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ePaper created 2017-11-09, 09:28:26 | version 1.38.0
| technology CeraRoot)45. A notable review proposed that in an ongo- ing clinical study, TZP- (ZrO2/Y2O3/Al2O3) experimental implants (n = 119) with an especially roughened surface presented a survival rate of 96.6 per cent after a one-year observation period.41 However, clinical and laboratory research data were scarce on safe recommendations for a widespread clinical application of Y-TZP implants.7 Mechanical tests In order to bring dental implants into markets, they should firstly pass several mechanical tests like fatigue and dynamical loading tests. These tests are mainly re- lated to the ability of an implant to withstand the loading strength as a simulation to what is comparable to the oral cavity. Fatigue is defined as the weakening of a ma- terial caused by repeatedly applied (mechanical) loads (repeated loading and unloading), normally below the ul- timate stress limit. Not only clinical loading scenarios are simulated including pressure or bending, but also torsion, shearing, or tensile forces are occurring. Fatigue stages are crack initiation, crack growth, and final failure. Cracks may, for example, initiate from structural or superficial de- fects (wear or processing traces). Stress level, rate, form, and frequency of the load situation are essential on the performance of the material as is the form of the speci- men or its surface condition. It seems important to select the loading parameters (force, frequency, etc.) in dependence on the material properties (e.g. viscoelastic behaviour) and application conditions (e.g. wet environment). Fatigue tests are of- ten performed by measuring the crack growth in a frac- ture mechanics approach or by determining the residual stability or strength after fatigue/aging tests. Therefore, short-term tests are required for each individual mate- rial or restoration, which lead to degradation or final fail- ure.46, 47 A number of publications underline the influence of the fatigue environment and synergetic corrosion fa- tigue on the performance of the materials, especially in Fig. 3 36 case of ceramic materials. Some studies indicated strong variations for the manuscripts available in literature, pro- viding no information, 20 °C or room temperature (dry), 37 °C (dry, in water or saliva), or thermal cycling (usually 5 °C/55 °C) as testing condition.48, 49 Loading tests for dental implants can be denoted according to predefined standards or norms (i.e. ISO, DIN, or EN). For instance, DIN 50100 describes a load-controlled fatigue testing design at constant load amplitudes on metallic specimens and components. The endurance limit can be displayed, for example, in a Wöhler curve or in fatigue strength diagrams.50 How- ever, this standard is not usually applicable for testing dental implants. ISO 13356:2015 specifies the require- ments and corresponding test methods for a biocom- patible and biostable ceramic bone-substitute mate- rial based on yttria-stabilised tetragonal for use as a material for surgical implants. This norm imposes that a maximum of 25 weight per cent of monoclinic phase is present in test specimens after an accelerated aging test (134 °C in a humid atmosphere with an air pressure of 0.2 MPa).51 ISO 14801:2016 (previously known as ISO 14801:2007) specifies a method of dynamic testing of single post en- dosseous dental implants of the transmucosal type in combination with their pre-manufactured prosthetic com- ponents,52, 53 and is used in 162 member countries around the world. It is most useful for comparing endosseous dental implants of different designs or sizes.54 This in- ternational standard is not a test of the fundamental fa- tigue properties of the materials from which the endos- seous implants and prosthetic components are made, and, moreover, is not applicable to dental implants with endosseous lengths shorter than 8 mm nor to magnetic attachments. While ISO 14801:2016 simulates the func- tional loading of an endosseous dental implant under “worst case” conditions, it is not applicable for predicting the in vivo performance of an endosseous dental implant or dental prosthesis, particularly if multiple endosseous dental implants are used for a dental prosthesis. Critics and possible modifications Although ISO standards are equipped to encounter all possible loading situations that could take place in the mouth, they still lack more real conditions that should be taken into consideration. ISO 13356 prescribes the evaluation of test specimens with a simplified geometry (bending bars) and a polished surface. However, com- plex geometries as well as postprocessing steps like mi- cro-roughening to enhance osseointegration are known to significantly compromise the mechanical properties Fig. 3: The testing facility allows for efficient testing of abutment and materials.