$144 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)

4475 Th, 17:15-17:30 (P45) Lubrication analysis of total disc arthroplasties A. Shaheen, D.E.T. Shepherd. Department ef Mechanical and Manufacturing Engineering, University of Birmingham, Birmingham, UK

The human spine consists of a series of bony vertebrae that are separated by intervertebral discs. The traditional surgical method of treating back pain is spinal fusion. The future of spinal surgery appears to be moving away from spinal fusion towards the use of medical devices that will partially or totally replace the intervertebral disc. The aim of this study was to predict the lubrication regimes that occur between the bearing surfaces of total disc arthroplasties. The disc arthroplasty was modelled as a ball and socket joint. Using the lubrication theory of Hamrock and Dowson, the minimum film thickness was calculated from: hmi n = 2.798Rx(tlu/E ~ Rx)°65(L/E ~ R2) -021 where Rx is the equivalent radius of the two bearing surfaces, t/ is the viscosity of the lubricant, L is the load, E ~ is the equivalent elastic modulus and u is the entraining velocity. The radius of the bearing surfaces, load, velocity and materials were varied to investigate the effect on lubrication regime. The lubrication regime that the disc arthroplasty operates under was determined from:

= hmin/([ , where hmi n is the predicted minimum film thickness and 6 is the compound surface roughness for the two bearing surfaces. If the lambda ratio is larger than 3 then there is separation between the two bearing surfaces and the lubrication regime is fluid film. A lambda ratio less than unity indicates boundary lubrication and values of lambda ratio between 1 and 3 indicate a mixed lubrication regime. The results indicate that metal-on-polymer arthroplasties will operate with a mixed or boundary lubrication regime. Therefore, the generation of wear debris in these implants may be a future problem. Arthroplasties that have ceramic-on- ceramic bearing surfaces offer the potential to operate with fluid film lubrication.

Oral Presentations

6018 Mo, 08:30-08:45 (P5) Non-ankle lower extremity injury distribution in NASS and CIREN frontal crashes R.W. Rudd 1 , R. Hanna 2, C. Sherwood 1 , C. Burke 2, S.M. Fakhry 2. 1University of Virginia Center for Applied Biomechanics, Charlottesville, VA, USA, 2Honda Ineva Fairfax Hospital CIREN Center, Falls Church, VA, USA

Lower extremity injuries in frontal vehicle crashes deserve considerable at- tention because of the frequency with which they occur and the disability that typically follows. An analysis of real-world crash data collected by the United States National Highway Traffic Safety Administration was conducted to determine the prevalence and risk of injury to different non-ankle lower extremity regions. Shared data fields among the National Automotive Sam- pling System Crashworthiness Data System (NASS/CDS) and Crash Injury Research Engineering Network (CIREN) cases were searched for frontal vehicle crashes with a principle direction of force between 11 and 1 o'clock involving drivers who sustained at least a moderate lower extremity injury. The lower extremity was broken down into four regions including tibia/fibula shaft, knee/patella, femur, and hip/pelvis, and vehicles were classified as either passenger cars or LTVs, which included sport utility vehicles and light trucks. Logistic regression was used to estimate odds ratios for injury risk to different regions based on driver height and gender. Males demonstrated higher incidence and risk of hip/pelvis and femur fractures than females overall, but were found to have a lower incidence and risk of tibia/fibula fractures. Considering only LTV crashes, men were nearly five times more likely than women to sustain hip/pelvis fracture. Drivers below 165cm in height were approximately twice as likely to sustain tibia/fibula fractures than those above 175cm in passenger cars. Conversely, taller drivers were found to be at higher risk than shorter drivers for femur and hip/pelvis fractures in passenger cars. These results suggest a trade-off in injury risk between the tibia/fibula region and the hip/pelvis/femur region based on gender and height, which may be related to factors such as geometric effects of seating position or anatomical differences between men and women.

