Bone Cement in Total Knee Replacement (TKR): Composition, Role, and Considerations

What is the bone cement used in TKR?

Bone cement, primarily polymethyl methacrylate (PMMA), is a crucial biocompatible acrylic polymer used in Total Knee Replacement (TKR) to firmly anchor the prosthetic components to the patient's natural bone, providing immediate stability for weight-bearing and rehabilitation.

The Role of Bone Cement in Total Knee Replacement (TKR)

In Total Knee Replacement (TKR), damaged knee joint surfaces are removed and replaced with artificial components, known as prostheses. For these components to function effectively and withstand the significant forces of daily movement, they must be securely fixed to the surrounding bone. Bone cement serves as the primary adhesive and filler material in a "cemented" TKR, creating a strong, stable interface between the metal and plastic prosthetic components and the patient's natural bone. This immediate mechanical interlock is critical for enabling early mobilization and successful rehabilitation post-surgery.

Composition and Chemical Nature of Bone Cement

The bone cement used in TKR is predominantly a polymethyl methacrylate (PMMA)-based substance. It is typically supplied as two separate components that are mixed together during the surgical procedure:

• Powder Component: This consists mainly of pre-polymerized PMMA beads. Other vital ingredients in the powder include:
  • Initiator: Such as benzoyl peroxide, which triggers the polymerization reaction when mixed with the liquid.
  • Radio-opacifier: Materials like zirconium dioxide or barium sulfate are added to make the cement visible on X-rays, allowing surgeons to assess its placement and integrity post-operatively.
  • Pigments: Small amounts of pigments may be included for easier visual identification during surgery.
• Liquid Component: This is primarily a monomer of methyl methacrylate (MMA). It also contains:
  • Activator: Such as N,N-dimethyl-p-toluidine, which reacts with the initiator to start the polymerization.
  • Stabilizer: Like hydroquinone, which prevents premature polymerization of the liquid component during storage.
  • Antibiotics: Increasingly, bone cements are pre-mixed with antibiotics (e.g., gentamicin, clindamycin) to help reduce the risk of periprosthetic joint infection.

The Polymerization Process: How Bone Cement Sets

The process by which bone cement hardens is a chemical reaction known as polymerization:

1. Mixing: The sterile powder and liquid components are combined, typically in a vacuum mixer to reduce porosity.
2. Dough Phase: As the MMA monomer begins to dissolve the PMMA beads, the mixture transitions into a viscous, dough-like consistency. This "working time" is crucial for the surgeon to apply the cement and position the prosthetic components.
3. Setting (Exothermic Reaction): The activator in the liquid reacts with the initiator in the powder, causing the individual MMA monomer molecules to link together, forming long chains of PMMA. This polymerization is an exothermic reaction, meaning it generates heat.
4. Hardening: Within minutes, the cement hardens into a rigid, solid mass. This solid cement creates a mechanical interlock by flowing into the microscopic irregularities and pores of the prepared bone surface and the porous or roughened surface of the prosthetic components.

Application Techniques and Types of Cement

Surgeons use specific techniques to optimize cement application and achieve strong fixation:

• Bone Preparation: Before cement application, the bone surfaces are meticulously cleaned and prepared. This often involves techniques like pulsed lavage (pressurized saline irrigation) to remove blood, fat, and debris, which can impede cement penetration and bonding.
• Cement Viscosity: Bone cements are available in different viscosities:
  • High-viscosity cements are thicker and easier to handle, often applied directly from a cartridge. They tend to penetrate less deeply into bone pores.
  • Low-viscosity cements are thinner and flow more readily, allowing deeper penetration into the cancellous bone for potentially stronger mechanical interlock.
• Vacuum Mixing: This technique is often employed to mix the cement components under vacuum conditions. This significantly reduces the incorporation of air bubbles, leading to a denser, stronger cement mantle with fewer voids, thereby improving its mechanical properties and reducing the risk of failure.

Advantages of Cemented Fixation in TKR

The use of bone cement offers several significant advantages, making it a highly successful and widely utilized method for TKR:

• Immediate Stability: The primary benefit is the immediate, rigid fixation it provides. This allows patients to bear weight and begin physical therapy much sooner after surgery, accelerating rehabilitation and recovery.
• Reliable Fixation in Varied Bone Quality: Cemented fixation is particularly advantageous for older patients or those with compromised bone quality (e.g., osteoporosis) where biological ingrowth (as in cementless fixation) might be less predictable. It can effectively fill gaps and irregularities in weaker bone.
• Long-Term Track Record: PMMA bone cement has been used in orthopedic surgery for decades, accumulating extensive clinical data demonstrating its long-term efficacy and durability in TKR.
• Versatility: It can be used for all components of the knee prosthesis (femoral, tibial, and patellar).

Potential Disadvantages and Complications

While highly effective, bone cement is not without potential drawbacks and associated complications:

• Aseptic Loosening: Over time, the bond between the cement and the bone, or the cement and the prosthesis, can weaken and fail without infection. This "aseptic loosening" is a common reason for revision surgery. Factors contributing to loosening include mechanical stress, wear debris, and micromotion.
• Infection: Although antibiotics are often incorporated, the presence of foreign material like cement can still provide a nidus for bacterial colonization, leading to periprosthetic joint infection, a serious complication.
• Allergic Reactions: Rare instances of allergic reactions to the cement components have been reported.
• Bone Cement Implantation Syndrome (BCIS): A rare but potentially life-threatening complication that can occur during cement polymerization, particularly when large volumes are used in confined spaces. It involves the release of vasoactive substances and/or fat/marrow emboli into the bloodstream, leading to cardiopulmonary compromise (e.g., hypotension, hypoxia, arrhythmias).
• Thermal Necrosis: The exothermic reaction during polymerization generates heat. While usually well-tolerated, excessive heat can theoretically cause localized damage to surrounding bone cells, though this is generally not a significant clinical issue in TKR.

Cementless and Hybrid Fixation as Alternatives

While cemented fixation remains the gold standard for many TKR procedures, alternative fixation methods exist:

• Cementless (Press-Fit) Fixation: These prostheses have a porous surface that encourages the patient's bone to grow into and onto the implant, creating a biological bond over time. This method is often preferred for younger, more active patients with good bone quality.
• Hybrid Fixation: This approach combines both methods, typically cementing the tibial component (where stability is crucial) and using a cementless femoral component.

The choice of fixation method depends on various factors, including the patient's age, bone quality, activity level, and the surgeon's preference and experience.

Conclusion

Bone cement, primarily composed of polymethyl methacrylate (PMMA), is a cornerstone material in Total Knee Replacement surgery. Its ability to provide immediate, rigid fixation of prosthetic components to bone has revolutionized the treatment of severe knee arthritis, enabling millions of individuals to regain mobility, reduce pain, and improve their quality of life. While associated with certain potential complications, ongoing research and refinements in materials and surgical techniques continue to enhance the safety and longevity of cemented TKR, underscoring its enduring importance in orthopedic practice.