Treatment options for spinal trauma, chronic disease and/or age degeneration vary based on the medical prognosis and severity of symptoms, with surgery being one treatment option. One of the many surgical options for stabilization is pedicle screw based fusion, where screws are placed into the spine and metal rods are fixed to these screws using set screws. The tightening torque is critical for these procedures, as under or over torquing leads to slippage, decreased strength, and ultimately poor fusion results. Numerous products in the market use a torque wrench or break-off set screws that ensure the correct tightening torque. The focus of this thesis is to develop techniques (based on geometrical alterations without changing the material) that modify the set screws so that they reduce shock (200-800 g-force) while still ensuring the correct tightening torque.
The hypothesis of this thesis is to show that geometric changes in the set screw tightening structure (i.e. device) can reduce the maximum shock by influencing how the stored energy is released, and yet keep the tightening torque the same. The objective is to increase the time period between yielding and fracture and channel more energy towards plastic deformation. The specific aim is to make geometric changes in the grooved region (designed for break off) to cause the ratio of maximum recoverable strain energy to maximum plastic energy dissipation to decrease. Computer models based on fracture mechanics are used to demonstrate this. Using a model for ductile metal Al 5083-H116, it was shown that wider, more gradual grooves lead to a 36% decrease in this ratio. A decrease in this ratio indicates that the maximum g-force shock will decrease since a greater percent of the energy will be dissipated during material deformation. In addition, there was a 74% increase in rotation before failure. This behavior is believed to be beneficial since broken bonds between molecules in the shear band are given more time to re-bond with an adjacent molecule, and this new bond consumes more energy when final shear separation occurs. Optimization, experimental validation, and extension of the models to titanium remain as future work.