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Lower extremity (knee) trauma is currently based on a bone fracture criterion derived from impacts of aged specimens. Recent clinical studies, however, indicate that a chronic disease (post-traumatic osteoarthritis), may be precipitated after mechanical insult without obvious bone fracture(1). It is hypothesized this is due to microcracking of subchondral bone under cartilage. This hard tissue layer is known to change with age and pathology. Ten ‘aged’ (71 years) and ten ‘young’ (47 years) cadaver knee joints were impacted to study the influence of age and pathology on the fracture load, and incidents of occult injury. Our results indicate that fracture load, per se, was independent of specimen age. On the other hand, severely pathological specimens required significantly higher loads to fracture bone. Occult microcraking was also observed in subfracture experiments, however, fewer incidents were recorded for the ‘aged’ specimens.

The location of the pelvis in the seated automobile operator is critical for proper packaging and seat comfort design. The pelvis is the skeletal structure which contains the hip joint (H-point) and ischial tuberosity (D-point). The orientation of the pelvis largely determines the curvature in the low back which is supported by lumbar supports in the seat back. A methodology has been developed that uses onboard video and pressure measurement systems to locate the pelvis. This system has been used in a mid-sized vehicle on seated operators driving the vehicle on the highway. This paper describes the methodology and the location of the pelvis in seated automobile operators.

This study showed that subfracture impact loading to a joint creates stresses in cartilage and bone which can initiate a chronic osteoarthrosis. The magnitude and location of the impact induced stresses are dependent on the orientation and the intensity of loading. Impact loading produced lesions on retro-patellar cartilage and their depths increased as the thickness of subchondral bone increased with time post-impact. Mechanical tests of cartilage indicated significant softening twelve months post-impact. These alterations are similar to those documented clinically as early OA. In vitro impacts of isolated limbs, together with mathematical models, showed that high mean stress generated during impact may help protect joint tissues from acute injury. This study and others are being used to develop stress-based tissue failure criteria for predicting an osteoarthrosis following subfracture impact loading.

A comparative investigation of airbag and HANS driver safety systems was carried out (HANS, is a Registered Trademark in the U.S.A.). With both systems, head and neck loads were reduced from potentially fatal values to values well below the injury threshold. Both systems performed similarly in reducing the potential for driver injury. For this reason and given the high costs of development and testing, there is no justification for further development of airbags for racing.

Lower extremity injuries due to automobile accidents are often overlooked, but can have a profound societal cost. Knee injuries, for example, account for approximately 10% of the total injuries. Fracture of the knee is not only an acute issue but may also have chronic, or long term, consequences. The criterion currently used for evaluation of knee injuries in new automobiles, however, is based on experimental impact data from the 70''s using seated human cadavers. These studies involved various padded and rigid impact interfaces that slightly alter the duration of contact. Based on these data and a simple mathematical model of the femur, it appears fracture tolerance increases as contact duration shortens. In contrast, more recent studies have shown mitigation of gross fractures of the knee itself using padded interfaces. The use of padded interfaces, however, result in coincidental changes in contact duration and knee contact area.

National Accident Sampling System (NASS) data, for the years 1993-1995, suggests a high frequency of tibiofemoral joint fractures among automotive accident victims. In addition, the NASS data also suggests that these injuries may be attributable to direct axial loading via the floor pan and/or the foot controls. Hirsch and Sullivan (1965), and Kennedy and Bailey (1968) conducted quasi-static fracture experiments axially compressing human tibiofemoral joints at low rates of loading and low angles of flexion. Hirsch and Sullivan observed a mean fracture load of approximately 8 kN compared to approximately 16 kN in the Kennedy and Bailey study. The current tibiofemoral joint injury criterion used in anthropomorphic dummies is based on Hrisch and Sullivan''s data. The current study involved impact experiments on human tibiofemoral joints (aged 71.4±11.2) directed in a superior direction along the axis of the tibia with the joint flexed 90°.

While the number of deaths from vehicle accidents is declining, probably because of mandatory seat belt laws and air bags, a high frequency of lower extremity injuries from frontal crashes still occurs. For the years 1979-1995 the National Accident Sampling System (NASS) indicates that knee injuries (AIS 1-4) occur in approximately 10% of cases. Patella and femur fractures are the most frequent knee injuries. Current literature suggests that knee fractures occur in seated cadavers for knee impact forces of 7.3 to 21.0 kN. Experimental data shown in a study by Melvin et al. (1975) further suggests that fracture tolerance of the knee may be reduced for an impact directed obliquely to the axis of the femur. The current study hypothesized that the patella is more vulnerable to fracture from an oblique versus an axial impact (directed along the femoral axis), and that the fracture pattern would vary with impact direction.