Child crash test dummies not crashworthy?

3 July 2008

in 2008

We’re not protecting young car passengers as well as we could, according to researchers at Sydney’s Prince of Wales Medical Research Institute.

They’ve shown that the spine of a young child is significantly different from that of an adult in ways which could influence the risk of spinal cord injury and the results of crash testing. And they’ve called for new crash dummy designs that better mimic what happens to a real child in a crash.

“Our studies have found huge differences in flexibility and stiffness between young and mature spines. And in a collision, a younger, more flexible, spine is likely to place greater strain on the spinal cord inside,” says Elizabeth (Liz) Clarke, a researcher at the Institute. 

Car crashes are the most common cause of spinal cord injuries in Australia. Such injuries can lead to permanent paralysis. Fortunately only about two percent of these injuries are in young children. The wide use of child restraints is probably reducing the risk.

Young children, however, may be more susceptible to spinal cord damage than these statistics would lead us to believe, says Liz. “We’ve found that the young spine is softer and about three times more flexible than that of an adult. Because the young spine allows more overall movement, the spinal cord inside may be stretched to a higher degree. So, it is possible that a much smaller impact would be required to cause spinal cord damage in a poorly or unprotected child than an adult.

“Also, while it is extremely rare in adults for the spinal cord to be damaged without fracturing the vertebrae, it is not uncommon in spinal cord injuries in young children, possibly due to this increased flexibility.”

Liz and her colleagues believe that the higher risk for younger children is hidden as they are less likely to be exposed to dangerous driving, and are better protected in the car by using child restraints. Other work at the Institute has shown the importance of an appropriate, approved child restraint in providing maximum protection to children. However, with better testing it may be possible to identify and/or design restraints which further reduce the risk of paralysing injuries, particularly in children too big for current child restraint designs.

The researchers have also discovered that flexibility along the length of a young spine varies markedly from flexibility along the adult spine. The young spine is very flexible in all directions and at all levels—the neck, and upper and lower back—whereas the adult spine is far less flexible in the middle of the back, and when twisting to the left or right.

 “Child crash test dummies have been modelled as scaled-down versions of adult crash test dummies,” Liz says, “but these research findings suggest that there is significant scope for improvement. Current child crash test dummies can tell us if a restraint will contain a child during an accident, but they don’t adequately model the spine.”

“We hope that our results will be used to improve crash test dummy design and eventually lead to a new generation of dummies. Better child crash test dummies will give us the tools we need to make travelling in cars safer for children of all ages.”

Elizabeth Clarke is one of 16 early-career scientists chosen for Fresh Science, a national program sponsored by the Federal and Victorian governments.

Abstract

Immature sheep spines are more flexible than mature spines: an in vitro biomechanical study.
Clarke EC, Appleyard RC, Bilston LE.,Spine,. 2007 Dec 15;32(26):2970-9

STUDY DESIGN: Dynamic triaxial biomechanical testing of immature and mature ovine spine motion segments. OBJECTIVE: To compare torque-deflection parameters of mature and immature spine motion segments and to investigate whether scaling relationships apply between mature and immature motion segment torque-deflection responses. SUMMARY OF BACKGROUND DATA: While previous studies have examined the cervical region in a limited number of loading directions, a comprehensive multiaxial study of the response of the pediatric spine at all 3 spinal levels (cervical, thoracic, and lumbar) has not been performed. METHODS: Motion segments from cervical, thoracic, and lumbar levels were tested under moment application about 3 axes for newborn and 2-year-old sheep. Range of motion, neutral zone, and stiffness were calculated for each motion segment and compared for immature and mature spine. RESULTS: Immature spine motion segments exhibited a significantly larger range of motion (P < 0.001) and neutral zone (P < 0.001) and significantly lower stiffness (P < 0.001) in comparison to mature spine segments about the 3 moment axes, at the 3 spinal levels tested. There were statistically significant interactions between specimen age and the moment axis and/or spinal level for some torque-deflection parameters. CONCLUSION: The significantly greater neutral zone of immature spine suggests greater ligament laxity. Significantly higher range of motion and lower stiffness of the immature spine may be implicated in spinal cord injury mechanisms and implies a change in relative tolerance of the spine to damage with spinal maturity. Significant statistical interactions between spinal maturity and moment axis or motion segment level suggest that scaling torque-deflection parameters from mature to immature spine may not be appropriate.

Abstract
Contrasting Biomechanics and Neuropathology of Spinal Cord Injury in Neonatal and Adult Rats Following Vertebral Dislocation
Clarke EC, Bilston LE., Journal of Neurotrauma, In Press (accepted April 1st, 2008)

Clinically, spinal cord injuries (SCI) in infants are different from SCIs in adults. SCI is rarer in infants, and the most common types of associated spinal column injury are different for adults and infants. Initially, infants tend to have higher injury severities and mortality, however young survivors of SCI typically have greater and more rapid functional recovery. The objective of this study was to contrast the biomechanics and neuropathology of SCI in adult and neonatal rats to investigate these differences. Thoracolumbar vertebrae of anaesthetized rats were dislocated laterally (T12 held stationary and L1 displaced laterally, with T13 between these levels) by 10mm at 250mm/s in adults and by 4mm at 100mm/s in neonates (13-15 days), and rats were euthanized 6 hours later. Spinal cord sections were stained to detect hemorrhage (H&E), axonal injury (ßAPP), and neuronal nuclei (NeuN). Maximum load was significantly higher in adults (25.7 ± 2.4N) than neonates (11.0 ± 2.4N) (p<0.001). Adult and neonatal hemorrhage volumes were not significantly different for either the raw or normalized data-sets (p=0.064 for normalized dataset). Un-normalized axonal injury densities were similar for adults and neonates but normalized axonal injury density was significantly higher in neonates (p<0.001). Reduction of NeuN immunoreactivity was significantly lower in neonates, for both un-normalized (p<0.004) and normalized (p<0.001) data-sets. The findings of this study may explain the different common types of spinal column injury associated with SCI, and the greater initial severity of SCI in infants.  

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