The Precarious Balance

The American Society of Biomechanics defines the term as "the broad interplay between mechanics and biological systems."¹ Although this definition certainly applies to dentistry and restorative dentistry as a whole, the interpretation within different segments of the industry provides interesting perspectives on how different dental professionals approach the issue.

M. Reed Cone, DMD, MS, CDT, 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."

According to Arian Deutsch, CDT, owner of Deutsch Dental Arts in Surprise, Arizona, it is important to incorporate physics and its effects on either the patient and/or the restorations. "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 of 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 that 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 restorative dentistry, this is the key approach 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 Lab-oratory 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 to help us choose the materials, etc?" Once the team has the answers to those questions, they then refocus to establish what they are being asked to restore, also taking into account the need for any future restorative work. Are they restoring one tooth, a segment of the teeth, an entire arch, or the whole mouth? If necessary, diagnostic work can be done along with the current restorative work to get everything planned accordingly. "For us, as we get cases in, we evaluate all of the variables, and then we select the best material choice for each individual patient. We rely heavily on photography analysis to assess many of these and return to the photos to ensure that proper protocols are followed," LeBeau says, although he admits that getting clear, accurate photographs often 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 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 20-year-old, low-value single central incisor 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 Frankfort-mandibular plane angle, and a heavily restored posterior dentition with significant tooth surface loss on the anterior teeth, I will immediately begin to consider the global rehabilitation of this individual with a group function occlusal scheme." Although Cone acknowledges that this brief explanation oversimplifies these processes, it illustrates the wide range of case types that he sees regularly.

Nonetheless, when establishing his treatment protocols 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 a restorative panacea for every patient with destructive parafunctional habits or limited interocclusal space. I find this to be 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 can be catastrophic," he says. "Many clinicians and technicians need to realize that the stronger materials do not always solve patients' problems, and physical characteristics are not always accurate predictors of positive clinical outcomes."

Those who work in implantology and fixed prosthodontics consider a variety of materials to determine which will help them achieve long-term viability of the restoration along with the highest level of esthetics. As Stephen J. Chu, DMD, MSD, CDT, an adjunct clinical professor in the Department of Prosthodontics at New York University College of Dentistry, shares, "There's been a lot of conversation about zirconia being such a strong material, but the question remains: If masticatory and/or musculature forces are going to translate directly to the implant fixtures, will they affect the bone around the implants and, ultimately, the implants' 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. "We have seen zirconia that will fracture if it is too thin," LeBeau adds. "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 dental professionals really need to consider the kind of material against which the restoration is functioning. "Ensuring material compatibility while keeping everything safe with both the proposed restoration and any existing restorations 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 that when most people assess an implant case, they look at implants as an ankylotic interface. "So, a single-tooth implant restoration should hardly touch in the mouth to protect the implant interface," he says. "However, you cannot do a full arch without the restoration touching the opposing dentition. You must understand occlusion really well, and making sure that the restoration is touching properly when it should be and not touching when it shouldn't be is the key to its longevity."

Whether for occlusal considerations, space limitations, or proper implant distancing, having a knowledge of physics offers dental professionals a unique perspective and a better understanding of the forces that need to be managed in our restorations. Understanding lever systems and fulcrum processes as they pertain to dentistry can help clinicians achieve significantly better outcomes.

Deutsch determines the 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 of that distance to determine how far he is willing to extend the cantilever.

Regarding occlusion, the same considerations must be adhered to when restoring both tooth-borne and implant-borne restorations. However, tooth-borne restorations are generally more forgiving because the ligaments attached allow for minor movement, whereas implants are rigid and allow absolutely no movement at all. Kois clarifies that "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, so the failure could be significant."

Occlusion, too, involves lever-system considerations and should be evaluated from the perspective of the whole system. Every element could potentially affect the others, and failures will occur at the weakest area, which could be a  poorly selected restorative material or components or, in a worst-case scenario, a combined violation of proper cantilever distance and minimum material thicknesses, according to Deutsch.

Occlusal management is difficult when the restorative process includes the use of 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 articulators. "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."

