Biomechanical Effects of a Dynamic Topping off Instrumentation in a Long Rigid Pedicle Screw Construct

STUDY DESIGN:: Biomechanical ex vivo study.

OBJECTIVE:: To determine if topping off instrumentation can reduce the hypermobility in the adjacent segments when compared to the classic rigid spinal instrumentation.

SUMMARY OF THE BACKGROUND DATA:: Long rigid instrumentation might increase the mechanical load in the adjacent segments, the resulting hyper-mobility, and the risk for adjacent segment disease. Topping off instrumentation intends to reduce the hypermobility at the adjacent level by more evenly distributing segmental motion and, thereby, potentially mitigating adjacent level disease.

METHODS:: Eight human spines (Th12-L5) were divided into two groups. In the rigid group, a three-segment metal rod instrumentation (L2-L5) was performed. The hybrid group included a two-segment metal rod instrumentation (L3-L5) with a dynamic topping off instrumentation (L2-L3). Each specimen was tested consecutively in three different configurations: native (N=8), two-segment rod instrumentation (L3-L5, N=8), three-segment instrumentation (rigid: N=4, hybrid: N=4). For each configuration the range of motion (ROM) of the whole spine and each level was measured by a motion capture system during 5 cycles of extension-flexion (angle controlled to ±5°, 0.1 Hz frequency, no preload).

RESULTS:: In comparison to the intact spine, both the rigid three-segment instrumentation and the hybrid instrumentation significantly reduced the ROM in the instrumented segments (L2-L5) while increasing the movement in the adjacent segment L1-L2 (P=0.002, η=0.82) and in Th12-L1 (P<0.001, η=0.90). There were no ROM differences between the rigid and hybrid instrumentation in all segments.

CONCLUSIONS:: Introducing the dynamic topping off did not impart any significant difference in the segmental motion when compared to the rigid instrumentation. Therefore, the current biomechanical study could not show a benefit of using this specific topping off instrumentation to solve the problem of adjacent segment disease.