On the Effects of Cyclic Transversal Forces on Osseointegrated Dental Implants: Experimental and Finite Element Shakedown Analyses

This paper is concerned with the mechanical strength of fixed osseointegrated dental implants subjected to cyclic external loads, applied mainly in a direction orthogonal to their axis. Such a loading condition, seen as a basic design action for the implant, has been given little attention so far. Experimental results and numerical simulations, performed on two- and three-dimensional Finite Element models, are discussed. The shakedown theory is used to show that a common implant design (threaded fixture-abutment-connection screw) is susceptible of low-cycle fatigue failure under loading conditions well within the working range, even if the same design is able to withstand loading of the same type, but applied monotonically, much in excess of the working values. The shakedown analyses give an indication of several possible failure modalities: the low-cycle fatigue either of the implant or of the connection screw, or the loosening of the connection screw itself. Experimental and numerical results are in good qualitative agreement, and both suggest that the issue of transversal cyclic loading on fixed dental implants should be carefully reconsidered in the design phase.     

Biomechanical analysis and comparison of 12 dental implant systems using 3D finite element study

Finite element analysis plays an important role in dental implant design. The objective of this study was to show the effect of the overall geometry of dental implants on their biomechanics after implantation. In this study, 12 dental implants, with the same length, diameter and screw design, were simulated from different implant systems. Numerical model of right mandibular incisor bone segment was generated from CT data. The von-Mises stress distributions and the total deformation distributions under vertical/lateral load were compared for each implant by scores ranking method. The implants with cylindrical shapes had highest scores. Results indicated that cylindrical shape represented better geometry over taper implant. This study is helpful in choosing the optimal dental implant for clinical application and also contributes to individual implant design. Our study could also provide reference for choice and modification of dental implant in any other insertion sites and bone qualities.     

An in vitro study of implant--tooth-supported connections using a robot test system.

Many unsolved problems in dental implant research concern the interfacial stress distributions between the implant components, as well as between the implant surface and contacting bone. To obtain a mechanical understanding of how vertical and horizontal occlusal forces are distributed in this context, it is crucial to develop in vitro testing systems to measure the force transmission between dental implants and attached prostheses. A new approach to such testing, involving a robotic system, is described in this investigation. The system has been designed to produce simulated mandibular movements and occlusal contact forces so that various implant designs and procedures can be thoroughly tested and evaluated before animal testing or human clinical trials. Two commonly used fixed prosthesis designs used to connect an implant and a tooth, a rigid connection and a nonrigid connection, were fabricated and used for experimental verification. The displacement and force distributions generated during simulated chewing activities were measured in vitro. Force levels, potentially harmful to human bone surrounding the connected dental implant and tooth, were analyzed. These results are useful in the design of prostheses and connecting components that will reduce failures and limit stress transfer to the implant/bone interface.     

Evaluation of bone remodeling around single dental implants of different lengths: a mechanobiological numerical simulation and validation using clinical data

Algorithmic models have been proposed to explain adaptive behavior of bone to loading; however, these models have not been applied to explain the biomechanics of short dental implants. Purpose of present study was to simulate bone remodeling around single implants of different lengths using mechanoregulatory tissue differentiation model derived from the Stanford theory, using finite elements analysis (FEA) and to validate the theoretical prediction with the clinical findings of crestal bone loss. Loading cycles were applied on 7-, 10-, or 13-mm-long dental implants to simulate daily mastication and bone remodeling was assessed by changes in the strain energy density of bone after a 3, 6, and 12 months of function. Moreover, clinical findings of marginal bone loss in 45 patients rehabilitated with same implant designs used in the simulation (n = 15) were computed to validate the theoretical results. FEA analysis showed that although the bone density values reduced over time in the cortical bone for all groups, bone remodeling was independent of implant length. Clinical data showed a similar pattern of bone resorption compared with the data generated from mathematical analyses, independent of implant length. The results of this study showed that the mechanoregulatory tissue model could be employed in monitoring the morphological changes in bone that is subjected to biomechanical loads. In addition, the implant length did not influence the bone remodeling around single dental implants during the first year of loading.     

