Finite Element Assessment of a Novel Patient-Specific Mandibular Implant for Severely Atrophic Ridge.

Purpose: Dental reconstruction for patients diagnosed with severe mandibular bone atrophy using common dental implants is a challenging process. In such cases, surgeons may encounter challenges such as insufficient available bone, soft tissue, damage to the inferior alveolar nerve, and even the risk of bone fracture. In this study, a new design concept of mandibular patient-specific implants for severely atrophic ridges followed by finite element evaluation was presented to investigate the mechanical functionality of the concept. Method: The implant is comprised of two modular parts including an inferior border cover and a horseshoe-shaped structure. This horseshoe segment fits into the cover and is then screwed to it using two screws on each side. A 1 mm deflection was applied to a reference point located between the two anterior posts to extract the resulting Von Mises stress distribution in each part and the reaction force on the reference point which corresponds to the chewing force that the patient must apply to deform the horseshoe. This 1 mm gap is a design consideration and critical distance that horseshoe contacts the gingiva and disturbs the alveolar nerve. Results: The results revealed that load was transmitted from the horseshoe to the cover, and there were no stress contours on the body of the mandible. However, stress concentration was observed in screw locations in the mandible, the amount of which was decreased by increasing the number of used screws. In horseshoe, stress concentration values were around 350 MPa, and the measured reaction force on the reference point was just under 200 N. Conclusion: The finite element analysis results showed that this concept would be functional as the minimum load would be transmitted to the mandibular ridge, and since the patients diagnosed with atrophic ridge are not able to apply load to an amount near 200 N, the horseshoe would not contact the gingiva. Also, it is concluded that increasing the number of bone screw fixations would decrease the risk of long-term screw loosening.

Biomechanical study of effect of tibial posteromedial defect depth and area on primary TKA implant stability.

BACKGROUND: Medial tibial defects are common in patients who underwent primary total knee arthroplasty for varus deformity. Previous clinical studies have categorized tibial defects according to the depth of the defects and recommended different ways of addressing them. This study aimed to perform a biomechanical FE analysis to investigate the role of depth and surface area of the medial tibial plateau defects in the stability of the tibial component in primary TKA implants. METHOD: Forty posteromedial tibial defect models with eight different depths (including 2, 4, 6, 8, 11, 13, 16, and 18 mm) and five different surface areas (including 10, 20, 30, 40, and 50% medial surface involvement) were used to create the FE models. Loads were applied to ellipses on tibial tray with 70-30% mediolateral distribution. The resulting relative motion of the bone and implant was measured to evaluate the tibial tray instability. RESULTS: For defects with less than 20% surface involvement, the amount of relative motion had a moderately increasing fashion; however, in more significant percentages of surface involvement of the medial tibial plateau, especially in 50%, the graphs revealed a nonlinear increasing pattern which means that the depth would affect the amount of relative motion only when defect area is large. CONCLUSION: In defects with less than 20% surface involvement approaches like graft and cement augmentation would suffice whereas it would be essential to consider a more appropriate method like stem or metal augments for defects with more than 20% of medial surface involvement as the instability increased nonlinearly.

Biomechanical study of using patient-specific diaphyseal femoral cone in revision total knee arthroplasty (rTKA).

Background: The primary objective of revision total knee surgery is to achieve solid bone fixation. Generally, this could be accomplished using sleeves and long stems, which require substantial remaining bone stock and may increase the risk of stem tip pain. An alternative approach involves the use of customized diaphyseal cones, which can preserve the integrity of the bone canal. This study evaluates the impact of employing femoral diaphyseal cones with various stem lengths on stress distribution and relative motion. Methods: CT scan data from five patients were used to generate the 3D model of the femur, cement, customized stems, and cones, along with assigning patient-specific material for each candidate's femur. Three different stem lengths, both with and without the customized cone, were assessed under three gait loading conditions to compare the resulting Von Mises stress distribution and relative motion. Results: Analysis indicated that the use of customized femoral cones moderately increases stress distribution values up to 30 % while significantly reducing relative motion at the femoral canal-cone interface by nearly 60 %. The presence of the cone did not significantly alter relative motion with varying stem lengths, although stem length variation without a cone substantially affected these values. Conclusion: Incorporating cones alongside stems enhances metaphyseal fixation, reduces stress shielding, potentially allowing for the use of shorter stems. Furthermore, cones promote osseointegration by minimizing relative motion, ultimately improving prosthetic stability.

