What are the early warning signs of periprosthetic joint infection after knee replacement?
Executive summary
Periprosthetic joint infection (PJI) after knee replacement most often first shows local signs — new or worsening joint pain, swelling, redness, warmth, effusion or persistent drainage — and may be accompanied by systemic features such as fever, chills or night sweats, especially in acute presentations [1] [2] [3]. Early recognition is critical because timely diagnosis increases the chance of retaining the prosthesis and guiding appropriate surgical and antimicrobial treatment [4] [5].
1. Local warning signs clinicians and patients commonly see first
The most consistent early indicators of PJI are localized symptoms around the operated knee: persistent or new-onset joint pain that is disproportionate to expected postoperative recovery; swelling or effusion; erythema (redness) and warmth of the skin over the joint; and any persistent wound drainage or a sinus tract communicating with the joint—each repeatedly described across reviews and clinical guidelines as cardinal local findings [3] [1] [6].
2. Systemic signs, timing and how acute versus chronic infections differ
Acute postoperative and hematogenous PJIs frequently present abruptly with swelling, erythema, warmth and tenderness and may show systemic toxicity—fever, chills or night sweats—whereas chronic infections can be subtler and present with pain, loosening or a draining sinus over time; distinguishing early (acute) from late (chronic or hematogenous) presentations matters for urgency and management [2] [7] [8].
3. When the picture is ambiguous: overlap with other causes and diagnostic pitfalls
Clinical presentation can be nonspecific and overlap with mechanical failure, aseptic loosening, metal reactions, deep vein thrombosis or normal postoperative inflammation, so isolated pain or stiffness is not diagnostic by itself; multiple lines of evidence—clinical, laboratory, imaging and synovial testing—are usually needed because no single test is perfect [7] [5].
4. Early tests clinicians use to confirm suspicion
When infection is suspected, initial lab screening typically includes serum CRP and ESR, followed by joint aspiration for synovial fluid white cell count, neutrophil percentage and culture; synovial leukocyte counts and neutrophil percentage have high reported sensitivity and specificity in TKA when using established cutoffs, and intraoperative tissue cultures and histology (multiple samples) remain central to definitive diagnosis [2] [3] [5] [9].
5. Who is at higher risk — context that changes the threshold for concern
Patient and surgical risk factors raise the pre-test probability of PJI and should lower the threshold for early investigation: obesity, diabetes, high ASA score, recent hematoma or wound dehiscence, prior intra‑articular injections or prior septic arthritis, prolonged operative time and use of non–antibiotic bone cement have all been associated with higher PJI rates in the literature [10] [11] [12] [13].
6. Practical takeaway: what early recognition enables and where uncertainty remains
Early vigilance for the constellation of persistent pain plus local inflammatory signs, rapid use of CRP/ESR and consideration of aspiration can markedly improve diagnostic yield and the chance of prosthesis retention, but clinicians must balance test timing (postoperative inflammation can elevate markers) and remember chronic PJIs can be subtle; the literature supports urgent evaluation of suspicious signs but also notes diagnostic complexity and the need for multidisciplinary assessment [4] [3] [5].