Probing the Influence of Hybrid Thread Design on Biomechanical Response of Dental Implants: Finite Element Study and Experimental Validation.

This study aimed to perform quantitative biomechanical analysis for probing the effect of varying thread shapes in an implant for improved primary stability in prosthodontics surgery. Dental implants were designed with square (SQR), buttress (BUT), and triangular (TRI) thread shapes or their combinations. Cone-beam computed tomography images of mandible molar zones in human subjects belonging to three age groups were used for virtual implantation of the designed implants, to quantify patient-specific peri-implant bone microstrain, using finite element analyses. The in silico analyses were carried out considering frictional contact to simulate immediate loading with a static masticatory force of 200 N. To validate computational biomechanics results, compression tests were performed on three-dimensional printed implants having the investigated thread architectures. Bone/implant contact areas were also quantitatively assessed. It was observed that, bone/implant contact was maximum for SQR implants followed by BUT and TRI implants. For all the cases, peak microstrain was recorded in the cervical cortical bone. The combination of different thread shapes in the middle or in the apical part (or both) was demonstrated to improve peri-implant microstrain, particularly for BUT and TRI. Considering 1500-2000 microstrain generates in the peri-implant bone during regular physiological functioning, BUT-SQR, BUT-TRI-SQR, TRI-SQR-BUT, SQR, and SQR-BUT-TRI design concepts were suitable for younger; BUT-TRI-SQR, BUT-SQR-TRI, TRI-SQR-BUT, SQR-BUT, SQR-TRI for middle-aged, and BUT-TRI-SQR, BUT-SQR-TRI, TRI-BUT-SQR, SQR, and SQR-TRI for the older group of human patients.

Biomechanical Analysis to Probe Role of Bone Condition and Subject Weight in Stiffness Customization of Femoral Stem for Improved Periprosthetic Biomechanical Response.

Stress shielding due to difference in stiffness of bone and implant material is one among the foremost causes of loosening and failure of load-bearing implants. Thus far, femoral geometry has been given priority for the customization of total hip joint replacement (THR) implant design. This study, for the first time, demonstrates the key role of bone condition and subject-weight on the customization of stiffness and design of the femoral stem. In particular, internal hollowness was incorporated to reduce the implant stiffness and such designed structure has been customized based on subject parameters, including bone condition and bodyweight. The primary aim was to tailor these parameters to achieve close to natural strain distribution at periprosthetic bone and to reduce interfacial bone loss over time. The maintenance of interfacial bone density over time has been studied here through analysis of bone remodeling (BR). For normal bodyweight, the highest hollowness exhibited clinically relevant biomechanical response, for all bone conditions. However, for heavier subjects, consideration of bone quality was found to be essential as higher hollowness induced bone failure in weaker bones and implant failure in stronger bones. Moreover, for stronger bone, thinner medial wall was found to reduce bone resorption over time on the proximo-lateral zone of stress shielding, while lateral thinning was found advantageous for weaker bones. The findings of this study are likely to facilitate designing of femoral stems for achieving better physiological outcomes and enhancement of the quality of life of patients undergoing THR surgery.

Effect of two-level pedicle-screw fixation with different rod materials on lumbar spine: A finite element study.

