House of Cards

The American Society of Biomechanics defines the term as "the broad interplay between mechanics and biological systems."¹ While this definition certainly applies to dentistry and restorative dentistry as a whole, the interpretation within our industry provides interesting perspectives on how different dental professionals approach the issue. Laboratory technicians often have a slightly more nuanced understanding. M. Reed Cone, DMD, MS, CDT, FACP, owner of Nuance Dental Specialists in Portland, Maine, thinks of biomechanics "as it relates to the human stomatognathic system: It is the scientific study of the movement and interactions that the muscles of mastication have with the dentition and supporting periodontium as they relate to optimum oral health and function." Arian Deutsch, CDT, owner of Deutsch Dental Arts in Surprise, Arizona, incorporates the importance of physics and its effects on either the patient and/or the restorations they fabricate. "Incorporating basic engineering principles is a must," he says. "How we determine the anterior-posterior spread of implants; what is our safe cantilever distance distal to a terminal implant; or how much can be loaded on angled implants—all these require us to consider engineering. It is essential that we account for biomechanics as they relate to our patients; otherwise, the biologic system or the materials we use could fail."

John C. Kois, DMD, MSD, whom many consider to be the authority on biomechanics in dentistry, has devoted his career to educating dental professionals on the effects of biomechanics in restorative dentistry at the Kois Center in Seattle, Washington. He defines biomechanics as the study of the structure, function, and motion of the mechanical aspect of biological systems. "For us, it is about what may happen to natural teeth and/or restored teeth in the mouth during normal physiological or parafunctional activity," he says. "The objectives are to see why we witness clinical failures and to learn what protocols we can use to optimize outcomes and improve the survival probability of what we do. We try to find predictors and identify how they work in the mouth." For laboratory technicians, this is the key piece to understanding and integrating concepts of biomechanics into our daily practices.

When working up a biomechanics protocol or sequence, the dental team cannot look at only one variable. "This is in spite of what many manufacturers advertise about their products, even though it makes absolute sense," Kois says. "Each variable is just one part of the system, but other things may or may not contribute to the success or failure of the case. Therefore, it is critically important for a dental professional to truly understand the biological system, principles of engineering, and how they behave within the mechanical properties to attempt to achieve the optimal results."

Robert LeBeau, of LeBeau Dental Laboratory in Renton, Washington, takes a similar approach when his laboratory examines new cases. "We work up restorative system assessments by answering questions designed to lead us to the best answer," he says. "Is it anterior or posterior work? Did the case come with information regarding a model of the provisionals, midline assessment and smile analysis, morphology, functional occlusion, bite records, and face-bow transfers, and do we have the restorative space needed, preparation designs, and shades that would help us choose the materials, etc?" Once they have the answers to those questions, the team then refocuses to establish what they are asked to restore, also taking into account any future restorative work: Is it one tooth? A segment of the teeth? Or the whole arch or mouth? Diagnostic work then can be done along with the current restorative work to get everything planned accordingly. "For us, as we get cases in, we evaluate those variables, and then we select the best material choice for this individual patient. We rely heavily on photography analysis to assess many of these and return to the photos to ensure proper protocols are followed," LeBeau says, though he admits that getting clear, accurate photographs requires educating their clients. Deutsch and his team take a similar approach. "Go back to basic engineering principles for starters and really look at strength, thicknesses, flexure, and fulcrum to understand the basics of what your restorations would need to endure while in function," Deutsch says.

Cone's treatment protocol work-up begins with having the patient's end result in mind. "If the final restoration is the elective replacement of a low-value single central incisor placed 20 years prior with no clinically detectable pathology or material failure," he says, "then my considerations tend to focus on esthetics. If, however, my patient exhibits robust masseter musculature, a large Frankfurt Mandibular Plane Angle, and a heavily restored posterior dentition with significant tooth surface loss on the anterior teeth, I will immediately begin to consider a global rehabilitation of this individual with a group function occlusal scheme." While Cone acknowledges that this brief explanation oversimplifies these processes, it illustrates the wide range of case types that he sees regularly.

