Courtesy: Orthopaedic Principles ICL Kochi
Definition
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Bone cement is an acrylic material that provides fixation through mechanical interlocking rather than biological bonding.
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It functions as a grout or filler that occupies the space between the implant and bone.
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Its primary role is to provide immediate stability and fixation of orthopaedic implants.
Components of Bone Cement
Bone cement consists of two components:
Liquid Component
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Methyl methacrylate monomer
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Activator such as dimethyl para-toluidine
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Stabilizers and inhibitors
Powder Component
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Pre-polymerized polymer beads of poly(methyl methacrylate) or copolymers of methyl methacrylate
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Initiator: Benzoyl peroxide
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Radiopacifier: Zirconium dioxide or barium sulfate
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Antibiotics (optional): Gentamicin, clindamycin, or tobramycin
Mixing Ratios
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Powder-to-liquid mixing system is used in some commercial preparations
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Liquid-to-powder mixing system is used in other formulations
Properties of Bone Cement
Mechanical Properties
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Influences the mechanical behavior of the implanted prosthesis
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High compressive strength
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Low tensile strength
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Low bending modulus of elasticity
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Acts as a viscoelastic polymer
Long-Term Mechanical Behavior
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Subject to creep under sustained load
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Susceptible to fatigue failure
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Exhibits stress relaxation over time
Functions of Bone Cement
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Fixation of artificial joints
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Anchoring of implants to bone
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Transfer of load from prosthesis to bone
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Optimization of stress and strain distribution
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Local delivery of antibiotics
Polymerization Characteristics
Exothermic Reaction
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Polymerization is associated with heat generation.
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In vitro temperatures are higher than those encountered in vivo.
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Lower in vivo temperatures are due to:
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Thin cement mantles
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Blood circulation
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Heat dissipation to the prosthesis
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Heat dissipation to surrounding viable tissues
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Polymerization Shrinkage
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Volume shrinkage ranges from 3 to 5 percent.
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Vacuum mixing results in greater volume shrinkage compared with hand mixing.
Residual Monomer
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Residual monomer may lead to cement implantation syndrome.
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This may manifest as sudden hypotension during implantation.
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Monomer can be detected in the bloodstream for several days.
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It is metabolized through the Krebs cycle.
Viscosity of Bone Cement
Definition
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Viscosity refers to resistance of a fluid to deformation under shear forces.
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It is commonly described as the thickness of a fluid.
Clinical Importance
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Determines handling characteristics and working time
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Influences penetration into cancellous bone
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Affects quality and longevity of fixation
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Adequate viscosity helps resist back-bleeding from cancellous bone
Requirements During Working Phase
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Viscosity must be low enough to allow delivery through a syringe
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Cement should penetrate trabecular bone interstices
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Must provide a comfortable and predictable working time
Types of Bone Cement Based on Viscosity
1. Low-Viscosity Cement
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Remains in a runny state for a longer duration
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Has a prolonged waiting phase
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True working time is relatively short
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Setting time may vary
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Commonly preferred by surgeons
2. High-Viscosity Cement
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Contains poly(methyl methacrylate) without methyl methacrylate–styrene copolymer
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Has no runny phase
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Immediately doughy after mixing
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Ready for hand application
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Requires close monitoring of working time
3. Medium-Viscosity Cement
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Combines properties of low and high viscosity cements
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Initially low viscosity during mixing
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Transitions to higher viscosity during application
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Allows easier and more homogeneous mixing
Preparation and Use of Bone Cement
Successful cementation depends on:
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Optimal mixing technique
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Proper bone preparation
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Appropriate cement delivery
Phases of Cement Mixing
There are 4 distinct phases:
1. Mixing Phase
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Begins with addition of liquid to powder
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Ends when the mixture becomes homogeneous
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Polymer beads swell and partially dissolve
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Viscosity increases but remains relatively low
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Cement consistency resembles toothpaste and is sticky
2. Waiting Phase
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Allows further swelling of polymer beads
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Polymerization progresses
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Viscosity increases and cement becomes doughy
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Cement is tested every 5 seconds with gloved fingers
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Waiting phase ends when cement is neither sticky nor stringy
3. Working Phase
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Begins when cement is no longer sticky
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Cement can be applied to bone or implant
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Polymerization and heat generation continue
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Cement expands thermally while undergoing volumetric shrinkage
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Very low viscosity during this phase may allow blood lamination
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Implant must be inserted before the end of this phase
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Ends when cement can no longer be kneaded smoothly
4. Setting Phase
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Cement hardens and polymerization stops
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Implant must already be in final position
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Temperature gradually returns to body temperature
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Cement continues volumetric and thermal shrinkage
Cement Readiness for Implantation
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Cement is ready when 2 cement balls adhere to each other when touched.
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If the balls do not stick, the cement has entered the curing stage.
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Implantation during curing can result in cement delamination from bone or prosthesis.
Timing of Cement Phases
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Dough time: Approximately 2 to 3 minutes after mixing begins
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Working time: Approximately 5 to 8 minutes
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Setting time: Approximately 8 to 10 minutes from start of mixing
Factors Affecting Cement Properties
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Heat of polymerization
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Type of cement formulation
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Powder-to-liquid ratio
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Each cement is supplied with a fixed ratio to ensure consistency
Bone Preparation
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Bone preparation is as important as cement preparation.
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Bone surfaces must be thoroughly cleaned and free of clots.
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Saline lavage is essential to create a clean bone–cement interface.
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Pulse lavage and brushes are recommended for optimal cleaning.
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In severely osteoporotic bone, pulse lavage may not be appropriate.
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Use of hydrogen peroxide–soaked sponges remains controversial.
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