Theresa Atkinson

Impact-Induced Fissuring of Articular Cartilage: An Investigation of Failure Criteria

Several candidate predictors for the occurrence of surface fissures in cartilage, including impact force, shear stress, and tensile strain have been previously proposed without an analytic basis. In this study a controlled impact experiment was performed where a dropped mass and three impact interfaces were used to identify loads associated with the initiation of fissuring. A Finite Element Model of each experiment was used to obtain stresses and strains associated with each impact event. The resulting experimental and analytical data were analyzed using logistic regression in order to determine the strongest predictor of a fissure, and thus to propose a failure criterion for articular cartilage during a blunt insult. The logistic regression indicated that shear stress, rather than impact force or drop height (an indicator of impact energy), was the strongest predictor for the occurrence of a fissure.

THE TENSILE AND STRESS RELAXATION RESPONSES OF HUMAN PATELLAR TENDON VARIES WITH SPECIMEN CROSS-SECTIONAL AREA

In order to provide insight into the mechanical response of the collagen fascicle structures in tendon, a series of constant strain rate and constant displacement, stress relaxation mechanical tests were performed on sequentially sectioned human patellar tendon specimens (protocol 1) and specimens with both small (approximately 1 mm2) and large (approximately 20 mm2) cross-sectional areas (protocol 2). These data described the stress relaxation and constant strain rate tensile responses as a function of cross-sectional area and water content. The experimental data suggested that small portions of tendon exhibit a higher tensile modulus, a slower rate of relaxation and a lower amount of relaxation in comparison to larger specimens from the same location in the same tendon. The decrease in relaxation response and the increase in tensile modulus with decreasing cross-sectional area was nonlinear. These data suggest that there may be structures other than the subfascicle, such as the epitenon and other connective tissue components, which influence the tensile and stress relaxation responses in tendon.

A poroelastic model that predicts some phenomenological responses of ligaments and tendons

Experimental evidence suggests that the tensile behavior of tendons and ligaments is in part a function of tissue hydration. The models currently available do not offer a means by which the hydration effects might be explicitly explored. To study these effects, a finite element model of a collagen sub-fascicle, a substructure of tendon and ligament, was formulated. The model was microstructurally based, and simulated oriented collagen fibrils with elastic-orthotropic continuum elements. Poroelastic elements were used to model the interfibrillar matrix. The collagen fiber morphology reflected in the model interacted with the interfibrillar matrix to produce behaviors similar to those seen in tendon and ligament during tensile, cyclic, and relaxation experiments conducted by others. Various states of hydration and permeability were parametrically investigated, demonstrating their influence on the tensile response of the model.

Knee injuries in motor vehicle collisions: a study of the National Accident Sampling System database for the years 1979–1995

A detailed study of knee injuries recorded in the 1979-1995 National Accident Sampling System database maintained by the National Highway Traffic Safety Administration was conducted. Injuries to other body regions were also considered in order to illustrate the relative frequency of knee injuries. This study demonstrated that knee injuries constitute approximately 10% of all injuries recorded every year. However, the majority of these injuries were of low severity (i.e. contusions, abrasions, lacerations) with an abbreviated injury score (AIS) of 1. Most knee injuries occurred following a frontal collision with no intrusion. The study also indicated most knee fractures occur in crashes where the vehicle velocity differences (deltaVs) were less than 45 kmph, with some occurring at deltaVs as low as 10 kmph. Serious non-fracture knee injuries (i.e. ligament tears) rated AIS 2 accounted for 20 out of every 1000 injuries and predominantly occurred at deltaVs below 25 kmph. In this study it was noted that women were more likely to experience a knee contusion than men. This study further suggests that knee impact scenarios have remained relatively constant over the years as the knee injury rates showed little variation. The rate of lap and shoulder belt use was lower in occupants who experienced a knee injury vs. the rate in the overall database and airbags were present in only a small number of cases. As this study largely included only vehicles without airbags it provides a good baseline for analysis of the influence of the airbag on knee injury trends in the future.

An Investigation of Biphasic Failure Criteria for Impact-Induced Fissuring of Articular Cartilage

Articular cartilage consists of both solid and fluid phases with fissures observed on the surface occurring in the solid portion. In order to determine which of the solid phase stresses provides the best predictor for the initiation of a fissure, elastic stresses from a series of in vitro impact experiments were used to derive stresses in the solid phase of the cartilage. This stress information was then analyzed using a logistic regression to identify the best predictor of fissuring. The mechanical analysis indicated that low-magnitude tensile solid hoop stress develops in the solid phase within the contact zone in impacts involving the two smaller radius interfaces. The logistic regression, however, indicated that maximum shear stress in the solid (which is equal to the shear stress from the elastic analysis) was the best predictor of the occurrence of a fissure. This study helps support the suggestion that in stress fields dominated by compression, the maximum shear stress from an elastic analysis may be used to predict fissure initiation in cartilage.

A method to increase the sensitive range of pressure sensitive film

Pressure-sensitive film is frequently used in biomechanics to document intra- and extra-articular contact pressures. This often involves the contact of two surfaces of varying curvature producing non-uniform pressure distributions. The purpose of this study was to determine the feasibility of using multiple films in such experiments to yield accurate pressure and contact area data. A composite arrangement of film was dynamically loaded using cylindrical indenters of five radii. An analytical model of each indentation was constructed to provide a standard for error analysis. The study showed that several ranges of pressure sensitive film can be used simultaneously to accurately transduce contact pressures arising from loading scenarios that produce contact pressure gradients and contact pressures that involve suprathreshold loading of a given film range.

