Practical and Theoretical Considerations for Dental Restorative Materials

Abstract

Dental materials are essential for most of dental treatment modalities. Understanding the science and chemistry behind the materials and their properties can enable the operator to employ the dental material to its maximum advantage. Contemporary dental materials have evolved significantly from the conventional variety, but there is always room for refinement since the inadequacies of the current dental materials in function are recognized only with the advent of advanced dental materials testing methods. As a result, continuous improvement and modification of dental materials is essential. Caries is a process of continuous demineralization and re-mineralization. Recurrent caries is a common occurrence around the tooth-restoration margin. It most likely indicates that the current dental materials are inadequate in their applications. As a result, augmenting conventional dental materials with additional advantageous properties is critical. This chapter aims to reflect on the empirical status of direct restorative materials frequently used in the field of restorative dentistry.

1. Introduction

Direct restorative materials are classified by the ADA Council on Scientific Affairs into four categories: amalgam, resin-based composites, glass ionomer, and resin-modified glass ionomer (Figure 1) [1].

Figure 1.

Direct restorative materials a. Class II amalgam restoration b. Class II composite restoration c. Class V Glass Ionomer restoration.

Amalgam is particularly well suited for Class I and II restorations in teeth subjected to high chewing forces [2, 3, 4]. Amalgam restorations have several advantages over other direct-placement materials, which include wear resistance, tolerance to a wide range of clinical placement conditions and excellent load-bearing properties contributing to its high survival rate [2, 3, 4]. However, some primary issues limiting the longevity of amalgam restorations are secondary caries, increased incidence of bulk and tooth fracture, cervical overhang, and marginal degradation [2]. Until the late 1960s, when resin-based composites were introduced, amalgam was the material of choice for all but the most esthetically demanding restorations. Dental composites are the most esthetic direct filling material available, as they mimic the color and translucency of natural teeth. Originally, these esthetic materials were only intended for anterior restorations. As their popularity grew and the materials improved, they were used in nearly all classes and types of dental restorations. Glass ionomers are tooth-colored filling materials that can be used to fill cavities with low load requirements. They are frequently used to repair non-carious erosion or abrasion defects in the tooth near the gingiva [5]. They are also used for pediatric restorations, which have low service longevity requirements [6]. Glass ionomers are also commonly used as cavity liners or bases, protecting the underlying tooth pulp in deep fillings. Resin-modified glass ionomers as compared to conventional glass ionomers have superior physical and mechanical properties and better handling characteristics [7]. Unlike traditional glass ionomers, which have short working and long setting times, the dentist has more control over the working and setting times of resin-modified glass ionomers. This removes some of the material’s technique sensitivity, making it easier to achieve a successful restoration. They are used for Class I, Class II, and Class III restorations, primarily in the primary dentition, as well as Class V restorations, liners, and bases. Other applications include fissure sealants and bonding agents for orthodontic brackets.

2. Properties of dental materials

The selection of a dental material is based on the physical properties required and the unique functional demands placed upon it in a specific clinical application. For instance, in evaluating a fiber post, a clinician would not be that interested in characteristics such as abrasion resistance, solubility, or even compressive strength, but would be highly interested in these characteristics if evaluating a composite filling material. Similarly, when a clinician is faced with the selection of dental materials for use in the permanent restoration of severely broken down teeth, esthetics should be secondary to the mechanical and physical properties necessary for that particular application [8].

Since many complex forces occur and tend to deform the material (tensile, compressive, shear, bending forces), the knowledge and interpretation of how these materials behave under such forces are relevant to understand the performance of the material. When a specific force or stress is applied to a body, it causes a reaction of equal intensity but in the opposite direction, which can be quantified. Stress can be calculated using the force-unit-area relationship because the shape and dimensions of the specimens under test can be measured. The stress can alter the original dimensions structurally. Strain is defined as the rate at which the original dimension gets altered due to deformation. The stress–strain ratio of a material is important in determining its mechanical behavior. Each material has a stress–strain proportional relationship, resulting in a stress–strain curve. If there is stress relief during loading and no permanent deformation occurs, it demonstrates elasticity. This proportion continues until a limit point, defined as a proportional limit, is reached, and deformation is defined as elastic deformation (Figure 2). This is the greatest amount of stress a material can withstand without permanently deforming. Because stress–strain is proportional until this point, there is a constant proportionality. It is the ratio of the stress–strain curve within the elastic limit that determines a material’s elasticity. The modulus of elasticity, also known as Young’s modulus, is a measure of this proportionality. Young’s modulus represents the stiffness of the material [9]. When the applied load exceeds this point, however, irreversible deformation takes place, resulting in permanent or plastic deformation. Each material has a resistance to deformation, and after that point, it will rupture. The ultimate strength value is obtained at this point.

