Aseptic Loosening and Foreign Body Response

Study Notes

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.

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.

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.

Polymers are broken down into low molecular weight fragments that can be taken up and metabolized by cells, then excreted by the body.

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.

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.

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.

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.