Study of the Behavior of a Bell-Shaped Colonic Self-Expandable NiTi Stent under Peristaltic Movements

Managing bowel obstruction produced by colon cancer requires an emergency intervention to patients usually in poor conditions, and it requires creating an intestinal stoma in most cases. Regardless of that the tumor may be resectable, a two-stage surgery is mandatory. To avoid these disadvantages, endoscopic placement of self-expanding stents has been introduced more than 10 years ago, as an alternative to relieve colonic obstruction. It can be used as a bridge to elective single-stage surgery avoiding a stoma or as a definitive palliative solution in patients with irresectable tumor or poor estimated survival. Stents must be capable of exerting an adequate radial pressure on the stenosed wall, keeping in mind that stent must not move or be crushed, guaranteeing an adequate lumen when affected by peristaltic waves. A finite element simulation of bell-shaped nitinol stent functionality has been done. Catheter introduction, releasing at position, and the effect of peristaltic wave were simulated. To check the reliability of the simulation, a clinical experimentation with porcine specimens was carried out. The stent presented a good deployment and flexibility. Stent behavior was excellent, expanding from the very narrow lumen corresponding to the maximum peristaltic pressure to the complete recovery of operative lumen when the pressure disappears

A methodology for the customized design of colonic stents based on a parametric model

The choice of necessary stent properties depends mainly on the length of the stenosis and degree of occlusion. So a stent design with variable radial stiffness along its longitudinal axis would be a good option. The design proposed corresponds to a tube-based stent with closed diamond-shaped cells made from a NiTi alloy. By acting independently on different geometric factors, variable geometries can be obtained with different radial force reactions. A design adjustment according to specific requirements, in order to get a better fit to ill-duct and reduces complications, is possible. A parametric analysis using finite element has been conducted to determine the influence of slot length, number of circumferential slots, tube thickness and shape-factor on stent mechanical behavior, which allow eliminating the need for extensive experimental work and knowing and quantifying the influence of those factors. The results of finite element simulations have been used, by means of least-squares fit techniques, to obtain analytical expressions for the main mechanical characteristics of the stent (Chronic Expansive Radial Force and Radial Compression Resistance) in terms of the different geometrical factors. This allows the stent geometry to be customized without launching an iterative and costly process of modeling and simulation for each case

Influence of screw combination and nail materials in the stability of anterograde reamed intramedullary nail in distal femoral fractures

Intramedullary nailing (IM) is a technique universally accepted to treat femoral diaphyseal fractures. The treatment of fractures located in the distal third remains a controversial issue though. A finite element model of the femur has been developed, analyzing distal fractures with several gap sizes combined with different interlocking combinations of distal screws with one oblique screw proximally to stabilize the intramedullary nail. The mechanical strength of the nail against bending and compression efforts was also studied. Beside the FE simulations, a clinical follow-up of 15 patients, 6 males and 9 females, with mean age of 53.2 years was carried out. Localizations of fractures were 10 in the right femur and 5 in the left femur, respectively. A fairly good correspondence agreement between clinical results and the simulated fractures in terms of gap size was found. Non-comminuted fractures had a mean consolidation time of 20.5 weeks (4.8 months), a tendency corresponding well to the mobility obtained in the FE simulations; Comminuted fractures on the other hand exhibited a higher mean consolidation period of 22.2 weeks (5.2 months) secondary to the excessive mobility at fracture site obtained by means of FE simulations. The best stability at fracture site was found for the system with three distal screws and the system with two distal screws placed medial lateral. The highest leverage of distal screws was obtained maximizing the distance between them and choosing the coronal plane for their orientation. The results obtained with both nail materials (stainless steel and titanium alloy) show a higher mobility when using titanium nails. Steel nails provide stiffer osteosyntheses than the titanium nails. In conclusion, the best screw combination in terms of stability to produce fracture healing and the least difficulties during treatment is the one which had one oblique proximal screw with two distal lateral screw implanted in the coronal plane

Influence of gap size, screw configuration, and nail materials in the stability of anterograde reamed intramedullary nail in femoral transverse fractures

Femoral shaft fractures are among the most severe injuries of the skeleton. They are associated with high morbidity and mortality. The most appropriate treatment depending on the type of fracture and location level should be chosen. A finite element model of the femur has been developed, analyzing various types of fractures in the subtrochanteric and diaphyseal supracondylar area, with several gap sizes, being stabilized with a single combination of screws for the intramedullary nail. The mechanical strength of the nail against bending and compression efforts was studied comparing two materials for the nail: stainless steel and titanium alloy. Beside the finite elements (FE) simulations, a clinical follow-up was carried out, considering a sample of 55 patients, 24 males, and 31 females, with mean age of 52.5 years. Localizations of fractures were 22 in the right femur and 33 in the left one, respectively. A good agreement between clinical results and the simulated fractures in terms of gap size was found. Non-comminuted fractures have a mean consolidation time of 4.1 months, which coincides with the appropriate mobility at fracture site obtained in the FE simulations, whereas comminuted fractures have a higher mean consolidation period estimated in 7.1 months, corresponding to the excessive mobility at fracture site obtained by means of FE simulations. The obtained results between both nail materials (stainless steel and titanium alloy) show a higher mobility when using titanium nails, which produce a higher rate of strains at the fracture site, amplitude of micromotions and bigger global movements compared to stainless-steel nails. Steel nails provide stiffer osteosyntheses than the titanium nails. In conclusion, anterograde locked nail is particularly useful in the treatment of a wide range of supracondylar fractures with proximal extension into the femoral diaphysis

