"Musculoskeletal Imaging" — Practice

Question 1

Which radiographic feature best indicates a slow-growing, probably benign bone lesion?

Accepted Answer

A

Question 2

Lucent bands running perpendicular to the cortex in the pubic rami and femoral neck indicate

Accepted Answer

B

Question 3

A 15-year-old has a metaphyseal knee lesion with a wide zone of transition, sunburst periosteal reaction, a Codman triangle and cloud-like calcification. Diagnose and explain each feature.

Accepted Answer

Osteosarcoma, and each feature is a readout of speed and of what the tumour makes.

The wide zone of transition means the lesion is destroying bone faster than the bone can respond, so no sharp margin forms.

Sunburst reaction reflects periosteum lifted so rapidly that new bone is laid down along Sharpey fibres perpendicular to the cortex rather than in orderly layers parallel to it.

The Codman triangle is the ossified edge of that lifted periosteum, and it is not specific to osteosarcoma since aggressive infection and subperiosteal haemorrhage produce it too.

Cloud-like calcification indicates osteoid matrix, meaning the tumour is producing bone.

Site and age complete it: metaphysis around the knee in the second decade, when growth is fastest. MRI defines local extent and skip lesions, chest CT stages the lungs, and biopsy is planned by the team that will operate.

Question 4

A 55-year-old man has multiple punched-out lytic skull lesions but a normal bone scan. Explain.

Accepted Answer

Multiple myeloma, and the normal bone scan is a positive finding rather than a reassuring one.

Bone scintigraphy uses a technetium-labelled diphosphonate that adsorbs onto hydroxyapatite at sites of active bone formation, so it images osteoblastic activity rather than bone destruction directly. Most metastases provoke a surrounding osteoblastic reaction and therefore light up.

Myeloma is different. Malignant plasma cells activate osteoclasts while simultaneously suppressing osteoblasts, so lesions are purely lytic with almost no compensatory bone formation. The tracer has nothing to bind to, and the lesions are cold or invisible.

The practical consequence is that bone scan is the wrong test for myeloma. Skeletal survey has traditionally been used, and whole-body low-dose CT, MRI or PET-CT are now preferred because they are considerably more sensitive.

Question 5

A 12-year-old has fever, raised inflammatory markers and a diaphyseal permeative lesion with onion-skin periosteal reaction. The team starts antibiotics. Comment.

Accepted Answer

The diagnosis may be right, but Ewing sarcoma produces an almost identical picture and must be excluded before the patient is committed to treatment for infection.

Both produce fever, raised white cell count and inflammatory markers, a permeative destructive pattern and a lamellated periosteal reaction, and both favour the diaphysis in this age group. Ewing sarcoma additionally tends to produce a large soft tissue mass, better appreciated on MRI than on the radiograph.

The consequence of getting this wrong is serious. Patients with Ewing sarcoma treated as osteomyelitis lose months while the tumour progresses, and an ill-placed incision for presumed drainage can compromise later limb salvage.

The correct approach is to obtain tissue for both histology and microbiological culture at the same procedure, planned with the orthopaedic oncology team so that the biopsy tract lies within the future resection field, with MRI performed before biopsy.

Question 6

A 68-year-old woman has an isolated destructive femoral lesion. The team requests a bone tumour panel to identify the primary sarcoma. Comment.

Accepted Answer

The priorities are inverted. Beyond the age of 40, a destructive bone lesion is a metastasis or myeloma until proved otherwise, and primary bone sarcoma is comparatively rare in this age group.

The appropriate workup therefore searches for a primary carcinoma and for myeloma first. The tumours that most commonly metastasise to bone are breast, prostate, lung, thyroid and kidney, so history, examination including breasts and thyroid, chest imaging, CT of chest, abdomen and pelvis, and serum and urine protein electrophoresis with free light chains are the initial steps.

A second consideration applies immediately. A destructive lesion in a weight-bearing bone raises the question of impending pathological fracture, which is assessed with a scoring system such as Mirels, since prophylactic fixation of an intact bone is a far better operation than fixation after it breaks.

If no primary is found, biopsy follows, again planned with the team that would operate.

Question 7

A 25-year-old has severe night pain in the thigh completely relieved by ibuprofen. The radiograph shows dense cortical sclerosis with a small central lucency. Diagnose and explain the pain.

Accepted Answer

Osteoid osteoma. The small lucency is the nidus, and the surrounding dense sclerosis is the bone's vigorous reactive response to it.

The pain is characteristic and mechanistically explained. The nidus contains nerve fibres and produces prostaglandins, particularly prostaglandin E2, at concentrations far above normal, which sensitise those fibres and cause vasodilatation. Because non-steroidal anti-inflammatory drugs inhibit prostaglandin synthesis, they relieve the pain dramatically and often completely, which is close to diagnostic when the response is that clear-cut.

Night pain reflects the same mechanism, since prostaglandin release is not suppressed by daytime activity or distraction.

CT is the best modality for demonstrating the nidus, which can be obscured on radiographs by surrounding sclerosis and is poorly seen on MRI, where extensive oedema may suggest a more aggressive process. Treatment options are continued anti-inflammatory therapy, since many lesions eventually burn out, or percutaneous radiofrequency ablation of the nidus.

Question 8

A 4-month-old presents with a spiral humeral fracture. The parents report a fall from a sofa. What raises concern and what is the correct investigation?

Accepted Answer

Several features raise concern here, and they should be stated explicitly rather than implied.

The child is not yet mobile. A four-month-old cannot generate the forces required to fracture a humerus, so any long bone fracture at this age is inherently suspicious, and the mechanism described does not fit a spiral fracture, which requires a twisting force.

The appropriate investigation is a full skeletal survey rather than a film of the injured limb alone, because findings with high specificity for inflicted injury occur at sites that are clinically silent. Metaphyseal corner or bucket-handle fractures result from shearing forces on a limb and do not occur in ordinary falls. Posterior rib fractures result from anteroposterior chest compression by adult hands. Fractures of differing ages indicate repeated injury.

Alongside imaging, the case is escalated through local child protection pathways, other children in the household are considered, and differential diagnoses including osteogenesis imperfecta and metabolic bone disease are excluded. The clinician's obligation is to raise the concern, not to determine culpability.

Question 9

A patient has destruction of two adjacent vertebral bodies with a relatively preserved intervening disc and a large paraspinal collection. Interpret.

Accepted Answer

This is spinal tuberculosis, and the disc appearance is the discriminating feature.

The intervertebral disc is avascular in adults, receiving nutrition by diffusion through the endplates, so any organism reaching it must arrive from adjacent bone and digest its way in. Pyogenic organisms, particularly Staphylococcus aureus, secrete abundant proteolytic enzymes that break down the disc early, which is why pyogenic infection is named spondylodiscitis and shows early loss of disc height.

Mycobacterium tuberculosis produces very few such enzymes. It destroys bone slowly through granulomatous inflammation and caseation, and it spreads preferentially beneath the anterior longitudinal ligament to reach the adjacent vertebral body, leaving the disc between them comparatively intact until late.

The large paraspinal collection is the cold abscess, which tracks along fascial planes and may present at a distance in the psoas sheath or groin.

MRI is the investigation of choice, tissue is obtained for culture and drug sensitivity, and treatment is antitubercular chemotherapy with surgery reserved for neurological deficit, instability or a large abscess.

Practice — "Musculoskeletal Imaging"

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