Lower Limb Injuries to Passenger Car Occupants

Federal Office of Road Safety - Contract Report 137

Full report in .pdf format [3.2MB]

Abstract:

Lower limb injuries occur to front scat occupants in more than one in three head-on casualty crashes. A study was undertaken to determine the various types of injuries, the sources of injury inside the vehicle, and the mechanisms of injury. This information was to help guide future regulation effort aimed at reducing the frequency and severity of these injuries and to make recommendations about how these injuries might be mitigated in future vehicle design. A detailed examination was undertaken of hospitalised or killed vehicle occupants who sustained a lower limb injury in a passenger car involved in a frontal crash. The findings showed that fractures occur in 88%of crashes where someone suffers a lower limb injury. Fractures to the ankle and foot were more common than other lower limb fractures and the floor and toepan area was especially involved in these fractures. There was no apparent age or sex effects among the injured occupants. Unrestrained occupants seemed more likely to sustain a thigh fracture from contact with the instrument panel than restrained occupants. The number of fractures was directly proportional to the impact velocity and roughly half these fractures occurred at a delta-V value of 48km/h or less. The most common mechanisms of injury was compression (axial loading) of the lower leg or thigh, perpendicular loading of the knee, and crushing or twisting of the foot. There is a need for additional regulation aimed at reducing the frequency and severity of these injuries and a number of countermeasures are available.

Executive Summary

Lower limb injuries to front seat occupants in frontal crashes are a major source of vehicle occupant trauma in this country, occurring in more than one in every three crashes where someone is either hospitalised or killed.

While not necessarily life threatening, they do cause considerable pain and suffering to the individuals involved, can require long-term treatment and rehabilitation and often result in permanent disability. They can be extremely costly to the people involved and to the community generally.

Lower Limb Study

To assist in future efforts aimed at reducing the frequency and severity of lower limb injuries, the Monash University Accident Research Centre was commissioned by the Federal Office of Road Safety to undertake a detailed examination of lower limb injuries to front seat occupants in frontal crashes involving current generation passenger cars.

The study set out to identify the range of lower limb injuries and contacts within the vehicle and to investigate the mechanisms of injury for the more serious and frequent of these injuries. The study was to make recommendations on the needs and priorities for further lower limb injury regulation and countermeasures aimed at reducing these injuries.

A literature review was initially conducted which reviewed previous research and recent developments in this area.

An analysis was then undertaken of lower limb injury cases contained in the Crashed Vehicle File at Monash University. This database comprised detailed inspections of 501 crashes that occurred in and around Melbourne involving 605 injured occupants where the occupant was either hospitalised or killed in the crash.

The injury analysis identified the most common severe lower limb injuries and their contact source within the vehicle in frontal crashes that required closer attention.

Mechanism of Injury Data

Mechanism of injury for these frequent injuries was then determined from the details of the injuries and sources of injury in the original case sheets as well as from additional details obtained from the patient files kept at the treating hospital.

An expert panel was formed consisting of a trauma surgeon, an epidemiologist, a biomechanical engineer and research staff to review each lower limb injury case in arriving at an agreed prognosis of the mechanism of injury.

Findings from the Study

There were a number of findings concerning the types and mechanisms of lower limb injuries to come from this study.

The most common severe lower limb injuries in frontal crashes are fractures (single or multiple). Fractures occur in 88% of cases where a front seat occupant is either hospitalised or killed in a frontal crash and sustains a lower limb injury.

The six most frequent lower limb fracture by contact source combinations were ankle/foot with the floor & toepan, lower leg with the floor & toepan, thigh with the instrument panel, lower leg with the instrument panel, knee with the instrument panel and knee with the steering column. The first of these was six times more likely than the last.

Current crash performance regulations only specify a maximum acceptable tolerance level for femur (thigh) loading. Moreover, there is a need for a new test dummy capable of measuring the full range of injurious forces to the lower limbs.

The average change of velocity during impact (delta-V) was slightly higher among cases where someone sustained a lower limb fracture than for all frontal crashes. Lower limb fracture cases had a higher minimum delta-V value than all injury cases.

Fifty percent of lower limb fractures occurred at 50km/h or below. The 80th percentile value was 70km/h. The number of fractures per fractured lower limb case was directly proportional to delta-V. Lower limb fractures were more common among occupants of smaller cars.

There were no marked age or sex related differences in lower limb fractures suggesting that any frailty effects due to ageing were more than offset by the level of lower limb trauma associated with frontal crashes.

There was a slight suggestion of over-involvement of unrestrained occupants among those sustaining a thigh fracture.

The three most common mechanisms of lower limb fracture include compression (axial loading) of the thigh and lower leg, perpendicular loading of the knee, and crushing or twisting of the foot. These three mechanisms (by body region) need to be emphasised in future efforts to measure and specify acceptable tolerances aimed at reducing lower limb injuries.

Foot and ankle movements of eversion and inversion and dorsiflexion were most common among foot and ankle fractures. Torsion forces were roughly equally distributed in either direction while perpendicular loading tended to be more medial than lateral.

Recommendations

There is a need for further effort aimed at reducing lower limb injuries for front seat occupants in frontal crashes. This could require additional regulation aimed at specifying a range of lower limb injury tolerance criteria.

As a prerequisite, there is a need for test dummies sufficiently sensitive to measure the types and ranges of lower limb injuries (fractures) apparent in real world crashes.

Manufacturers, too, need to consider ways in which car design could be improved to protect these lower extremities.

Possible countermeasures for lower limb injuries include:

  • more forgiving lower instrument panel designs,
  • knee bars,
  • removing injurious fittings in the regions likely to come into contact with lower limbs in frontal crashes,
  • the use of more sturdy materials in dashboards (such as sheetmetal, rather than brittle plastics),
  • innovative pedal designs to minimise the likelihood of ankle and foot fracture, and
  • structural improvements in the floor and toepan regions to minimise intrusions and deformations likely to injure occupants' feet and lower legs.