Track 5

Occupational and Impact Injury Biomechanics

5.1. Real World Injuries 7271 Mo, 08:15-08:30 (P5) A new protocol for documenting the causes and biomechanics of injury in crashes J.D. Rupp, L.W. Schneider, J.D. MacWilliams, The Members of the CIREN Engineering Committee. University of Michigan Transportation Research Institute, Ann Arbor, ML USA

The Crash Injury Research and Engineering Network (CIREN) Biomechanics Table, or BioTab, provides a means to accurately and objectively analyze and document the physical causes of injury in motor-vehicle crashes based on data from in-depth crash investigations and the medical and biomechanical literature. Using the BioTab involves determining a scenario by which it is believed an injury occurred. While this injury causation scenario (ICS) can be "caused by another injury", it is generally the set of crash, vehicle, occupant, and restraint conditions that were necessary for an AIS 3+ to have occurred as well as the factors that affected the likelihood and severity of the injury. The elements of an ICS include the source of energy producing the injury, the involved physical component(s) (IPCs), the body region(s) contacted by each IPC, the internal body-component path from the injured body region to each IPC, and all factors that may have affected the likelihood or severity of the injury. The BioTab documents the evidence supporting these elements uses it to determine confidence levels for each IPC and ICS. The BioTab also documents the specific "mechanisms" by which an injury is believed to have occurred. Importantly, the BioTab distinguishes between injury causation scenarios and injury mechanisms. While the latter are necessarily a function of a particular ICS, the mechanisms that produced an injury are specific descriptions of the physical response of occupant to the applied loading. In the BioTab, mechanisms can be documented at the body-region and organ/component level and include physical events such as compression, torsion, acceleration, and bending. As with IPCs and ICSs, evidence for injury mechanisms is documented in the BioTab and may include specific injury data obtained from the crash investigation (e.g., a butterfly fracture to a long bone indicates a bending mechanism), as well as information in the biomechanical and medical literature (e.g., ribs break in bending). As with ICSs and IPCs, confidence levels are assigned to injury mechanisms based on evidence.

5129 Mo, 08:45-09:00 (P5) Lower extremity injuries in children seated in forward facing child restraint systems J.S. Jermakian, K.B. Arbogast. Center for Injury Research and Prevention, The Children's Hospital of Philadelphia, Philadelphia, USA

Children restrained in forward facing child restraint systems (FFCRS) have an estimated reduction in the risk of serious injury of 71% over children of similar age restrained by seatbelts. For crashes resulting in serious injuries to children in FFCRS, the lower extremity is among the most injured body regions resulting in 28% of these injuries when considering all crash configurations. These children are at greater risk of lower extremity injury compared to slightly older children restrained by seatbelts. These injuries do not often present a threat to life but they can be devastating to growth and development, having significant long-term implications for the children and their families. Despite the prevalence of these injuries, the current US and European motor vehicle safety standards governing FFCRS (FMVSS 213 and ECE 44) do not address lower extremity injury risk nor do the current pediatric test devices provide adequate instrumentation to detect such injuries. This research examined the crash circumstances, types and mechanisms of AIS2+ lower extremity injuries sustained by children restrained by FFCRS through retrospective review of in- depth crash investigations. The most common crash scenario resulting in lower extremity injury involved children seated in the rear seat of passenger cars in frontal impact crashes. The tibia/fibula was the most common fracture location, accounting for more than 50% of all lower extremity fractures to children in FFCRS. These injuries were frequently caused by direct impact with the front seatback. In developing countermeasures for these lower extremity injuries, consideration should be given to the nature of the contact between the lower extremity and the vehicle seatback and thus any test configuration designed to address mitigation of this injury pattern should be assessed in an environment that includes both a rear and front seat.

5914 Mo, 09:00-09:15 (P5) The incidence of upper and lower extremity injuries from far side crashes H.C. Gabler 1, B. Fildes 2, M. Fitzharris 2, K. Digges 3. 1Virginia Tech, Blacksburg, Virginia, USA, 2 Monash University Accident Research Centre, Melbourne, Australia, 3 George Washington University, Ashbum, Virginia, USA

In a side impact, the occupants on both the struck, or near side, of the vehicle and the occupants on the opposite, or far side, of the vehicle are at risk of injury. Previous studies on the risk of side crash injury have made the surprising finding that the upper and lower extremities accounted for a combined 37% of the Harm and 25% of the serious injuries (MAIS3+) in far side crashes.

