Knee Replacement vs. Prosthetic Knee: Understanding the Key Differences

What is the difference between a prosthetic knee and a knee replacement?

While both terms relate to artificial components designed to restore knee function, a knee replacement (Total Knee Arthroplasty) is an internal surgical procedure that replaces damaged joint surfaces within the body, whereas a prosthetic knee refers to an external device used to replace a missing limb after an amputation.

Understanding Total Knee Arthroplasty (Knee Replacement)

A knee replacement, medically known as Total Knee Arthroplasty (TKA), is a major surgical procedure performed to alleviate severe knee pain and disability. It involves removing damaged bone and cartilage from the thighbone (femur), shinbone (tibia), and sometimes the kneecap (patella), and replacing them with artificial components.

• Purpose: The primary goal of a knee replacement is to restore the knee's natural gliding motion, reduce pain, and improve mobility and quality of life for individuals suffering from conditions like osteoarthritis, rheumatoid arthritis, or post-traumatic arthritis.
• Components: The artificial components used in a knee replacement are themselves called implants or internal prostheses. These typically consist of:
  • A metal component (often cobalt-chromium or titanium alloy) that resurfaces the end of the femur.
  • A metal component that resurfaces the top of the tibia.
  • A high-grade plastic (polyethylene) spacer inserted between the metal components to create a smooth gliding surface.
  • Sometimes, a plastic button is placed on the back of the kneecap.
• Nature: It is an internal surgical intervention, meaning the artificial joint is implanted inside the body, becoming an integral part of the skeletal system.

Understanding a Prosthetic Knee (External Prosthesis)

A prosthetic knee, in common usage, refers to a component of an external prosthetic limb designed for individuals who have undergone an above-knee (transfemoral) amputation. This device replaces the missing lower limb, including the knee joint, lower leg, and foot.

• Purpose: The primary goal of an external prosthetic knee is to enable an individual with limb loss to stand, walk, and engage in various daily activities by providing an artificial substitute for the lost limb.
• Components: A full lower limb prosthesis typically consists of:
  • A socket: Custom-fitted to the residual limb, providing the interface and support for the entire prosthesis.
  • A pylon (or shank): Connects the socket to the prosthetic foot, often made of lightweight materials like carbon fiber or aluminum.
  • A prosthetic knee joint: An articulated mechanism that mimics the natural knee's ability to bend and extend, allowing for walking, sitting, and other movements. These can range from simple hinge joints to highly sophisticated micro-processor controlled units.
  • A prosthetic foot: Designed to mimic the natural foot's ability to absorb shock and provide propulsion.
• Nature: It is an external medical device that is worn by the individual, rather than surgically implanted.

The Core Distinction: Internal vs. External

The fundamental difference lies in their location and function:

• Knee Replacement (Total Knee Arthroplasty): An internal surgical implant that replaces the diseased or damaged surfaces of a natural knee joint within the body. The individual still possesses their biological leg.
• Prosthetic Knee (External Prosthesis): An external artificial device that replaces a missing lower limb, including the knee joint, due to amputation.

Materials and Design Considerations

The distinct applications of these devices necessitate different material choices and design philosophies.

• Knee Replacement Implants:

  • Materials: Primarily medical-grade metals (cobalt-chromium, titanium alloys) and ultra-high molecular weight polyethylene (UHMWPE). These materials are chosen for their biocompatibility, durability, and low friction properties, designed to withstand the body's internal environment and continuous weight-bearing.
  • Design: Engineered for smooth articulation, stability, and long-term integration with bone (often through cementing or bone ingrowth surfaces). The design closely mimics the anatomical shape and function of a natural knee joint.

• External Prosthetic Knee Joints:

  • Materials: Often incorporate lightweight yet strong materials such as carbon fiber, aluminum alloys, and various plastics. Microprocessor-controlled knees may also contain complex electronics and hydraulic systems.
  • Design: Focuses on optimizing gait mechanics, energy efficiency, weight distribution, and user comfort. Designs vary widely based on the user's activity level, ranging from basic single-axis hinges to multi-axis, hydraulic, or pneumatic systems offering varying levels of stability, swing control, and energy return.

Functional Implications and Rehabilitation

Rehabilitation for both conditions is crucial but targets different functional outcomes.

• Knee Replacement Rehabilitation:

  • Focus: Regaining range of motion, restoring muscle strength around the new joint, improving balance, and normalizing gait patterns.
  • Goal: To enable the individual to return to activities with a pain-free, stable, and mobile knee joint that functions as closely as possible to a healthy natural knee.

• External Prosthetic Knee Rehabilitation:

  • Focus: Learning to don and doff the prosthesis, achieving proper socket fit, mastering balance and coordination with an artificial limb, developing efficient gait mechanics, and adapting to the unique biomechanics of the prosthesis.
  • Goal: To maximize functional independence, mobility, and quality of life by effectively using the artificial limb to navigate daily environments. This often involves significant energy expenditure compared to walking with a biological limb.

Key Takeaways

• A knee replacement is a surgical procedure that implants artificial components inside your existing leg to resurface a diseased knee joint.
• A prosthetic knee is a component of an external artificial limb worn by someone who has had an amputation, replacing the entire lower leg.
• One is an internal surgical implant for joint restoration; the other is an external device for limb replacement.

Conclusion

Understanding the precise terminology around medical devices and procedures is vital for patients, caregivers, and fitness professionals alike. While both knee replacements and external prosthetic knees aim to restore mobility and improve quality of life, they address fundamentally different medical conditions and involve distinct approaches to rehabilitation and long-term management. Clarifying these terms helps ensure effective communication and appropriate expectations for recovery and functional outcomes.