Benign Ethnic Neutropenia is more common in males.

Acute knee injuries requiring operative treatment

Figure 11 shows the radiograph of a 2-year-old child with marked genu varum and tibial bowing. Based on these findings, what is the best initial course of action?

Obtain serum phosphorous, calcium, and alkaline phosphatase levels.

When performing an inside-out lateral meniscal repair, capsule exposure is provided by developing the

iliotibial band and biceps tendon interval, then retracting the lateral head of the gastrocnemius posteriorly.

Figures A and B are radiographic images of an 85-year-old woman with isolated left hip pain. She describes a non-syncopal fall from standing 4 hours ago. Physical examination reveals pain with log-rolling the left thigh and the inability to bear weig

Stress view radiographs of the left hip

The above surgery was performed with recombinant human bone morphogenetic protein-2 (rhBMP-2). She reports immediate relief of her leg pain and is discharged home on postoperative day 2. The patient reports new-onset radicular pain and weakness in he

Bony overgrowth from the use of BMP protein in the disk space

A 10-year-old child with cerebral palsy undergoes bilateral hamstring lengthening for severe knee flexion contractures, and knee immobilizers are applied postoperatively. Examination at the initial postoperative check 2 hours after surgery reveals th

remove the left knee immobilizer and flex the knee.

A 56-year-old man has a chief complaint of leg weakness and inability to walk. Examination reveals 5 out of 5 motor strength in all lower extremity muscle groups tested and normal sensation to light touch in both lower extremities. The patient is slo

MRI of the thoracic and cervical spine

Orthopedic MCQs: Joint Infection Diagnosis & Aspiration

Question 1

According to the 2018 International Consensus Meeting (ICM) criteria for periprosthetic joint infection (PJI), which of the following is considered a definitive major criterion for diagnosing PJI?

Explanation

Under the 2018 ICM criteria, there are only two major criteria that definitively confirm a PJI: 1) Two positive periprosthetic cultures with phenotypically identical organisms, or 2) A sinus tract communicating with the joint. All other findings (including elevated WBC, positive alpha-defensin, single positive culture, and purulence) are considered minor criteria that contribute to an aggregate diagnostic score.

Question 2

Which of the following statements regarding the synovial fluid alpha-defensin test for periprosthetic joint infection (PJI) is most accurate?

Explanation

Alpha-defensin is an antimicrobial peptide released by human neutrophils in response to pathogens. A major clinical advantage of the alpha-defensin test is that it maintains its high sensitivity and specificity for diagnosing PJI even when patients have been previously treated with antibiotics. It is also unaffected by concurrent systemic inflammatory conditions like rheumatoid arthritis.

Question 3

A 68-year-old male presents with chronic knee pain and stiffness 3 years after a total knee arthroplasty (TKA). Aspiration of the joint yields 3 cc of clear synovial fluid. Which of the following synovial fluid analysis thresholds most strongly supports a diagnosis of chronic PJI?

Explanation

For chronic PJI (typically defined as occurring >90 days postoperatively), the generally accepted threshold is a synovial fluid WBC count > 3,000 cells/uL and/or a PMN percentage > 80%. This is significantly lower than the threshold for a native septic joint (WBC > 50,000 cells/uL) because biofilm formation in PJI attenuates the typical robust leukocytic response. Acute post-op PJI (<90 days) utilizes a higher threshold of approximately > 10,000 cells/uL and > 90% PMNs.

Question 4

A 2-year-old child presents with a suspected native joint septic arthritis of the knee. Given the high prevalence of Kingella kingae in this age group, which of the following microbiological techniques will maximize the likelihood of isolating this fastidious organism?

Explanation

Kingella kingae is a fastidious Gram-negative organism that is a leading cause of pediatric septic arthritis in children under 4 years old. Standard solid agar plating often yields false-negative results. Directly inoculating the synovial fluid into aerobic blood culture vials (such as BACTEC) significantly increases the diagnostic yield for this organism.

Question 5

A patient with a painful metal-on-metal (MoM) total hip arthroplasty undergoes diagnostic joint aspiration to rule out infection. Which of the following is a critical modification required during the synovial fluid analysis?

Explanation

In metal-on-metal bearings or cases with severe metallosis, suspended metal debris in the synovial fluid is often misread by automated cell counters as leukocytes, leading to falsely elevated WBC counts. Therefore, a manual count using a hemocytometer is required to obtain an accurate WBC count.

Question 6

You are following the postoperative inflammatory markers of a patient who underwent an uncomplicated primary total hip arthroplasty (THA). Assuming a normal recovery without infection, at what postoperative timeframes do C-reactive protein (CRP) and Erythrocyte Sedimentation Rate (ESR) typically peak?

Explanation

In normal, uncomplicated postoperative recovery after total joint arthroplasty, CRP is an acute responder that peaks rapidly around postoperative day 2 and typically returns to baseline within 3 weeks. ESR is a slower responder, peaking around postoperative days 5 to 7, and can take up to 6 to 12 weeks to normalize.