BACKGROUND: Pedicle-screw-rod fixation system is very popular surgical remedy for degenerative disc disease. It is important to observe load vs. spinal motion characteristic for better understanding of clinical problems and treatment of spinal instability associated with low-back pain. OBJECTIVE: The objective of this study is to understand the effect [range of motion (ROM) and intervertebral foramen height] of pedicle-screw fixation with three rod materials on lumbar spine under three physiological loading conditions. METHOD: A three-dimensional finite element (FE) model of lumbar to sacrum (L1-S) vertebrae with pedicle-screw-rod fixation at L3-L5 level is developed. Three rod materials [titanium alloy (Ti6Al4V), ultra-high molecular weight poly ethylene (UHMWPE) and poly-ether-ether-ketone (PEEK)] are used for two-level fixation and the FE models are simulated for axial rotation, lateral bending and flexion-extension under ±10 Nm moment and 500 N compressive load and compared with the intact (natural) model. RESULT & DISCUSSION: For axial rotation, lateral bending and flexion, ROM increased 2.8, 4.5 and 1.83 times respectively for UHMWPE, and 3.7, 7.2 and 2.15 times respectively for PEEK in comparison to Ti6Al4V. As ROM is 49, 29 and 31% of the intact model during axial rotation, lateral bending and flexion respectively, PEEK rod produced better motion flexibility than Ti6Al4V and UHMWPE rod. Foramen height increased insignificantly by 2.21% for the PEEK rod with respect to the intact spine during flexion. For the PEEK rod, maximum stress of 40 MPa during axial rotation is much below the yield stress of 98 MPa. CONCLUSION: Ti6Al4V pedicle-screw-rod fixation system highly restricted the ROM of the spine, which is improved by using UHMWPE and PEEK, having lower stiffness. The foramen height did not vary significantly for any implant materials. In terms of ROM and maximum stress, PEEK rod may be considered for a better implant design to get better ROM and thus mobility.

Prediction of micro-scale bone adaptation of human trabecular bone under different implanted conditions.

BACKGROUND AND OBJECTIVE: Different bone remodeling algorithms are used to predict bone adaptation and to understand how bones respond to the mechanical stimuli altered by implants. This paper introduces a novel micro-scale bone remodeling algorithm, which deviates from conventional methods by focusing on structure-based bone adaptation instead of density-based approaches. METHODS: The proposed model simulated cellular activities such as bone resorption, new bone formation, and maturation of newly formed bone. These activities were assumed to be triggered by mechanical stimuli. Model parameters were evaluated for the 3D geometries of trabecular bone from intact femur developed from micro computed tomography (CT) scan data. Two different hip implants, solid and porous were used, and two different bone remodeling methods were performed using the proposed and conventional methods. RESULTS: Results showed that micro CT scan-based finite element (FE) models accurately captured the microarchitecture and anisotropy of trabecular bone. The predicted bone resorption rate at the peri-prosthetic regions for the solid and porous implants was in the range of 17-27% and 4.5-7.3%, respectively, for a simulated period of four years. CONCLUSIONS: The results obtained from FE analysis strongly align with clinical findings, confirming the effectiveness of the proposed algorithm. By emphasizing the structural aspect of bone adaptation, the proposed algorithm brings a fresh perspective on bone adaptation at the peri-prosthetic bone. This method can help researchers and clinicians to improve implant designs for better clinical outcomes.

Design and manufacturing of patient-specific Ti6Al4V implants with inhomogeneous porosity.

Stress shielding remains a challenge in orthopaedic implants, including total hip arthroplasty. Recent development in printable porous implants offers improved patient-specific solutions by providing adequate stability and reducing stress shielding possibilities. This work presents an approach for designing patient-specific implants with inhomogeneous porosity. A novel group of orthotropic auxetic structures is introduced, and their mechanical properties are computed. These auxetic structure units were distributed at different locations on the implant along with optimized pore distribution to achieve optimum performance. A computer tomography (CT) based finite element (FE) model was used to evaluate the performance of the proposed implant. The optimized implant and the auxetic structures were manufactured using laser powder bed-based laser metal additive manufacturing. Validation was done by comparing FE results with experimentally measured directional stiffness and Poisson's ratio of the auxetic structures and strain on the optimized implant. The correlation coefficient for the strain values was within a range of 0.9633-0.9844. Stress shielding was mainly observed in Gruen zones 1, 2, 6, and 7. The average stress shielding on the solid implant model was 56%, reduced to 18% when the optimized implant was used. This significant reduction in stress shielding can decrease the risk of implant loosening and create an osseointegration-friendly mechanical environment on the surrounding bone. The proposed approach can be effectively applied to the design of other orthopaedic implants to minimize stress shielding.

Simulation and experimental investigation of the surgical needle deflection model during the rotational and steady insertion process.

