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OR3.1-3 | Musculoskeletal Infection — Practice Quiz
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A 7-year-old boy presents with 3 days of high fever, inability to bear weight on the right leg, and tenderness over the distal femur metaphysis. WBC is 18,000/mm³ and ESR is 65 mm/hr. Blood cultures are pending. Which organism is the most likely causative agent?
Correct. S. aureus is the most common organism in acute haematogenous osteomyelitis across all age groups. H. influenzae was previously important in children under 5 but is now rare due to vaccination.
Staphylococcus aureus is the most common causative organism of acute haematogenous osteomyelitis in all age groups beyond the neonatal period. The metaphysis of long bones is the most common site due to its rich sinusoidal blood supply and sluggish blood flow.
S. aureus remains the most common pathogen in acute osteomyelitis. H. influenzae is now rare post-vaccination; Pseudomonas is associated with puncture wounds through trainers; Group B Strep is important in neonates.
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A 6-year-old girl presents with fever, right hip pain, and holds the hip in flexion, abduction, and external rotation. She refuses passive internal rotation. WBC is 20,000/mm³, ESR 75 mm/hr, CRP 12 mg/dL, temperature 38.9°C. Ultrasound confirms a hip effusion. Which clinical decision tool best guides urgent surgical drainage?
Correct. Kocher criteria use WBC >12,000, ESR >40, temperature >38.5°C, and inability to bear weight. With all four positive, the predicted probability of septic arthritis approaches 99.6%, mandating urgent surgical drainage.
The Kocher criteria (WBC >12,000, ESR >40, fever >38.5°C, inability to weight-bear) are used to differentiate septic arthritis from transient synovitis. All four criteria being positive confers a >99% probability of septic arthritis requiring urgent arthrotomy and washout.
The Kocher criteria are specifically designed to differentiate septic arthritis of the hip from transient synovitis in children, helping determine urgency of surgical intervention.
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A 12-year-old boy presents with a 3-week history of dull, aching pain in the proximal tibia with minimal systemic upset. X-ray reveals a well-circumscribed, round radiolucent lesion in the metaphysis with sclerotic margins and no periosteal reaction. What is the most likely diagnosis?
Correct. Brodie's abscess is the classic presentation of subacute osteomyelitis — a walled-off metaphyseal abscess with sclerotic rim and minimal periosteal reaction, low-grade systemic symptoms, typically caused by S. aureus.
Brodie's abscess is the hallmark of subacute osteomyelitis, typically caused by S. aureus in a partially immune host. It appears as a well-defined metaphyseal lucency with sclerotic rim on X-ray, often in the proximal tibia, with minimal systemic inflammatory response.
The combination of subacute course, well-circumscribed sclerotic-rimmed metaphyseal lucency, and minimal systemic signs is pathognomonic of Brodie's abscess (subacute haematogenous osteomyelitis).
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A 45-year-old patient with chronic osteomyelitis of the tibia following an open fracture 5 years ago develops a discharging sinus. Plain radiograph shows a dense bony fragment surrounded by a lucent zone within a thickened cortex. The fragment represents which pathological structure?
Correct. A sequestrum is dead, avascular, infected bone that cannot be resorbed due to absent blood supply. It is the key pathological substrate of chronic osteomyelitis and must be removed (sequestrectomy) for cure.
A sequestrum is necrotic, avascular, infected bone that has separated from living bone in chronic osteomyelitis. It appears radiologically as a dense fragment surrounded by a radiolucent zone (the involucrum surrounds it). The sinus tract allows drainage but perpetuates the infection.
The sequestrum (dead bone), involucrum (new bone shell around it), and cloaca (opening through which pus discharges) are the three classic pathological features of chronic osteomyelitis.
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A 35-year-old HIV-positive man presents with 6 weeks of low back pain, mild fever, and weight loss. ESR is 80 mm/hr. MRI shows vertebral body destruction at L3-L4 with a large paravertebral abscess containing calcifications. Compared to pyogenic spondylodiscitis, which feature is more characteristic of spinal tuberculosis?
Correct. Pott's disease presents with an indolent, months-long course, large paravertebral cold abscess that may contain calcifications, relative preservation of disc space until late, multilevel involvement, and a risk of kyphotic gibbus deformity.
Spinal tuberculosis (Pott's disease) characteristically shows an indolent/insidious onset, relative disc preservation until late, large paravertebral cold abscess with calcifications, multilevel involvement, and kyphotic deformity (gibbus). Pyogenic spondylodiscitis has acute onset, early disc destruction, and smaller abscesses.
TB spondylitis is insidious (weeks to months) vs pyogenic spondylodiscitis which is acute (days). TB produces large cold abscesses with calcification; pyogenic produces smaller, hotter abscesses. Disc preservation is a key early TB feature.
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During a diagnostic joint aspiration of a swollen, hot right knee in a 25-year-old woman with suspected septic arthritis, synovial fluid is aspirated. Which finding on synovial fluid analysis is most consistent with septic arthritis rather than crystal arthropathy?
Correct. Septic arthritis produces a markedly inflammatory fluid with WBC typically >50,000–100,000/mm³, >75% neutrophils, and ideally a positive Gram stain. This demands immediate empirical antibiotics and surgical/arthroscopic drainage.
Septic arthritis synovial fluid characteristically shows WBC >50,000/mm³ (often >100,000) with >75% neutrophils and a positive Gram stain or culture. Crystal arthropathies also produce inflammatory fluids, but culture is negative and crystals are identified under polarised light.
Septic arthritis: WBC >50,000, >75% PMNs, positive Gram stain/culture. Gout: negatively birefringent needle-shaped (monosodium urate) crystals. Pseudogout: positively birefringent rhomboid (calcium pyrophosphate) crystals.
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A 50-year-old diabetic man with chronic osteomyelitis of the right distal tibia undergoes sequestrectomy and saucerisation. Which is the primary goal of saucerisation?
Correct. Saucerisation converts the closed infected cavity (with its poor antibiotic penetration and anaerobic environment) into an open saucer wound. This eliminates dead space, allows free drainage, and permits wound care with granulation from below.
Saucerisation involves converting an enclosed infected medullary cavity into an open saucer-shaped wound by removing the outer cortex and infected granulation tissue. This eliminates dead space, improves access for wound care, and promotes drainage, allowing granulation tissue to fill in from the base.
Saucerisation is a surgical technique for chronic osteomyelitis that eliminates the closed-cavity dead space by converting it to an open saucer wound, enabling secondary healing by granulation.
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A neonate (10 days old) presents with fever, irritability, and refusal to move the left arm. Examination shows pseudoparalysis of the limb. Ultrasound reveals a hip joint effusion. Which organism is the most likely causative agent in this age group?
Correct. Group B Streptococcus is the leading organism in neonatal bone and joint infections, reflecting peripartum or early neonatal acquisition. S. aureus is the other major pathogen. Multi-focal disease is characteristic of neonates.
In neonatal osteomyelitis/septic arthritis (under 3 months), Group B Streptococcus (Streptococcus agalactiae) and Staphylococcus aureus are the predominant organisms. Multi-focal involvement is common. The periosteum is loosely attached in neonates, allowing rapid spread. Immediate drainage is essential to protect the cartilaginous epiphysis.
Neonatal septic arthritis/osteomyelitis is predominantly caused by Group B Streptococcus and S. aureus. Kingella kingae is important in toddlers (6 months–3 years). Salmonella is associated with sickle cell disease.
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