| Summary: | 碩士 === 國立臺灣大學 === 醫學工程學研究所 === 93 === The degeneration of intervertebral discs is usually a result of abnormal load on the spine. One of the important biomechanical function of the spine is to absorb the impact energy during daily activities. Clinically, in the case of disc degeneration, besides using drugs, therapists may suggest performing disc prosthesis replacements for severely degenerated discs. However, the real condition of the impact energy shared by discs during high speed loading is still not well understood, and it is discussed in few reports. Therefore, the purpose of this study is to explore how intervertebral discs affect the distribution of impact energy and the shock absorbing phenomenon of the spine during impact loading.
Twelve fresh-frozen porcine lumbar three-motion segments (L1~L4) were used in the experiment. All passive elements including muscles and ligaments were removed. However discs, capsules, and intra-capsular structures were preserved intact. Four dual axial accelerometers were mounted on the left lateral side of each L1~L4 vertebral bodies. A “drop-tower type” impact testing apparatus was used for the testing. This testing apparatus is called the “Continuous Impact Testing Apparatus”, CITA. Triggered by a systematic program control system, the impactor drops from the set height, and the energy is transmitted to the specimen through the impounder. The top and bottom part of the specimen were firmly fixed on the CITA. There were uniaxial loadcells and six-dimensional loadcell recording the top and bottom force during impact. The parameters of the experiment were three magnitudes of impact energy and two different contact times.
The downward compressing acceleration (gd) is measured on the specimen in the process of being impacted, while an upward rebounding acceleration (gu) with the occurrence of stress-relaxation is produced. When we add both absolute values of gd and gu together, the result is the total acceleration (gt) on which the whole specimen is impacted. Judging by the varying trend of gt, the absolute value of gd in each corresponding point is smaller than that of gu. That is, the upward rebounding acceleration gu is greater than the downward compressing acceleration gd after the spine is compressed and then stress-relaxed. Moreover, the value of gt will increase according to the increase of energy. Then, as to the attenuate value of gt of the whole specimen, the whole attenuating trend is able to increase distinctly when the location is much further from the side of impact. Therefore, in order to maintain the stability of the spine, more shock-absorbing ability must be provided to attenuate the impact energy when the intervertebral disc is much further from the side of impact.
From the gt distribution measured on the intervertebral bodies, the range of compression will be limited by the prerequisite that the specimen is not destroyed. In the process of upward rebounding, if energy still remains after recovering from the previous deformation, the impounder will drag the top of the specimen and continue to move upwardly, producing a rending of soft tissue (intervertebral disc, facet joint capsule). Furthermore, we can observe that the facet joint capsule may be destroyed due to rending, after the experiment is completed. This condition can also be verified by observing the trend of attenuate value. In the original deformation, the attenuate value of compressing should be equal to that of rebounding. When the specimen enters a condition of excessive rending, the attenuate value of rebounding is partly increased on account of the intervention of the soft tissue.
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