Bone cement

Components

Part Composition
Liquid Monomer - Methyl Methacrylate (MMA)
Accelerator – N-Dimethyltoludine
Inhibitor – Hydroquinone
Powder Polymer - Polymethyl Methacrylate (PMMA)
Initiator – Benzoyl Peroxide
Barium Sulphate
Dye – Chlorophyll
Antibiotics

Chemical Reaction

  • Polymerization occurs on mixing.
  • The reaction is exothermic (releases heat).

Phases of Cement Polymerization

Phase Features
Mixing Phase Performed homogeneously in a vacuum to reduce porosity.
Waiting (Dough) Phase From mixing until cement is no longer doughy. Viscosity increases. Too low viscosity allows blood to mix into the cement.
Working Phase Cement can be manipulated. This is the difference between doughy and setting times.
Hardening Phase Cement completely hardens. Temperature of stem, cement, and environment influences this.
Setting Time From mixing until the cement reaches half of its maximal heat.

Macroscopic Stages

  1. Sandy: Powder and liquid mix appear grainy and separate.
  2. Stringy: Cement sticks to gloves.
  3. Doughy: Cement no longer sticks to gloves, marking the start of its working time.
  4. Final: Cement becomes hard and hot.

Mechanical Properties of Cement

  1. Weak tensile strength.
  2. Moderate shear strength.
  3. Strong compressive strength.
  4. Young’s modulus between cortical and cancellous bone.
  5. Brittle nature.
  6. Highly notch-sensitive.
  7. Viscoelastic – shows creep and stress relaxation.
  • Reaches ultimate strength within 24 hours.
  • Functions as a grout, not an adhesive.
  • Depends on mechanical interlocking with bone.

Factors Affecting Cement Properties

  1. Atmospheric
    • Temperature & Humidity: Increased humidity and temperature reduce setting time.
  2. Mixing Technique
    • Vacuum or centrifuge mixing reduces porosity.
    • Reduced porosity increases mechanical strength.
  3. Implantation Technique
    • Pressurization increases interdigitation.
    • Retention of cancellous bone.
    • Blood or soft tissue interposition can interfere.
    • Voids or stress riser formation can lead to failure.
    • Sharp-edged implants and bone-implant contact can cause issues.
    • Centralization improves stability.
  4. Cement Composition
    • Barium for radiopacity.
    • Antibiotics may increase porosity.
    • Less than 2:1 ratio of powder to liquid (more liquid) reduces setting time.

Complications of Bone Cement Use

  1. Heat Generation
    • Thicker mantles or smaller surface areas increase heat generation.
    • Excessive heat can cause bone necrosis.
  2. Systemic Effects
    • Cement implantation can cause transient but severe hypotension.
    • Likely due to micro-embolization of fat and cement rather than chemical mediation.
  3. Cement Fracture
    • Occurs due to crack formation and propagation through the mantle.
    • Cement is highly notch-sensitive.

Commonly Used Cements

Cement Details
Palacos R Contains Zirconium Dioxide as its radio-opaque element. Antibiotic version: Palacos RG (Gentamicin).
Simplex P Contains Barium Sulphate as the radio-opaque agent. Antibiotic version: Simplex PT (Tobramycin).
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