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What are the signs of infection after knee replacement surgery?
Executive summary
Common signs of infection after knee replacement include increasing or continuous pain, persistent or worsening swelling, redness and warmth at or around the incision, drainage from the wound, and systemic signs such as fever, chills or night sweats; many professional sources note infections occur in roughly 0.5–2% of primary knee replacements and are more common after revision surgery (risk estimates vary) [1] [2]. Early (acute) infections usually appear within weeks of surgery while late infections can arise months or years later; persistent wound drainage beyond a few days is singled out as a warning sign [1] [3].
1. What clinicians look for — the visible and the felt
Surgeons and orthopaedic organizations list the same core local signs: redness, persistent or increasing swelling, warmth of the knee, drainage or an opening in the incision, and continuous or worsening pain — especially pain that does not improve with rest or expected recovery timelines [4] [1] [5]. The American Association of Orthopaedic Surgeons emphasizes inspecting the skin carefully before surgery because breaks in the skin give bacteria access; similarly, post-op visible incision problems are a primary red flag [6].
2. Systemic symptoms that suggest a deeper problem
Beyond the knee itself, systemic symptoms such as fever, chills, sweats, fatigue and general malaise may indicate that an infection has spread or is more than a superficial wound issue. Sources note that septic arthritis and prosthetic joint infections can cause fever and severe joint discomfort, and that these systemic signs warrant urgent medical assessment [5] [1].
3. Timing matters — acute vs. late infections
Infections are commonly categorized by timing: acute infections typically occur within the first 4–6 weeks (or up to 90 days in some descriptions) and often stem from perioperative contamination; late infections can present months to years later and may arise from bacteria traveling through the bloodstream from another infection site (for example, a urinary tract infection) or from low-grade colonization of the implant [1] [3] [7]. Clinical approach and urgency differ by timing, so when symptoms start matters for treatment planning [1].
4. How common and how serious are these infections?
Multiple sources characterize prosthetic knee infections as uncommon but serious. Reported incidence ranges in the provided material — for primary knee replacement roughly 0.5–2% is cited, with revision procedures carrying higher rates (around 4–5% in one source); institutional figures often summarize it as about 1 in 100 [1] [2] [3]. When infections are deep or delayed, they frequently require further surgery and prolonged antibiotics; staged revision procedures are a common approach for late or entrenched infections [6] [1].
5. Why some symptoms are tricky — normal recovery vs. infection
Sources warn that some redness, warmth, mild swelling and itching can be normal for months after surgery; distinguishing expected post-operative healing from infection depends on persistence, worsening trends, and associated signs like fever or drainage [4] [1]. Prolonged wound drainage beyond about three days is singled out by at least one source as increasing the risk of infection and meriting evaluation [1].
6. Risk factors and routes of infection to watch for
Diabetes, obesity, smoking, prolonged surgery time, prior joint surgery, immunodeficiency and other chronic conditions increase infection risk; bacteria may enter at the time of surgery through the incision or later via the bloodstream from another infection (for example a UTI or dental source) [4] [3] [7]. Professional guidance recommends preoperative skin checks and avoiding breaks in skin integrity to reduce risk [6].
7. What to do if you suspect infection
All sources uniformly advise contacting your surgeon or primary care provider promptly for evaluation if you have increasing pain, spreading redness, significant swelling, drainage, fever or any systemic symptoms after knee replacement [4] [1] [6]. Diagnostic steps commonly include physical exam, wound assessment, blood tests and imaging; treatment ranges from antibiotics and local wound care to surgical washout or staged revision depending on severity and timing [6] [3].
Limitations and competing viewpoints: the available sources agree on the main signs and the seriousness of infections but give somewhat different numerical risk ranges (0.5–2% vs. “about 1 in 100” vs. higher revision rates), and they emphasize different alarm thresholds (for example, “drainage for more than 3 days” is noted specifically in one source) — readers should treat incidence numbers as approximate and rely on their surgeon’s local data and advice [1] [2] [3]. Available sources do not mention alternative unverified signs beyond those listed here.