Proreflex 1000

To account for each segment’s unique kinematics and minimize coupled forces and moments imposed by test boundary conditions, we applied the test’s compressive loads at the segment’s center of force balance (CFB). Detailed in Appendix A.1, the segment’s CFB was identified (Figure 2), and its location about the segment’s mid-sagittal and mid-coronal axes marked on the cement embedding using indelible ink. These marks were used to co-register the segment between consecutive test stages. The segment’s middle vertebra was instrumented with a cluster of four passive optical markers (7mm, Qualysis AG, Sweden), and the segment secured to a six-degrees-of-freedom testing frame (Figure 3) with its mid-sagittal and mid-coronal axes aligned with the frame’s X and Y axes. As detailed in Appendix A.2, the testing frame was used to apply a 15N axial compressive (AC) follower load (i.e., a compressive load applied in line with the segment’s CFB), the segment allowed to equilibrate for 60sec under load, and the optical marker configuration acquired using cameras (ProReflex 1000, Qualisys AB, Sweden). This configuration establishes the marker- and anatomical-based vertebral coordinate systems for computing vertebral body motion about the segment’s anatomical axes^{27 (link), 28 (link)}. The measurement error for angles and translations were 0.2deg and 0.1mm, respectively.