Knee Replacement Attachment: Cemented, Cementless, and Hybrid Fixation

How is a knee replacement attached to the bone?

A knee replacement, also known as total knee arthroplasty (TKA), is primarily attached to the thigh bone (femur) and shin bone (tibia) using specialized bone cement (polymethyl methacrylate) or through a biological process called osseointegration, where the patient's own bone grows into a porous surface on the implant.

Understanding Total Knee Arthroplasty (TKA)

Total knee arthroplasty (TKA) is a surgical procedure that replaces the damaged surfaces of the knee joint with artificial components, or prostheses. This intervention is most commonly performed to alleviate pain and restore function in individuals suffering from severe osteoarthritis, rheumatoid arthritis, or post-traumatic arthritis. The knee joint comprises the distal end of the femur, the proximal end of the tibia, and the patella (kneecap). In a TKA, the damaged cartilage and a small amount of underlying bone are removed from these surfaces, and metal and plastic components are precisely fitted to recreate the joint.

The typical components of a knee replacement include:

• Femoral Component: A curved metal component that caps the end of the thigh bone.
• Tibial Component: A flat metal plate with a stem that is inserted into the shin bone.
• Polyethylene Insert (Spacer): A high-density plastic component that fits between the femoral and tibial components, mimicking the function of cartilage.
• Patellar Component: A plastic button that may be placed on the back of the kneecap.

The primary challenge in TKA is ensuring a durable and stable interface between these artificial components and the patient's living bone.

The Primary Methods of Prosthetic Fixation

The attachment of the prosthetic components to the bone is critical for the long-term success and stability of the knee replacement. There are two main methods of fixation, each with distinct mechanisms and considerations: cemented fixation and cementless (press-fit) fixation. A hybrid approach combining both methods is also common.

Cemented Fixation

Mechanism: This is the most widely used method. Polymethyl methacrylate (PMMA) bone cement, a fast-curing acrylic polymer, is used to bond the prosthetic components to the prepared bone surfaces. It acts as a grout, filling the microscopic irregularities on both the implant and bone surfaces, creating a mechanical interlock. It is not an adhesive in the traditional sense, but rather a filler that hardens to physically lock the components in place.

Process:

1. The bone surfaces are meticulously prepared by shaping them to precisely match the implant's contours.
2. The bone is cleaned and dried, and sometimes a pulsatile lavage is used to remove marrow and debris, enhancing cement penetration.
3. The bone cement is mixed and applied in a dough-like consistency to both the implant's undersurface and the prepared bone.
4. The implant is then pressed firmly into place, and excess cement is removed.
5. The cement cures rapidly, typically within 10-15 minutes, providing immediate mechanical stability.

Advantages:

• Immediate Stability: Provides robust initial fixation, allowing for early weight-bearing and rehabilitation.
• Versatility: Can be used effectively in a wide range of bone qualities, including osteoporotic bone.
• Established Track Record: Decades of clinical success and long-term data support its efficacy.

Disadvantages:

• Aseptic Loosening: Over time, the cement mantle can degrade or fracture, leading to loosening of the implant.
• Thermal Necrosis: The curing process of PMMA is exothermic, generating heat that can potentially cause localized bone cell damage, though this is usually minor.
• Fatigue Failure: The cement itself can undergo fatigue failure over many years of cyclic loading.

Cementless (Press-Fit) Fixation

Mechanism: Also known as uncemented or biological fixation, this method relies on the patient's own bone growing directly into a porous surface on the implant. The implant surface is typically coated with a porous material (e.g., titanium beads, fiber mesh) or a bioactive substance like hydroxyapatite, which mimics the mineral component of bone and encourages bone ingrowth (osseointegration).

Process:

1. The bone is prepared with extreme precision to create a "press-fit" where the implant fits very tightly into the bone.
2. The implant is then hammered or pressed into place, achieving significant initial mechanical stability due to the tight fit.
3. Over weeks to months, the patient's bone cells grow into the porous surface of the implant, creating a strong, biological bond.

