Biomechanical comparison of the end plate design of three vertebral body replacement systems.

Introduction: Compression fractures at the thoracolumbar junction are frequently treated by reconstruction with vertebral body replacement systems. Modern cage implants have been developed which respect the anatomy and angulation of the adjacent bony endplates. The objective of this study was to investigate the biomechanical performance of anatomic endplate design and variable endplate angulation.

Materials And Methods: Three cage systems [Hydrolift (HYL), Aesculap; Synex II (SYN), Synthes; Obelisc (OBC), Ulrich] were compared employing a composite bone substitute material at two levels of endplate angulation (0°, 3°). Their load-bearing capacity was assessed in a physiologic test with human vertebral specimens in a misalignment situation (3°). The HYL and SYN offered anatomically shaped endplates. The endplates of the HYL had variable angulation during insertion and were then mechanically fixated. The OBC had fixed and circular endplates. The load to failure and system stiffness were evaluated by an axial compression test. The bone mineral density (BMD) and the area of the bony endplates were measured via CT.

Results: None of the mechanical properties differed between 0° and 3° for the HYL cage using bone substitute material, while the OBC lost 19% of the failure load (p = 0.001) and 55% of stiffness (p = 0.001) in case of misalignment. In human bone specimens, failure loads were comparable among all implants (p > 0.1) with the HYL showing the largest system stiffness (p < 0.05). Furthermore, a strong correlation between stiffness and BMD (R(2) = 0.82) and failure load and BMD (R(2) = 0.87) was found.

Conclusion: Anatomically shaped and continuously variable endplates provide mechanical advantages under imperfect alignment and may thus reduce secondary dislocation and the loss of correction. This is achieved by retaining an optimal contact area between the implant and the bony endplates. Conventional cage design with circular endplates offer adequate stability in optimal contact situations.