Twist and... shout!

By EMMA YOUNG in SYDNEY

The first comprehensive model of the human spine is challenging assumptions about the causes of back pain.

Contrary to the idea that spinal injuries are caused by a combination of compression, bending, tension and shear forces, the 3D animated model suggests many injuries are the result of quick twists of the vertebrae, making the joints between them rotate.

Dr Nick Beagley and Dr Vladimir Ivancevic of the Defence Science and Technology Organisation in Edinburgh, South Australia, have spent the past 18 months developing their mathematical model, called the Full Spine Simulator (FSS).

Existing models of the spine evaluate forces placed on a single joint, or a simple series of joints, and allow each just a few degrees of freedom. But the FSS represents all 25 movable joints of the spine, and gives each its full six degrees of freedom.

The FSS also takes into account the presence of soft tissue, and mimics the way the body moves to stabilise itself when bending or in response to an impact.

The FSS is at least 10 times as complex as any other model, says Dr Ivancevic. To evaluate the risk of spinal injury, it can be varied to account for body size and strength, as well as the nature of an impact, such as a car crash.

Beagley and Ivancevic say that the prevailing way to estimate the risk of spinal injury, the "principal loading hypothesis", is inadequate. Although it can explain gross injuries, such as fractures in vertebrae or slipped discs, it does not explain the vast majority of cases of back pain.

The researchers say the spine should be considered not as a column, but as a dynamic chain of segments that can rotate. When viewed as such, it becomes clear that torque can damage the joints and muscle between and around vertebrae.

Dr Ivancevic got a mixed response when he presented the FSS to the International Society of Biomechanics Congress in Dunedin last month. "The strong supporters of the principal loading hypothesis obviously don't agree with us."

Richard Appleyard, director of biomechanics at the Orthopaedic Research Institute in Sydney, says the model looks exciting, but its ability to predict injury has not been validated.

Dr Beagley and Dr Ivancevic are now trying to confirm the accuracy of their model. Ethical and technological constraints make it very difficult to measure forces inside the spine of a living person, but sensors are being developed. In the meantime, they are working on a 3D neuromusculature model of the entire skeleton.