Three-dimensional finite element analysis of stress distribution in retention screws of different crown–implant ratios

The retaining screw of the implant-supported dental prosthesis is the weakest point of the crown/implant system. Furthermore, crown height is another important factor that may increase the lever arm. Therefore, the aim of this study was to assess the stress distribution in implant prosthetic screws with different heights of the clinical crown of the prosthesis using the method of three-dimensional finite element analysis. Three models were created with implants (3.75 mm × 10 mm) and crowns (heights of 10, 12.5 and 15 mm). The results were visualised by means of von Mises stress maps that increased the crown heights. The screw structure exhibited higher levels of stresses in the oblique load. The oblique loading resulted in higher stress concentration when compared with the axial loading. It is concluded that the increase of the crown was damaging to the stress distribution on the screw, mainly in oblique loading.     

An intra-oral hydraulic system for controlled loading of dental implants

This study reports a method for controlling loads on an in vivo dental implant and its application for the investigation of early loading versus delayed loading of dental implants. The method was developed for the purpose of studying an ongoing hypothesis that amounts to bone loss around dental implants are related to mechanical-mediated adaptation of the alveolar bone. Using a customized intra-oral hydraulic system, the daily loading over a dental implant has been completed and recorded for six Sinclair swine. Each pig had a 5-month duration implant loading. During the experiments (loading), no analgesic treatment was supplied. The mean of the in vivo daily loadings was confirmed through an in vitro bench test after each animal was euthanized. Variations of the averaged loading input among the six animals were smaller than 10%. Preliminary data produced by the model suggests that cervical bone loss is less for early loading than for delayed loading. The current system is expected to provide a useful load control model for the study of alveolar bone adaptation around dental implants in relation to various loadings.     

A finite element comparison between the mechanical behaviour of rigid and resilient oral implants with respect to immediate loading

In this paper, a qualitative comparison between two types of dental implants with respect to their behaviour under immediate loading is presented. This analysis has been carried out using the finite element method. Since micromotions (and not the load) are responsible of the appearance of a fibrous interface avoiding osseointegration, the relative displacement between the bone surface and the implant has been the main variable analyzed at different loading states and for the two implant types here considered. The implants analyzed differ in their mechanical behavior: rigid or resilient. Their main difference lies in the joining between the different pieces that make up the dental system. While in the rigid implant all the pieces are screwed, in the resilient implant a relative displacement between the pieces is allowed, with the additional introduction of a silicone gasket that acts like the periodontal ligament. Both implants were considered with a similar geometry and under two different loading scenarios, one equivalent to the force of chewing applied to a molar and another which corresponds to a premolar. For the resilient implant, a hyperelastic behaviour for the silicone and contact conditions between the different mobile parts of the implant are considered. The displacements of the emerging-body in both designs are also compared with the values obtained by several authors. However, the results show that both implants fulfill the constraint of the immediate loading protocol. The micromotions of the resilient implant are lower to those of the rigid one, favouring therefore a good osseointegration process while keeping the stresses in the implant under admissible maximum values.     

Prospects of implant with locking plate in fixation of subtrochanteric fracture: experimental demonstration of its potential benefits on synthetic femur model with supportive hierarchical nonlinear hyperelastic finite element analysis

Background

Effective fixation of fracture requires careful selection of a suitable implant to provide stability and durability. Implant with a feature of locking plate (LP) has been used widely for treating distal fractures in femur because of its favourable clinical outcome, but its potential in fixing proximal fractures in the subtrochancteric region has yet to be explored. Therefore, this comparative study was undertaken to demonstrate the merits of the LP implant in treating the subtrochancteric fracture by comparing its performance limits against those obtained with the more traditional implants; angle blade plate (ABP) and dynamic condylar screw plate (DCSP).

