Novel design of a crosslink for pedicle screw constructs
Background
Unstable thoracolumbar fractures are a challenge to treat but the use of internal fixation has allowed not only for these fractures to be treated but for patients to be mobilized earlier thus reducing patient morbidity. Internal fixation has evolved over the past decade or so with the introduction of pedicle screw constructs. Research has shown that these constructs are more rigid than previous constructs such as wires and hooks. Furthermore, cross-link systems were introduced to increase the rigidity of the pedicle screw constructs. The cross-links attach to the rod constructs bridging the two rods so that the rods do not act independently of each other. These cross-link systems helped to improve the stiffness of the constructs in torsional stability rather than flexion/extension or lateral movements. A more rigid construct has been shown to improve the fusion rates in the spine in clinical and biomechanical studies. Rigid instrumentation also helps to maintain correction that is obtained intraoperatively.
Current Study
It is the objective of the current study to design a novel cross-link that will provide superior rigidity in torsional loading thereby increasing fusion rates.
Finite Element Analysis
The anatomical setup was modeled using 3-D vertebrae obtained from the BEL Repository. These vertebrae were assembled in AUTODesk Inventor and intervertebral disks were modeled using the vertebral geometry. Pedicle screws were modeled as 4mm cylinders and were inserted into the vertebrae. 4mm rods were then attached to these screws to create the construct. Several construct setups were used including no cross-link, one and two regular cross-links and the novel cross-link. The rigidity of each construct was tested using ABAQUS CAE in 4 modes of loading; Compression, Flexion/Extension, Lateral Bending/ Torsion. The FEA results were used to refine the design of the novel cross-link and to confirm its effectiveness in increasing the rigidity in torsional loading.
Torsional Stress on Two Cross-links Displacement of a Construct with No Cross-links
Experimental Procedure
When the FEA was completed the results required verification by experimental data. Porcine spines were instrumented with pedicle screws and rods and then the most superior and inferior vertebrae were potted in acrylic pots using screws and cerrobend. The superior vertebrae was attached to the end of a robot arm and the inferior vertebrae was held fixed. The arm was equipped with a JR3 6 degree of freedom sensor which recorded force data during the experiment. The novel cross-link was fabricated using a CNC machine and each setup was tested in torsion using 6 degrees of rotation from the resting position.
Current Progress
The novel design is currently being modified and a second set of experiments is being prepared. The experimental results are anticipated to match the data from the FEA model.
Researchers
TTU, Mechanical Engineering Dept., 7th and Boston, Lubbock, TX 79409
(806) 742-3563