Orthopedic Board Review MCQs: Reconstruction & Infection

Question 1

A 68-year-old male with a metal-on-metal total hip arthroplasty (THA) presents with a new onset of groin pain and swelling. Blood tests reveal elevated serum cobalt and chromium ions. A joint aspiration is performed, and synovial fluid analysis shows a positive alpha-defensin test but negative bacterial cultures. Which of the following is the most likely cause of a false-positive alpha-defensin result in this clinical scenario?

Explanation

Alpha-defensin is an antimicrobial peptide released by neutrophils and is a highly sensitive and specific biomarker for periprosthetic joint infection (PJI). However, false-positive results can occur in the setting of metallosis or adverse local tissue reaction (ALTR) associated with metal-on-metal implants or mechanically failing components. The severe local inflammatory response and extensive tissue necrosis in ALTR can lead to neutrophil recruitment and subsequent alpha-defensin release in the absence of an infectious organism.

Question 2

A 72-year-old female undergoes a debridement, antibiotics, and implant retention (DAIR) procedure for an acute hematogenous periprosthetic joint infection of her total knee arthroplasty. Cultures yield methicillin-susceptible Staphylococcus aureus (MSSA). According to current infectious disease guidelines, what is the optimal timing for the initiation of adjunctive Rifampin therapy?

Explanation

Rifampin is highly efficacious against staphylococcal biofilms. However, staphylococci can rapidly develop resistance to Rifampin if it is used as monotherapy or if it is initiated when the bacterial burden is very high (such as during active bacteremia or immediately pre/post-op before source control is achieved). Current guidelines recommend delaying the initiation of Rifampin for 3 to 5 days after the start of active intravenous therapy and surgical debridement. This reduces the bacterial burden and minimizes the risk of rapid resistance development.

Question 3

According to the 2018 International Consensus Meeting (ICM) criteria for Periprosthetic Joint Infection, which of the following is considered a 'Major Criterion' that definitively diagnoses a PJI without the need for further scoring?

Explanation

Under the 2018 ICM criteria for PJI, there are two major criteria that definitively establish the diagnosis of infection: 1) Two positive periprosthetic cultures with phenotypically identical organisms, and 2) A sinus tract communicating with the joint. The other options (elevated synovial WBC, elevated synovial CRP, positive leukocyte esterase, and a single positive culture) are considered minor criteria that contribute points toward an aggregate diagnostic score.

Question 4

A surgeon is preparing antibiotic-loaded bone cement (ALBC) to create a temporary articulating spacer for a two-stage exchange of an infected total knee arthroplasty. To maximize the elution profile of the antibiotics, which of the following preparation techniques is most appropriate?

Explanation

To maximize antibiotic elution from a PMMA spacer, the cement should be highly porous. This is best achieved by hand-mixing (non-vacuum mixing) the cement and adding high doses of antibiotics (typically >10% of the cement weight, e.g., 4 to 8 grams of antibiotic per 40-gram bag of PMMA). Vacuum mixing decreases porosity and improves mechanical strength, which is desired for definitive fixation but counterproductive for a temporary spacer where maximum antibiotic elution is the primary goal.

Question 5

Which of the following is considered an absolute contraindication for attempting a debridement, antibiotics, and implant retention (DAIR) procedure in a patient presenting with an acute periprosthetic joint infection?

Explanation

A loose prosthesis is an absolute contraindication for a DAIR procedure. Successful eradication of infection with implant retention requires components that are stably fixed to the bone. If a component is loose, the interface between the implant and bone is compromised, allowing biofilm and purulence to harbor in areas inaccessible to debridement, necessitating implant removal (1-stage or 2-stage exchange). Symptom duration < 3-4 weeks is an indication for DAIR, not a contraindication.

Question 6

A 62-year-old male undergoes revision of a total shoulder arthroplasty for presumed aseptic loosening. Intraoperative tissue cultures are obtained. On postoperative day 12, the microbiology lab reports growth of Cutibacterium acnes. Which of the following best describes this organism and its management?

Explanation

Cutibacterium acnes (formerly Propionibacterium acnes) is a slow-growing, anaerobic to microaerophilic gram-positive rod commonly found on the skin, particularly around the shoulder and chest. It is a frequent cause of indolent, low-grade periprosthetic joint infections, especially in shoulder arthroplasty. Because of its slow growth, cultures must often be held for up to 14 days. It is generally highly susceptible to Penicillin and Ceftriaxone, but its ability to form biofilms on implants makes surgical intervention (such as revision) necessary.