Oftentimes, in order to mitigate these circumstances, Chu practices the "why not" of putting in additional or extra implants to absorb and share the forces or even segments the case. "Some are now suggesting that for a full-arch mandibular restoration, there is flexure that we need to observe. The mandible has flexure, whereas the maxilla does not because it is a part of the skull. In an article published in 1981, Gordon N. Gates, DDS, MSD, and Jack I. Nicholls, PhD, demonstrated that there is 3 mm or 4 mm of flexure on the mandible at maximum opening.² We need to be careful and choose wisely." Zirconia cannot flex, and if it is splinted on the mandible, it can ultimately break 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 occurs only in the posterior region, not so much in the anterior because the symphysis is very rigid anteriorly. You do not typically see any flexural problem from the second bicuspid forward."

"I have had a patient who complained of tightness on a mandibular full-arch zirconia restoration, and eventually, the bridge broke," Chu elaborates. "I remade the restoration, but this time in three segments. The patient was very comfortable, and the restoration sustained." A mandibular full-arch restoration supported by multiple implants does not need to be made in one piece; many issues can be avoided by segmenting it. That is a very important point regarding the effects of biomechanics on our work, as Kois, Chu, and LeBeau all agree.

According to Deutsch, a good conceptualization of biomechanics and its effects on our restorative processes can be derived from a firm understanding of complete dentures and what will be sustained in the oral environment. 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 can be very difficult for patients to understand what is being asked of them to record a jaw relationship record. At times, when bite rims are being used, patients struggle with locating a true centric position. My experience has been to do the best that we can to restore patients 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 patients in the long run." For all fully edentulous patients, Deutsch recommends that clinicians use arch tracers rather than wax rims. "This technique permits you to 99% record the centric and vertical in the wax try-in," he says.

With removable partial dentures, if biomechanical principles are not adhered to properly, some detrimental consequences can result, such as the creation of an unintended extraction device. All of this is directly dependent on the design of the denture and the technician's knowledge of the fulcrum and lever system as it relates to retention, reciprocation, and support for clasping. Deutsch is optimistic about 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 outcomes. Digital dentistry is great and provides a means to do some sophisticated work, but it does not mean we can skip learning 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 about biomechanics from his father and ventured beyond to learn more. "I took a manufacturer's course in 2012 that backed up my experience-based learning with a guideline for a very detailed cast analysis," he says. "We looked 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; defined a posterior stop point for any occlusal contact areas of the residual ridge that ascend beyond 22.5° off the Camper's plane; and established static lines on the ridge between anatomic landmarks as well as 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 of the factors and their interplay, 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 materials, the load is driven down deeper into the restorations, which oftentimes affects or leads to failure in the luting agent or the substructures underneath," he 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 further into the restoration, potentially causing failure further down the system. You will need to manage where the system could potentially fail and determine if a restoration is a better choice over an implant or a prepared tooth. We have to be careful where we drive the forces."

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

"All of the materials that we use today were once new," Kois notes. "However, I still remain very cautious and choose to wait until robust data are 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," he says. "You need to have some data and clinical research to support the claims out there."

LeBeau feels a similar level of biomechanical unease about utilizing titanium bases for implant-supported restorations. "We don't rely on Ti-bases or cements; in fact, all of our cases are done over custom abutments, and all bridges and splinted cases are completed with non-engaging custom abutments," he 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 develop a quality emergence profile without worrying about the subgingival zirconia breaking because it is too thin."

Regarding the adoption of new materials, Deutsch notes that "because they are new and the consequences are unknown, we really need to pull back and be very careful. We need to understand the manufacturer's instructions for use and follow them because that is what has been tested. We should not stretch the boundaries because that is how we get into 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. Many of us have 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 materials 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."

The concept of biomechanics, as it relates to the human oral environment, is incredibly important to the understanding of dentistry. The adequacy of the entire restorative team's knowledge of these principles will undoubtedly affect the ultimate success, or lack thereof, of our restorative work as well as patients' function, esthetics, and overall well-being. The assessment and treatment protocols of every case must be undertaken with proper biomechanical considerations in mind. "Muscles will win every time," Cone says, "and, given enough time, every restoration will eventually need to be replaced. The oral environment is harsh and unforgiving, and the human jaw is an extremely effective Class 3 lever that is oblivious to buzz words such as tensile strength, modulus of elasticity, or transformation toughening. The basic principles of case selection, tooth preparation design, material manufacturing, cementation/bonding, and patient follow-up are all necessary for successful patient outcomes and good long-term prognoses."