Fatigue is the most common mechanical failure type in dental implants. ISO 14801 standardizes fatigue testing of dental implants, providing the load-life curve which is most useful for comparing the fatigue behavior of different dental implant designs. Based on it, many works were published in the dental implant literature, comparing different materials, component geometries, connection types, surface treatments, etc. These works are useful for clinicians in order to identify the best options available in the market. The present work is intended not for clinicians but for dental implant manufacturers, developing a design tool that combines Finite Element Analysis, fatigue formulation and ISO 14801 experimental tests. For that purpose, 46 experimental tests were performed on BTI INTERNA® IIPSCA4513 implants joined with INPPTU44 abutments by means of INTTUH prosthetic screws under three different tightening torque magnitudes. Then, the load case was reproduced in a FE model from where the nominal stress state in the fatigue critical section was worked out. Finally, Walker criterion was used to represent accurately the effects of mean stress and predict fatigue life of the studied dental implant assembly, which can be extended to most of the products of BTI manufacturer. By means of this tool, dental implant manufacturers will be able to identify the critical design and assembly parameters in terms of fatigue behavior, evaluate their influence in preliminary design stages and consequently design dental implants with significantly better fatigue response which in turn will reduce future clinical incidences.
Self-loosening of the prosthetic screws is a major mechanical problem affecting roughly 10% of dental implants, according to the literature. This phenomenon may lead to micro-movements that produce crestal bone loss, peri-implantitis, or structural failure of the implant assembly. In this paper, a simple and effective tool to predict self-loosening under masticatory loads is presented. The loads acting on the screw are obtained from a simple finite element (FE) model, and introduced in a mathematical formula that calculates the torque needed to loosen the screw; self-loosening will occur when this torque becomes zero. In this sense, all the parameters involved in self-loosening phenomenon can be easily identified, and their effect quantified. For validating purposes, 90 experimental tests were performed in a direct stress test bench. As a result, a powerful tool with a maximum experimental error of 7.6% is presented, allowing dental implant manufacturers to predict eventual occurrence of self-loosening in their developed dental implant products and take corrective actions at preliminary design stage. Furthermore, the following clinical implications can be directly derived from the methodology: a higher screw preload, that is a higher tightening torque, improves self-loosening response of the dental implant and, similarly, for a given preload force, higher friction coefficient and screw metric, as well as lower pitch and thread angle values, are also found to be beneficial.
Dental implants are widely used to replace missing teeth. 1 In direct-to-implant restorations, implant and abutment are joined by the preload obtained through the tightening torque applied to the prosthetic screw 2 as illustrated in Figure 1, providing the necessary structural integrity to the restoration. [3][4][5][6][7][8] The recommended torque value is provided by manufacturers based on different implant design factors. 9 Clinical studies suggest that the failure of implant restorations is often induced by fatigue because of the variable load conditions during their life span. [10][11][12] These failures typically occur in the prosthetic screw, with abutment or implant failures being less common. 13,14 The screw fracture is usually located at the first thread engaged with the implant. 15,16 Under given loading conditions, the number of cycles to fatigue failure depends on several parameters, including component dimensions, screw metric, material, manufacturing processes, and preload level. 17 Most prosthetic screws are made of pure or alloyed titanium, as these are less expensive than gold alloy screws while having excellent biocompatibility, corrosion resistance, machinability, and desirable physical and mechanical properties. 18,19 Two processes are widely used for screw thread manufacturing in industry: cutting and cold rolling. In thread cutting, material is removed from a cylindrical blank by machining, and in thread rolling, a matched set of dies displaces material of the manufacturing part to produce external threads on the
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