Making the Diagnosis of Infection in TKA – The Latest Tests

1. High-Yield Summary

• Core Diagnostic Mandate: Periprosthetic joint infection (PJI) remains one of the most devastating complications following total knee arthroplasty (TKA), accounting for approximately 25% of all revision procedures. Every painful or poorly functioning TKA must be considered infected until definitively proven otherwise through systematic clinical, serological, and synovial fluid evaluation.
• Stepwise Diagnostic Hierarchy: Initial screening begins with serum inflammatory biomarkers (erythrocyte sedimentation rate [ESR] and C-reactive protein [CRP]). If either marker is elevated, or if clinical suspicion is high despite normal serum markers, knee arthrocentesis is mandatory prior to the initiation of empirical antibiotic therapy.
• Synovial Fluid Differential Thresholds: Aspiration fluid analysis forms the cornerstone of diagnosis. In chronic presentations (beyond 90 days from index surgery), diagnostic thresholds are synovial white blood cell (WBC) count greater than 3,000 cells/µL or polymorphonuclear neutrophil percentage (PMN%) greater than 70%. In acute postoperative settings (within 90 days), cutoffs elevate to WBC greater than 10,000 cells/µL or PMN% greater than 90%.
• Modern Advanced Biomarkers: Synovial fluid alpha-defensin (both laboratory ELISA and qualitative lateral flow strip) and leukocyte esterase (LE) colorimetric reagent strips offer rapid diagnostic accuracy (sensitivity greater than 95%, specificity 97% to 100%) and maintain high diagnostic fidelity even in the setting of prior antibiotic administration.
• Catastrophic Diagnostic Error: Administering empirical oral or intravenous antibiotics prior to obtaining synovial fluid aspiration and multiple tissue cultures, which drastically reduces culture sensitivity, obscures biofilm recovery, and delays targeted microbiological eradication.

2. Anatomy

Neurovascular Danger Zones

Diagnostic arthrocentesis and surgical debridement of an infected total knee arthroplasty require precise anatomical orientation to prevent iatrogenic neurovascular injury. During diagnostic knee aspiration, the superolateral approach to the patellofemoral pouch is preferred because it maximizes synovial fluid yield and provides the lowest risk of cartilaginous or neurovascular impingement. Needle entry must remain 1 cm superior and 1 cm lateral to the superolateral pole of the patella. Medial aspirations risk inadvertent trauma to the infrapatellar branch of the saphenous nerve (IPBSN), which courses across the anteromedial joint line, potentially causing painful postoperative neuroma formation or medial numbness.

In operative revisions and radical synovectomy for PJI, the popliteal neurovascular bundle represents the preeminent vascular danger zone. The popliteal artery is closely tethered to the posterior joint capsule and posterior cruciate ligament insertion site. In chronically infected knees, severe proliferative synovitis and pericapsular fibrotic scarring obscure tissue planes, drawing the popliteal artery closer to the posterior femoral condyles and posterior tibial cortex. All posterior retractors placed during revision arthroplasty must remain strictly subperiosteal directly against osseous margins, and knee flexion to 90 degrees must be maintained during posterior capsular debridement to permit posterior translation of the neurovascular bundle.

The common peroneal nerve is located adjacent to the posterior aspect of the biceps femoris tendon before curving around the fibular neck. Extensive lateral synovectomy, posterolateral capsular release, or acute correction of valgus deformity during revision for PJI places the peroneal nerve under substantial traction or direct compression, creating significant risk of postoperative foot drop.

Internervous Plane and Surgical Approaches

The standard operative approach for surgical debridement or revision TKA is the anterior midline cutaneous incision utilizing a medial parapatellar arthrotomy. This approach traverses the interval between the rectus femoris/quadriceps tendon medially and the vastus medialis, entering through the medial patellar retinaculum and capsule, all innervated by terminal branches of the femoral nerve. Prior surgical incisions must be carefully inspected. When multiple previous longitudinal incisions exist, the most lateral usable incision is typically selected to safeguard medial cutaneous blood supply, and a minimum cutaneous bridge of 5 to 7 cm must be preserved between incisions to prevent skin flap necrosis.

