Biomechanical Analysis Of An Anterior Angular Stable Locking Rod Thoracolumbar Spinal Instrumentation (D-rod)

Background:Many different types of instrumentation systems are available to fix the thoracolumbar spine in clinical. Anterior thoracolumbar instrumentations were evoled to either rods or plates. Both types have been used widely in the treatment of several diseases such as fractures,tumors and infections. Briefly, dual rod designs has been suggested to offer superior biomechanical stability than plate designes. However, the profile of anterior instrmentation is important to consider,bulky implants have led to catastrophic complications secondary to aortic erosions.With these considerations in mind,we sought to modify the configuration of the bulky rods system with lower profile design (D-rod system) to achieving biomechanical stability.Objective:To introduce a lower profile angular stable locking rod anterior thoracolumbar spinal instrumentation and biomechanical in vitro evalution it compared with Z-plate and Kaneda system.Methods:Eighteen pig spinal specimens (T14-L3) were divided into three groups (6 per group) and subjected to L1 corpectomy. One of three fixation methods was applied to the left of the vertebra(T15-L2) in each group:The D-rod system, Z-plate and Kaneda system.Each spine was tested in three different scenarios:1)intact spine;2)after partial L1 corpectomy with graft and stabilization with three alternative instrumentations; 3) after flexion-extension cyclic fatiguing for 5000 cycles at a load of±3.0Nm. T15-L2 range of motion (ROM) was measured in a 6-degree-of-freedom (flexion-extension, lateral bending, and axial rotation) spine simulator under pure moments of 6.0 Nm.Results:All three fixation stabilized spine showed superior stability compared to the intact spine (P<0.05).The D-rod system and Kaneda system stabilized spine showed superior stability compared to Z-plate in axial rotation (P< 0.05).Flexion/extension loading demonstrated no statistical difference between the systems in ROM (P>0.05). After fatiguing test, there have no implants fail, the D-rod system and Kaneda system instrumented spine can still restored the stability of the intact spine (P<0.05) in axial rotation. The Z-plate stabilized spine was more rigid in right lateral bending than left lateral bending before and after fatigue (P< 0.05).Conclusion:All three model anterior thoracolumbar instrumentations can restored the stability. The D-rod system and Kaneda system showed superior stability compared to the Z-palte in axial rotation. The D-rod system could provide adequate stability for anterior thoracolumbar with low profile.