Abstract
INTRODUCTION:
As a consequence from cervical arthroplasty, spine structural stiffness, loading and kinematics are changed, resulting in issues like adjacent segment degeneration and altered range of motion. However, complex anatomical structures and lack of adequate precision to study the facet joint (FJ) segmental motion in 3D have prevented proper quantitative analyses. In the current study, we investigate the innovative use of a local coordinate system on the surface of the superior articular process of the caudal vertebral body in order to analyze FJ segmental motion using CT-based 3D vertebral models in flexion/extension.
METHODS:
CT images were obtained from six patients (2F/4M, mean age: 53 y.o.) with cervical degenerative disc disease in neutral, flexion and extension positions. CT data was used to create subject-specific surface mesh models of each vertebral body. From these, mean normal vectors were calculated for all FJ surfaces and posterior walls from C3/4 down to C6/7 (Fig. 1). The global coordinate system (x, y, z) corresponds to the CT scanner. Within this system, a new local coordinate system (u, v, w) was set on the centroid of each FJ surface (Fig. 1), where the u-, v-, and w- axes correspond to the normal-to-the-FJ, right-left and cranio-caudal directions, respectively. In flexion/extension, translations in mm were calculated as differences in the FJ centroid position and rotations were calculated in degrees as angular differences of the vector of the opposing surface in flexion/extension. Results are presented as mean ± SD. Differences within vertebral levels and left/right FJs were sought using 1- or 2-way ANOVA, respectively.
RESULTS:
The flexion/extension segmental motion was described in its six degrees-of-freedom. Among the three translations, the largest movement was observed in the cranio-caudal direction (u = −0.22 ± 0.47 mm, v = 0.11 ± 0.89 mm, w = −2.06 ± 1.60 mm); while the three rotations about the (u, v, w) axes showed a dominant rotation about the v-axis (u = −0.41 ± 4.42°, v = −5.12 ± 5.61°, w = −0.01 ± 2.71°). Comparing translational and rotational motions by cervical level, movements at C6/7 were shown to be smaller than those at the other levels (p < 0.05) (Figs. 2, 3). There were no significant differences in the movement of the FJ between left and right sides (p > 0.05).
DISCUSSION:
A key finding of this study was that along with the expected translation in the w-axis, there was rotation about the v-axis consistent with the overall neck flexion-to-extension motion. If the rotation about the v-axes were negligible, the FJ motion could be considered as a pure translation (sliding), but the data suggests otherwise. This finding supports the hypothesis of a rolling-sliding type of facet segmental motion that might be influenced by the facet surface curvature. Future studies will focus on analyses of the changes in FJ gap with motion and characterization of the facet surfaces' curvature and congruence.
SIGNIFICANCE: An innovative look into flexion/extension motion from the FJ point of view describes FJ segmental motion as a sliding-rolling motion instead of the traditional concept of sliding-only mechanism.