Question 7

A 70-year-old female is diagnosed with an Enterococcus faecalis periprosthetic joint infection of her total hip arthroplasty. Which of the following is a hallmark characteristic of this organism that complicates systemic antibiotic therapy?

Explanation

Enterococci, including E. faecalis, have intrinsic resistance to all cephalosporins, as well as relatively low susceptibility to many other beta-lactams due to the expression of low-affinity penicillin-binding proteins (PBPs). They are notoriously difficult to treat in PJI and often require combination therapy (such as Ampicillin + Ceftriaxone or an aminoglycoside) to achieve synergistic bactericidal activity, or agents like Linezolid or Daptomycin. They are facultative anaerobes and grow readily on standard media.

Question 8

A surgeon is considering a single-stage (one-stage) exchange arthroplasty for a patient with an infected total knee arthroplasty. According to international consensus guidelines, which of the following is a strict contraindication to proceeding with a one-stage exchange?

Explanation

A single-stage exchange relies on a meticulous debridement and immediate reimplantation. Contraindications include unknown or culture-negative infections (where targeted antibiotic therapy cannot be planned), highly resistant organisms (where no effective oral/IV suppressive or targeted therapy exists), and severely compromised soft tissues or a sinus tract that precludes adequate primary closure and healthy tissue coverage. Well-controlled comorbidities and known susceptible organisms are prerequisites for a successful one-stage procedure.

Question 9

A 65-year-old male is 3 weeks status post primary total knee arthroplasty and presents with increased pain, swelling, and erythema. A joint aspiration is performed. According to recent literature and consensus guidelines, which of the following synovial fluid white blood cell (WBC) thresholds is most appropriate to suggest an acute periprosthetic joint infection in this early post-operative period?

Explanation

In the acute post-operative period (typically defined as within the first 6 weeks following surgery), the baseline synovial fluid WBC count is naturally elevated due to surgical trauma and hematoma. The traditional threshold used to diagnose a chronic PJI (>3,000 cells/µL) is inadequate in this setting. Literature has established that a cutoff of >10,000 cells/µL (with >90% PMNs) is highly suggestive of an acute PJI in the early post-operative period. Some newer studies suggest a threshold as high as 27,800 cells/µL, but >10,000 is the classic threshold to distinguish from normal post-op inflammation.

Question 10

A patient with a culture-proven Candida albicans periprosthetic joint infection is planned for a two-stage exchange arthroplasty. Which of the following antifungal agents is most appropriate to incorporate into the polymethylmethacrylate (PMMA) cement spacer due to its thermal stability and proven elution properties?

Explanation

Amphotericin B (particularly in its liposomal or deoxycholate forms) and Voriconazole are the antifungal agents most commonly added to PMMA cement spacers for fungal PJIs. They possess the necessary thermal stability to withstand the exothermic reaction of PMMA polymerization and have demonstrated effective elution profiles. Fluconazole, while systemically effective, has poor thermal stability and highly variable elution from PMMA.

Question 11

During pre-operative nutritional optimization for a patient scheduled for revision arthroplasty, the surgeon orders serum biomarkers to assess acute changes in nutritional status. Which of the following markers has the shortest half-life (approximately 2 to 3 days) and is best suited for monitoring short-term nutritional improvements?

Explanation

Prealbumin (also known as transthyretin) has a half-life of approximately 2 to 3 days, making it a highly sensitive marker for assessing acute changes in nutritional status and the efficacy of nutritional supplementation. In contrast, serum albumin has a half-life of about 21 days and transferrin has a half-life of about 8 days, making them better indicators of chronic malnutrition but poor markers for short-term recovery.

Question 12

During the second stage of a revision total knee arthroplasty for a prior infection, the surgeon evaluates the joint spaces using trial spacer blocks. The knee is noted to be unacceptably tight in flexion but excessively loose in extension. Which of the following surgical adjustments is most appropriate to balance the gaps?

Explanation

Gap balancing in revision TKA is critical. If the knee is tight in flexion, the flexion gap must be increased. This is done by downsizing the femoral component, which translates the posterior condyles anteriorly. If the knee is loose in extension, the extension gap must be decreased. This is achieved by adding distal femoral augments, which translates the femoral component distally. Therefore, downsizing the femur and adding distal augments addresses both imbalances simultaneously.

