Biomedical Engineering students stress-test bones and ligaments in a sheep’s leg

Some people may have raised their eyebrows when a sheep’s leg made its way into a laboratory at Reykjavik University rather than into the kitchen of the university’s Málid cafeteria. However, there was a perfectly good explanation for this, at least for the students and lecturer in Advanced Biomechanics, a course taught as part of RU’s MSc programme in Biomedical Engineering.

One of the practical activities students undertake in the course is stress-testing the bones and ligaments in a sheep’s leg.

“We get the students to be a little more hands-on, cutting through ligaments and tendons, and try to give them a feel for just how strong these tissues are and how strong these materials really are,” says Magnús Kjartan Gíslason, Associate Professor at RU’s Department of Engineering, who teaches Advanced Biomechanics.

“In the course, we discuss the material properties of bones, tendons and cartilage. We also look at the material properties of substances that are implanted in the body, such as various types of metals, ceramics and polymers. Finally, we discuss the design of artificial joints and what happens in the body when a metal joint is implanted,” Magnús Kjartan explains.

He says the experiment is intended to verify what students have already learned through lectures and research in which the material properties of bones have been measured.

Nearly 200 kg of force was not enough to tear the ligaments

How did you carry out the experiment?

“We obtained a femur from a recently unalived sheep from the University of Iceland’s Keldur Institute for Experimental Pathology, through the assistance of Professor Halldór Jónsson Jr., an orthopaedic surgeon who works at the Institute of Biomedical and Neural Engineering.

Under his supervision, the students prepared the knee by removing all the muscles and cutting them away. We then drilled through both bones, inserted a steel wire and pulled.

Unfortunately, we were unable to tear the ligaments because the specimen slipped out of its fixtures before it broke. However, we did manage to reach 1,800 N, equivalent to approximately 180 kg of force, and the ligaments still did not tear.

Just imagine: you attach a weight of around 200 kg to the bone, and it still holds. When the same experiment is carried out on a younger lamb, the bones break along the growth plates because the animal has not yet fully matured. Ligaments are extremely strong, although people do sometimes manage to tear them, most often while participating in sports.

The experiment is partly about confirming that our results are in the same ballpark as previously published figures, while also giving students some insight into just how difficult it is to measure materials like these. It is quite different from working with a steel beam, for example, which can be manufactured to precise, standardised dimensions and shapes. Here, you are dealing with bones and soft tissues of all kinds of shapes and sizes,” explains Magnús Kjartan.

“After we had pulled on the bones while the knee was still intact, we cut the knee apart. We then separated the femur and tibia and tested each bone individually using a three-point bending test. This experiment is used to calculate the stiffness of the bone.

We also attached a strain gauge to the bone to measure its deformation. Hannes Páll Þórðarson, supervisor of the Electrical Lab at RU, helped us attach the strain gauge.

We then calculate the stresses based on the force measured by the tensile testing machine, which allows us to calculate the bone’s stiffness modulus in a relatively straightforward way.

What is remarkable is that our results come surprisingly close to the published values used in the course material, even though those values were obtained using much more sophisticated equipment and considerably more effort,” Magnús Kjartan adds.

Links to bone density and joint replacement surgery

Links to bone density and joint replacement surgery

When asked about the potential benefits of an experiment like this, Magnús Kjartan points, for example, to its connection with IBNE, where 3D models are produced for joint replacement procedures and other types of surgery.

“The benefit lies in understanding the material properties of bone. If we know its stiffness modulus, we can relate that to both bone density and bone strength.

Another way to assess the distribution of bone density is through computed tomography (CT) scans, which provide a much more detailed picture of the distribution of the bones’ material properties. DXA scans are also commonly used to measure bone density in specific areas, particularly for women showing signs of osteoporosis.

This experiment shows us how stiffness behaves at the macroscopic level, whereas medical imaging provides a microscopic picture of the distribution of bone density and, consequently, bone stiffness and ultimately bone strength.

This also connects to a project that Paolo [Gargiulo, Professor at RU’s Department of Engineering and Director of IBNE], Halldór and I have been working on. We are examining bone density in the femur after a patient receives a hip replacement. We want to understand how bone density changes after one year or five years, because bone tissue deteriorates.

When a foreign object such as a metal implant is introduced into the body, a proportionally greater share of the load is transferred through the metal rather than the bone. Bones need to experience mechanical loading in order to maintain themselves and grow. This is one of the central ideas underlying our work on bone,” concludes Magnús Kjartan.