Comparative finite element analysis of contact and stress distribution in tibiotalar articular cartilage: Healthy versus varus ankles.

Background: The distribution of forces within the ankle joint plays a crucial role in joint health and longevity. Loading disorders affecting the ankle joint can have significant detrimental effects on daily life and activity levels. This study aimed to enhance our understanding of the mechanical behavior of tibiotalar joint articular cartilages in the presence of varus deformity using finite element analysis (FEA) applied to patient-specific models. Methods: Two personalized ankle models, one healthy and another with varus deformity, were created based on CT scan images. Four static loading scenarios were simulated at the center of pressure (COP), coupled to the hindfoot complex. The contact area, contact pressure, and von Mises stress were computed for each cartilage. Results: It was found that the peak contact pressure increased by 54% in the ankle with varus deformity compared to the healthy ankle model. Furthermore, stress concentrations moving medially were observed, particularly beneath the medial malleolus, with an average peak contact pressure of 3.5 MPa and 4.7 MPa at the tibial and talar articular cartilages, respectively. Conclusion: Varus deformities in the ankle region have been consistently linked to elevated contact pressure, increasing the risk of thinning, degeneration, and eventual onset of osteoarthritis (OA), emphasizing the need for prompt interventions aimed at mitigating complications.

Ipsilateral Concurrent Knee Arthroplasty and Tibial Osteotomy with 3D-Printed Patient-Specific Instrumentation: A Case Report.

CASE: A 70-year-old woman presented with knee pain and instability and was diagnosed with advanced knee osteoarthritis and bifocal tibial deformities. The complexity of the case challenged our team to perform a significant sagittal correction (>60°) and restore her ability to walk independently. We performed ipsilateral total knee arthroplasty and anterior closed wedge tibial osteotomy using virtual planning and 3D-printed patient-specific instrumentation. CONCLUSION: Using 2 separate 3D-printed patient-specific cutting guides for this patient with a complex deformity and managing the whole planning process in close collaboration between the surgeons and engineers resulted in a satisfactory postoperative outcome, optimal implant positioning and leg alignment, and minimal soft-tissue damage.

Management of tibial nonunion and osteoarthritis using a 3D-printed titanium cone: A case report.

The use of customized 3D-printed structures has been gaining popularity in non-union management, as it allows for bypassing the defect while promoting osseointegration. Additionally, porous titanium implants minimize stress shielding due to their stiffness and elastic modulus being closer to that of bone. The interconnected channels increase the surface area and provide space for cell adhesion and proliferation. This study presents the case of a 62-year-old female patient with concomitant knee osteoarthritis recalcitrant aseptic atrophic nonunion in the tibial proximal metaphysis. Due to the small distance between the nonunion site and the joint line, nonunion treatment had to be included in the treatment plan, as it would result in a lack of mechanical stability of the tibial component, and techniques such as plating were not an option. A customized 3D-printed porous titanium cone was used to bypass the fracture site and support the stem used with the CCK prosthesis, allowing for simultaneous nonunion and osteoarthritis management.

Tablet Geometry Effect on the Drug Release Profile from a Hydrogel-Based Drug Delivery System.

