Effects of storage temperature on the mechanical properties of porcine kidney estimated using shear wave elastography

The objective of this study was to evaluate the effects of different conservation techniques on the mechanical properties of the ex vivo porcine kidney in order to select an appropriate conservation protocol to use prior to mechanical testing. Five groups of eight kidneys each were subjected to different methods of conservation: storage at 4°C, -18°C, -34°C and -71°C, for 7 days, or storage at 20°C for 2 days only (as the tissues degraded quickly). Their shear modulus as a function of depth in the organ was evaluated before (fresh) and after conservation using shear wave elastography. Results obtained on fresh kidneys were collected within 6h of death. Freezing lead to a significant decrease (p<0.05) of the shear modulus in the most superficial zone (renal cortex), irrespectively of the freezing temperature (-18°C, -34°C, -71°C). There were no significant change (p>0.05) in the properties of the renal cortex when stored at 4°C or 20°C. The average moduli in the central region of the kidney (medulla) were much higher than in the cortex and exhibited also exhibited larger specimen to specimen variations. The effects of the conservation method on the central region were not significant. Overall, the results suggest that kidney tissues should not be frozen prior to biomechanical characterization and that inhomogeneity may be important to consider for in biomechanical models.

Mechanical response of animal abdominal walls in vitro: evaluation of the influence of a hernia defect and a repair with a mesh implanted intraperitoneally

Better mechanical knowledge of the abdominal wall is requested to further develop and validate numerical models. The aim of this study was to characterize the passive behaviour of the abdominal wall under three configurations: intact, after creating a defect simulating an incisional hernia, and after a repair with a mesh implanted intraperitonally. For each configuration, controlled boundary conditions were applied (air pressure and then contact loading) to the abdominal wall. 3D local strain fields were determined by digital image correlation. Local strains measured on the internal and external surfaces of the intact abdominal wall showed different patterns. The air pressure and the force applied to the abdominal wall during contact loading were measured and used to determine stiffness. The presence of a defect resulted in a significant decrease of the global stiffness compared to the intact abdominal wall (about 25%). In addition, the presence of the mesh enabled to restore the stiffness to values that were not significantly different from those of the intact wall. These results suggest that intraperitoneal mesh seems to restore the global biomechanics of the abdomen.

Sensitivity of the tibio-femoral response to finite element modeling parameters

A generic finite element (FE) model of the lower limb was used to study the knee response in-vivo during a one-legged hop. The approach uses an explicit FE code and a combination of estimated muscle forces and measured three-dimensional tibio-femoral kinematics and ground reaction force as input to the FE model. The sensitivity of the simulated tibio-femoral response to variations of key geometric and material parameters was investigated by performing a total of 38 different simulations. The amplitudes of both kinematic and kinetic responses were affected by the change of these parameters. For the current approach, the results suggest that while cartilage mechanical and geometric properties are very important for the estimation of tibio-femoral cartilage pressure, they have limited effects on the overall kinematic response. The study may help to better define the relative importance of modeling parameters for the development of subject-specific models.

A new method to investigate in vivo knee behavior using a finite element model of the lower limb

Several finite element models have been developed for estimating the mechanical response of joint internal structures, where direct or indirect in vivo measurement is difficult or impossible. The quality of the predictions made by those models is largely dependent on the quality of the experimental data (e.g. load/displacement) used to drive them. Also numerical problems have been described in the literature when using implicit finite element techniques to simulate problems that involve contacts and large displacements. In this study, a unique strategy was developed combining high accuracy in vivo three-dimensional kinematics and a lower limb finite element model based on explicit finite element techniques. The method presents an analytical technique applied to a dynamic loading condition (impact during hopping on one leg). The validation of the lower limb model focused on the response of the whole model and the knee joint in particular to the imposed 3D femoral in vivo kinematics and ground reaction forces. The approach outlined in this study introduces a generic tool for the study of in vivo knee joint behavior.

Lower extremity injuries in lateral impact: a retrospective study

ABSTRACT A retrospective analysis of the NASS/CDS database from 1993 to 2000 was used to investigate lower extremity injury in lateral impact. The analysis includes the study of the injury patterns, crash characteristics and the interactions between the occupant and the vehicle interior, including injuries to the farside occupants. The findings include significantly different injury patterns for the nearside and farside impacts. In particular, while the proportion of pelvis/hip injuries, with respect to AIS2 and AIS3 lower extremity skeletal injuries and 2-4 and 10-8 o'clock side impacts, was higher in nearside (70.4%) than farside (38.3%), the opposite trend was observed for the thigh (2.8% vs 4.5%), knee (6.2% vs 16.7%), leg (10.1% vs 19.5%) and foot/ankle (5.6% vs 14.7) injuries. Analysis of the PDOF suggested that a large proportion the impacts occurred obliquely, at approximately 10 and 2 o'clock, with a rearward component of force. It is hoped that the findings of the current study can help to investigate injury mechanisms.

Lower Limb: Advanced FE Model and New Experimental Data

The Lower Limb Model for Safety (LLMS) is a finite element model of the lower limb developed mainly for safety applications. It is based on a detailed description of the lower limb anatomy derived from CT and MRI scans collected on a subject close to a 50th percentile male. The main anatomical structures from ankle to hip (excluding the hip) were all modeled with deformable elements. The modeling of the foot and ankle region was based on a previous model Beillas et al. (1999) that has been modified. The global validation of the LLMS focused on the response of the isolated lower leg to axial loading, the response of the isolated knee to frontal and lateral impact, and the interaction of the whole model with a Hybrid III model in a sled environment, for a total of nine different set-ups. In order to better characterize the axial behavior of the lower leg, experiments conducted on cadaveric tibia and foot were reanalyzed and experimental corridors were proposed. Future work will include additional validation of the model using global data, joint kinematics data, and deformation data at the local level.