In this review, we compare the reported values of Young's modulus (YM) obtained from indentation and tensile deformations of soft biological tissues. When the method of deformation is ignored, YM values for any given tissue typically span several orders of magnitude. If the method of deformation is considered, then a consistent and less ambiguous result emerges. On average, YM values for soft tissues are consistently lower when obtained by indentation deformations. We discuss the implications and potential impact of this finding.

http://chemotaxis.semmelweis.hu/CHTXhpg/Durotaxis/DT2/McKeeCT_TissueEng_2011.pdf

Indentation versus tensile measurements of Young's modulus for soft biological tissues.

Recent research has suggested a possible link between sports-related concussions and neurodegenerative processes, highlighting the importance of developing methods to accurately quantify head impact tolerance. The use of kinematic parameters of the head to predict brain injury has been suggested because they are indicative of the inertial response of the brain. The objective of this study is to characterize the rotational kinematics of the head associated with concussive impacts using a large head acceleration dataset collected from human subjects. The helmets of 335 football players were instrumented with accelerometer arrays that measured head acceleration following head impacts sustained during play, resulting in data for 300,977 sub-concussive and 57 concussive head impacts. The average sub-concussive impact had a rotational acceleration of 1230 rad/s2 and a rotational velocity of 5.5 rad/s, while the average concussive impact had a rotational acceleration of 5022 rad/s2 and a rotational velocity of 22.3 rad/s. An injury risk curve was developed and a nominal injury value of 6383 rad/s2 associated with 28.3 rad/s represents 50% risk of concussion. These data provide an increased understanding of the biomechanics associated with concussion and they provide critical insight into injury mechanisms, human tolerance to mechanical stimuli, and injury prevention techniques.

/pdf/rotational-head-kinematics-in-football-impacts-an-injury-2swmxzfyn0.pdf

Rotational Head Kinematics in Football Impacts: An Injury Risk Function for Concussion

Recent research has suggested possible long term effects due to repetitive concussions, highlighting the importance of developing methods to accurately quantify concussion risk. This study introduces a new injury metric, the combined probability of concussion, which computes the overall risk of concussion based on the peak linear and rotational accelerations experienced by the head during impact. The combined probability of concussion is unique in that it determines the likelihood of sustaining a concussion for a given impact, regardless of whether the injury would be reported or not. The risk curve was derived from data collected from instrumented football players (63,011 impacts including 37 concussions), which was adjusted to account for the underreporting of concussion. The predictive capability of this new metric is compared to that of single biomechanical parameters. The capabilities of these parameters to accurately predict concussion incidence were evaluated using two separate datasets: the Head Impact Telemetry System (HITS) data and National Football League (NFL) data collected from impact reconstructions using dummies (58 impacts including 25 concussions). Receiver operating characteristic curves were generated, and all parameters were significantly better at predicting injury than random guessing. The combined probability of concussion had the greatest area under the curve for all datasets. In the HITS dataset, the combined probability of concussion and linear acceleration were significantly better predictors of concussion than rotational acceleration alone, but not different from each other. In the NFL dataset, there were no significant differences between parameters. The combined probability of concussion is a valuable method to assess concussion risk in a laboratory setting for evaluating product safety.

/pdf/brain-injury-prediction-assessing-the-combined-probability-10frx8age8.pdf

Brain Injury Prediction: Assessing the Combined Probability of Concussion Using Linear and Rotational Head Acceleration

This study is aimed to develop a next-generation, high quality, extensively validated finite element (FE) human head model for enhanced head injury prediction and prevention. The geometry of the model was based on CT and MRI scans of an adult male. A new feature-based multi-block technique was adopted to develop hexahedral brain meshes including the cerebrum, cerebellum, brainstem, corpus callosum, ventricles, and thalamus. Conventional meshing methods were used to create the bridging veins, cerebrospinal fluid (CSF), skull, facial bones, flesh, skin, and membranes - including falx, tentorium, pia, arachnoid, and dura. The head model has 270,552 elements in total. A total of 49 loading cases were selected from a range of experimental and real world head impacts to check the robustness of the model predictions based on responses including the brain pressure, relative skull-brain motion, intracranial strain, skull response, facial response, and bridging vein elongation. The brain pressure was validated against intracranial pressure data reported by Nahum et al. (1977) and Trosseille et al. (1992). The brain motion was validated against brain displacements under sagittal, coronal, and horizontal blunt impacts performed by Hardy et al. (2001, 2007). The facial bone responses were validated under nasal impact (Nyquist et al., 1986), zygoma and maxilla impact (Allsop et al., 1988). The skull bones were validated under frontal angled impact, vertical impact, and occipital impact (Yoganandan et al., 1995) and frontal horizontal impact (Hodgson et al., 1970). The FE head model was further used to study injury mechanisms and tolerances for brain contusion (Nahum et al., 1976), bridging vein rupture (Depreitere et al., 2006), and brain strains for real-world brain injury cases (Franklyn et al. 2005). Studies from 49 loading cases demonstrated that the FE head model had good biofidelity in predicting head responses under various impact scenarios. Furthermore, tissue-level injury tolerances were proposed. A maximum principal strain of 0.42% was adopted for skull cortical layer fracture and maximum principal stress of 20 MPa was used for skull diploe layer fracture. Additionally, a plastic strain threshold of 1.2% was used for facial bone fracture. Average of 17% of engineering tensile strain indicates bridging vein rupture. For brain contusion, 277 kPa of brain pressure was calculated from reconstruction of one contusion case. Lastly, the high strains predicted by the FE head model match the trend of brain injuries reported in four real-world cases. Language: en

Development of a finite element human head model partially validated with thirty five experimental cases.

