Ceramics

Overview

Ceramics are biologic compounds made up of metallic and non-metallic elements.

Types of Ceramics Used in Orthopaedics

• Bioinert
  • Alumina
  • Zirconia
  • Oxinium
• Bioactive
  • Hydroxyapatite (HA)
  • Tricalcium Phosphate

Specific Ceramics

• Alumina:
  • Main ceramic used, with improved quality compared to the 1st generation.
• Zirconia:
  • Currently not favored due to high fracture rates and expense.
• Oxinium:
  • Oxidized zirconia.
• Hydroxyapatite (HA):
  • Bioactive and osteoconductive ceramic.
  • Provides bidirectional bone growth.
  • Speeds up the bonding process, aiding bone growth and bonding.

Manufacturing

• The strength of ceramics is dependent on the original grain size:
  • Smaller grain size results in stronger ceramic.
• Produced by pressing ceramic powder and water into a pre-fabricated cast.

Modern Processes in 4th Generation Ceramic Production

1. Sintering via Hot Isostatic Pressing (Hipping):
   • Binds individual grains more tightly.
   • Increases density, toughness, and strength.
2. Platelet-like Crystals:
   • Addition of an oxide creates platelet-shaped crystals within the alumina matrix.
   • These crystals help dissipate energy and deflect cracks.
3. Transformational Toughening:
   • Involves adding small amounts of zirconia to increase toughness and decrease fracture susceptibility.
   • Used in the production of Biolox Delta (4th generation ceramic).

Material Properties

Good Properties

1. Inert:
   • Particles are not biologically active.
2. Stiff:
   • High modulus of elasticity.
3. Hard:
   • The 3rd hardest material known to man.
4. Smooth:
   • Low surface roughness, beneficial for articulating surfaces.
5. Good Scratch Profile:
   • Properties not significantly affected by scratches, which do not result in heaped edges.
6. Wettable:
   • Aids joint lubrication, allowing fluid film lubrication.
7. Low Wear Characteristics:
   • Less abrasive and linear wear; not subject to creep.
8. Less Osteolysis:
   • Small, less biologically active particles are virtually inert.
9. Lower Infection Rates:
   • Supported by Swedish registry data for Total Hip Replacements (THR).
   • More wettable surfaces make it harder for bacteria to adhere.
10. Very Strong in Compression.

Bad Properties

1. Brittle:

   • Virtually no plastic deformation before failure, leading to fracture risks and sharp debris particles.
   • Less common with 4th generation ceramic (Biolox Delta), which reports a 0.04% fracture risk.

2. Weak in Tension.

3. More Expensive.

4. Lack of Long-term Data.

5. Squeaking:

   • Etiology not fully understood; reported risk of 1-3%, though less than in the past.
   • Component positioning, age, weight, and height are implicated.
   • Likely caused by stripe wear due to edge loading during high flexion activities.

Oxidised Zirconium (OXINIUM)

• A metallic alloy of zirconium with a ceramic surface.
• The alloy is oxidized in air, converting the surface into ceramic.
• The ceramic is integrated into the material, not just a coating.

Properties

• Hard, smooth, scratch-resistant, inert, and wettable.
• Suitable for individuals with metal allergies due to minimal nickel presence.
• In vitro studies show greatly reduced polyethylene wear.
• No long-term in vivo trials available yet, and it remains expensive.

Ceramic Fracture

• Generates numerous third bodies, causing accelerated wear of plastic and particulate debris.
• Typically, ceramic particles are small and inert; however, third body wear produces larger, more biologically active debris, accelerating osteolysis.

Revision Considerations

• Revision should include synovectomy to clear all ceramic debris.
• Damage to the Morse taper may necessitate revision to prevent catastrophic failure.
• Incidence of complications is significantly lower with 4th generation ceramics and modern manufacturing techniques:
  • Hot isostatic pressing.
  • Transformational strengthening.
  • Platelet-shaped crystal addition.
• Current quoted risk of fracture stands at 0.04%.