Question 13

A 40-year-old female presents with a chronic, culture-negative periprosthetic joint infection. Synovial fluid is sent for Next-Generation Sequencing (NGS). Which of the following molecular targets is primarily utilized by NGS to identify bacterial species in this setting?

Explanation

Next-Generation Sequencing (NGS) for bacterial identification in culture-negative PJI primarily targets the 16S ribosomal RNA (rRNA) gene. The 16S rRNA gene is highly conserved among all bacteria but contains hypervariable regions that are species-specific, allowing for precise identification. The 18S rRNA gene is targeted for fungal identification. The mecA and vanA genes are markers of antibiotic resistance (MRSA and VRE, respectively), not broad species identification.

Question 14

Staphylococcal species are the most common pathogens in periprosthetic joint infections, largely due to their ability to form resilient biofilms. Which phase of biofilm formation is critically mediated by the synthesis of polysaccharide intercellular adhesin (PIA) via the icaADBC operon?

Explanation

Biofilm formation occurs in multiple stages: 1) Initial attachment, 2) Irreversible adherence (mediated by MSCRAMMs), 3) Accumulation and maturation, and 4) Detachment. The accumulation and maturation phase is characterized by cellular proliferation and the production of an extracellular polymeric substance (EPS) matrix. In staphylococci, a major component of this matrix is polysaccharide intercellular adhesin (PIA), which is synthesized by the products of the icaADBC operon.

Question 15

A patient is identified as a nasal carrier of methicillin-resistant Staphylococcus aureus (MRSA) during routine preoperative screening for a total hip arthroplasty. According to evidence-based protocols, which of the following is the most effective decolonization regimen to reduce the risk of surgical site infection?

Explanation

The gold standard for MRSA decolonization prior to elective total joint arthroplasty is a 5-day regimen consisting of intranasal 2% mupirocin ointment applied twice daily coupled with daily bathing using chlorhexidine gluconate. This protocol has been shown in numerous studies to significantly decrease the MRSA carrier burden and reduce the incidence of postoperative periprosthetic joint infections.

Question 16

A 68-year-old male undergoes the second stage of a two-stage exchange arthroplasty for a previously infected total knee. Post-operative cultures remain negative, and the patient recovers well. Based on recent multi-center randomized controlled trials (such as the POET trial), what is the recommended duration of extended oral antibiotic prophylaxis to minimize the risk of reinfection?

Explanation

Recent evidence, prominently including the Prophylactic Oral Antibiotics in Two-Stage Exchange (POET) trial, has demonstrated that 90 days (3 months) of extended oral antibiotic prophylaxis following the reimplantation stage of a two-stage exchange arthroplasty significantly reduces the rate of reinfection compared to patients who receive only standard immediate postoperative IV antibiotics. Lifelong suppression is reserved for retained implants in medically unfit patients.

Question 17

When evaluating a patient with a suspected periprosthetic joint infection, an implant sonication protocol is utilized on the explanted prosthesis. What is the primary mechanism by which sonication improves microbiological yield compared to standard periprosthetic tissue cultures?

Explanation

Implant sonication involves placing the explanted prosthesis in a sterile fluid and applying low-frequency ultrasound. The acoustic cavitation physically disrupts and dislodges the bacterial biofilm from the surface of the implant, releasing the bacteria into a planktonic state in the fluid. This fluid is then cultured, significantly increasing the sensitivity and microbiological yield compared to standard tissue cultures, especially in patients who have received prior antibiotic therapy.

Question 18

Which of the following intraoperative factors is most strongly associated with the failure of a Debridement, Antibiotics, and Implant Retention (DAIR) procedure for an acute periprosthetic joint infection of a total knee arthroplasty?

Explanation

In a DAIR procedure, retention of the modular components (such as failing to exchange the polyethylene liner in a TKA) is a well-established, independent risk factor for failure. The modular interfaces harbor biofilm that cannot be adequately debrided unless the components are disassembled and exchanged. Therefore, exchanging the modular liner and thoroughly cleaning the baseplate is a critical step in maximizing the success of a DAIR.

Question 19

A 79-year-old male with severe cardiopulmonary comorbidities presents with a chronic periprosthetic joint infection of his total hip arthroplasty. The organism is identified as a multi-drug susceptible Staphylococcus epidermidis. The implant is radiographically well-fixed, and the patient is deemed medically unfit for revision surgery. He is placed on chronic suppressive oral antibiotic therapy. What is the primary clinical endpoint of this treatment strategy?