Materials and Methods

Nine standard composite femurs were acquired, divided into three groups and fixed with LP (n?=?3), ABP (n?=?3) and DCSP (n?=?3). The fracture was modeled by a 20?mm gap created at the subtrochanteric region to experimentally study the biomechanical response of each implant under both static and dynamic axial loading paradigms. To confirm the experimental findings and to understand the critical interactions at the boundaries, the synthetic femur/implant systems were numerically analyzed by constructing hierarchical finite element models with nonlinear hyperelastic properties. The predictions from the analyses were then compared against the experimental measurements to demonstrate the validity of each numeric model, and to characterize the internal load distribution in the femur and load bearing properties of each implant.

Results

The average measurements indicated that the constructs with ABP, DCPS and LP respectively had overall stiffness values of 70.9, 110.2 and 131.4?N/mm, and exhibited reversible deformations of 12.4, 4.9 and 4.1?mm when the applied dynamic load was 400?N and plastic deformations of 11.3, 2.4 and 1.4?mm when the load was 1000?N. The corresponding peak cyclic loads to failure were 1100, 1167 and 1600?N. The errors between the displacements measured experimentally or predicted by the nonlinear hierarchical hyperelastic model were less than 18?%. In the implanted femur heads, the principal stresses were spatially heterogeneous for ABP and DCSP but more homogenous for LP, meaning LP had lower stress concentrations.

Conclusion

When fixed with the LP implant, the synthetic femur model of the subtrochancteric fracture consistently exceeds in the key biomechanical measures of stability and durability. These capabilities suggest increased resistance to fatigue and failure, which are highly desirable features expected of functional implants and hence make the LP implant potentially a viable alternative to the conventional ABP or DCSP in the treatment of subtrochancteric femur fractures for the betterment of clinical outcome.     

Importance of diameter-to-length ratio in selecting dental implants: a methodological finite element study

Implant dimensions greatly influence load transfer characteristics and the lifetime of a dental system. Excessive stresses at peri-implant area may result in bone failure. Finding the critical point at the implant–bone interface and evaluating the influence of implant diameter-to-length ratio on adjacent bone stresses makes it possible to select implant dimensions. For this, different cylindrical implants were numerically analysed using geometrical models generated from computed tomography images of mandible with osseointegrated implants. All materials were assumed to be linearly elastic and isotropic. Masticatory load was applied in its natural direction, oblique to occlusal plane. Maximum von Mises stresses were located around the implant neck at the critical point of its intersection with the plane of loading and were functions of implant diameter-to-length ratio. It was demonstrated that there exists a certain spectrum of diameter-to-length ratios, which will keep maximum bone stresses at a preset level chosen in accordance with patient's bone strength.     

Oral health and oral implant status in edentulous patients with implant‐supported dental prostheses who are receiving long‐term nursing care

Aim: The aim of this study was to investigate oral health and oral implant status in a group of edentulous patients receiving long‐term residential or nursing care (LTC), all of whom had implant‐supported fixed or removable dental prostheses. Material and methods: A dental examination was performed on a total of 3310 patients receiving LTC and from this population 35 edentulous patients in whom dental implants had been placed formed the cohort for this study. All examinations were performed by a specialist in hospital dentistry and took place in the patients’ own home environment. Oral health was assessed by means of a protocol which evaluated oral hygiene status, possible oral mucosal inflammation and oral mucosal friction levels. Any problems with the implant‐supported prosthesis, implant mobility or other complications were also assessed. In addition, patients were asked about any oral symptoms and their usual oral hygiene procedures. Results: About half of the subjects (17/35) were registered as having no/mild inflammation with 18 of 35 having moderate/severe inflammation. Twelve of the 35 patients had good/acceptable oral hygiene and 23 of 35 had poor/bad oral hygiene. Twenty‐one of the 35 patients depended on help from the nursing personnel for their daily oral hygiene procedures. Obvious problems with food impaction were noted in 11 patients. A total of 229 implants had been placed in 43 jaws supporting 40 full arch‐fixed prostheses and three implant‐borne overdentures. There was no evidence of mobility or fractures of either the implants or the prostheses. Fifteen implants showed some exposed screw threads. Pus was exuding from one implant site and general peri‐implant gingival hyperplasia was noted in two patients. Twenty‐four patients were completely satisfied with the function and appearance of their implant‐supported prostheses. Two patients were totally dissatisfied. Conclusion: This study indicates that oral implant therapy can be considered as a treatment of choice in elderly patients, even if oral hygiene is sub‐optimal.     

