How an imaging technique helped me understand the mechanics of a biological soft tissue

Living bodies consist of complex biological tissues. In a healthy body, these tissues work to keep us going in the best way nature allows. For example, our bodies are built to withstand a certain intake of oxygen, resist earth’s gravitational force, temperature fluctuations due to change in seasons, and so on. Sometimes these external factors affect how biological tissues in our body behave and scientists are still working to further determine behaviour of these tissues under different factors. In this blog post, I am focusing on how I determined the mechanical behaviour of anterior cruciate ligaments in the knee joint. Firstly, let me clarify a few terminologies here.

The anterior cruciate ligament, what is it?

The anterior cruciate ligament is a fibrous tissue located in the human knee joint that assists in controlling excessive motion in the joint (Figure 1). There are several ligaments within the knee joint; however the anterior cruciate ligament is highly susceptible to rupture, hence my interest in investigating this tissue.

Figure 1: This is a simplified image of the right knee joint in human showing some of the anatomical structures in the joint (https://bit.ly/2YVTNsD. Accessed 07/04/2019).

And what do I mean by mechanical behaviour of the anterior cruciate ligament?

It is a study of how the ligament behaves under certain mechanical forces. For example, you can think of the mechanical behaviour as a study to find out how this tissue responds when excessively stretched at different speeds. This takes me to the ultimate question.

How did I measure the response of these tissues under certain forces?

Well, there are certainly lots of ways to measure tissue response to stretching forces, all of which have benefits and limitations. One of the imaging techniques I used in my previous work is called digital image correlation method.

Digital image correlation is a non-contact method using digital cameras for measuring deformation on the surface of an object (i.e. ligaments). You can think of this method as a tracking system such that you can take an initial image of an object before deformation, then continuously take images of the same object while it is deforming over a period of time. Then you can compare all of these images to each other to find out how and by how much your object is deformed (Figure 2).

Figure 2: Here, we have a typical step-by-step illustration of how the digital image correlation method was used while an object was stretched. In most cases, the surface of the object we are testing does not have a suitable texture for tracking; therefore, we have to create a pattern to distinguish this movement as shown in step one (speckle pattern). The second step is capturing images of the sample during deformation and finally comparing the reference image to the deformed images so that we can calculate the displacement of the pattern. (http://digitalimagecorrelation.org. Accessed 07/04/2019)

The digital image correlation process can be used on two- or three-dimensional images. For my investigation I needed to obtain three-dimensional images because my object (the ligament) was a complex structure with a curved surface. Briefly, my test set-up consisted of:

1. A tensile testing machine — a mechanical testing machine to apply stretching forces specified by a user, in this case I was the user! (Figure 3)

2. Six digital cameras — I set up these cameras in a way to replicate stereoscopic vision so that I could reproduce three dimensional images later on (Figure 4).

Figure 3: This is a tensile testing machine I used to study mechanical behaviour of the cranial cruciate ligaments (Rosti Readioff (2018) — https://bit.ly/2U4W8hr).

Figure 4: This is the exciting part! This image shows my Digital Image Correlation setup which consists of six cameras around a ligament sample I was planning to test (Rosti Readioff (2018) — https://bit.ly/2U4W8hr).

To carry out my experimental work, I initially collected knee ligaments from cadavers[1]. Then I mounted these ligaments on the tensile testing machine in order to stretch them by a certain amount. I used my camera set up to continuously capture images of the ligament while it was deforming under stretching forces. I was able to capture the shape of the ligament deforming all around it (360°). I obtained an all-round view of my test samples (ligaments) by using six cameras which was very important given the non-geometrical shape of the ligaments.

To make things more exciting, I also used the three-dimensional images to create computer models of the ligaments (Figure 5). I used the computer models to further understand mechanical behaviour of the ligaments in different environments. Maybe you already know, but I just want to point out that nowadays scientists are pushing for the reduction of the use of biological samples in research and one way to do so is by creating a computer model to replace/reduce the need for further biological sample tests. With this in mind, I have created computer models of the ligaments that consisted of information about their mechanical behaviour collected during the tensile tests. Therefore, if you for example, decided to test these ligaments under different forces or environments, then hopefully you wouldn’t need to carry out further experimental tests on new cadavers.

Figure 5: This is simply an illustration of a computer model I created for one of my ligament samples. I do not wish to overwhelm you with technicality of how I created this other than I used the three-dimensional images from my test set-up (Rosti Readioff (2018) — https://bit.ly/2U4W8hr).

In brief, the three-dimensional images helped me capture tissue deformation in an all-round view and create computer models which I then used to get a better understanding of how the anterior cruciate ligaments behave under different forces. This information is valuable for tissue engineers and biomaterials researchers. Or potentially the outcome could also become useful for medical interventions when helping people with injured knee ligaments.

[1] My cadavers were from dogs and these were collected with full ethical approval.