Experimental Evaluation of the Developmental Mechanism Underlying Fractures at the Adjacent Segment

Compression fractures at adjacent mobile segments have been reported as adjacent segment disease under trauma in several studies. In this study, the occurrence of fractures at the adjacent segment was evaluated experimentally under trauma.

Static testing of different fixation systems was performed to show their biomechanical performances. The ovine vertebrae fixed with rigid, dynamic, and semirigid systems were used as test samples. The stiffness values of the systems were obtained by testing the vertebrectomy models under compression bending, lateral bending, and torsion tests. In addition, their effects on the adjacent segments were experimentally evaluated within a drop mechanism. A free-fall drop mechanism was designed and manufactured. Next, 3.5-kg, 5-kg, and 7-kg weights were released from 1 m above the test samples to generate compression fractures. The occurrence of compression fractures was observed with the use of radiograph of test samples, which were obtained before and after the drop test.

Dynamic and semirigid systems have advantages compared with rigid systems as the result of their lower stiffness values. Radiographs showed that epiphysis fractures occurred at fixed and adjacent mobile segments, which were fixed with semirigid fixation. In addition, dynamic fixation well preserved the fixed and adjacent mobile segments under trauma.

The dynamic system with a polyetheretherketone rod can better preserve both adjacent and fixed segments. However, because of the cantilever beam effect, the semirigid system exhibits a great disadvantage.