Screening for Vascular Injury After Knee Dislocation
A missed popliteal artery injury after a knee dislocation can cost the leg, and a reflexive angiogram on every normal exam exposes a stable patient to unneeded risk. The ABI and the pulse exam are what let a student follow the vascular workup on rounds instead of waiting on someone else's call.
Key terminology
- Hard sign
- Absent pulse, active hemorrhage, expanding hematoma, or a bruit or thrill on vascular exam - mandates immediate surgical exploration, no imaging needed first
- Soft sign
- A diminished but present pulse, a nerve deficit, or a history of bleeding near the injury - prompts further workup such as ABI rather than immediate surgery
- Ankle-brachial index (ABI)
- Systolic blood pressure at the ankle divided by systolic blood pressure at the arm - a quick bedside test for arterial flow to the leg
- Popliteal fossa
- The diamond-shaped space behind the knee containing the popliteal artery, popliteal vein, and the tibial and common peroneal nerves
- Warm ischemia time
- How long a limb survives without blood flow at body temperature before the damage becomes irreversible
- Selective arteriography
- Obtaining an angiogram only when the exam or ABI is abnormal, rather than imaging every patient
Takeaways
- Popliteal artery injury is less common in modern series than the classic teaching figure suggests. A 2024 review of 43,869 dislocations found popliteal injury in 7.8%, while the frequently quoted 32% comes from a single 1977 series of 245 dislocations that predates current selective-imaging management (Green and Allen, J Bone Joint Surg Am 1977;59:236-239, PMID 845209; Randall et al, Injury 2024;55:111904, PMID 39357194).
- An ABI under 0.90 predicts a surgical arterial injury with near-perfect accuracy. In a prospective study of 38 knee dislocations, all 11 patients with an ABI below 0.90 had an arterial injury confirmed at arteriography, and all 27 with an ABI of 0.90 or higher had no vascular injury on serial exam and duplex, giving sensitivity, specificity, PPV and NPV of 100% (Mills et al, J Trauma 2004;56:1261-1265, PMID 15211135).
- Pulse exam alone misses too many injuries to stand on its own. A pooled analysis of 7 studies and 284 knee injuries found abnormal pedal pulses carried sensitivity of only 79% and negative predictive value of 93%, well short of the ABI's 100% - ABI supplements the pulse exam rather than replacing it (Barnes et al, J Trauma 2002;53:1109-1114, PMID 12478036).
- Selective arteriography guided by serial exam missed zero injuries in exam-negative patients. Among 130 consecutive patients with multiligament knee injury, 10 had an abnormal vascular exam (9 true positive, 1 false positive), and every patient with a normal serial exam proved to have no popliteal injury, supporting selective imaging over routine arteriography for every dislocation (Stannard et al, J Bone Joint Surg Am 2004;86:910-915, PMID 15118031).
- A hard sign sends the patient straight to the OR, and the whole hard-sign/soft-sign framework is borrowed from penetrating trauma, not written for blunt knee dislocation. A hard sign (absent pulse, active hemorrhage, expanding hematoma, bruit or thrill) carried specificity and PPV of 100%, and a normal exam plus normal ABI together dropped the post-test probability of injury to 0% (Simsek and Gur, Emerg Med Int 2024;2024:9979585, PMID 39758587; deSouza et al, Acad Emerg Med 2017;24:994-1017, PMID 28493614).
- Revascularization delay turns a salvageable leg into an amputation. In the original 245-dislocation series, repair completed within 6 to at most 8 hours of injury avoided amputation in most patients, while 86% of those treated after that window lost the limb (Green and Allen, J Bone Joint Surg Am 1977;59:236-239, PMID 845209).
- Even with treatment, a confirmed popliteal injury can still end in amputation. In the LEAP cohort, 4 of 18 knees with a popliteal artery injury after dislocation required amputation despite treatment, a rate measured only in patients already known to have a vascular injury and not comparable to the 2.3% amputation rate reported across all knee dislocations in a 2024 systematic review (Patterson et al, J Trauma 2007;63:855-858, PMID 18090017; Randall et al, Injury 2024;55:111904, PMID 39357194).
- Vascular injury rarely travels alone, check the peroneal nerve too. The same 2024 review of 43,869 knee dislocations found peroneal nerve injury in 15.3% of patients, roughly twice the 7.8% rate of popliteal artery injury, and a separate series put common peroneal nerve injury as high as 10-40% in multiligament knee injuries (Randall et al, Injury 2024;55:111904, PMID 39357194; Lehane et al, Arthrosc Tech 2025;14:103445, PMID 40548046).