Bioceramics

Metallic Implant Materials: 

Metallic Implant Materials 생체재료로서 금속 Ni-Cr alloy 이나 Co-Cr alloy 등 내식, 내열 합금에 대한 연구로 Stainless steel 이 개발된 이후 본격적으로 이용 Vanadium Steel : 최초의 금속생체재료로 개발 ; bone fracture plates(Sherman plates), screws Most Metals (Fe, Cr, Co, Ni, Ti, Ta, Mo, W) - can be tolerated by the body in minute amounts - Naturally occurring metallic elements are essential ex.) red blood cell functions (Fe), synthesis of a vitamin B12 (Co) - cannot be tolerated in large amounts in the body

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Main application : 치과용 치관, 의치상용, ; 골절부분의 고정용, 인공골 인공관절 등의 보철용 생체 내에 매입된 금속재료의 상태 - 고농도 염소이온이 존재하는 가혹한 부식환경 - 인공고관절 등과 같이 하반신에 사용할 경우, 체중의 수배에 이르는 반복적이고 지속적인 하중 Effects of corrosion - 금속원소나 부식 생성물 (corrosion products)이 생체 내에 용출 (loss of material) - 부식된 곳이 파괴의 원인으로 작용하거나 metal implant를 약화시킴 Biocompatibility of the implant metals - 생체 적합성 등의 생물학적으로 생체에 무해할 것 - 재료학적으로 뛰어난 내식성과 충분한 기계적 강도를 가질 것

몇 가지 생체금속재료의 기계적 특성: 

몇 가지 생체금속재료의 기계적 특성

탄성계수: 

탄성계수

ASTM규격에서 정하고 있는 생체금속재료의 화학조성: 

ASTM규격에서 정하고 있는 생체금속재료의 화학조성

Stainless steel: 

Stainless steel The first stainless steel used for implant - 18Cr-8Ni (type 302 in modern classification) - stronger than the vanadium steel - more resistant to corrosion 18-8sMo stainless steel (316 stainless steel) - contains molybdenum - improve the corrosion resistance in salt water 316L - carbon content was reduced from 0.08 wt% to 0.03 wt% - better corrosion resistance in chloride solution

Types and Compositions of Stainless Steels: 

Types and Compositions of Stainless Steels Chromium(Cr) : major component of corrosion-resistant stainless steel - minimum effective concentration of chromium : 11 wt% - chromium is a reactive element ; but can be passivated to give an excellent corrosion resistance Austenitic stainless steels (types 316 and 316L) - most widely used for implants ; ASTM recommends type 316L rather then 316 for implant - not hardenable by heat treatment - hardenable by cold-working - nonmagnetic - possesses better corrosion resistance

Composition of 316 and 316L stainless steels: 

Composition of 316 and 316L stainless steels

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Molybdenum : enhances resistance to pitting corrosion in salt water Nickel : serves to stabilize the austenitic phase at room temperature : enhance corrosion resistance - austenitic phase stability can be influenced by both the Ni and Cr contents (Fig) The effect of Ni and Cr contents on the austenitic phase of stainless steels containing 0.1 wt% C

Properties of Stainless Steel: 

Properties of Stainless Steel Mechanical Properties of Stainless Steel Surgical Implants

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Stainless steel can be obtain a wide range of properties depending on the heat treatment - 316L may corrode inside the body under highly stressed and oxygen-depleted region ; suitable to use in temporary devices (ex. Fracture plates, screws, hip nails) Characteristics of austenitic stainless steel - work-harden very rapidly - need intermediate heat treatments - heat treatment ; formation of chromium carbide (CCr4) in the grain boundaries - chromium carbide may cause corrosion - therefore austenitic stainless steel implants are not usually welded

Co-Based Alloys: 

Co-Based Alloys CoCrMo alloy ; cast products - castable CoCrMo alloy has been in use for many decades in dentistry and recently in making artificial joints CoNiCrMo alloy ; wrought by (hot) forging - wrought CoNiCrMo alloy is used for making the stems of prostheses for heavily loaded joints (knee and hip) 2 types of cobalt-chromium alloys

Types and Compositions of Co-Based Alloys: 

Types and Compositions of Co-Based Alloys ASTM lists four types of Co-based alloys for surgical implant applications: - cast CoCrMo alloy (F76) - wrought CoCrWNi alloy (F90) - wrought CoNiCrMo alloy (F562) - wrought CoNiCrMoWFe alloy (F563) Only two of the four alloys are used extensively in implant fabrications - castable CoCrMo - wrought CoNiCrMo alloy

Chemical compositions of Co-based alloys: 

Chemical compositions of Co-based alloys

Properties of Co-Based Alloys: 

Properties of Co-Based Alloys Form a solid solution - up to 65 wt% Co and the remainder is Cr - molybdenum is added to produce finer grains : results in higher strengths after casting or forging Most promising wrought Co-based alloys : CoNiCrMo alloy - called MP35N (Standard Pressed Steel Co.) - contains 35 wt% Co and Ni each - has a high degree of corrosion resistance to seawater (containing chloride ions) under stress - cold-working can increase the strength of the alloy considerably (Fig) - considerable difficulty of cold-working for large devices (ex.) hip joint stems - only hot-forging can be used to fabricate an implant with the alloy

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Relationship between UTS and amount of cold working

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Abrasive wear properties - wrought CoNiCrMo alloy are similar to cast CoCrMo alloy - about 0.14 mm/year in joint simulation test Wrought CoNiCrMo alloy - not recommended for the bearing surfaces of a joint prosthesis ; because of its poor frictional properties with itself or other materials - superior fatigue strength and ultimate ultimate tensile strength : very suitable for long service life without fracture or stress fatigue (ex.) stems of the hip joint prosthesis - easy to remove when implant has to be replaced with another one ; If implants are failed into pieces, it is quite difficult to remove the pieces embedded deep in the femoral medullary canal - revision arthroplasty : usually inferior to the original function owing to poorer fixation of the implant

Mechanical properties of Co-based alloys: 

Mechanical properties of Co-based alloys the increased strength is accompanied by decreased ductility both the cast and wrought alloys have excellent corrosion resistance The modulus of elasticity for the cobalt-based alloys does not change with the changes in their ultimate tensile strength The moduli range from 220 to 234 GPa, which are higher than other materials such as stainless steels (200 GPa) - implies different load transfer modes to the bone ; although the effect of modulus is not yet established clearly

Rate of nickel release from the CoNiCrMo alloy and  316L stainless steel: 

Rate of nickel release from the CoNiCrMo alloy and 316L stainless steel - experimental determination in 37°C Ringer's solution - cobalt alloy has more initial release of nickel ions - rate of release was about the same (3×10-10 g/cm3/day) for both alloys - nickel content of the CoNiCrMo alloy is about three times that of 316L stainless steel