Stability Evaluation Of A New Lumbar Dynamic Non-fusion Systems Made Of A New Type Titanium-alloy

PrefaceSpinal fusion with the rigid fixation device is frequently performed to treat low back pain related to degenerative changes and instability of the lumbar spine. The stress-shielding properties of the rigid pedicle screw systems, the risk of device-related osteopenia and accelerated adjacent and/or fixed level degeneration have been reported after lumbar arthrodesis surgery. Nonetheless, ideas of dynamic non-fusion systems became more and more popular in order to provide flexible restabilization and spinal alignment, while maintaining the intervertebral disc and facet joints to avoid fusion and to prevent negative effects on adjacent segments. We designed a new dynamic internal stabilization system (DIS I, DIS II) composed of bilateral pedicle screws, connected with a rod measuring 3.5mm in diameter on each side. The screws were made of the new type titanium-alloy with 75GPa Young's modulus and the rods were made of the same type titanium-alloy with two type of Young's modulus: 43GPa and 72GPa. Our goal of this in vitro study was to investigate whether the new dynamic non-fusion system was capable to restore the physiologic motion and stability characteristic of the instable spinal segment.MethodsIn vitro biomechanical study to characterize the flexibility and stability of the DIS system when instrumented to L3-L4 segments of calf specimens, through measuring motions at the fixed and adjacent segments. Six calf specimens (L2-L6) (median age 6 months) were collected and used for this in vitro experiment. The specimens were loaded with pure moments of±10 Nm in flexion/extension, lateral bending, and axial rotation. The range of motion (ROM) and neutral zone (NZ) of the fixed and adjacent segments were measured. The specimens were tested under the following conditions:(1) intact;(2) dissection of the supraspinous ligament, the interspinous ligament, ligamenta flava and transaction of facet joint capsules at L3-L4;(3) #2 with the DIS I system;(4) #2 with the DIS II system;(5) #2 with the SINO system.The angular motion values at the fixed and adjacent segments were analyzed using analysis of variance and multiple comparisons with Bonferroni correction.ResultsFor the fixed segment L3-L4, the defect increased the median values of the ROM and NZ in flexion-extension, lateral bending and axial rotation, comparing with other conditions (P<0.05). After fixation of the implants, the DIS I had no significantly difference with the intact condition, DIS II and SINO system, but the DIS II and SINO system decreased the ROM and NZ in all directions compared with the intact condition (P<0.05). The DIS II had no significantly difference with SINO system. The two DIS systems and SINO system restored the ROM and NZ back to 65%, 55% and 40% of the intact spine.The ROM and NZ in adjacent segments L2-L3 and L4-L5 were not influenced by none stabilization method.DiscussionBecause of the better elasticity characteristic of the new type titanium-alloy, the DIS system had enough elastic to achieve the goal of flexible restabilization and also to prevent the failure of fixation earlier.The DIS I system could provide the stabilization similar to the SINO system, and restore the fixed segmental ROM and NZ to the level of the intact. The DIS II system could decrease the fixed segmental ROM and NZ below the intact, but provide better stabilization for the instable segments.The DIS system could reduce the increased segmental ROM and NZ of flexion-extension, lateral bending and axial rotation in the similar degree. It was the evidence that the instantaneous center of rotation for the DIS system was consistent with the fixed segments. So the DIS system would meanly unload for the fixed segments during the motion of lumbar spine. It was useful for restabilize the function of the lumbar physiologically.ConclusionThe DIS I system almost ideally provided the motion and stabilization for the instable segments in flexion-extension, lateral bending and axial rotation, and achieved the goal of dynamic stabilization; the character of DIS II system was more similar to the SINO system. The systems made of all titanium-alloy provided the foundation to prevent the fatigue of fixation.