In cases of severe arthrofibrosis or extensive extensor contracture complicating chronic infection, utilitarian exposure maneuvers such as a quadriceps snip, V-Y quadricepsplasty, or tibial tubercle osteotomy (TTO) may become necessary. When performing a tibial tubercle osteotomy in the setting of infection, a generous osseous fragment measuring 8 to 10 cm in length and 1.5 to 2 cm in thickness is preserved to ensure adequate vascularity and solid fixation upon closure.

Bony Landmarks and Reference Geometry

Accurate identification of anatomical landmarks is critical during both diagnostic workup and reconstruction. The joint line is determined relative to the medial epicondyle (approximately 25 mm distal), lateral epicondyle (approximately 20 mm distal), and the fibular head (approximately 10 to 15 mm proximal). Chronic PJI frequently leads to catastrophic osteolysis, bone resorption, and structural metaphyseal defects. Preserving or accurately reconstructing these bony benchmarks ensures appropriate joint line restoration and collateral ligament tensioning during second-stage reimplantation.

3. Pathogenesis

The fundamental biological mechanism underlying periprosthetic joint infection is the rapid formation of bacterial biofilm on foreign implant surfaces. Upon implantation of prosthetic components, host proteins (fibronectin, fibrinogen, vitronectin) coat the surfaces of cobalt-chromium, titanium, and ultra-high-molecular-weight polyethylene. Circulating or inoculated bacteria adhere to this conditioning film through non-specific physical interactions, followed by specific adhesin-mediated receptor attachment.

Following irreversible attachment, bacteria proliferate and synthesize a complex extracellular polymeric substance (EPS) matrix consisting of polysaccharides, extracellular DNA, and structural proteins. This dense, hydrated glycocalyx matrix encases microcolonies and functions as a physical and biological fortress. Bacteria within the biofilm downregulate their metabolic activity, entering a stationary growth phase characterized by phenotypic “persister cells.” These dormant metabolically quiescent organisms exhibit tolerance to antimicrobial concentrations up to 1,000 times greater than the minimum inhibitory concentration (MIC) required for planktonic free-floating counterparts.

The microbiological spectrum of PJI in total knee arthroplasty is dominated by Gram-positive cocci, with Staphylococcus aureus (methicillin-sensitive and methicillin-resistant) and coagulase-negative staphylococci (principally Staphylococcus epidermidis and Staphylococcus lugdunensis) accounting for 60% to 70% of confirmed cases. Streptococcus species, Enterococcus species, and Gram-negative bacilli (such as Pseudomonas aeruginosa, Escherichia coli, and Klebsiella pneumoniae) comprise the majority of remaining cases. Indolent, slow-growing organisms such as Cutibacterium acnes and fungal pathogens represent important diagnostic challenges, while culture-negative PJI occurs in 10% to 20% of series, frequently resulting from prior empirical antibiotic exposure or fastidious biofilm encapsulation.

4. Natural History

The natural history of an unrecognized or improperly managed periprosthetic joint infection is characterized by progressive local tissue destruction, osteolysis, component loosening, and substantial systemic morbidity. In the absence of surgical intervention, bacterial biofilm expansion stimulates intense chronic macrophage and osteoclast activation via pro-inflammatory cytokine cascades (interleukin-1, interleukin-6, tumor necrosis factor-alpha). This persistent osteoclastic stimulation precipitates progressive bone resorption at the bone-cement or bone-implant interface, culminating in aseptic-appearing gross loosening, catastrophic metaphyseal bone loss, and extensor mechanism incompetence.

Epidemiologically, PJI occurs following 1% to 2% of primary total knee arthroplasties and increases to 5% to 10% following revision procedures. Patient-specific risk factors include poorly controlled diabetes mellitus (glycated hemoglobin HbA1c greater than 7.5% to 8.0%), severe obesity (body mass index BMI greater than 40 kg/m²), active tobacco use, chronic inflammatory arthropathies, malnutrition (serum albumin less than 3.5 g/dL), and chronic renal or hepatic dysfunction.

Chronologically, PJI manifests across three distinct temporal phases: acute postoperative infections occurring within 4 to 12 weeks of the index procedure, chronic low-grade indolent infections presenting after 3 months with vague pain and subtle functional decline, and acute hematogenous infections characterized by sudden, catastrophic acute pain and effusion in a previously well-functioning, pain-free joint secondary to distant bacteremia.

