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.

Are the unreamed nails indicated in diaphyseal fractures of the lower extremity? A biomechanical study.

INTRODUCTION: Intramedullary nailing is generally accepted as the first choice for the treatment of diaphyseal fractures of femur and tibia, with a gradual incease in the use of unreamed nails. Different studies during last years show controversial outcomes. Some authors strongly favor unreamed nailing, but most of the authors conclude that reamed nailing have proved to be more successful. MATERIAL AND METHODS: This study simulates unreamed intramedullary nailing of four femoral and three tibial fracture types by means of Finite Element (FE) models, at early postoperative stages with a fraction of physiological loads, in order to determine whether sufficient stability is achieved, and if the extent of movements and strains at the fracture site may preclude proper consolidation. RESULTS: The behavior observed in the different fracture models is very diverse. In the new biomechanical situation, loads are only transmitted through the intramedullary nail. Mean relative displacement values of fractures in the femoral bone range from 0.30 mm to 0.82 mm, depending on the fracture type. Mean relative displacement values of the tibial fractures lie between 0.18 and 0.62 mm, depending on the type of fracture. Concerning mean strains, for femoral fractures the maximum strains ranged between 12.7% and 42.3%. For tibial fractures the maximum strains ranged between 10.9% and 40.8%. CONCLUSIONS: The results showed that unreamed nailing provides a very limited mechanical stability, taking into account that analyzed fracture patterns correspond to simple fracture without comminution. Therefore, unreamed nailing is not a correct indication in femoral fractures and should be an exceptional indication in open tibial fractures produced by high-energy mechanism.

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.

Biomechanical analysis of the stability of anterograde reamed intramedullary nails in femoral spiral fractures.

Femoral shaft fractures present high morbidity and important complications and consequences, being spiral fractures the most complicated from a biomechanical point of view, being unstable and without possibility of getting a good contact between nail and femoral endosteum. Femoral diaphyseal fractures are treated, usually, by means of intramedullary nailing. So, it is necessary to know the osteosynthesis stability and which locking screws combination is optimal. This work studies the use of reamed locked intramedullary nails in spiral femoral fractures located along zones 2 and 4 of wiss, depending on the spire length, corresponding to 32-A spiral type in AO/OTA classification, which represent a percentage of 23% within the total of diaphyseal fractures. A three-dimensional finite element model of the femur was developed, modeling a spiral fracture with different spiral lengths and gaps. A femoral nail was used, considering two transversal screws both at the proximal and the distal parts. The study was focused on the immediately post-operative stage, verifying the appropriate stability of the osteosynthesis. Reamed intramedullary blocked nails provide appropriate stability of femoral spiral fractures, considering global mobility of femoral head with respect to femoral condyles, relative displacements between fragments at fracture site, stresses at nail and locking screws, and stresses at cortical bone. The obtained results show that the use of blocked reamed nails in spiral femoral fractures can be considered as an appropriate surgical technique, providing sufficient stability in order to obtain an adequate fracture healing.

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=0.97, and a normalized root mean square error of εNRMS=0.0814 for proximal spiral samples, and R2=0.89 , ÎµNRMS=0.1600 and R2=0.94 , ÎµNRMS=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.

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.

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.

Comparison between DEXA and finite element studies in the long-term bone remodeling of an anatomical femoral stem.

The implantation of a cemented or cementless femoral stem changes the physiological load transfer on the femur producing an effect on the bone called adaptative remodeling. The patterns of this remodeling are attributed to mechanical and biological factors, and those changes in bone mineral density have been determined in long-term densitometry studies. This technique has proved to be a useful tool able to quantify small changes in bone density in different femoral areas, and it is considered to be ideal for long-term studies. On the other hand, the finite element (FE) simulation allows the study of the biomechanical changes produced in the femur after the implantation of a femoral stem. The aim of this study was to contrast the findings obtained from a 5 year follow-up densitometry study that used a newly designed femoral stem (73 patients were included in this study), with the results obtained using a finite element simulation that reproduces the pattern of load transfer that this stem causes on the femur. In this study we have obtained a good comparison between the results of stress of FE simulation and the bone mass values of the densitometry study establishing a ratio between the increases in stress (%) versus the increases in bone density (%). Hence, the changes in bone density in the long term, compared with the healthy femur, are due to different load transfers after stem implantation. It has been checked that in the Gruen zone 7 at 5 years, the most important reduction in stress (7.85%) is produced, which coincides with the highest loss of bone mass (23.89%). Furthermore, the simulation model can be used with different stems with several load conditions and at different time periods to carry out the study of biomechanical behavior in the interaction between the stem and the femur, explaining the evolution of bone density in accordance to Wolff's law, which validates the simulation model.

