[Effects of novel angled cervical disc replacement on facet joint stress].

OBJECTIVE

To analyze the biomechanical changes of the adjacent cervical facet joints when the angled cervical prosthesis is replaced.

METHODS

A total of 400 northwestern people were involved, with an age of 40 years or older. The cervical vertebra lateral X-ray films were taken, and the cervical angles were measured by professional computer aided design software, then the cervical intervertebral disc prosthesis with 10 degree angle was designed. The finite element models of C4, 5 and C4-6 segments with intact cervical discs were developed; the C4, 5 disc was replaced by the cervical prosthesis with 0 degrees and 10 degrees angle respectively; and then all models were subjected to axial loading, flexion/extension, lateral bending, and torsion loading conditions; the stress effects on adjacent facet joints after replacement were observed by comparing with that of the intact model.

RESULTS

The cervical angles were (9.97 +/- 3.64) degrees in C3, 4, (9.95 +/- 4.34) degrees in C4, 5, (8.59 +/- 3.75) degrees in C5, 6, and (8.49 +/- 3.39) degrees in C6, 7, showing no significant difference between C3, 4 and C4, 5, C5, 6 and C6, 7 (P > 0.05) and showing significant differences between the other cervical angles (P < 0.05). When C4, 5 model was axially loaded, no significant difference in equivalent shearing stress were observed in intact, 0 degrees, and 10 degrees groups; at flexion/extension loading, the stress was biggest in intact group, and was smallest in 10 degrees group; at lateral bending, the stress got the high rank in intact group, and was minimum in 10 degrees group; at torsion loading, the stress state of 10 degrees group approached to the intact one condition. When C4-6 model was loaded, the facet joint stress of the replaced segment (C4, 5) decreased significantly at axial loading, flexion/extension, and lateral bending; while no obvious decrease was observed at torsion loading; the stress of the adjacent inferior disc (C5, 6) decreased significantly at axial loading and lateral bending condition, while less decrease was observed at torsion loading, no significant change at flexion/extension condition, it approached to that of the intact one.

CONCLUSION

The finite element analysis reveals that the biomechanical properties of 10 degrees designed prosthesis is approximate to that of the intact cervical disc, thus the 10 degrees designed prosthesis can meet the requirements of biomechanical function reconstruction of the cervical spine.