Question 7

During a diagnostic aspiration of a suspected infected total knee arthroplasty, the surgeon encounters a 'dry tap'. The surgeon injects 10 mL of sterile, non-bacteriostatic normal saline, ranges the knee, and re-aspirates the fluid. Which of the following synovial fluid parameters remains the most diagnostically reliable despite the saline dilution?

Explanation

When a saline lavage is performed during a 'dry tap', parameters relying on absolute concentrations (such as the absolute WBC count, leukocyte esterase, and protein) become artificially lowered due to dilution and lose their established diagnostic thresholds. However, the PMN percentage represents a ratio of cell types and remains largely reliable and unaffected by the dilution.

Question 8

In the evaluation of pediatric septic arthritis of the hip, the original Kocher criteria utilized four predictors. Subsequent studies (e.g., Caird et al.) added a fifth predictor which was found to be the strongest independent predictor of septic arthritis. Which biomarker is this?

Explanation

Caird et al. modified the original Kocher criteria by adding C-reactive protein (CRP) > 2.0 mg/dL (20 mg/L) as a fifth variable. In their study, a CRP > 2.0 mg/dL was identified as the single strongest independent clinical predictor of septic arthritis in pediatric patients.

Question 9

A 72-year-old female presents 4 weeks post-total knee arthroplasty with an acute onset of severe pain, erythema, and a large effusion. She undergoes a joint aspiration. What is the accepted synovial WBC threshold indicating acute periprosthetic joint infection (<90 days post-op)?

Explanation

For acute periprosthetic joint infections (defined typically as within the first 6-12 weeks or <90 days postoperatively), the diagnostic threshold is higher than for chronic PJI due to expected postoperative inflammation. The accepted threshold for acute PJI is > 10,000 cells/uL and a PMN percentage of > 90%.

Question 10

A 55-year-old male complains of insidious, progressive shoulder pain and stiffness 14 months after a primary anatomic total shoulder arthroplasty. Serum ESR and CRP are normal. An ultrasound-guided aspiration is performed. To accurately diagnose Cutibacterium acnes (formerly Propionibacterium acnes), which specific microbiological protocol must be requested?

Explanation

Cutibacterium acnes is an indolent, slow-growing anaerobic Gram-positive bacillus that is a leading cause of shoulder periprosthetic joint infections. It frequently presents with normal serum inflammatory markers and no frank purulence. Because of its slow growth rate, routine 3-5 day cultures will often yield false negatives; standard protocol requires holding anaerobic cultures for up to 14 days.

Question 11

A 22-year-old healthy female presents with an acutely swollen, painful left wrist. She reports a history of migratory polyarthralgia over the past week and multiple scant vesiculopustular skin lesions. Synovial fluid aspiration shows a WBC count of 40,000 cells/uL. If this condition is suspected, what is the optimal culture medium required for diagnosis?

Explanation

The clinical presentation (migratory polyarthralgia, tenosynovitis, skin lesions, and an infected joint with slightly lower WBC than typical S. aureus septic arthritis) strongly suggests gonococcal arthritis (Neisseria gonorrhoeae). N. gonorrhoeae is a fastidious organism that requires specialized chocolate agar enriched with antibiotics, specifically Thayer-Martin agar, to selectively grow while inhibiting normal flora.

Question 12

A 9-year-old boy from New England presents with a massive, minimally painful knee effusion and is able to bear weight without significant difficulty. Synovial aspiration reveals a WBC count of 35,000 cells/uL with 80% PMNs. You suspect Lyme arthritis. What is the most appropriate initial diagnostic test for confirming this specific etiology?

Explanation

The diagnosis of Lyme arthritis is established via a two-tiered serologic algorithm. The initial step is a sensitive enzyme immunoassay (ELISA) for B. burgdorferi antibodies. If positive or equivocal, it is followed by a confirmatory Western blot (the second tier). Synovial fluid PCR is highly specific but lacks sensitivity compared to serology and is not recommended as the initial screening test. Culture is exceedingly difficult and rarely performed clinically.

Question 13

The leukocyte esterase (LE) colorimetric strip test is an effective, rapid diagnostic tool for PJI. However, false or unreadable results frequently occur in the setting of hemarthrosis. To obtain a valid LE strip reading in heavily blood-stained synovial fluid, what preparatory step must be taken?

Explanation

Leukocyte esterase strips rely on a color change to indicate the presence of neutrophil esterase. Gross blood in the synovial fluid heavily obscures this color change, making the strip unreadable. The validated method to overcome this is centrifuging the sample to pellet the red blood cells and then testing the clear supernatant.