Estudio densitométrico y con elementos finitos de la remodelación ósea tras la implantación de un vástago femoral anatómico no cementado / Densitometric and finite-element analysis of bone remodeling further to implantation of an uncemented anatomical femoral stem

Introducción. La implantación de un vástago femoral cambia las condiciones de transmisión de carga de la cadera, produciendo el denominado remodelamiento adaptativo. El objetivo de todos los vástagos (cementados y no cementados) ha sido conseguir una perfecta transmisión de cargas que evite los fenómenos de puenteo de fuerzas o stressshielding, que a su vez producen una desvitalización ósea proximal.Material y método. Para cuantificar las variaciones de lamasa ósea en las 7 zonas de Gruen se ha realizado un estudio seriado a 10 años con DEXA en 80 pacientes, con mediciones en el pre y posoperatorio, 6 meses posperatorio, y a 1, 3, 5, 7 y 10 años tras la implantación de la prótesis.Resultados y conclusiones. La simulación con elementos finitos (EF) permite caracterizar los cambios biomecánicos que se producen en el fémur tras la implantación de un vástago protésico, así como su comportamiento a largo plazo. El objetivo de nuestro estudio es comprobar si los resultados de la simulación explican los cambios biomecánicos que justifiquen la evolución de la densidad ósea obtenida mediante el estudio con DEXA, tras la implantación de unvástago anatómico no cementado. Los resultados de la simulación con EF presentan un perfecto paralelismo entre las pérdidas de masa ósea detectadascon la DEXA y la evolución tensional en cada zona deGruen, lo que confirma que aunque el diseño de la prótesis es de apoyo metafisario, se produce un claro fenómeno de puenteo de fuerzas en la zona proximal del fémur, todo lo cual produce una desvitalización ósea en las zonas 1 y 7 de Gruen

Introduction. Implantation of a femoral stem changes theload transmission dynamics in the hip and gives rise tothe so-called adaptive remodeling. The goal pursued by all stems, whether cemented or not, is to achieve a perfect load transmission mechanism in order to avoid the phenomenon of stress-shielding, which may cause proximal bone devitalization.Materials and methods. In order to quantify bone mass variations in the 7 Gruen zones, a serial DEXA analysis was carried out in 80 patients, with preoperative measurements as well as postoperative measurements at 6 months and 1, 3, 5, 7 and 10 years post implantation.Results and conclusions. Finite-element (FE) simulationsmake it possible to characterize the biomechanical changes that occur in the femur further to implantation of a prosthetic stem, as well as the stem’s long-term performance. The purpose of our study is to determine whether the results of the simulation can explain the biomechanical changes that may lie behind the evolution of bone density observed through DEXA scanning after implantation of an uncemented anatomical stem.The results of the FE simulation show an excellent matchbetween the bone loss observed on DEXA scans and theevolution of stress patterns observed in each of the Gruen zones, which confirms that even if the stem implanted was metaphyseal, stress shielding was manifest in the proximal femoral area, giving rise to the devitalization of bone in Gruen zones 1 and 7

Activation of plant plasma membrane H(+)-ATPase by the non-ionic detergent Brij 58.

Plasma membrane vesicles were purified from tobacco callii and the modulation of H(+)-ATPase by detergents was investigated. The nonionic detergent Brij 58 not only activated ATP hydrolysis (2-fold) but also proton pumping (more than 4-fold). Triton X-100, within a more limited concentration range, produced a similar effect. The simultaneous activation of ATP hydrolysis and proton pumping is not compatible with current interpretations of effects of nonionic detergents on the H(+)-ATPase based on latency of the enzyme and opening of vesicles.