Keren Shemtov-Yona

Mechanical assessment of grit blasting surface treatments of dental implants.

This paper investigates the influence of surface preparation treatments of dental implants on their potential (mechanical) fatigue failure, with emphasis on grit-blasting. The investigation includes limited fatigue testing of implants, showing the relationship between fatigue life and surface damage condition. Those observations are corroborated by a detailed failure analysis of retrieved fracture dental implants. In both cases, the negative effect of embedded alumina particles related to the grit-blasting process is identified. The study also comprises a numerical simulation part of the grit blasting process that reveals, for a given implant material and particle size, the existence of a velocity threshold, below which the rough surface is obtained without damage, and beyond which the creation of significant surface damage will severely reduce the fatigue life, thus increasing fracture probability. The main outcome of this work is that the overall performance of dental implants comprises, in addition to the biological considerations, mechanical reliability aspects. Fatigue fracture is a central issue, and this study shows that uncontrolled surface roughening grit-blasting treatments can induce significant surface damage which accelerate fatigue fracture under certain conditions, even if those treatments are beneficial to the osseointegration process.

Modeling dental implant insertion

The success of dental implantation is connected with the so-called implant primary stability, a synonym for the implant anchoring inside the bone. The primary stability is related to the applied peak torque to the implant during the insertion process. This work simulates the process of insertion of a typical commercial implant into the jaw bone (mandible) using a 3D dynamic non-linear finite-elements software. The model considers the geometrical and mechanical properties of the implant, the bone-implant friction, and the insertion procedure parameters, namely angular velocity and normal load. The numerical results assess the influence of those parameters on the evolution of the insertion torque and the resulting bone damage. It is found that, within the models assumptions, the angular insertion velocity (up to 120rpm) has little or no effect on the process. The application of a normal load, in addition to the implant rotation, enforces an extrusion process in addition to the screwing one. The respective contribution of the cortical and trabecular bone components to the insertion torque reveals that, despite its significantly lower strength, the trabecular bone has a definite contribution to the insertion process. This work shows that if the various physical, geometrical and mechanical parameters of the bone-implant system are well-defined, the insertion process can be simulated prior to the surgical act, and predict, tailor and contribute to maximize the success of dental implantation in a personalized manner.

Random spectrum loading of dental implants: An alternative approach to functional performance assessment.

The fatigue performance of dental implants is usually assessed on the basis of cyclic S/N curves. This neither provides information on the anticipated service performance of the implant, nor does it allow for detailed comparisons between implants unless a thorough statistical analysis is performed, of the kind not currently required by certification standards. The notion of endurance limit is deemed to be of limited applicability, given unavoidable stress concentrations and random load excursions, that all characterize dental implants and their service conditions. We propose a completely different approach, based on random spectrum loading, as long used in aeronautical design. The implant is randomly loaded by a sequence of loads encompassing all load levels it would endure during its service life. This approach provides a quantitative and comparable estimate of its performance in terms of lifetime, based on the very fact that the implant will fracture sooner or later, instead of defining a fatigue endurance limit of limited practical application. Five commercial monolithic Ti-6Al-4V implants were tested under cyclic, and another 5 under spectrum loading conditions, at room temperature and dry air. The failure modes and fracture planes were identical for all implants. The approach is discussed, including its potential applications, for systematic, straightforward and reliable comparisons of various implant designs and environments, without the need for cumbersome statistical analyses. It is believed that spectrum loading can be considered for the generation of new standardization procedures and design applications.

Fatigue of Dental Implants: Facts and Fallacies

Dental implants experience rare yet problematic mechanical failures such as fracture that are caused, most often, by (time-dependent) metal fatigue. This paper surveys basic evidence about fatigue failure, its identification and the implant’s fatigue performance during service. We first discuss the concept of dental implant fatigue, starting with a review of basic concepts related to this failure mechanism. The identification of fatigue failures using scanning electron microscopy follows, to show that this stage is fairly well defined. We reiterate that fatigue failure is related to the implant design and its surface condition, together with the widely varying service conditions. The latter are shown to vary to an extent that precludes devising average or representative conditions. The statistical nature of the fatigue test results is emphasized throughout the survey to illustrate the complexity in evaluating the fatigue behavior of dental implants from a design perspective. Today’s fatigue testing of dental implants is limited to ISO 14801 standard requirements, which ensures certification but does not provide any insight for design purposes due to its limited requirements. We introduce and discuss the random spectrum loading procedure as an alternative to evaluate the implant’s performance under more realistic conditions. The concept is illustrated by random fatigue testing in 0.9% saline solution.

The Failure Envelope Concept Applied To The Bone-Dental Implant System

Dental implants interact with the jawbone through their common interface. While the implant is an inert structure, the jawbone is a living one that reacts to mechanical stimuli. Setting aside mechanical failure considerations of the implant, the bone is the main component to be addressed. With most failure criteria being expressed in terms of stress or strain values, their fulfillment can mean structural flow or fracture. However, in addition to those effects, the bony structure is likely to react biologically to the applied loads by dissolution or remodeling, so that additional (strain-based) criteria must be taken into account. While the literature abounds in studies of particular loading configurations, e.g. angle and value of the applied load to the implant, a general study of the admissible implant loads is still missing. This paper introduces the concept of failure envelopes for the dental implant-jawbone system, thereby defining admissible combinations of vertical and lateral loads for various failure criteria of the jawbone. Those envelopes are compared in terms of conservatism, thereby providing a systematic comparison of the various failure criteria and their determination of the admissible loads.

Fatigue failure of dental implants in simulated intraoral media.

