Biomaterial Lecture 3 PDF

LECTURE(3) Metallic Implant Materials 1  Metallic Implant Materials Dental Metals 1) Dental Amalgam An amalgam is an alloy in which one of the component metals is mercury. The rationale for using amalgam as a tooth filling material i...

LECTURE(3) Metallic Implant Materials 1  Metallic Implant Materials Dental Metals 1) Dental Amalgam An amalgam is an alloy in which one of the component metals is mercury. The rationale for using amalgam as a tooth filling material is that since mercury is a liquid at room temperature it can react with other metals such as silver and tin and form a plastic mass that can be packed into the cavity, and which hardens with time. To fill a cavity the dentist mixes solid alloy, supplied in particulate form, with mercury in a mechanical triturator. The resulting material is readily deformable and is then packed into the prepared cavity. The solid alloy is composed of at least 65% silver, and not more than 29% tin, 6% copper, 2% zinc, and 3% mercury. Dental amalgams typically contain 45 to 55% mercury, 35 to 45% silver, and about 15% tin, when fully set. The strength of the restoration increases during the setting process, so that the amalgam has attained one quarter of its final strength after one hour, and almost all of its final strength after one day. 2  Metallic Implant Materials 2) Gold Gold and gold alloys are useful metals in dentistry as a result of their durability, stability, and corrosion resistance. Gold fillings are introduced by two methods: casting and malleting. Cast restorations are made by taking a wax impression of the prepared cavity, making a mold from this impression in a material such as gypsum silica, which tolerates high temperature, and casting molten gold in the mold. The patient is given a temporary filling for the intervening time. Gold alloys are used for cast restorations, since they have mechanical properties superior to those of pure gold. Corrosion resistance is retained in these alloys provided they contain 75 w/o or more of gold and other noble metals. Copper, alloyed with gold, significantly increases its strength. Platinum also improves strength, but no more than about 4% can be added. Silver compensates for the color of copper. A small amount of zinc may be added to lower the melting point and to scavenge oxides formed during melting. Gold alloys of different composition are available. Softer alloys containing more than 83% gold are used for inlays, which are not subjected to much stress. Harder alloys containing less gold are chosen for crowns and cusps, which are more heavily stressed. 3  Metallic Implant Materials 3) Nickel-Titanium Alloys The nickel-titanium alloys show an unusual property in that after the metal is deformed, they can snap back to their previous shape following heating. This phenomenon is called the shape memory effect. The shape memory effect (SME) of Ni-Ti alloy was first observed by Buehler and Wiley at the U.S. Naval Ordnance Laboratory (NOL). The equiatomic Ni-Ti alloy (Nitinol) exhibits an exceptional SME near room Austenite temperature: if it is plastically deformed below the transformation temperature, it reverts back to its original shape as the temperature is raised. The SME can be generally related to a diffusionless martensitic phase transformation that is also thermoelastic in nature, the thermoelasticitv being attributed to ordering in the parent and martensitic phases. Martensite Martensite 4  Metallic Implant Materials Shape memory alloys are used in orthodontic dental arch wires. They also are used in arterial blood vessel stents, and may be used in vena cava filters, intracranial aneurysm clips, and orthopedic implants. On a more speculative level, they might find use in contractile artificial muscles for an artificial heart. In order to develop such devices, it is necessary to understand fully the mechanical and thermal behavior associated with the martensitic phase transformation. A widely known Ni-Ti alloy is 55-Nitinol (55 weight % (w/o) or 50 atomic % (a/o) Ni), which has a single phase and “mechanical memory* plus other properties — for example, high acoustic damping, direct conversion of heat energy into mechanical energy, good fatigue properties, and low temperature ductility. Deviation from the 55-Nitinol in the Ni-rich direction yields a second group of alloys that are also completely nonmagnetic but differ from 55- Nitinol in their capability of being thermally hardened to higher hardness levels. Shape recovery capability decreases and heat-treatability increases rapidly as Ni content approaches 60 w/o. Both 55- and 60-Nitinols have relatively low moduli of elasticity and can be tougher and more resilient than stainless steel, Ni-Cr, or Co-Cr based alloys. 5  Metallic Implant Materials The efficiency of 55-Nitinol shape recovery can be controlled by changing the final annealing temperatures during preparation of the alloy device (Annealing is the process of heating a metal or alloy to a temperature below its melting point in order to make it softer). For the most efficient recovery, the shape is fixed by constraining the specimen in a desired configuration and heating to between 482 and 510°C. If the annealed wire is deformed at a temperature below the shape recovery temperature, shape recovery will occur upon heating, provided the deformation has not exceeded crystallographic strain limits (~8% strain in tension). The Ni-Ti alloys also exhibit good biocompatibility and corrosion resistance in vivo. Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable. The mechanical properties of Ni-Ti alloys are especially sensitive to the stoichiometry (the relationship between the relative quantities of substances taking part in a reaction or forming a compound) of composition and to the individual thermal and mechanical history. 6  Metallic Implant Materials Ni-Ti alloy wire specimens were tested at 0°C and room temperature. The samples deformed at room temperature recovered almost completely to their original states, indicating that the transformation temperature is close to room temperature. The results indicate that the elastic modulus is higher at a higher temperature. 7  Metallic Implant Materials 4) Other Metals Several other metals have been used for a variety of specialized implant applications. Tantalum has been subjected to animal implant studies and has been shown very biocompatible due to the thin oxide layer formed that prevents further oxygen penetration as is the case with titanium. Due to its relatively poor mechanical properties and its high density (16.6 g/cm3), it is restricted to a few applications, such as wire sutures used by plastic surgeons and neurosurgeons. Radioactive tantalum (Ta 182) has been used to treat head and neck tumors. Arterial stents made from a single woven tantalum filament help open occluded vessels. Also, porous tantalum (72-81% porosity) has been tested as a bone graft substitute for the femoral head, as shown in Figure. 8  Metallic Implant Materials NiCu and CoPd alloys are of interest in cancer treatment since their magnetic properties enable them to be heated by an oscillating magnetic field. In this form of cancer treatment, referred to as hyperthermia, a “seed” (1 cm long, 1 mm diameter) is implanted in the tumor, then subjected to induction heating to kill tumor cells without harming adjacent tissue. This method is used for prostate cancer. Induction heating is by a Helmholtz coil at radio frequency (RF). These alloys, however, have a Curie temperature above which ferromagnetic behavior becomes paramagnetic and induction heating ceases. Therefore, one could design self-regulating thermal seeds that can be heated whenever necessary from outside of the body. The maximum temperature is the same as the Curie temperature. The Curie temperatures of Ni and Co can be manipulated mainly by varying the amount of alloying element and to a lesser degree by annealing the drawn wires. The Pd-6.15%Co and Ni28Cu give the Curie temperature of about 60°C. 9  Metallic Implant Materials Platinum and other noble metals in the platinum group (Pd, Rh, and Ir) are extremely corrosion resistant but have poor mechanical properties. They are mainly used as alloys for electrodes such as pacemaker tips because of their high resistance to corrosion and low threshold potential. 10