Needle insertion is executed in numerous medical and brachytherapy events. Exact needle insertion into inhomogeneous soft biological tissue is of useful importance due to its significance in clinical diagnosis (especially percutaneous) and treatments. The surgical needles used in such processes can deflect during the percutaneous process. Needle deflecting which affects needle - soft tissue interface and needle controllability have a crucial role in establishment precision. In this paper, we have analyzed a mechanics-based model both rotational and non-rotational needle insertion, and studied the deflection phenomenon in both insertion cases, we validated it with a real-time nonlinear Dassault Systèmes® ABAQUS simulation model. For definite contact force, the maximum the contact stiffness was, the minimum it inserted, the cohesive surface model was used to investigate the needle insertion analysis, where the fracture point was defined by a failure strain and with the help of the in, the fully failed components would be removed. Using living tissue comparable PVA gel materials, the needle insertion force model is developed from insertion experimentations with the help of two different processes (rotational and non-rotational needle insertion). In a rotational needle, deflection is less than in a non-rotational needle. The preliminary insertion was observed in the rotational needle at 1.261 mm (experiment), and 1.538 mm (simulation), and for non-rotational needle insertion, the initial insertion was noticed at 1.756 mm (experiment) and 1.982 mm (simulation). The main aim of this study is to navigate the surgical needle in an accurate way to reduce the erroneousness for a clinical diagnosis like anesthesia, brachytherapy, biopsy, and modern microsurgery operation.

3D Printing of Ti-6Al-4V-Based Porous-Channel Dental Implants: Computational, Biomechanical, and Cytocompatibility Analyses.

Objective: Loosening of dental implants due to resorption of the surrounding bone is one of the challenging clinical complications in prosthetic dentistry. Generally, stiffness mismatch between an implant and its surrounding bone is one of the major factors. In order to prevent such clinical consequences, it is essential to develop implants with customized stiffness. The present study investigates the computational and experimental biomechanical responses together with cytocompatibility studies of three-dimensional (3D)-printed Ti-6Al-4V-based porous dental implants with varied stiffness properties. Methods: Additive manufacturing (direct metal laser sintering, DMLS) was utilized to create Ti-6Al-4V implants having distinct porosities and pore sizes (650 and 1000 µm), along with a nonporous (solid) implant. To validate the compression testing of the constructed implants and to probe their biomechanical response, finite element models were employed. The cytocompatibility of the implants was assessed using MG-63 cells, in vitro. Results: Both X-ray microcomputed tomography (µ-CT) and scanning electron microscopy (SEM) studies illustrated the ability of DMLS to produce implants with the designed porosities. Biomechanical analysis results revealed that the porous implants had less stiffness and were suitable for providing the appropriate peri-implant bone strain. Although all of the manufactured implants demonstrated cell adhesion and proliferation, the porous implants in particular supported better bone cell growth and extracellular matrix deposition. Conclusions: 3D-printed porous implants showed tunable stiffness properties with clinical translational potential.

Patient-specific femoral implant design using metamaterials for improving load transfer at proximal-lateral region of the femur.

Loading configuration of hip joint creates resultant bending effect on femoral implants. So, the lateral side of femoral implant which is under tension retracts from peri­implant bone due to positive Poisson's ratio. This retraction of implant leads to load shielding and gap opening in proximal-lateral region, thereby allowing entry of wear particle to implant-bone interface. Retraction of femoral implant can be avoided by introducing auxetic metamaterial to the retracting side. This allows the implant to push peri­implant bone under tensile condition by virtue of their auxetic (negative Poisson's ratio) nature. To develop such implants, a patient-specific conventional solid implant was first designed based on computed-tomography scan of a patient's femur. Two types of metamaterials (2D: type-1) and (3D: type-2) were employed to design femoral meta-implants. Type-1 and type-2 meta-implants were fabricated using metallic 3D printing method and mechanical compression testing was conducted. Three finite element (FE) models of the femur implanted with solid implant, type-1 meta-implant and type-2 meta-implant were developed and analysed under compression loading. Significant correlation (R2 = 0.9821 and R2 = 0.9977) was found between the experimental and FE predicted strains of the two meta-implants. In proximal-lateral region of the femur, an increase of 7.1% and 44.1% von-Mises strain was observed when implanted with type-1 and type-2 meta-implant over the solid implant. In this region, bone remodelling analysis revealed 2.5% bone resorption in case of solid implant. While bone apposition of 0.5% and 7.7% was observed in case of type-1 and type-2 meta-implants, respectively. The results of this study indicates that concept of introduction of metamaterial to the lateral side of femoral implant can prove to provide higher osseointegration-friendly environment in the proximal-lateral region of femur.

