In vitro biomechanical evaluation of a bi-directional expandable cage

English Keywords:Biomechanical  Spine  Expandable Cage  Bi-directional

English Abstract:

  【Abstract】 Objectives: To perform an in vitro biomachanical evaluation of a self-developed bi-directional expandable fusion device. Methods: 20 fresh calf spines(L3/4) were randomly divided into 4 groups of 5 each and were modeled into normal group(intact), unstable group (discectomy), conventional fusion group[implantation of traditional posterior bullet-type Polyether-ether-ketone(PEEK) cage], and new fusion group(implantation of a self-developed bi-directional expandable cage). Using MTS multi-degree of freedom(spinal motion) simulation test system, 400N preload was applied on the experimental specimen, and at the same time 8Nm moment was also applied in the directions of flexion, extension, left bending, right bending, left rotation and right rotation, and the range of motion(ROM) in each direction was tested. Another 20 calf spines(L3/4) were selected, among which 10 were subjected to sink tests and 10 to pull-out tests, and each test was molded according to the conventional fusion group and the new fusion group respectively and tested using the Bose Electroforce dynamic mechanical testing system to compare and analyze the stability of the two kinds of fusion cages in the intervertebral space. Results: The unstable and conventional fusion groups showed significantly higher ROM values in all directions compared with the normal group, with statistically significant differences(P<0.05), and the new fusion group showed higher ROM values in extension, left bending, left rotation and right rotation working conditions compared with the normal group, with statistically significant differences(P<0.05). The ROM values in all directions were significantly lower in the conventional fusion group and new fusion group compared with unstable group, and the differences were statistically significant(P<0.05). The ROM values in the new fusion group were lower in flexion and extension, right and left bending, and left rotation conditions compared with the conventional fusion group, and the differences were statistically significant(P<0.05). In the sink test, the maximum compressive load of the bi-directional expandable cage(2265.67±13.01N) was significantly higher than that of the PEEK cage(1665.67±10.83N), and the difference was statistically significant(P<0.05); In the pull-out test, the pull-out strength of the bi-directional expandable cage(759.87±15.17N) was higher than that of the conventional PEEK cage(93.37±3.15N), and the difference was statistically significant(P<0.05). Conclusions: The newly self-developed bi-directional expandable cage has better mechanical stability than the conventional PEEK intervertebral cage under the conditions of flexion-extension, lateral bending, and left rotation, and is superior to the conventional PEEK cage in terms of anti-displacement ability and anti-sinkage ability, meeting the needs of clinical applications in terms of mechanical properties.