126 - Translational and Rotational Response of a Keeled Total Disc Replacement After One-, Two-, and Three-Level Implantation

Introduction: While motion associated with cervical disc arthroplasty has been studied, few investigations have documented the physical separation between the translational and rotational components associated with disc arthroplasty devices.

Aims/

Objectives: The purpose of this study was to examine the effects of segment motion above, below, and at the surgical index level induced by a keeled artificial disc implantation with distinction between translation and rotation.

Methods: Eight human cervical spines from (C2-C7) with age range from 38 to 66 were subjected to 20 loading cycles in flexion, extension, and lateral bending under displacement control of 3mm at a rate of 0.1Hz. 3-D motion data was acquired at 60 frames per second from sensors affixed at (C4-C5), (C5-C6), and (C6-C7). The testing configuration permitted orientation and loading in various modalities by rotating the specimen into the plane of the loading axis without removal from the testing apparatus (Figure 1A). Furthermore, the design of the loading apparatus did not constrain the specimen to the plane of loading. The index level was located at the center of the loading apparatus. Testing conditions included the intact specimen followed by sequential artificial disc implantations by experienced spine surgeons at the index (C5-C6), inferior (C6-C7), and superior (C4-C5) levels. Motion was evaluated by calculating the translational magnitude for each sensor at each vertebral level. Angular data was combined into a single angular value through planar addition in X, Y, and Z orientations. The translational and rotary changes in motion between intact versus one-level, two-level, and three-level implantations were expressed as fraction of the intact specimen and compared using 1 sample t-test (Intact = 1) within each loading mode. Significance was set at P< 0.05.

Results: In flexion (Figure 1B), compared to the intact specimens, all three vertebral levels displayed a significant increase in translation (P < 0.025) with increased rotation distal to the index surgery (P < 0.035). Continued implantations resulted in statistically increased translations at (C5-C6) with increased rotations at (C5-C6) and (C6-C7), (P < 0.036). Superior implantation resulted in increased translation at the index level with increased but non-statistically significant rotations at all levels. Extension resulted in increased translations at index and distal segments (P < 0.022) for one- and two-level surgeries. A three-level surgery only increased translation distal to the index level. Rotation was not different from intact. Lateral bending displayed increased translations for the index (P < 0.017, all segments) and two-level surgery (P < 0.025, (C5-C6), (C4-C5)). No statistical differences were detected for a three-level implantation.

Conclusions: Implantation of this keeled total disc replacement device placed inferior, superior, and at index level increased, or retained the translation and rotation associated with the intact specimens. The study represents the physical separation of motion into the translational and rotational components manifested by a keeled cervical disc replacement. One-, two-, or three-levels of keeled device implantations resulted in increased and/or retained specimen motion relative to intact specimens.