Question 14

A patient with suspected chronic PJI of the hip has been empirically placed on oral antibiotics by their primary care physician. To maximize the diagnostic yield of pre-operative joint aspiration cultures, what is the minimum recommended 'antibiotic holiday' (duration to withhold antibiotics) prior to obtaining the aspiration?

Explanation

According to the American Academy of Orthopaedic Surgeons (AAOS) guidelines and the International Consensus Meeting (ICM) on PJI, an 'antibiotic holiday' of at least 14 days (2 weeks) is recommended prior to obtaining diagnostic cultures to minimize the risk of false-negative culture results.

Question 15

During a revision total knee arthroplasty for suspected culture-negative PJI, the surgeon extracts the femoral and tibial components and sends them for sonication. What is the primary mechanism by which sonication improves the microbiological yield in PJI?

Explanation

Sonication involves submersing explanted hardware in a fluid bath and applying low-frequency ultrasound. The acoustic cavitation physically dislodges and disrupts the protective extracellular polymeric substance (EPS) of the biofilm, releasing viable sessile bacteria into the planktonic state, which significantly increases the sensitivity of subsequent cultures.

Question 16

Under the 2018 International Consensus Meeting (ICM) minor criteria scoring system for PJI, which of the following synovial fluid or serum findings is assigned a weight of exactly 3 points?

Explanation

In the 2018 ICM scoring algorithm for PJI, a positive synovial fluid leukocyte esterase (++) OR an elevated synovial WBC count (>3000) yields 3 points. Positive Alpha-defensin also yields 3 points. Serum D-dimer (>860 ng/mL) or CRP (>10 mg/L) yields 2 points. Synovial PMN percentage (>80%) yields 2 points, and elevated ESR (>30) yields 1 point. A total score of 6 or more confirms PJI.

Question 17

Which of the following is considered the primary advantage of utilizing Next-Generation Sequencing (NGS) over standard culture techniques in the diagnosis of periprosthetic joint infections?

Explanation

Next-Generation Sequencing (NGS) is highly sensitive as it sequences all microbial DNA present in a sample. Its primary advantage is the ability to identify the precise causative organism in culture-negative PJI scenarios (e.g., when the patient is on active antibiotics or when the bacteria are dead or non-culturable). It cannot directly determine standard MICs and has a risk of detecting clinically irrelevant contaminant DNA.

Question 18

A 65-year-old male with a history of recurrent gout presents with acute severe pain, swelling, and redness in his native left knee. Aspiration yields cloudy fluid with 65,000 WBCs/uL. Polarized light microscopy confirms the presence of abundant needle-shaped, strongly negative birefringent crystals. Gram stain is negative. What is the most appropriate next step in management regarding the diagnosis of infection?

Explanation

The presence of monosodium urate crystals strongly supports an acute gout flare; however, gout and septic arthritis can coexist (found concurrently in ~1.5% to 5% of cases). Given the extremely high WBC count (which can be seen in severe gout but also indicates infection) and clinical picture, one cannot definitively rule out infection based on crystals alone. The fluid must always be sent for definitive bacterial cultures.

Question 19

The 2018 International Consensus Meeting (ICM) introduced a novel serum biomarker to the diagnostic criteria for PJI. This marker is valued for its high sensitivity and comparable performance to serum CRP. Which biomarker is this?

Explanation

Serum D-dimer was formally introduced as a minor criterion in the 2018 ICM diagnostic algorithm for PJI. It was established as an equivalent alternative to serum CRP, where an elevated D-dimer (>860 ng/mL) or an elevated CRP (>10 mg/L) equally contributes 2 points to the aggregate PJI score.

Question 20

A 50-year-old immunocompromised patient from the San Joaquin Valley presents with chronic swelling of the knee. Native joint aspiration yields a WBC count of 25,000 cells/uL with a lymphocytic predominance. A fungal etiology is suspected. Which of the following culture media is the most appropriate for isolating the likely pathogen?

Explanation

The patient's demographics (San Joaquin Valley) and presentation suggest a Coccidioides immitis fungal joint infection. Sabouraud dextrose agar is specifically designed for the cultivation of fungi (dermatophytes, yeasts, and dimorphic fungi like Coccidioides) and is the correct selective medium for this suspected pathogen.

Question 21

A 65-year-old female undergoes knee aspiration for suspected periprosthetic joint infection (PJI). The synovial fluid is noticeably blood-tinged. How does this gross appearance affect the use of a leukocyte esterase (LE) test strip?

Explanation

Gross blood contamination can interfere with the colorimetric reading of a leukocyte esterase strip, leading to false-positive results. Centrifuging the sample and testing the clear supernatant mitigates this issue and restores test accuracy.

Question 22

In the diagnostic algorithm for periprosthetic joint infection (PJI), which of the following scenarios is the most widely accepted indication for utilizing Next-Generation Sequencing (NGS) on synovial fluid?

Explanation

Next-Generation Sequencing (NGS) detects microbial DNA and is highly sensitive. It is most beneficial in cases of culture-negative PJI where traditional cultures fail to grow an organism, often due to prior antibiotic administration.

