Experimental analysis on the resistance of fixing nails for spine cages

Arthrodesis is one of the most common surgical treatment for the management of pathologies concerning the intervertebral discs. Special cages replace the intervertebral disc and realize the fusion between the vertebral body, often together with a fixing plate, screws, and rods. An alternative method is a cage that contains integrated screws, designed with suitable holes, it allows an easier insertion of fixing system during surgery. Mt Ortho company offers a cage equipped with two diverging locking nails in Ti6Al4V-ELI titanium alloy. These nails are placed directly inside the cage inclined by 30° with respect to the horizontal. This paper, performed in collaboration with Mt Ortho, reports the experimental tests aimed at evaluating the performance of locking nails under pull-out stresses. To evaluate the nails pull-out in a more physiological way, together to the standard condition, tests were performed on a composite layer made of wood and foam. Furthermore, the notch effects generated by the locking nails are qualitatively evaluated by photoelastic analysis, in view of a geometry able to reduce the stress concentration factor on the bone tissue, without reducing the pull-out resistance.