Design Criteria for Restorative Dental Materials

C H A P T E R

3 Design Criteria for Restorative Dental Materials O U T L I N E Design Cycle Evidenced Used in Product Design Evidence-Based Dentistry Patient Evidence Laboratory (In Vitro) Evidence

Creating the Plan Building the Restoration

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CRAIG’S RESTORATIVE DENTAL MATERIALS

As discussed in Chapter 2, restorative materials are exposed to chemical, thermal, and mechanical challenges in the oral environment. The combination of forces, displacements, bacteria, biofilm, fluids, thermal fluctuations, and changing pH contribute to the degradation of natural and synthetic biomaterials. Each patient has a unique combination of these factors. When considering a new or replacement restoration for a patient, the performance history of the patient’s existing restorations can provide insight into the prognosis of the new restoration. The performance of materials in controlled conditions, in vitro and in vivo, is also useful when selecting materials and predicting their service life. Making the final materials choices involves a complex decision-making process that can be informed by principles of product design.

DESIGN CYCLE Considering many factors and integrating many specifications into one final product is a requirement of any object that requires fabrication. In product design, a cyclical approach of analyzing and then testing problems is used to determine the best design for the final production piece. Three categories of problem-solving are used in the design cycle: observe, plan, and build. Then the steps are repeated as the time, number of problems, and difficulty of problems allow (Figure 3-1). To illustrate how this process can be applied to the design of a materials-sensitive product for dental hygiene, we use the simple example of dental floss. The job this product needs to accomplish is removal of interproximal plaque and debris. All interproximal regions and surfaces are not the same. Some interproximal contacts are tight, others are open, and

Observe

Build

Plan

FIGURE 3.1  The design cycle: Observe, Plan, Build, …. Repeat.

some regions might have proximal restorations with varying degrees of marginal adaptation. The development of a new dental floss product might start with the problem of a potential customer who has a two-surface posterior restoration with an overhang. Current floss products on the market shred or tear when flossing in such a region. This main observation is analyzed and deemed significant, because many people with this problem and similar problems could be helped by a design change to this dental floss. Multiple and varied ideas are generated to address the problem: (1) the dental floss cross section could be a ribbon rather than a rope to ease the floss over the overhang; (2) the floss could be a single strand rather than a braid of multiple strands to reduce the number of surfaces on the floss that could catch; or (3) the floss could be made of a different material or a slippery coating could be added to reduce friction. (Note that all of these designs have been presented to consumers at one time or another.) Based on these possible design changes, a plan is made that incorporates a method or combination of methods that appear to be most promising in regard to addressing the observed problem. All of the possibilities could have merit, but by selecting those that address the observed problem most directly, one can test the solutions most directly. In this example, we will say that the floss will be formed as a ribbon cross section and a change of material will be made to reduce friction. The new floss is built and tested in simulated and actual environments. One cycle of our design process for a new dental floss has been completed. We hope to find in our testing that we solved the observed problem. That would be an effective solution. What we may observe through testing our built product, however, is that the material is too slippery to remove plaque effectively, or the ribbon is too wide to stay flat when drawn through the interproximal contact and into the gingival sulcus. Based on these observations, a new plan is made, a new product version is created, and we find that we have completed another design cycle. We repeat this process creating more refined versions of the product that provide more exacting solutions to the observed problems. We also observe use of the product in as broad a range of consumer groups as possible to ensure the product addresses the needs of the target market. The design cycle for developing new products can be used in the planning of restorations as well. When selecting materials for a restoration, one observes the patient’s oral and medical condition and prioritizes the observed problems. The observation data are integrated with valid materials performance data to create a plan of treatment. A restoration is built and tested for occlusion, compatibility, esthetics, feel, and so forth. Adjustments are made in recurring observe,

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3.  DESIGN CRITERIA FOR RESTORATIVE DENTAL MATERIALS

plan, build steps, refining the restoration to satisfy both patient and clinician. Scientific evidence

EVIDENCE USED IN PRODUCT DESIGN The entire design cycle is based on evidence. Observation provides evidence about the history of performance of existing materials and solutions and identifies the job that new solutions must perform. The thoroughness of the observation phase depends on the skills and experience of the designer. In the plan phase, material properties and characteristics and test data for performance of materials in controlled conditions are added to the observation data. The build phase integrates knowledge of the job or problem with the skill and experience of the designer and considers variations in the operating conditions and properties and known performance of the materials. Without this systematic and integrative approach, the design process would be haphazard and wasteful. The evidence-based design cycle just described is analogous to evidence-based decision making in health care and evidence-based dentistry.

