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°. One joint from each cadaver was impacted sequentially at increasing velocities to establish a range of loads and impact energies needed to cause gross fracture. The contralateral joints were impacted only once at a lower velocity that generated fracture and nonfracture data. The location of damage on the tibial plateau and femoral condyles coincided with the location of highest tibiofemoral contact pressure. Repeated impacts resulted in tibiofemoral fracture to the medial and lateral tibial plateau, medial femoral condyle, and femoral notch for a maximum load of 8.0 ± 1.8 kN. Single impact experiments resulted in a 33% frequency of fracture at 5.8 ± 1.5 kN. A microscopic analysis of the remaining eight single impact joints indicated occult damage at the cartilage/subchondral bone interface for a load of 7.1 ± 2.4 kN. These damages were consistent with the location of gross fracture. The location of these micro-cracks were also consistent with the location of occult bone trauma in a limited number of motor vehicle accident MRI trauma patients referred to Michigan State University. This study helps establish impact injury thresholds for the flexed human tibiofemoral joint that will be relevant to the automotive industry.