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Background Femoral bone remodeling in response to stress shielding induces periprosthetic bone loss. Computerized finite element analysis (FEA) is employed to demonstrate differences in initial stress distribution. However, FEA is often performed without considering the precise sites at which the stem was fixed. We determined whether FEA reflects mid-term radiological examination exactly as predicted following long-term stress shielding. Methods Femur–stem fixation sites were evaluated radiologically according to the location of spot welds in two anatomical cementless stem designs. Based on mid-term radiological results, four femur–stem bonding site conditions were defined as: (Condition A) no bonding; (Condition B) bonding within the 10 mm area proximal to the distal border of the porous area; (Condition C) bonding of the entire porous area; and (Condition D) bonding of the entire femoral stem, prior to conducting FEA analysis. Furthermore, we radiographically evaluated mid- and long-term stress shielding, and measured bone mineral density of the femur 10 years after total hip arthroplasty. Results Spot welds appeared frequently around the border between the porous and smooth areas. FEA showed that, based on mid-term radiological evaluation, von Mises stress was reduced in condition B in the area proximal to the femur–stem bonding sites for both stem designs compared with condition A (no bonding). Conversely, von Mises stress at all areas of the femur–stem bonding sites in conditions C and D was higher than that in condition A. With respect to stress shielding progression, there was no significant difference between the two types of stem designs. However, stress shielding progressed and was significantly higher in the presence of spot welds ( p  = 0.001). In both stem designs, bone mineral density in zone VII was significantly lower than that in the contralateral hips. Conclusions These results indicate that FEA based on mid-term radiological evaluation may be helpful to predict the influence of long-term stress shielding more precisely.

Introduction Intraoperative navigation during liver resection remains difficult and requires high radiologic skills because liver anatomy is complex and patient-specific. Augmented reality (AR) during open liver surgery could be helpful to guide hepatectomies and optimize resection margins but faces many challenges when large parenchymal deformations take place. We aimed to experiment a new vision-based AR to assess its clinical feasibility and anatomical accuracy. Patients and Methods Based on preoperative CT scan 3-D segmentations, we applied a non-rigid registration method, integrating a physics-based elastic model of the liver, computed in real time using an efficient finite element method. To fit the actual deformations, the model was driven by data provided by a single RGB-D camera. Five livers were considered in this experiment. In vivo AR was performed during hepatectomy ( n  = 4), with manual handling of the livers resulting in large realistic deformations. Ex vivo experiment ( n  = 1) consisted in repeated CT scans of explanted whole organ carrying internal metallic landmarks, in fixed deformations, and allowed us to analyze our estimated deformations and quantify spatial errors. Results In vivo AR tests were successfully achieved in all patients with a fast and agile setup installation (< 10 min) and real-time overlay of the virtual anatomy onto the surgical field displayed on an external screen. In addition, an ex vivo quantification demonstrated a 7.9 mm root mean square error for the registration of internal landmarks. Conclusion These first experiments of a markerless AR provided promising results, requiring very little equipment and setup time, yet providing real-time AR with satisfactory 3D accuracy. These results must be confirmed in a larger prospective study to definitively assess the impact of such minimally invasive technology on pathological margins and oncological outcomes.

En el presente trabajo se formula un modelo basado en el método de los elementos finitos, con el propósito de medir la presión facial generada por las mascarillas de uso cotidiano. El análisis incluye la determinación computacional de la presión generada por la mascarilla sobre dos formas de cabeza que contemplan las dimensiones antropométricas de la población colombiana entre 20 y 59 años, una representa el sexo masculino y la otra el sexo femenino. El modelo de la cabeza se divide en cinco partes (dos mejillas, la frente, el mentón y posterior de la cabeza), algunas de ellas contemplan capas de piel, musculo, tejido graso y hueso, acorde a la anatomía de la cabeza humana. La mascarilla está compuesta por tres capas de materiales diferentes, un clip metálico y dos bandas elásticas que permiten ajustar la mascarilla a la cara. El proceso de simulación consiste en colocar la mascarilla totalmente centrada a la cara y estirar las bandas elásticas hasta ubicarlas en la parte posterior de las orejas, de esta forma se genera el contacto facial entre mascarilla y cabeza. Los resultados obtenidos indican que los valores máximos de presión se encuentran concentrados en cinco puntos específicos de la cabeza.

Detailed cervical spine models are necessary to better understand cervical spine response to loading, improve our understanding of injury mechanisms, and specifically for predicting occupant response and injury in auto crash scenarios. The focus of this study was to develop a C4–C5 finite element model with accurate representations of each tissue within the segment. This model incorporates more than double the number of elements of existing models, required for accurate prediction of response. The most advanced material data available were then incorporated using appropriate nonlinear constitutive models to provide accurate predictions of response at physiological levels of loading. This tissue-scale segment model was validated against a wide variety of experimental data including different modes of loading (axial rotation, flexion, extension, lateral bending, and translation), and different load levels. In general, the predicted response of the model was within the single standard deviation response corridors for both low and high load levels. Importantly, this model demonstrates that appropriate refinement of the finite element mesh, representation at the tissue level, and sufficiently detailed material properties and constitutive models provide excellent response predictions without calibration of the model to experimental data. Load sharing between the disc, ligaments, and facet joints was investigated for various modes of loading, and the dominant load-bearing structure was found to correlate with typical anatomical injury sites for these modes of loading. The C4–C5 model forms the basis for the development of a full cervical spine model. Future studies will focus on tissue-level injury prediction and dynamic response.

The mechanical characteristics of ultrastructures of rat sciatic nerves were investigated through animal experiments and finite element analyses. A custom-designed dynamic testing apparatus was used to conduct in vitro transverse compression experiments on the nerves. The optical coherence tomography (OCT) was utilized to record the cross-sectional images of nerve during the dynamic testing. Two-dimensional finite element models of the nerves were built based on their OCT images. A hyper-viscoelastic model was employed to describe the elastic and stress relaxation response of each ultrastructure of the nerve, namely the endoneurium, the perineurium and the epineurium. The first-order Ogden model was employed to describe the elasticity of each ultrastructure and a generalized Maxwell model for the relaxation. The inverse finite element analysis was used to estimate the material parameters of the ultrastructures. The results show the instantaneous shear modulus of the ultrastructures in decreasing order is perineurium, endoneurium, and epineurium. The FE model combined with the first-order Ogden model and the second-order Prony series is good enough for describing the compress-and-hold response of the nerve ultrastructures. The integration of OCT and the nonlinear finite element modeling may be applicable to study the viscoelasticity of peripheral nerve down to the ultrastructural level.

Lucrarea prezintă rezultatele analizei numerice obținute în urma utilizării metodei elementului finit (MEF), efectuată pe un corp cilindric cu geometrie variabilă, solicitat la o presiune interioară, p = 3 MPa. Lucrarea de faţă ia în discuţie cazul unei treceri fără racordare de la o grosime a peretelui la una mai redusă, caz în care se investighează valorile deformaţiilor liniare specifice şi tensiunile corespunzătoare. În acest scop, a fost realizat un model geometric construit în Inventor Professional și apoi importat în Nastran In-CAD pentru calculul MEF al obiectului.