Track 5. Occupational and Impact Injury Biomechanics 5.2. Whiplash and Neck Injury Biomechanics $145

Serious injuries to the upper and especially the lower extremities are frequently disabling, and are a challenging issue for countermeasure design. This paper evaluates the risk of upper and lower extremity injury for vehicle occupants exposed to far side crashes. Based on the analysis of U.S. crash data from NASS/CDS 1995-2004 and Australian crash data from MIDS 1989- 2004, this study examines the injury outcome of over 4500 car, light truck, and van occupants subjected to far side impact. The analysis was restricted to 3-point belted occupants. The paper evaluates the nature of these upper and lowers extremity impact injuries by examining the relevant prevalence of injury by skeletal, vascular, or other soft tissue region injured in each crash. As a means of developing countermeasure development priorities, the paper presents injury risk as a function of injury contact source, struck body type, collision partner, delta-V, crash direction (PDOF), and occupant compartment intrusion. Injury risk is evaluated using the maximum injury severity for each occupant, by injury severity for each body region, and by Harm, a social cost measure.

6641 Mo, 09:15-09:30 (P5) Characterizing occupant injuries in vehicle crashes with a deployed side airbag F.A. Pintar, N. Yoganandan. Medical College of Wisconsin and VA Medical Center, Neuroscience and Biomechanics Research Laboratories, Milwaukee, Wl, USA

To gather a sufficient data set to examine the characteristics of real-world side airbag deployment crashes, both the United States National Automotive Sampling System (NASS) and the US Crash Injury Research and Engineering Network (CIREN) databases were queried. The weighted NASS data produced 7812 crashes between 1997 and 2004. AIS >~2 level injuries occurred to 5071 occupants. There were 3828 cases of torso only airbags, 955 cases of torso-head bag combination, 288 inflatable tubular structure/curtain systems. A preponderance of the side airbag systems deployed from the seat, followed by some from the door and very few from the roof side rail. The CIREN data set yielded a similar division between type of side airbag system. Side airbags were not attributed to be the cause of head or chest injury to any occupant at this level of severity. The predominance of torso airbags followed by torso- head airbag combination systems reflected vehicle model years and changing technology. Greater than 90% of occupants in these vehicles wore three-point belts. The injuries were similar to those described in the literature for side impacts without airbags; head and chest trauma predominated the data set. The velocity at which equivalent trauma occurred however, appeared higher for occupants in side airbag-deployed crashes. The additional detail included in the CIREN database was beneficial in determining mechanisms of injury and potential effectiveness of side airbag systems in real-world crashes. Because airbag technology continues as a safety feature in motor vehicles, data from this study may assist in future analysis of side airbag efficacy and countermeasure design. This study presents the largest known real-world data set on side airbag crashes.

7049 Mo, 09:30-09:45 (P5) Predictors of traumatic rupture of the aorta in near-side impacts J.M. Cavanaugh 1 , J. Augenstein 2, J. Stratton 2, J. Mackinnon 2, L. Labiste 2, E. Perdeck 2. 1Wayne State University, Bioengineering Center, Detroit, USA, 2University of Miami, Ryder Trauma Center, Miami, USA

Traumatic rupture of the aorta (TRA) is a leading cause of death in high velocity motor vehicle accidents. The purpose of this study was determine occupant, vehicle and injury parameters that predict aortic injury. Methods: Near-side impacts occurring between 1996 and 2003 were analyzed in the Crash Care database of the University of Miami Ryder Trauma Center. Inclusion of subjects involved a consent process, clinical data collection, vehicle inspection and case review and analysis. Occupant parameters included age, height, weight and gender. Vehicle parameters included delta V, maximum crush, maximum intrusion, and vehicle damage pattern. A damage pattern to the side of the vehicle that included the occupant compartment and was in the shape of a V when viewed from the top of the vehicle was termed V damage. Injury parameters included the AIS and ISS values for rib fracture, internal chest injury, abdominal injury, pelvic injury and combinations of these. Logistic regression and Receiver-Operating Characteristics (ROC) were used to determine the relationships between TRA and these parameters. Results: Of the 167 near side impact victims, 84 were males and 83 females. The average age was 38.6 years and average weight was 162 pounds. Forty- four occupants (26%) had aortic injury. Occupant age, weight and height did not predict aortic injury. Delta V, vehicle crush, vehicle intrusion and V damage each had some ability to predict aortic injury. Delta V averaged 46.3kph, maximum crush 64.4 cm and maximum intrusion 46.3cm in the aortic injury group. These values were 39.1, 53.7 and 37.6, respectively, in the group without aortic trauma. AIS and ISS values of rib fracture, internal chest injury and abdominal injury were more predictive of aortic injury than vehicle crash

parameters. Internal chest injury AIS averaged 3.5 in the aortic injury group and 1.5 in the non-injury group. Linear combinations of delta V, intrusion, V damage, age and chest injury severity improved the ability to predict aortic injury compared to the individual parameters.

