Artificial disc and vertebra system: a novel motion preservation device for cervical spinal disease after vertebral corpectomy

Abstract

Objective: To determine the range of motion and stability of the human cadaveric cervical spine after the implantation of a novel artificial disc and vertebra system by comparing an intact group and a fusion group.

Methods: Biomechanical tests were conducted on 18 human cadaveric cervical specimens. The range of motion and the stability index range of motion were measured to study the function and stability of the artificial disc and vertebra system of the intact group compared with the fusion group.

Results: In all cases, the artificial disc and vertebra system maintained intervertebral motion and reestablished vertebral height at the operative level. After its implantation, there was no significant difference in the range of motion (ROM) of C(3-7) in all directions in the non-fusion group compared with the intact group (p>0.05), but significant differences were detected in flexion, extension and axial rotation compared with the fusion group (p<0.05). The ROM of adjacent segments (C(3-4), C(6-7)) of the non-fusion group decreased significantly in some directions compared with the fusion group (p<0.05). Significant differences in the C(4-6) ROM in some directions were detected between the non-fusion group and the intact group. In the fusion group, the C(4-6) ROM in all directions decreased significantly compared with the intact and non-fusion groups (p<0.01). The stability index ROM (SI-ROM) of some directions was negative in the non-fusion group, and a significant difference in SI-ROM was only found in the C(4-6) segment of the non-fusion group compared with the fusion group.

Conclusion: An artificial disc and vertebra system could restore vertebral height and preserve the dynamic function of the surgical area and could theoretically reduce the risk of adjacent segment degeneration compared with the anterior fusion procedure. However, our results should be considered with caution because of the low power of the study. The use of a larger sample should be considered in future studies.

Figure 1

The ADVS is made of a titanium alloy (Ti6Al4V), which has three major parts, two discs and a vertebra. There is a hemisphere socket joint between the disc and vertebra that theoretically allow 12° in flexion, extension and lateral bending, 360° in axial rotation. A) ADVS in flexion; B) ADVS in extension; C) the artificial vertebra part; D) the artificial disc part; E) the lateral view of ADVS. 1) the base part of disc, it has a smooth surface on the top that fits the adjacent vertebra; 2) the front wing part; 3) the articular fossa; 4) bone graft tubes in different diameter; 5) the articular head; 6) the “L” structure, which was designed for dislocation between the disc part and the vertebra part; 7) the self-drilling trapping screw; F) upper, middle and lower sagittal diameters of C5 vertebra; G) anterior, media and posterior height from C4∼5 disc to C5∼6 disc. The ADVS was implanted after corpectomy was performed at C5 level, X-ray films of posteroanterior view H) and lateral I) views. The anterior plate fusion performed after cervical discectomy and vertebral corpectomy of C5 level, X-ray films of posteroanterior view J) and lateral K) views.

Figure 2

ROM of C3∼C7, C3∼4, C4∼6, C6∼7. The ROM were measured under 2 N·m torque loading pressure. A) the fusion group showed a significantly restricted ROM of C3∼C7 regarding flexion, extension and axial rotation ROM of C3∼C7 compared to the intact group and non-fusion group. No significant difference of ROM in any direction was detected when compared the intact group and non-fusion group. B) the fusion group and non-fusion group showed a significantly increased ROM of C3∼4 regarding extension and lateral bending ROM of C3∼4 compared to the intact group. Significant difference of ROM of C3∼4 in axial rotation was detected when compared the intact group and non-fusion group. C) the fusion group showed a significantly restricted ROM of C4∼6 in all directions compared to the intact group and non-fusion group. Comparing with the intact group, the extension and lateral bending ROM of C4∼6 in non-fusion group increased significantly. D) the fusion group showed a significantly restricted ROM of C6∼7 in extension compared to the intact group and non-fusion group. The axial rotation ROM of C6∼7 in non-fusion group decreased significantly comparing with the fusion group. No significant difference of ROM of C6∼7 in any direction was detected when compared the intact group and non-fusion group. *p<0.05