Assessing traumatic brain injuries using EEG power spectral analysis and instantaneous phase.

Although mild traumatic brain injury (mTBI) occurs commonly, little is known about how multiple mTBI incidents accumulate over time to produce serious morbidity or how the extent of injury can be quantified. This work presents a rat model that uses deceleration-induced brain trauma and an implantable EEG system for recording injury-induced changes in brain activity. Specifically, we present an analysis method to assess and quantify mTBI by combining information derived from EEG power spectral analysis and EEG phase shifts. We found that in different frequency bands, both EEG power spectra and the instantaneous phases of the two EEG channels before the impact were different from those measured after the impact. This study shows that EEG analysis can be used as a tool to identify and assess brain related injuries.

The effect of tip geometry on the mechanical performance of unused and reprocessed orthopaedic drill bits.

Although reprocessed drill bits have been in clinical use as a cost-saving measure, their performance has not been critically evaluated in comparison with the performance of unused drill bits. The effect of three commonly used reprocessing methods on the geometry and mechanical performance of 2.5 mm orthopaedic drill bits was investigated and compared with that for unused drill bits. Four mechanically significant drill parameters including chisel edge, chisel edge angle, point angle, and lip length of 36 drill bits in four groups were measured and compared. Group A included unused drill bits. Group B included drill bits reprocessed once by one company whereas group C included those reprocessed twice by the same method. Group D included drill bits reprocessed once by another company. For mechanical performance, a test set-up was developed in which the time of travel of the drill bits through layers of cortical and trabecular synthetic bone materials under constant compressive force were measured. The geometrical parameters were found to be significantly altered as a result of the reprocessing methods. A linear relationship was derived to relate the chisel edge to the drill time through cortical and trabecular bones. The mechanical performance of drill bits is correlated largely to the chisel edge length. A larger chisel edge is correlated to a reduced drill time, particularly in the cortical bone.

Frequency dependence of complex moduli of brain tissue using a fractional Zener model.

Brain tissue exhibits viscoelastic behaviour. If loading times are substantially short, static tests are not sufficient to determine the complete viscoelastic behaviour of the material, and dynamic test methods are more appropriate. The concept of complex modulus of elasticity is a powerful tool for characterizing the frequency domain behaviour of viscoelastic materials. On the other hand, it is well known that classical viscoelastic models can be generalized by means of fractional calculus to describe more complex viscoelastic behaviour of materials. In this paper, the fractional Zener model is investigated in order to describe the dynamic behaviour of brain tissue. The model is fitted to experimental data of oscillatory shear tests of bovine brain tissue to verify its behaviour and to obtain the material parameters.

Nonlinear viscoelastic effects in oscillatory shear deformation of brain tissue.

Two nonlinear constitutive models were used to describe the dynamic viscoelastic behavior of brain tissue. Small disc-shaped samples of bovine brain tissue were tested in simple shear using forced vibrations (0.5 to 200 Hz) with finite amplitudes (up to 20% Lagrangian shear strain). The samples response to simple, double, and triple harmonic inputs was determined in order to characterize the nonlinearities up to the third-order. A quasilinear viscoelastic model was proposed to describe the spatial nonlinearity. A fully nonlinear viscoelastic model with product-form multiple hereditary integrals was proposed to describe the spatial as well as the temporal nonlinearities. The fully nonlinear model demonstrated superiority at high frequencies (above 44 Hz). Under finite strains, the linear complex modulus showed nonrecoverable asymptotic strain conditioning behavior. Discrepancies observed in previously published studies and the threshold of functional failure of the neural tissue were shown to be related to this strain conditioning effect.

Material properties for modeling traumatic aortic rupture.

Traumatic aortic rupture is a significant cause of fatalities in frontal automobile crashes. However, such ruptures are difficult to reproduce experimentally in cadaveric surrogates, and it is difficult to observe dynamic aortic response in situ. So, the aortic injury mechanism or mechanisms remains in dispute. This study is a staged investigation of the physical parameters and mechanisms of human aortic rupture. The investigation includes both experimental study of local and global viscoelastic properties and failure properties of aortas using aortic tissue samples, excised aortas in vitro, and whole human aortas in situ in cadaver thoraxes. This study is the first phase in a staged programme to develop a finite element computer model of aorta injury to examine the mechanisms of aorta injury in automobile crashes. The high-rate local biaxial properties of porcine aorta tissue are determined from samples taken from the isthmus region, the most common area of failure in traumatic aorta injury. Using porcine aortas, similar in structure and physical characteristics to human aortic tissue, biaxial oscillatory response is determined at large strains and high strain rates. From this data, a hyperelastic material model with a failure threshold is developed that is in good agreement with local property data determined from oscillatory tests at 20 Hz and 65 Hz. Further, whole aorta tests are performed using pressure application with aortic pressure time histories similar in onset rate to those seen in cadaveric sled testing. These tests establish the ultimate stretch ratio and strain to failure for human aorta specimens. The specimens show no significant difference in response between the in situ tests and the in vitro tests. This indicates either that the internal thoracic boundary conditions may not be important in the stress and strain level of aorta failure or that the number of in situ tests (3) was too small to establish a difference. A Weibull survival analysis of the whole aorta failure tests shows significant dependence of aortic ultimate stretch ratio on age. A 50% risk of failure is 852 kPa in the circumferential direction and 426 kPa in the longitudinal direction. For pressure, the 50% risk of failure for all the tests is approximately 101 kPa. This increases to greater than 120 kPa for subjects below 68 years.