The What, Where, and Why
Understanding the diagnostic value of today’s material options
Peter Pizzi, MDT, CDT, and Jason Mazda
The evolution of restorative dental materials has led to a multitude of options, ranging from esthetic choices to products known for their hardness/breaking points, bondability, and/or processing requirements—either manually or via CAD.
Every material, whether new or old, has both benefits and drawbacks that can provoke difficulties in achieving the level of functional and esthetic results expected by today's educated patient and clinical teams. However, the selection of these materials sometimes seems driven more by promotion than by our understanding of optical properties, functional limitations, and preparations or edentulous spacing. The materials that are marketed the most successfully are requested by clinicians, and laboratories often simply fulfill those requests. Our opinions matter as we are the artisans of each case. Our clinical partners can respect our restorative choices if we have a true understanding of the restorative options, such as functional determinations and optical properties, and how to provide the proper information, such as full-arch impressions or scans for the functional needs and preparation color, facial and intraoral photographs, and the provisional template of the final incisal edge position and horizontal reference.
The other trap we can easily fall into involves the constant quest to be minimally invasive. Our job is to protect and save teeth, and to be as minimally invasive as possible. We want to prepare less tooth structure and use less restorative material. The flaw occurs when that dictates the material options. For example, if we are restoring a full maxillary arch for either esthetic or functional reasons, and we decide we want to be minimally invasive by preparing veneers in the anterior and vonlays in the posterior—not preparing the teeth 360 degrees by taking away a small amount on the facial surface and a small amount on the occlusal surface—then the only tooth structure we have saved is on the lingual wall, and yet we have driven the material option because the only material that will work well on a vonlay posterior preparation is lithium disilicate, due to its bondability and functional attributes. Although feldspathic porcelain could be used on the occlusal surfaces, depending on the thickness, that could create functional challenges unless there is enough enamel to which to bond. We cannot put zirconia there unless we are very comfortable with bonding it. The decision to be minimally invasive dictated the material decision, which instead should be made based on the evaluation of the patient. There may be functional concerns that would make metal-ceramics, zirconia, or layered lithium disilicate better options. Conversely, function may be such a nonfactor that additive bonding with feldspathic ceramics might be preferable. The patient's priorities might dictate different strategies as well. When the minimal invasiveness instead drives the material selection, rather than the case diagnostics driving the preparation, we are reversing the process.
Our knowledge base is the most critical factor in the success of our case outcome. As healthcare professionals, we have a responsibility to provide patients with options that fit their environment without predetermined choices. Each choice is based on the individual need of the patient's environment, supporting attributes, and functional and esthetic concerns.
Material selection starts with learning the etiology of the human on whom we are working. Is it a functional issue, an esthetic issue, or an accidental issue? Did the patient suffer trauma to the face and break their teeth, or was there a constant wear cycle with an esthetic/functional component risk factor? In the latter scenario, our job is to understand the etiology of what happened and why. Does the problem involve tooth position? Or is it a brain issue? Is there an underlying cause? Our challenge is to figure out how to fix those problems with a material that works best in that particular environment. The preparations should be planned to fit the ideal material.
People have certain products on their mind because they hear about them constantly, and then they feel the need to be bioesthetic and minimally invasive, so they think, "This material will be perfect." They are getting funneled into a choice, when really they need to think through the options objectively. In the end, for the previously mentioned scenario, we very likely may choose a minimally invasive lithium disilicate vonlay. That is fine; lithium disilicate is a good material and a good restoration. However, we should be using it for the right reasons—not because we were funneled into it. There is not a material that does not have limitations. They all have advantages and disadvantages, and our job is to know those factors and think through them in order to come to the best conclusion.
Where to Start
Everything should start from a diagnostic perspective: the patient's face and the incisal edge position of the final restorative process. Is the incisal edge position correct? If not, how would we like to change it, shorten it, lengthen it, move it forward, or move it back? Everything starts from the esthetic parameters determined by where we would like to see the teeth in the patient's face. Most of the time, when we are restoring, the incisal edge position will need to be adjusted. Clinically adding composite and impressioning is one way to show the new position. Suggesting the new incisal edge for the technician to recreate must be met with precise information such as the exact position to start from; for example, "Please add 1.5 mm to the mesial-incisal edge of tooth No. 9 and follow for horizontal reference."
Once that determination has been made, we need to start thinking about the etiology—why are the teeth not in the space where we would like them to be? Was it a functional factor? Was it poor orthodontics? Or was it an accidental factor? Of course, the fourth possibility is that the patient simply decided they would like their teeth to look different. Regardless, however, we need to make those determinations before starting the case. These determinations will help us understand the functional parameters and whether we are placing our restorative materials in areas of risk.
Once we understand the diagnostic evaluation, each direction has its own caveats. For a functional concern, our job is to figure out the functional paralysis. Where did the problem originate? That could snowball into a lot of corners, but to put it simply, the issue will be either parafunctional—meaning the patient is doing something that is causing the problem—or truly functional, meaning the tooth position or the engram (the brain thought process) is causing it. Engram issues are typically fixed with mental health treatment and/or medication. The vast majority of functional errors, however, involve misaligned teeth, inclined ridges, or other issues that need to be addressed from an occlusal standpoint.
If a case is determined to involve a functional concern, those can be addressed in many ways. It could be an orthodontic treatment—and this is where understanding the value of the interdisciplinary team is so important—or it could be a restorative option. Restoratively, there are limits to. how much movement can be generated without true orthodontic movement. We can, though, create restorative orthodontics for limited tooth movements.
