THE first comprehensive model of the human spine is challenging our 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.
Nick Beagley and 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. That movement is either caused by signals from the cerebellum in the brain or spinal reflex action. “You see a lot of studies that use crash-test dummies,” Beagley says, but because these do not take account of any nervous system signals, they are no better than studying cadavers. In contrast, the FSS is at least 10 times as complex as any other model, says 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. This model estimates the risk of injury by considering the combination of compression, bending, tension and shear forces acting on the spine. “It is implicitly assumed that the spine is a column – that it is one complex stick,” says Ivancevic. But while the principal loading hypothesis 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. The model can reveal, for instance, whether equipment added to a soldier’s helmet could result in excessive “torque jolts” – the kind of quick rotational jerks that Beagley and Ivancevic blame for spinal injuries – as the soldier performs manoeuvres.
Ivancevic got a mixed response when he presented the FSS to the International Society of Biomechanics Congress in Dunedin, New Zealand, 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. “This can only be done by comparing the model with in-vivo and in-vitro data,” he says.
Beagley and 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 that should help validate the model, says Ivancevic. In the meantime, they are working on a 3D neuro-musculature model of the entire skeleton.