Nonetheless, in establishing his treatment protocol and material selections, Cone admits, "I always try to remain as conservative as possible while maximizing the final esthetics for the case. Many dental professionals have been misled and persuaded to believe that many of the new high-strength ceramics are the restorative panacea for every patient with destructive parafunctional habits or limited interocclusal space. I find this a disconcerting and unfortunate trend. In my experience, there is still nothing that can replace the biological and long-term mechanical benefits of a properly executed gold restoration."

Kois agrees that the interface interconnectivity can be more important to the success of the restoration. "When the tooth structure becomes the weakest link in the restoration itself, then the outcome would be catastrophic," he says. "Many clinicians and technicians must realize that the stronger materials do not always solve the patients' problems, and physical characteristics are not always accurate predictors of positive clinical outcomes."

Those who work in implantology and fixed restoratives consider a variety of materials to determine which will help them achieve the long-term viability of the restoration along with the highest level of esthetics. As Stephen J. Chu, DMD, MSD, CDT, an Adjunct Clinical Professor at New York University College of Dentistry in the Department of Prosthodontics, shares, "There's been a lot of conversation about zirconia being such a strong material that the question remains: If masticatory and/or musculature forces are going to translate directly to the implant fixtures, will it affect the bone around the implants and ultimately affect the implant's success? Unfortunately, that has not been very well documented or established in the dental literature, although many believe it to be true." Some companies claim that their products offer the solution, but "there is just no science to really support these claims," Chu says.

LeBeau agrees with Chu's concern and adds to it. "We have seen zirconia that will fracture if it is too thin," LeBeau says. "If the anatomy was ground into it and if the bite is questionable, you can unknowingly propagate fractures as well." With zirconia being so rigid, LeBeau believes technicians really need to know the kind of material against which the restoration is functioning. "Ensuring material compatibility while keeping everything safe with both the proposed restoration and the existing restoration; or keeping in mind that if we would have a failure, how we could control where our failure would be—these are my main concerns as I go through the biomechanical assessment process," LeBeau says.

Kois says when most people assess an implant case, they look at implants as an ankylotic interface. "So a single-tooth implant should hardly touch in the mouth to protect the implant interface," he says. "However, you cannot do a full arch without the restoration touching. You must understand occlusion really well, and making sure it is touching properly when it should, or avoiding it when it shouldn't, is the key to the longevity of the restoration."

Whether for occlusal consideration, space limitations, or proper implant distancing, having knowledge of physics offers dental professionals a unique perspective and a better understanding of the forces needing to be managed in our restorations. Knowing the lever system and fulcrum processes as they pertain to dentistry will yield a significantly better outcome.

Deutsch identifies placement and design of an implant-supported bar—and specifically the distal cantilever—by adhering to the anterior-posterior (AP) spread. "AP spread is among the first things we look at to see what the safe distance to cantilever the restoration is—meaning, where are we stopping the dentition?" Deutsch's rule of thumb is to measure from the most anterior implant to the most posterior implant linearly and then calculate half that distance to determine how far he is willing to extend the cantilever.

For occlusion, the same considerations must be adhered to when restoring tooth-borne or implant-borne restorations. However, the tooth-borne restorations are generally more forgiving since the ligaments attached allow minor movement, whereas implants are rigid and allow absolutely no movement at all. Kois clarifies, "As you get to more rigid systems, the occlusal considerations become significantly more contributory to the longevity of the restoration. With implant systems, you are dealing with a rigid biological system often against another rigid material; the failure could be significant."

Occlusion, too, has lever-system considerations and must be considered from the perspective of a whole system. Everything could potentially affect the other and failures will occur at the weakest area, which may be a poorly selected restorative material or components, or in a worst-case scenario, a combination of violated cantilever distance and minimum material thicknesses, according to Deutsch.