Making the Selection
Once the diagnostic evaluation is complete, the next step is choosing the best material for the esthetic parameters of the case. What will allow us to match the esthetic and functional needs of the patient? There is a strength component, an esthetic component, and a natural component.
Strength is a complicated consideration. We must not forget that it is a combination of rigidity and flexibility, because without both components, a tooth is unlikely to last long in the oral environment. Another way to think about it is by considering that some of the tallest buildings in the world must manage the elements of their height and environment, and as we journey to the top of these structures, we feel the movement that takes place; yet these structures continue to stand. The combination of rigidity and flexability allows this.
Megapascals (MPa) have become a standard for measuring strength in restorative dental materials, but they were initially used to determine the breaking point of concrete. The goal was to determine how much pressure breaks the material and how thick the material needs to be to allow taller buildings. That hardness standard is not a functional standard, however. If we measure natural teeth the exact same way, their strength is approximately 120 MPa. The difference between that and a material like zirconia is that, as pressure is applied to zirconia, there is almost no movement up until the point when it snaps in half; a natural tooth, meanwhile, will eventually elongate so much that it will slip out of the pressure. There is a flexural component that is critical.
Despite the limitations of our measurements, strength still must be a consideration. Feldspathic molars would not be a good option for a patient with functional concerns; it would be silly not to use a material with more strength, especially if we are being minimally invasive with only 0.2 mm to 0.3 mm of material. We must always consider the functional environment the material is going into, and the chances of that material being durable in the long-term.
Strength requirements are not always completely straightforward, either. Interocclusal space, preparation height, bony/tissue architecture, and more can impact the strength of a material. For example, using zirconia for a long-span bridge would seem obvious. However, the connector between the tooth and the pontic needs to be 4 mm2 in order to achieve the full strength of the zirconia. If either the connector size or the thickness of the material is compromised, the restoration itself will be compromised. For a four-unit bridge, if the peak of the papilla has a sharp point and the adjacent tooth is very short, the connector size cannot be 4 mm, and thus zirconia is not an ideal material.
The next considerations are esthetics and nature. Is a monolithic, layered, or micro/fully layered material best? Do we want to raise the bar on esthetics, in which case layering or micro-layering might be best? Or are we restoring posterior teeth with a limited smile opening, in which case making the second molar beautiful should be less of a priority? Additive materials are still the best option for diastemas, spaces, or retroclined teeth that need to be brought forward; when we are working in that type of environment, we might not need any preparations at all if we utilize feldspathic ceramic or lithium disilicate.
Once we have decided what the ideal material is, only then should we determine the ideal preparation for it. The important part of "what," "where," and "why" is thinking. Our job as clinicians and technicians is to fully evaluate the process and not be funneled into a decision because of what is happening around us. Our job is to think differently. In the end, scrutinizing everything and choosing a minimally invasive lithium disilicate vonlay is perfectly viable, if that is what is best for the case.
In a minimally invasive world, any time we can preserve as much tooth structure as possible and use the most esthetic materials, that is ideal. In many cases, that is feldspathic porcelain. It could be one minimally invasive preparation, or even no preparations when it comes to newer concepts such as window veneers, window chips, contact lenses, and other additive restorations to enhance the patient's smile without disturbing their natural teeth. If every patient could be treated that way, with perhaps only some orthodontics to create ideal tooth positioning, that would be a wonderful way to work. Unforunately, that is not always the case. Realistically, the majority of cases we handle are for patients who have already undergone previous restorative treatment 5, 10, 20, or more years ago; they might be full-coverage restorations that were done very well but have run the course of their normal lifespan, or they might not have been done as well as they could have been, and they might have even caused some of the etiology of the challenges now being presented. Regardless, the concept of staying minimally invasive is no longer an option if pre-existing conditions apply.
In these cases, interdisciplinary teamwork is very important, and choosing when to involve periodontal and orthodontic treatment is critical. Perhaps a patient needs to have gingival architecture restored for implants, which changes the esthetic parameters of where the bone needs to be, where the implants need to be, and how we restore the process. Periodontally, tissue position and bony architecture must be considered; are they in the right place esthetically and do they interfere with anything functionally, such as the bone being impacted or the tissue not being in the best esthetic position? Along with orthodontics, those are all diagnostic processes that must be evaluated from the beginning. We must reach a point where we have diagnosed our way through the whole interdisciplinary team and explored all options; we have chosen tooth position and esthetic parameters; we have worked through the functional parameters based on where the patient was and where we are trying to bring them; and then we have dictated, based on the materials we want to use, what kind of preparations or support they need. Then the next step is test driving it all via the provisional process. That is an important step because the time when the patient is provisionalized either confirms that everything we did was proper or reveals that we missed something. With everything we do, we need to maintain the ability to change course if necessary. We should not be overly resolute in our choices. For example, when choosing an incisal edge for a patient's face, the laboratory might think it needs to be 1 mm longer than previously, and the dentist might suggest 2 mm. A diagnostic waxup at 2 mm is created, and after the patient wears it for some time, we re-evaluate it. Perhaps the 2 mm is great, or maybe the patient has a phonetic issue and every time they smile it looks as if they have buck teeth. Even during diagnostics, there are always evaluations.
Once we have provisionalized and we feel comfortable that the process is functionally proper, it is just a matter of improving the esthetics through natural contour, embrasure spaces, etc. Then it is about the technician's skill. It is about being artistic and creative in either a digital or analog world—or hopefully both.
Our value and our viability in the profession lie in our ability to think. That is the overarching theme of material selection. If we want to be sought after for our skills in creating functional and esthetic restorations, then the ability to think and make astute judgements is what will separate us from the pack.