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.