Finite element modelling of implant designs and cortical bone thickness on stress distribution in maxillary type IV bone

The aims of this study were to examine the effect of implant neck design and cortical bone thickness using 3D finite element analysis and to analyse the stability of clinical evidence based on micromotion and principal stress. Four commercial dental implants for a type IV bone and maxillary segments were created. Various parameters were considered, including the osseointegration condition, loading direction and cortical bone thickness. Micromotion and principal stresses were used to evaluate the failure of osseointegration and bone overloading, respectively. It was found that the maximum stress of the peri-implant bone decreased as cortical bone thickness increased. The micromotion level in full osseointegration is less than that in non-osseointegration and it also decreases as cortical bone thickness increases. The cortical bone thickness should be measured before surgery to help select a proper implant. In the early stage of implantation, the horizontal loading component induces stress concentration in bone around the implant neck more easily than does the vertical loading component, and this may result in crestal bone loss.     

Influence of custom-made implant designs on the biomechanical performance for the case of immediate post-extraction placement in the maxillary esthetic zone: a finite element analysis

Due to the increasing adoption of immediate implantation strategies and the rapid development of the computer aided design/computer aided manufacturing technology, a therapeutic concept based on patient-specific implant dentistry has recently been reintroduced by many researchers. However, little information is available on the designs of custom-made dental implant systems, especially their biomechanical behavior. The influence of the custom-made implant designs on the biomechanical performance for both an immediate and a delayed loading protocol in the maxillary esthetic zone was evaluated by means of the finite element (FE) method. FE models of three dental implants were considered: a state of the art cylindrical implant and two custom-made implants designed by reverse engineering technology, namely a root-analogue implant and a root-analogue threaded implant. The von Mises stress distributions and micro-motions around the bone-implant interfaces were calculated using ANSYS software. In a comparison of the three implant designs for both loading protocols, a favorable biomechanical performance was observed for the use of root-analogue threaded implant which approximated the geometry of natural anterior tooth and maintained the original long-axis. The results indicated that bone-implant interfacial micro-motion was reduced and a favorable stress distribution after osseointegration was achieved.     

Development and design of a novel loading device for the investigation of bone adaptation around immediately loaded dental implants using the reindeer antler as implant bed

The assessment of the behavior of immediately loaded dental implants using biomechanical methods is of particular importance. The primary goal of this investigation is to optimize the function of the implants to serve for immediate loading. Animal experiments on reindeer antlers as a novel animal model will serve for investigation of the bone remodeling processes in the implant bed. The main interest is directed towards the time and loading-dependant behavior of the antler tissue around the implants. The aim and scope of this work was to design an autonomous loading device that has the ability to load an inserted implant in the antler with predefined occlusal forces for predetermined time protocols. The mechanical part of the device can be attached to the antler and is capable of cyclically loading the implant with forces of up to 100 N. For the calibration and testing of the loading device a biomechanical measuring system has been used. The calibration curve shows a logarithmic relationship between force and motor current and is used to control the force on the implant. A first test on a cast reindeer antler was performed successfully.     

On high-cycle fatigue of 316L stents

This paper deals with fatigue life prediction of 316L stainless steel cardiac stents. Stents are biomedical devices used to reopen narrowed vessels. Fatigue life is dominated by the cyclic loading due to the systolic and diastolic pressure and the design against premature mechanical failure is of extreme importance. Here, a life assessment approach based on the Dang Van high cycle fatigue criterion and on finite element analysis is applied to explore the fatigue reliability of 316L stents subjected to multiaxial fatigue loading. A finite element analysis of the stent vessel subjected to cyclic pressure is performed to carry out fluctuating stresses and strain at some critical elements of the stent where cracks or complete fracture may occur. The obtained results show that the loading path of the analysed stent subjected to a pulsatile load pressure is located in the safe region concerning infinite lifetime.     

