THE REACTION OF INTERVERTEBRAL DISCS TO COMPRESSION FORCES

¹ Orthopaedic Department of Karolinska Institutet and King Gustav V. Research Institute, Stockholm

1. Assuming intervertebral disc lesions to be the causative factor of low-back pain, our present knowledge does not allow us to explain the pathological mechanism involved.

2. Since at least 40 per cent. of all patients with backache claim that the pain started because the back was subjected to extra strain, it is suggested that biomechanical studies may give a new approach for pathological and mechanical discussions.

3. An experimental approach has been adopted by which the mechanical responses of intervertebral discs can be recorded with a high degree of accuracy.

4. Two quantitatively different responses of the disc have been described. The reaction to a statically applied load takes place in the course of minutes, while the response to the dynamic load occurs within fractions of a second. The latter is characterized by vibrations in the disc.

5. Vibrations were registered with a disc under steady load. Even if a disc has reached a static equilibrium, additional rapid forces can increase the deformations to a great extent, even if these forces are relatively small.

6. Apparently insignificant traumata are not recognized by the insurance companies in many countries as cases for compensation. On the basis of the observations recorded in the experiments, it is not the violence itself that decides the extent of the damage but the trauma combined with the condition of the disc at the time of injury. The more the disc is compressed, the less additional trauma may be required to cause lesions. Degenerated discs may, under certain circumstances, show a decreased tolerance to stress.

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