A1183. VARIABILITY AND REPRODUCIBILITY OF RANGE OF MOTION FOR TRI-LOBE VS. BALL & TROUGH CERVICAL TOTAL DISC DESIGNS

Abstract

A critical objective of cervical total disc replacement (TDR) is to restore predictable reproducible range-of-motion (ROM) with correct kinematics, while maintaining stability of the segment. Current articulating cervical TDR devices feature fixed centers of rotation, sometimes coupled with unconstrained translation in one or more vectors. The difficulty they have in restoring reproducible, kinematically correct motion has manifest as subsequent facet degeneration as well as other problems. A Tri-Lobe articulating cervical TDR has been developed to recreate predictable, kinematically correct motion, as well as to address other common TDR problems including placement sensitivity, excess wear, instability, and imaging compatibility. The Tri-Lobe TDR design features three incongruent, self-centering, hard-on-hard articulations arranged in a tripod configuration -three hemispherical lobes oriented in a tripod configuration on the superior component articulating against mating non-congruent hemispherical pockets on the inferior component. The diameter and spacing of these articulations determines a specific -kinematic -envelope, and has been designed to match the 6-D anatomic motion data from available published sources. It has diamond-on-diamond articulations to sustain the elevated Hertzian stresses of its incongruent bearing geometry, and is engineered to couple motions in a physiologic manner. This study was designed to compare the variability and reproducibility of a Tri-Lobe cervical TDR as compared to the intact spine, and compared to a ball & trough control TDR design.

Seven human cervical spines (C2-C7) were studied (two pilot and five test specimens) utilizing a 7-Axis spinal testing system. A hybrid load/position control protocol was used to test the specimens. The intact spine was tested first in flexion/extension, lateral bending, and axial rotation to 1.5Nm. Then the C4-C5 segment was implanted with the test and control TDRs utilizing an implant placement fixture that provided accurate reproducible placement of the device in the spine. The order of test and control device placement was randomly varied. Data collected included applied moments, forces, and rotations at C2 and C7, and 3D vertebral movements via an optical tracking system (Optotrak). Statistical analysis of kinematic data was performed with paired-ANOVA followed by a Tukey-Kramer HSD post hoc test.

The ROM for flexion/extension (FE), lateral bending (LB), and axial rotation (AR) are as follows: Intact cervical motion segment FE ROM averaged 4.6±1.0 degrees (max 7.5, min 2.6, range 5.0), LB ROM averaged 1.6±0.6 degrees (max 2.5, min 1.3, range 1.2), and AR ROM averaged 9.3±0.8 degrees (max 11.7, min 6.8, range 4.8). For the Tri-Lobe TDR FE ROM averaged 4.7±0.7 degrees (max 6.5, min 2.5, range 4.0), LB ROM averaged 1.9±0.3 degrees (max 2.5, min 1.2, range 1.3), and AR ROM averaged 10.7±0.3 degrees (max 11.9, min 8.4, range 3.5). For the Ball & Trough TDR FE ROM averaged 4.9±1.6 degrees (max 9.3, min 1.5, range 7.8), LB ROM averaged 2.1±0.5 degrees (max 3.1, min 0.7, range 2.4), and AR ROM averaged 11.0±1.3 degrees (max 13.6, min 8.3, range 5.3). While there was not a statistically significant difference between the Average ROM for the intact, Tri-Lobe, or ball & trough design (p=.96), this is misleading. The variance for motion in all three categories for the ball & trough was significantly greater than for both the intact and Tri-Lobe case. Further, for the minima and maxima, the ball and trough had values that were significantly outside the intact values, while, the Tri-Lobe had values close to that of the intact. The ball & trough design exhibited 1.95, 1.84, and 1.51 times the Range of the ROM compared to the Tri-Lobe in FE, LB, and AR respectively.

Critical surgical objectives in cervical TDR include restoring predictable inematicallycorrect motion to the segment while maintaining stability. Both incorrect and excess motion can lead to instability or facet degeneration. Too little motion fails to relieve adjacent segments of the increased stresses occurring with fusion, and can lead to auto-fusion as well. With conventional articulating cervical TDR, issues such as TDR placement within the disc space as well as variations in normal anatomy can adversely affect reconstructed kinematics. The Trilobe cervical TDR studied in this experiment was able to accommodate variations in anatomy and placement providing a highly predictable and reproducible ROM matching very closely the kinematic envelope for the intact spinal motion segment. Its incongruent bearings are the key to its tolerance of variation in anatomy and placement. Its tripod design contributes to its intrinsic stability and self-centering. It may be more forgiving to surgical variability. This is not only desirable in providing the surgeon with flexibility in selecting implantation position to address deformity and bone defects, but also in providing tolerance to unpredictable variations in facet anatomy permitting acceptable motion with stability for a broad range of conditions.