Question 23

A 55-year-old male presents with stiffness and vague pain 2 years following a total shoulder arthroplasty. A joint aspiration is performed to rule out infection. To optimize the detection of Cutibacterium acnes, what specific instruction must be given to the microbiology laboratory?

Explanation

Cutibacterium acnes is a slow-growing anaerobic organism commonly responsible for shoulder PJIs. Cultures must be held in anaerobic conditions for a minimum of 14 days to prevent false-negative results.

Question 24

According to the 2018 International Consensus Meeting (ICM) criteria, what is the accepted threshold for synovial fluid white blood cell (WBC) count to meet criteria for an acute periprosthetic joint infection (within 90 days of surgery)?

Explanation

The 2018 ICM recognizes different thresholds for acute versus chronic PJI. For an acute PJI (within 90 days post-op), a synovial fluid WBC > 10,000 cells/uL or PMN > 90% is highly suggestive of infection.

Question 25

You are attempting to aspirate a painful total hip arthroplasty for suspected PJI but obtain a "dry tap" despite fluoroscopic confirmation of intra-articular needle placement. What is the most appropriate next step to obtain fluid for microbiological analysis?

Explanation

In the event of a "dry tap" during joint aspiration, instilling sterile, non-bacteriostatic saline and re-aspirating the lavage fluid is the recommended technique to recover organisms for culture.

Question 26

A patient with a suspected chronic knee PJI has been taking oral amoxicillin for a recent dental infection. To maximize the diagnostic yield of synovial fluid cultures, what is the recommended minimum duration to withhold antibiotics prior to aspiration?

Explanation

Current guidelines recommend an "antibiotic holiday" of at least 14 days prior to obtaining synovial fluid or tissue cultures to minimize the risk of false-negative culture results.

Question 27

Serum D-dimer was incorporated into the 2018 ICM scoring system for periprosthetic joint infection (PJI). Which of the following statements regarding its role is most accurate?

Explanation

In the 2018 ICM criteria, an elevated serum D-dimer (> 850 ng/mL) is considered a minor criterion and is awarded 2 points, which is the same weight given to an elevated serum CRP (> 10 mg/L).

Question 28

A 60-year-old male presents with an acutely swollen, erythematous native knee. Aspiration yields cloudy fluid with a WBC count of 65,000 cells/uL. Polarized microscopy reveals negatively birefringent needle-shaped crystals. What is the most critical next step?

Explanation

The presence of crystals does not rule out septic arthritis, as concurrent crystal arthropathy and joint infection can occur in up to 2% of cases. Synovial fluid must still be cultured.

Question 29

During a revision total knee arthroplasty for presumed aseptic loosening, suspicious periprosthetic tissue is sent for intraoperative frozen section. According to the 2018 ICM criteria, which histological finding is considered a positive minor criterion for PJI?

Explanation

Positive histology, defined as greater than 5 polymorphonuclear neutrophils (PMNs) per high-power field in at least 5 separate fields (at 400x magnification), is a minor criterion highly suggestive of PJI.

Question 30

When analyzing synovial fluid from a painful total hip arthroplasty to evaluate for a chronic periprosthetic joint infection (PJI), which polymorphonuclear (PMN) cell percentage is the accepted minor criterion threshold in the 2018 ICM scoring system?

Explanation

For chronic PJI, the 2018 ICM utilizes a synovial fluid PMN percentage > 80% (or a synovial WBC > 3,000 cells/uL) as a minor criterion, awarding 3 points toward the diagnosis.

Question 31

A 24-year-old sexually active female presents with migratory polyarthralgia and an acutely swollen wrist. Aspiration reveals a WBC count of 45,000 cells/uL. Routine synovial cultures remain negative at 48 hours. What is the most sensitive test to confirm the suspected diagnosis?

Explanation

Disseminated gonococcal infection is notoriously difficult to culture from synovial fluid. NAAT of mucosal surfaces (cervix, urethra, pharynx) provides the highest diagnostic sensitivity for confirming Neisseria gonorrhoeae.

Question 32

Which of the following best describes the primary advantage of implant sonication over standard periprosthetic tissue cultures in the diagnosis of periprosthetic joint infection?

Explanation

Sonication uses low-frequency ultrasound waves to dislodge bacteria embedded within the biofilm on explanted hardware. This significantly increases the sensitivity of cultures compared to standard periprosthetic tissue sampling.

Question 33

A patient with a history of latent tuberculosis presents with chronic loosening of a hip prosthesis. Routine testing is negative. If mycobacterial periprosthetic joint infection is suspected, which of the following is true regarding diagnostic testing?

Explanation

Mycobacterium tuberculosis is a fastidious, slow-growing organism. Routine cultures will not detect it, and specific AFB cultures must be held for 6 to 8 weeks. AFB smears are notoriously insensitive.