5. Clinical Presentation

History

A rigorous clinical history is the first line of diagnostic detection. Clinicians must specifically inquire regarding persistent or progressive knee pain that has been present since the index surgery (“a knee that was never right”). Pain associated with PJI is classically present at rest and at night, contrasting with the mechanical start-up pain typical of aseptic loosening. Clinicians must meticulously document the surgical timeline, index wound healing characteristics, prolonged wound drainage (drainage extending beyond 48 to 72 hours postoperatively), secondary wound interventions, and any empirical oral antibiotic prescriptions administered in the postoperative period. A thorough review of recent bacteremic events—including dental extractions without antibiotic prophylaxis, urinary tract infections, colonoscopy, or indwelling intravenous lines—must be systematically cataloged.

Physical Exam

Physical examination must begin with visual inspection of the operative extremity, assessing for periprosthetic erythema, localized calor, tense effusion, and wound compromise. The presence of a sinus tract communicating directly with the prosthetic joint is pathognomonic for PJI.

• Direct Sinus Tract Probing: Gentle sterile exploration of cutaneous sinus opening using a blunt sterile probe -> Communication with joint capsule or metallic hardware [Sensitivity 30% – 40%, Specificity 100%].
• Effusion Palpation & Ballottement: Compression of the suprapatellar pouch while depressing the patella against the trochlea -> Demonstrable intra-articular fluid wave and tense ballottable patella [Sensitivity 75% – 85%, Specificity 60% – 70%].
• Local Thermal Assessment: Comparative tactile evaluation of cutaneous temperature against the contralateral healthy knee using the dorsum of the hand -> Marked localized calor [Sensitivity 65% – 75%, Specificity 70% – 80%].
• Range of Motion Arc: Measurement of active and passive flexion-extension arc -> Progressive loss of terminal extension or acute reduction in flexion arc accompanied by severe capsular pain [Sensitivity 70% – 80%, Specificity 50% – 60%].
• Extensor Mechanism Continuity: Active straight leg raise testing against gravity -> Extensor lag or complete inability to sustain extension indicating patellar tendon or quadriceps tendon compromise [Sensitivity 20% – 30%, Specificity 95%].

6. Imaging and Diagnostic Studies

Evaluating potential infection demands a structured multimodal diagnostic pathway progressing from basic radiography and serology to synovial fluid biochemistry and molecular microbiology.

Plain Radiography

Weight-bearing anteroposterior (AP), lateral, Merchant patellar, and full-length standing alignment radiographs are mandatory. Serial comparative radiographs represent the most effective radiographic tool. While radiographs are frequently normal in acute PJI, chronic infection manifests with progressive radiolucent lines greater than 2 mm at the bone-cement or bone-implant interface, patchy osteolysis occurring within the first 1 to 2 years after surgery, periosteal new bone formation (an aggressive periosteal reaction highly suggestive of infection), and rapid implant migration, subsidence, or catastrophic cement mantle fracture.

First-Line Serum Biomarkers

Initial laboratory screening must include serum erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). Serum CRP is the more sensitive and responsive marker, peaking on postoperative day 2 to 3 and returning to normal physiological levels (less than 10 mg/L) within 30 to 45 days. Serum ESR peaks on postoperative day 5 and may remain elevated for up to 90 to 180 days. In chronic presentations, thresholds of ESR greater than 30 mm/h and CRP greater than 10 mg/L provide a combined sensitivity of 91% to 95% and a specificity of 85% to 88%. When both markers are negative, the probability of PJI is low, although indolent low-virulence organisms can present with normal serum indices.

Serum D-dimer has emerged as a high-yield adjunct biomarker. Demonstrating a diagnostic cutoff of 850 ng/mL, serum D-dimer exhibits sensitivity of 89% and specificity of 93%, providing superior diagnostic performance compared to ESR and CRP in patients with inflammatory systemic diseases or low-virulence indolent infections.

Synovial Arthrocentesis & Cell Count

Aspiration of the knee joint is the gold standard diagnostic procedure. Aspiration should ideally be performed in a dedicated procedure room under sterile preparation without prior local anesthetic infiltration into the joint space. In chronic presentations, a synovial fluid white blood cell (WBC) count greater than 3,000 cells/µL or a polymorphonuclear neutrophil percentage (PMN%) greater than 70% is diagnostic of PJI. In the acute postoperative period (within 90 days of surgery), normal physiological inflammation necessitates higher diagnostic thresholds: synovial WBC greater than 10,000 cells/µL or PMN% greater than 90%.