Dental implant abutment resembling the two-phase tooth mobility

Natural teeth and dental implants have differing degrees of mobility thus causing a potential biomechanical problem when connected by fixed bridgework. The clinical follow-up often discloses marginal bone loss around an implant neck probably due to high stress factors. An implant with a built-in compliance resembling the tooth mobility could be advantageous for stress distribution. With axial loading the proposed 'elastic'-test model accomplishes this demand. By means of theoretical and experimental studies this 'elastic'-test model is optimized and compared with a stiff implant-model. The results show a 20 times reduction of stress accumulation in bone with the 'elastic'-test model.     

Finite element investigation of implant-supported fixed partial prosthesis in the premaxilla in immediately loaded and osseointegrated states

The aim of this study was to gain insight into the behaviour of the stresses and strains at the bone–implant interface of an implant-supported fixed partial prosthesis (FPP) in the premaxilla under immediate loading and osseointegrated conditions. Finite element models of a four-unit FPP were generated. An extreme condition was simulated, using only two immediately loaded implants in order to derive recommendations for possible clinical application. Straight and 20°-angled abutments and bonded or sliding contact between the bridge and abutment were simulated. In addition, two models were generated with two completely osseointegrated implants. A 150 N load to the prosthesis at a 45° angle to the long axis of each implant was applied. Minor differences were observed in implant displacements, stress and strain distributions of the two abutment designs. However, bone loading exceeded the physiological limits, including a risk of bone atrophy. A considerable decrease in implant displacements and bone loading was observed in the osseointegrated cases. An FPP supported by only two implants cannot be recommended for immediate loading.     

Numerical investigation of bone remodelling around immediately loaded dental implants using sika deer (Cervus nippon) antlers as implant bed

This study combines finite element method and animal studies, aiming to investigate tissue remodelling processes around dental implants inserted into sika deer antler and to develop an alternative animal consuming model for studying bone remodelling around implants. Implants were inserted in the antlers and loaded immediately via a self-developed loading device. After 3, 4, 5 and 6 weeks, implants and surrounding tissue were taken out. Specimens were scanned by μCT scanner and finite element models were generated. Immediate loading and osseointegration conditions were simulated at the implant-tissue interface. A vertical force of 10 N was applied on the implant. During the healing time, density and Young’s modulus of antler tissue around the implant increased significantly. For each time point, the values of displacement, stresses and strains in the osseointegration model were lower than those of the immediate loading model. As the healing time increased, the displacement of implants was reduced. The 3-week immediate loading model (9878 ± 1965 μstrain) illustrated the highest strains in the antler tissue. Antler tissue showed similar biomechanical properties as human bone in investigating the bone remodelling around implants, therefore the use of sika deer antler model is a promising alternative in implant biomechanical studies.     

The discrete nature of trabecular bone microarchitecture affects implant stability

Small endosseous implants, such as screws, are important components of modern orthopedics and dentistry. Hence they have to reliably fulfill a variety of requirements, which makes the development of such implants challenging. Finite element analysis is a widely used computational tool used to analyze and optimize implant stability in bone. For these purposes, bone is generally modeled as a continuum material. However, bone failure and bone adaptation processes are occurring at the discrete level of individual trabeculae; hence the assessment of stresses and strains at this level is relevant. Therefore, the aim of the present study was to investigate how peri-implant strain distribution and load transfer between implant and bone are affected by the continuum assumption. We performed a computational study in which cancellous screws were inserted in continuum and discrete models of trabecular bone; axial loading was simulated. We found strong differences in bone-implant stiffness between the discrete and continuum bone model. They depended on bone density and applied boundary conditions. Furthermore, load transfer from the screw to the surrounding bone differed strongly between the continuum and discrete models, especially for low-density bone. Based on our findings we conclude that continuum bone models are of limited use for finite element analysis of peri-implant mechanical loading in trabecular bone when a precise quantification of peri-implant stresses and strains is required. Therefore, for the assessment and improvement of trabecular bone implants, finite element models which accurately represent trabecular microarchitecture should be used.