Estimation of the in-plane ultimate stress of lamellar tissue as a function of bone mineral density and osteocyte lacunae porosity.

BACKGROUND AND OBJECTIVE: Detailed finite element models based on medical images (µ-CT) are commonly used to analyze the mechanical behavior of bone at microscale. In order to simulate the tissue failure onset, isotropic failure criteria of lamellar tissue are often used, despite its non-isotropic and heterogeneous nature. The main goal of the present work is to estimate the in-plane ultimate stress of lamellar bone, considering the influence of mineral content and the porosity due to the osteocyte lacunae concentration. METHODS: To this aim, a representative volume cell of lamellar tissue is modeled numerically, including: (1) non-isotropic elastic properties of tissue as a function of the bone mineral density and (2) explicit modeling of the osteocyte lacunae, considering the range of porosity content, size and orientation of ellipsoid-shaped lacunae. Firstly, the element size for the finite element models have been defined from a preliminary convergence analysis. Bounds on the ultimate stress of non-porous lamellar tissue are estimated for two values of bone mineral density, considering the results of tensile and compressive tests of wet osteons from the literature. Subsequently, the ultimate stress of lamellar tissue considering several values of micro-porosity is addressed. RESULTS: Results obtained in this work show that the strength of lamellar bone decreases exponentially with the increase of lacunae porosity concentration. Ultimate stress of non-porous tissue (p=0%) increases with high mineral content, reaching a value of S¯transc=355.40±39.80 MPa for compression in the transversal direction of the fiber bundles, being BMD=1.246g/cm3. The mean value for the longitudinal to transverse strength ratio evaluated for porosity p=0%,1% and 5% and a mineral content BMD=1.2g/cm3, is 2.47:1 for tension and 1.55:1 for compression. These values are in agreement with literature. CONCLUSIONS: Osteocyte lacunae act as stress concentrators, acting as potential stimulus for the bone regeneration process. A novel micromechanical model for the in-plane ultimate stress of lamellar tissue as a function of mineral content and lacunae concentration is presented. Additional considerations about the intralamellar shear stress evolution are also given.

Numerical analysis of the influence of triply periodic minimal surface structures morphometry on the mechanical response.

BACKGROUND AND OBJECTIVE: Design of bone scaffolds requires a combination of material and geometry to fulfil requirements of mechanical properties, porosity and pore size. Triply Periodic Minimal Surface (TPMS) structures have gained attention due to their similarities to cancellous bone. In this work, we aim at exploring relationships between morphometry and mechanical properties for TPMS configurations. METHODS: Eight TPMS structures are defined considering six porosity levels and their morphometry is characterized. The stiffness matrix of each structure is assessed and related to morphometry through a statistical analysis. RESULTS: An orthotropic mechanical behavior has been derived from the numerical homogenization. Properties decay exponentially for decreasing volume fraction. Through volume fraction variation, TPMS mechanical properties can be selected to match bone properties in a range of 0.2% to 70% of the bulk material properties. CONCLUSIONS: The comparison between cancellous bone and TPMS morphometry, considering a unit cell size of 1.5 mm, reveals that the configurations analyzed in this work match the requirements of volume fraction, mean thickness and pore size. However, the TPMS studied in this work differ from cancellous bone anisotropy. The results in this paper provide a framework to select the proper TPMS configuration and its geometry for patient-specific applications.

Numerical modelling of cancellous bone damage using an orthotropic failure criterion and tissue elastic properties as a function of the mineral content and microporosity.

