Introduction: Legislation driven & technology aided reductions in mortality have been documented over the past 10 years for road traffic accidents (RTAs). However many authors have noted an increasing morbidity as a result of serious lower limb injuries. In collaboration with the Transport Research Laboratory (TRL) a 2 stage research programme has been carried out on fresh frozen PHMS lower limbs. This programme, has culminated in a specific series of PMHS tests to reproduce the most disabling lower limb injuries seen in real world accident data. The authors aimed to establish force thresholds for failure (fracture) of the calcaneus, talus and tibial plafond in frontal and frontal offset RTAs. This data is considered essential to support new pan-European legislation for better lower limb protection structures in new motor vehicles which is currently under discussion.

Methods: A 5m bungee driven sled test facility capable of creating a validated and repeatable dynamic crash pulse was used to subject 15 PMHS lower limb specimens to, axial impact loading. The pulse was modelled on the accelerometer toe-pan recordings from a full-scale automotive crash test in frontal impact. To represent brake pedal intrusion at an impact velocity of up to 14ms⁻¹, a staggered double impact, delaying application of axial loading was used. Impact loading was achieved via a modelled brake pedal to the mid-foot. All specimens were preloaded through the Achilles tendon and by knee extension to simulate the plantar flexing response seen in the foot & ankle in driving simulator studies. Delaying the application of axial loading after the initial impact and sled deceleration effectively imparts momentum into the specimen, further preloading the foot and ankle and thus increasing pre-impact bracing. Transducer data were recorded using high frequency (20 & 100 KHz) capture systems (K-Trader and Prosig). High-speed cinematography enabled additional kinematic analysis. Each specimen was tested once only. Specimens were selected at random for five impact severity groups. All specimens underwent pre impact BMD evaluation using protocols previously designed for this type of work. Post impact analysis included X-rays and necropsy.

Results: The specimens used varied in BMD and age similar to specimens used in other centres for similar testing. In the 15 final test specimens 8 calcaneal fractures were generated, one with an additional talar neck fracture. Seven specimens did not sustain injury. Measured BMD did not appear to be a useful predictor of load to failure. Peak axial forces ranged from 5KN up to 14kN. Toe pan and foot accelerations up to 200g were generated.

Discussion: This test method appears to predispose the calcaneus to injury. It failed to create either a Pilon fracture or an isolated talus fracture. Previous research investigating axial impact loading have applied a direct impact with varing levels of pre-load. They resulted in a range of injuries and suggested pre-loading reduced injury thresholds for talar and tibial injuries. This has not been our experience.

Conclusions: This data is invaluable, enabling thresholds for legislative car crash testing to be authoritatively stated and incorporated into national and international standards.

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.