In order to achieve the optimal level of effectiveness and safety of drugs, it is necessary to control the drug release rate. Therefore, it is important to discover the factors affecting release profile from a drug delivery system. Geometry is one of these effective factors for a tablet-shaped drug delivery system. In this study, an attempt has been made to answer a general question of how the geometry of a tablet can affect the drug release profile. For this purpose, the drug release process of theophylline from two hundred HPMC-based tablets, which are categorized into eight groups of common geometries in the production of oral tablets, was simulated using finite element analysis. The analysis of the results of these simulations was carried out using statistical methods including partial least squares regression and ANOVA tests. The results showed that it is possible to predict the drug release profile by knowing the geometry type and dimensions of a tablet without performing numerous dissolution tests. Another result was that, although in many previous studies the difference in the drug release profile from several tablets with different geometries was interpreted only by variables related to the surface, the results showed that regardless of the type of geometry and its dimensions, it is not possible to have an accurate prediction of the drug release profile. Also, the results showed that without any change in the dose of the drug and the ingredients of the tablet and only because of the difference in geometry type, the tablets significantly differ in release profile. This occurred in such a way that, for example, the release time of the entire drug mass from two tablets with the same mass and materials but different geometries can be different by about seven times.

Is a Complete Anatomical Fit of the Tomofix Plate Biomechanically Favorable? A Parametric Study Using the Finite Element Method.

Background: The opening wedge high tibial osteotomy (HTO) fixation using the Tomofix system is at the risk of mechanical failure due to unstable fixation, lateral hinge fracture, and hardware breakage. This study aimed to investigate the effect of the level of anatomical fit (LOF) of the plate on the failure mechanisms of fixation. Methods: A finite element model of the HTO with a correction angle of 12 degrees was developed. The LOF of the TomoFix plate was changed parametrically by altering the curvature of the plate in the sagittal plane. The effect of the LOF on the fixation performance was studied in terms of the factor of safety (FOS) against failure mechanisms. The FOSs were found by 1) dividing the actual stiffness of the plate-bone construct by the minimum allowable one for unstable fixation, 2) dividing the compressive strength of the cortical bone by the actual maximum pressure at the lateral hinge for the lateral hinge fracture, and 3) the Soderberg criterion for fatigue fracture of the plate and screws. Results: The increase of the LOF by applying a larger bent to the plate changed the fixation stiffness slightly. However, it reduced the lateral hinge pressure substantially (from 182 MPa to 71 MPa) and increased the maximum equivalent stresses in screws considerably (from 187 MPa to 258 MPa). Based on the FOS-LOF diagram, a gap smaller than 2.3 mm was safe, with the highest biomechanical performance associated with a 0.5 mm gap size. Conclusion: Although a high LOF is necessary for the Tomofix plate fixation to avoid mechanical failure, a gap size of 0.5mm is favored biomechanically over complete anatomical fit.

Influence of the angle between tibial and prosthesis mechanical axes on tibial bone remodeling in total knee arthroplasty.

Osteoarthritis of the knee is one of the most common diseases that affect the quality of life in the elderly population, and Total Knee Arthroplasty is considered the only real treatment for it, and as with any other surgery, a suboptimal technique may lead to an undesirable outcome. This paper aims to investigate the effects of the angle between mechanical axes of the tibia and the implant on the bone remodeling process. A 3D model was reconstructed using CT images, which was then used in an ABAQUS model with a USDFLD subroutine to simulate bone remodeling post TKA. The USDFLD subroutine compares the strain energy density from each increment to that of the previous increment to determine how the bone density will change. Simulation results suggest that when the prosthesis is inclined to one side, stress and density distribution increase, whereas stress and bone density decrease substantially on the opposite side. This decrease in bone density can be as much as 35% in the coronal plane. Sagittal malalignment results suggest that the effect would be relatively localized to the vicinity of the cutting plane. Results suggest uniform load distribution may be achieved when the two mechanical axes are kept parallel, which in turn can lead to decreased prosthesis loosening and bone fractures.

Investigation of the effect of the angle between femoral and prosthesis mechanical axes on bone remodeling of femur in total knee arthroplasty.