Advantages:

• Long-Term Biological Fixation: Creates a durable, living bond between the bone and implant, potentially reducing the risk of aseptic loosening over very long periods.
• No Cement-Related Issues: Eliminates concerns related to cement fatigue, fragmentation, or thermal effects.
• Easier Revision Surgery: If a revision is needed years later, removing a cementless implant can sometimes be less complex than removing a cemented one.

Disadvantages:

• Requires Good Bone Quality: Optimal bone ingrowth requires healthy, dense bone, making it less suitable for patients with severe osteoporosis.
• Initial Stability: While designed for a tight fit, initial stability might theoretically be slightly less than cemented implants, though clinical outcomes are comparable.
• Slower Osseointegration: The biological bonding process takes time (weeks to months), which may influence early rehabilitation protocols.

Hybrid Fixation

Mechanism: This approach combines both cemented and cementless techniques within the same knee replacement. A common configuration is to cement the tibial component (shin bone) and cementless fix the femoral component (thigh bone).

Rationale: This strategy aims to leverage the benefits of each method. The tibial component often experiences higher shear stresses, making cemented fixation advantageous for immediate stability. The femoral component, with its larger surface area and different loading patterns, may benefit from the long-term biological bond of cementless fixation. The patellar component, if replaced, is almost always cemented due to its small size and specific loading.

Factors Influencing Fixation Choice

The decision between cemented, cementless, or hybrid fixation is complex and depends on several patient-specific and surgical factors:

• Patient Age and Activity Level: Younger, more active patients with good bone quality may be candidates for cementless fixation due to the potential for longer-lasting biological bonds. Older, less active patients, or those with compromised bone, often benefit from the immediate stability of cemented implants.
• Bone Quality: Osteoporosis significantly impacts bone strength and the ability for bone ingrowth. Cemented fixation is generally preferred in cases of poor bone quality.
• Surgeon Preference and Experience: Surgeons often develop expertise and preference for certain techniques and implant designs based on their training and clinical experience.
• Implant Design: Specific prosthetic designs are engineered for either cemented or cementless application, influencing the choice.
• Underlying Condition: Certain inflammatory conditions or previous surgeries may influence the optimal fixation method.

The Role of Bone Preparation and Implant Design

Regardless of the fixation method, precise surgical technique and advanced implant design are paramount for successful attachment:

• Surgical Precision: Accurate bone cuts are crucial to ensure the implant fits perfectly. Modern surgical tools, including computer navigation and robotic assistance, enhance this precision.
• Implant Surface Technology: For cementless implants, the porosity, pore size, and surface chemistry (e.g., hydroxyapatite coating) are engineered to optimize bone ingrowth. For cemented implants, the surface is designed to maximize the mechanical interlock with the cement.
• Component Design: Features like pegs, fins, and keels on the underside of the metallic components are designed to enhance initial stability and resist rotational forces, whether they are embedded in cement or rely on a press-fit.

Long-Term Durability and Potential Complications

The longevity of a knee replacement is significantly influenced by the integrity of the bone-implant interface. While both cemented and cementless fixation methods have excellent long-term success rates, the most common reason for revision surgery (re-doing the replacement) is aseptic loosening. This occurs when the bond between the implant and the bone fails without infection, often due to micromotion at the interface, wear debris, or fatigue of the fixation material (cement or bone ingrowth interface).

Other potential complications affecting the attachment include:

• Infection: Bacterial infection can compromise the bone-implant interface, often requiring removal of the prosthesis.
• Wear of Polyethylene Insert: While not directly related to bone attachment, excessive wear of the plastic spacer can lead to inflammatory reactions and secondary loosening.

Rehabilitation and Recovery

Post-operative rehabilitation plays a crucial role in the long-term success of knee replacement, irrespective of the fixation method. Early, controlled weight-bearing and range-of-motion exercises are initiated to promote healing, strengthen surrounding muscles, and maintain joint mobility. While cemented implants allow for immediate full weight-bearing due to their robust initial stability, cementless implants also permit early weight-bearing, as the initial press-fit provides sufficient stability for guarded activity while osseointegration occurs. Adherence to physical therapy protocols is essential to optimize functional outcomes and ensure the durability of the implant's attachment to the bone.