gms | German Medical Science

Objective: Hybrid stabilization with a dynamic implant has been suggested to avoid adjacent segment disease by creating a smoother transition zone from the instrumented segments to the untreated levels above. This study evaluates ex vivo the range of motion (RoM) to characterize the transition zones of two-level posterior instrumentation strategies for elucidating biomechanical differences between rigid fixation and the hybrid stabilization approach with a pedicle screw-based dynamic implant.

Method: Eight human lumbar spines (L1-5) were loaded in a spine tester with pure moments of 7.5 Nm in lateral bending (LB), flexion/extension (FE) and axial rotation (AR) as well as with a hybrid loading protocol in lateral bending and flexion/extension. The following states were tested: (a) intact, (b) laminectomy L4 with rigid fixation of L4-5 and dynamic instrumentation L3-4 (HPS, Paradigm Spine, Wurmlingen, Germany). (c) Laminectomy L4 with rigid fixation of L3-5. The RoM for all segments was evaluated with both loading protocols and normalized to the intact segmental RoM.

Results: For pure moment loading, RoMs of the segments cranial to both instrumentations (L1-2 and L2-3) were not affected by the type of instrumentation (p>0.5) in FE, LB and AR. The dynamic instrumentation in L3-4 reduced the RoM compared to intact (p<0.05) but allowed more motion than the rigid fixation of the same segment (p<0.05) in LB and FE. Under the hybrid loading protocol, the cranial segments (L1-2 and L2-3) had a significant higher RoM (p<0.05) for both instrumentations compared to the intact (ranging from 162%–227% of the intact RoM in FE and LB). Comparing the two instrumentation approaches with each other, the hybrid stabilization showed a smaller increase of RoM than the rigid fixation (L1-2 Δ=15% and 26%; L2-3 Δ=8% and 11% for FE and LB, respectively).

Conclusions: Regardless of the approach, two-level posterior instrumentation was accompanied by a considerable amount of compensatory movement in the cranial untreated segments under the hybrid protocol. Hybrid stabilization, however, showed a significant reduction of this compensatory movement in comparison to rigid fixation, along with more than 50% of movement preservation of the dynamically instrumented level. The clinical relevance of these differences cannot be deduced from this model. From a biomechanical point of view, however, the hybrid stabilization could offer advantages because of a slightly smoother transition zone.