Explanation

Chronic suppressive antibiotic therapy is a palliative strategy used when curative surgery (like a 2-stage exchange) is contraindicated due to unacceptable surgical risk. The goal of suppressive therapy is NOT the eradication of the infection or biofilm, which is biologically impossible without removing the hardware. Rather, the goal is to control clinical symptoms (pain, swelling), prevent systemic dissemination (sepsis), and halt progressive periprosthetic osteolysis and implant loosening.

Question 20

A patient with a culture-negative PJI is found to have acid-fast bacilli on a specialized synovial fluid smear. Cultures eventually grow Mycobacterium fortuitum. Which of the following statements most accurately characterizes this infection?

Explanation

Mycobacterium fortuitum is a non-tuberculous, 'rapid-growing' mycobacterium. Despite being termed 'rapid-growing' (colonies appear in < 7 days on specialized media, unlike M. tuberculosis which takes weeks), it is notoriously difficult to eradicate from prosthetic joints. It typically requires removal of the implants (two-stage exchange) combined with prolonged, tailored multidrug antimicrobial therapy. It is generally resistant to standard first-line anti-TB drugs like Isoniazid and Pyrazinamide, and can occur in immunocompetent hosts.

Question 21

A 65-year-old male presents with a painful total knee arthroplasty. A joint aspirate is grossly bloody. Which of the following rapid synovial fluid tests for periprosthetic joint infection is most likely to be unreliable or uninterpretable due to the presence of significant erythrocytes?

Explanation

The leukocyte esterase test utilizes a colorimetric strip that can be difficult to interpret or yield false results when obscured by grossly bloody synovial fluid. Centrifugation may help, but heavy hemolysis still severely limits visual interpretation.

Question 22

A 60-year-old male undergoes revision reverse total shoulder arthroplasty for presumed aseptic loosening. On postoperative day 11, routine cultures grow Cutibacterium acnes. Which of the following best characterizes the typical histologic finding of the periprosthetic tissue in a C. acnes joint infection?

Explanation

Cutibacterium acnes PJI typically presents with a lack of acute inflammatory cells (neutrophils) on histology, instead showing a chronic mononuclear infiltrate or normal-appearing tissue. This makes traditional histological PMN criteria for PJI unreliable for diagnosing C. acnes.

Question 23

During a two-stage exchange for a periprosthetic joint infection, the surgeon prepares an articulating antibiotic-loaded polymethylmethacrylate (PMMA) spacer. To maximize the local elution of antibiotics, which of the following preparation techniques should be employed?

Explanation

Hand-mixing cement increases its porosity, which significantly enhances the surface area and subsequent elution profile of the incorporated antibiotics. Vacuum mixing decreases porosity and therefore decreases antibiotic elution.

Question 24

A 55-year-old female presents with acute onset knee pain and swelling 14 days after an uncomplicated primary total knee arthroplasty. Aspiration yields 65,000 WBCs/mcL with 95% neutrophils. Which of the following is an absolute contraindication to performing a Debridement, Antibiotics, and Implant Retention (DAIR) procedure with polyethylene exchange?

Explanation

The presence of a sinus tract communicating with the prosthesis is an absolute contraindication to a DAIR procedure, as it represents a chronic infection with established mature biofilm. In such cases, component removal via a single- or two-stage exchange is required.

Question 25

A 62-year-old female with a total hip arthroplasty utilizing a large diameter cobalt-chromium head on a titanium stem presents with thigh pain. MRI with MARS reveals a large cystic mass adjacent to the hip. What biomechanical factor most increases the risk of mechanically assisted crevice corrosion (MACC) at the head-neck junction?

Explanation

Mechanically assisted crevice corrosion (trunnionosis) is exacerbated by increased bending moments at the head-neck junction. This increased stress typically occurs with increased femoral offset (longer head-neck tapers) and the use of larger diameter metal heads.

Question 26

Based on the results of the Oral Versus Intravenous Antibiotics for Bone and Joint Infection (OVIVA) trial, how does early transition to oral antibiotics compare to standard 6-week intravenous therapy in the management of periprosthetic joint infection?

Explanation

The OVIVA trial demonstrated that a highly bioavailable oral antibiotic regimen is non-inferior to intravenous antibiotics for the treatment of bone and joint infections when evaluating treatment failure at 1 year. This paradigm shift supports early transition to oral therapy in appropriately selected patients.