EVIDENCE-BASED DENTISTRY The American Dental Association (ADA) defines evidence-based dentistry as an approach to oral health care that requires the judicious integration of systematic assessments of clinically relevant scientific evidence relating to the patient’s oral and medical condition and history, along with the dentist’s clinical expertise and the patient’s treatment needs and preferences (http://ebd.ada.org). This approach is patient centered and tailored to the patient’s needs and preferences. Our goal is to practice at the intersection of the three circles (Figure 3-2).

Patient Evidence Patient needs, conditions, and preferences are considered throughout the diagnostic and treatment planning process. Observation of patient needs and medical/dental history occurs first. In this phase, performance of prior and existing restorations, in terms of success or failure, should be noted. This is often a good indicator of conditions in the oral environment and the prognosis of success of similar materials in this environment. The patient’s facial profile and orofacial musculature is a good indicator of potential occlusal forces. Wear patterns on occlusal surfaces are indicators of bruxing, clenching, occlusal forces, and mandibular movements. Cervical

Clinician experience and expertise

Patient needs, conditions and preferences

FIGURE 3.2  The elements of evidence-based dentistry. abfractions may indicate heavy occlusal contact accompanied by bruxing or occlusal interferences. Erosion on anterior teeth suggests elevated levels of dietary acids, and generalized wear without occlusal trauma could involve a systemic disorder such as gastroesophageal reflux disease (GERD). Any of these conditions would compromise the longevity of restorative therapy. Unusually harsh environments require careful restoration design and selection of materials, sometimes different from the norm. The options for material to be selected then need to be considered in accord with the problems and needs exhibited by the patient. These data are found in the scientific literature. The integration of patient data and materials data helps make a more fully considered plan for treatment.

Laboratory (In Vitro) Evidence When searching for scientific evidence, the best available evidence, usually compiled from a review of the scientific literature, provides scientific evidence to inform the clinician and patient. The highest level of validity is chosen to minimize bias. These studies are typically meta-analyses of randomized controlled trials (RCTs), systematic reviews, or individual RCTs. Lower levels of evidence are found in case studies, cohort studies, and case reports. Laboratory studies are listed as “other evidence” because a clinical correlation can be made only as an extrapolation of the laboratory data. The listing of bench or laboratory research as “other evidence” should not be construed as meaning that bench research is not valid. The hierarchy of evidence as presented for evidence-based data (EBD) is based on human clinical data, for which bench data can only be a surrogate. When searching for scientific evidence, the best available, or most valid, data should be chosen. New

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material developments that are enhancements to existing products are not required to undergo clinical testing by the Food and Drug Administration (FDA). Published laboratory or in  vitro studies are often the only forms of scientific evidence available for materials. This does not mean that no evidence is available. It is simply an indication that laboratory studies should be admitted into evidence for making the clinical decision (Table 3-1). Researchers in dental materials science have sought to correlate one or two physical or mechanical properties of materials with clinical performance. Although it is possible to use laboratory tests to rank the performance of different formulations of the same class of material, the perfect clinical predictor remains elusive. Often the comparison of laboratorybased materials studies is difficult because of an incomplete description of methods and materials. Researchers in dental materials are encouraged to provide a complete set of experimental conditions in their publications to enable the comparison of data among studies. This process will facilitate systematic reviews of laboratory studies that can be used as a source of scientific evidence when clinical studies are not available. Every patient is unique, including the patient’s oral environment and general physiology. This provides a unique set of circumstances and challenges

for implementing successful materials choices in a treatment plan. The elements of EBD and material properties should be considered as a system to provide the best patient-centered care. The observed evidence in an assessment of the patient, the analyzed evidence of the laboratory data, the experience of the clinician, and the needs and wants of the patient are all related and all impact the prognosis of the restoration. Although it might be tempting to categorize a patient’s needs by age, gender, or general clinical presentation, careful data gathering, planning, and analysis provides the best solution. This assessment is the basis for the complex process of oral rehabilitation (Figure 3-3).