Question 34

A 65-year-old female presents with acute knee pain, swelling, and erythema 3 weeks following a primary total knee arthroplasty. Aspiration of the joint is performed. According to the 2018 International Consensus Meeting (ICM) criteria, which of the following synovial fluid white blood cell (WBC) thresholds supports a diagnosis of an acute postoperative periprosthetic joint infection?

Explanation

In the acute postoperative period (less than 90 days), the threshold for diagnosing PJI is higher due to expected postoperative inflammation. The 2018 ICM cutoff for acute PJI is a WBC count greater than 10,000 cells/mcL with greater than 90% PMNs.

Question 35

What is the primary diagnostic advantage of utilizing next-generation sequencing (NGS) of synovial fluid in the evaluation of suspected periprosthetic joint infection (PJI)?

Explanation

Next-generation sequencing (NGS) targets the 16S rRNA gene for bacteria, offering high sensitivity for detecting organisms in culture-negative PJI or when patients have received prior antibiotics. However, it does not currently replace traditional phenotypic susceptibility testing.

Question 36

Aspiration of a persistently painful metal-on-metal total hip arthroplasty yields fluid with a WBC count of 4,500 cells/mcL and a polymorphonuclear (PMN) cell differential of 45%. Based on this synovial fluid profile, what is the most likely diagnosis?

Explanation

Metal-on-metal arthroplasties with ALTR typically present with elevated synovial WBC counts (often 3,000 to 10,000) but a remarkably low PMN percentage (usually less than 50%). This macrophage predominance helps differentiate ALTR from a chronic PJI, which is PMN-predominant.

Question 37

When evaluating a painful shoulder arthroplasty for a suspected Cutibacterium acnes infection, which of the following statements regarding the diagnostic workup is most accurate?

Explanation

Cutibacterium acnes is a slow-growing, Gram-positive, anaerobic rod frequently responsible for shoulder PJI. Cultures must be held for at least 14 days to minimize false negatives, and systemic inflammatory markers (ESR/CRP) are frequently normal.

Question 38

A 5-year-old boy presents with a temperature of 38.8 C, refusal to bear weight on the right leg, an ESR of 55 mm/hr, and a peripheral WBC of 14,000 cells/mcL. According to the Kocher criteria, what is the most appropriate next step in management?

Explanation

This patient meets 4 out of 4 Kocher criteria, predicting a 99% probability of septic arthritis. The next most appropriate step is an ultrasound-guided hip aspiration to confirm the diagnosis and secure cultures prior to surgical intervention.

Question 39

During the evaluation of a suspected periprosthetic joint infection, a point-of-care leukocyte esterase strip test is utilized. The aspirated synovial fluid is grossly bloody. What is the recommended technique to properly utilize the leukocyte esterase strip in this scenario?

Explanation

Grossly bloody synovial fluid visually obscures the colorimetric change on leukocyte esterase strips, rendering them uninterpretable. Centrifuging the sample and testing the clear supernatant effectively restores the diagnostic accuracy of the test.

Question 40

Synovial fluid calprotectin has recently gained attention in the workup of periprosthetic joint infection (PJI). What is the primary clinical utility of this biomarker?

Explanation

Synovial fluid calprotectin is an antimicrobial peptide released by activated neutrophils. It has emerged as a rapid, inexpensive, and highly sensitive point-of-care biomarker for diagnosing PJI, performing comparably to alpha-defensin.

Question 41

A patient with a painful total knee arthroplasty has been taking oral cephalexin for a suspected urinary tract infection. You plan to aspirate the knee to rule out chronic PJI. According to major consensus guidelines, how long should antibiotics ideally be held prior to aspiration to optimize culture yield?

Explanation

Concurrent or recent antibiotic therapy suppresses bacterial growth and significantly increases the rate of culture-negative PJI. Systemic antibiotics should ideally be held for at least 14 days prior to obtaining diagnostic joint aspirations.

Question 42

Which of the following synovial fluid profiles is most characteristic of disseminated gonococcal infection of a native joint?

Explanation

Gonococcal septic arthritis typically presents with a moderately elevated synovial WBC count (30,000 to 50,000 cells/mcL), which is lower than non-gonococcal bacterial arthritis. Gram stains and standard synovial cultures are frequently negative, often requiring nucleic acid amplification testing (NAAT) from mucosal sites.

Question 43

While attempting to aspirate a suspected chronically infected total hip arthroplasty, you obtain a 'dry tap'. You decide to inject 10 mL of sterile normal saline and re-aspirate the fluid. Which of the following tests remains the most reliable diagnostic option when performed on this lavage fluid?

Explanation

Injecting normal saline to lavage a dry joint significantly dilutes the returned fluid, rendering cell counts, PMN percentages, and biomarker concentrations inaccurate. However, microbiological culture of the lavage fluid remains valid and is a crucial diagnostic step.

