Sinan Müftü

Mechanics of the tapered interference fit in dental implants.

In evaluation of the long-term success of a dental implant, the reliability and the stability of the implant-abutment interface plays a great role. Tapered interference fits provide a reliable connection method between the abutment and the implant. In this work, the mechanics of the tapered interference fits were analyzed using a closed-form formula and the finite element (FE) method. An analytical solution, which is used to predict the contact pressure in a straight interference, was modified to predict the contact pressure in the tapered implant-abutment interface. Elastic-plastic FE analysis was used to simulate the implant and abutment material behavior. The validity and the applicability of the analytical solution were investigated by comparisons with the FE model for a range of problem parameters. It was shown that the analytical solution could be used to determine the pull-out force and loosening-torque with 5-10% error. Detailed analysis of the stress distribution due to tapered interference fit, in a commercially available, abutment-implant system was carried out. This analysis shows that plastic deformation in the implant limits the increase in the pull-out force that would have been otherwise predicted by higher interference values.

Predictions of bone remodeling around dental implant systems

This study presents the implementation of a mathematical bone remodeling algorithm to bone adaptation in the premolar area of the mandible around various dental implant systems, and thus sheds a new perspective to the complex interactions in dental implant mechanics. A two-dimensional, plane strain model of the bone was built from a CT-scan. The effect of implant contour on internal bone remodeling was investigated by considering four dental implant systems with contours similar to commercially available ones and another four with cylindrical and conical cross-sections. The remodeling algorithm predicts non-homogeneous density/elastic modulus distribution; and, implant contour has some effect on how this is distributed. Bone density is predicted to increase on the tips of the threads of the implants, but to decrease inside the grooves. Threadless implants favor to develop a softer bone around their periphery, compared to implant systems that have threads. The overall contour (dimensions and the shape) of an implant affect the bone density redistribution, but the differences between different implant systems are relatively small.

Combined effects of implant insertion depth and alveolar bone quality on periimplant bone strain induced by a wide-diameter, short implant and a narrow-diameter, long implant

STATEMENT OF PROBLEM Strain levels in periimplant bone are affected by implant dimensions, bone quality, and implant insertion depth, resulting in different bone maintenance characteristics. PURPOSE The purpose of this study was to evaluate the biomechanical response of the jaw bone to a wide-diameter, short (WDS) implant, and a narrow-diameter, long (NDL) implant for various simulated clinical scenarios. MATERIAL AND METHODS The finite element method was used to evaluate periimplant bone strain distribution for 5 × 6-mm (WDS) and 3.5 × 10.7-mm (NDL) implants. A 3-dimensional segment of the mandible was constructed from a computerized tomography image of the premolar region. Occlusal force was simulated by applying a 100-N oblique load on the abutment. Bone strain distributions for 5 different implant insertion depths and 2 different levels of alveolar bone quality were evaluated. RESULTS For an NDL implant, approximately 60% to 80% of the bone volume surrounding the implant was subjected to 200-1000 μstrain (μɛ), and 15% to 35% was subjected to 1000-3000 μɛ, regardless of the alveolar bone quality. For a WDS implant, the bone volume subjected to 1000-3000 μɛ increased, and the bone volume subjected to 200-1000 μɛ decreased in lower quality alveolar bone. For both implant types, bone volume experiencing strain levels less than 200 μɛ, and/or greater than 3000 μɛ, was predicted to be relatively small. CONCLUSIONS In general, the thread design promoted relatively high strain around the thread tips, and the bone inside grooves was less strained. A more even and higher strain distribution in the periimplant bone was generated by the WDS implant as compared to the NDL implant. Regardless of the implant dimensions and simulated clinical scenarios, the development of high strain in the alveolar region was inevitable. Strain levels in periimplant bone were reduced as the insertion depth of the implant was increased.

Load transfer along the bone–dental implant interface

In this paper the variation of normal and shear stresses along a path defined on the bone-dental implant interface is investigated. In particular, the effects of implant diameter, collar length and slope, body length, and the effects of four different types of external threads on the interfacial stress distribution are studied. The geometry of the bone is digitized from a CT scan of a mandibular incisor and the surrounding bone. The bone and the implant are assumed to be perfectly bonded. The finite element method with 2D plane strain assumption is used to compute interfacial stresses. Highest continuous interfacial stresses are encountered in the region where the implant collar engages the cortical region, and near the apex of the implant in the subcortical region. Stress concentrations in the interfacial stresses occur near the geometric discontinuities on the implant contour, and jumps in stress values occur where the elastic modulus of the bone transitions between the cortical and trabecular bone values. Among the six contour parameters, the slope and the length of the implant collar, and the implant diameter influence the interfacial stress levels the most, and the effects of changing these parameters are significantly noticed only in the cortical bone (alveolar ridge) area. External threads cause significant stress concentrations in interfacial stresses in otherwise smoothly varying regions. This work shows that the presence of external threads could cause significant variations in both normal and shear stresses along the bone-implant interface, but not reduction in shear stress as previously thought.