In contrast to the publicly available data on the safety of automobiles, consumers have no analytical mechanism to evaluate the protective performance of football helmets. The objective of this article is to fill this void by introducing a new equation that can be used to evaluate helmet performance by integrating player head impact exposure and risk of concussion. The Summation of Tests for the Analysis of Risk (STAR) equation relates on-field impact exposure to a series of 24 drop tests performed at four impact locations and six impact energy levels. Using 62,974 head acceleration data points collected from football players, the number of impacts experienced for one full season was translated to 24 drop test configurations. A new injury risk function was developed from 32 measured concussions and associated exposure data to assess risk of concussion for each impact. Finally, the data from all 24 drop tests is combined into one number using the STAR formula that incorporates the predicted exposure and injury risk for one player for one full season of practices and games. The new STAR evaluation equation will provide consumers with a meaningful metric to assess the relative performance of football helmets.

Development of the STAR Evaluation System for Football Helmets: Integrating Player Head Impact Exposure and Risk of Concussion

The current understanding of the tolerance of the frontal bone to blunt impact is limited. Previous studies have utilized vastly different methods, which limits the use of statistical analyses to determine the tolerance of the frontal bone. The purpose of this study is to determine the tolerance of the frontal bone to blunt impact. Acoustic emission sensors were used to provide a noncensored measure of the frontal bone tolerance and were essential due to the increase in impactor force after fracture onset. In this study, risk functions for fracture were developed using parametric and nonparametric techniques. The results of the statistical analyses suggest that a 50% risk of frontal bone fracture occurs at a force between 1885 N and 2405 N. Subjects that were found to have a frontal sinus present within the impacted region had a significantly higher risk of sustaining a fracture. There was no association between subject age and fracture force. The results of the current study suggest that utilizing peak force as an estimate of fracture tolerance will overestimate the force necessary to create a frontal bone fracture.

The Tolerance of the Frontal Bone to Blunt Impact

Dynamic material properties of the human sclera

Dynamic tensile properties of human placenta

Objective To determine the dynamic rupture pressure of the human eye by using an in vitro high-rate pressurization system to investigate blunt-impact eye injuries. Methods Internal pressure was dynamically induced in the eye by means of a drop-tower pressurization system. The internal eye pressure was measured with a small pressure sensor inserted into the eye through the optic nerve. A total of 20 human eye tests were performed to determine rupture pressure and characterize rupture patterns. Results The high-rate pressurization resulted in a mean (SD) rupture pressure of 0.97 (0.29) MPa (7275.60 [2175.18] mm Hg). A total of 16 eyes ruptured in the equatorial direction, whereas 4 ruptured in the meridional direction. There was no significant difference in the rupture pressure between the equatorial and meridional directions ( P  = .16). Conclusion As the loading rate increases, the rupture pressure of the human eye increases. Clinical Relevance Eye injuries are expensive to treat, given that the estimated annual cost associated with adult vision problems in the United States is $51.4 billion. Determining globe rupture properties will establish injury criteria for the human eye to prevent these common yet devastating injuries.

High-Rate Internal Pressurization of Human Eyes to Predict Globe Rupture

This study reports the results of 38 infraorbital maxilla impacts performed on male cadavers. Impacts were performed using an unpadded, cylindrical impactor (3.2 kg) at velocities between 1 and 5 m/s. The peak force and acoustic emission data were used to develop a statistical relationship of fracture risk as a function of impact force. Acoustic emission sensors were used to provide a noncensored measure of the maxilla tolerance and were essential due to the increase in impactor force after fracture onset. Parametric and nonparametric techniques were used to estimate the risk of fracture tolerance. The nonparametric technique produced an estimated 50% risk of fracture between 970 and 1223 N. The results obtained from the parametric and nonparametric techniques were in good agreement. Peak force values achieved in this study were similar to those of previous work and were unaffected by impactor velocity. The results of this study suggest that an impact to the infraorbital maxilla is a load-limited event due to compromise of structural integrity.

Jill A. Bisplinghoff

Papers

The Tolerance of the Frontal Bone to Blunt Impact

Dynamic material properties of the human sclera

Dynamic tensile properties of human placenta

High-Rate Internal Pressurization of Human Eyes to Predict Globe Rupture

The Tolerance of the Maxilla to Blunt Impact