Knee replacement surgery offers a long-term solution for joint pain. While most replacements function well for decades, infection remains a concern even years after the procedure. An infection involving the prosthetic joint is called a periprosthetic joint infection (PJI), and it is a leading cause of revision surgery. Though relatively uncommon, PJI can occur long after the initial recovery, necessitating an understanding of its causes, symptoms, and treatment.
The Definition of Late-Onset Prosthetic Joint Infection
A late-onset prosthetic joint infection occurs when the infection manifests more than 12 to 24 months following the initial joint replacement. This distinguishes it from infections that occur closer to the time of surgery. The primary mechanism driving late infections is the formation of a biofilm. Bacteria adhere to the implant components, creating a protective matrix of proteins and sugars. This biofilm shields the bacteria from the body’s immune system and prevents antibiotics from working effectively. This chronic, low-grade bacterial presence often makes late-onset PJI subtle and persistent.
Sources and Mechanisms of Bacterial Spread
Late-onset PJI is most frequently caused by hematogenous spread, where bacteria travel through the bloodstream from a distant site of infection to the knee implant. The prosthetic joint remains susceptible to colonization by circulating microorganisms, even years after implantation. This mechanism differs from early infection, which usually involves contamination during the initial surgery. Several common infections can serve as the source for this bacterial seeding:
- Skin and soft tissue infections, such as cellulitis or infected wounds, are frequent contributors. The organism Staphylococcus aureus is often associated with these infections and is a common culprit in hematogenous PJI. Prompt and effective treatment of any significant skin infection is important to mitigate this risk.
- The oral cavity is a recognized source, especially following dental procedures that cause temporary bacteremia (bacteria in the blood). Bacteria like viridans streptococci can enter the bloodstream during extractions or deep cleanings. Maintaining good oral hygiene and treating dental abscesses quickly helps reduce the possibility of spread.
- The urinary tract. Urinary tract infections (UTIs) caused by organisms like Escherichia coli can lead to PJI if the bacteria enter the bloodstream.
- The respiratory tract. Severe respiratory tract infections, such as pneumonia, can introduce high levels of bacteria into the circulation.
Recognizing and treating seemingly unrelated infections immediately is a primary preventative measure for the long-term health of the joint replacement.
Recognizing the Warning Signs
The clinical presentation of late-onset PJI is often insidious and less dramatic than an acute infection. The most prominent symptom is the onset of new or worsening joint pain in a knee that was previously functioning well. This pain is persistent and typically not relieved by rest or standard medication. Late-onset PJI usually does not involve a high fever or pus drainage, which distinguishes it from early infections.
Instead, patients may experience chronic symptoms such as joint stiffness, reduced range of motion, fatigue, or general malaise. The joint may feel warm to the touch or appear subtly swollen, but these signs are often mistaken for a flare-up of arthritis or mechanical loosening. A definitive sign of infection is the development of a sinus tract, which is a small opening in the skin that drains fluid and communicates directly with the infected joint. New joint pain combined with a history of a previous distant infection should prompt a specialist evaluation.
Diagnostic Procedures and Treatment Protocols
Diagnosing late-onset PJI requires laboratory tests and direct sampling of the joint fluid. Initial steps involve blood tests to check for elevated markers of inflammation, specifically the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). If these markers are elevated, the next step is joint aspiration, where a needle withdraws synovial fluid from the knee. This fluid is analyzed for white blood cell (WBC) count and differential, as a high count of polymorphonuclear cells indicates infection. The fluid is also sent for culture to identify the specific bacteria, which guides antibiotic therapy. The standard treatment protocol for late-onset PJI is the two-stage revision arthroplasty.
Stage One
The first stage involves the complete removal of the infected prosthetic components and thorough debridement of all infected tissue. An antibiotic-loaded cement spacer is then placed. This spacer is designed to release high concentrations of antibiotics directly into the joint space for an extended period, treating the local infection.
Stage Two
Following a course of intravenous and oral antibiotics, the patient is monitored until infection markers return to normal levels, typically over several weeks to months. The second stage involves removing the antibiotic spacer and implanting a new, sterile knee replacement. This procedure aims to restore function once the infection is fully eradicated.
A less common treatment, Debridement, Antibiotics, and Implant Retention (DAIR), is generally reserved for acute hematogenous infections that are caught very early and involve a well-fixed, stable implant.