A Material Removal Model for CMP Based on the Contact Mechanics of Pad, Abrasives, and Wafer

Applied pressure in chemical mechanical polishing CMP is shared by the two-body pad–wafer and the three-body pad–abrasive– wafer contacts. The fraction of applied pressure transferred through the particle contacts is a significant factor as most of the material removal is due to abrasive particles trapped in the pad–wafer interface. In this work, the contact of a rough, deformable pad and a smooth, rigid wafer with rigid particles in the contact interface is investigated by using contact mechanics and finite element modeling. The interactions between the pad, wafer, and abrasive particles are modeled at different scales of contact, starting from the particle–pad level and gradually expanding to the multiasperity contact of the pad and the wafer. Wear rate due to each abrasive particle is calculated based on the wafer–abrasive particle contact force and by considering adhesive and abrasive wear mechanisms. A thin passivated layer on the wafer surface is modeled to consider the effect of chemical reactions between slurry and wafer. Good agreement between the model and the experimental literature is found for the relationships between material removal rate and applied pressure, pad elastic modulus and porosity, particle size and concentration, and pad roughness and wafer hardness. Chemical mechanical polishing CMP is a polishing method commonly used in the manufacturing of wafer-based integrated circuits ICs. Since CMP was first introduced to the IC manufacturing in the mid-1980s by IBM, CMP became a key technology in generating planar surfaces for several semiconductor manufacturing processes. 1 CMP operation involves forcing a rotating wafer attached to a wafer carrier against a rotating polishing pad. The polishing pad is covered with liquid slurry, which contains abrasive particles. The chemical reactions between the CMP slurry and the wafer are the primary driver for preparing the surface for polishing. The chemical composition of the surface is modified by the chemically reactive slurry to favor higher wear rates. In addition to the chemical interactions, the pad–wafer interface experiences the effects of contact and lubrication. Three-body contact due to the abrasive particles caught between the pad and the wafer and two-body contact between the pad and the wafer provide the necessary physical force to remove the material from the wafer surface. While solidbody contact is taking place, the pad–wafer interface experiences the lubricating effect of the slurry flow. The abrasive particles used in conventional polishing techniques are 2.5–3 times harder than the workpiece material. These techniques cause scratches and pitting on the surface and cracks in the subsurface of the workpiece. The passivation of the surface layer of the wafer workpiece is important to achieve smooth and planarized surfaces without any surface and subsurface defects. 2 For this reason, an effective CMP process should provide a balance between the chemical and mechanical effects. The Preston equation is widely used to calculate the material removal rate MRR, which is given as follows 3 RR = kPPoVr 1 where RR is the removal rate with units of material depth/time, kP is the Preston constant, Po is the average push-down pressure, and Vr is the relative sliding velocity of the surfaces. The Preston equation indicates a linear variation in MRR with respect to applied pressure and relative velocity. There are various experimental studies indicating both linear and nonlinear applied pressure Po relationships. 4-10 Also, kP is typically determined experimentally, and it conveniently includes the effects of other parameters that influence the outcome of polishing. In this paper, we present a mechanistic model of the MRR in CMP. A hierarchical model of the particle–wafer–pad interactions, presented recently, 11 was combined with abrasive and adhesive wear models to obtain a wafer-level MRR model based on particle-level interactions. The predictions of the model were compared with published experimental data. Material removal models were developed by considering a contact regime in the pad–wafer interface. 2,12-14

Mechanical and electrical evaluation of parylene-C encapsulated carbon nanotube networks on a flexible substrate

Carbon nanotube networks are an emerging conductive nanomaterial with applications including thin film transistors, interconnects, and sensors. In this letter, we demonstrate the fabrication of single-walled carbon nanotube (SWNT) networks on a flexible polymer substrate and then provide encapsulation utilizing a thin parylene-C layer. The encapsulated SWNT network was subjected to tensile tests while its electrical resistance was monitored. Tests showed a linear-elastic response up to a strain value of 2.8% and nearly linear change in electrical resistance in the 0%–2% strain range. The networks’ electrical resistance was monitored during load-unload tests of up to 100 cycles and was hysteresis-free.