Advanced & Emerging Synovial Biomarkers

Synovial fluid alpha-defensin is an antimicrobial peptide released by activated neutrophils in response to intra-articular pathogens. Available as both a quantitative laboratory enzyme-linked immunosorbent assay (ELISA) and a rapid qualitative lateral flow cassette, alpha-defensin demonstrates sensitivity of 97% and specificity of 97% to 100%. Critically, alpha-defensin maintains high diagnostic accuracy even in patients who have received recent antibiotic therapy, overcoming the major diagnostic limitation of standard microbiological cultures.

Synovial leukocyte esterase (LE) reagent test strips provide a rapid, inexpensive point-of-care colorimetric assessment of neutrophil esterase activity. A reading of 2+ (or 1+ in appropriate clinical contexts) yields a sensitivity of 88% to 93% and specificity of 93% to 98%. If the aspirate is heavily contaminated with blood, centrifugation or dilution is required to prevent visual obscuration of the reagent color pad. Additional emerging synovial biomarkers include synovial calprotectin, synovial CRP, and synovial interleukin-6 (IL-6).

Microbiological Culture & Molecular Diagnostics

Synovial aspirates must be inoculated directly into pediatric blood culture bottles at the bedside, which substantially enhances the recovery of fastidious organisms. Cultures must be incubated under aerobic and anaerobic conditions for a minimum of 14 days to permit the identification of slow-growing pathogens such as Cutibacterium acnes. During operative exploration, a minimum of 3 to 5 distinct tissue specimens must be obtained using separate clean instruments from the implant-bone interface, neo-capsule, and pseudocapsule.

In culture-negative PJI, advanced molecular diagnostics offer diagnostic breakthroughs. Next-generation sequencing (NGS)—including targeted 16S ribosomal RNA sequencing and metagenomic NGS—identifies microbial DNA fragments with remarkable sensitivity (85% to 90% in culture-negative cohorts), pinpointing causative pathogens and guiding targeted antimicrobial regimens when conventional cultures fail.

7. Classification

Standardized classification criteria are mandatory to harmonize clinical practice, guide therapeutic decision-making, and validate clinical research.

8. Treatment

Medical / Nonoperative

Nonoperative medical management alone is never curative for established periprosthetic joint infection due to the impervious nature of bacterial biofilm. Long-term chronic suppressive oral antibiotic therapy without surgical intervention is strictly reserved for patients with prohibitively high surgical risk, severe terminal comorbidities, or patients who decline operative reconstruction. Essential prerequisites for chronic suppressive therapy include well-fixed components without radiographic loosening, a low-virulence pathogen with verified oral antimicrobial susceptibility, absence of systemic sepsis, and an intact soft-tissue envelope without a draining sinus tract.

Operative

The primary goals of surgery in PJI are complete eradication of infection, alleviation of pain, and restoration of a stable, functional lower extremity. Operative strategies include:

• Debridement, Antibiotics, and Implant Retention (DAIR): Indicated strictly for early acute postoperative infections (less than 3 to 4 weeks from index surgery) or acute hematogenous infections with symptom duration under 3 weeks. Procedure requires radical synovectomy, copious pulsatile irrigation (9 to 12 liters), and mandatory exchange of modular polyethylene inserts to access posterior capsular compartments.
• Two-Stage Revision Arthroplasty (Gold Standard): The established benchmark in North America. Stage 1 consists of complete hardware explantation, radical synovectomy, aggressive osseous debridement, and insertion of an antibiotic-impregnated polymethylmethacrylate (PMMA) spacer (static vs. articulating). Patients complete 6 to 12 weeks of targeted systemic antimicrobials followed by a 2 to 4 week antibiotic holiday and repeat joint aspiration. Stage 2 involves spacer explantation, frozen section histology, and definitive modular revision reconstruction.
• One-Stage (Single-Stage) Revision: Hardware explantation, radical debridement, and immediate reimplantation. Applicable in strictly selected patients: known low-virulence pathogen with known antibiotic susceptibility, intact soft-tissue envelope without sinus tract, minimal bone loss, and absence of prior revision failures.
• Surgical Salvage Procedures: For recalcitrant, multi-drug resistant PJI or severe extensor mechanism loss, salvage options include knee arthrodesis (utilizing intramedullary arthrodesis nails or modular fusion systems), resection arthroplasty, or above-knee amputation in life-threatening sepsis.

