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Questions and Answers
What causes aseptic loosening?
Wearing damage
What is the initial stage of aseptic loosening?
Debris production
The process that occurs due to fatigue failure is characterized by ________, internal stress concentrations, and catastrophic failure.
defect
Which of the following factors help resist cracking under fatigue?
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How can polymers be strengthened or hardened?
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What is the main mechanism of biodegradation for polymers?
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What are titanium (Ti) alloys known for?
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Match the types of polyurethane with their characteristics:
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What is a disadvantage of cobalt (Co) alloys?
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Abiotic degradation is initiated by _______ mechanisms, which do not involve living organisms.
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Study Notes
Aseptic Loosening
- Aseptic loosening is the failure of a joint replacement implant without infection.
- It is caused by wear damage, resulting in the generation of tiny particles around the implant.
- The process involves four stages:
- Debris production: Metal or particulate debris is generated due to wear.
- Macrophage activated osteolysis: Macrophages respond to the debris, triggering inflammation.
- Prosthesis micromotion: The implant moves, increasing wear particle production.
- Particulate debris dissemination: Wear particles spread throughout the body.
Foreign Body Response
- The body's immune system recognizes implants as foreign entities and initiates a response:
- Protein adsorption: Plasma and interstitial fluid proteins bind to the implant surface.
- Exudation: Blood cells, proteins, and fluid leak from the vasculature to the surrounding tissue.
- Cell adhesion: Cells attach to the implant surface, influencing their response.
- Macrophage fusion: Macrophages engulf small biomaterial particles. Larger particles lead to the formation of foreign body giant cells (FBGCs).
- Fibrous capsule formation: Fibroblast differentiation into myofibroblasts and collagen production results in a fibrous capsule surrounding the implant.
Fatigue Failure
- Occurs through a progressive process:
- Defect formation: Initial crack or defect arises.
- Internal stress concentration: Stress concentrates around the defect.
- Permanent defect accumulation: Repeated loading cycles accumulate damage around the initial defect.
- Catastrophic failure: The defect grows to a critical size, causing the material to fail.
Fatigue Resistance
- Materials with the following properties resist fatigue cracking:
- High elastic modulus: Ability to deform elastically under stress.
- High yield strength: Resistance to permanent deformation.
- High fracture toughness: Resistance to crack propagation.
- Resistance to deformation: Ability to withstand repeated cycles of stress without significant deformation.
Polymer Strengthening and Hardening
- Methods to enhance polymer strength and hardness include:
- Hinder chain motion: Restricting molecular movement through crosslinking or bulky side groups.
- Increase molecular weight: Longer polymer chains lead to stronger entanglement.
- Add hard particles or fibers: Reinforcing the matrix with rigid fillers.
- Increase crosslinking: Forming chemical bonds between polymer chains for greater rigidity.
- Increase crystallinity: More ordered arrangement of polymer chains improves strength.
Biodegradation of Polymers
- Polymers are broken down into low molecular weight fragments that can be taken up and metabolized by cells, then excreted by the body.
Tuning Biodegradation Kinetics
- Factors influencing polymer biodegradation include:
- Chemical bonds in polymer chains: Weaker bonds like esters degrade faster.
- Steric interference: Bulky side groups hinder degradation.
- Crosslink density: Higher density slows down degradation.
- Crystallinity: More ordered chains resist degradation.
Titanium Alloys (Ti Alloys)
- Pros:
- High strength, particularly for Cr/Ni-allergic patients.
- Bone-bonding ability for permanent implants.
- Cons:
- Poor bending ductility due to HCP structure.
- Poor wear resistance, limiting use in high wear areas.
- Avoid use in shear-stressed regions due to poor bending strength.
- Avoid long-term temporary use due to bone-bonding tendency.
Polyurethane (PU)
- PU is a thermoplastic elastomer with reversible physical crosslinks or a thermoset elastomer with irreversible chemical crosslinks.
- Properties are modified by using chain extenders:
- Diol (HO-R-OH) or diamine (H2N-R-NH2) molecules connect polymer chains.
- The final PU's properties depend primarily on:
- The chemical nature of diol, diamine, and isocyanate building blocks.
- The relative proportions used during synthesis.
Cobalt Alloys (Co-alloys)
- Pros:
- High strength and fatigue resistance due to FCC-HCP Co structure.
- Excellent corrosion resistance from Cr, Mo, and Ni.
- Good stress corrosion cracking (SCC) and fatigue resistance.
- Cons:
- Expensive.
- Stress shielding due to high Young's modulus (220-230 GPa) compared to bone (10-30 Gpa).
- Metal toxicity: Ni, Cr, and Co can cause allergic reactions and inflammation.
Biodegradation Mechanisms
- Abiotic degradation: Hydrolysis or oxidation, driven by water or oxygen diffusion.
- Biotic degradation: Involves living organisms (cells).
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Description
This quiz covers the mechanisms of aseptic loosening in joint replacements and the body's foreign body response to implants. Explore the stages of debris production, macrophage involvement, and the immune system's reaction to foreign materials. Understand how these processes affect implant longevity and performance.
