The first stainless steel utilized for implant fabrication was the 18-8 (type 302), which is stronger and more resistant to corrosion than the vanadium steel. Vanadium steel is no longer used in implants since its corrosion resistance is inadequate in vivo. Later 18-8sMo stainless steel was introduced which contains a small percentage of molybdenum to improve the corrosion resistance in chloride solution (salt water). This alloy became known as type 316 stainless steel. In the 1950s the carbon content of 316 stainless steel was reduced from 0.08 to a maximum amount of 0.03% for better corrosion resistance to chloride solution and to minimize the sensitization, and hence became known as type 316L stainless steel. The minimum effective concentration of chromium is 11% to impart corrosion resistance in stainless steels. The chromium is a reactive element, but it and its alloys can be passivated by 30% nitric acid to give excellent corrosion resistance.
Types 316 and 316L, are most widely used for implant fabrication. These cannot be hardened by heat treatment but can be hardened by cold-working. This group of stainless steels is nonmagnetic and possesses better corrosion resistance than any others. The inclusion of molybdenum enhances resistance to pitting corrosion in salt water. The American Society for Testing and Materials (ASTM) recommends type 316L rather than 316 for implant fabrication. The specifications for 316L stainless steel are given in Table 1.1.
TABLE 1.1 Compositions of 316L Stainless Steel [American Society for Testing and Materials ASTM]
Element Composition (%)
Carbon
Manganese
Phosphorus
Sulfur
Silicon
Chromium
Nickel
Molybdenum 0.03 max
2.00 max
0.03 max
0.03 max
0.75 max
17.00–20.00
12.00–14.00
2.00–4.00
The nickel stabilizes the austenitic phase [?, face centered cubic crystal (fcc) structure] at room temperature and enhances corrosion resistance. The minimum amount of Ni for maintaining austenitic phase is approximately 10%. Table 1.2 gives the mechanical properties of 316L stainless steel. A wide range of properties exists depending on the heat treatment (annealing to obtain softer materials) or cold working (for greater strength and hardness).
The engineer must consequently be careful when selecting materials of this type.
Even the 316L stainless steels may corrode inside the body under certain circumstances in a highly stressed and oxygen depleted region, such as the contacts under the screws of the bone fracture plate. Thus, these stainless steels are suitable for use only in temporary implant devices such as fracture plates, screws, and hip nails. Surface modification methods such as anodization, passivation, and glow-discharge nitrogen implantation are widely used in order to improve corrosion resistance, wear resistance, and fatigue strength of 316L stainless steel.
CoCr Alloys:
There are two types of cobalt-chromium alloys: (1) the castable CoCrMo alloy and (2) the CoNiCrMo alloy, which is usually wrought by (hot) forging. The castable CoCrMo alloy has been used for many decades in dentistry and, relatively recently, in making artificial joints. The wrought CoNiCrMo alloy is relatively new, now used for making the stems of prostheses for heavily loaded joints such as the knee and hip. Table 1.3 shows, the compositions are quite different from each other.
TABLE 1.3 Chemical Compositions of CoCr Alloys [American Society for Testing and
Materials, F75–87, p.42; F562–84, p.150, 1992]
The two basic elements of the CoCr alloys form a solid solution of up to 65% Co. The molybd
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|