Suspected internal derangement of the knee

ACR AC Acute Trauma to the Knee; ESSR knee MRI recommendations (2024)

Radiographs first — they answer the fracture question and grade the arthritis that often makes MRI unnecessary — and then MRI without contrast for the meniscal, ligamentous and cartilage question in a knee whose management would change.

Twisting injury or a fall with an effusion, locking, giving way or persistent mechanical symptoms; or chronic knee pain with mechanical features.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

Limb radiograph — two orthogonal views

Limb radiograph

What we'd amend, and why

Radiographs are the initial study after acute knee trauma when a validated decision rule is positive, and they remain the right first test in chronic mechanical pain: they show fracture, lipohaemarthrosis, a Segond fragment indicating ACL injury, loose bodies and the degree of joint space loss. That last finding is what most often determines whether MRI will change anything.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayany — All patients
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(1)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.

Worth asking the referrer (1)

None of these hold the request up. They sharpen the protocol or the plan that follows.

What was the mechanism, and is the knee locking, giving way or unable to bear weight?
It separates the urgent locked knee from routine degenerative pain, and it tells the reporter which structures to prioritise.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

All patients

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	Limb radiograph Limb radiograph — two orthogonal views usually appropriate	Radiographs are the initial study after acute knee trauma when a validated decision rule is positive, and they remain the right first test in chronic mechanical pain: they show fracture, lipohaemarthrosis, a Segond fragment indicating ACL injury, loose bodies and the degree of joint space loss. That last finding is what most often determines whether MRI will change anything.
Second line	MRI Knee MRI Knee — routine internal derangement	MRI without intravenous contrast is the next study once radiographs show no fracture and internal derangement is suspected: it is the only modality that shows the menisci, cruciate and collateral ligaments, cartilage and marrow together, and the bone bruise pattern alone frequently reveals the mechanism. It is also more sensitive than CT for the occult fracture that a normal radiograph missed.
Problem solving	CT Extremity / Musculoskeletal CT Extremity — Unenhanced, Thin Section	CT is for the bony question the radiograph raised — tibial plateau depression and comminution before fixation — and for the patient who cannot have MRI. It contributes little to a meniscal or ligamentous assessment.

Pitfalls

Going straight to MRI without radiographs and missing the fracture, the loose body or the advanced arthritis that would have redirected the whole pathway.
Reporting a degenerate meniscal tear in an arthritic knee as if it were the treatable lesion; in that group arthroscopic meniscectomy has repeatedly failed to outperform conservative care.
Not flagging a displaced bucket-handle tear as urgent when the history says the knee is locked.
Requesting contrast for a routine internal derangement study.
Forgetting the extensor mechanism and the proximal tibiofibular joint, both of which sit at the edge of the standard field of view.

Priors — what to pull first

Previous knee surgery — meniscectomy, reconstruction — changes both the appearances and the diagnostic criteria for a re-tear, so the operation note is worth more than the previous report.
Earlier radiographs establish whether the arthritis is new or long-standing.

What makes a good request

A truly locked knee is a semi-urgent question: a displaced bucket-handle meniscal tear is a surgical problem, and the request should say the knee is locked.
In a knee with advanced radiographic osteoarthritis, degenerate meniscal tears are close to universal and rarely change management. Radiographs are what identify that group.
Intravenous contrast has no role in routine internal derangement; it belongs to suspected infection, tumour and synovial disease.
After acute trauma the radiograph itself is a decision, not a reflex: a validated rule — the Ottawa knee rule is the one on this site — identifies who needs a film at all, and an alert patient who can bear weight, flex to 90 degrees and has no isolated patellar or fibular head tenderness usually does not. Over-imaging a well knee is a failure of the same kind as missing a fracture.
A tense haemarthrosis within hours of injury is a different question from a slow effusion: it means an ACL rupture, an osteochondral fracture or a patellar dislocation until shown otherwise, and lipohaemarthrosis on the lateral film means an intra-articular fracture whatever the rest of the radiograph looks like.

Scoring this once it is done

The classification and risk tools this question ends in.

Outerbridge / ICRS Classification (Articular Cartilage)Kellgren-Lawrence Grade Schatzker Classification (Tibial Plateau)Ottawa Ankle / Knee / Foot Rules

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

Limb radiograph — two orthogonal views MRI Knee — routine internal derangement CT Extremity — Unenhanced, Thin Section

Confirm locally

Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.