Question 44

According to the 2018 International Consensus Meeting (ICM) criteria for periprosthetic joint infection, what is the role of the serum D-dimer test?

Explanation

The 2018 ICM criteria include serum D-dimer as a minor diagnostic biomarker for PJI. Evidence demonstrates that it offers better sensitivity and specificity than traditional markers like ESR for detecting chronic PJI.

Question 45

When processing explanted orthopedic hardware, sonication of the components is utilized to improve microbiological yield. By what mechanism does sonication primarily enhance bacterial detection?

Explanation

Sonication involves applying low-frequency ultrasound to explanted hardware in a fluid bath. This physical process disrupts the protective biofilm, dislodging sessile bacteria into a planktonic state, thereby significantly increasing culture sensitivity.

Question 46

Aspiration of a swollen, erythematous first metatarsophalangeal joint yields cloudy fluid. Microscopy reveals negatively birefringent, needle-shaped crystals, and the WBC count is 65,000 cells/mcL. What is the most appropriate next step regarding the synovial fluid analysis?

Explanation

The identification of monosodium urate crystals establishes a diagnosis of gout, but it does not rule out a concomitant septic arthritis. Because crystal arthropathy and joint infection can coexist, especially with WBC counts exceeding 50,000, Gram stain and culture must always be obtained.

Question 47

A 65-year-old female is suspected of having a culture-negative periprosthetic joint infection (PJI) of the knee. Standard cultures are negative at 7 days. Which of the following represents the primary advantage of Next-Generation Sequencing (NGS) over standard culture in this scenario?

Explanation

Next-Generation Sequencing (NGS) detects the DNA of both viable and non-viable bacteria. This makes it highly sensitive for diagnosing culture-negative PJI, particularly when the patient has recently received antibiotics.

Question 48

A surgeon uses a leukocyte esterase (LE) colorimetric strip to evaluate a knee aspirate for suspected PJI. The aspirate is grossly bloody. What is the most significant limitation of the LE strip test in this specific scenario?

Explanation

High concentrations of red blood cells or tissue debris in synovial fluid obscure the colorimetric reading of the LE strip. Centrifugation of the fluid is often required prior to testing to obtain an interpretable result.

Question 49

A patient is referred with a painful total knee arthroplasty while currently taking oral cephalexin. To optimize the diagnostic yield of a synovial fluid culture, for what minimum duration should the antibiotics be discontinued prior to joint aspiration?

Explanation

The 2018 International Consensus Meeting (ICM) recommends an antibiotic holiday of at least 14 days prior to obtaining synovial cultures. This minimizes suppression of bacterial growth and maximizes the chances of identifying the causative organism.

Question 50

A knee aspiration for a suspected PJI yields a grossly bloody sample. Synovial fluid analysis reports 50,000 RBCs/mcL and 3,500 WBCs/mcL. What is the approximate adjusted WBC count to account for peripheral blood contamination?

Explanation

The standard correction for a bloody aspirate is subtracting 1 WBC for every 500 RBCs to account for peripheral blood. Subtracting 100 WBCs (50,000 divided by 500) from the initial 3,500 yields an adjusted count of 3,400 WBCs/mcL.

Question 51

A 72-year-old male presents with a swollen, painful knee 3 weeks after an uncomplicated primary total knee arthroplasty. Which of the following synovial fluid WBC thresholds is most appropriate for diagnosing an acute PJI in this early postoperative period?

Explanation

In the acute postoperative period (within 6 weeks of surgery), normal postoperative inflammation is still present. Therefore, the threshold for diagnosing PJI is elevated to a synovial WBC count of > 10,000 cells/mcL.

Question 52

A patient is undergoing aspiration and tissue biopsy prior to revision of a reverse total shoulder arthroplasty for suspected Cutibacterium acnes infection. How many days should the cultures be held to ensure optimal detection of this specific organism?

Explanation

Cutibacterium acnes is a slow-growing, anaerobic Gram-positive bacillus common in shoulder PJI. Protocols recommend holding these cultures for 14 days to maximize detection of this indolent pathogen.

Question 53

Which of the following best describes the primary mechanism by which sonication of explanted orthopedic hardware improves the diagnostic yield for periprosthetic joint infection?

Explanation

Sonication uses low-frequency ultrasound to physically disrupt the extracellular polymeric substance of the biofilm on explanted hardware. This releases the sessile bacteria into the surrounding fluid, significantly improving culture yields.

Question 54

Alpha-defensin has emerged as a reliable synovial biomarker for PJI. Which of the following cells is the primary source of alpha-defensin in an infected joint space?

Explanation

Alpha-defensin is a naturally occurring antimicrobial peptide released by activated neutrophils in response to pathogens. Its presence in synovial fluid strongly correlates with a neutrophil-driven response to infection.

