Shashi Kuppa

Dynamic Axial Tolerance of the Human Foot-Ankle Complex

Dynamic axial impact tests to isolated lower legs were conducted at the Medical College of Wisconsin laboratory in the USA. The aim is to develop a more definitive and quantitative relationship between biomechanical parameters such as specimen age, axial force, and injury. Twenty-six intact adult lower legs excised from unembalmed human cadavers were tested under dynamic loading using a mini-sled pendulum device. Results from these tests were combined with the data from the studies by Wayne State University and Calspan Corporation, both in the USA. The total sample size available was 52. Statistical analysis of these data was performed using Weibull techniques. Age and dynamic axial force were the most significant discriminant variables that defined the injury risk function. Consequently, the probability of foot-ankle injury was described in terms of specimen age and force. The findings are a first step towards the quantification of the dynamic tolerance of the human foot-ankle complex under the axial impact modality.For the covering abstract of the conference see IRRD 891635.

Biomechanical study of pediatric human cervical spine : A finite element approach

Although considerable effort has been made to understand the biomechanical behavior of the adult cervical spine, relatively little information is available on the response of the pediatric cervical spine to external forces. Since significant anatomical differences exist between the adult and pediatric cervical spines, distinct biomechanical responses are expected. The present study quantified the biomechanical responses of human pediatric spines by incorporating their unique developmental anatomical features. One-, three-, and six-year-old cervical spines were simulated using the finite element modeling technique, and their responses computed and compared with the adult spine response. The effects of pure overall structural scaling of the adult spine, local component developmental anatomy variations that occur to the actual pediatric spines, and structural scaling combined with local component anatomy variations on the responses of the pediatric spines were studied. Age- and component-related developmental anatomical features included variations in the ossification centers, cartilages, growth plates, vertebral centrum, facet joints, and annular fibers and nucleus pulposus of the intervertebral discs. The flexibility responses of the models were determined under pure compression, pure flexion, pure extension, and varying degrees of combined compression-flexion and compression-extension. The pediatric spine responses obtained with the pure overall (only geometric) scaling of the adult spine indicated that the flexibilities consistently increase in a uniform manner from six- to one-year-old spines under all loading cases. In contrast, incorporation of local anatomic changes specific to the pediatric spines of the three age groups (maintaining the same adult size) not only resulted in considerable increases in flexibilities, but the responses also varied as a function of the age of the pediatric spine and type of external loading. When the geometric scaling effects were added to these spines, the increases in flexibilities were slightly higher; however, the pattern of the responses remained the same as found in the previous approach. These results indicate that inclusion of developmental anatomical changes characteristic of the pediatric spines has more of a predominant effect on biomechanical responses than extrapolating responses of the adult spine based on pure overall geometric scaling.

DEVELOPMENT OF AN IMPROVED THORACIC INJURY CRITERION

In this study, seventy-one frontal impact sled tests were conducted using post-mortem human subjects in the drivers position in an effort to better understand thoracic trauma in frontal impacts. Various contemporary automotive restraint systems were used. The resulting injury from the impact was determined through radiography and detailed autopsy, and its severity was coded according to the Abbreviated Injury Scale (AIS). The measured mechanical responses were analyzed using statistical procedures. In particular, linear logistic regression was used to develop models which associate the measured mechanical parameters to the observed thoracic injury response. Univariate and multivariate models were developed taking into consideration potential confounders and effect modifiers. The risk factors used in the models were normalized concerning the size and weight of the specimen. The gender and age of specimen at time of death were found not to be confounders in this data set. A linear combination of the 3-msec clip value of maximum resultant spine acceleration and maximum normalized chest deflection from an array of five measurements provided the goodness of fit measure. This linear combination was found to have significantly better injury predictive ability, for thoracic trauma in human subjects under any restraint environment, than other existing injury criteria such as VCmax ( Maximum Viscous Criterion), chest deflection, or chest acceleration alone. For the covering abstract of the conference see IRRD E201429.