BACKGROUND AND OBJECTIVE: Elastic and strength properties of lamellar tissue are essential to analyze the mechanical behaviour of bone at the meso- or macro-scale. Although many efforts have been made to model the architecture of cancellous bone, in general, isotropic elastic constants are assumed for tissue modelling, neglecting its non-isotropic behaviour. Therefore, isotropic damage laws are often used to estimate the bone failure. The main goals of this work are: (1) to present a new model for the estimation of the elastic properties of lamellar tissue which includes the bone mineral density (BMD) and the microporosity, (2) to address the numerical modelling of cancellous bone damage using an orthotropic failure criterion and a discrete damage mechanics analysis, including the novel approach for the tissue elastic properties aforementioned. METHODS: Numerical homogenization has been used to estimate the elastic properties of lamellar bone considering BMD and microporosity. Microcomputed Tomography (µ-CT) scans have been performed to obtain the micro-finite element (µ-FE) model of cancellous bone from a vertebra of swine. In this model, lamellar tissue is orientated by considering a unidirectional layer pattern being the mineralized collagen fibrils aligned with the most representative geometrical feature of the trabeculae network. We have considered the Hashin's failure criterion and the Material Property Degradation (MPDG) method for simulating the onset and evolution of bone damage. RESULTS: The terms of the stiffness matrix for lamellar tissue are derived as functions of the BMD and microporosity at tissue scale. Results obtained for the apparent yield strain values agree with experimental values found in the literature. The influence of the damage parameters on the bone mechanics behaviour is also presented. CONCLUSIONS: Stiffness matrix of lamellar tissue depends on both BMD and microporosity. The new approach presented in this work enables to analyze the influence of the BMD and porosity on the mechanical response of bone. Lamellar tissue orientation has to be considered in the mechanical analysis of the cancellous bone. An orthotropic failure criterion can be used to analyze the bone failure onset instead of isotropic criteria. The elastic property degradation method is an efficient procedure to analyze the failure propagation in a 3D numerical model.

Open cell polyurethane foam compression failure characterization and its relationship to morphometry.

Open cell polyurethane foams are often used as cancellous bone surrogates because of their similarities in morphology and mechanical response. In this work, open cell polyurethane foams of three different densities are characterized from morphometric and mechanical perspectives. The analysis of micro-computed tomography images has revealed that the high density foams present the greatest inhomogeneities. Those inhomogeneities promoted the failure location. We have used the finite element models as a tool to estimate elastic and failure properties that can be used in numerical modeling. Furthermore, we have assessed the anisotropic mechanical response of the foams, whose differences are related to the morphometric inhomogeneities. We found significant relationships between morphometry and the elastic and failure response. The detailed information about morphometry, elastic constants and strength limits provided in this work can be of interest to researchers and practitioners that often use these polyurethane foams in orthopedic implants and cement augmentation evaluations.

Ultra-high-molecular-weight polyethylene sublaminar tape as semirigid fixation or pedicle screw augmentation to prevent failure in long-segment spine surgery: an ex vivo biomechanical study.

OBJECTIVE: Complications after adult spinal deformity surgery are common, with implant-related complications occurring in up to 27.8% of cases. Sublaminar wire fixation strength is less affected by decreasing trabecular bone density in comparison to pedicle screw (PS) fixation due to the predominant cortical bone composition of the lamina. Sublaminar fixation may thus aid in decreasing implant-related complications. The goal of this study was to compare fixation characteristics of titanium sublaminar cables (SCs), ultra-high-molecular-weight polyethylene (UHMWPE) tape, PSs, and PSs augmented with UHMWPE tape in an ex vivo flexion-bending setup. METHODS: Thirty-six human cadaver vertebrae were stratified into 4 different fixation groups: UHMWPE sublaminar tape (ST), PS, metal SC, and PS augmented with ST (PS + ST). Individual vertebrae were embedded in resin, and a flexion-bending moment was applied that closely resembles the in vivo loading pattern at transitional levels of spinal instrumentation. RESULTS: The failure strength of PS + ST (4522 ± 2314 N) was significantly higher compared to the SC (2931 ± 751 N) and PS (2678 ± 827 N) groups, which had p values of 0.028 and 0.015, respectively (all values expressed as the mean ± SD). Construct stiffness was significantly higher for the PS groups compared to the stand-alone sublaminar wiring groups (p = 0.020). In contrast to SC, ST did not show any case of cortical breach. CONCLUSIONS: The higher failure strength of PS + ST compared to PS indicates that PS augmentation with ST may be an effective measure to reduce the incidence of screw pullout, even in osteoporotic vertebrae. Moreover, the lower stiffness of sublaminar fixation techniques and the absence of damage to the cortices in the ST group suggest that ST as a stand-alone fixation technique in adult spinal deformity surgery may also be clinically feasible and offer clinical benefits.

Some Practical Considerations for Compression Failure Characterization of Open-Cell Polyurethane Foams Using Digital Image Correlation.