Figure 2.

Stress–Strain curve. Proportional limit (A), elastic deformation (point A), and plastic deformation (between points A and B). Point B represents the moment of rupture of the material under tensile condition.

Other clinically relevant properties of direct restorative materials include tensile strength, diametral compressive strength, compressive strength, Poisson’s ratio, flexural strength, resistance to fatigue, and hardness strength.

Tensile strength refers to the resistance of the material to a load when a body is subjected to axial forces in a straight line and opposite directions (Figure 3) [8, 10].

Figure 3.

The direction of forces in tensile, compressive, and shear stress.

It is an important feature of metallic materials because they can deform under tensile forces until fracture occurs, indicating the workability of an alloy. Brittle rupture under low tension is a characteristic of fragile materials [11]. Tensile strength is not recommended in such cases to evaluate the material’s reaction due to its low cohesive condition. An alternative method of tensile strength is calculated by compressive testing. It is also known as the diametral compression test for tension or the indirect tension test [7, 8, 9]. Materials must be investigated under this condition because the majority of mastication forces are compressive in nature [10]. As a result, this test is used to contrast dental amalgam, impression materials, investments, and cement [12]. When a material is subjected to axial loading, it also experiences lateral strain [10, 11]. Poisson’s ratio indicates that during the elastic range, cross-sectional change is proportional to deformation. Brittle materials show little permanent reduction in cross-section during tensile test situations than more ductile materials (Figure 4) [7, 8].

Figure 4.

Diagrammatic representation of Poisson’s ratio.

A material’s flexural strength is its ability to bend before breaking. It is obtained when a material’s ultimate flexibility is reached before its proportional limit [13, 14]. Clinical situations generate flexural forces, and dental materials must withstand repeated flexing, bending, and twisting. Because dental materials are subjected to chewing stresses that can result in permanent deformation, high flexural strength is desirable. The behavior of these materials under the action of relatively low but intermittent stresses demonstrates their fatigue resistance [7]. Cracks form when defects in the microstructure of a restoration or specimen are subjected to high or low stresses, and these cracks can lead to material fracture. Hardness is not a material property that can be precisely defined in terms of fundamental mass, length, and time units. A specific measurement procedure is used to determine the value of a hardness property. The depth or area of an indentation left by a specific shape indent with a specific force applied for a specific time is the most common way to determine hardness.

The four most common standard test methods for expressing a material’s hardness are Brinell, Rockwell, Vickers, and Knoop. Each of these methods is divided into scales based on the applied load and indenter geometry. Hardness tests are widely used in dentistry and have important applications. A hardness test, for example, can determine how mineralized a dental substrate is [15, 16]. This test can also be used to determine the polymerization level of resin composites and resin cement.

Another property relevant to composite resin is the thermal expansion coefficient, which may influence composite adaptation to cavity walls [17]. When there is a mismatch between the thermal expansion coefficients of composites and enamel and dentin, stresses can be generated at the interfacial bond during exposure to hot and cold food and drinks [18]. Color stability and viscosity are also important properties for composites because they affect the esthetics, handling, and placement of the material, respectively [19].

The complexity of the oral environment, as well as the geometric diversity of cavities filled with restorations, make it difficult to precisely define clinical failure processes and associate routinely measured mechanical properties with dental material performance. It is not an easy task to identify the relevant laboratory tests to predict the clinical performance of restorative material. The clinical difficulties are multifactorial, and there are likely to be significant interactions between the factors. As a result, one must have a thorough understanding of the clinical factors and the magnitude of their impact on long-term performance [13]. Even though mechanical tests have not yet reached the level of clinical simulation, they are an important parameter in analyses, so familiarity with the major laboratory tests for evaluating dental materials is essential [12].