Improvements to a scale-dependent model for contact and friction

In this investigation three effects are included in a recently developed scale-dependent multi-asperity model of elastic contact and friction. First, a Weibull distribution of asperity heights is used, which allows the skew and kurtosis to be varied, but not independently of each other. Second, the effect of non-constant radii of curvature of the asperity summits, with the curvature varying with asperity height, is examined. Finally, the influence of noncontacting asperities on the normal force and hence on contact and friction is included. It is noted that the contact and friction model used (Adams et al 2003 ASME J. Tribol. 125 700–8) includes the effects of adhesion and scale-dependent friction. It is demonstrated that positive/negative skew decreases/increases both the friction coefficient and its dependence on the normal load. The results also indicate that for radii of curvature that increase/decrease with height, the friction coefficient increases/decreases as does its dependence on load.

Simulation of peri-implant bone healing due to immediate loading in dental implant treatments.

The goal of this work was to investigate the role of immediate loading on the peri-implant bone healing in dental implant treatments. A mechano-regulatory tissue differentiation model that takes into account the stimuli through the solid and the fluid components of the healing tissue, and the diffusion of pluripotent stem cells into the healing callus was used. A two-dimensional axisymmetric model consisting of a dental implant, the healing callus tissue and the host bone tissue was constructed for the finite element analysis. Poroelastic material properties were assigned to the healing callus and the bone tissue. The effects of micro-motion, healing callus size, and implant thread design on the length of the bone-to-implant contact (BIC) and the bone volume (BV) formed in the healing callus were investigated. In general, the analysis predicted formation of a continuous layer of soft tissue along the faces of the implant which are parallel to the loading direction. This was predicted to be correlated with the high levels of distortional strain transferred through the solid component of the stimulus. It was also predicted that the external threads on the implant, redistribute the interfacial load, thus help reduce the high distortional stimulus and also help the cells to differentiate to bone tissue. In addition, the region underneath the implant apex was predicted to experience high fluid stimulus that results in the development of soft tissue. The relationship between the variables considered in this study and the outcome measures, BV and BIC, was found to be highly nonlinear. A three-way analysis of variance (ANOVA) of the results was conducted and it showed that micro-motion presents the largest hindrance to bone formation during healing.

The Self-Acting, Subambient Foil Bearing in High Speed, Contact Tape Recording with a Flat Head

The mechanics and tribology of a flat head for high-speed, contact tape recording is presented. Experiments performed on a “row-bar” of thin-film disk heads where the tape is wrapped only on the edge opposite to the heads showed very stable contact for a wide range of tape speeds and very low wear. A model of the interface showed that a self-acting, subambient air bearing forms near the leading wrapped corner. This suction is caused by the expansion of air into the diverging gap on the upstream side of the head-tape interface, which is unique to this wrap geometry, and it is responsible for the stability and low contact pressures. A bidirectional version of a flat head geometry is analyzed via modeling and suggestions are made for that design. This work also showed strong evidence of a threshold of contact pressure below which wear becomes negligible.

Frictional Characteristics of Ultra–Thin Polytetrafluoroethylene (PTFE) Films Deposited by Hot Filament–Chemical Vapor Deposition (HFCVD)

Frictional characteristics of polytetrafluoroethylene (PTFE) films deposited by a hot filament chemical vapor deposition (HFCVD) method on glass substrates was investigated. A universal micro-tribotester (UMT) was used in the ball-on-flat configuration to examine the effect of coefficient of friction (COF). The effects of normal force (2.5–15 N), sliding speed (1.4 × 10−3 to 14 mm/s) and film thickness (0.3, 1, 5, 10 μm) on the frictional behavior of the PTFE film were tested. Optical microscopy was used to examine the wear tracks of the PTFE films. In addition, durability of the PTFE films was examined by monitoring the change of COF in ball-on-disk tests. The COF values of the tested PTFE films were found to be in the range 0.03–0.18 depending on the measurement conditions. In particular, PTFE exhibits a low COF at slow speeds and high normal forces compared to a high COF at high speeds and small normal forces. Modified Hertzian contact equations and finite element analysis were used to analyze the indentation process. Comparison with the width of the experimental wear tracks indicates that the PTFE layer has a small contribution in carrying the normal load yet provides good protection against tangential forces.