A comparative study of hyperelastic constitutive models for colonic tissue fitted to multiaxial experimental testing

For colonic stents design, the interaction with colonic tissue is essential in order to characterize the appropriate radial stiffness which provides a minimum lumen for intestinal transit to be maintained. It is therefore important to develop suitable constitutive models allowing the mechanical behavior of the colon tissue to be characterized. The present work investigates the biomechanical behavior of colonic tissue by means of biaxial tests carried out on different parts of the colonic tract taken from several porcine specimens. Samples from the colonic tract were quasi-statically tensioned using a load-controlled protocol with different tension ratios between the circumferential and the axial directions. Fitting techniques were then used to adjust specific hyperelastic models accounting for the multilayered conformation of the colonic wall and the fiber-reinforced configuration of the corresponding tissues. It was found that the porcine colon changed from a more isotropic to a more anisotropic tissue and became progressively more flexible and compliant in circumferential direction depending on the position along the duct as it approaches the rectum. The best predictive capability of mechanical behavior corresponds to the Four Fiber Family model showing mean values of coefficient of determination R2 ¼ 0:97, and a normalized root mean square error of eNRMS ¼ 0:0814 for proximal spiral samples, and R2 ¼ 0:89 ; eNRMS ¼ 0:1600 and R2 ¼ 0:94 ; eNRMS ¼ 0:1227 for distal spiral and descending colon samples, respectively. The other analyzed models provide good results for proximal spiral colon specimens, which have a lower degree of anisotropy. The analyzed models with the fitted elastic parameters can be used for more realistic and reliable FE simulations, providing the appropriate framework for the design of optimal devices for the treatment of colonic diseases

Comparative analysis of the biomechanical behavior of anterograde/retrograde nailing in supracondylar femoral fractures

Supracondylar femoral fractures account for a noticeable percentage of the femoral shaft fractures, affecting two etiological groups: high energy trauma in young men, with good bone quality, and older women with osteoporotic femur. Surgical treatment of those kind of fractures remains controversial, with different surgical options such as plate and sliding barrel locking condylar plate, less invasive stabilization system (LISS) or intramedullary nailing, which has emerged as a new fixation choice in the treatment of that type of fractures. The present work performs a comparative study about the biomechanical behavior of anterograde and retrograde nailing in supracondylar femoral fractures type A, in order to determine the best choice of nailing and locking configuration. A three-dimensional finite element model of the femur was developed, modeling femoral supracondylar fracture and different nailing configurations, both for anterograde and retrograde nails. The study was focused on the immediately post-operative stage, verifying the appropriate stability of the osteosynthesis. The obtained results show a better biomechanical behavior for anterograde nails, providing a better stability from the point of view of global movements, lower stresses in screws, and less stress concentration in cortical bone. So, for the analyzed fractures and osteosyntheses types, anterograde nailing has demonstrated to be a better surgical option, being an excellent indication in supracondylar fractures of femur, with clear benefits compared to retrograde nailing, providing a better stabilization which enables for a more satisfactory fracture healing

A new approach for initial callus growth during fracture healing in long bones

The incidence of bone fracture has become a major clinical problem on a worldwide scale. In the past two decades there has been an increase in the use of computational tools to analyse the bone fracture problem. In several works, various study cases have been analysed to compare human and animal bone fracture healing. Unfortunately, there are not many publications about computational advances in this field and the existing approaches to the problem are usually similar. In this context, the objective of this work is the application of a diffusion problem in the model of the bone fragments resulting from fracture, working together with a mesh-growing algorithm that allows free growth of the callus depending on the established conditions, without a pre-meshed domain. The diffusion problem concerns the different biological magnitudes controlling the callus growth, among which Mesenchymal Stem Cells and chondrocytes concentrations were chosen, together with Tumour Necrosis Factor α and Bone Morphogenetic Protein as the factors influencing the velocity in the callus formation. A Finite Element approach was used to solve the corresponding diffusion problems, obtaining the concentration values along the entire domain and allowing detecting the zones in which biological magnitudes reach the necessary thresholds for callus growth. The callus growth is guided by a geometrical algorithm which performs an additional mesh generation process (self-added mesh) at each step of the iterative procedure until complete callus formation. The proposed approach was applied to different types of diaphyseal femoral fractures treated by means of intramedullary nailing. Axisymmetric models based on triangular quadratic elements were used, obtaining results in good agreement with clinical evidence of these kinds of fractures. The algorithm proposed has the advantage of a natural callus growth, without the existence of a previous mesh that may affect the conditions and direction of growth. The approach is intended for the initial phase of callus growth. Future work will address the implementation of the corresponding formulations for tissue transformation and bone remodelling in order to achieve complete fracture healing

