Back pain: A question of mechanics

Together with the University of Pittsburgh and Balgrist University Hospital, Empa is decoding the mechanics of the lower back vertebrae. The researchers can now show how wear and tear on the vertebral bodies and intervertebral discs occurs. This will make it easier to select the right therapy.

Some say back pain is the price of walking upright. The others say that the problem of back pain started when man sat down to think: lack of exercise weakens the muscles, plus stress in private life or at work. The back muscles cramp and hurt.

In most cases, the problem can be solved by loosening and strengthening the back muscles. But this does not work for one in seven sufferers; even the administration of opiates then no longer helps. Only surgery can end the suffering. In severe cases, defective vertebrae or intervertebral discs are bridged with a metal construction (intervertebral fusion). The fixed segment ossifies and can initially no longer cause pain. But such repair operations bring patients relief for only a few years, after which the problem recurs in the adjacent vertebrae. The question is: Why is this, and how could it be prevented?

Bernhard Weisse and his team at Empa are researching precisely these mechanical issues. To understand why and how quickly an intervertebral disc wears out, researchers need to know the forces acting in this area. And that, in turn, requires precise knowledge of the shape, elasticity and mobility of the individual elements - it's a question for mechanical engineers.

The skeleton simulator
In a first step, the Empa researchers fine-tuned the theoretical basis: Weiss' team fed spinal geometry data from 81 patients into the computer program Open Sim - a simulation program for the human musculoskeletal system developed by Stanford University and used worldwide. Then the task was to map the biomechanics as accurately as possible in the computer simulation: Does an intervertebral disc behave like a ball-and-socket joint? Or more like a rubber bearing? What influence do the muscles have on this - does the rubber bearing always remain the same stiffness, or does the stiffness change, depending on the angle of bending? For this purpose, Empa collaborated with the Laboratory of Orthopedic Biomechanics at Balgrist University Hospital (University of Zurich) and the Institute of Biomechanics at ETH Zurich.
With the help of the computer model, the scientists succeeded in reproducing the mechanics. The result: in people with a certain spinal misalignment, the intervertebral discs are already under up to 34 percent more strain in a healthy state. If an intervertebral disc breaks down and is bridged, the load in the neighboring joints increases even further and can be up to 45 percent higher than in people without this malposition.

Individual therapy recommendations become possible
But computer analysis of a health problem alone is not enough. The goal is to make an individual diagnosis for each patient and recommend the appropriate therapy. A collaboration with U.S. scientists, funded by the Swiss National Science Foundation, helped here: researchers at the University of Pittsburgh have developed a novel 3-D X-ray video system. Called "Digital Stereo-X-Ray Imaging" (DSX), it can reproduce the movement of the spine at 250 frames per second, while the position of the vertebrae can be seen with an accuracy of 0.2 millimeters. The trick is that the blurred X-ray images of the movement are combined with sharp CT images of the patient lying still in the computer.

One of the researchers working there, Ameet Aiyangar, was already a guest scientist at Empa in 2009 and is now returning to Empa. In Pittsburgh, he had twelve healthy people lift weights and produced high-resolution films of their spinal movement. Currently, Aiyangar is in the process of matching the captured X-ray films with computer models of each subject. Once the model is consistent for healthy people, the researchers plan to use this method to study the problem of spondylodesis (vertebral body locking). To do this, patients are filmed before and after surgery with the DSX system and the movement of their vertebrae is analyzed. This makes it possible to determine what forces were acting in the lower spine before surgery and what the bridging of the vertebrae has changed about this distribution of forces. The study will help to better understand the wear and tear of spinal vertebrae and more accurately localize the cause of lower back pain.In the future, this type of computer analysis could be available for all back surgery patients.

(Text: Empa)