CREATING THE PLAN The plan phase integrates elements of evidencebased decision making and a consideration of material properties and performance. The process of treatment planning is familiar to clinicians, but the practice of designing restorations with material properties in mind might not be done routinely. To begin, performance requirements are analyzed. The environment in which the restoration will serve is used as a modifier to the performance requirements. For example, when treatment planning a three-unit

TABLE 3.1  Assessing the Quality of Evidence Study Quality

Diagnosis

Treatment/Prevention/ Screening

Level 1: goodquality, patientoriented evidence

Validated clinical decision rule SR/meta-analysis of high-quality studies High-quality diagnostic cohort study*

SR/meta-analysis or RCTs with consistent findings High-quality individual RCT† All-or-none study‡

SR/meta-analysis of ­good-quality cohort studies Prospective cohort study with good follow-up

Level 2: limitedquality patientoriented evidence

Unvalidated clinical decision rule SR/meta-analysis of lower quality studies or studies with inconsistent findings Lower quality diagnostic cohort study or diagnostic case-control study

SR/meta-analysis of lower quality clinical trials or of studies with inconsistent findings Lower quality clinical trial Cohort study Case-control study

SR/meta-analysis of lower quality cohort studies or with inconsistent results Retrospective cohort study or prospective cohort study with poor follow-up Case-control study Case series

Level 3: other evidence

Consensus guidelines, extrapolations from bench research, usual practice, opinion, ­ isease-oriented evidence (intermediate or physiologic outcomes only), or case series for d ­studies of diagnosis, treatment, prevention, or screening

Prognosis

From Newman MG, Weyant R, Hujoel P: J. Evid. Based Dent. Pract. 7, 147-150, 2007. *High-quality diagnostic cohort study: cohort design, adequate size, adequate spectrum of patients, blinding, and a consistent, ­well-defined reference standard. †High-quality RCT: allocation concealed, blinding if possible, intention-to-treat analysis, adequate statistical power, adequate follow-up (greater than 80%). ‡In an all-or-none study, the treatment causes a dramatic change in outcomes, such as antibiotics for meningitis or surgery for appendicitis, which precludes study in a controlled trial. SR, Systematic review; RCT, randomized controlled trial.

3.  DESIGN CRITERIA FOR RESTORATIVE DENTAL MATERIALS

posterior fixed dental prosthesis, the usual considerations will include the length of the span, location, condition of the abutments, opposing occlusion, periodontal support of the abutments, parafunctional habits, oral hygiene, existing restorations in the abutment teeth, shape of the edentulous area, and esthetic concerns. Other elements for the integrative plan and design are the three-dimensional geometry of the edentulous area, potential occlusal force that can be generated, history of other restorations in the region, the cause of tooth loss, and potential materials and their properties. Tooth preparation occurs after the restoration is conceptually designed

Observe

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because the design of the restoration will determine the amount and shape of the tooth reduction. When creating a plan, the goal is to be succinct, which is difficult to do considering a goal of the observe phase is to be comprehensive in the gathering of data. Some pieces of information will be extraneous to the treatment problem and can conceal the best plan of treatment. There will be synergistic and contradictory materials solutions for the possible plans of action. There will be features and constraints presented by each possible materials choice. Prioritization of this information will guide the clinician toward the solutions that will be most effective.

Data collection 1. Patient information 2. Chief complaint 3. Medical and dental history 4. Physical evaluation 5. Clinical examination 6. Radiographic examination 7. Performance of existing restorations/materials 8. Risk assessment

Data integration and system analysis Plan

Patient modifiers 1. Patient conditions and activation level 2. Patient preferences 3. Materials performance

Clinician modifiers 1. Clinician experience 2. Materials handling

Scientific evidence 1. Systematic reviews 2. Clinical trials 3. Laboratory studies

Build

Summary of significant and relevant data and conclusions

Diagnoses

Problem list 1. Medical/systemic 2. Oral

Integrative treatment objectives

Treatment design 1. Procedure options 2. Restoration and prep design options 3. Materials options 4. Forecasted outcomes

Definitive treatment

Treatment plan 1. Procedures 2. Restoration & prep design 3. Selected materials 4. Rationale 5. Prognosis

Provisional treatment

Performance review, maintenance, and prevention

FIGURE 3.3  A process diagram that illustrates the integration of clinical decision making, evidence, and the design cycle.

