Knee Replacements: Understanding Cemented, Cementless, and Hybrid Attachment Methods

How are knee replacements attached?

Knee replacements are primarily attached to the existing bone through two main methods: cemented fixation, which uses a specialized bone cement to bond the prosthetic components, and cementless (or press-fit) fixation, which relies on the bone growing directly into a porous surface on the implant for biological integration.

Understanding Total Knee Arthroplasty (TKA)

Total knee arthroplasty, commonly known as total knee replacement, is a surgical procedure that resurfaces a damaged knee joint with artificial components. This intervention aims to alleviate pain and restore function in individuals suffering from severe arthritis or significant knee damage. For the artificial joint to function effectively and provide long-term stability, its components must be securely fixed to the patient's existing bone. The method of attachment is a critical determinant of the surgery's success and the implant's longevity.

The Components of a Knee Replacement

Before delving into attachment methods, it's essential to understand the typical components of a total knee replacement:

• Femoral Component: This metal (often cobalt-chromium or titanium alloy) component replaces the end of the thigh bone (femur). It is shaped to mimic the natural femoral condyles, allowing smooth articulation.
• Tibial Component: This component replaces the top of the shin bone (tibia). It typically consists of a metal tray that fits onto the resected tibia and a durable polyethylene (plastic) insert that serves as the new articular surface, allowing the femoral component to glide smoothly.
• Patellar Component: In some cases, the kneecap (patella) is also resurfaced with a polyethylene button that is attached to its posterior surface.

Each of these components requires a robust method of fixation to the underlying bone to ensure stability and proper biomechanical function.

Primary Attachment Methods

The two predominant methods for attaching knee replacement components are cemented and cementless fixation, with hybrid approaches also being utilized.

Cemented Fixation

Polymethyl methacrylate (PMMA) bone cement is the most widely used material for securing knee replacement implants. It functions not as an adhesive in the traditional sense, but as a filler that interdigitates with the microscopic irregularities of both the bone and the implant surface, creating a mechanical interlock.

• Mechanism: After careful preparation of the bone surfaces (removing damaged cartilage and bone, creating a precise fit), the bone cement is mixed into a dough-like consistency and applied to the implant's bone-facing surfaces and/or the prepared bone. The implant is then pressed into place, and the cement hardens within minutes, creating a strong, immediate bond.
• Advantages:
  • Immediate Stability: Provides excellent initial fixation, allowing for early weight-bearing and rehabilitation.
  • Versatility: Effective across a wide range of bone qualities, including osteoporotic bone, where cementless fixation might be less reliable.
  • Long-Standing Efficacy: Has a long and proven track record of clinical success.
• Disadvantages:
  • Cement Mantle Failure: The cement can degrade or fracture over time, potentially leading to loosening.
  • Thermal Necrosis: The polymerization process of the cement generates heat, which can cause localized bone cell death (necrosis) if not properly managed.
  • Difficulty in Revision: Removing cemented components during revision surgery can be more complex, potentially leading to greater bone loss.

Cementless (Press-Fit) Fixation

Also known as biologic fixation or porous-coated fixation, this method relies on the body's natural ability to grow bone into the implant's surface.

• Mechanism: Cementless components have a porous surface (e.g., beads, fibers, or a roughened texture) that encourages bone ingrowth. The implant is precisely shaped to achieve a "press-fit" into the prepared bone, providing initial mechanical stability. Over time, the patient's bone grows into the porous surface of the implant, creating a strong, biological bond.
• Advantages:
  • Long-Term Biological Fixation: Once bone ingrowth occurs, it provides a durable, living interface between the implant and the bone, potentially leading to longer implant survival by avoiding issues associated with cement fatigue.
  • Easier Revision: In some cases, cementless components may be easier to remove during revision surgery, preserving more bone stock.
  • Reduced Risk of Cement-Related Complications: Eliminates concerns related to cement mantle failure or thermal necrosis.
• Disadvantages:
  • Requires Good Bone Quality: Optimal bone ingrowth requires healthy, dense bone, making it less suitable for patients with severe osteoporosis.
  • Initial Stability Concerns: While a press-fit provides initial stability, full biological ingrowth takes weeks to months, meaning the implant is vulnerable to micromotion and potential early loosening before full integration.
  • Thigh Pain: Some patients report anterior thigh pain with cementless femoral components, though the exact cause is not fully understood.

Hybrid Fixation

Hybrid fixation combines elements of both cemented and cementless techniques. A common hybrid approach involves cementing the tibial component and patellar component (if resurfaced) while using a cementless femoral component.

• Rationale: This approach leverages the advantages of each method. The tibial component often benefits from the immediate stability of cement, especially given the complex load distribution on the tibia, while the femoral component can achieve long-term biological fixation.

Factors Influencing Choice of Attachment Method

The decision of which attachment method to use is made by the orthopedic surgeon and depends on several factors:

• Patient Age: Younger, more active patients with good bone quality are often candidates for cementless fixation due to the potential for longer-lasting biological fixation. Older patients or those with poorer bone quality may benefit more from the immediate stability of cemented implants.
• Bone Quality: The density and health of the patient's bone are crucial. Cementless implants require robust bone for successful ingrowth.
• Activity Level: Higher activity levels may favor biological fixation for its potential long-term durability.
• Surgeon Preference and Experience: Surgeons often develop expertise with specific techniques and implant designs.
• Implant Design: Some implant designs are specifically engineered for one type of fixation over another.

The Surgical Procedure and Attachment

Regardless of the chosen attachment method, the surgical procedure involves precise steps to prepare the bone:

1. Bone Resection: Damaged bone and cartilage are carefully removed from the ends of the femur and tibia, and sometimes the back of the patella, using specialized instruments and cutting guides to ensure accurate alignment.
2. Component Seating: The prosthetic components are then meticulously seated onto the prepared bone surfaces.
3. Fixation:
   • For cemented components, bone cement is applied to the implant and/or bone, and the component is pressed firmly into place until the cement sets.
   • For cementless components, the implant is press-fit into the bone, relying on the tight mechanical fit and subsequent biological ingrowth for long-term stability. Screws may also be used in some cementless designs for additional immediate stability.

Post-Surgical Considerations and Longevity

Following knee replacement surgery, regardless of the attachment method, rehabilitation is crucial for optimal recovery and function. The choice of attachment method can influence the initial weight-bearing protocols and the long-term longevity of the implant. While both cemented and cementless knee replacements have excellent long-term success rates, typically lasting 15-20 years or more, ongoing research continues to refine materials and techniques to improve outcomes further.

Conclusion

The secure attachment of artificial components to the patient's bone is fundamental to the success of a knee replacement. Whether through the immediate mechanical interlock of bone cement or the durable biological integration of bone into a porous surface, each method offers distinct advantages. The selection of the appropriate attachment technique is a highly individualized decision, carefully considered by the orthopedic surgeon to optimize outcomes based on the patient's unique anatomy, bone quality, and lifestyle.