DEVELOPMENT OF INJURY CRITERIA FOR HUMAN SURROGATES TO ADDRESS CURRENT TRENDS IN KNEE-TO-INSTRUMENT PANEL INJURIES

Lower extremity injuries during car accidents are common; the lower extremities are typically the first point of contact between the occupant and the car interior. Lower extremity injuries are not normally life threatening, but can represent a large societal burden through treatment costs, lost work days, and reduced quality of life. The purpose of this research was to study injuries of the knee and propose a methodology to prevent future knee injuries. Data from the National Accident Sampling System (NASS) showed that 10% of all injuries were to the knee, second only to head and neck injuries. Most knee injuries are a result of knee-to-instrument panel and subfracture injuries were most common, followed by gross fracture injuries. Cadaver data show that increasing the contact area for a given contact force over the knee greatly reduces acute injury in fracture and subfracture experiments. However, cadaver force-area data cannot be applied to the Hybrid III dummy, which is the most used human surrogate in car crash simulations. This study also sought to develop a transformation of the cadaver contact force-area relationship to the dummy. Numerous experiments were conducted on the dummy to establish a comparison with companion experiments conducted on cadavers. Data points representing a 50% risk of gross fracture were calculated for the cadaver and transformed into the dummy response to yield data directly relevant to sled testing with dummies. Several sled tests were run using an idealized instrument panel to show the utility of the data in predicting joint injury for depowered air bags and various restraint scenarios. Mathematical models were used to show a theoretical scenario in which load and area could be estimated without the need for sled testing. This study shows that a simple measure of knee contact load and area could be used to predict injuries in the cadaver knee from blunt insult via dummy test data and hopefully provide increased knee injury protection for car occupants.

Breaking strength retention and histologic effects around 1.3-mm. ORTHOSORB® polydioxanone absorbable pins at various sites in the rabbit

Absorbable 1.3-mm polydioxanone (ORTHOSORB) pins were implanted in 75 New Zealand White rabbits in three sites: within the lateral subcutaneous tissue parallel to the femur, down the femoral intramedullary canal, and mediolaterally across the femoral condyles (transcondylar). Pins were harvested at periodic intervals up to 56 and 365 days for mechanical and histologic analyses, respectively. Mechanical analyses were performed by loading the pin in double shear. Histologic analyses were performed on the pin and surrounding tissue. Histologic observations revealed a typical nonspecific foreign-body reaction at all implant sites that resolved at 1 year after resorption of the pin. On histologic examination, there was complete resorption of the pin material in the subcutaneous site by day 182, and there was complete resolution of all response to the pin in six of nine rabbits by day 365. In the intramedullary site, pin material was completely resorbed, based on histologic examination, in five of six rabbits by day 182, and there was complete resolution of the response to the pin in eight of nine rabbits by day 365. The pin material was completely resorbed based on histologic examination of the transcondylar site by day 210, and there was complete resolution of the response to the pin in four of six rabbits by day 270 and in four of nine rabbits by day 365. No enlarged pin tracks or sinus formations were observed in or near the implants sites. The average initial shear strength as 171.4+/ 5.1 MPa, and the breaking strength retention decreased with increasing implantation time. Pins from the subcutaneous regions maintained above 97% of their initial strengths at 28 days, and those from the intramedullary canals maintained above 92%. At later times the strength of the pins implanted in the intramedullary canal decreased more rapidly than those from the subcutaneous region. Overall, the average breaking strength of the subcutaneous pins was significantly greater than that of the intramedullary pins at all time points beyond 14 days. These data indicate that the pins exhibited a strength retention profile sufficient to allow normal healing of bone without enlarged pin tracts, allergic reactions, or sinus formations.

Lower extremity injuries in motor vehicle collisions: a survey of NASS 1997-1999

Lower extremity injury is common in front seated occupants of motor vehicles involved in collisions. The recent introduction of air bags, while intended to reduce injury to vital body regions, namely the head and chest, may influence the incidence of lower limb injury. The current study examined lower extremity injuries recorded in the National Automotive Sampling System in order to identify injury rates and trends. NASS data from 1995-1999 suggests drivers sustain a lower extremity injury in 35% of collisions. However, the rate of moderate-to-severe injury is low occurring in approximately 11% of front seat occupants. The most common site for moderate-to-serious injury was the foot/ankle. The thigh was most frequently the site of serious lower extremity injuries, with the majority of these injuries due to instrument panel contact. Air bag deployment does not appear to alter the risk of lower extremity injury, while seat belt use tends to slightly decrease the risk.

The Influence of Occupant Anthropometry and Seat Position on Ejection Risk in a Rollover

Purpose: During rollover crashes, ejection increases an occupants risk of severe to fatal injury as compared to risks for those retained in the vehicle. The current study examined whether occupant anthropometry might influence ejection risk. Factors such as restraint use/disuse, seating position, vehicle type, and roll direction were also considered in the analysis. Methods: The current study examined occupant ejections in 10 years of National Automotive Sampling System (NASS) single-event rollovers of passenger vehicles and light trucks. Statistical analysis of unweighted and weighted ejection data was carried out. Results: No statistically significant differences in ejection rates were found based on occupant height, age, or body mass index. Drivers were ejected significantly more frequently than other occupants: 62 percent of unrestrained drivers were ejected vs. 51 percent unrestrained right front occupants. Second row unrestrained occupants were ejected at rates similar to right front–seated occupants. There were no significant differences in ejection rates for near- vs. far-side occupants. Conclusions: These data suggest that assessment of ejection prevention systems using either a 50th or 5th percentile adult anthropomorphic test dummy (ATD) might provide a reasonable measure of system function for a broad range of occupants. They also support the development of ejection mitigation technologies that extend beyond the first row to protect occupants in rear seat positions. Future studies should consider potential interaction effects (i.e., occupant size and vehicle dimensions) and the influence of occupant size on ejection risk in non-single-event rollovers.