Lecture 11/12 - Corrosion of Metal Implants

Question 1

Which of Eight Forms of Corrosive Attack Occur Most Frequently in Biomedical Implants?

Answer 1

• Major: uniform attack

- Minor: galvanic corrosion, crevice corrosion

Question 2

Which Implants Most Susceptible to Corrosion?

Answer 2

• Metals

- All exposed to oxidative degredation

Question 3

Modular (Hip Implant)

Answer 3

• 2 or 3 (double-modular) parts

- Can customize for patient (size and chemistry)

Question 4

Why Double-Modular?

Answer 4

• Larger height/weight
• Distribute mechanical load as evenly as possible
• Younger/active (lower amount of wear particles)

Question 5

Double-Modular Case Study: Materials

Answer 5

• Femoral head: zirconia (ceramic)
• Acetabular cup: zirconia (ceramic)
• Zirconia: high hardness, low wear, better chemistry match for patient

Question 6

Double-Modular Case Study: Material Failure

Answer 6

• Catastrophic failure at stem/neck interface

- Mechanical forces on stem could alter susceptibility of material to corrosion

Question 7

Double-Modular Case Study: Corrosion Mechanism

Answer 7

• Pitting, crevice, fretting (cyclic, small distance)
• Materials produce passivating oxide layer (protection) but micromotion scrapes off passivating layer (fretting)
• Mating surface between two portions of implant

Question 8

Double-Modular Case Study: Solution

Answer 8

• Perform surgery and recover fractured parts
• X-ray to examine placement of acetabular cup (migration with time)
• Visual/physical inspection of cup ( have to remove cup if signs of wear, balance with loss of more tissue in another surgery if have to remove cup, have to get rid of cup if want to use different material)
• Removal of stem which means loss of more tissue (may need bigger stem to fit into bone)
• Look now at single-modular design (weight/height makes double-modular not ideal)

Question 9

Corrosion in Implants

Answer 9

• Not always bad (use instead of drug)

- Corrosion rate: change in alloying addition comes with cost (add corrosion resistance, change mechanical properties)

Question 10

Copper Intrauterine Device

Answer 10

• Dissolution of copper
• Formation of cupric ions inactivate sperm (surface of sperm broken down which is needed to penetrate eggs) and inflame uterine lining causing temporary sterilitys
• No consensus on how much ion release needed to prevent pregnancy

Question 11

Copper IUD Case Study: Corrosion Rate

Answer 11

• Corrosion rate dependent on uterine enviroment (protein in fluid and pH of fluid)
• Huge variance in corrosion rates for different people (changes efficacy and possible side effects)

Question 12

Copper IUD Case Study: Side Effects

Answer 12

• Excessive bleeding and spotting
• Excessive pelvic pain
• Excessive uterine cramping
• Excessive inflammation of endometrium
• Caused by strong burst release because large surface area of pristine surface attacked all at once

Question 13

Copper IUD Case Study: Author Solutions

Answer 13

• Concerned about effects of inhibitors on surrounding environment
• Measure cupric ion release via electrochemical cell and performed cytotoxicity assay with Hela cells (immortalized)

Question 14

Copper IUD Case Study: Alternative Solutions

Answer 14

• Pre-burst or package in fluid (would alter shelf life)
• Porous polymer coating (areas where exposed to corrosion/not exposed, degradable)
• Conversion coating (mask portion of it)
• In vivo: soak in fluids and test copper ion release, toxicity assay (Hela then primary uterine cells)
• In vitro: smaller scale (rat, rabbit)