"Infections" — Practice

Question 1

Acute haematogenous osteomyelitis in children characteristically begins in the

Accepted Answer

B

Question 2

In chronic osteomyelitis, the sleeve of new bone formed by lifted periosteum around dead bone is called

Accepted Answer

B

Question 3

A 5-year-old refuses to walk. Temperature is 39 degrees Celsius, ESR 55 mm/hour, white cell count 15,000/mm3. What is the management?

Accepted Answer

All four Kocher predictors are present, giving a probability of septic arthritis of about 99.6 per cent. Manage as a septic hip: urgent aspiration under imaging guidance for Gram stain, cell count, glucose and culture, followed by surgical drainage and empirical antibiotics covering Staphylococcus aureus with attention to local methicillin resistance.

Waiting for culture before drainage allows enzymatic destruction of cartilage that cannot be reversed, and in the hip specifically raised intracapsular pressure can tamponade the retinacular vessels and infarct the femoral head.

Question 4

A 14-month-old with proximal femoral osteomyelitis develops a septic hip. Give the two anatomical reasons this occurred at this site and this age.

Accepted Answer

First, under about 18 months transphyseal vessels still cross the growth plate, so metaphyseal infection can spread directly into the epiphysis rather than being contained by the physis. After those vessels involute, spread tends to be subperiosteal instead.

Second, the proximal femoral metaphysis lies inside the hip capsule, so pus breaking through the metaphyseal cortex enters the joint directly. The proximal humerus, radial neck and distal lateral tibia share this arrangement, and the hip is the most dangerous because intracapsular pressure tamponades the retinacular vessels.

Question 5

A 40-year-old has 4 months of back pain, evening fever and weight loss. MRI shows destruction of two adjacent vertebral bodies with a relatively preserved intervening disc and a large paraspinal collection. What is the diagnosis, and why does the disc appearance help?

Accepted Answer

Spinal tuberculosis. The disease begins paradiscally in the vertebral body next to the endplate and spreads under the anterior longitudinal ligament to the adjacent vertebra.

The intervertebral disc is avascular and mycobacteria produce few proteolytic enzymes, so disc height is preserved relatively late while the bodies collapse. Pyogenic spondylodiscitis destroys the disc early and produces smaller collections. The large paraspinal collection is the cold abscess, which tracks along fascial planes and may present in the psoas sheath or groin.

Question 6

A total knee replacement becomes infected at 4 years with 3 weeks of pain and swelling. Culture grows Staphylococcus aureus reported fully sensitive. Why will six weeks of that sensitive antibiotic alone probably fail?

Accepted Answer

Because sensitivity is measured on planktonic bacteria growing in broth, whereas the organism on the implant is sessile within a mature biofilm. Sessile bacteria divide slowly, so agents that kill dividing cells work poorly, the extracellular matrix impedes penetration, and neutrophils cannot phagocytose an adherent film. Tolerance can reach hundreds of times the reported minimum inhibitory concentration.

Three weeks of symptoms at four years after implantation indicates a mature biofilm rather than an acute one, so treatment requires implant removal, typically a two-stage revision with an antibiotic-loaded cement spacer, rather than antibiotics with retention.

Question 7

Two patients have chronic tibial osteomyelitis. One is a healthy 16-year-old with a medullary focus. The other is a 62-year-old smoker with diabetes and peripheral vascular disease and a diffuse segmental lesion. Using Cierny-Mader, explain why their management differs so greatly.

Accepted Answer

The classification pairs anatomical type with host class precisely because both determine what surgery is reasonable. The adolescent is a type I medullary infection in a class A host: debridement of the canal leaves a stable bone in a patient who will heal, and cure is a realistic goal.

The older patient is a type IV diffuse infection, meaning segmental resection will leave the bone unstable and requiring reconstruction, in a class B host compromised both systemically by diabetes and smoking and locally by vascular disease. Healing capacity and soft-tissue cover are both poor. Realistic options range from staged reconstruction to long-term suppression, and if the treatment would be worse than the disease the patient is class C, where amputation or palliation may genuinely be the better choice.

Question 8

Two patients with spinal tuberculosis develop paraplegia. One presents with active disease, fever and a large paraspinal collection. The other presents 8 years after completing treatment with radiologically healed disease and a fixed kyphosis. Compare the mechanisms and prognoses.

Accepted Answer

The first has early-onset Pott paraplegia, occurring during active disease. The cord is compressed by abscess, granulation tissue or caseous material, all of which are soft and can resolve with chemotherapy and drainage, so the prognosis for neurological recovery is good.

The second has late-onset paraplegia arising from healed disease, where compression comes from a transverse bony ridge, dense fibrosis or progressive deformity at the gibbus. These are structural and do not regress with any drug, so recovery is far less likely and surgical decompression with correction of the deformity is required. The dividing question in both is simply whether the compressing material is capable of disappearing.

Question 9

A 6-year-old has 3 days of fever, refuses to bear weight, and is tender over the distal femoral metaphysis. Radiographs are reported as normal. Outline the diagnostic reasoning and the first three steps.

Accepted Answer

The normal radiograph is uninformative rather than reassuring, because visible lysis requires loss of roughly 30 to 40 per cent of bone mineral and takes 10 to 14 days to appear. The clinical picture of fever, refusal to bear weight and metaphyseal tenderness is acute haematogenous osteomyelitis until proved otherwise.

First, take blood cultures, which are positive in a substantial minority. Second, arrange MRI, which shows marrow oedema within days and also identifies subperiosteal collection and adjacent joint effusion, both of which change the operation. Third, aspirate or biopsy the metaphysis before antibiotics where the child is stable, since pre-antibiotic sampling gives the highest microbiological yield and the short delay is usually justified.

Practice — "Infections"

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