5.2. Whiplash and Neck Injury Biomechanics 4298 Mo, 11:00-11:15 (Pa) The biomechanical and kinematic differences between rear impact and frontal impact automobile crashes at low velocities A.C. Croft. Spine Research Institute of San Diego; Southern California University of Health Sciences, USA

Background: Numerous factors are believed to influence the risk for injury to the cervical spine in low speed automobile crashes. Several studies have indi- cated that rear impact crashes are associated with greater risk for injury and, perhaps, a worse prognosis. We sought to answer the question concerning the possible reasons for the disparity in risk between front vector crashes and rear vector crashes using human subject crash testing. Materials and Methods: Instrumented human subjects were placed in in- strumented crash test vehicles. Occupant accelerations were recorded. Force and moment analysis were calculated. Vehicle acceleration and speed metrics were recorded. Volunteers were subjected to three rear impact crashes, two of which were conducted in the unaware mode, and one in the aware mode in which the subjects were allowed to brace for the impact. All variables were held constant between frontal and rear impact sequences except the relative roles of the two vehicles (e.g., striking or struck). Thus, the only variables with this crossover study design were the impact vectors. Results: Volunteers rated their subjective experiences in the rear impact crashes as markedly more traumatic or physically unpleasant than in the frontal crashes. When holding vehicle mass, crash speeds, occupant variables and their interactions constant, the acceleration of the subjects' heads was nearly three times higher in rear impact crash vectors vs. frontals. The resulting occupant kinematics were more complex in the rear impact crash. Additionally, it appears that the mechanism of injury in whiplash may vary with occupant mass. Conclusions: The results suggest that more attention should be given to crashworthiness in the rear impact crash vector. The small study size does not allow a high level of confidence in regards to our findings of differential forces and resulting kinematics, but our results were always consistent and the observed differences were quite large. To our knowledge, this is the first study to look specifically at differential effects of rear vs. frontal crashes at low speeds, holding all other variables constant.

6151 Mo, 11:15-11:30 (Pa) Comparison of ATD upper and lower neck flexion/extension moments, and implications for neck injury criteria

C. Raasch 1 , M. Carhart 1,2. 1Exponent Failure Analysis Associates; 2 Harrington Department of Bioengineering, Arizona State University, USA

Recent research on neck injury thresholds has focused on upper cervical spine injury in airbag deployments; however, the lower cervical spine may often be injured by other mechanisms. Anthropomorphic Test Device (ATD) injury assessment reference values (IARVs) have been suggested for the lower neck, and were derived by scaling upper neck values. We have undertaken a study to independently develop ATD lower neck IARVs for extension and flexion by matching ATD tests with cadaver studies where lower cervical spine injuries were obtained (e.g., Clemens and Burow, SAE 720960). Pendulum and sled tests simulating frontal and rear impacts were performed using Hybrid III 50 th- percentile male and 5th-percentile female ATDs with upper and lower neck load cells. Delta-Vs at the T1 level (or sled delta-V for rigid seatback tests) ranged from 5 to 9 m/s, with pulse durations of 30 to 90 ms. For tests of similar severity, the 50th-male upper and lower neck peak moments were higher than those of the 5th-female, while head/neck angles were lower. Peak neck moments for the 50th-male and 5th-female were found to be related by a scaling factor of approximately 1.6 for lower neck, and 1.3 for upper neck. Comparison of lower and upper neck moments within each test produced non-linear relationships. At lower severities, lower and upper neck peak moments for both ATDs appeared to be related by a factor of 2, as would be expected in longitudinal loading of the upright head and neck, due to geometrical considerations alone. However, for higher severities, peak lower neck moments were modulated more strongly than upper neck moments, following power relationships. This may occur because upper neck loading transitions from bending to tension with greater head/neck angular excursion. Our results, in addition to preliminary threshold values derived from matched cadaver tests, indicate that simple scaling of upper neck injury thresholds may overestimate lower neck injury thresholds.