Question 55

During a fluoroscopy-guided aspiration of a painful total hip arthroplasty, no fluid is obtained initially (a "dry tap"). Which of the following is the most appropriate next step to evaluate for PJI?

Explanation

In the event of a "dry tap," instilling a small volume of sterile, non-bacteriostatic saline allows for fluid retrieval for culture. While cell counts will be diluted and unreliable, cultures and alpha-defensin assays may still be diagnostic.

Question 56

Which of the following systemic serum biomarkers has been identified as highly sensitive and specific for the diagnosis of PJI, performing comparably to or better than ESR and CRP in recent consensus literature?

Explanation

Serum D-dimer has been validated as a highly promising biomarker for PJI. Multiple studies show it outperforms traditional inflammatory markers like ESR and CRP in both sensitivity and specificity for diagnosing periprosthetic joint infection.

Question 57

According to the ICM criteria, positive histological analysis of periprosthetic tissue is a minor criterion for PJI. What is the specific cellular threshold required to meet this criterion?

Explanation

The established histological criterion for PJI is the presence of greater than 5 polymorphonuclear neutrophils (PMNs) per high-power field in 5 distinct tissue fields at 400x magnification.

Question 58

Synovial fluid C-reactive protein (CRP) has been increasingly utilized in the diagnostic workup for PJI. Which of the following statements is most accurate regarding synovial CRP?

Explanation

Synovial fluid CRP is an excellent localized marker for PJI that offers better sensitivity and specificity than systemic serum CRP. It is rapidly becoming a standard test when evaluating a painful arthroplasty.

Question 59

When performing an anterior ultrasound-guided aspiration of a native hip for suspected septic arthritis, the needle trajectory must carefully avoid which critical structures located medially?

Explanation

The femoral neurovascular bundle (femoral vein, artery, and nerve) lies medial to the standard anterior hip aspiration pathway. Ultrasound guidance safely directs the needle lateral to these structures into the anterior joint recess.

Question 60

A 55-year-old male presents with an acute, swollen knee. Aspiration yields cloudy fluid with 65,000 WBCs/mcL and intracellular monosodium urate crystals. Gram stain is negative. What is the most appropriate management plan?

Explanation

The presence of crystals confirms gout but does not definitive rule out concomitant septic arthritis. The safest management is to treat the acute flare while observing the patient and awaiting final synovial fluid culture results.

Question 61

Following an uncomplicated total joint arthroplasty, at what time point does the serum C-reactive protein (CRP) typically peak, and when does it generally return to its normal baseline?

Explanation

The normal postoperative kinetic profile for serum CRP demonstrates a peak at day 2 or 3 postoperatively, followed by a steady decline to normal baseline levels within approximately 21 days.

Question 62

A patient is undergoing workup for an indolent, chronic total knee arthroplasty infection. Routine bacterial and fungal cultures are negative, and a mycobacterial infection is strongly suspected. What specific culture medium must be requested?

Explanation

Lowenstein-Jensen medium is the specialized solid lipid-based medium required for the isolation and growth of mycobacteria. Sabouraud dextrose is for fungi, and Thayer-Martin is for Neisseria.

Question 63

Synovial fluid calprotectin has emerged as a rapid biomarker for PJI. Which of the following accurately describes its primary clinical utility?

Explanation

Calprotectin is a major protein found in the cytosol of neutrophils. A rapid point-of-care calprotectin lateral flow assay strongly correlates with the synovial neutrophil count, providing a highly sensitive and specific immediate marker for PJI.

Question 64

The Kocher criteria are utilized to risk-stratify children presenting with an irritable hip. Which of the following is NOT one of the classic four Kocher criteria used to differentiate septic arthritis from transient synovitis?

Explanation

The classic four Kocher criteria are non-weight-bearing, ESR > 40, WBC > 12,000, and Temp > 38.5 C. Although CRP > 2.0 mg/dL was later identified as a strong independent predictor by Caird et al., it is not one of the original four Kocher criteria.

Question 65

In the setting of diagnosing periprosthetic joint infection, how does the diagnostic performance of serum procalcitonin generally compare to that of serum C-reactive protein (CRP)?

Explanation

Serum procalcitonin is highly specific for systemic bacterial infections but demonstrates significantly lower sensitivity than CRP for localized infections like PJI. It is not currently recommended as a first-line screening test for PJI.

Question 66

Although the synovial fluid alpha-defensin test is highly accurate for diagnosing PJI, false positives can occasionally occur. Which of the following non-infectious conditions is most likely to produce a false-positive alpha-defensin test?

Explanation

Severe adverse local tissue reactions (ALTR) or metallosis can cause an intense local inflammatory response that interferes with lateral flow assays. This is a recognized, albeit rare, cause of a false-positive alpha-defensin test.

Question 67

A 72-year-old female with a painful total hip arthroplasty has a high clinical suspicion of periprosthetic joint infection (PJI). Standard cultures at 14 days are negative. What is the primary advantage of next-generation sequencing (NGS) of synovial fluid in this scenario?