Investigating the morphology of the proximal femur of the Indian population towards designing more suitable THR implants.

There are considerable variations in the femoral geometry of populations across different geographical locations and ethnic groups. The osteological parameters of the proximal femur are very important for the design of suitably sized prostheses of total hip replacement (THR), especially for cementless implantation. Though total hip prostheses in different sizes are available from manufacturers, best-fit implants are often unavailable for Indian patients. To produce hip prostheses of suitable sizes and shapes for Indian patients, important osteological parameters of the proximal femur in the Indian population are needed. In this study, 100 computed tomography (CT) images of hip joints of members of the Indian population were collected, and 20 anatomical parameters of the proximal femur were analyzed. The mean values of these parameters were compared with those of the populations of a few other countries that were available from the literature. The parameter comparison was also performed between males and females in our subsample of the Indian population. Finally, values of the important parameters were grouped suitably for future design of standard sizes of THR implants for the Indian population. We found variations in the morphology of the proximal femur between the Indian population and that of other countries, which illustrates a need for standardizing THR implant sizes for the Indian population, especially for cementless implantation. The variations of a few important parameters of the proximal femur also occur between the male and female Indian populations. This study is likely to be a significant step toward designing suitably sized cementless THR implants for the Indian population.

Study of the surgical needle and biological soft tissue interaction phenomenon during insertion process for medical application: A Survey.

The insertion of the surgical needle in soft tissue has involved significant interest in the current time because of its purpose in minimally invasive surgery (MIS) and percutaneous events like biopsies, PCNL, and brachytherapy. This study represents a review of the existing condition of investigation on insertion of a surgical needle in biological living soft tissue material. As observes the issue from numerous phases, like, analysis of the cutting forces modeling (insertion), tissue material deformation, analysis of the needle deflection for the period of the needle insertion, and the robot-controlled insertion procedures. All analysis confirms that the total needle insertion force is the total of dissimilar forces spread sideways the shaft of the insertion needle for example cutting force, stiffness force, and frictional force. Various investigations have analyzed all these kinds of forces during the needle insertion process. The force data in several measures are applied for recognizing the biological tissue materials as the needle is penetrated or for path planning. The deflection of the needle during insertion and tissue material deformation is the main trouble for defined needle placing and efforts have been prepared to model them. Applying existing models numerous insertion methods are established that are discussed in this review.

Probing combinational influence of design variables on bone biomechanical response around dental implant-supported fixed prosthesis.

This study aimed to understand the effect of physiological and dental implant-related parameter variations on the osseointegration for an implant-supported fixed prosthesis. Eight design factors were considered (implant shape, diameter, and length; thread pitch, depth, and profile; cantilever [CL] length and implant-loading protocol). Total 36 implantation scenarios were simulated using finite element method based on Taguchi L36 orthogonal array. Three patient-specific bone conditions were also simulated by scaling the density and Young's modulus of a mandible sample to mimic weak, normal, and strong bones. Taguchi method was employed to determine the significance of each design factor in controlling the peri-implant cortical bone microstrain. For normal bone condition, CL length had the maximum contribution (28%) followed by implant diameter (18%), thread pitch (14%), implant length (8%), and thread profile (5%). For strong bone condition, CL and implant diameter had equal contribution (32%) followed by thread pitch (7%) and implant length (5%). For weak bone condition, implant diameter had the highest contribution (31%) followed by CL length (30%), thread pitch (11%) and implant length (8%). The presence of distal CL in dental framework was found to be the most influential design factor, which can cause high strain in the cervical cortical bone. It was seen that implant diameter had more effect compared to implant length toward peri-implant bone biomechanical response. Implant-loading time had no significant effect towards peri-implant bone biomechanical response, signifying immediate loading is possible with sufficient mechanical retention.

