Figure 1 Clinical uses of inorganic biomaterials (Hench 1985).
Related Figures (16)
Table 1. Mechanical properties of the implant alloys and human bone. Also, Ti6A14V alloy contains aluminum which is well-known to cause certain bone diseases and neurological disorders. Vanadium is considered to be an essential element in the body, but may become toxic at excessive levels. The toxicity of vanadium is well-known, and can be aggravated when an implant is fractured and subsequently undergoes fretting. Release of titanium ions into the tissues adjacent to the implants results in discolouration of the tissues. This may also be detrimental to the bone attachment and further bone growth on the implant surface. Table 2. Properties of bone. Figure 3. Generalised fibrous capsule forming in response to intramuscularly implanted metal. Figure 4. Two principles of breakdown of passivity: (a) Metal undergoes general corrosion with fil dissolution, (b) metal undergoes pitting corrosion. Figure 5. Idealized polarization curves indicating pitting and crevice potentials. Table 3. Electrochemical parameters for modified stainless steel materials in Hank’s solution. Figure6. Potentiodynamic anodic polarization curves for (a) type 316L and nitrogen bearing stainless steels, and (b) type 316L superferritic, duplex and 316 Ti modified stainless steel. The modified materials have shown improvement in high pitting corrosion resistance as compared to the currently used type 316L SS (implant alloy). For comparison, the pitting potential of the failed implants, 316L (ASTM standard specification for implant alloy) and the modified stainless steels are shown in figure 7. Figure 7. Pitting behaviour of failed implants (10 cases studied), 316L stainless steel and modified stainless steels. Table 4. Cyclic polarization data for modified stainless steel materials in Ringer’s solution. To compare the in vivo tests with those obtained under in vitro conditions, the polarisation curves of the reference material type 316L SS under both conditions are presented in fig- ure 8. The cyclic polarisation curves obtained using the electrode cell assembly showed good reproducibility under in vivo and in vitro experimental conditions. The pitting potential of in vivo experiment was about +120mV on the anodic region as compared to the in vitro experi- ments, and the repassivation potential also exhibited a shift of about +100 mV towards noble direction under in vivo condition. Many factors can be considered to explain the differences between in vivo and in vitro corrosion resistances. Amino compounds present under in vivo function as inhibitors (Arumugam 1998). Further, when a foreign material is inserted into a living system, almost instantaneously, specific blood proteins, consisting mainly of fibrino- gen, adsorb onto the solid surface. This pattern of initial protein adsorption is encountered on all types of foreign materials inserted into a living system. Then, after continued adsorption of proteinaceous components, the surface of the solid becomes covered by a ‘conditioning’ film stainless contains higher percentage of Cr, Ni and Mo, lower carbon content and inclusions Localized corrosion resistance behaviour was evaluated by in vitro electrochemical techniques namely open circuit potential (OCP) versus time measurements, pitting and crevice corrosion resistance studies and accelerated leaching studies were carried out for nitrogen ion implanted and unimplanted specimens in simulated human body fluid (Ringer’s Solution) environment in order to optimizes the dose for producing maximum corrosion resistance to localized corrosion (Veerabadran 1999). unimplanted specimen. The passive current density and area of the repassivation loop were found to decrease as the dose values increased. The electrochemical impedance spectroscopic results indicate that the polarization resistance was higher for the of dose of 2.5 x 10!” ions/cm? implanted at an energy of 70keV and at 5 x 10'° ions/cm? for an energy of 100 keV. Nitrogen ion implantation enhanced the passivablity and reduced the corrosion kinetics of the alloy surface with increasing tendency for repassivation and thus enhanced corrosion resistance. This improvement arises from the formation of precipitates of TiN and Ti2N, which screen underlying titanium atoms, avoiding their migration and stabilizing the growth of the oxide film. GIXRD studies have shown that the implanted nitrogen present in the solid solution started to form nitride at the dose of 1 x 10!’ ions/cm? whereas at the higher dose 2-5 x 10'” ions/cm? stoichiometric nitride had been formed. SIMS studies established the changes in the nature, composition of the implanted area as well as of the passive film formed after implantation that explain the enhanced corrosion resistance of the nitrogen ion implanted Ti-6A1-7Nb alloys in simulated body fluid. The study showed the depletion of Al, V and Nb in the passive film and the incorporation of nitrogen in the passive film which blocks the kink sites on the surface by the formation of oxynitrides along with oxides to prevent further dissolution (Thair et al 2001). Nitrogen implantation for both higher dose and energy (> 2-5 x 10!” ions/cm”) was found to be detrimental from the electrochemical point of view, owing to the carbonitrides formed during nitrogen ion implantation. Table5. Electrochemical parameters of uncoated and HAP coated type 316L SS for various potentials at constant time duration of 3 minutes. affects the stability of HAP coatings on type 316L SS. No changes in stoichiometric structure of HA P (figure 11) were detected after deposition and subsequent heat treatments. The coating weight and thickness of the coatings increased with the increase in applied potential and time of the coatings. The coatings obtained above a potential of 70V and 5 minutes resulted in thick porous deposits. Loss of thicker coatings from the metal surface was observed due to decohesion that occurs between the surface and the coating. Particle congestion at the cathode results in weakly bonded coatings. Higher electric field drives the particles towards the electrode and exerts pressure on the deposited layer. Figure 12. SIMS depth profile of HAP coated 316L SS.
