Joel D. Stitzel

Introduction: Finite element (FE) models are powerful tools for injury analysis and prediction. Their performance should be assessed with a standardized system to validate their use before employing them for injury prediction. It is necessary to develop an FE model that is able to reproduce the local response of a wide range of experimental conditions for the intended use. Because there is no standard for quantifying model performance, different validated brain models can produce varying regional pressure and strain responses, which motivates the use of a standard validation protocol, as used in the current study. Methods: In the current study, biofidelity improvements were made to an existing atlas-based brain FE model developed from the International Consortium for Brain Mapping (ICBM) brain atlas. The brain-skull interface was enhanced by decoupling the current skull and cerebrospinal fluid (CSF) parts and implementing a smooth skull geometry and a sliding with no separation contact at the skull interface. This new model is an enhancement of the atlas-based brain model (ABM) and will be referred to as ABMv4. The new, decoupled model was tested in various experimental impacts and its performance against validation data assessed using CORA (CORrelation and Analysis), an objective rating method. Results: A smooth surface at the skull interface was implemented to create the ABMv4. The ABMv4 was then validated against 23 experimental cadaver impacts - sixteen ICP tests, nine strain tests, and seventeen brain displacement tests. The improved model achieved CORA ratings of 0.733, 0.563, 0.846, and 0.605 for ICP, brain strain, revised brain strain, and brain displacement validation tests, respectively. Discussion: The ABMv4 performs well in all types of validation tests and achieves a good biofidelity rating. Additionally, the ABMv4 results in increased relative brain-skull motion (average of 0.54 mm for ABMv2 compared to average of 1.06 mm for ABMv4) and reduced brain strain on the surface of the model, with the surface strain being on average 61.7% of the surface strain value predicted for the ABMv2.

Femoral head core decompression is an efficacious joint-preserving procedure for treatment of early stage avascular necrosis. However, postoperative fractures have been described which may be related to the decompression technique used. Femoral head decompressions were performed on 12 matched human cadaveric femora comparing large 8mm single bore versus multiple 3mm small drilling techniques. Ultimate failure strength of the femora was tested using a servo-hydraulic material testing system. Ultimate load to failure was compared between the different decompression techniques using two paired ANCOVA linear regression models. Prior to biomechanical testing and after the intervention, volumetric bone mineral density was determined using quantitative computed tomography to account for variation between cadaveric samples and to assess the amount of bone disruption by the core decompression. Core decompression, using the small diameter bore and multiple drilling technique, withstood significantly greater load prior to failure compared with the single large bore technique after adjustment for bone mineral density (p< 0.05). The 8mm single bore technique removed a significantly larger volume of bone compared to the 3mm multiple drilling technique (p< 0.001). However, total fracture energy was similar between the two core decompression techniques. When considering core decompression for the treatment of early stage avascular necrosis, the multiple small bore technique removed less bone volume, thereby potentially leading to higher load to failure.

A computational finite element analysis was performed to evaluate the effects of C5-6 cervical total disc replacement (CTDR) on cross-sectional neck loading and cervical spine kinematics during a simulated rotary-wing aircraft ground impact. The neck of a human body finite element model was modified to include a C5-6 interbody arthroplasty with either a Prestige ST or ProDisc-C CTDR. The adjacent-level, cross-sectional loading for the C5-6 segment was not greatly altered by the CTDRs, as indicated by CORrelation and Analysis (CORA) ratings of 0.988 for the Prestige ST and 0.909 for the ProDisc-C. The CTDRs increased the interbody range of motion, altering both the interbody and cervical facet loading. While the facet capsules experienced increased tension in both CTDR simulations, established injury threshold levels were not reached. Overall, cervical arthroplasty at the C5-6 level did not appear to have a deleterious effect on the dynamic neck response during a simulated rotary-wing aircraft impact.

Traumatic brain injury is a leading cause of disability and injury-related death. To enhance our ability to prevent such injuries, brain response can be studied using validated finite element (FE) models. In the current study, a high-resolution, anatomically accurate FE model was developed from the International Consortium for Brain Mapping brain atlas. Due to wide variation in published brain material parameters, optimal brain properties were identified using a technique called Latin hypercube sampling, which optimized material properties against three experimental cadaver tests to achieve ideal biomechanics. Additionally, falx pretension and thickness were varied in a lateral impact variation. The atlas-based brain model (ABM) was subjected to the boundary conditions from three high-rate experimental cadaver tests with different material parameter combinations. Local displacements, determined experimentally using neutral density targets, were compared to displacements predicted by the ABM at the same locations. Error between the observed and predicted displacements was quantified using CORrelation and Analysis (CORA), an objective signal rating method that evaluates the correlation of two curves. An average CORA score was computed for each variation and maximized to identify the optimal combination of parameters. The strongest relationships between CORA and material parameters were observed for the shear parameters. Using properties obtained through the described multiobjective optimization, the ABM was validated in three impact configurations and shows good agreement with experimental data. The final model developed in this study consists of optimized brain material properties and was validated in three cadaver impacts against local brain displacement data.

