Knee Spacers: Purpose, Types, and Applications

What is a Knee Spacer?

A knee spacer is a medical device, typically an implant, designed to create or maintain space within the knee joint. Its primary purposes include alleviating pain and reducing friction, often in the context of osteoarthritis, managing infection in a previously replaced knee, or promoting cartilage healing through joint distraction.

Understanding the Knee Joint

The knee is a complex hinge joint, crucial for mobility, supporting the body's weight, and enabling movements like walking, running, and jumping. It is formed by the articulation of three bones: the femur (thigh bone), tibia (shin bone), and patella (kneecap). Between the femur and tibia are two crescent-shaped pieces of cartilage called menisci, which act as shock absorbers. All articulating surfaces are covered with smooth articular cartilage, which allows for frictionless movement. When this cartilage wears down, often due to osteoarthritis, bone-on-bone contact can occur, leading to pain, stiffness, and reduced function.

What Exactly is a Knee Spacer?

A knee spacer, in its broadest medical sense, refers to an implantable device or a technique used to establish or maintain a gap between the articulating surfaces within the knee joint. Unlike a total knee replacement (TKR) which replaces the entire joint, a spacer typically aims to address a specific issue by creating separation or filling a void, rather than completely resurfacing the joint. The design and material of a knee spacer can vary significantly depending on its intended purpose and duration of implantation.

Primary Purposes of Knee Spacers

Knee spacers are utilized for distinct clinical objectives, each addressing a specific pathology or stage of treatment:

• Pain Reduction and Friction Alleviation in Osteoarthritis: In cases of mild to moderate osteoarthritis, where cartilage erosion leads to bone-on-bone rubbing, certain types of spacers (e.g., unicompartmental spacers or joint distraction techniques) are designed to increase the joint space. By creating a physical gap, they can reduce direct bone-on-bone contact, alleviate pain, improve joint mechanics, and potentially delay the need for a total knee replacement.
• Infection Management (Antibiotic Spacers): One of the most common applications of a "knee spacer" is in the two-stage revision surgery for an infected total knee arthroplasty. If a prosthetic knee joint becomes infected, the original implants must be removed. An antibiotic-loaded cement spacer is then temporarily implanted into the joint. This spacer serves multiple critical functions:
  • It delivers high concentrations of antibiotics directly to the infected area, fighting the bacterial infection.
  • It maintains the joint space, preventing scarring and contracture, which would make the subsequent reimplantation of a new knee prosthesis more difficult.
  • It allows for some limited mobility during the interim period (typically 6-12 weeks) while the infection is being treated.
• Joint Distraction for Cartilage Repair: In some experimental or specialized treatments for severe cartilage defects or early osteoarthritis, an external fixation device is used to apply continuous traction to the knee, effectively "distracting" or pulling the joint surfaces apart. While not an internal implant, this technique creates and maintains a significant joint space, reducing load on the damaged cartilage and potentially stimulating natural healing or regeneration processes.

Types of Knee Spacers

The material and form of knee spacers vary based on their application:

• Antibiotic-Loaded Cement Spacers: These are typically custom-molded by the surgeon during the first stage of revision surgery using bone cement mixed with various antibiotics. They can be articulated (allowing some knee motion) or static (fixed in place).
• Metal or Polymer Implants: For long-term or permanent use in managing osteoarthritis, some "spacer" devices are small, unicompartmental implants made of materials like cobalt-chrome alloys or specialized polymers. These are designed to be placed in one compartment of the knee to create space and reduce friction.
• External Fixators: While not an internal implant, external fixators are used in joint distraction procedures to mechanically create and maintain the joint space.

Who is a Candidate?

Candidacy for a knee spacer depends entirely on the specific type and purpose:

• Antibiotic Spacers: Patients with an infected total knee replacement are primary candidates.
• Osteoarthritis Spacers/Distraction: Individuals with early to moderate unicompartmental osteoarthritis who wish to delay or avoid full knee replacement, or those who are not candidates for other surgical interventions, may be considered.
• Joint Distraction: Patients with focal cartilage defects or early-stage osteoarthritis, often younger individuals, seeking cartilage regeneration.

The Procedure: What to Expect

The implantation of a knee spacer is a surgical procedure performed under anesthesia.

• For Antibiotic Spacers: This is the first stage of a two-stage revision. The existing infected knee prosthesis is removed, the joint is thoroughly cleaned (debridement), and the antibiotic-loaded cement spacer is custom-molded and implanted.
• For Other Implants: The surgeon makes an incision, prepares the bone surfaces, and precisely positions the spacer device within the joint.
• For Joint Distraction: This involves inserting pins into the bones around the knee (femur and tibia) and attaching them to an external frame that slowly pulls the joint apart.

Post-procedure, patients will typically have a period of limited weight-bearing and will require pain management and physical therapy.

Benefits of Utilizing a Knee Spacer

The advantages of knee spacers are specific to their purpose:

• Effective Infection Control: Antibiotic spacers are highly effective in treating periprosthetic joint infections.
• Pain Relief: By reducing bone-on-bone contact or inflammation.
• Maintenance of Joint Space: Prevents contracture and stiffness, crucial for future definitive surgery.
• Delaying or Avoiding Major Surgery: For osteoarthritis, they can potentially postpone or eliminate the need for total knee replacement.
• Less Invasive (in some cases): Compared to full knee replacement, some spacer implants may involve less bone removal.
• Improved Function: Can restore some degree of mobility and quality of life during treatment or as a long-term solution.

Potential Risks and Considerations

As with any surgical procedure, there are potential risks:

• Infection: Despite antibiotic delivery, infection can persist or recur.
• Loosening or Fracture of the Spacer: The device may become unstable or break.
• Limited Mobility: Especially with static antibiotic spacers.
• Nerve or Vascular Damage: Rare, but possible.
• Allergic Reactions: To materials or antibiotics.
• Need for Further Surgery: Spacers are often temporary or may eventually fail, requiring definitive treatment.
• Hardware Complications: For external fixators, pin site infections or discomfort.

Recovery and Rehabilitation

Recovery involves a structured rehabilitation program tailored to the type of spacer and the individual's condition. This typically includes:

• Pain Management: To ensure comfort.
• Physical Therapy: To restore range of motion, strength, and function. This is critical for preventing stiffness and preparing for the next stage of treatment (if applicable).
• Weight-Bearing Restrictions: Often necessary to protect the joint and spacer.
• Monitoring: Regular follow-up appointments to assess healing, infection status, and spacer integrity.

Knee Spacers in the Context of Knee Health

It's important to understand that a knee spacer is rarely a standalone, permanent cure for advanced knee degeneration. Instead, it serves as a crucial component within a broader treatment strategy. For infection, it's a vital bridge to a new, healthy joint. For osteoarthritis, it's a tool to manage symptoms and potentially extend the life of the natural joint, or delay more invasive surgery. For joint distraction, it's part of a regenerative approach. The decision to use a knee spacer is made by an orthopedic surgeon, considering the patient's specific condition, goals, and overall health.

Conclusion

A knee spacer is a versatile and valuable orthopedic tool, specifically designed to create or maintain space within the knee joint. Whether it's an antibiotic-loaded implant fighting infection, a device reducing friction in an osteoarthritic knee, or a method promoting cartilage healing, its application is precise and purposeful. While not a universal solution, understanding the role of knee spacers is essential for grasping the comprehensive strategies employed in modern knee health management and rehabilitation.