PSI - Issue 2_A

Guido La Rosa et al. / Procedia Structural Integrity 2 (2016) 1244–1251 G. La Rosa et alii/ Structural Integrity Procedia 00 (2016) 000 – 000

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indicated in the figures is the nominal bending stress considering the core diameter of the screws, without taking into account the notch effect. Even with a limited number of tests, the thermographic technique allows the prediction of the fatigue limit, estimated around 270 MPa for the VC screws and 340 MPa for the VS screws. From bending tests, that well simulate the load conditions of the implant, the fatigue limit is substantially comparable, even better for VS screws. The fatigue curves, as only in the first approach, put in evidence how, in the LCF range, the commercial (VC) screws show a better behavior but, in the HCF range, the performances are comparable between the two types of screws. This behavior allows to revalue the performances of the VS screw, mainly for the secondary stabilization. Consequently, the VS screws, that ensuring a better osseointegration grace to the higher roughness, could offer increased security in the post-operative phase. 6. Conclusions This work, conducted in collaboration with the company Mt Ortho, has the purpose of carrying out a preliminary analysis to assess the viability of a future commercialization of pedicle titanium screws obtained by additive manufacturing process EBM. To this aim, experimental and numerical tests have been performed to compare the EBM screws (VS) with commercial screws currently in clinical use (VC). The tests carried out have shown encouraging results. The VS screws exhibited a lower resistance to pull-out tests with respect to VC screws, but it is known that a rough and microporous surface, such as that of the VS screws, provides to the implant osseoinductive capacity, improves cell adhesion, proliferation and the differentiation of osteoblasts, thus ensures a better secondary stability. It was also demonstrated that the geometry of the VS screws allows a better distribution of the stresses at the bone-implant interface and reduces the notch effect, with obvious benefits on the host bone tissue. VS screws have a resistance to bending comparable with the VC screws, but the first show a brittle behavior, while the second ductile. ASTM Designation: F 543 – 02 Standard Specification and Test Methods for Metallic Medical Bone Screws. ASTM Designation: F 1839 – 01 Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments. Daftari, T.K.; Horton, W.C.; Hutton, W.C., 1994, Correlations Between Screw Hole Preparation, Torque of Insertion, and Pullout Strength for Spinal Screws, Journal of Spinal Disorders, 7, 2, 139-145. Fargione, G., Geraci, A., La Rosa, G., Risitano A., 2002, Rapid determination of the fatigue curve by the thermographic Method, International Journal of Fatigue, 24, 11 – 19. Gaines, R.W. Jr., 2000, The Use of Pedicle-Screw Internal Fixation for the Operative Treatment of Spinal Disorders, J Bone Joint Surg Am, 82 (10), 1458 -1458. Geetha, M., Singh, A.K., Asokamani, R., Gogia, A.K., 2009, Ti based biomaterial, the ultimate choice for orthopedic implants. A review, Progress in Materials Science, 54, 397-425. La Rosa, G., Risitano A., 2000, Thermographic methodology for rapid determination of the fatigue limit of materials and mechanical components. International Journal of Fatigue, 22, 65 – 73. Marin, E., Pressacco, M., Lanzutti, A, Turchet, S. , Fedrizzi L., 2013, Characterization of grade 2 commercially pure Trabecular Titanium structures, Materials Science and Engineering C, 2648-2656. McLain, R.F., Fry, M.F., Moseley, T.A., Sharkey, N.A., 1995, Lumbar Pedicle Screw Salvage: Pullout Testing of Three Different Pedicle Screw Design, Journal of Spinal Disorders, 8, 1, 62-68. Pfeiffer, M., Gilbertson, L.G., Goel, V.K., Griss, P., Keller, J.C., Ryken, T.C., Hoffman, H.E., 1996, Effect of Specimen Fixation Method on Pullout Tests of Pedicle Screws, Spine, 21, 9, 1037-1044. Risitano, A., Corallo , D., Risitano, G., 2012, Cumulative damage by Miner’s rule and by energetic analysis, SDHM Structural Durability and Health Monitoring, 8, 2, 91-109. Risitano, A., La Rosa, G., Geraci, A., Guglielmino, E., 2015, The choice of thermal analysis to evaluate the monoaxial fatigue strength on materials and mechanical components, Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 229, 7, 1315-1326. Rolander, S.D, 1966, Motion of the lumbar spine with special reference to the stabilizing effect of posterior fusion. Acta Orthop Scand; 90, 1-144. Weinstein, J.N., Rydevik, B.L., Rauschning, W., 1992, Anatomic and Technical Considerations of Pedicle Screw Fixation, Clinical Orthopaedics & Related Research, 284, 34-46. Yuan, H.A., Garfin, S.R., Dickman, C.A., Mardjetko, S.M., 1994, A Historical Cohort Study of Pedicle Screw Fixation in Thoracic, Lumbar, and Sacral Spinal Fusion, Spine 19, 20, 2279S-2296S. References