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ePaper created 2017-11-09, 09:28:26 | version 1.38.0
| research Ageing of titanium vs zirconia Ageing of titanium implants is a not widely known phenomenon and starts four weeks after their production which decreases dramatically the osseointegra- tion potential.15–18 Ageing of zirconia (Low Temperature Degradation LTD, i.e. the slow transformation of the metastable tetragonal crystals to the stable monoclinic structure in the presence of water or water vapour) on the m o k.c c other hand is quite well investigated. uttersto h © vinai chunkhajorn/S Degradation rates at room or body temperature of Y-TZP ceramics are currently not available, and acceler- ated tests at intermediate temperature (100 to 300 °C) are the only basis for extrapolating an estimate of the trans- formation rate and, hence, of the product lifetime. This approach relies on the assumption that the transforma- tion rate follows the same Arrhenius-like trend down to room/body temperature. Unfortunately, such extrapola- tion could lead to a significant error in estimating room/ body temperature lifetimes.9 Still this is the method that is used in researches. Monzavi M. et al. (2017) examined 36 zirconia implants of four different brands and found that the effect of ageing was minimal in all systems.19 They suggested though that in vivo studies are needed to investigate the effect of mastication force on the extent of LTD and the influence of surface changes such as delam- ination of the grains on surrounding hard- and soft-tissue. Still a certain degree of transformation from tetragonal to monoclinic phase can actually improve the mechanical properties of Y-TZP. Under stress, i.e. at the tip of a crack, the Y-TZP undergoes a phase transformation from tetrag- onal to monoclinic phase. This phase transformation re- sults in a 3 to 4 per cent volumetric expansion inducing a compressive stress in the area of the crack and theoreti- cally prevents crack propagation.1 An implant which exhib- its phase transformation in case of microcracks and high forces is desirable. Still it is not sure whether the already existing microcracks that are produced (for instance, during handling) during mastication or parafunctional ac- tivities, don’t propagate, leading to a possible fracture. One- vs two-piece zirconia implants Zirconia appears in two varieties, one- and two-piece im- plants. One-piece implants offer the absence of a microgap between implant and abutment which seems to be of ben- efit. The surgical placement of the implant, though may not always meet the prosthodontic requirements and angled abutments in order to correct misalignment, is not com- mon. Secondary corrections of the shape by grinding must be avoided, as this severely affects the fracture strength of zirconia.20 Protection by use of splints is also required, though not always possible. So, two-piece implants were ZrO2 is a highly bio compatible material that needs to osseointegrate and withstand masticatory force without fracturing. provides the advantage of easier milling than the fully sin- tered ZrO2. It requires less milling time and causes less wear of the cutting tools.10, 11 In hard machining of fully sintered ZrO2, no sintering shrinkage is expected and there is no need for a sintering oven. However, microcracks maybe introduced.10 Since diamond zirconia is known as the toughest material ex- isting, only diamond tools are used for cutting sintered zirconia. The grinding of the fully sintered ZrO2 causes a certain degree of transformation (from tetragonal to monoclinic phase) in the surface of this material.12 When comparing the final surface of the soft machined ZrO2 to the hard machined ZrO2, it is expected that the former will have a more consistent final state, given that it is left in- tact (no sandblasting or grinding) after the final sintering.13 The implants that are produced need to be roughened in order to be osseointegrated. Question arises what is the optimal roughness and surface that is produced af- ter it, in order for zirconia implants to be successfully os- seointegrated in any of the aforementioned production methods. It seems that the rougher the body, the better the odds for osseointegration.14 This though should not be the goal for the head of the implant in case that it is vis- ible in the mouth—it could favour bacteria colonisation. The best method to achieve the optimal roughness as well as the moment that this should be realised with re- spect to the material’s properties is also not established. Finally, depending on the procedure, the roughened sur- face needs to be totally clean, free of all foreign bodies. 08 implants 1 2017