Biomechanical comparison of expandable cages for vertebral body replacement in the thoracolumbar spine

Study Design. An in vitro biomechanical study of expandable cages for vertebral body replacement in the human thoracolumbar spine. Objectives: The purpose of this study was to compare the in vitro biomechanical properties of 3 different expandable cages with a nonexpandable cage. Summary and Background Data. Recently, there has been a rapid increase in the use and the commercial availability of expandable cages for vertebral body replacement in the thoracolumbar spine. Although all 3 expandable cages, evaluated in this study, are approved for clinical use in Europe, little information is available concerning the biomechanical properties of these implants. Material and Methods. Thirty-two human thoracolumbar spines (T11 to L3) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive loading technique using an unconstrained testing apparatus. Three-dimensional displacement was measured using an optical measurement system. First, all motion segments were tested intact. After complete corporectomy of L1, cages were implanted according to producer's information. The following implants ( n = 8/group) 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 using the Universal Spine System ( Synthes), posterior-anterior stabilization using the Universal Spine System ( Synthes), and anterior plating ( Locking Compression Plate, Synthes) was applied to each test specimen. The mean apparent stiffness values, range of motion, and neutral and elastic zone were calculated from the corresponding load-displacement curves. Results. No significant differences could be determined between the in vitro biomechanical properties of expandable and nonexpandable cages. In comparison to the intact motion segment, isolated anterior stabilization using cages and anterior plating significantly decreased stiffness and increased range of motion in all directions. In contrast, additional posterior stabilization significantly increased stiffness and decreased range of motion in all directions compared to the intact motion segment. The combined anterior-posterior stabilization demonstrated greatest stiffness results. Conclusion. Biomechanical results indicate that design variations of expandable cages for vertebral body replacement are of little importance. Additionally, no significant difference could be determined between the biomechanical properties of expandable and nonexpandable cages. After corporectomy, isolated implantation of expandable cages plus anterior plating was not able to restore normal stability of the motion segment. Therefore, isolated anterior stabilization using cages plus Locking Compression Plate should not be used for vertebral body replacement in the thoracolumbar spine.