Issue 35

Š. Major et alii, Frattura ed Integrità Strutturale, 35 (2016) 379-388; DOI: 10.3221/IGF-ESIS.35.43 387 This figure shows also curves of theoretical lifespan obtained with help of a model based on Navarro method. Other methods of lifespan forecast have been tested for a comparison with our new method. In article [9], there are described various multiaxial criteria, adapted for lifespan forecast of notched samples, which corresponds to the case of the screw. A criterion proposed by Goncalves gives the best forecast of all criteria described in [9]. Goncalves criterion has been modified with help of A G and B G parameters for sample analysis. 5 2 1.max 1 1 G GAM i G GAM i A a d B b f          (8) and           max min / 2 i i i d s t s t   , (9) where parameters d i can be determined from minimum and maximum values of the transformed deviatoric stress tensor. The material variables are set from fatigue limits as:   1 2 1 1 3 G a     , (10) 3 3 1 G b     . (11) If we perform a comparison of theoretical lifespan forecast of the screw based on Navarro method [6, 7, 8], and older methods [9] (Goncalves method is best of them), it can be said that Navarro method gives 50 % better result than Goncalves method. Navarro method respects conditions of fixation of the screw in the bone better than older methods. The most important advantage of Navarro method is a possibility of including an influence of filling cement. C ONCLUSIONS ollow screws with the performance show higher fatigue resistance with the same load, which can be explained by stronger deposition over the entire length of the screw. Results of fatigue test were compared with theoretical predictions and theoretical model predicts that initiation phase is about 10% of fatigue life of implant. R EFERENCES [1] Chen, C.S., Chen, W.J., Cheng, C.K., Jao, S.H., Chuech, S.C., Wang, S.C., Failure analysis of broken pedicle screws on spinal instrumentation, Medical Engineering & Physics, 27 (2005) 487–496. DOI:10.1016/j.medengphy.2004.12.007. [2] Griza, S., de Andrade, C.E.C., Batista, W.W, Tentardini, E.K., Strohaecker, T.R., Case study of Ti6Al4V pedicle screw failures due to geometricand microstructural aspects, Engineering Failure Analysis, 25 (2012) 133–143. DOI:10.1016/j.engfailanal.2012.05.009. [3] Amaritsakul, Y., Ching-Kong , Ch., Jinn, J., Biomechanical evaluation of bending strength of spinal pedicle screws, including cylindrical, conical, dual core and double dual core designs using numerical simulations and mechanical tests, Medical Engineering & Physics, 36 (2014) 1218–1223. DOI: 10.1016/j.medengphy.2014.06.014. [4] Krag, M. H., Biomechanics of thoracolumbar spinal fixation: a review, Spine, 16 (1991) 84–99. [5] Ormsby, R. McNally, T., Oharre, P., Burke, G., Mitchell, G., Fatigue and biocompability of properties of poly(methyl methacrylate) bone cement with multi-waled carbon nanotubes, Acta Biamaterialia 8 (2012) 1201–1212. DOI: 10.1016/j.actbio.2011.10.010. [6] Ayllón, J.M, Navarro, C., Vázquez, J., Domínguez, J., Fatigue life estimation in dental implants, Engineering Fracture Mechanics, 123 (2014) 34–43. DOI:10.1016/j.engfracmech.2014.03.011. H