Quantifying the ranges of relative motions of the intervertebral discs and facet joints in the normal cervical spine.

Functional neck motion is achieved by the cervical segments with each composed of an intervertebral disc (IVD) and two facet joints (FJs). Using biplane fluoroscopic imaging, we investigated the ranges of motion (ROMs) of the three joints in the cervical spines (from C3 to C7) of eighteen asymptomatic subjects. Three functional neck motions were examined, including flexion-extension (FE), lateral bending (LB) and axial twisting (AT). Our measurements showed that the translations of both IVD and FJs primarily occurred in the sagittal plane during all neck motions, and the anteroposterior translations of IVDs were significantly smaller than those of the corresponding FJs (p < 0.05) at all segments. For example, the ranges of IVD and FJ anteroposterior translations at C3/4 were 2.7 ± 0.7 mm vs. 3.5 ± 1.1 mm in FE, 0.9 ± 0.5 mm vs. 4.6 ± 1.1 mm in LB, and 1.0 ± 0.5 mm vs. 3.1 ± 1.0 mm in AT. Furthermore, we introduced an IVD-FJ translation ratio, which represents the ratio of the IVD to FJ translational ROMs. In FE neck motion, the IVD-FJ anteroposterior translation ratios decreased from 0.81 ± 0.18 at C3 to 0.52 ± 0.19 at C3, indicating gradually increasing resistances of IVDs compared to FJs from the proximal to distal levels. In LB neck motion, the smallest IVD-FJ translation ratios (0.14 ± 0.09 and 0.43 ± 0.30) occurred at C4/5 for both anteroposterior and left-right translations. In AT neck motion, the largest IVD-FJ anteroposterior translation ratio (0.42 ± 0.21) occurred at C3/4, and was significantly different from those at C4/5 and C5/6 (p < 0.05). These data could be used as references for improving motion-preserving cervical treatment methods that aimed to achieve the normal ranges of translational motions of both IVD and FJs.