Issue 18

F. Felli et alii, Frattura ed Integrità Strutturale, 18 (2011) 14-22; DOI: 10.3221/IGF-ESIS.18.02 17 It is known that the load stress regime on the implant during service cannot be simply reduced to the elementary compressive stress. Upon mandible closing, horizontal forces arise. These forces depend both on the dental cusps profiles, differently inclined relatively to the occlusive force which is substantially vertical, and on the mastication motion including side and protrusion movements of the mandible. Moreover, the vector sum of vertical forces acting on a tooth is not directed along the longitudinal axis of the tooth itself, thus bending moments arise. These bending moments added to those resulting from the horizontal forces, determine a flexural stress which is not negligible. Since the horizontal force components generally do not lay on a plane formed by their points of application and the symmetry axis of the implant (the tooth longitudinal axis) a torsion stress arises in addition to existing compressive and flexural stresses. In the cyclic fatigue regime, bending moments determine the necessary conditions for the initiation and subsequent propagation of fatigue cracks in the portion of the tooth which is in tension. Torsion moments, besides contributing to overall tension stress, can lead implant to mobilizing problems due to the decrease of the screw clamping torque and shear actions on the fixture-bone interface. Tests description The fatigue tests performed on the two dental implants are executed using a hydraulic-drive fatigue test machine, while the fracture surface analyses are carried out by a scanning electron microscope (SEM). The first implant (hollow and bored cylinder) is incorporated in an aluminium cylinder for a depth corresponding to the "implantantion-depth" (that is the same depth at which the implant should be inserted in the bone when in service) using an epoxy resin. The aluminium cylinder allows the connection with the test machine grippers. The adhesion between implant and cylinder is also increased by means of steel wires connected to the implant and immersed in the interface epoxy resin which is, specifically, a double-component epoxy resin Mecaprex MA. Firstly, this implant system is tested in air; three fatigue tests are performed using sinusoidal tension loads having increasing maximum values equal to 500, 750, 1000 N. The tests are repeated 3 times in order to achieve an acceptable statistical value considering the limited number of samples available. The selected maximum tension loads take into account the real average stress in the mastication which is, on the most stressed teeth (molar and pre-molar), about 770 N. The minimum value of the sinusoidal load is fixed and constant for all the performed tests and it is equal to 100 N. The loading frequency, fixed at 1 Hz, is comparable with the real average mastication frequency. Then, other three tests are performed using the same set-up and loading system, but in a different environment; the implant works in aqueous solution containing 3%wt NaCl in order to simulate, to a certain extent , the corrosive environment established in the mouth during the mastication. The tension load is selected in order to simulate the stress regime that occurs in the tension area of an implant when subjected to bending moments. As far as the second type of implant (screw type) is concerned, the complex stress regime to which it is subjected is firstly analysed by means of a finite elements solver provided by a software tool (ANSYS 7.0) for the Branemark type implant; the loading scheme and the results are shown in Figs. 3, 4, and 5. In the left part of Fig. 3, the 3D-mesh used for the finite elements calculation is shown. On the right part it is possible to observe the loading scheme with the punch having the contact plane, transmitting the load, inclined of 30° on the horizontal, on which is applied a 1000 N force. The surface in light blue is the part of the implant (fixture) that is constrained by the bone. Figure 3 : 3D mesh and loading scheme.