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Some diagnoses of a clinical situation may have a strong basis in the care that was previously provided. A patient may have an upper posterior tooth that is exhibiting an excessive amount of wear. A lower posterior crown with a porcelain occlusal surface could be the culprit. Porcelain was chosen for esthetic reasons. However, the hard porcelain occlusal surface is abrading the upper tooth, resulting in severe occlusal wear of the enamel surface. This prognosis could be addressed by restoring the upper tooth, and/or by changing the restoration on the lower tooth to a softer material or less abrasive material. Because the preservation of natural tooth material is a high priority, replacement of the lower crown with a material that is more harmonious with the occlusal scheme would provide the best service for the patient. The importance of a systems approach to observation and planning is illustrated here, where all factors are to be considered together, followed by prioritization. Prioritization is also an opportunity to provide education and to enhance the patient’s level of activation. It is not uncommon for patients to be unaware of serious oral problems. For example, interproximal caries or oral lesions might not be evident to the patient, whereas a single discolored anterior tooth can be very noticeable. Although the patient’s desire for care could be the discolored tooth, prioritizing the care and first acting to treat the immediate danger of infection and disease progression helps the patent’s overall oral health. The priorities and expectations can be altered so that the most serious issues are addressed first. The patient may arrive with a materials-specific treatment in mind, such as a ceramic veneer on the discolored tooth mentioned earlier. If the tooth is in malocclusion or potentially abutting an opposing natural enamel surface, a ceramic material could be an unwise choice because of the potential for chipping or wear of the opposing tooth. In this case, composite is a better choice for longevity of the natural dentition. Discussion of the materials available and the potential plans of action can shift the patient’s understanding of the problem to include the unique nature of the case, the desire for a long-lasting, naturally colored restoration, and the materials that are best suited for the case to meet the patient’s initial goal. The planning then shifts away from the predetermined material specification of a ceramic veneer. Including the patient in prioritizing his or her care provides an immediate and personal feedback loop that can be incorporated into the planning phase. The materials used are both features and constraints of a restoration. Composites, ceramics, and metals offer features and constraints that allow their applications to vary slightly in different treatments. The observe, plan, build…repeat cycle is a process that aids the identification and analysis of features and

constraints. Each step of the cycle offers opportunities to prioritize the features and constraints of materials for the case and select those that best fit the occlusal scheme. In every design cycle, the solution and problem become more convergent and the quality of the final product or service increases.

BUILDING THE RESTORATION Building is the next phase. The building required for the restoration may be directly applied to the tooth or may require several iterative steps to create the final product, including a laboratory procedure. In many intracoronal restorations and some veneers, the restoration is applied directly to the tooth and typically completed in one visit. These are referred to as direct procedures. For these procedures, final material decisions are made prior to any “building” procedures. The plan of treatment must include all materials selections. Fine adjustments are made by adding and/or removing material based on assessment of the occlusion, questioning the patient about tactile feel, and evaluating the esthetics and harmony of the restoration with the rest of the dentition and oral environment. Some restorative materials are more sensitive to technique variations than others. For example, placement of resin composite restorations in posterior teeth requires more steps than for an amalgam. Each of these steps require a specified level of precision, that when totaled, equate to a more complex process. An error in any step could affect the success of the restoration. Clinical expertise therefore is an important factor when developing a treatment plan and selecting restorative materials, particularly when the restoration is a direct application to the tooth. The handling properties of a material are an important consideration that is often difficult to measure and describe. In indirect restorations that require several appointments and a laboratory procedure, prototyping occurs before building. Prototyping can also be done with direct restorations, for example, by simulating the shape and color of a composite veneer without curing the material. For indirect restorations, prototyping is done routinely. Creating models of the final restoration is helpful because that allows the clinician and patient to discuss and agree on treatment outcomes. This early discussion reduces surprises when the final restoration is delivered to the patient. The use of models is also an excellent aid for designing tooth preparations that optimally transfer occlusal force through the restoration to the tooth and supporting tissues. The concept of prototyping is also useful in the fabrication of provisional or transitional restorations

