Researchers at the Empa are investigating the biomechanical causes of pain in the lumbar region. The forces which put a strain on the back can be identified by combining computer simulations with 3D X-ray videos. The objective is to plan operations more effectively and to develop new implants.

Mechanical engineers Ameet Aiyangar (left) and Bernhard Weisse. (Photos: Kellenberger Kaminski Photographie)

"We are mechanical engineers and we want to know how the joints in the spine work," explained Ameet Aiyangar, who is studying the human locomotor system at Empa using an innovative method. He does this by performing computer simulations that demonstrate the distribution of forces in the back when a person lifts a load. How well these calculations reflect reality depends significantly on how accurate the inputs to the computer model are. "And that is what the problem has been up until now," said Bernhard Weisse, Head of the Biomedical Engineering and Structural Mechanics research group in the Mechanical Systems Engineering department at Empa.

"The combination of experimental data and computer simulations is the innovative aspect of this study," says Bernhard Weisse.

How do the individual vertebrae move in relation to one another when a person bends down and lifts a load? The precise nature of this was unknown up to now because it was impossible to measure the dynamic movement of the bones directly. All they had at their disposal was static X-rays. "We needed a partner in order to improve the quality of the inputs for the simulations," said Ameet Aiyangar, who used to work at the University of Pittsburgh and who is now supported by an Ambizione grant from the Swiss National Science Foundation (SNSF). He was involved in the construction of a unique 3D X-ray video system, the Digital Stereo X-Ray (DSX), in the US, which enables the movements of the spine to be visualised. Therefore, it seemed obvious to nurture cooperation between Empa researchers and the University of Pittsburgh.

The research group examined twelve healthy adults using the special apparatus in Pittsburgh. The test subjects were required to lift loads of between 4.5 and 13.5 kilograms. The apparatus recorded the process, which each lasted about two seconds, from two directions at 30 frames per second; the X-ray radiation dose was very low. "This allows us to see how the bones move in relation to one another with a degree of accuracy of 0.2 millimetres," Ameet Aiyangar revealed. In a follow-up project, DSX measurements were performed on ten patients before and six months after spinal surgery.

Centres of rotations migrate

The X-ray video reveal something astonishing: Migration of the centres of rotation around which the lumbar vertebrae rotate when bending and straightening, and the translational movements can now be measured very accurately, which is especially beneficial to people with back problems. The engineers observed that the test subjects experienced translational movements of up to ten millimetres during a 75° bending movement; this was rather more in patients with degeneration, although the migration path of the centres of rotation is very irregular. When the Empa researcher fed this data into his simulation program, the displacement in the internal forces had an even greater effect. "Translational movements within the joints had been ignored previously," explained Ameet Aiyangar, "but our simulations show that the migration of the centres of rotation has a significant bearing on the forces in the intervertebral disc.”

Mechanical engineers Ameet Aiyangar (left) and Bernhard Weisse. (Photos: Kellenberger Kaminski Photographie)

The engineer performed his calculations using software from the University of Stanford which he has adapted. Based on the calculated distribution of forces, he uses a second simulation method to determine the local stress on the intervertebral discs in the lumbar region and compares the result with the values measured. "The combination of experimental data and computer simulations is the innovative aspect of this study," Bernhard Weisse explained. "This enables us to verify the results." This is decisive for the implementation of the results in the treatment of back patients.

If movement therapies and painkillers no longer help, spondylodesis is often performed, i.e. vertebral fusion surgery. However, the problems often return a few years later because the surgery has increased the stress on the neighbouring segments of the spine. "We hope that our research will help with the planning of surgical intervention in the next five to ten years," said Bernhard Weisse. Patients could benefit from more individualised and targeted treatment from doctors – in keeping with the idea of personalised medicine – and doctors could identify those cases where vertebral fusion is less advisable or where it would make sense to insert a special implant. However, the findings could also help in the development of new disc prostheses which are better at simulating the natural movement patterns of a healthy back.