An uncomplicated bunion is primarily a problem of bone and joint alignment. Weight-bearing X-rays generally provide the most useful structural information because they show the deformity under load. MRI performed while lying down does not replace this functional assessment.
Ultrasound may be useful when symptoms suggest a soft-tissue problem near the bunion rather than, or in addition to, the deformity. Examples can include a superficial bursa, tendon disorder, cyst, soft-tissue mass or local nerve irritation. It can also permit dynamic assessment and guide an injection when an injection is clinically appropriate.
MRI provides more detailed information about bone marrow, cartilage, tendons, ligaments and other soft tissues. It may be considered when pain is unexplained, when an occult stress injury, osteochondral lesion, avascular change, inflammatory process or soft-tissue disorder is suspected, or when symptoms do not fit the clinical and X-ray findings.
CT, including weight-bearing CT in selected centres, can provide three-dimensional information and may be useful for complex, recurrent or rotational deformity. It is not routinely required for every bunion.
The decision to order additional imaging should begin with a clinical question: what diagnosis is being considered, and how would the result alter management? Scanning without a clear purpose can reveal incidental findings that may not be responsible for the patient’s symptoms.