Junction kinematics between proximal mobile and distal fused lumbar segments: biomechanical analysis of pedicle and hook constructs

Background context: Biomechanical studies have demonstrated increased motion in motion segments adjacent to instrumentation or arthrodesis. The effects of different configurations of hook and pedicle screw instrumentation on the biomechanical behaviors of adjacent segments have not been well documented. Purpose: To compare the effect of three different fusion constructs on adjacent segment motion proximal to lumbar arthrodesis. Methods: Seven human cadaver lumbar spines were tested in the following conditions: 1) intact; 2) L4-L5-simulated circumferential fusion (CF); 3) L4-L5-simulated fusion extended to L3 with pedicle screws; and 4) L4-L5-simulated fusion extended to L3 with sublaminar hooks. Rotation data at L2-L3, L3-L4, and L4-L5 were analyzed using both load limit control (±7.5 N·m) and displacement limit control (truncated to the greatest common angular motion of the segments for each specimen). Results: Both the L3-L4 and L2-L3 motion segments above the L4-L5-simulated CF had significantly increased motion in all loading planes compared with the intact spine, but no significant differences were found between L3-L4 and L2-L3 motion. When the L3-L4 segment was stabilized with pedicle screws, its motion was significantly smaller in flexion, lateral bending, and axial rotation than when stabilized with sublaminar hooks. At the same time, L2-L3 motion was significantly larger in flexion, lateral bending, and axial rotation in the pedicle screw model compared with the sublaminar hook construct. Conclusions: The use of sublaminar hooks to stabilize the motion segment above a circumferential lumbar fusion reduced motion at the next cephalad segment compared with a similar construct using pedicle screws. The semiconstrained hook enhancement may be considered if a patient is at a risk of adjacent segment disorders.