Design of patient specific basal dental implant using Finite Element method and Artificial Neural Network technique.

The bone conditions of mandibular bone vary from patient to patient, and as a result, a patient-specific dental implant needs to be designed. The basal dental implant is implanted in the cortical region of the bone since the top surface of the bone narrows down because of aging. Taguchi designs of experiments technique are used in which 25 optimum solid models of basal dental implants are modeled with variable geometrical parameters, viz. thread length, diameter, and pitch. In the solid models the implants are placed in the cortical part of the 3D models of cadaveric mandibles, that are prepared from CT data using image processing software. Patient-specific bone conditions are varied according to the strong, weak, and normal basal bone. A compressive force of 200 N is applied on the top surface of these implants and using finite element analysis software, the microstrain on the peri-implant bone ranges from 1000 to 4000 depending on the various bone conditions. According to the finite element data, it can be concluded that weak bone microstrain is comparatively high compared with normal and strong bone conditions. A surrogate artificial neural network model is prepared from the finite element analysis data. Surrogate model assisted genetic algorithm is used to find the optimum patient-specific basal dental implant for a better osseointegration-friendly mechanical environment.

Design of patient specific bone stiffness mimicking scaffold.

The difference in stiffness of a patient's bone and bone implant causes stress shielding. Thus, implants which match the stiffness of bone of the patient result in better bone growth and osseointegration. Variation in porosity is one of the methods to obtain implants with different stiffness values. This study proposes a novel method to design biomimetic bone graft implant based on computed tomography (CT) scan data, that creates similar pre- and post-implant mechanical environment on peri-implant bone. The design methodology is demonstrated by taking three different sections of human femur bone, greater trochanter, diaphysis and epicondyle, with two different implant materials, Ti-6Al-4V and Ti-Mg. Bones from these three sections were replaced with porous implants of effective stiffness of replaced bone, as would have been required after a resection surgery. Models were simulated with physiological loading condition using finite element (FE) method. Variation of maximum von Mises stress and average strain on peri-prosthetic bone were found to be in the range of -6% to 10.7% and -7% to -17.9% for porous implants and 26% to 50% and -36% to -59% for solid implant respectively compared to natural bone. The results revealed that the porous implants, which have been designed based on CT scan data, can effectively produce mechanical response at peri-implant bone, which is very close to pre-implanted condition. Following this methodology, more osseointegration friendly mechanical environment can be achieved at peri-implant bone for any anatomical location independent of implant materials.

Finite element and experimental analysis to select patient's bone condition specific porous dental implant, fabricated using additive manufacturing.

BACKGROUND: Differences in patients' bone conditions lead to variations in the bio-mechanical environment at the peri-implant bone after implantation. It is therefore imperative to design patient-specific dental implants with customized stiffness to minimize stress shielding and better osseointegration. METHOD: Nine Ti-6Al-4V implants with pore sizes of 500, 700, 900 µm and 10, 20, 30% porosity each and one non-porous (solid) implant were modelled for experimental and finite element (FE) analysis. Using computed tomography (CT) data of the mandible, five different bone conditions were considered by varying bone density. Implants were fabricated using additive manufacturing, and micro-CT analysis was performed for assessing accuracy of fabricated implants and further modelling for FE analyses. The FE results were also compared with experimental results. RESULTS: Under a 200 N static load, the average difference between the experimental and FE observations of deformation was 9.7%. The peri-implant bone micro-strain revealed statistically significant interactions between percentage porosity (%porosity) and bone condition, as well as between pore size and %porosity (p < 0.05). In contrast, no statistically significant interaction between pore size and bone condition (p > 0.05) was observed. Together, %porosity and bone conditions contributed about 45.22% of the overall peri-implant bone micro-strain. CONCLUSIONS: Considering 1500-2000 as the maximum generated peri-implant bone micro-strain during regular physiological functioning, implants with 700 and 900 µm pore size and 10% porosity were deemed suitable for a 'very weak' bone condition. Contrarily, implants with 900 µm pore size and 30% porosity generated the highest peri-implant bone micro-strain for a 'normal' bone condition. Overall, the study establishes the necessity for considering the patient's bone condition as an important factor for the design of dental implants.

