Background: A new anchored cervical interbody PEEK spacer was devised that uses only 2 integrated variable angle screws diagonally into the adjacent vertebral bodies instead of the established device that uses 4 diagonal fixed-angle screws.
Objective: To compare in vitro the stability provided by the new 2-screw interbody spacer to that of the 4-screw spacer and a 4-screw anterior plate plus independent PEEK spacer.
Methods: Three groups of cadaveric specimens were tested with 2-screw anchored cage (N=8), 4-screw anchored cage (N=8), and standard plate/cage (N=16). Pure moments (1.5Nm) were applied to induce flexion, extension, lateral bending, and axial rotation while measuring 3-D motion optoelectronically.
Results: In all three groups, the mean range of motion (ROM) and lax zone (LZ) were significantly reduced relative to intact after discectomy and fixation. The 2-screw anchored cage allowed significantly greater ROM (p<0.05) than the standard plate during flexion, extension, and axial rotation and allowed significantly greater ROM than the 4-screw cage during extension and axial rotation. The 4-screw anchored cage did not allow significantly different ROM or LZ than the standard plate during any loading mode.
Conclusion: The 2-screw variable angle anchored cage significantly reduces ROM relative to intact. Greater stability can be achieved, especially during extension and axial rotation, by using the 4-screw cage or standard plate plus cage.
From: Biomechanical Assessment of Anchored Cervical Interbody Cages: Comparison of 2-screw and 4-screw Designs by Reis et al.