(1) Background: Open-cell polyurethane foam mechanical behavior is highly influenced by microstructure. The determination of the failure mechanisms and the characterization of the deformation modes involved at the micro scale is relevant for accurate failure modeling. (2) Methods: We use digital image correlation (DIC) to investigate strain fields of open-cell polyurethane foams of three different densities submitted to compression testing. We analyze the effect of some DIC parameters on the failure pattern definition and the equivalent strain magnification at the apparent ultimate point. Moreover, we explore speckle versus non-speckle approaches and discuss the importance of determining the pattern quality to perform the displacement correlation. (3) Results: DIC accurately characterizes the failure patterns. A variation in the subset size has a relevant effect on the strain magnification values. Step size effect magnitude depends on the subset size. The pattern matching criterion presented little influence (3.5%) on the strain magnification. (4) Conclusion: The current work provides a comprehensive analysis of the influence of some DIC parameters on compression failure characterization of foamed structures. It highlights that changes of subset and step sizes have a significant effect on the failure pattern definition and the strain magnification values, while the pattern matching criterion and the use of speckle have a minor influence on the results. Moreover, this work stands out that the determination of the pattern quality has a major importance for texture analysis. The in-depth, detailed study carried out with samples of three different apparent densities is a useful guide for DIC users as regards texture correlation and foamed structures.

An Experimental and Numerical Investigation to Characterize an Aerospace Composite Material with Open-Hole Using Non-Destructive Techniques.

In this study, the open-hole quasi-static tensile and fatigue loading behavior of a multidirectional CFRP thick laminate, representative of laminates used in the aerospace industry, is studied. Non-destructive techniques such as infrared thermographic (IRT) and digital image correlation (DIC) are used to analyze the behavior of this material. We aim at characterizing the influence of the manufacturing defects and the stress concentrator through the temperature variation and strain distribution during fatigue and quasi-static tests. On the one hand, the fatigue specimens were tested in two main perpendicular directions of the laminate. The results revealed that manufacturing defects such as fiber waviness can have a major impact than open-hole stress concentrator on raising the material temperature and causing fracture. In addition, the number of plies with fibers oriented in the load direction can drastically reduce the temperature increment in the laminate. On the other hand, the quasi-static tensile tests showed that the strain distribution around the hole is able to predict the crack initiation and progression in the external plies. Finally, the experimental quasi-static tests were numerically simulated using the finite element method showing good agreement between the numerical and experimental results.

Explicit expressions for the estimation of the elastic constants of lamellar bone as a function of the volumetric mineral content using a multi-scale approach.

In this work, explicit expressions to estimate all the transversely isotropic elastic constants of lamellar bone as a function of the volumetric bone mineral density (BMD) are provided. The methodology presented is based on the direct homogenization procedure using the finite element method, the continuum approach based on the Hill bounds, the least-square method and the mean field technique. Firstly, a detailed description of the volumetric content of the different components of bone is provided. The parameters defined in this step are related to the volumetric BMD considering that bone mineralization process occurs at the smallest scale length of the bone tissue. Then, a thorough description provides the details of the numerical models and the assumptions adopted to estimate the elastic behaviour of the forward scale lengths. The results highlight the noticeable influence of the BMD on the elastic modulus of lamellar bone. Power law regressions fit the Young's moduli, shear stiffness moduli and Poisson ratios. In addition, the explicit expressions obtained are applied to the estimation of the elastic constants of cortical bone. At this scale length, a representative unit cell of cortical bone is analysed including the fibril orientation pattern given by Wagermaier et al. (Biointerphases 1:1-5, 2006) and the BMD distributions observed by Granke et al. (PLoS One 8:e58043, 2012) for the osteon. Results confirm that fibril orientation arrangement governs the anisotropic behaviour of cortical bone instead of the BMD distribution. The novel explicit expressions obtained in this work can be used for improving the accuracy of bone fracture risk assessment.

[Approach to the present state of proctology in Andalusia (Spain): results of a community survey]. / Aproximación al estado actual de la proctología en Andalucía: resultados de una encuesta comunitaria.

