Introduction: DDD at the level adjacent to fusion is a well-recognized condition, believed to result from the supraphysiological biomechanical stress and increased ROM created by the rigid fixation. It is hypothesized that topping a fusion with flexible constructs may diminish these increased stress and ROM and subsequently, reduce the incidence of adjacent level disc disease. The objective of this work was therefore to determine the biomechanical effect of topping off devices by measuring facet loading, intradiscal pressure and ROM of a segment adjacent to a fusion prior and after topping off.
Methods: A 3D muscular ligamentous L1-S1 FE-Model validated based on in-vivo data was used in this study. The model was adapted to simulate fusion at L4-L5 with topping off at L3-L4 using the following analogue systems:
- Dynesys® (Zimmer)
- PEEK rod with fixed screws at the fusion level, topped with screws that retain their polyaxial feature (VIPER™ SC Screw, DePuy Spine Inc.).
Pure moment of ±7.5 Nm was applied in flexion/extension, axial rotation and lateral bending. Facet loads, disc pressure, ROM and were determined at the index level for the following modalities:
- Intact as a control
- PLIF cages at L4-L5 with 5.5mm PEEK and Dynesys systems
- PLIF cages at L4-L5 from previous models extended to L3 with VIPER SC and Dynesys
Results: All results were expressed relative to a normal spinal segment (referred to as “intact”), without adjacent instrumentation. Analyses included ROM as well as facet loads and intradiscal pressures, as shown in Table 1.
Conclusions: PDS topping off a fusion was shown in these models to control motion, relieve facet loading and reduce intradiscal pressure on adjacent segment to fusion. Assuming that these stresses contribute to adjacent-level disc disease, these data indicate that PDS topping off a fusion may provide some protection against fusion-induced adjacent-level degeneration. In addition, reduced bone screw interface loading further suggests that PDS may be an option to avoid adjacent segment disc disease.
Patient Care: Topping off fusion may be an option to avoid the degeneration cascade.
Learning Objectives: Topping off fusion may be an option to avoid the degeneration cascade.