Issue 18

F. Felli et alii, Frattura ed Integrità Strutturale, 18 (2011) 14-22; DOI: 10.3221/IGF-ESIS.18.02 20 For the second implant type ( screw implant), the solution of the loading scheme is reported in Fig 4, showing the cross- section view of the deformed implant and the tension/compression stress in the direction of the implant long axis (symmetry axis). This image highlights that the most stressed areas, both in tension (red) and in compression (blue), are the contact regions between the fixture and the stump. Upon breaking down this interface, there will occur a re- distribution of the stresses and strains in which the maximum stress intensification will be on the connecting screw. The connecting screw, before interface failure, is already subjected to a tension of about 100 MPa both in tension and compression. This stress intensification, in the portion of the implant subjected to tensile stress, may easily lead to crack initiatiation when the applied load becomes cyclic and high enough. Fig. 5 shows the deformed implant and the stresses in the direction perpendicular to the implant long axis, torsion moments and shear stresses in the horizontal plane, due to horizontal forces. Shear stresses reach the maximum value on the contact surfaces between the fixture and the stump causing the implant to be mobilized and subjected to partial rotation around his long axis. These shear actions can loosen the screw and act on the implant-bone tissue interface to rotate the whole implant. The first fatigue test carried out on this implant with the flat punch (0° angled), extended for 250000 cycles does not lead to any damage and mobilizing effect on the implant. The other two tests, performed with inclined punches, showed a mobilizing effect after the same number of cycles (250000). By using the 15° angled punch this mobilizing appears to be slight and caused by the screw-fixture clamping torque decrease. With the 30° angled punch, the axial deformation due to the bending moment is evident (Fig. 11) and mobilizing phenomena due to the torsion moment are still noticeable and similar to the previous case. Figure 11 : Screw implant, deformed after the test. The other six tests are performed using the most severe loading scheme (30° angled punch) and sinusoidal stress maximum values growing over the range 700 N - 3500 N, with minimum load set to zero. The maximum value (3500 N) represents the stress at which the implant fails under static load, as in common tensile test (it would be the static resistance of the implant). The test results are summarized in Wohler-like plot, reported in Fig. 12 and relative to this implant. From the diagram analysis it can be noticed the existence of a fatigue limit at about 600-700 N. The fracture analysis showed that the damage has firstly affected the connecting screw, thus the fracture itself is promoted by a decrease in the clamping torque of the screw and by the subsequent deformation of the hexagonal head. Then the crack triggers and starts advancing in the proximity of the threads corresponding to the first section immersed in the bone (in the resin in