that the selection of restorative materials should be dictated by the indication and individual patient/case characteristics (eg, risk factors, occlusal needs, oral habits, prognosis), and choices are best made collaboratively between the dentist and the laboratory. This helps to ensure that the materials selected satisfy form, function, and esthetic expectations while simultaneously contributing to overall clinical success.

The Decision Tree

Once dental caries has been diagnosed, a treatment option is chosen in a process that includes active participation from the informed patient. Generally, the three basic options are to restore the tooth, extract the tooth, or do nothing. Too often, clinicians extract teeth when endodontic therapy, crown-lengthening surgery, forced orthodontic eruption, or regenerative therapy could be used with predictable results.¹⁵ If the decision is made to place a restoration, the next decision concerning which procedure and material to use is made primarily by the dentist. The main variables to consider when determining the best approach to preparing a tooth include the preparation design, the type of restoration, the type of coverage (ie, partial or full), the restorative material, the amount of mechanical retention, and the bonding agent. "Care, skill, judgment, past experience, and education all come together to inform that decision tree and determine the best preparation method for a given tooth," notes Mark Malterud, DDS, a private practitioner from Minneapolis, Minnesota.

There are a few restorative considerations that factor into preparation design. The type of restoration, location, and retention form will influence the choice of traditional cementation or adhesive bonding. The restorative material will be chosen based on its handling properties and long-term esthetic stability. There are a variety of materials to choose from. Dental amalgam has historically been the gold standard of restorative care; however, the use of composites as a restorative option has grown in popularity.¹⁶ When choosing a material for a restoration, it is important to take into consideration the patient's history and the tooth's specific conditions as well as the problem, its location, and the cause of the problem. The optimal color matching of restorations with adjacent natural teeth is especially important in the anterior region.¹⁷ The thicker the restorative material, the more it will block out the color of the tooth underneath.^18,19 Thickness also plays into how fracture resistant various materials can be, and it must be accounted for in preparation design. "Occlusion is at the foundation of all of this," says Malterud. "We must create excellent occlusal patterns and control parafunctional forces if we want our patients to keep our restorations for the long haul." Increasing the longevity of restorations is an ongoing goal.

The degree of preparation required in any clinical situation is dependent upon the ultimate goal of the restoration (ie, restoring carious lesions, correcting alignment issues, closing a gingival embrasure or diastema, etc). "The best way to determine preparation design is to work with your ceramist," suggests Lee Ann Brady, DMD, a private practitioner in Glendale, Arizona. "Send them all of the preoperative models and photographs, tell them your postoperative goals, and then they can wax-up the case and make recommendations regarding the placement of the margins and the thickness of the materials to accomplish your goals."

Essential Armamentarium

Depending on the procedure and individual preferences, dentists may employ any number of tools from the array of diamond burs, carbide burs, and other instruments available for completing tooth preparations. Burs are used to access carious lesions. The removal of carious tissue is traditionally accomplished with a spoon excavator followed by a conventional handpiece with a round bur. A gingival margin trimmer, enamel chisel, or enamel hatchet can be used to remove unsupported enamel. "I prefer a new, small carbide bur for my preparations and will use either a No. 330 or a No. 329 if I really am able to keep the preparation small," says Malterud. "Whether I am using a bur, a laser, or even an ultrasonic system for a preparation, I will always finish it by cleaning the area with air abrasion using aluminum oxide, and if the situation is right, I can use air abrasion exclusively for the entire preparation," says Malterud.

When using these tools, it is important to remain as minimally invasive as possible. Tooth structure can be preserved and aggressive preparations avoided when careful diagnosis and considerate treatment planning are undertaken to identify opportunities for conservatively restoring patients to improved function and esthetics.²⁰ For example, orthodontics and additive direct composite resin techniques may be deemed appropriate not only in cases of misalignment but also in those involving wear and improper guidance, offering patients a less invasive treatment alternative.²⁰ Similarly, enamel can be left intact and adhesive dentistry and etchable ceramic materials can be employed in cases in which opening the vertical dimension of occlusion is required.²¹

Ultimately, the process of assessing current tooth condition, establishing a complete diagnosis/prognosis, and weighing patient expectations during the planning phase can enable dentists to avoid removing excessive amounts of healthy tooth structure while still accomplishing functional and esthetic goals.^21,22

Thereafter, thoughtfully designed preparations executed using preparation guides (ie, reduction matrices) can help to realize the preservation of tooth structure and, when indirect treatments are indicated, enable laboratories to fabricate restorations with ideal morphology.²² Sufficient tooth reduction is necessary to ensure the function, esthetics, and longevity of indirect restorations; however, preparation guides or matrices can help to prevent overly aggressive reductions as well as establish ideal tooth angulation and maintain periodontal health.²³ Ideal preparation requires a fine balance. "Under-reducing can be as inappropriate as removing more tooth than you need," Brady notes. Of course, preparations should be planned according to the needs of each individual case.

Preparation Guides

Preparation guides are fabricated from diagnostic wax-ups, which facilitate and enhance the treatment planning process.²⁴ The most common preparation guides are indices made from silicone putties. The matrix is used to create an intraoral mock-up of the final restoration that a dentist can make uniform depth cuts into. This matrix can be further modified by cutting windows to visualize reduction during or after preparation. When it is well adapted to the waxed-up model, the minute details of contour from the wax-up are accurately captured. Preparation guides should be fabricated from a prototype of the final restorations that is either printed or waxed and then duplicated in stone. This ensures the accurate and even reduction of single or multiple teeth.

Generally, multiple indices are fabricated for use because different index styles are utilized depending on the preparation design and the need for visibility. Vertical and window indices are very versatile, and other index styles include palatal and facial. "The key is to always start preparation into an intraoral mock-up of the final restoration," says Jason Olitsky, DMD, a private practitioner in Ponte Vedra Beach, Florida.

"Design is critical," adds Bakeman. "Prep-aration guides can be prepared by the laboratory or in the dental office, as long as the guide is designed in a thoughtful, purposeful way." Preparation guides are used to verify design, shape, contours, and depths throughout the process of restoration.

To overcome the difficulties associated with conventional techniques, technologies such as intraoral scanning and CAD/CAM were developed for dental practice.²⁵ These technologies facilitate easier treatment planning, case acceptance, and communication with laboratories as well as reduced operative time, storage requirements, and treatment times.^26-28Regarding preparations, the ability to use these technologies to instantaneously review them, both magnified and from every angle, and then modify them, if needed, before sending them to the laboratory improves accuracy and efficiency down the line.

"Perhaps one of the best benefits is the ability to accurately measure occlusal clearance. How many times do we check the clearance and still end up needing a reduction coping?" asks Olitsky. "Measuring the clearance on screen enables us to provide ideal preparation guidelines for the different materials available based on monolithic or layering laboratory fabrication."

Peter Pizzi, MDT, CDT, the editor-in-chief of Inside Dental Technology, notes, "The goal of dentistry is not to prepare teeth but to save the health of the tooth. With this in mind, we want to be as minimally invasive as possible. The guidelines for preparing should be based on previously prepared structure and the functional and esthetic concerns." Each scenario is unique, so although the aforementioned recommendations and tips can be useful to guide preparation, every restorative situation requires its own analysis based on the unique specifications of the tooth, the materials available, and the clinician's skills and experience.