Indications & Contraindications

DAIR is strictly contraindicated in the presence of chronic symptoms (greater than 4 weeks), component loosening, compromised soft-tissue envelope, or multi-drug resistant/fungal organisms. Single-stage revision is contraindicated in culture-negative PJI, multi-drug resistant organisms (methicillin-resistant Staphylococcus aureus [MRSA], vancomycin-resistant Enterococcus [VRE], Pseudomonas), active sinus tract, or severe uncontained bone loss requiring massive structural allografting.

9. Outcome

Clinical success following surgical treatment of PJI is defined by infection eradication, absence of subsequent surgical intervention for infection, and preservation of functional joint articulation. Two-stage revision arthroplasty achieves infection eradication rates between 85% and 92% at intermediate to long-term follow-up. However, functional outcomes—measured by the Knee Society Clinical Rating Score (KSS) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)—remain lower than primary or aseptic revision TKA, with mean post-reimplantation KSS scores typically ranging from 72 to 80 points and mean knee flexion averaging 90° to 100°.

Debridement, Antibiotics, and Implant Retention (DAIR) in properly selected acute cohorts yields success rates of 65% to 80%. When DAIR is inappropriately applied to chronic infections or performed without modular insert exchange, failure rates exceed 60% to 70%, predisposing patients to substantial bone loss and compromised subsequent two-stage revision outcomes. The systemic mortality burden of PJI is substantial: the 5-year mortality rate following PJI diagnosis is estimated at 20% to 25%, surpassing that of many common solid organ malignancies.

10. Complications

The diagnostic workup and surgical management of infected total knee arthroplasty carry substantial complication profiles that demand proactive multidisciplinary surveillance:

11. Pearls and Pitfalls

Surgical Pearls

• Strict Antibiotic Holiday Before Aspiration: Always hold systemic antibiotics for a minimum of 14 days prior to diagnostic arthrocentesis unless the patient presents in florid systemic sepsis. Even a single dose of oral fluoroquinolone or beta-lactam can suppress synovial culture yield by up to 50%.
• Bedside Blood Culture Bottle Inoculation: Inoculate 5 to 10 mL of synovial fluid directly into pediatric BACTEC aerobic and anaerobic blood culture bottles at the bedside. This significantly accelerates organism detection and increases culture positivity by 20% compared to standard sterile swab transport.
• Employ Point-of-Care Leukocyte Esterase: Keep urine reagent test strips in the orthopaedic clinic. When synovial fluid is aspirated, place one drop on the leukocyte esterase reagent pad. An immediate 2+ purple reading provides a rapid, inexpensive 95% sensitive ruling in of PJI within 90 seconds.
• Modular Polyethylene Exchange Is Mandatory in DAIR: Never perform a DAIR procedure without exchanging the modular polyethylene insert. Retaining the liner prevents radical debridement of the posterior capsule, leaves biofilm-harboring backside polyethylene surfaces untouched, and results in unacceptably high failure rates.

Technical Pitfalls

• Relying Exclusively on Negative Plain Radiographs: Assuming a joint is not infected because radiographs show intact component fixation and no radiolucent lines. Early and indolent PJI routinely presents with completely normal plain radiographs.
• Overlooking Blood Dilution in Synovial WBC Counts: Failing to correct synovial WBC counts when the aspiration is traumatic or hemarthrotic. A simple correction formula (subtract 1 synovial WBC for every 500 to 1,000 RBCs present) prevents misinterpreting a bloody aspirate as acute infection.
• Premature Reimplantation Based on Normalizing Serum ESR/CRP: Relying solely on normal serum ESR or CRP to dictate the timing of second-stage reimplantation. Up to 25% of patients with persistent intra-articular infection exhibit normal or near-normal serum inflammatory markers. Repeat joint aspiration before Stage 2 is essential.
• Failure to Debride Behind Posterior Metaphyseal Flares: During hardware explantation, leaving necrotic fibrous tissue or retained cement fragments behind the posterior femoral condyles or posterior tibial cortex, which guarantees persistent infection.