Development and kinematic verification of a finite element model for the lumbar spine: Application to disc degeneration

The knowledge of the lumbar spine biomechanics is essential for clinical applications. Due to the difficulties to experiment on living people and the irregular results published, simulation based on finite elements (FE) has been developed, making it possible to adequately reproduce the biomechanics of the lumbar spine. A 3D FE model of the complete lumbar spine (vertebrae, discs, and ligaments) has been developed. To verify the model, radiological images (X-rays) were taken over a group of 25 healthy, male individuals with average age of 27.4 and average weight of 78.6 kg with the corresponding informed consent. A maximum angle of 34.40° is achieved in flexion and of 35.58° in extension with a flexion-extension angle of 69.98°. The radiological measurements were 33.94 ± 4.91°, 38.73 ± 4.29°, and 72.67°, respectively. In lateral bending, the maximum angles were 19.33° and 23.40 ± 2.39, respectively. In rotation a maximum angle of 9.96° was obtained. The model incorporates a precise geometrical characterization of several elements (vertebrae, discs, and ligaments), respecting anatomical features and being capable of reproducing a wide range of physiological movements. Application to disc degeneration (L5-S1) allows predicting the affection in the mobility of the different lumbar segments, by means of parametric studies for different ranges of degeneration

Aplicación de aleaciones con memoria de forma en el diseño y fabricación de ferulajes para corrección de las deformidades articulares de los dedos de la mano

OBJETIVO: Se propone un prototipo de férula para la corrección de las deformidades articulares de los dedos de la mano, basados en las propiedades de las aleaciones de níquel-titanio, con el fin de mejorar la aplicación de las ortesis dinámicas de acción constante, empleadas habitualmente en ortopedia. MATERIAL Y MÉTODOS: Para ello, se han diseñado diferentes prototipos de láminas y varillas, variando sus grosores. Los prototipos se han aplicado a 15 pacientes con su consentimiento, deformidad en Boutonnière, entre 40 y 60º de angulación de la articulación interfalángica proximal (IFP) del dedo y edades comprendidas entre 18 y 39 años (media 31 años), midiendo cuando empieza a notarse un cambio en la fuerza tensil de la férula sobre la deformidad del dedo con el equipo Xpresion. RESULTADOS: Los valores obtenidos, son de 3291,67 ± 222,30 gr./mm 2, guardando relación directa con el diámetro del dedo. Los resultados obtenidos hasta conseguir una corrección superior al 75 %, han sido buenos en 9 pacientes, 5 regulares (corrección entre 30 % y 75 %) y uno malo (inferior al 30 %). CONCLUSIONES: Las férulas de Níquel Titanio con tensiones constantes durante largos periodos de tiempo, hasta recuperar la posición original, con su diseño, mejoran el ajuste inicial facilitando el manejo por parte del paciente y del especialista.Peer Reviewe

A predictive mechanical model for evaluating vertebral fracture probability in lumbar spine under different osteoporotic drug therapies

Osteoporotic vertebral fractures represent a major cause of disability, loss of quality of life and even mortality among the elderly population. Decisions on drug therapy are based on the assessment of risk factors for fracture from bone mineral density (BMD) measurements.A previously developed model, based on the Damage and Fracture Mechanics, was applied for the evaluation of the mechanical magnitudes involved in the fracture process from clinical BMD measurements. BMD evolution in untreated patients and in patients with seven different treatments was analyzed from clinical studies in order to compare the variation in the risk of fracture. The predictive model was applied in a finite element simulation of the whole lumbar spine, obtaining detailed maps of damage and fracture probability, identifying high-risk local zones at vertebral body.For every vertebra, strontium ranelate exhibits the highest decrease, whereas minimum decrease is achieved with oral ibandronate. All the treatments manifest similar trends for every vertebra. Conversely, for the natural BMD evolution, as bone stiffness decreases, the mechanical damage and fracture probability show a significant increase (as it occurs in the natural history of BMD). Vertebral walls and external areas of vertebral end plates are the zones at greatest risk, in coincidence with the typical locations of osteoporotic fractures, characterized by a vertebral crushing due to the collapse of vertebral walls.This methodology could be applied for an individual patient, in order to obtain the trends corresponding to different treatments, in identifying at-risk individuals in early stages of osteoporosis and might be helpful for treatment decisions