Metallic dental implants are exposed to various intraoral environments and repetitive loads during service. Relatively few studies have systematically addressed the potential influence of the environment on the mechanical integrity of the implants, which is therefore the subject of this study. Four media (groups) were selected for room temperature testing, namely dry air, saliva substitute, same with 250ppm of fluoride, and saline solution (0.9%). Monolithic Ti-6Al-4V implants were loaded until fracture, using random spectrum loading. The study reveals that the only aggressive medium of all is the saline solution, as it shortens significantly the spectrum fatigue life of the implants. The quantitative scanning electron fractographic analysis indicates that all the tested implants grew fatigue cracks of similar lengths prior to catastrophic fracture. However, the average crack growth rate in the saline medium was found to largely exceed that in other media, suggesting a decreased fracture toughness. The notion of a characteristic timescale for environmental degradation was proposed to explain the results of our spectrum tests that blend randomly low and high cycle fatigue. Random spectrum fatigue testing is powerful technique to assess and compare the mechanical performance of dental implants for various designs and/or environments.

Modelling dental implant extraction by pullout and torque procedures

Dental implants extraction, achieved either by applying torque or pullout force, is used to estimate the bone-implant interfacial strength. A detailed description of the mechanical and physical aspects of the extraction process in the literature is still missing. This paper presents 3D nonlinear dynamic finite element simulations of a commercial implant extraction process from the mandible bone. Emphasis is put on the typical load-displacement and torque-angle relationships for various types of cortical and trabecular bone strengths. The simulations also study of the influence of the osseointegration level on those relationships. This is done by simulating implant extraction right after insertion when interfacial frictional contact exists between the implant and bone, and long after insertion, assuming that the implant is fully bonded to the bone. The model does not include a separate representation and model of the interfacial layer for which available data is limited. The obtained relationships show that the higher the strength of the trabecular bone the higher the peak extraction force, while for application of torque, it is the cortical bone which might dictate the peak torque value. Information on the relative strength contrast of the cortical and trabecular components, as well as the progressive nature of the damage evolution, can be revealed from the obtained relations. It is shown that full osseointegration might multiply the peak and average load values by a factor 3-12 although the calculated work of extraction varies only by a factor of 1.5. From a quantitative point of view, it is suggested that, as an alternative to reporting peak load or torque values, an average value derived from the extraction work be used to better characterize the bone-implant interfacial strength.

On stress/strain shielding and the material stiffness paradigm for dental implants

BACKGROUND Stress shielding considerations suggest that the dental implant materials compliance should be matched to that of the host bone. However, this belief has not been confirmed from a general perspective, either clinically or numerically. PURPOSE To characterize the influence of the implant stiffness on its functionality using the failure envelope concept that examines all possible combinations of mechanical load and application angle for selected stress, strain and displacement-based bone failure criteria. Those criteria represent bone yielding, remodeling, and implant primary stability, respectively MATERIALS AND METHODS: We performed numerical simulations to generate failure envelopes for all possible loading configurations of dental implants, with stiffness ranging from very low (polymer) to extremely high, through that of bone, titanium, and ceramics. RESULTS Irrespective of the failure criterion, stiffer implants allow for improved implant functionality. The latter reduces with increasing compliance, while the trabecular bone experiences higher strains, albeit of an overall small level. Micromotions remain quite small irrespective of the implants stiffness. CONCLUSION The current paradigm favoring reduced implant materials stiffness out of concern for stress or strain shielding, or even excessive micromotions, is not supported by the present calculations, that point exactly to the opposite.

Modeling the effect of osseointegration on dental implant pullout and torque removal tests

BACKGROUND Osseointegration of dental implants is a key factor for their success. It can be assessed either by destructive (eg, pullout or torque extraction), or nondestructive methods (eg, resonant frequency analysis). However, as of today there is a scarcity of models that can relate the outcome of destructive tests to the level of osseointegration. PURPOSE To study various percentages of bone to implant bonding (tie) using finite element simulations. While evolutions of the bone mechanical properties are not explicitly taken into account, emphasis is put on the 3-dimensional variable extent of the bone-implant bonding, its statistical distribution, and its influence on the measurable extraction and torque loads, seeking to obtain a quantitative relationship. MATERIALS AND METHODS We performed numerical simulations of randomly tied implants and calculated the evolution of the pullout force as well as that of the extraction torque. CONCLUSION Within simplifying assumptions for the osseointegration represented by a tie (as opposed to frictional) constraint, the results of this work indicate that the torque test is more discriminant than the extraction one, while both cannot really discriminate osseointegration levels below a relative variation of 20%.

Engineering Dental Implants

Purpose of ReviewImplant dentistry is traditionally viewed as a clinical subject. However, the integration of a foreign metallic structure into a living bone involves several engineering considerations. This paper aims at reviewing and discussing recent basic issues and developments pertaining to the engineering aspects of dental implant development.Recent FindingsWe consider the three components of the system, namely the implant itself, the bone, and their interaction. We start with the implant material and its geometrical and surface condition parameters. Next, we discuss the long-term mechanical survivability of the implant, namely its resistance to fatigue cracking, outlining the uncertainty on the applied loads, and surrounding atmosphere. Following a summary of the jawbone from a mechanical standpoint, we discuss the dental implant-bone interaction, as modeled analytically or numerically, with emphasis on the bone damage and evolution. The contribution of high resolution observations to enriched numerical simulations is discussed.SummaryProgress in both experimental characterization techniques and numerical simulation methods brings engineering and dentistry closer, allowing for more focused clinical work that will ultimately lead to personalized implant dentistry.