3. Dental materials science: research, testing, and standards

Because they serve different purposes, the materials used in the mouth interact with different tissues and are exposed to different environments, resulting in a wide range of chemistries. Such materials must be manufactured, processed, and tested to be clinically safe. A ‘test’ is used in the context of a statistical examination of data to determine the probability of an outcome against a formal ‘null hypothesis,’ and thus potentially to falsify it. The term “test” can be used to characterize a dental material or product in direct comparison with others, but it can also be used to compare with some defined or threshold value of the tested property, and it is in this context that international standards are frequently and correctly used. The term “research” can refer to a very basic approach, such as developing new chemistry or discovering new mechanisms of action. A clear demarcation between the two domains is not always possible [20].

When designing the testing, careful consideration must be given. Test planning for all materials must be meticulous and specific to the material type and use. Blanket or routine testing with no understanding of why the testing is being performed is not cost-effective and does not provide useful information to readers or users of these materials. There are certain interactions of the material being tested with the substrate which can be difficult to simulate in vitro [21]. The context of the material used, location, and exposure are important during laboratory testing and experiment design, and this must be recognized and implemented through simulation for all tests, whether physical, chemical, mechanical, or biological.

The test suite must be comprehensive and integrated to cover all relevant aspects—single-factor work that ignores system complexity and interdependence is rarely helpful and may result in the “salami slicing” of the research [22]. Material characterization and reference to literature about the material’s composition are essential in every experimental plan, as is covering theoretical expectations for the subsequent testing. Hence surface testing should also be included in addition to bulk (object) testing because changes and effects may occur only at the surface.

The longevity of the material should also be taken into account since the age at which failure occurs is also important, hence data from surveys of the ages of functional restorations should also be scrutinized. Ideally, longitudinal studies should be performed to study the longevity of a specified population of restorations. The longevity of restorations has been recorded in different ways previously e.g., by noting the percentage of restorations remaining after a specified number of years or by recording the mean or median age. All methods may be useful, but a common method needs to be selected to allow for comparison between and among data from different surveys. The median longevity has been concluded to be the most common way of recording longevity, and it is recommended that this value should always be recorded to allow for comparative estimates [23].

For biological testing, the choice of bacterial strains and cell lineages should be appropriate to the location of material placement, and, whenever possible, both microbiological and biological testing is required to ensure that, while the material may be antimicrobial, it is not toxic to the host [24]. Hence one has to keep in mind to modify or update the currently established protocols for testing newer materials.

4. Failure of restorations

A complex of bacterial cells adhering to one another in an enclosed polymeric extracellular matrix is known as a biofilm [25]. Replacement of the existing restoration and further tooth structure loss are two effects of oral infections brought on by biofilms involving previous or old restorations. Dental caries is caused by pathogenic biofilms that attach to the tooth surface or restorative materials using specific binding proteins [26]. Plaque buildup at the tooth/restoration interface enables microbial invasion and biofilm formation that results in another carious lesion. The recurrence of carious lesions around restorations is influenced by a variety of factors, including dysbiotic biofilm growth, difficult access for cleaning between teeth, and surface characteristics of the dental restorative material [27]. The production of acids by plaque biofilms near the restoration margins is a risk factor for restoration failure [28]. Failure-related restoration replacement accounts for 50–70% of all routine restorative procedures. Many approaches have been investigated to reduce biofilm formation over polymeric restorative materials and at tooth/material interfaces [27]. For example dental composite restorations are the first line of minimally invasive options for the treatment of dental caries in tooth structure However, polymeric materials are highly susceptible to bacterial attachment and colonization, leading to dental diseases. In previous reports, the prevalence of secondary caries associated with polymeric restorative materials has reached 60%, and it has been recognized as the most common reason for resin composite restoration failure and replacement [27].

One strategy to provide a durable, long-lasting restoration is the incorporation of antimicrobial agents to control and/or eliminate these secondary infections. Similarly, any strategy that could disrupt the formation of biofilms would be considered clinically valuable as a route to control infections related to biofilm accumulation [28, 29]. Different classes of agents in the development of antibacterial materials are- Contact based anti-bacterial materials, release-based anti-bacterial materials, dual contact, and release-based anti-bacterial materials, on-demand anti-bacterial materials, materials with bacterial resistant surfaces, and materials with bacterial release surfaces (Figure 5) [27].

Figure 5.