Design of Patient Specific Spinal Implant (Pedicle Screw Fixation) using FE Analysis and Soft Computing Techniques.

BACKGROUND: This work uses genetic algorithm (GA) for optimum design of patient specific spinal implants (pedicle screw) with varying implant diameter and bone condition. The optimum pedicle screw fixation in terms of implant diameter is on the basis of minimum strain difference from intact (natural) to implantation at peri-prosthetic bone for the considered six different peri-implant positions. METHODS: This design problem is expressed as an optimization problem using the desirability function, where the data generated by finite element analysis is converted into an artificial neural network (ANN) model. The finite element model is generated from CT scan data. Thereafter all the ANN predictions of the microstrain in six positions are converted to unitless desirability value varying between 0 and 1, which is then combined to form the composite desirability. Maximization of the composite desirability is done using GA where composite desirability should be made to go up as close as possible to 1. If the composite desirability is 1, then all 'strain difference values in 6 positions' are 0. RESULTS: The optimum solutions obtained can easily be used for making patient-specific spinal implants.

Measurement of strain in the rod for lumbar pedicle screw fixation: An experimental and finite element study.

Spinal fusion with pedicle-screw-rod is being used widely for treating spinal deformities diseases. Several biomechanical studies on screw rod based implant failure through screw pullout, bending, screw breakage have been performed. But few studies are available regarding the effect of strain for breakage of rod. So, the purpose of the present study is to observe strain at the rod connected with the pedicle screw for different loading condition. The strain in stainless steel (SS) connecting rods for pedicle screw fixation were measured using strain gauge. In order to investigate the bio-mechanical response of lumbar spine with reference to strain in the rod, a simple experimental setup was developed using a specimen of L1-S spine segment. SS rods were used for pedicle screw implant on prototyped lumbar Spine. Prior to testing with pedicle screw, the lumbar spine specimen was also compared with FE results. The strain measured using strain gauges at L3-L4 level on SS rod were within a range of 85 to 310 microstrain under 6, 8, 10 Nm flexion and extension, and for L4-L5 level, these values were within a range of 95 to 440 microstrain. It was found that FE result was higher than the strain gauge result and the error varied between 10.5% to 33% with average error of 22.8%. However similar stain behavior was observed by the FE analysis. The proposed method, as well as the qualitative data, might be helpful for the researchers to understand biomechanical behavior of pedicle-screw implanted spine.

Application of the finite element method in orthopedic implant design.

The finite element method (FEM) was first introduced to the field of orthopedic biomechanics in the early 1970s to evaluate stresses in human bones. By the early 1980s, the method had become well established as a tool for basic research and design analysis. Since the late 1980s and early 1990s, FEM has also been used to study bone remodeling. Today, it is one of the most reliable simulation tools for evaluating wear, fatigue, crack propagation, and so forth, and is used in many types of preoperative testing. Since the introduction of FEM to orthopedic biomechanics, there have been rapid advances in computer processing speeds, the finite element and other numerical methods, understanding of mechanical properties of soft and hard tissues and their modeling, and image-processing techniques. In light of these advances, it is accepted today that FEM will continue to contribute significantly to further progress in the design and development of orthopedic implants, as well as in the understanding of other complex systems of the human body. In the following article, different main application areas of finite element simulation will be reviewed including total hip joint arthroplasty, followed by the knee, spine, shoulder, and elbow, respectively.

