First and second year — the floor first, then every step
A knee dislocation often reduces before imaging. Suspect it from the injury pattern (multiple ligaments, fibular head, Segond, tibial spine), and treat the popliteal artery as injured until CTA or an arterial assessment says otherwise.
Orient first
The popliteal artery is tethered above (adductor hiatus) and below (soleal arch), so it tears when the tibia displaces on the femur — even when the pulse is palpable.
Spontaneous reduction is common: a normal-looking alignment on the radiograph does not exclude a dislocation that happened.
A multiligament injury (both cruciates, or a cruciate plus a collateral/corner) IS a knee dislocation for vascular purposes.
Acquire the study
AP and horizontal-beam lateral of the knee.
The manoeuvre
Lateral view: tibiofemoral alignment — anterior or posterior translation of the tibia.
AP view: medial or lateral joint space widening compared with the other compartment.
Avulsion markers: Segond fracture (lateral tibial rim), arcuate sign (fibular head), tibial spine and PCL-insertion avulsions.
Lipohaemarthrosis on the horizontal-beam lateral (an intra-articular fracture).
What confirms it
Tibiofemoral displacement, or a multiligament injury on MRI, or an avulsion pattern that implies both.
What licenses you to exclude it
Normal pulses do not exclude an intimal injury; CTA or an ankle–brachial index per protocol is required.
The classic misread
Calling a reduced knee normal when the avulsion pattern says it dislocated.
Reporting the injury
Classification to use
Schenck anatomical classification (KD I–V by which ligaments are torn); direction of dislocation (anterior, posterior, medial, lateral, rotatory).
Measurements — and how to take them
Tibial translation in mm on the lateral view; length of any arterial narrowing and percentage stenosis.
What to report
Current alignment; avulsion fractures; the popliteal artery status by segment; each ligament; the peroneal nerve; associated fractures.
How to report it
CT: "Focal intimal flap in the P2 segment of the right popliteal artery with 30% luminal narrowing; distal run-off opacifies symmetrically with the left. No extravasation."
MRI: "Complete tears of the ACL and PCL with a grade III posterolateral corner injury — Schenck KD III-L."
What not to report
Do not report "no dislocation" on a reduced knee with a multiligament pattern.
Do not call an arterial abnormality "likely spasm" without recommending follow-up.
Associated injuries to look for
Popliteal artery injury, common peroneal nerve injury, tibial plateau and fibular head fractures, compartment syndrome (clinical).
What changes management
Any popliteal artery injury → vascular surgery before ligament surgery.
Irreducible dislocation (posterolateral, "dimple sign") → open reduction.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Diagnostic criteria
Popliteal artery · Popliteal calibre and entrapment / aneurysm
similar to the distal SFA; a popliteal aneurysm is a focal dilatation (commonly discussed from about 1.5–2 cm) and is bilateral often enough that the other side must be seen
Entrapment is a POSITIONAL attenuation with plantarflexion / gastrocnemius contraction. An aneurysm conversation is about thrombus and the runoff, not a millimetre alone.
USG · CT
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.