[Expandable cages: biomechanical comparison of different cages for ventral spondylodesis in the thoracolumbar spine]

Due to a recent increase in the commercial availability of expandable cages for vertebral body replacement, this study was designed to gain more information about their biomechanical properties. All three expandable cages evaluated in this study are approved for clinical use, but little knowledge about their biomechanical properties exists.

Human thoracolumbar spines (T11 to L3) ( n=32) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive stiffness method. Three-dimensional displacement was measured using an optical measurement system. All motion segments were tested intact. After L1 corporectomy, cages were implanted and the following groups ( n=8 each) were tested: (1) meshed titanium cage (nonexpandable cage, DePuy Acromed), (2) X-tenz (expandable cage, DePuy Acromed), (3) Synex (expandable Cage, Synthes), and (4) VBR (expandable cage, Ulrich). Finally, posterior stabilization and posterior-anterior stabilization, both using USS (Synthes), and anterior plating (LCDCP, Synthes) was applied. The mean apparent stiffness values, ranges of motion, and neutral and elastic zones were calculated from the corresponding load/displacement curves.

No significant differences were found between the in vitro biomechanical properties of expandable and nonexpandable cages. Compared to the intact motion segment, isolated anterior stabilization using cages and anterior plating significantly decreased stiffness and increased range of motion in all directions. Additional posterior stabilization significantly increased stiffness and decreased range of motion in all directions compared to the intact motion segment. Combined anterior-posterior stabilization demonstrated the greatest stiffness results.

Design variations of expandable cages for vertebral body replacement do not show any significant effect on the biomechanical results. There was no significant difference found, between the biomechanical properties of expandable and non-expandable cages. After corporectomy, isolated implantation of expandable cages plus anterior plating was not able to restore normal stability of the motion segment. As a consequence, isolated anterior stabilization using cages plus LCDCP should not be used for vertebral body replacement in the thoraco-lumbar spine.