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ePaper created 2017-12-06, 12:03:50 | version 1.38.0
| research Titanium and its alloys in dental implantology Authors: Drs Roland Masa & Gábor Braunitzer, Hungary Pure titanium and titanium alloys for dental purposes properties without reducing biocompatibility is still a challenge.6 In contemporary prosthodontics, the use of dental implants is as self-evident as any other established method. Titanium (Ti) and its alloys are still the most widely used materials for dental and orthopaedic ap- plications.1, 2 Titanium has good mechanical stability, low density (4.5 g/cm3), a high strength-to-weight ratio and favourable biocompatibility.3 Titanium and its alloys have excellent corrosion resistance owing to the thick, insoluble titanium dioxide (TiO2) layer that forms on the surface within nanoseconds. This layer can restore itself immediately in the presence of water or air should damage occur.4 Four grades of unalloyed, commercially pure (CP) Ti are available for dental applications, designated as Grades 1 to 4. These grades are defined by their oxygen and iron content, as these elements have a substantial effect on the mechanical and physical properties of the metal, even in very small concen- trations. As the concentration of oxygen or iron in- creases, the mechanical strength increases in paral- lel, while ductility decreases.5 The Ti-6Al-4V alloy (described later) is also referred to as Grade 5. Grades over 5 are not used in dentistry. A compari- son of the mechanical properties of CP Ti and its alloys is given in Table 1. Grades 1 to 4 As already mentioned, the physical characteristics of CP Ti are predominantly influenced by the oxygen and iron content of the material. The increasing grade number expresses a decreasing amount of these “impurities”. Therefore, Grade 1 is the softest and most ductile type of CP Ti, while Grade 4 is sig- nificantly stronger and less malleable than the lower grades.5 Of the unalloyed CP Ti grades, Grade 4 has the highest tensile strength and yield strength. Some disadvantages that Grades 1 to 4 have are relatively low mechanical strength, a high Young’s modulus and poor wear resistance. Improving the mechanical Grade 5 (Ti-6Al-4V) CP Ti is not preferable when high stress tolerance is required. Mechanical properties such as implant strength, creep resistance and formability can be im- proved by alloying Ti with a wide range of elements (e.g. aluminium, Al; vanadium, V; tantalum, Ta; zirco- nium, Zr). As shown in Table 1, the mechanical prop- erties of Ti alloys are superior to those of Grades 1 to 4, and therefore it comes as no surprise that Grade 5 is the most widely used Ti alloy for biomedical appli- cations.3 In spite of its good mechanical features, corrosion wear and ion release (Al, V) initially raised concerns about its applicability in implant dentistry. De Morais et al. investigated the level of these ions released from orthodontic mini-implants and the potential toxicity of these elements.7 They concluded that, despite the detectable amounts of Ti, Al and V ions, these values remained below the average nutrition uptake of these ions and did not reach the level of toxicity.7 A high Young’s modulus is also a problem with Grade 5, but the exact value (115 GPa) does not sig- nificantly differ from that of the CP grades. Therefore, this should not raise specific concerns regarding this alloy. Different alloying elements have been used to replace Al and V in the Ti-6Al-4V alloy. One example is the use of niobium (Nb) and Zr in the alloy Ti-13Nb-13Zr. This offers the highest strength-to- weight ratio and a reduced Young’s modulus (77 GPa), making Ti-13Nb-13Zr optimal for orthopaedic im- plants.8 Ti-13Nb-13Zr’s possible dental applicability is still under investigation.9, 10 Adverse reactions to titanium and titanium alloys Since Ti is a transition metal, allergy or metal hypersensitivity may be a matter of concern.11–13 In 06 implants 4 2017