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° 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° and 10° 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)° in C3,4,(9.95 ± 4.34)° in C4,5,(8.59 ± 3.75)° in C5,6,and(8.49 ± 3.39)° 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°,and 10° groups;at flexion/extension loading,the stress was biggest in intact group,and was smallest in 10° group;at lateral bending,the stress got the high rank in intact group,and was minimum in 10° group;at torsion loading,the stress state of 10° 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° designed prosthesis is approximate to that of the intact cervical disc,thus the 10° designed prosthesis can meet the requirements of biomechanical function reconstruction of the cervical spine.