Herniated discs can have very different effects: some cause no discomfort and are only discovered by accident; others can cause paralysis or cause patients to be in great pain. For the most part, these problems develop suddenly after an awkward movement – at least that is what patients report.
Yet herniated discs do not appear to come suddenly, but rather develop over an extended period of time. Researchers in Ulm, Germany, are investigating the effects of many dynamic motion cycles on one individual spinal disc with a stress simulator. Professor Joachim Wilke explains in his interview at MEDICA.de what they want to find out about the origin of herniated discs and how this simulator works.
Professor Wilke, where did you get the idea to build a stress simulator for spinal discs?
Hans-Joachim Wilke: Although herniated discs are very common, we actually do not know exactly where and why they develop. Based on descriptions by patients however, you could infer for instance that a rotary motion under strain, for example when you bend forward, is not good and can lead to a failed lumbar disc. Yet in experiments that simulated these types of movements, it was detected that a herniated disc is not so easy to recreate artificially – meaning it does not happen as suddenly as patients report. One of two theories was deduced from this, which pertains to the development of herniated discs: an accumulation of micro-traumas in a spinal disc.
What does this theory look like?
Wilke: The spinal disc consists of two different parts. There is a jelly-like nucleus in its interior that is surrounded by a ring of fibers. There is a series of 25 to 28 concentric fiber layers around the nucleus. The skew fiber direction alternates between the rings, so the fibers always run counter directionally; at the same time, the layers are strongly connected and cross-linked. The theory is this: if there is a weak spot in the fibers in the innermost ring, the nucleus eventually pushes the fibers apart and tears the layer open even more. This subsequently increases the pressure on the next layer and later on the one behind it, so that the damage continues to expand outwards like a run. Ultimately, this creates a protrusion, a bulging spinal disc. If the patient then makes such a movement, the last fiber layers also tear and the nucleus is pushed out. This is then called a herniated disc.
Several years ago, we developed a finite element model of a spinal disc segment ourselves. This is a mathematical model to describe a spinal disc. With it, we were able to calculate that excessive flexion combined with a high axial load and rotary motion indeed leads to the most strain in the outer ring of fibers in the rear part of the spinal disc. This explains the theory in part.
Wilke: In the model, we also detected high shear stress between the fibers and the cartilaginous portion of the end plate in which the ring of fiber is anchored. The theory here would be that the defect does not go from the inside out, but that the damage accumulates from the outside inwards, when the fibers are separated from the end plate. In this case, the herniated disc is near the end plate. A clinical trial from India was introduced last year, which also supports this theory: in two-thirds of patients there, herniation occurred near the end plate, while in one third of the patients, it took place more towards the middle of the spinal disc.
Now we would like to simulate the damage patterns in spinal discs with our simulator. That means we want to stress spinal disc preparations with numerous cycles in a relatively short amount of time to where we can achieve the type of damage accumulation in approximately one day that would take years in a human being.
What exactly does this simulator look like?
Wilke: We can rotate the spinal disc preparation around three axles via a kind of Cardan joint. Rotations around each individual axle or in any combination are also possible. Amplitude and speed can be fully varied. There are also shear forces in the spinal disc: when you bend over, a force vector is applied to the spinal disc due to body weight, the various force components are determined by the assumed body posture. We can simulate this with three other motors, also individually or in combination. Basically, we are able to systematically break down what conditions accumulate the largest damages. In Ulm, we also have the advantage of being able to then examine the preparations with high-resolution magnetic resonance imaging. We are subsequently able to exactly quantify the localization, diffusion and type of damages with 3D analysis.
How true-to-life do you rate the simulator? Do you not have to consider the spine as a whole?
Wilke: Essentially yes. However, the spine is a complex structure that consists of many individual segments that we do not fully understand in their entirety yet. We have looked into this subject for more than 20 years, since we built a spine simulator. You can mount longer preparations, meaning longer spinal segments with this simulator. However, the device is only intended for slow motions. The new spinal disc simulator is more dynamic, which is why we had to make a compromise and are only able to illustrate an individual motion segment. You can understand the spine per se with the spine simulator.
What are other applications for the spinal disc simulator?
Wilke: The search for causes of herniated discs is just the primary intention. The goal is to find the weak spots of surgical techniques that perhaps only cause problems in years to come such as the loosening or sagging of implants for example. I believe there will be many applications in the future for spine surgery or physiotherapy.