An analysis of the National Automotive Sampling System/ Crashworthiness Data System (NASS/CDS) for the years 1993-1999 was conducted to determine the risk of injury to different body regions in frontal crashes. Lower extremities were the leading injured body region. The risk of lower limb injuries was significant in all crash modes. A detailed examination of these lower extremity injuries was then conducted using the AIS-90 injury codes. The long term consequence of lower extremity injuries was estimated using the Functional Capacity Index (FCI) associated with each AIS-90 injury code. The effect of a particular injury on society was reported in terms of total Functional Life-years Lost to Injury (LLI) which is defined as the product of FCI and the injured personis life expectancy.

Using existing biomechanical data on lower extremity injuries, injury criteria and associated injury risk curves were synthesized for different regions of the lower extremity, namely 1) knee-thigh- hip complex fractures, 2) knee ligaments tears, 3) tibial plateau/condyle fractures, 4) tibia/ fibula shaft fractures, 5) calcaneus, ankle, and midfoot fractures, 6) malleolar, ligament, and ankle injuries. The threshold for a 25% probability of injury for the 50th percentile male were then scaled to obtain the corresponding threshold for other adult sizes.