3.  DESIGN CRITERIA FOR RESTORATIVE DENTAL MATERIALS

for indirect restorations. When the provisional restoration accurately simulates the final design in form and appearance, the patient and clinician can discuss design outcomes, expectations, and required modifications. Esthetics is particularly important to simulate as accurately as possible because of its subjectivity. Color, shape, size, and position are all important factors to evaluate to ensure the patient’s satisfaction with the restoration. Provisional restorations also act as important diagnostic aids for studying occlusion, occlusal forces, parafunctional habits, oral hygiene, and soft tissue response. Analysis of the performance of carefully fabricated transitional restorations can provide many clues for optimal design and fabrication of the permanent restoration. The transitional restoration is the place for testing and making iterative modifications to design concepts before the permanent restoration is fabricated. Observations of occlusal wear facets, cracks, dislodgements, discoloration, and discomfort from the provisional restoration are all indicators of conditions that might be beyond the usual design limits. Studying the cause of these events can help specify material properties for the dental laboratory. Material selection is best made during planning and design rather than after tooth preparation. Material options for a particular restorative scenario will differ in their mechanical and physical properties. For example, casting alloys for a fixed dental prosthesis differ in their stiffness, hardness, malleability, and corrosion resistance. Higher stiffness alloys will transfer more occlusal stress to the abutments, whereas lower stiffness alloys will deform and cause the prosthesis to deflect. Corrosion resistance is important when patients have diets high in acids and consume foods and fluids with high staining potential. In another example, ceramics might satisfy the esthetic requirements of the restoration, but might not be suitable for the high occlusal loads of patients who brux and clench. The concepts of the design process—observe, plan, build…repeat—integrate well with evidence-based decision making and materials selection. When all of

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these elements are considered together, an integrative treatment plan and design can be achieved that provides the optimal outcome for the patient. The iterative cycle of design also provides many opportunities for discussion between the clinician and patient to facilitate agreement on expectations well ahead of the delivery of the final restoration. Chapter 4 presents concepts of material science, including the physical and mechanical properties of materials. A good understanding of the fundamental properties of materials enables the clinician to design treatment and prepare oral tissues to best distribute forces in the oral environment.

Bibliography American Dental Association: ADA Center for Evidence-Based Dentistry. http://ebd.ada.org/. Accessed August 28, 2011. Ashby MF: Materials Selection and Process in Mechanical Design. 1999, Butterworth Heinemann, Oxford. Ashby MF, Johnson K: Materials and Design, the Art and Science of Materials Selection in Product Design, 2002, ­Butterworth Heinemann, Oxford. Bader JD: Stumbling into the age of evidence, Dent Clin North Am 53(1):15, 2009. Brown T: Design thinking, Harv Bus Rev 86(6):84, 2008. Forrest JL: Introduction to the basics of evidence-based dentistry: concepts and skills, J Evid Based Dent Pract 9(3):108, 2009. Forrest JL, Miller SA: Translating evidence-based decision making into practice: EBDM concepts and finding the evidence, J Evid Based Dent Pract 9(2):59, 2009. Martin R: How successful leaders think, Harv Bus Rev 85(6):60, 2007. Miller SA, Forrest JL: Translating evidence-based decision making into practice: appraising and applying the evidence, J Evid Based Dent Pract 9(4):164, 2009. Newman MG, Weyant R, Hujoel P: JEBDP improves grading system and adopts strength of recommendation taxonomy grading (SORT) for guidelines and systematic reviews, J Evid Based Dent Pract 7:147–150, 2007. Sakaguchi RL: Evidence-Based Dentistry: Achieving a Balance, J Am Dent Assoc 141(5):496–497, 2010. Vossoughi S: Designing the ‘care’ into health care, Business Week Nov 21, 2007.