01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Ultrasound and MRI answer overlapping but different questions. High-frequency ultrasound offers dynamic examination, comparison, compressibility, Doppler and immediate interaction with the point of symptoms. It works best for superficial structures that can be followed along their course. Its field of view and penetration are limited, anisotropy can create false defects, and performance depends on the operator and patient position.
MRI offers a wider and more reproducible map of deep and intra-articular structures. It can demonstrate bone marrow oedema, occult fracture, cartilage and several ligaments or tendons in one examination. The trade-offs are cost, access, motion, device conditions and static imaging. Increased tendon signal, degenerative tearing and ligament scarring can occur without being the current pain generator.
Imaging should refine a clinical diagnosis or alter treatment. A convincing complete acute Achilles rupture should be protected immediately rather than held unprotected for an MRI. Conversely, a locked or unstable knee with suspected internal derangement benefits from MRI once fracture and vascular danger are addressed. The report should state whether fibres are intact, partially disrupted or completely discontinuous, where the lesion lies, how far tissue has retracted and what associated injury is present.
Key points
- 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.
- Obtain trauma radiographs first when fracture, avulsion or joint malalignment is possible; advanced soft-tissue imaging should follow immediate limb-threat assessment.
- Acute Achilles rupture is primarily a clinical diagnosis; use ultrasound when the examination is equivocal or anatomy remains uncertain, and do not delay protective immobilisation for imaging.
- For suspected ACL injury and associated internal derangement, MRI is the modality of choice after clinical and radiographic assessment when the result will guide treatment.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Fibre discontinuity, a fluid-filled gap, retraction and abnormal dynamic movement support complete rupture, but surrounding haematoma can obscure the ends.
Focal fibre disruption or thinning with some continuity remains; anisotropy and tendinosis can mimic a defect on ultrasound.
Thickening, altered echotexture or MRI signal and neovascularity may support tendinopathy but correlate imperfectly with pain severity.
Discontinuity, abnormal orientation, oedema and secondary translation signs on MRI support ACL injury and prompt assessment of menisci, cartilage and collateral structures.
A small bony fragment at a tendon or ligament attachment can be the clearest sign of major soft-tissue injury and should be sought on radiographs.
Multiligament instability with perfusion abnormality is managed as a possible knee dislocation before MRI characterises the ligament pattern.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Plain radiographyFirst step - Why
- Identify fracture, avulsion, joint malalignment, calcification and important chronic bony context after trauma.
- Interpretation and limitations
- A normal radiograph does not exclude tendon or ligament rupture because most of these structures are not directly visible.
- 02
Diagnostic ultrasound - Why
- Assess accessible tendons dynamically for continuity, retraction, subluxation, fluid and Doppler change.
- Interpretation and limitations
- Accuracy varies with expertise, depth, body habitus, pain and anisotropy; central ligaments and many intra-articular structures are poorly assessed.
- 03
Magnetic resonance imaging - Why
- Map deep or complex tendon, ligament, meniscal, labral, cartilage, muscle and marrow injury.
- Interpretation and limitations
- Morphological abnormalities require clinical correlation, and MRI should not delay acute protection or vascular assessment.
- 04
Stress or dynamic radiography - Why
- Assess selected instability under controlled load in a specialist protocol.
- Interpretation and limitations
- Pain, guarding and technique alter measurements; unsafe acute injuries must not be stressed solely to obtain an image.
- 05
Vascular imaging - Why
- Evaluate suspected arterial injury when perfusion findings or a high-risk dislocation pattern warrants it.
- Interpretation and limitations
- This answers limb perfusion rather than ligament anatomy and may take priority over MRI.
04Clinical next stepsHow the result changes management or prompts escalation.
01Achilles pathwayProtect before imagingFirst stepSudden posterior ankle pain and weakness raises possible acute Achilles rupture.+
- 1Examine resting plantar-flexion angle and calf-squeeze response while noting that residual active plantar flexion does not exclude rupture.
- 2Immobilise in plantar flexion and arrange the urgent specialist pathway when the clinical findings are convincing.
- 3Use experienced ultrasound if the examination is equivocal, the level is atypical or anatomy will change the plan.
- 4DefinitiveDo not allow routine MRI scheduling to delay protection, thrombosis assessment or definitive review.
02Knee pathwayMap internal derangementAfter trauma the knee remains unstable, locked or swollen and internal structural injury is suspected.+
- 1Assess fracture risk, obtain radiographs when indicated and document pulse, perfusion and named nerve functions.
- 2EscalationEscalate immediately for vascular assessment if multiligament instability or a possible reduced dislocation is present.
- 3Request MRI when defining cruciate, meniscal, cartilage and associated injury will guide specialist treatment.
- 4Correlate each MRI lesion with mechanism, examination and functional instability before selecting rehabilitation or surgery.
03Ultrasound pathwayUse dynamic superficial assessmentA superficial tendon or ligament can be accessed and a focused dynamic answer will change management.+
- 1Give the operator the exact structure, site, mechanism and required decision rather than requesting an unspecified scan.
- 2Examine in two planes and through movement where safe, adjusting probe angle to avoid anisotropy.
- 3Describe continuity, location, extent, retraction, vascularity and relevant adjacent findings.
- 4EscalationEscalate to MRI or specialist review when ultrasound is limited, discordant or cannot assess the deeper suspected lesion.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Reassess neurovascular status and skin after splintage or any manipulation for acute tendon or ligament injury.
- Track completion and ownership of advanced imaging when return to work, sport or operative planning depends on the result.
- Follow pain, range, strength, stability and function rather than repeating scans solely for persistent signal abnormality.
- After conservative care, image again only when recovery is discordant or a new intervention requires anatomical clarification.
- Document the ultrasound operator, technique and limitations so a negative result is interpreted in the correct confidence range.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Anisotropy creates artefact
A tendon can look falsely hypoechoic when the beam is not perpendicular, so heel-toe correction and orthogonal review are essential.
Dynamic imaging adds function
Real-time movement may demonstrate subluxation or preserved continuity that a static scan cannot show as clearly.
MRI sees beyond the target
Bone bruising and associated meniscal or cartilage damage can explain severity and change management after ligament trauma.
Residual movement misleads
Accessory plantar flexors can preserve weak ankle movement despite complete Achilles rupture, making tension tests more informative.
Signal does not equal symptoms
Degenerative tendon and ligament abnormalities occur in asymptomatic people and must match examination and mechanism.
Radiographs still matter
Avulsion fragments and malalignment can reveal a mechanically important injury that advanced soft-tissue imaging should not obscure.
07Common pitfallsFrequent interpretation and management errors.
- 01
Using a negative radiograph to exclude tendon or ligament injury.
- 02
Requesting MRI before protecting a clinically obvious acute tendon rupture.
- 03
Treating an operator-limited ultrasound as definitive exclusion of a deep lesion.
- 04
Ignoring vascular risk in an apparently reduced multiligament knee injury.
- 05
Calling every MRI signal abnormality the symptomatic lesion without clinical correlation.