Biomechanical Analysis and Mid-Term Clinical Outcomes of the Dynamic-Transitional Optima Hybrid Lumbar Device

Background/Objectives: Spinal fusion with static fixation-surgically joining two or more vertebrae to eliminate motion-is commonly employed to treat degenerative spinal disease. However, the rigidity imposed by static constructs and the increased load on the adjacent segments frequently result in complications such as disc or facet degeneration, spinal stenosis (SS), and segmental instability. This study investigates the effectiveness of pedicle-based dynamic stabilization using the Dynesys system, particularly in a dynamic-transitional optima (DTO) hybrid configuration, in mitigating adjacent segment disease (ASD) and improving clinical outcomes. In this work, we analyzed the mechanical performance and intermediate-term clinical effects of the DTO hybrid lumbar device, focusing on how the load-sharing properties of the Dynesys cord-spacer stabilizers may contribute to junctional complications in individuals with diverse grades of intervertebral disc degeneration.

We designed a combined biomechanical finite element (FE) and experimental analysis to predict the clinical outcomes.

Among 115 patients with lumbar SS enrolled for analysis, 31 patients (mean age: 68.5 ± 7.5 years), with or without grade I spondylolisthesis (18/13), underwent a two-level DTO hybrid procedure-L4-L5 static fixation and L3-L4 dynamic stabilization-with minimal decompression to preserve the posterior tension band. Post-surgical follow-ups were conducted for over 48 months (range: 49-82).

Radiological assessments were performed by two neurosurgeons, one orthopedic surgeon, and one neuroradiologist. The posterior disc height, listhesis distance, and dynamic angular changes were measured pre- and postoperatively to evaluate ASD progression. Methods: Dynamic instrumentation was assigned to the L3-L4 motion segment with lesser disc deterioration, in contrast to the L4-L5 segment, where static fixation was applied due to its greater degree of degeneration. FE analysis was performed under displacement-controlled conditions. Intersegmental motion analysis was conducted under load-controlled conditions in a synthetic model. Results: The DTO hybrid devices reduced stress and motion at the transition segment. However, compensatory biomechanical effects were more pronounced at the adjacent cephalad than the caudal segments. In the biomechanical trade-off zone-where balance between motion preservation and stabilization is critical-the flexible Dynesys cord significantly mitigated stiffness-related issues during flexion. At the L3-L4 transition level, the cord-spacer configuration enhanced dynamic function, increasing motion by 2.7% (rotation) and 12.7% (flexion), reducing disc stress by 4.1% (flexion) and 12.9% (extension), and decreasing the facet contact forces by 4.9% (rotation) and 15.6% (extension). The optimal cord stiffness (50-200 N/mm) aligned with the demands of mild disc degeneration, whereas stiffer cords were more effective for segments with higher degeneration. The pedicle screw motion in dynamic Dynesys systems-primarily caused by axial translation rather than vertical displacement-contributed to screw-vertebra interface stress, influenced by the underlying disc or bone degeneration. Conclusions: Modulating the cord pretension in DTO instrumentation effectively lessened the interface stress occurring at the screw-vertebra junction and adjacent facet joints, contributing to a reduced incidence of pedicle screw loosening, ASD, and revision rates. The modified DTO system, incorporating minimal decompression and preserving the posterior complex at the dynamic level, may be biomechanically favourable and clinically effective for managing transitional degeneration over the mid-term.