Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.
2 min synopsisUK scopeSources checked 13 Sept 2026Clinical review pending
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Stabilise before soft-tissue imaging
Vascular compromise, open injury, compartment syndrome or a potentially reduced knee dislocation takes priority over ultrasound or routine MRI.
Action: Protect the limb, document named neurovascular findings, obtain urgent orthopaedic or vascular assessment and use emergency radiography or angiographic imaging as the immediate clinical question requires.
Synopsis
Choose between ultrasound and MRI for suspected tendon or ligament injury by anatomy and decision, understand technique limitations, and act on limb-threatening mimics before advanced imaging.
Choose the test from the structure and decision: ultrasound is real-time and dynamic for accessible superficial tendons, while MRI maps deep, intra-articular and multistructure injury.
Ultrasound can show tendon continuity, fibre disruption, retraction, movement and hyperaemia, but depends heavily on operator skill, probe angle and access.
MRI provides a reproducible overview of ligaments, tendons, menisci, labrum, cartilage, muscle and bone marrow, but static signal change may be incidental.
Key red flags
A cool foot, abnormal perfusion or gross multiligament knee instability may represent a spontaneously reduced knee dislocation with arterial injury.
An open tendon injury, skin compromise or contamination requires surgical assessment; ultrasound gel and probe pressure must not be applied casually to the wound.
Sudden loss of active extension at the knee or finger, or loss of plantar flexion with a positive calf-squeeze test, needs prompt protection and specialist review.
Fever, marked erythema, systemic illness or a collection changes the question from mechanical injury to infection and may require aspiration or urgent surgery.
Vascular mimic
Multiligament instability with perfusion abnormality is managed as a possible knee dislocation before MRI characterises the ligament pattern.
Investigation priorities
01
Plain radiographyFirst step
Identify fracture, avulsion, joint malalignment, calcification and important chronic bony context after trauma.
Management branches
Achilles pathwayProtect before imaging
Sudden posterior ankle pain and weakness raises possible acute Achilles rupture.
Examine resting plantar-flexion angle and calf-squeeze response while noting that residual active plantar flexion does not exclude rupture.
Immobilise in plantar flexion and arrange the urgent specialist pathway when the clinical findings are convincing.
National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 13 Sept 2026; clinical approval remains outstanding.
BOFAS advanced imagingCurrent BOFAS professional Hyperbook page checked 13 September 2026; ultrasound and MRI modality features in foot and ankle practice.
BOFAS acute Achilles ruptureCurrent professional Hyperbook body checked 13 September 2026; acute clinical assessment and limits of gap-based ultrasound management.
BOA/BASK-BOSTAA ACL best practiceSeptember 2020 best-practice book, current BOA listing checked 13 September 2026; MRI for ACL and associated internal derangement.
ACR acute trauma to the kneeCurrent final professional criteria checked 13 September 2026; radiography first where indicated and MRI after negative radiographs for suspected internal injury; US setting.
BOASt arterial injuries associated with musculoskeletal traumaPublished June 2026 by BOA/BAPRAS/Vascular Society; patients with arterial injury associated with musculoskeletal trauma. Requires documented arterial examination, urgent realignment of a pulseless deformed limb, repeat examination and CTA when suspicion persists; excludes paediatric supracondylar fractures.