The biomechanical effects on adjacent segments after two-level ACDF with reduced lordosis

Objective To study the biomechanical effect on adjacent segments of anterior cervical discectomy and fusion (ACDF). Methods 6 adult human cervical spine specimens (C2-C7) were tested under the following 3 conditions: intact cervical spine group, C4-6 ACDF with normal lordosis group, C4-6 ACDF with reduced lordosis group. The specimens were tested in flexion, extension, bending and rotation on the spine 3D motion experiment system. The Cobb angle, overall range of motion (ROM), ROM and loads of facet joints at adjacent segments were measured. Results The lordosis Cobb angle was 13.5°±4.1° in intact cervical spine group, 15.7°±4.6° in C4-6 ACDF with normal lordosis group, 9.0°±4.0° in C4-6 ACDF with reduced lordosis group, respectively. After operation the overall ROM of specimens reduced in flexion, extension, bending and rotation, but the ROM of upper and lower adjacent segments increased. The ROM of upper segments (C3-4) in flexion and extension, as well as that of lower segments (C6-7) in flexion, extension and rotation, had significant difference. The extension ROM of upper segment (C3-4) of C4-6 ACDF with reduced lordosis group was found greater than that of C4-6 ACDF with normal lordosis group and the flexion and extension ROM at lower segment (C6-7) of C4-6 ACDF with reduced lordosis group were also greater. Increased max and average stress on C3-4 facet joints was observed in all the 4 types of motion after ACDF and the differences in extension and bending were statistically significant. Comparing C4-6 ACDF with reduced lordosis group and C4-6 ACDF with normal lordosis group, a stress decreasing trend was observed along with the reduced lordosis angle but had no significant difference. Conclusion After 2 level ACDF on cervical spine specimens, ROM of adjacent segments and stress on facet joints increased. The ROM of specimens with reduced lordosis is higher than those with normal lordosis, which may accelerate ASD. Key words: Cervical vertebrae; Spinal fusion; Biomechanics