The main objective of this study is to determine the damage tolerance and describe the damage mechanisms of the extended human knee when it is exposed to lateral impact loads in car-pedestrian accidents, particularly those that occur at a low velocity (20 km/h), and compare the results with those obtained at a high velocity (40 km/h). In-vitro experiments with human subjects were conducted where only the purest possible shearing deformation or the purest possible bending deformation affected the knee region at the time. Five experiments were performed in the shearing setup and another five in bending setup. The peak values of the shearing force and the bending moment related to the damage of knee ligaments and bone fractures were calculated at the knee joint level. Damages were assessed by dissecting the lower extremity.
When the knee joint was exposed to the “purest possible shearing deformation”, the common initial damagemechanism was ligament damage related to ACL (60% of cases). This type of damage occurred when mean values of the peak shearing force and the peak bending moment acting at the knee joint level were 2.4 kN (SD 0.2) and 418 Nm (SD 100), and the shearing displacement and bending angle were 16 mm (SD 4) and -2.9° (SD 0.2), respectively.
When the knee joint was exposed to the “purest possible bending deformation”, the most common initial damage mechanism (40% of cases) was ligament damage related to MCL. The mean values of the peak shearing force and the peak bending moment calculated when this damage occurred were 1.6 kN (SD 0.4) and 358 Nm (SD 167), respectively. This type of initial damage occurred when the knee was bent 12.3° (SD 2.9). The initial meta-physis fracture of the femur due to bending deformation of the knee was observed in only 20% of the cases. The mean values of the peak shearing force and the peak bending moment developed at knee joint level that correspond to this damage were 0.9 kN and 205 Nm. This type of damage occurred when the knee was bent 12.3°.
The physical values of the shearing force and the bending moment at the time of initial damage for low-speed lateral loading were found to be similar with those from previously performed experiments at high-speed lateral loading (40 km/h).
The ratio of bone fracture to ligament damage was 0.3 in the purest possible shearing deformation test, and 0.5 in the purest possible bending deformation at low speed. In previously performed experiments with high-speed lateral loading, the ratio was 1.3 in the purest possible shearing deformation test, and 2.7 in the purest possible bending deformation test.