Issue 18

F. Felli et alii, Frattura ed Integrità Strutturale, 18 (2011) 14-22; DOI: 10.3221/IGF-ESIS.18.02 15 similarity between the fracture mode of the tested implants and of the actual implants broken after a certain operating period. K EYWORDS . Dental implants; Fatigue resistance; Titanium alloys. I NTRODUCTION owadays dental prosthetic-implant systems are widely used for treatment of partial and total edentulism. This wide diffusion has been promoted by numerous studies which have demonstrated the "biologic" effectiveness of the used materials (in particular Titanium and some of its alloys) [1-4]. In spite of this diffusion some aspects have not been deeply investigated, like the mechanical resistance properties of implants and the stability of the connection between implants and prosthesis elements [5-6]. Although it is rather unusual to observe a clinical failure due to the biological reaction of the bone and/or gingival tissues after the installation of a dental implant, some mechanical problems frequently arise, like the fracturing of fixture or other prosthesis components (abutment, the implant-abutment connecting screws) [7-13]. Another commonly observed problem is represented by the progressive loosening of the screws connecting the implant with the abutment, this event often leads to the fracture of such screws, or even worse, to the fracture of the whole implant [6, 14]. Thus, the study of the mechanical characteristic of dental prosthesis implant systems is important to achieve long-term clinical success of implant-supported prosthesis. This kind of evaluations can be executed on a certain implant both "in vivo" on an installed prosthesis, and by using a mechanical system to accurately simulate the mastication effect. Carrying out "in vivo" studies has many technical difficulties, besides understandable ethic issues. First of all, forces exerted on natural and prosthetic dental elements during physiological functions (mastication, deglutition, phonation) and pathological functions (bruxism and other para-functions) have characteristics of intensity, direction and frequency which are extremely variable depending on the individual. This variability, although included in a well studied and well known range of values, makes difficult to extend results obtained on a limited sample of individuals to all possible clinical situations. To increase the validity of such tests, a much wider number of samples should be studied. To obtain reliable results, it should be considered the long duration of such testing: for a "normal" dental functionality, it would be necessary months, or most likely years [15]. To overcome those difficulties, mechanical systems have been designed and then improved in order to simulate the mastication effect [16-17]. Such systems are able to accurately reproduce: - set of mechanical loads (compression, bending, tension, torsion loads) acting on a prosthesis implant system during functional and para-functional activities. To achieve that, the developed occlusion forces depending on occlusal plane morphology, tooth position in the dental arch, size and hardness of food are schematized and reproduced. - connection between implant and bone tissue. As it is known the biological relationship between the chemically pure surface of dental materials, like titanium, and the living bone tissue is defined as "osseointegration" [18-23]. The contact area between bone and tissue varies according to several parameters such as the permanence time of the fixture in the tissue after the installation, the bone density (variable according to histologic type, cortical or spongy, and anatomic position of the implant in the dental arch) and kind and history of applied loads. - functionality of the prosthesis-implant system, which is the capability of the system to maintain a stable osseointegration (and then the capability of efficiently transmitting the applied load to the bone tissue) and the correct and stable connection between the implant and the prosthetic elements. This work concerns with the study of some clinically failed implants, which fractured at different times after the clinical installation: this study is of interest because those implants are still used from many patients. After observing the fracture surface morphology, a mechanical system has been setup to reproduce the mastication loading conditions to perform more accurate evaluations on implants similar to the ones on which the clinical failures have occurred. Although several methods are described in literature for fatigue testing [24], an specifically designed equipment has been set up in this work to assess the implants in terms of test loads and cycles. This paper analyses some clinically failed implants, which are of the bore cylinder type, and tries to investigate the failure mode. SEM analyses highlighted that fatigue failures occurred in such implants with initiations in the areas near to the bores, whose function is to promote osseointegration. Thus, since fatigue failure is known to initiate and propagate under tensile stress conditions, it is clear that the retrieved implants were subjected to traction load in their service life. So in this N