Occlusal management is difficult for a laboratory technician only because technicians work on articulators, which do not load the forces the way they are biologically. There is also no flexure and there is no chewing envelope or physiologic movement associated with the articulator. "Laboratory technicians primarily adjust occlusion with excursive movements, which are unloaded movements," Kois says. "The actual way the mouth is loaded is from the outside in, not the way it is on an articulator with the inside out. Occlusion is not just an axial load; it is rather a non-axial load and tends to flex what is under it; flexure and fatigue create a lot of the problems we see, even more than just compressive loads."

Often, in order to mitigate these circumstances, Chu practices the "why not" of putting in more or extra implants to absorb and share the forces, and even segment the case. "Some people are now suggesting that for a full-arch mandibular restoration, there is flexure we need to observe. The mandible has flexure, where the maxilla does not, because it is a part of the skull. Gordon N. Gates, DDS, MSD, and Jack I. Nicholls, PhD, showed in a 1981 article that there is 3 or 4 mm of flexure on the mandible due at maximum opening.² We need to be careful and choose wisely." Zirconia cannot flex and ultimately can break on the mandible if splinted, due to mandibular flexure. Kois reinforces those sentiments by adding, "We are concerned with flexure of the mandible, but that comes into the picture if we are splinting a second molar to another second molar. A lot of the flexure is only in the posterior region, not so much in the anterior because the symphysis is very rigid anteriorly, so you do not typically see any flexural problem from the second bicuspid forward." Chu continues to elaborate that, "I have had a patient who complained of tightness on a full-zirconia mandibular-arch restoration and eventually the bridge broke. I remade the restoration, but this time in three segments; the patient was very comfortable and the restoration sustained." A mandibular-arch restoration with multiple implants does not need to be made in one piece; just segment them and there will not be an issue. That is a very important point when speaking of biomechanics and its effects on our work, as Kois, Chu, and LeBeau all agree.

A good guide to biomechanics and its effects on our restorative process is a concept that is often derived from a firm understanding of complete dentures and what will be sustained in the oral environment, according to Deutsch. With full dentures, most importantly, we need to be aware of the limitations, particularly for skeletal class. "The patient's vertical dimension might be closed," he says, "and if the clinician is not using a gothic tracer device, it is very difficult for the patient to understand what is being asked of them to record a jaw relationship record. At times, when bite rims are being used, the patient struggles with locating a true centric position. My experience has been to do the best we can to restore the patient in the class they were originally in, because you are talking about musculature, prolonged or arrested mandibular bone growth, etc. Keeping them in the same class will work better for the patient in the long run." For all fully edentulous patients, Deutsch prefers for the clinician to use arch tracers rather than wax rims. "This technique affords you to 99% record the centric and vertical in the wax try-in," he says.

With removable partial dentures (RPDs), if biomechanical principles are not adhered to properly, some detrimental consequences can result, such as creating an unintended extraction device, which all relates directly to RPD design and the technician's knowledge of the fulcrum and lever system as they relate to retention, reciprocation, and support for clasping. Deutsch is optimistic with digital mediums for design and says that, "With the evolution of digital mediums for fabrication, we need to hone in on opportunities to attain greater information, such as photography and measurements, to achieve the best restorative outcome. Digital is great and is a means to do some sophisticated work, but it does not mean we can skip the critical and basic knowledge of the biological oral system."

When setting teeth on an edentulous patient, the placement and its effects on how the denture will function are of critical importance. Deutsch, a second-generation technician, learned a lot of biomechanics concepts from his father and ventured beyond to learn more. "I took a manufacturer's course back in 2012 called ‘Total Prosthetic and Function' that backed up my experience-based learning with a guideline for a very detailed cast analysis," he says. "We look at landmarks such as the lowest point on the residual ridge anatomy in the mandible—elaborated on by Gerber—which is where the last tooth should be placed. Defining a posterior stop point for any occlusal contact areas of the residual ridge that ascend beyond 22.5° off the Camper's Plane. Establishing static lines on the ridge between anatomic landmarks and establish outer and inner connections, which provide the safest contact points lingual and buccal to the ridge. That put everything together for me."