Antibacterial killing strategies can be achieved via different approaches. The contact-killing mechanism can be provided by quaternary ammonium compounds posing highly positive charged surfaces to disrupt accumulated microorganisms (A). Antimicrobial peptides (AMPs) and antimicrobial enzymes (AMEs) can conduct contact-killing by invading the cellular membrane and targeting the main cellular components (B). Antimicrobial peptides can also conduct antibacterial action via their positively-charged surface. The antibacterial action via ion release can be provided by release-based (C) and on-demand (D) antibacterial materials. Materials interfering with bacterial adhesion can be designed using bacterial-resistance and bacterial-release surfaces (E, F) [30].

5. Incorporation of practice-based groups

Given that the majority of dental treatments in the world are performed in dental offices, there is therefore an imbalance between the treatment output and research output. It can be assumed that dental practice can have an increasing impact on clinical dental research. Since dental practice is “real world”, a technique or material must be accepted under practical conditions for it to be successful and hence they should be evaluated in this context [31].

Research methods such as meta-analyses, systematic reviews, or randomized controlled clinical trials are less likely to be used by general dentists, as they require knowledge of statistics. and research methods which are often not within the reach of the physician. The use of practice-based networks to evaluate the effectiveness of materials and techniques in dental practice can be very productive. Several practice-based research groups are presently in operation in the UK and the USA, generally carrying out evaluations of the handling of materials, with increasing emphasis on the clinical evaluation of restorations. PREP (Product Research and Evaluation by Practitioners), BRIDGE (Birmingham Research in Dental General Practice), and GRID (Glasgow Research Initiative in Dental Practice, A West of Scotland-based practitioner research group) are a few of the practice-based groups. Perhaps the best-known group of practice-oriented researchers is the Clinical Research Association (CRA), founded by Gordon Christensen 30 years ago. To undertake research work in dental practice, trainees must be trained in the standardization of procedures, calibration in prosthetic evaluation, and the scientific method. All these groups include general practitioners and they hold annual meetings to propose and discuss ideas for new projects. They perform fact-based assessments of a variety of dental materials, as well as laboratory assessments [32].

Hence randomized clinical trials and retrospective and prospective clinical evaluations which can be readily carried out in dental practice, where the patient base is likely to be substantially greater than in dental schools or hospitals can prove to be very helpful. Moreover, the patient base in dental practice represents patients of various patterns of attendance, from different walks of life, and different levels of oral hygiene and caries experience. Whereas patients in dental hospitals may not be considered to represent a typical patient population, since they generally elect to attend such institutions because they have the time available for treatment by students (i.e., the retired or unemployed) and may often have attended many courses of treatment, in which their oral hygiene will be reinforced (i.e., their oral hygiene may be better than that in the general population).

Cross-sectional studies have also been used in general dental practice to produce useful and meaningful results, with Ivar Mjör being the main proponent of this research approach [33]. These cross-sectional studies have the advantage over randomized controlled clinical trials in terms of generating data that may include large numbers of recoveries in many different patients, performed by dentists with many different qualifications and experience. A recent example of this type of study examined the impact of different UK rehabilitative care funding methods on the age of recovery at replacement, with rehabilitations placed in the National Health Service being replaced at a lower age than the restoratives placed in other funding methods [34].

It is essential to follow an established protocol to standardize and eliminate any deviations. Board members must use the materials as directed. This way, the manufacturer can get feedback on the company’s material handling from this particular group of practitioners. Members can place the restoration under typical routine conditions, perform a baseline assessment, and arrange to recall the patient for review by a trained and calibrated independent assessor who will review the restoration using the modified United States Public Health Service criteria. Patients and practitioners can be reimbursed for their costs, and the time the practitioner’s office is inactive during a patient examination can be considered the primary expense incurred by the practitioner [32].

6. Conclusions

The way dentistry is practiced worldwide is changing as a result of the daily advances made in science. Without scientific research, clinical trials to evaluate the benefits and drawbacks of particular materials and methods would not be feasible, and we might end up using therapies that are ineffective and potentially harmful. Since dental well-being is crucial for general health, maintaining national and international standards for dental health depends on the development of new treatments and medications. The creation of a new generation of enhanced biomaterials with the potential to change how dental caries are currently managed has a bright future.

Conflict of interest

The author declares no conflict of interest.