Question 27

A 70-year-old diabetic male undergoes an aspiration of a painful TKA. Cultures grow Candida albicans. A two-stage exchange is planned. Which of the following is the most appropriate local antibiotic spacer strategy for this patient?

Explanation

Fungal periprosthetic joint infections require local antifungal delivery to effectively clear the joint space. Voriconazole and Amphotericin B maintain their stability and elute effectively from polymethylmethacrylate (PMMA) cement, unlike echinocandins.

Question 28

During a two-stage exchange for a chronic TKA periprosthetic joint infection, the surgeon notes massive femoral bone loss, severe collateral ligament insufficiency, and a deficient extensor mechanism. Which of the following is the most appropriate management regarding the antibiotic spacer?

Explanation

A static spacer is indicated in the setting of massive bone loss, severe collateral ligament incompetence, or a deficient extensor mechanism. An articulating spacer in this scenario risks dislocation, extensor mechanism damage, and further bone loss in a highly unstable joint.

Question 29

In revision total knee arthroplasty, isolated distal femoral augmentation is utilized to lower the joint line. What is the expected biomechanical effect of lowering the joint line on the flexion and extension gaps?

Explanation

Distal femoral augmentation adds material to the distal aspect of the femur, which directly tightens the extension gap. Because it does not alter the geometry of the posterior femoral condyles, it does not significantly affect the flexion gap.

Question 30

A 74-year-old female requires an acetabular revision for aseptic loosening. Radiographs demonstrate superior migration of the component >3 cm, a broken Kohler's line, and severe ischial osteolysis, representing a Paprosky Type IIIB defect. Which of the following reconstructive options is most appropriate?

Explanation

A Paprosky IIIB defect involves massive bone loss with pelvic discontinuity or severe structural compromise of the anterior and posterior columns. A cup-cage construct or custom triflange component is required to provide immediate rigid fixation and span the discontinuity.

Question 31

Bacteria residing within a mature biofilm on a prosthetic joint surface exhibit high tolerance to systemic antibiotics. Which of the following mechanisms best explains this phenotypic antibiotic resistance?

Explanation

Within a mature biofilm, deep-layer bacteria transition into a metabolically dormant, stationary phase. Since many antibiotics (like beta-lactams) rely on active cell wall synthesis and metabolic activity, these dormant 'persister' cells become highly tolerant to antimicrobial therapy.

Question 32

Highly cross-linked polyethylene (HXLPE) is widely used in total hip arthroplasty to reduce wear. After irradiation to create cross-links, the polyethylene is often subjected to either melting or annealing. Which of the following best describes the effect of remelting compared to annealing?

Explanation

Remelting polyethylene above its melting point eliminates free radicals, virtually stopping oxidation. However, it decreases crystallinity, which reduces yield strength and fatigue strength compared to annealing.

Question 33

A 65-year-old male presents with acute knee swelling and pain 4 weeks after a primary total knee arthroplasty. A joint aspiration is performed to rule out periprosthetic joint infection (PJI). According to current consensus guidelines, what synovial fluid white blood cell (WBC) count threshold is most suggestive of an acute PJI in this post-operative timeframe?

Explanation

In the acute post-operative period (typically defined as < 6 weeks), the normal inflammatory response causes elevated cell counts. The accepted synovial WBC threshold for diagnosing acute PJI is > 10,000 cells/µL, compared to > 3,000 cells/µL for chronic PJI.

Question 34

A 70-year-old female with long-standing ankylosing spondylitis and a completely fused lumbosacral spine is scheduled for a primary total hip arthroplasty. Her pelvis is fixed in a retroverted position. Which of the following component positioning strategies is most appropriate to minimize the risk of anterior impingement and posterior dislocation during sitting?

Explanation

A fused spine prevents the normal posterior pelvic tilt that accommodates hip flexion when sitting. To compensate and prevent anterior impingement with subsequent posterior dislocation, the acetabular cup must be placed in increased anteversion.

Question 35

A 68-year-old male develops an acute hematogenous periprosthetic joint infection of his total hip arthroplasty. He is considered for a debridement, antibiotics, and implant retention (DAIR) procedure. Which of the following factors is incorporated in the KLIC score and is most predictive of DAIR failure?

Explanation

The KLIC score predicts DAIR failure in early PJI. Its components include Kidney disease, Liver cirrhosis, Index surgery type (revision vs primary), Cemented prosthesis, and CRP > 115 mg/L.