When considering new materials, Kois believes we must examine all factors and how they behave with one another, not just how they may resolve a specific issue. "The problem we find from our previous research is that when forces are being absorbed by the materials, the load is driven down deeper into the restorations, which many times affects or leads to failure in the luting agent or the substructure(s) underneath even more," Kois says. "Even with porcelain, for example, if the forces are overloading the material, then it is likely you may get a chip or a crack that will cause the material to fail. However, if the material can sustain the load, then many times the forces are driven more into the restoration and would therefore fail further down the system. You will need to manage where it could potentially fail and determine if the restoration is a better choice over the implant or the prepared tooth. We have to be careful where we drive the forces."

Similarly, LeBeau adds, "How you treat the materials and ensuring that you use them for the right purposes are key in biomechanics. We may use materials that we think can function well in the particular environment, but in reality, sometimes the interphase fails because the material was never developed for that purpose and to withstand the stresses of the mechanics in which it was used."

Some technicians, including LeBeau, have expressed excitement about high-impact polymers. "PEKK (polyetherketoneketone) seems to be resilient and gives a little extra cushion to function better and not fracture," LeBeau says. "We have a few in the mouth and think this is a good material to solve some of the issues inherited with implants."

However, Kois advises caution. "All the materials we use today were once new," he says. "However, I still remain very cautious and choose to wait until more data is available and the materials are validated before I jump to utilize new options." Chu agrees with Kois's apprehension. "The problem that we are facing is that there is not a lot of research to substantiate some of the claims made by companies and experts," Chu says. "You need to have some data and clinical research to support the claims out there."

LeBeau feels a similar level of biomechanical unease when utilizing titanium bases for their implant-supported restorations. "We don't rely on Ti-bases or cements; in fact, all our cases are done over custom abutments, and all bridges and splinted cases are used with non-engaging custom abutments," LeBeau says. "It certainly requires a bit more work, but it is a measure we are willing to take to ensure that our cases do not fail at the Ti-base interface. That way, you can control your interface and truly get a quality emergence profile without worrying about the subgingival zirconia breaking because it is too thin."

On establishing new materials, Deutsch says, "Since they are new and the consequences are unknown, we really need to pull back and be very careful. We need to know the manufacturer's instruction for use and follow it, since that is what has been tested. We should not stretch the boundaries because that is how we get in trouble with the restoration."

LeBeau emphasizes common-sense caution. "We just need to be very mindful of the materials we use," he says. "New materials often sound promising, but we cannot afford to try them all. We have all had materials go wrong and paid dearly for it. Those failures impacted our patients and businesses greatly. That is why we rely on evidence-based studies published in authoritative publications to test these and inform us of best practices."

Nonetheless, it is possible to approach new developments with both careful consideration and excited optimism. "Material evolution is occurring so rapidly," Cone says. "Ceramics are more esthetic, polymers are stronger, and CAD/CAM strategies coupled with radiographic diagnostic tools are allowing increasing precision in the delivery of care to our patients. It is a great time to be involved in the dental profession."

Biomechanics, as it relates to the human oral environment, is incredibly important to learn in dentistry. Adequate knowledge of these principles by the restorative team will undoubtedly affect the ultimate success, or lack thereof, for our restorative work and patients' well-being, function, and esthetics. The assessment and treatment protocols of every case must be established with proper biomechanical considerations in mind. "Muscles will win every time," Cone says, "and, given enough time, every restoration will need to be replaced eventually. The oral environment is harsh and unforgiving, and the human jaw is an extremely effective Class 1 lever that is oblivious to buzz words such as tensile strength, modulus of elasticity, or transformation toughening. Basic principles of case selection, tooth preparation design, material manufacturing, cementation/bonding protocol, and patient follow-up are all necessary for successful patient outcomes and good long-term prognoses."