The bone remodeling is the process in which the bone adapts its structure to the variation of environmental loads. The joint might be broken or damaged as a result of aging or an accident. To remedy this situation, Total Knee Arthroplasty (TKA) and prosthesis implantation is recommended. The main goal of this research is to investigate the effects of femur implanting angle on the bone remodeling process after TKA in the Coronal, Sagittal and horizontal planes over seven years. First, the 3D CAD model from CT images is created then the bone behavior is simulated using a model with a USDFLD subroutine. The results show that as the implant rotates in one direction, the stress and density distribution increases in the same direction whereas the load and consequently the bone density decrease substantially in the opposite direction. Consequently, the bone density might even decrease 77 and 31 percent in the coronal and sagittal plane respectively, so in the total knee arthroplasty, the mechanical axes of prosthesis and femur should be parallel. The active bone which occurs as a result of mechanical axes of prosthesis and femur parallelism and consequently uniform load distribution, can protect the implant from prosthesis loosening and fracture.

Numerical study of patient-specific ankle-foot orthoses for drop foot patients using shape memory alloy.

Drop foot is a nerve-muscle disorder that affects the muscles that lift the foot. The two main side effects of drop foot are slapping/kicking the foot after heel strike (foot) and dragging the foot during the swing (toe drag). Treatment methods such as ankle-foot orthoses (AFO) have some biomechanical benefits, but are not applicable to all walking conditions and cannot mitigate significant gait complications. This study introduces the design of a passive AFO system, which combines an ordinary AFO and a shape memory alloy (SMA) element. OpenSim was used to simulate patients with muscle weakness and to calculate the torque needed to imitate normal ankle joint stiffness. The calculated torque was then reproduced for different levels of muscle weakness by the superelasticity of SMAs. The study showed that the normal joint stiffness profile for each patient with a certain level of muscle weakness can be restored by designing a patient-specific orthosis.

Aortic dissection simulation models for clinical support: fluid-structure interaction vs. rigid wall models.

BACKGROUND: The management and prognosis of aortic dissection (AD) is often challenging and the use of personalised computational models is being explored as a tool to improve clinical outcome. Including vessel wall motion in such simulations can provide more realistic and potentially accurate results, but requires significant additional computational resources, as well as expertise. With clinical translation as the final aim, trade-offs between complexity, speed and accuracy are inevitable. The present study explores whether modelling wall motion is worth the additional expense in the case of AD, by carrying out fluid-structure interaction (FSI) simulations based on a sample patient case. METHODS: Patient-specific anatomical details were extracted from computed tomography images to provide the fluid domain, from which the vessel wall was extrapolated. Two-way fluid-structure interaction simulations were performed, with coupled Windkessel boundary conditions and hyperelastic wall properties. The blood was modelled using the Carreau-Yasuda viscosity model and turbulence was accounted for via a shear stress transport model. A simulation without wall motion (rigid wall) was carried out for comparison purposes. RESULTS: The displacement of the vessel wall was comparable to reports from imaging studies in terms of intimal flap motion and contraction of the true lumen. Analysis of the haemodynamics around the proximal and distal false lumen in the FSI model showed complex flow structures caused by the expansion and contraction of the vessel wall. These flow patterns led to significantly different predictions of wall shear stress, particularly its oscillatory component, which were not captured by the rigid wall model. CONCLUSIONS: Through comparison with imaging data, the results of the present study indicate that the fluid-structure interaction methodology employed herein is appropriate for simulations of aortic dissection. Regions of high wall shear stress were not significantly altered by the wall motion, however, certain collocated regions of low and oscillatory wall shear stress which may be critical for disease progression were only identified in the FSI simulation. We conclude that, if patient-tailored simulations of aortic dissection are to be used as an interventional planning tool, then the additional complexity, expertise and computational expense required to model wall motion is indeed justified.