Question 36

During a primary total knee arthroplasty, trial components are inserted. The knee demonstrates appropriate mediolateral stability but is found to be tight in extension and loose in flexion. Which of the following surgical adjustments is the best definitive option to balance this knee?

Explanation

A tight extension gap with a loose flexion gap indicates the distal femoral cut is insufficient relative to the posterior cut. Removing more distal femur will open the extension gap without affecting the flexion gap.

Question 37

A 62-year-old male with a conventional metal-on-polyethylene total hip arthroplasty presents with unexplained groin pain. Workup reveals an adverse local tissue reaction (ALTR) due to mechanically assisted crevice corrosion (MACC) at the head-neck junction. Which combination of implant characteristics confers the highest risk for this complication?

Explanation

Mechanically assisted crevice corrosion (trunnionosis) is driven by increased fretting and micromotion. The highest risk occurs with a large, heavy cobalt-chrome head on a high-offset titanium stem, which maximizes torsional forces at the taper junction.

Question 38

A patient undergoes revision total hip arthroplasty for a painful metal-on-metal bearing. Intraoperatively, extensive soft tissue necrosis and a large cystic mass are noted. Histological examination of the periprosthetic tissue demonstrates an aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL). This finding is characteristic of which type of immune response?

Explanation

ALVAL is characterized by perivascular lymphocytic infiltration, indicative of a Type IV delayed-type hypersensitivity response to metal wear debris, specifically cobalt and chromium ions.

Question 39

Fluoroscopic kinematic analysis of a patient with a posterior cruciate-retaining (CR) total knee arthroplasty demonstrates paradoxical anterior sliding of the femur on the tibia during deep flexion. This kinematic abnormality is most strongly associated with which of the following?

Explanation

In a CR TKA, an insufficient posterior tibial slope (or a tight PCL) can restrict normal posterior femoral rollback. This kinematic mismatch leads to paradoxical anterior translation of the femur on the tibia during flexion.

Question 40

A 70-year-old female undergoes a 2-stage revision for a Staphylococcus epidermidis periprosthetic joint infection. Her antibiotic regimen includes Rifampin, which is highly effective against biofilm-associated staphylococci. What is the mechanism of action of Rifampin?

Explanation

Rifampin is a potent bactericidal agent against staphylococci in biofilms because it easily penetrates the biofilm matrix. It exerts its effect by inhibiting bacterial DNA-dependent RNA polymerase, thereby suppressing RNA synthesis.

Question 41

When preparing antibiotic-loaded polymethylmethacrylate (PMMA) for a structural articulating spacer in a two-stage revision, what is the maximum recommended weight of antibiotic powder per 40-gram bag of cement to avoid catastrophic mechanical failure?

Explanation

For structural PMMA spacers, adding more than 4 grams (approx 10%) of antibiotics per 40-gram bag significantly compromises the mechanical compressive strength of the cement. Non-structural beads can accommodate higher ratios up to 20% without clinical consequence.

Question 42

A 65-year-old male presents with a Candida albicans periprosthetic joint infection of his total knee arthroplasty. If the surgeon plans a two-stage exchange using an antimicrobial-loaded spacer, which of the following antifungal agents is heat-stable and most appropriate for incorporation into the PMMA?

Explanation

Amphotericin B and voriconazole are heat-stable and can be safely incorporated into PMMA cement for local elution in fungal PJI. Echinocandins and many azoles degrade during the exothermic polymerization process.

Question 43

A 70-year-old female undergoes joint aspiration for a suspected PJI, showing elevated synovial WBC counts, but multiple cultures are negative at 14 days. Which of the following diagnostic technologies detects microbial cell-free DNA directly from the synovial fluid and is best utilized in this culture-negative scenario?

Explanation

Metagenomic next-generation sequencing (mNGS) detects microbial cell-free DNA directly from synovial fluid, offering high sensitivity for identifying pathogens in culture-negative PJI. MALDI-TOF requires a positive culture to identify the grown organism.

Question 44

A diabetic patient presents for a primary total hip arthroplasty. Her Hemoglobin A1c is 8.5%. The surgery is delayed for optimization. Which of the following laboratory markers best reflects her glycemic control over the preceding 2 to 3 weeks and is most useful to confirm recent improvement?