OBJECTIVE: Proctology has acquired such complexity that it requires a specific qualification, as well as the creation of specialized groups. In Andalusia there are proctologists throughout the public health system. Measurement of the quality of the distinct processes is essential. The aim of the present study was to determine the resources available, as well as to identify the management of these patients, with a view to identifying areas requiring improvement. MATERIALS AND METHOD: A descriptive, multicenter, cross sectional study of the current situation of proctology in general surgery services in 25 hospitals in Andalusia (20 public and 5 private) was performed. A survey was used to identify the general characteristics of the hospital, the tests available, and the treatments used. RESULTS: A total of 65.38% of the hospitals have a coloproctology unit, while a drop-in outpatient clinic is available in only 38.46%. Manometry and anal endosonography are available in 50% and 58%, respectively. Treatment of hemorrhoids is predominantly hemorrhoidectomy (92.30%). Medical treatment and sphincterotomy are the most widely used treatments for anal fissure, and anal dilation is still performed in 30.76%. Seton is used in 92.30%, as well as fistulotomy (88.46%), to treat fistula. The most frequent treatment of sinus is en-bloc resection and primary closure (54.16%). Between 30% and 44% report little experience of pelvic floor disorders. CONCLUSION: Although there is a willingness to achieve progress and reach an adequate scientific level, a multitude of obsolete practices should be corrected. Tools such as scientific evidence, information management and evaluation by means of comparison with standards should be diligently included in our clinical practice.

Aproximación al estado actual de la proctología en Andalucía: resultados de una encuesta comunitaria / Approach to the present state of proctology in Andalusia (Spain): results of a community survey

Objetivos. La proctología ha adquirido tal complejidad que requiere una capacitación específica, así como la creación de grupos especializados. En Andalucía hay proctólogos en toda la red sanitaria pública. Resulta imprescindible medir el nivel de calidad ofrecido en los diferentes procesos, con el fin de poder mejorarlos. Nuestro objetivo es conocer los medios disponibles y cómo se está llevando a cabo el manejo de estos pacientes para poder aplicar las mejoras. Materiales y método. Estudio descriptivo, multicéntrico y transversal sobre el estado de la proctología en los servicios de cirugía general en 25 hospitales de Andalucía (20 públicos y 5 privados). Se empleó una encuesta que reflejaba generalidades del hospital, pruebas disponibles y tratamientos empleados. Resultados. En el 65,38% hay una unidad de coloproctología. Hay consulta monográfica sólo en el 38,46%. Disponen de manometría y ecografía endoanal el 50 y el 58%, respectivamente. El tratamiento de hemorroides es predominantemente con hemorroidectomía (92,30%). El tratamiento médico y la esfinterotomía son los más empleados en la fisura, y la dilatación anal aún se practica en el 30,76%. El sedal se utiliza en el 92,30%, así como la fistulotomía (88,46%) para tratar la fístula. El tratamiento del sinus es la exéresis en bloque y cierre primario (54,16%). Respecto a las enfermedades del suelo pelviano, entre el 30 y el 44% reconocen que la experiencia es escasa. Conclusiones. Aunque existe una gran voluntad de avanzar y alcanzar un nivel científico adecuado, debemos corregir multitud de gestos que han quedado obsoletos. Herramientas como la evidencia científica, la gestión de la información y la evaluación mediante la comparación con estándares, se debe incluirlas con diligencia en nuestra práctica clínica (AU)

Objective. Proctology has acquired such complexity that it requires a specific qualification, as well as the creation of specialized groups. In Andalusia there are proctologists throughout the public health system. Measurement of the quality of the distinct processes is essential. The aim of the present study was to determine the resources available, as well as to identify the management of these patients, with a view to identifying areas requiring improvement. Materials and method. A descriptive, multicenter, cross sectional study of the current situation of proctology in general surgery services in 25 hospitals in Andalusia (20 public and 5 private) was performed. A survey was used to identify the general characteristics of the hospital, the tests available, and the treatments used. Results. A total of 65.38% of the hospitals have a coloproctology unit, while a drop-in outpatient clinic is available in only 38.46%. Manometry and anal endosonography are available in 50% and 58%, respectively. Treatment of hemorrhoids is predominantly hemorrhoidectomy (92.30%). Medical treatment and sphincterotomy are the most widely used treatments for anal fissure, and anal dilation is still performed in 30.76%. Seton is used in 92.30%, as well as fistulotomy (88.46%), to treat fistula. The most frequent treatment of sinus is en-bloc resection and primary closure (54.16%). Between 30% and 44% report little experience of pelvic floor disorders. Conclusion. Although there is a willingness to achieve progress and reach an adequate scientific level, a multitude of obsolete practices should be corrected. Tools such as scientific evidence, information management and evaluation by means of comparison with standards should be diligently included in our clinical practice (AU)