Abstract
There is an increasing focus on lower limb injuries occurring in motor vehicle accidents, as passive safety measures reduce the relative risk of fatal head and thorax injuries. With this comes an increase in significance and impact of joint injuries. Subchondral and interfacial tidemark injuries caused by motor accidents can be attributed largely to the high loading speed involved.
It is therefore important, for health reasons, to establish a threshold loading speed above which debilitating injury is likely. Joint injuries can lead to later joint conditions such as osteoarthritis and osteoarthrosis. In the younger human beings osteochondritis dissecans could also eventuate. These joint conditions have heavy psychological and financial implications, and enforce a resulting burden on health services.
Pure shear loading configuration at different rates of loading were employed in the study to adequately model the physiological loading conditions such as those resulting from severe knee-dashboard and knee-bumper impacts resulting from high speed vehicle impacts.
Data obtained from the series of experiments was used to establish the hypothesis that under sustained pure shear loading an age-dependent threshold level of loading speed, with concomitant severity of impact loading, exists beyond which the osteochondral junction would be severely impaired for both load-bearing purposes and resistance to fracture.
A threshold loading speed of approximately 83.3x10 -3 m/s was found to exist beyond which the osteochondral regions ability to resist fracture was significantly diminished. The average energy to initiate fracture at this speed was 23.1 J ±1.5(P95) for the mature specimens and 18.4 J ±3.84(P95) for the immature specimens.
This threshold loading speed is related to the size of the tested specimens. It is obviously not possible to conduct pure shear tests on full joints and the results from the small specimens (0.01m cubed) used require scaling to identify the fracture resistance and loading speed threshold for the osteochondral junctions of complete joints.
The influence of level of maturity on fracture of the osteochondral junction under sustained, predominantly pure shear, loading was small but significant.