Explanation

Fructosamine reflects glycemic control over the preceding 2 to 3 weeks, making it highly useful for assessing short-term preoperative optimization in diabetic patients. Hemoglobin A1c reflects longer-term control over 2 to 3 months.

Question 45

During a revision total knee arthroplasty for suspected PJI, the explanted components are sent for sonication. What is the primary mechanism by which sonication improves the yield of microbiological cultures?

Explanation

Sonication uses low-frequency acoustic waves to disrupt the extracellular polymeric substance of the biofilm. This dislodges persister cells and planktonic bacteria from explanted hardware, significantly improving culture yield.

Question 46

A 55-year-old male presents with acute hematogenous PJI of a total hip arthroplasty. Which of the following is considered an absolute contraindication to attempting a Debridement, Antibiotics, and Implant Retention (DAIR) procedure?

Explanation

A loose implant is an absolute contraindication for a DAIR procedure because the infection cannot be cleared from the unstable interface. Successful DAIR requires well-fixed components and ideally a short symptom duration.

Question 47

A 72-year-old female presents with a loose cemented total hip arthroplasty and massive proximal osteolysis. Radiographs reveal absence of the medial calcar and extensive metaphyseal bone loss, with only 3 cm of intact diaphyseal bone at the isthmus. What is the most appropriate femoral component choice for revision?

Explanation

This describes a Paprosky 3B femoral defect, characterized by extensive metaphyseal and diaphyseal bone loss with less than 4 cm of intact diaphyseal isthmus. A modular fluted tapered stem relies on rigid distal diaphyseal fixation, making it the optimal choice.

Question 48

During a complex revision total hip arthroplasty, an extended trochanteric osteotomy (ETO) is performed to aid in cement removal. To ensure optimal osseous healing of the osteotomized fragment, preservation of which of the following muscle attachments is most critical for its vascular supply?

Explanation

The ETO fragment relies heavily on the vastus lateralis muscle pedicle and its blood supply from the first perforating branch of the profunda femoris. Preserving this attachment is critical to ensure osseous healing of the osteotomy site.

Question 49

A 62-year-old male with a primary total hip arthroplasty (titanium stem, cobalt-chromium head, highly cross-linked polyethylene liner) presents 5 years postoperatively with vague groin pain. Radiographs show well-fixed components. Aspiration yields sterile fluid with a macrophage predominance. Blood tests show elevated cobalt but normal chromium levels. What is the most likely diagnosis?

Explanation

Elevated serum cobalt with normal chromium in a metal-on-polyethylene THA indicates mechanically assisted crevice corrosion (trunnionosis) at the head-neck junction. This can cause adverse local tissue reactions (ALTR) presenting as pain and sterile effusions.

Question 50

A 68-year-old female with a history of a multi-level lumbar fusion (L2-Pelvis) is undergoing a primary total hip arthroplasty. How should the acetabular cup positioning be altered to prevent impingement and posterior dislocation?

Explanation

A fused lumbar spine reduces spinopelvic mobility, meaning the pelvis cannot naturally retrovert during sitting to increase acetabular anteversion. To prevent anterior impingement and posterior dislocation when sitting, the cup must be placed in greater anteversion and inclination than the traditional safe zone.

Question 51

When formulating an antibiotic-loaded polymethylmethacrylate (PMMA) spacer for a two-stage exchange arthroplasty, which of the following mixing techniques is recommended to maximize antibiotic elution?

Explanation

Hand-mixing PMMA cement in an open bowl at atmospheric pressure increases the porosity of the cement mantle. This high porosity is undesirable for structural long-term fixation but is necessary to maximize the elution of antibiotics in a temporary spacer.

Question 52

An 81-year-old, low-demand female sustains a Vancouver B3 periprosthetic femur fracture around a cemented stem. Radiographs demonstrate a loose stem with profound loss of proximal femoral bone stock extending into the subtrochanteric region. What is the most appropriate surgical management?

Explanation

A Vancouver B3 fracture is characterized by a loose stem and severely deficient proximal bone stock. In elderly, low-demand patients, a proximal femoral replacement provides immediate stability and allows for early, unrestricted weight-bearing.

Orthopedic Surgery Board Review MCQs: Arthroplasty, Trauma & Spine Part 255

Orthopedic Surgery Board Review MCQs: Adult Reconstruction & Infection | Part 7

Key Takeaway

Orthopedic Surgery Board Review MCQs: Adult Reconstruction & Infection | Part 7

Chapter Index