Knee Spacers: Composition, Purpose, and Role in Joint Infection Treatment

What is a knee spacer made of?

A knee spacer, most commonly used in two-stage revision surgery for periprosthetic joint infection (PJI), is primarily composed of polymethyl methacrylate (PMMA), a type of bone cement, specifically designed to incorporate and deliver antibiotics directly to the infected joint space.

Understanding Knee Spacers in Orthopedics

In the realm of orthopedic surgery, a "knee spacer" typically refers to a temporary implant utilized as an intermediate step in the treatment of periprosthetic joint infection (PJI) following a total knee arthroplasty (TKA). PJI is a severe complication where bacteria colonize the prosthetic joint, necessitating a multi-stage approach for eradication.

The primary purposes of a knee spacer are:

• Antibiotic Delivery: To provide a high, localized concentration of antibiotics directly to the infected site, combating the bacterial biofilm.
• Joint Space Maintenance: To preserve the joint space and prevent contracture of surrounding soft tissues, which facilitates the subsequent definitive revision surgery.
• Pain Management: To offer some stability and reduce pain compared to a completely disarticulated joint.
• Limited Mobility: Many modern spacers are articulating, allowing for some range of motion, which aids in rehabilitation and maintains muscle strength.

Types of Knee Spacers and Their Composition

The composition of a knee spacer is critical to its function, primarily revolving around the ability to elute antibiotics and provide mechanical support.

• Antibiotic-Loaded Cement Spacers (ALCS) This is the most common type of knee spacer.

  • Matrix Material: Polymethyl Methacrylate (PMMA) PMMA, commonly known as bone cement, forms the structural backbone of the spacer. It is a biocompatible acrylic polymer that, when mixed from its powder and liquid components, undergoes an exothermic polymerization reaction to form a hard, durable material. PMMA's porous structure allows for the sustained release of antibiotics.
  • Antibiotics Antibiotics are mixed directly into the PMMA powder prior to polymerization. The choice of antibiotics is crucial and depends on the specific bacteria identified (or suspected) in the infection. Common antibiotics include:
    • Vancomycin: Effective against Gram-positive bacteria, particularly Methicillin-resistant Staphylococcus aureus (MRSA).
    • Gentamicin and Tobramycin: Aminoglycosides effective against a broad spectrum of Gram-negative bacteria and some Gram-positive strains.
    • Other broad-spectrum antibiotics may be used based on culture sensitivities. These antibiotics are chosen for their heat stability (to withstand the exothermic reaction of PMMA polymerization) and their ability to elute effectively from the cement matrix over weeks to months.
  • Radiopacifiers Substances like barium sulfate are often incorporated into the PMMA to make the spacer visible on X-ray images, allowing surgeons to monitor its position and integrity.
  • Forms: ALCS can be static (non-articulating, essentially a block of cement filling the joint space) or articulating (designed to mimic the joint surfaces, allowing for some movement). Articulating spacers are often custom-molded intraoperatively or pre-fabricated.

• Temporary Articulating Spacers (Pre-fabricated) While still often involving PMMA with antibiotics, some pre-fabricated articulating spacers may utilize other biocompatible polymers, such as ultra-high molecular weight polyethylene (UHMWPE) for articulating surfaces, sometimes combined with a PMMA core or shell for antibiotic delivery. These are designed for more precise fit and improved articulation.

It is important to differentiate these temporary spacers from permanent implants used in total knee replacement, which are made of highly specialized alloys (e.g., cobalt-chromium, titanium) and medical-grade plastics (UHMWPE).

Key Material Properties and Their Importance

The selection of materials for knee spacers is driven by several critical properties:

• Biocompatibility: The materials must be non-toxic and not elicit an adverse immune response or inflammation in the body. PMMA has a long history of safe use in orthopedic surgery.
• Mechanical Strength: While temporary, the spacer must possess sufficient compressive and shear strength to maintain joint space, withstand partial weight-bearing, and resist fracture during the interim period.
• Drug Elution Kinetics: The ability of the PMMA matrix to release antibiotics effectively and consistently over an extended period is paramount for infection eradication. Factors like antibiotic concentration, porosity of the cement, and surface area influence elution.
• Ease of Fabrication: PMMA's ability to be mixed and molded intraoperatively allows surgeons to create custom-fit spacers tailored to the patient's anatomy and the specific requirements of the revision.
• Radiopacity: Essential for post-operative imaging to confirm placement and monitor the spacer's condition.

The Role of Materials in Patient Outcomes

The materials used in knee spacers play a direct role in the success of two-stage revision surgery for PJI. The effective local delivery of antibiotics from the PMMA matrix is crucial for sterilizing the joint and preventing recurrence of infection. Simultaneously, the mechanical integrity of the spacer, provided by the PMMA, ensures that soft tissue contractures are minimized, and the joint's anatomy is preserved, thereby simplifying the subsequent definitive implantation of a new total knee prosthesis. This meticulous approach, underpinned by material science, significantly improves the chances of a successful long-term outcome for patients suffering from periprosthetic joint infection.

Conclusion

Knee spacers are vital temporary orthopedic devices, predominantly crafted from polymethyl methacrylate (PMMA) bone cement, imbued with potent antibiotics. This unique material composition allows them to serve a dual purpose: providing mechanical support to maintain joint space and delivering a targeted, high-concentration dose of antibiotics to combat persistent infections. The careful selection and engineering of these materials are foundational to the successful management and eventual resolution of complex periprosthetic joint infections, highlighting the intricate interplay between material science and advanced orthopedic care.