Injury risk curves for the skeletal knee-thigh-hip complex for knee-impact loading

Injury risk curves for the skeletal knee-thigh-hip (KTH) relate peak force applied to the anterior aspect of the flexed knee, the primary source of KTH injury in frontal motor-vehicle crashes, to the probability of skeletal KTH injury. Previous KTH injury risk curves have been developed from analyses of peak knee-impact force data from studies where knees of whole cadavers were impacted. However, these risk curves either neglect the effects of occupant gender, stature, and mass on KTH fracture force, or account for them using scaling factors derived from dimensional analysis without empirical support. A large amount of experimental data on the knee-impact forces associated with KTH fracture are now available, making it possible to estimate the effects of subject characteristics on skeletal KTH injury risk by statistically analyzing empirical data. Eleven studies were identified in the biomechanical literature in which the flexed knees of whole cadavers were impacted. From these, peak knee-impact force data and the associated subject characteristics were reanalyzed using survival analysis with a lognormal distribution. Results of this analysis indicate that the relationship between peak knee-impact force and the probability of KTH fracture is a function of age, total body mass, and whether the surface that loads the knee is rigid. Comparisons between injury risk curves for the midsize adult male and small adult female crash test dummies defined in previous studies and new risk curves for these sizes of occupants developed in this study suggest that previous injury risk curves generally overestimate the likelihood of KTH fracture at a given peak knee-impact force. Future work should focus on defining the relationships between impact force at the human knee and peak axial compressive forces measured by load cells in the crash test dummy KTH complex so that these new risk curves can be used with ATDs.

The Feasibility and Effectiveness of Belt Pretensioning and Load Limiting for Adults in the Rear Seat

The US Fatality Analysis Reporting System (FARS) and State Data System (SDS) for Florida, Pennsylvania and Maryland were utilised to estimate relative fatality rates and injury risk ratios between the front and rear-seat passengers, and a parametric study of rear-seat restraint parameters was performed to assess chest deflection and head excursion trends. The fatality and serious injury risk in frontal crashes is found to be higher for older occupants in rear seats than for those in front seats. In addition, the relative effectiveness of rear seats is lower in newer vehicle models, presumably due to the advances in front-seat restraint technology. The simulations demonstrate that injury risk in the rear seat can be reduced by incorporating front-seat restraint technology (load limiting and pretensioning), even in the absence of an air bag and knee bolster. A force-limiting belt with a pretensioner can maintain or reduce head excursion relative to a standard belt, while reducing thoracic injury risk. In fact, 42 sets of restraint parameters were identified that reduced both head excursion and chest deflection relative to the baseline belt.

An Injury Risk Curve for the Hip for Use in Frontal Impact Crash Testing

To facilitate the assessment of hip injury risk in frontal motor-vehicle crashes, an injury risk curve that relates peak force transmitted to the hip to the probability of hip fracture was developed by using survival analysis to fit a lognormal distribution to a recently published dataset of hip fracture forces. This distribution was parameterized to account for the effect of subject stature, which was the only subject characteristic found to significantly affect hip fracture force (X(2)(1)=6.03, p=0.014). The distribution was further parameterized to account for the effects of hip flexion and abduction from a standard driving posture on hip fracture force using relationships between mean hip fracture force and hip flexion/abduction reported in the literature. The resulting parametric distribution was used to define relationships between force applied to the hip and the risk of hip fracture for the statures associated with the small female, midsize male, and large male crash-test dummies, thus allowing these dummies to assess hip fracture/dislocation risk in frontal crashes, provided that such dummies are sufficiently biofidelic. For the midsize male crash test dummy, a 50% risk of hip fracture was associated with a force of 6.00kN. For the small female and large male dummies, a 50% risk of hip fracture was associated with forces of 4.46 and 6.73kN, respectively.