Explanation

Next-generation sequencing (NGS) is highly sensitive and can identify pathogen DNA in culture-negative cases, especially when prior antibiotics were given. However, its specificity is lower due to potential contamination or the detection of non-viable organisms.

Question 68

You attempt a knee aspiration for suspected PJI but obtain a "dry tap." What is the most appropriate next step to obtain a fluid sample for diagnostic testing?

Explanation

Injecting non-bacteriostatic sterile normal saline and re-aspirating (lavage) is recommended for a dry tap. Bacteriostatic saline contains preservatives that inhibit bacterial growth and can lead to false-negative cultures.

Question 69

According to the 2018 International Consensus Meeting (ICM) criteria, which of the following serum markers has been newly introduced as a minor criterion for diagnosing PJI?

Explanation

Serum D-dimer was included in the 2018 ICM criteria as a valuable serum biomarker for PJI. It has shown comparable sensitivity and specificity to standard markers like ESR and CRP.

Question 70

A synovial fluid sample from a symptomatic TKA is placed on a leukocyte esterase (LE) strip, which reads "++". What is the significance of this test in the context of PJI?

Explanation

Leukocyte esterase is a rapid, inexpensive test that detects neutrophil activity and acts as a minor criterion for PJI. However, grossly bloody aspirates can obscure the color change on the test strip, limiting its utility in those instances.

Question 71

A 45-year-old male presents with an acutely swollen native knee. Synovial fluid analysis reveals an elevated WBC count. Above what threshold is native joint septic arthritis highly suspected?

Explanation

In native joints, a synovial WBC count greater than 50,000 cells/uL with >90% polymorphonuclear cells is highly suggestive of septic arthritis. Lower thresholds (e.g., 3,000 cells/uL) are typically used for chronic periprosthetic joint infections.

Question 72

A 55-year-old male presents with stiffness and vague pain 1 year after a total shoulder arthroplasty. Aspiration is performed. Given the most common indolent organism in this location, how long should the cultures be held?

Explanation

Cutibacterium acnes is a slow-growing anaerobe commonly responsible for shoulder PJI. Cultures should be held for a minimum of 14 days to minimize the risk of false-negative results.

Question 73

A patient with suspected PJI was started on empiric oral antibiotics by their primary care provider. The antibiotics were stopped yesterday. To optimize the yield of a synovial fluid aspiration for culture, how long should you wait before performing the aspiration?

Explanation

To reduce the risk of false-negative cultures, it is recommended to discontinue antibiotic therapy for at least 14 days prior to obtaining synovial fluid or tissue cultures for PJI diagnosis.

Question 74

Which of the following statements regarding synovial fluid C-reactive protein (CRP) in the diagnosis of PJI is most accurate?

Explanation

Synovial fluid CRP has been shown to be highly accurate for diagnosing PJI, providing comparable sensitivity and specificity to alpha-defensin. Furthermore, it is less affected by systemic inflammation than serum CRP.

Question 75

A patient presents with a culture-negative PJI of the hip and a history of prolonged antibiotic use. You suspect a fungal infection. Which specific synovial fluid test is most appropriate to aid in diagnosing a Candida PJI?

Explanation

1,3-beta-D-glucan is a component of the fungal cell wall, and its presence in synovial fluid or serum can be a useful biomarker for diagnosing invasive fungal infections, including Candida PJI.

Question 76

According to the 2018 ICM, how do the diagnostic thresholds for synovial fluid WBC count and PMN percentage in an acute PJI (within 90 days of surgery) compare to those for chronic PJI?

Explanation

The 2018 ICM thresholds for acute PJI (e.g., >10,000 cells/uL and >90% PMN) are significantly higher than those for chronic PJI (>3,000 cells/uL and >80% PMN) to account for normal post-surgical inflammation.

Question 77

Synovial fluid calprotectin has emerged as a biomarker for PJI. It is primarily released by which of the following cell types during a bacterial infection?

Explanation

Calprotectin is a calcium-binding protein complex predominantly found in the cytoplasm of neutrophils. It is released during neutrophil activation or death, making it a reliable marker of acute inflammation.

Question 78

A patient with a metal-on-metal THA presents with pain and a large joint effusion. Aspiration yields thick, opaque fluid. Synovial WBC is 1,200 cells/uL with 90% monocytes/macrophages. Cultures are pending. Which of the following diagnoses is most likely?

Explanation

Adverse local tissue reactions (ALTR) often present with massive effusions and a low synovial WBC count predominantly composed of macrophages/monocytes, distinguishing it from the neutrophil-predominant fluid of PJI.

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Orthopedic Hyperguide: Advanced MCQs on Joint Infection Diagnosis & Aspiration

Key Takeaway

Orthopedic Hyperguide: Advanced MCQs on Joint Infection Diagnosis & Aspiration

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