: Head injuries resulting from motor vehicle crashes (MVC) are extremely common, yet the details of the mechanism of injury remain to be well characterized. Skull deformation is believed to be a contributing factor to some types of traumatic brain injury (TBI). Understanding biomechanical contributors to skull deformation would provide further insight into the mechanism of head injury resulting from blunt trauma. In particular, skull thickness is thought be a very important factor governing deformation of the skull and its propensity for fracture. Previously, age- and sex-based skull cortical thickness changes were difficult to evaluate based on the need for cadaveric skulls. In this cross-sectional study, skull thickness changes with age and sex have been evaluated at homologous locations using a validated cortical density-based algorithm to accurately quantify cortical thickness from 123 high-resolution clinical computed tomography (CT) scans. The flat bones of the skull have a sandwich structure; therefore, skull thickness was evaluated for the inner and outer tables as well the full thickness. General trends indicated an increase in the full skull thickness, mostly attributed to an increase in the thickness of the diploic layer; however, these trends were not found to be statistically significant. There was a significant relationship between cortical thinning and age for both tables of the frontal, occipital, and parietal bones ranging between a 36% and 60% decrease from ages 20 to 100 years in females, whereas males exhibited no significant changes. Understanding how cortical and full skull thickness changes with age from a wide range of subjects can have implications in improving the biofidelity of age- and sex-specific finite element models and therefore aid in the prediction and understanding of TBI from impact and blast injuries. © 2015 American Society for Bone and Mineral Research.

This study developed a parametric methodology to robustly predict occupant injuries sustained in real-world crashes using a finite element (FE) human body model (HBM). One hundred and twenty near-side impact motor vehicle crashes were simulated over a range of parameters using a Toyota RAV4 (bullet vehicle), Ford Taurus (struck vehicle) FE models and a validated human body model (HBM) Total HUman Model for Safety (THUMS). Three bullet vehicle crash parameters (speed, location and angle) and two occupant parameters (seat position and age) were varied using a Latin hypercube design of Experiments. Four injury metrics (head injury criterion, half deflection, thoracic trauma index and pelvic force) were used to calculate injury risk. Rib fracture prediction and lung strain metrics were also analysed. As hypothesized, bullet speed had the greatest effect on each injury measure. Injury risk was reduced when bullet location was further from the B-pillar or when the bullet angle was more oblique. Age had strong correlation to rib fractures frequency and lung strain severity. The injuries from a real-world crash were predicted using two different methods by (1) subsampling the injury predictors from the 12 best crush profile matching simulations and (2) using regression models. Both injury prediction methods successfully predicted the case occupant's low risk for pelvic injury, high risk for thoracic injury, rib fractures and high lung strains with tight confidence intervals. This parametric methodology was successfully used to explore crash parameter interactions and to robustly predict real-world injuries.

Skull deformation is believed to be a contributing factor in traumatic brain injury (TBI). Furthermore, skull thickness is thought to be an important factor governing deformation of the skull and its susceptibility to fracture. Although many studies have been done to understand the mechanisms of brain injury and skull fracture, the majority of the cadaveric and finite element (FE) modeling efforts are comprised of older males and 50th percentile male skulls, respectively, which do not accurately represent the population as a whole. This study employed a set of skull table thickness regressions defined at homologous landmarks on the skull which were calculated from 123 pre-existing head CT scans (ages 20-100) using a cortical density-based algorithm. A method was developed to morph the Global Human Body Models Consortium (GHBMC) 50th percentile male skull model to age and gender specific geometries based on the full thickness regressions using a Thin Plate Spline algorithm. A quantitative measure of morphing error was devised and measured using the morphed and desired full thickness values at the homologous landmark locations. This methodology can be used to create gender and age-specific FE models of the skull and will ultimately be used to understand the relationship between cortical thickness, skull deformation, and head injury.