Cement Knee: Understanding Total Knee Replacement, Fixation, and Recovery

What is a Cement Knee?

A "cement knee" refers to a total knee replacement (TKR or TKA) where the prosthetic components are fixed to the thigh bone (femur) and shin bone (tibia) using a specialized bone cement, providing immediate and stable fixation.

Understanding Total Knee Arthroplasty (TKA)

Total Knee Arthroplasty (TKA), commonly known as total knee replacement, is a surgical procedure performed to alleviate pain and restore function in a severely damaged knee joint. This damage is most frequently caused by osteoarthritis, but can also result from rheumatoid arthritis, post-traumatic arthritis, or other degenerative conditions. During the procedure, the damaged cartilage and bone surfaces are removed and replaced with prosthetic components typically made of metal alloys, high-grade plastics, and sometimes ceramics.

The Role of Cement in Knee Replacement

When discussing a "cement knee," the focus is on the method of fixation used to secure these prosthetic components within the bone. Bone cement, specifically polymethyl methacrylate (PMMA), acts as a grout or filler, creating a strong mechanical interlock between the implant and the patient's bone.

• Mechanism of Action: PMMA is a self-curing acrylic polymer that starts as a powder and liquid, mixed by the surgeon during the operation. It sets rapidly, typically within 10-15 minutes, conforming to the intricate contours of both the implant and the prepared bone surface. This immediate hardening provides exceptional initial stability.
• Advantages of Cemented Fixation:
  • Immediate Stability: Provides strong, stable fixation right after surgery, allowing for early weight-bearing and rehabilitation.
  • Versatility: Effective across a wide range of bone qualities, including osteoporotic bone where uncemented options might be less reliable.
  • Proven Track Record: Cemented fixation has a long history of successful clinical outcomes, with decades of data supporting its durability and efficacy.
  • Reduced Risk of Micromotion: The immediate rigid fixation minimizes micromotion at the bone-implant interface, which can be beneficial for pain control and long-term implant survival.
• Disadvantages of Cemented Fixation:
  • Potential for Cement Failure: While durable, cement can, over time, degrade or fracture, leading to implant loosening.
  • Thermal Necrosis: The curing process of PMMA is exothermic (generates heat), which can theoretically cause localized thermal damage to surrounding bone tissue, though this is rare with modern techniques.
  • Biological Response: Some individuals may have a mild inflammatory response to cement particles, although significant adverse reactions are uncommon.
  • Revision Surgery Complexity: Removing cemented implants during revision surgery can be more challenging and may result in greater bone loss compared to uncemented implants.

Types of Knee Replacement Fixation

Beyond cemented fixation, there are other methods used in total knee arthroplasty:

• Uncemented (Press-Fit/Porous-Coated): These implants have a porous surface designed to allow the patient's bone to grow directly into the implant over time, forming a biological bond. They rely on an initial "press-fit" for stability. This method is often preferred for younger, more active patients with good bone quality.
• Hybrid Fixation: This approach combines both methods. For example, the femoral component (thigh bone) might be cemented, while the tibial component (shin bone) is uncemented, or vice versa. The choice depends on the specific design of the implant and the surgeon's preference based on patient factors.

The decision to use cemented, uncemented, or hybrid fixation is made by the orthopedic surgeon based on various factors, including the patient's age, bone quality, activity level, and the surgeon's experience and preference.

The Surgical Procedure

The cemented knee replacement procedure involves several key steps:

1. Preparation: The patient is positioned, and the knee area is sterilized and draped.
2. Incision: An incision is made over the front of the knee to expose the joint.
3. Bone Resection: Specialized instruments are used to precisely remove the damaged bone and cartilage from the ends of the femur and tibia, and sometimes the back of the kneecap (patella).
4. Implant Sizing and Placement: Trial components are used to ensure proper fit and alignment.
5. Cement Application: Once the final components are selected, bone cement is mixed and applied to the prepared bone surfaces and/or the implant's underside.
6. Implant Insertion: The prosthetic components are then firmly pressed into place, allowing the cement to cure and create a rigid bond.
7. Closure: The knee joint is thoroughly irrigated, and the incision is closed in layers.

Post-Operative Recovery and Rehabilitation

Recovery from a cemented knee replacement typically begins immediately after surgery.

• Early Mobilization: Due to the immediate stability provided by cement, patients are often encouraged to begin moving their knee and bearing weight on the operated leg within hours or days of surgery.
• Physical Therapy: A structured physical therapy program is crucial. It focuses on reducing swelling, restoring range of motion, strengthening the muscles around the knee, and improving balance and gait.
• Expected Timeline: While initial recovery of pain and mobility can be rapid, full recovery and return to most activities can take several months to a year. Adherence to the rehabilitation program is paramount for optimal outcomes.
• Long-Term Care: Regular follow-up appointments with the orthopedic surgeon are important to monitor the implant's condition and address any potential issues.

Potential Complications

While total knee replacement is a highly successful procedure, like any surgery, it carries potential risks:

• General Surgical Risks: Infection, blood clots (deep vein thrombosis - DVT), nerve or blood vessel damage, and anesthesia-related complications.
• Implant-Specific Complications:
  • Loosening: Over time, the bond between the implant and bone (or cement and bone) can weaken, leading to implant loosening, which may require revision surgery.
  • Wear: The plastic component between the metal parts can wear out, necessitating revision.
  • Instability: The knee may feel unstable or give way.
  • Stiffness: Scar tissue can sometimes limit the knee's range of motion.
  • Periprosthetic Fracture: A fracture around the implant can occur.
  • Cement-Related Issues: While rare, issues like an allergic reaction to cement components or localized bone necrosis due to the exothermic reaction can occur.

Who is a Candidate for a Cemented Knee Replacement?

Cemented knee replacement is a widely applicable and effective technique, often preferred for:

• Older Patients: Patients with lower bone density or osteoporosis, where biological ingrowth for uncemented implants might be compromised.
• Patients with Poorer Bone Quality: Conditions that affect bone strength or healing.
• Patients Requiring Immediate Weight-Bearing: The rapid stability is advantageous for early rehabilitation.

While cemented fixation has historically been the gold standard, uncemented and hybrid options are increasingly used, particularly in younger, active individuals with robust bone quality, as they avoid the potential long-term issues associated with cement fatigue.

The "Cement Knee" in Context: Longevity and Performance

A well-performed cemented knee replacement has an excellent prognosis. Modern knee implants, whether cemented or uncemented, are designed to last 15-20 years or more in a majority of patients.

• Factors Influencing Longevity: Patient activity level, weight, adherence to post-operative guidelines, and the quality of the surgical technique all play a role in how long a cemented knee replacement will last.
• Activity Levels: While a "cement knee" allows for a return to many daily activities and low-impact sports (e.g., walking, swimming, cycling, golf), high-impact activities (e.g., running, jumping, contact sports) are generally discouraged as they can accelerate wear and loosening of the prosthetic components.

In conclusion, a "cement knee" represents a robust and proven method of total knee replacement, offering immediate stability and excellent long-term outcomes for countless individuals seeking relief from debilitating knee pain.