Proposed frequencies of a vibrator used for implant retrieval at the time of hip joint revision surgery.

The number and the rate of success of hip implantation surgeries have increased significantly during last thirty years, not only in the USA, but also throughout the world. It has been reported that the failure rates of implanted hip joints are less than 8% after 10 years, and less than 20% after twenty years. Failures occur directly or indirectly due to wear, stress shielding and infection. Revision surgery is needed for those failed implant replacements. In the future, as the elderly population increases, the frequency of this type of revision surgery will also increase. At the time of revision surgery, removal of the existing cemented femoral implant can be a problem for the surgeon. Use of a vibrator for loosening of the existing cement layer between the bone and the implant may be a helpful solution. In this study, we investigated the optimum resonance frequencies of such a vibrator that might be used to loosen the cement layer easily and efficiently. Natural frequencies of different-sized implants and of different materials were determined. For harmonic analysis, CT scan data of a femur was processed in the image processing software MIMICS. Then the outline of the total hip was modeled and was analyzed by the finite element software ANSYS. The required portion of the femoral part was edited, implant and cement layer were introduced in that model, and elements were generated in that FEA software. Then elements of the femoral part, except the cement layer and the implant, were sent to MIMICS software again for assignment of different Youngs modulus of each element, which are proportionate to their densities. Then the elements were brought back to the FEA software. The harmonic analysis was performed for the total model in the FEA software ANSYS. For that particular boundary condition, the first three natural frequencies of the three types of implant sizes and materials varied by a maximum of 7-8%. Results of the numerical harmonic analysis showed that at the bone-cement interface, the resonance frequencies were at the ranges of 4 to 6 Hz, 26 to 29 Hz, and 43 to 49 Hz. The vibration response was similar for three cement-bone interface locations examined. This suggests that a vibrator that will produce a resonance frequency response may cause cracks in the bone-cement mantle and thus may facilitate the removal of the failed femoral component. Retrieval of hip implant may be easier using a vibrator in that band of frequencies with a moderate amplitude. The magnitude of those frequencies may not differ significantly from implant to implant as the natural frequencies of different types of implant, for that particular boundary conditions, are within a close range.

Biomechanical Analysis of CT Scan-based Hip Prosthesis With an Optimal Hip Ball by Using Finite Element Analysis.

Bone loss around the femoral stems during the insertion of a standard prosthesis is a major problem in hip arthroplasty. Moreover, long periods of use of the standard metallic prosthesis often lead to revision surgery because of disuse osteoporosis (stress shielding). The main factor behind this problem is the material-stiffness mismatch of the bone and implant, with the latter consisting of metals such as stainless steel, Co-Cr-Mo alloy, or Ti6Al4V alloy. Our study aimed to decrease the factor of geometric mismatch by designing and making customized hip prostheses from computed tomography scan data and finite element analysis. Therefore, the inner medullar cavity of the femur would match exactly with the prosthesis. Our results showed that the desired stress-strain values were close to the physiological level. We observed that the maximum Von Mises stresses for the bone and implant were 41.8 MPa and 197 MPa, respectively. An optimization analysis of the taper angle of the prosthesis hip ball for fixation with the stem has also been performed, in which the angle was found to be approximately 2 deg. The taper angle plays an important role in load transfer and safe levels of stress-strain using various ball materials.

Computational intelligence based design of implant for varying bone conditions.

The objective is to make the strain deviation before and after implantation adjacent to the femoral implant as close as possible to zero. Genetic algorithm is applied for this optimization of strain deviation, measured in eight separate positions. The concept of composite desirability is introduced in such a way that if the microstrain deviation values for all eight cases are 0, then the composite desirability is 1. Artificial neural network (ANN) models are developed to capture the correlation of the microstrain in femur implants using the data generated through finite element simulation. Then, the ANN model is used as the surrogate model, which in combination with the desirability function serves as the objective function for optimization. The optimum achievable deviation was found to vary with the bone condition. The optimum implant geometry varied for different bone condition, and the findings act as guideline for designing patient-specific implant.