Artificial Knee Attachment: Components, Surgical Process, and Fixation Methods

How is an artificial knee attached?

An artificial knee, or knee prosthesis, is surgically attached by precisely removing damaged bone and cartilage from the femur (thigh bone), tibia (shin bone), and sometimes the patella (kneecap), and then fitting durable metal and plastic components to these prepared bone surfaces, typically secured with specialized bone cement or through a biological ingrowth process.

Understanding Total Knee Arthroplasty (TKA)

Total Knee Arthroplasty (TKA), commonly known as total knee replacement surgery, is a highly effective orthopedic procedure designed to alleviate chronic knee pain and restore function in individuals suffering from severe knee joint damage. This damage is most frequently caused by conditions such as osteoarthritis, rheumatoid arthritis, or post-traumatic arthritis. The procedure involves replacing the diseased or damaged parts of the knee joint with artificial components, collectively known as a prosthesis. From an exercise science perspective, the goal is to restore a pain-free, stable joint capable of supporting functional movement patterns essential for daily activities and, eventually, low-impact exercise.

Components of an Artificial Knee Joint

A typical artificial knee joint is comprised of three primary components, often with a fourth optional component:

• Femoral Component: This is a metal cap, usually made of cobalt-chromium or titanium alloy, designed to resurface the end of the thigh bone (femur). It mimics the natural shape of the femoral condyles and allows for smooth articulation with the tibial component.
• Tibial Component: Consisting of two parts, this component replaces the top surface of the shin bone (tibia). It has a flat metal tray (also titanium or cobalt-chromium) that is fixed to the tibia and a high-grade plastic (polyethylene) insert that snaps into or is cemented onto the metal tray. The polyethylene insert acts as the new cartilage, providing a smooth bearing surface for the femoral component and absorbing shock.
• Patellar Component (Optional): If the undersurface of the kneecap (patella) is also damaged, it may be resurfaced with a dome-shaped polyethylene button. This component ensures smooth gliding of the patella within the trochlear groove of the femoral component.
• Polyethylene Insert (Spacer): As mentioned, this is the crucial plastic component positioned between the femoral and tibial components. It is made from highly durable, ultra-high molecular weight polyethylene (UHMWPE), designed to minimize friction and withstand years of wear and tear.

The Surgical Attachment Process: A Step-by-Step Overview

The attachment of an artificial knee is a meticulous surgical procedure that involves precise bone preparation and component implantation:

• Incision and Joint Exposure: The surgeon makes an incision, typically down the front of the knee, to access the knee joint. The quadriceps muscle and patella are carefully moved aside to expose the damaged bone surfaces.
• Bone Resection: Using specialized instruments, cutting guides, and jigs, the surgeon precisely removes the damaged cartilage and a small amount of underlying bone from the end of the femur and the top of the tibia. The bone is shaped to perfectly match the contours of the prosthetic components. For the patella (if resurfaced), the diseased portion of its undersurface is removed.
• Trial Components: Before definitive implantation, trial components (temporary prosthetics) are often used. These allow the surgeon to assess the fit, alignment, stability, and range of motion of the knee joint. Adjustments can be made to bone cuts or component sizes at this stage to optimize the knee's mechanics.
• Implantation of Femoral Component: The metal femoral component is secured to the prepared end of the femur.
• Implantation of Tibial Component: The metal tray of the tibial component is attached to the prepared top surface of the tibia. Following this, the polyethylene insert is placed onto or cemented into the tibial tray.
• Patellar Resurfacing (if applicable): The polyethylene patellar button is attached to the prepared undersurface of the kneecap.
• Final Assessment: Once all components are in place, the surgeon flexes and extends the knee, checks for proper alignment, stability, and range of motion. Ligament balance is crucial to ensure the knee functions optimally.

Fixation Methods: Cemented vs. Cementless

The primary methods for attaching the prosthetic components to the bone are cemented fixation and cementless (press-fit) fixation:

• Cemented Fixation:
  • This is the most common method. A fast-curing bone cement, typically polymethyl methacrylate (PMMA), is used to bond the metal components directly to the prepared bone surfaces.
  • Advantages: Provides immediate stability and weight-bearing capability, often preferred for older patients or those with poorer bone quality.
  • Disadvantages: While highly durable, the cement can theoretically loosen over many years, potentially requiring revision surgery.
• Cementless (Press-Fit) Fixation:
  • These components have a porous surface coating (e.g., titanium beads, hydroxyapatite) that encourages the patient's own bone to grow into the pores over time. This process, called osseointegration, creates a strong biological bond between the bone and the implant.
  • Advantages: Creates a more natural, long-lasting biological bond, potentially reducing the risk of aseptic loosening associated with cement.
  • Disadvantages: Requires healthy, strong bone for successful ingrowth, and initial weight-bearing may be more restricted until osseointegration occurs. It may not be suitable for all patients.
• Hybrid Fixation:
  • Some surgeons may use a combination, such as a cemented tibial component and a cementless femoral component, based on patient factors and surgeon preference.

Post-Surgical Considerations and Rehabilitation

Once the artificial knee is attached, the surgical site is closed, and recovery begins. Immediate post-operative care focuses on pain management, infection prevention, and deep vein thrombosis (DVT) prophylaxis.

The Role of Exercise Science in Recovery

From an exercise science and kinesiology perspective, the successful integration and long-term function of an artificial knee heavily rely on a structured rehabilitation program.

• Early Mobilization: Initiating gentle range of motion exercises soon after surgery is critical to prevent stiffness and scar tissue formation, ensuring the newly attached components move freely.
• Strength Training: Progressive strengthening exercises for the quadriceps, hamstrings, and gluteal muscles are essential. Strong surrounding musculature provides dynamic stability to the artificial joint and improves functional capacity.
• Gait Training: Re-education of proper walking mechanics is vital. Physical therapists guide patients to re-establish a symmetrical and efficient gait pattern, reducing stress on the new joint and preventing compensatory movements.
• Proprioception and Balance: Exercises that challenge balance and proprioception (the body's sense of its position in space) help improve neuromuscular control around the knee, enhancing stability and reducing the risk of falls.
• Functional Movement Patterns: Rehabilitation progresses to incorporate exercises that mimic daily activities, such as climbing stairs, standing from a seated position, and navigating uneven terrain, ensuring the artificial knee supports a return to a high quality of life.

The precise attachment of an artificial knee, combined with dedicated rehabilitation, allows individuals to regain mobility, reduce pain, and return to an active lifestyle, leveraging the principles of biomechanics and exercise physiology for optimal outcomes.