BIOMECHANICS OF ANKLE AND HINDFOOT INJURIES IN DYNAMIC AXIAL LOADING

Abstract

A research programme has been directed at the mechanism by which car occupants sustain ankle and hind-foot injuries. The severe injuries that are most associated with long term disability and high socio-economic cost have been investigated. Although seat belts and air bags have had a beneficial effect on injuries to most body regions including pelvic, femur and knee injuries, no protective effect has been demonstrated for below knee injuries. Only by understanding the mechanism of injuries to the leg below the knee will it be possible to design improved protection in the future.

Twenty three post mortem human surrogate (PMHS) limbs were impacted using a test set up that was developed to simulate the loading conditions seen in a frontal collision in 3 different positions – A, B & C. The impactor head (5cm x 10cm wide), was instrumented with an accelerometer and linear potentiometer. The impacting force was generated using a bungee-powered sled mounted on steel bearings. Three PMHS legs were tested In Position A (impactor head centred in line with the tibial axis), 9 PMHS legs were tested in Position B (impactor head centred on the anterior tibial margin) and 11 PMHS legs were tested in Position C (impactor head centred 2.5cm anterior to the anterior tibial margin). Active dorsiflexion was simulated through the Achilles tendon and prior to the application of Achilles tension a tibial pre-load (500 to1500N) was applied via a ‘jacking-plate’ applied to the proximal end of the tibia.

During impact testing, bone failure (fractures) occurred at impact loads of 5.7+/−1.9 kN (resultant tibial failure load 6.4+/−1.9 kN) and the following injuries were generated: 9 intra-articular calcaneal fractures; 1 talar neck and 2 talar body fractures; 3 intra-articular distal tibial (pilon) fractures; 2 malleolar fractures; 3 soft tissue injuries and in 3 cases there was no detectable injury. The impact test conditions were replicated with a Hybrid III leg in a first attempt at developing injury risk functions for the dummy.

This study has demonstrated the importance of preload through muscle tension in addition to the intrinsic properties of PMHS specimens in the generation of severe ankle and hindfoot injury.