Experimental analysis of pedicle screws

Aim of this study is to examine the mechanical properties of pedicle screws in titanium alloy obtained by the process of additive manufacturing (Electron Beam Melting, EBM) and compare them with those of screws obtained with traditional techniques. EBM is a methodology able to process metallic powders layer-by-layer. Pedicle screw-produced by the EBM process by the MT Ortho (VS), and commercial screws (VC and VT) were examined. VS screws showed lower resistance to pull-out tests than VC and VT screws. This behavior can be attributed to a lower sharpness and a lower depth of thread. These features, which may be negative from the point of view of the resistance to pull-out tests, could have positive effects from the point of view of stress concentration on bone. To analyze these effects, fatigue pull-out tests were also carried out and the different screw-bone interface behavior was evaluated using photoelasticity and finite element analysis. The results obtained by numerical simulation confirm what was previously stated, a thread with a geometry less sharpened ensures a better distribution of the loads and reduces the notch effect, allowing a balance of the disadvantage due to the lower primary stabilization. The fatigue limit and fatigue curve were obtained by the thermographic (Risitano) method. Even with a limited number of tests, the thermographic technique allows the prediction of the limit of fatigue and of the whole fatigue curve