Imaging Modalities in Orthopaedics
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Direct answer
Plain radiographs are the first-line investigation in orthopaedics, usually taken as two views across the joints above and below the injury. CT gives the best bony detail for complex fractures, MRI shows ligaments, menisci, marrow and early avascular necrosis, and ultrasound suits superficial soft tissues and infant hips. The right modality depends on the clinical question, whether that is fracture pattern, soft tissue damage, occult bone injury or infection.
What you must remember
- Radiographs: first-line; two views at right angles, including the joints above and below the injury.
- CT: best bony detail for complex intra-articular, pelvic, acetabular and calcaneal fractures, and useful for assessing union.
- MRI: ligaments, menisci, cartilage, marrow oedema, spinal and bone infection, and the most sensitive test for early avascular necrosis.
- Bone scan: technetium-99m uptake reflects increased osteoblastic activity, useful for occult fracture, osteomyelitis and metastasis screening.
- Ultrasound: superficial tendons, joint effusions, guided injections and hip screening in young infants before the femoral head ossifies.
- DEXA: measures bone mineral density for osteoporosis; it does not diagnose fractures.
- MRI limits: unsafe pacemakers and some metallic implants exclude MRI, which is why CT is used as the alternative.
Matching the scan to the clinical question
A good imaging choice starts with the clinical question rather than the most advanced machine. Each modality answers a different part of the problem.
| Modality | Best for | Main limitation |
|---|---|---|
| Radiograph | Fracture pattern, alignment and first-line screening | Two-dimensional and insensitive to occult injury |
| CT | Complex intra-articular, pelvic, acetabular and calcaneal fractures | Radiation dose and poor soft tissue contrast |
| MRI | Ligaments, menisci, cartilage, marrow oedema and early avascular necrosis | Cost, time and contraindicated implants |
| Bone scan | Occult fracture, osteomyelitis, metastasis screening and CRPS | Non-specific uptake that persists for weeks |
| Ultrasound | Tendons, effusions, guided injection and infant hip screening | Operator dependent and limited by bone |
| DEXA | Bone mineral density in osteoporosis | Does not diagnose fractures |
The table is a starting point, and the clinical question decides the choice. A negative early radiograph in a scaphoid injury with snuffbox tenderness is not the end of the work-up, so MRI or CT is used to detect an occult fracture.
Where the exam tries to catch you
Imaging questions usually test the modality that answers the stem, not the most expensive one.
- Trap: choosing CT for a suspected meniscal tear Correct: MRI is the investigation of choice for menisci and ligaments, because CT shows bone rather than soft tissue.
- Trap: ordering a bone scan to confirm early avascular necrosis of the femoral head Correct: MRI is the most sensitive test for early avascular necrosis, while plain films may look normal in the early stage.
- Trap: using ultrasound to assess the hip of an older infant Correct: after the femoral head ossifies, radiographs are the preferred method for assessing developmental dysplasia of the hip.
Frequently asked questions
Which imaging is best for a suspected meniscal tear?
MRI is the imaging of choice for a suspected meniscal tear, because it shows the menisci, ligaments and cartilage in detail. Plain radiographs and CT show bone and are not used to diagnose meniscal injury.
Why is MRI the most sensitive test for early avascular necrosis?
MRI detects marrow oedema and other changes in the femoral head before collapse, and often before plain films show any bony change. This makes MRI the most sensitive investigation for early avascular necrosis.
When is ultrasound used to screen the hip in infants?
Ultrasound, commonly by the Graf method, screens the hips of young infants for developmental dysplasia before the femoral head ossifies. After ossification, radiographs become the preferred way to assess the hip in these children.
What is a bone scan used for in orthopaedics?
A technetium-99m bone scan detects increased osteoblastic activity and is used for occult fracture, osteomyelitis, metastasis screening and complex regional pain syndrome. Its uptake is non-specific, so a positive scan needs correlation with other imaging.