A temporary knee spacer is a specialized medical device used during a two-stage revision total knee arthroplasty (TKA), the standard treatment for managing an infection in an existing knee replacement joint. The spacer serves as an interim solution to treat the deep-seated infection. This approach involves two separate surgeries: the first stage removes the infected components and places the spacer, and the second stage replaces the spacer with a new implant once the infection is cleared. The process addresses periprosthetic joint infection, a serious complication requiring intervention to eradicate colonized bacteria. The period the spacer remains in place is highly variable and depends on the patient’s biological response to the treatment.
What Is a Temporary Knee Spacer and Its Purpose
A temporary knee spacer is a molded structure, most commonly created from polymethylmethacrylate (PMMA) bone cement. This material is capable of delivering high concentrations of medication. The cement is mixed with specific, high-dose antibiotics tailored to fight the identified bacteria. This allows for a direct and concentrated delivery of the antibiotic payload right into the infected joint space, which is more effective than relying solely on systemic antibiotics.
The spacer performs a dual function, working both chemically and mechanically to prepare the knee for the final implant. Mechanically, the spacer maintains the joint space, preventing surrounding soft tissues, such as the joint capsule and ligaments, from contracting or scarring down. This preservation of the soft tissue envelope makes the second-stage surgery much easier to perform.
There are two primary types of spacers: static and articulating. A static spacer is a block of cement that locks the knee joint in a fixed position, preventing all movement between the thigh and shin bones. An articulating spacer is designed to mimic the shape of the original knee joint, allowing for some degree of motion during the interim period. The choice between these two types influences the patient’s mobility during the waiting period.
Factors Determining Spacer Duration
The duration of a temporary knee spacer is not determined by a fixed schedule but by the successful eradication of the infection. The waiting period is highly individualized. While many patients undergo the second-stage surgery in as little as 6 to 8 weeks, the range can extend to 12 weeks or longer in complex cases. In some studies, the mean time for spacer retention has been observed to be around 15 to 19 weeks.
The decision to remove the spacer and proceed with the definitive implant hinges on specific medical evidence confirming the infection has been cleared. Surgeons rely on the normalization of inflammatory markers in the patient’s blood, primarily C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR). These markers, which are elevated during an active infection, must return to normal or near-normal levels before the second surgery is considered safe.
The type of bacteria identified and the specific antibiotic regimen selected also influence the timeline. Infections caused by drug-resistant organisms or those requiring a longer course of intravenous antibiotics may prolong the waiting period. A joint fluid aspiration is often performed to collect a sample from the knee. The culture from this fluid must be negative for any bacterial growth to confirm the infection is gone. If the infection is not fully cleared, the spacer must remain in place, or a new spacer may be inserted.
Patient Expectations While the Spacer Is In Place
During the waiting period, patient expectations center on managing pain, restricted movement, and rigorous medical monitoring. Patients often experience a reduction in infection-related pain soon after the first-stage surgery, as the source of the infection has been removed and the antibiotic-loaded cement is working locally. Pain management is crucial to ensure comfort.
Mobility and physical therapy goals are different from those following a standard total knee replacement. Weight-bearing restrictions vary based on the type of spacer used. Static spacers typically require the patient to be non-weight-bearing. Articulating spacers, which allow for some movement, may permit partial or full weight-bearing, depending on the surgeon’s protocol and the spacer’s stability.
The range of motion will also be limited, particularly with a static spacer, which may not allow the knee to bend. Even with an articulating spacer, the goal is to maintain a limited, functional range of motion to prevent severe stiffness and muscle atrophy, rather than achieving a full recovery. Patients must attend ongoing clinical appointments for physical examinations and regular blood tests to monitor inflammatory markers, ensuring the infection remains suppressed and the body is ready for the second stage.