Knee aspiration is a useful tool and safely performed under maximum aseptic conditions. However, in certain circumstances (morbid obesity, infection, and cellulitis near the joint line), arthrocentesis can be considered a real challenge [6]. Several different approaches have been described to obtain synovial fluid of the knee [7] (Figs. 10.1 and 10.2).

The lateral midpatellar approach is the most commonly used. The needle is directed at a 45° angle toward intra-articular space. A superolateral approach (2 cm above and 1 cm lateral to patellar surface) may be used, especially in large effusion cases. A 15 ml of joint fluid should be sufficient for the analysis. Aspiration in the supine position yields more fluid and is recommended for suspected infection [8].

Many reviews have shown ultrasound-guided aspiration as an alternative to traditional joint aspiration. Radiologists have described the success of several techniques of USG (ultrasound-guided) joint and soft tissue injection. Several clinical studies suggested that ultrasound could be used as an adjuvant tool for intra-articular injections in the knee joint via the suprapatellar bursa [9].

One of the drawbacks of this technique is dry aspiration which has an incidence of up to 30% of the cases in the hip. Other limitations include the inability to identify microorganisms that have formed biofilms or have been internalized by osteoblasts [10].

Fink et al. [11] demonstrated that preoperative synovial biopsy with samples obtained at arthroscopy increased sensitivity (100%) and specificity (98.2%) compared with knee aspiration (72.5% and 95.2%, respectively). Williams [12] considered that this practice is not recommended because of its more invasive nature. Other techniques have been described such as the percutaneous interface biopsy with a sensitivity of 88.2% and specificity of 100% based on the hypothesis that a sample interface of the periprosthetic membrane could complement the results of the knee aspiration [10].

The role of arthrocentesis in the evaluation of the painful TKA has evolved over past decades. Aspiration is not only useful in diagnosing but also in analyzing the sensitivity profile and antibiogram of the microorganism causing the infection.

Most authors seem to agree with the fact that in the clinical suspicion of infection and high and inflammatory parameters (CRP and ESR), the practice of aspiration should be mandatory [13]. Gredianus et al. [14] found ESR with a cutoff point of 30 mm/h had a sensitivity of 93% and specificity of 84%. Similarly, CRP had a sensitivity of 91% and specificity of 86% (10 mg/dl). In a level 1 study of 207 consecutive TKA revisions, if both ESR and CRP were normal, the probability of periprosthetic joint infection was 3%. Sensitivity of combined ESR, CRP, and aspiration was 99.7% [15]. After considering the benefits, opportunities, costs, and risks of the different available alternatives, preclinical models support the AAOS recommendations regarding the use of serum markers (ESR/CRP) before arthrodesis as the best diagnostic strategy for periprosthetic joint infection (PJI) [16].

A recent literature review of Meermans et al. [17] including 29 studies of joint infection in hip and knee prostheses reported a mean sensitivity of cultures of 71%. Joint aspiration should be performed 2 weeks after any antibiotic treatment has been discontinued. The odds of a negative culture increase 4.7-fold if the patient has received antibiotics in the last 3 months [18].

White blood cell (WBC) count with analysis of polymorphonuclear (PMN) percentage has gained popularity for differentiating periprosthetic joint infection from aseptic total knee arthroplasty. Normal joint fluid contains <200 white blood cells and <25% segmented cells. In patients with inflammatory and degenerative diseases, this figure can rise to 1000 WBC/ml. WBC counts lower than 1100 WBC/μl containing less than 64% PMNs resulted in 99.6% negative predictive value for excluding periprosthetic joint infection [19]. Similarly, a synovial fluid WBC count greater than 9000 WBC/μl combined with elevation of either the ESR or CRP resulted in a 100% positive predictor value and 98% accuracy for identifying PJI [20].

Some studies have focused on analyzing the WBC count and PMN %. AAOS establishes a cutoff point of 1760/ml white blood cells and 73% PMN if performed after 6 weeks from surgery and 10,700 cells/ml and 89% PMN if earlier [21]. Ghanem argues that the finding in the joint aspiration of >1100 ml⁻¹ cells and 64% PMN confirms the diagnosis [22]. Other reports raise this figure up to 2382 ml⁻¹ [23]. Synovial fluid WBC count and PMN percentage are both elevated during the early postoperative period. The two markers behave differently, with synovial fluid WBC counts demonstrating an earlier return to baseline levels. WBC and PMN percentage may not be reliable for a patient with inflammatory arthropathy [24].

Gram staining has a low sensitivity of 10–67% [25]. In most cases, 5 days of culture should be enough, although it recommended that cultures remain incubated for 14 days to allow the identification of other less virulent microorganisms (propionibacterium species, aerobic gram-positive bacilli, and Peptostreptococcus); otherwise, up to 30% of infections would go undetected [18].

Additional mycobacterial and fungal cultures can be obtained for at-risk patients or when clinical suspicion dictates such action. It is unclear if anaerobic, fungal, or acid-fast bacilli cultures should be routinely sent because of the additional cost [26].

Blood culture bottles (BCBs) have proven to be more effective than the conventional swabs in transporting samples to the lab. BCB is a low-cost, easy-access method that significantly improves the ability to positively identify bacterial cultures in PJI [27]. Hughes et al. [28] have shown the use of BCB is superior to conventional methods in the setting of native joint infection. Furthermore, Minassian et al. [29] showed that BCBs had comparable sensitivity, specificity, and shorter time to positivity than cooked meat enrichment broth methods.

Synovial fluid should be stored in tubes containing ethylenediaminetetraacetic acid (EDTA) and kept at 4 °C before analysis [30]. Synovial fluid white cell count decreases by 47% after 48 h when stored in heparin, compared with 5.1% when stored in EDTA. EDTA is a much more suitable preservative than heparin [31].