12. Controversy

The foremost surgical controversy in the management of PJI is the debate between single-stage and two-stage revision arthroplasty. While two-stage revision has historically been regarded as the gold standard in North America, single-stage revision is widely performed in specialized European centers with reported infection control rates exceeding 85% to 90%. Proponents of single-stage revision highlight the avoidance of an interim period of disability, lower total healthcare expenditure, and preservation of joint tissue planes and bone stock. However, opponents caution that single-stage revision requires absolute preoperative organism identification and antibiotic sensitivity, flawless radical debridement, and dedicated institutional infectious disease infrastructure. In the setting of culture-negative PJI or antibiotic-resistant organisms, two-stage revision remains the overwhelmingly safer strategy.

A second major controversy concerns the clinical threshold and utility of next-generation sequencing (NGS) and molecular diagnostics. While metagenomic NGS identifies microbial DNA in up to 90% of culture-negative PJI cases, debate persists regarding the clinical differentiation between viable pathogenic organisms, dead bacterial DNA fragments, and normal skin flora contamination. Current clinical consensus positions NGS as an invaluable second-line diagnostic tool in culture-negative clinical presentations rather than a universal first-line screening modality.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Parvizi J, Tan T, Goswami K et al. The 2018 Definition of Periprosthetic Hip and Knee Infection: An Evidence-Based and Validated Criteria. The Journal of Arthroplasty. 2018. doi:10.1016/j.arth.2018.02.078 [Level I (Diagnostic / Consensus): Evidence-Based Diagnostic Consensus / Multicenter Validation]
• Parvizi J, Zmistowski B, Berbari E et al. New Definition for Periprosthetic Joint Infection: From the Workgroup of the Musculoskeletal Infection Society. Clinical Orthopaedics & Related Research. 2011. doi:10.1007/s11999-011-2102-9 [Level V (Expert Consensus): Seminal Workgroup Consensus Definition]
• Deirmengian C, Kardos K, Kilmartin P et al. Combined Measurement of Synovial Fluid α-Defensin and C-Reactive Protein Levels: Highly Accurate for Diagnosing Periprosthetic Joint Infection. Journal of Bone and Joint Surgery. 2014. doi:10.2106/jbjs.m.01316 [Level II (Diagnostic): Prospective Diagnostic Validation Cohort]
• Shahi A, Kheir M, Tarabichi M et al. Serum D-Dimer Test Is Promising for the Diagnosis of Periprosthetic Joint Infection and Timing of Reimplantation. Journal of Bone and Joint Surgery. 2017. doi:10.2106/JBJS.16.01395 [Level II (Diagnostic): Prospective Diagnostic Comparative Study]
• Tarabichi M, Shohat N, Goswami K et al. Diagnosis of Periprosthetic Joint Infection: The Potential of Next-Generation Sequencing. Journal of Bone and Joint Surgery. 2018. doi:10.2106/JBJS.17.00434 [Level II (Diagnostic): Prospective Diagnostic Investigation]

15. Master Class Pro-Tip

When evaluating a painful, stiff, or poorly functioning total knee arthroplasty, never succumb to the temptation of initiating empirical oral antibiotics before obtaining an aspiration. The single greatest diagnostic obstacle in modern revision knee arthroplasty is the culture-negative joint created by a well-intentioned 7-day course of antibiotics. In the clinic, establish an absolute protocol: if a patient presents with unexplained persistent pain, effusion, or delayed wound drainage, perform an immediate aseptic superolateral arthrocentesis, send the fluid for cell count with differential and 14-day extended cultures, and perform an instant bedside leukocyte esterase strip test. If the knee aspirate yields a dry tap, never inject saline to wash the joint, as saline dilution falsely lowers the leukocyte count and renders biomarker thresholds uninterpretable; instead, refer the patient for fluoroscopically or ultrasound-guided aspiration. Treat every unexplained painful knee as infected until proven otherwise, and let the objective microbiology guide every operative incision.

Master Class Clinical Pearl: Diagnostic Evaluation of Periprosthetic Joint Infection in TKA