Knee Replacement Surgery: Understanding Bone Cement, Fixation, and Types

What kind of glue is used in knee replacement surgery?

The "glue" used in knee replacement surgery is specifically polymethyl methacrylate (PMMA) bone cement, a biocompatible acrylic polymer that provides immediate mechanical fixation by interlocking with the bone and prosthetic components.

Understanding Bone Cement in Orthopedic Surgery

When discussing the "glue" in knee replacement surgery, it's crucial to clarify that it's not an adhesive in the traditional sense, like an epoxy or superglue. Instead, it's a specialized material known as bone cement, predominantly polymethyl methacrylate (PMMA). Its primary function is to provide immediate and stable mechanical fixation between the prosthetic components (such as the femoral, tibial, and patellar implants) and the patient's existing bone structure. This fixation allows for immediate weight-bearing and facilitates early rehabilitation, which are critical for successful recovery.

The Composition of Polymethyl Methacrylate (PMMA) Bone Cement

PMMA bone cement is a sophisticated biomaterial that undergoes a chemical reaction when its two main components are mixed:

• Powder Component: This typically consists of:
  • Polymethyl methacrylate (PMMA) beads: The pre-polymerized form of the material.
  • A co-polymer: Often styrene or methyl acrylate, which helps optimize the mechanical properties of the final cement.
  • An initiator: Such as benzoyl peroxide, which triggers the polymerization process when mixed with the liquid.
  • An opacifier: Commonly zirconium dioxide or barium sulfate, added to make the cement visible on X-rays, allowing surgeons to assess its distribution and integrity post-operatively.
• Liquid Component: This typically consists of:
  • Methyl methacrylate (MMA) monomer: The raw liquid form that polymerizes to form PMMA.
  • A stabilizer: Such as hydroquinone, which prevents premature polymerization of the monomer during storage.
  • An activator: Such as N,N-dimethyl-p-toluidine, which reacts with the initiator in the powder to start the polymerization process.

When the powder and liquid are mixed, an exothermic (heat-producing) polymerization reaction occurs, transforming the liquid monomer into a solid polymer, creating the hardened bone cement.

How Bone Cement Achieves Fixation

Unlike traditional adhesives that rely on chemical bonding, PMMA bone cement achieves its fixation primarily through a mechanical interlock. Here's how it works:

• Preparation: Before cement application, the bone surfaces are meticulously prepared by cleaning and drying. This often involves pulsatile lavage (a high-pressure saline wash) to remove marrow and blood, creating a clean, porous bone bed.
• Application: The mixed cement, in a dough-like consistency, is applied to the prepared bone surfaces and/or the back of the prosthetic components.
• Pressurization: The surgeon then presses the implant firmly into the bone, creating pressure that forces the viscous cement into the microscopic pores and irregularities of the bone surface (trabecular bone) and into any roughened or porous surfaces on the implant itself.
• Hardening: As the cement hardens, it molds precisely to these intricate contours, effectively creating a tight, stable mechanical interlock between the bone and the implant. It's analogous to pouring concrete into a mold or using grout to fill gaps.

Advantages of Cemented Fixation in Knee Replacement

The widespread use of PMMA bone cement in knee replacement surgery is due to several significant advantages:

• Immediate Stability: One of the most critical benefits is that it provides immediate, rigid fixation, allowing patients to bear weight on the operated leg almost immediately after surgery. This is crucial for early mobilization and rehabilitation.
• Reliability and Proven Track Record: Cemented fixation has been used for decades and has an excellent long-term success rate, particularly in older patients.
• Versatility: It is highly effective across a wide range of bone qualities, including osteoporotic (weakened) bone, where bone ingrowth for cementless implants might be compromised.
• Reduced Pain: The immediate stability can help reduce micromotion between the implant and bone, which can be a source of post-operative pain.

Potential Disadvantages and Considerations

While highly effective, PMMA bone cement does have potential considerations and disadvantages:

• Cement Mantle Integrity: Over time, the cement mantle (the layer of cement) can degrade, crack, or debond from the bone or implant dueishing to mechanical stresses, which can lead to implant loosening.
• Thermal Necrosis: The exothermic reaction during polymerization generates heat. While usually controlled, excessive heat can potentially cause localized damage (necrosis) to surrounding bone tissue, though this is rare with modern techniques.
• Allergic Reactions: Although extremely rare, some patients may experience an allergic reaction to components of the cement.
• Cement Embolism Syndrome (rare): During cement application and pressurization, there's a very small risk of bone marrow contents or cement particles entering the bloodstream, potentially leading to complications, though this is extremely uncommon.
• Revision Surgery Challenges: If a revision surgery is needed years down the line, removing hardened bone cement can be challenging, potentially leading to more bone loss compared to removing cementless implants.

Cemented, Cementless, and Hybrid Fixation Strategies

While PMMA bone cement is a cornerstone of knee replacement, it's important to understand that it's not the only fixation method. Surgeons consider various factors, including the patient's age, bone quality, activity level, and their own surgical preference, when choosing a fixation strategy:

• Cemented Fixation: All components (femoral, tibial, patellar) are secured with bone cement. This is the most common method, especially for older patients or those with poorer bone quality.
• Cementless (Biologic/Press-Fit) Fixation: These implants have a porous or roughened surface designed to allow the patient's own bone to grow directly into the implant, creating a biological bond. This method requires good bone quality and is often preferred for younger, more active patients.
• Hybrid Fixation: This approach combines both methods. For example, the femoral component might be cementless (allowing for bone ingrowth), while the tibial component is cemented (providing immediate stability to the larger weight-bearing surface).

Optimizing Outcomes: The Role of Surgical Technique and Rehabilitation

Regardless of the fixation method chosen, the success of a knee replacement hinges on several factors:

• Surgical Precision: The surgeon's skill in preparing the bone, mixing the cement, and applying it correctly is paramount for a durable fixation. Proper cement thickness and complete filling of the bone-implant interface are crucial.
• Patient Compliance: Adherence to post-operative instructions, including weight-bearing restrictions (if any) and participation in a structured physical therapy program, is vital for long-term implant longevity and functional recovery.

Conclusion

The "glue" used in knee replacement surgery is not a simple adhesive but a highly engineered biomaterial: polymethyl methacrylate (PMMA) bone cement. It functions by creating a robust mechanical interlock between the prosthetic components and the patient's bone, providing immediate stability essential for successful outcomes. While it has a long and successful history, understanding its properties, advantages, and potential considerations helps appreciate the complexity and ingenuity behind modern orthopedic surgery.