Knee Dislocation 

Updated: Aug 24, 2015
Author: H Brendan Kelleher, MD; Chief Editor: Barry E Brenner, MD, PhD, FACEP 



Knee dislocation is a relatively rare injury but an important one to recognize because coexistent vascular injury, if missed, often leads to limb loss. In addition, knee dislocation often presents in the context of multisystem trauma or spontaneous relocation, which makes detection more difficult.[1, 2, 3, 4, 5, 6, 7, 8]

Patients with knee dislocation should undergo radiography and MRI, as well as angiography. The American College of Radiology has published the ACR Appropriateness Criteria for knee trauma to rate the appropriateness of imaging and treatment procedures.[9]




Knee dislocations are described using either positional or anatomical classification systems. Positional classifications describe the position of the tibia relative to the femur and thus require the dislocation to be witnessed for proper classification. Many knee dislocations spontaneously reduce prior to ED presentation, making the positional classification system difficult to apply. For this reason, the anatomical classification system is generally preferred because it specifically describes the injury by its ligamentous, arterial, and neural involvements.

The positional classification system was developed by Kennedy and describes 5 major types of positional dislocation, which are illustrated in the image below:

Types of knee dislocation. Types of knee dislocation.

See the list below:

  • Anterior: Anterior dislocation often is caused by severe knee hyperextension. Cadaver research has shown that approximately 30 degrees of hyperextension is required before dislocation will occur.

  • Posterior: Posterior dislocation occurs with anterior-to-posterior force to the proximal tibia, such as a dashboard type of injury or a high-energy fall on a flexed knee. The image below shows a radiograph of a posterior dislocation.

    Posterior knee dislocation. Posterior knee dislocation.
  • Medial, lateral, or rotatory: Medial, lateral, and rotatory dislocations require varus, valgus, or rotatory components of applied force. A lateral dislocation is illustrated in the image below.

    Lateral knee dislocation (before reduction). Lateral knee dislocation (before reduction).
  • More than half of all dislocations are anterior or posterior, and both of these have a high incidence of popliteal artery injury. Twenty to thirty percent of all knee dislocations are complicated further by open joint injury (see the image below).

    Open knee dislocation. Open knee dislocation.

The anatomical classification system was developed by Schenck and modified by Wascher. It describes the injury by its ligamentous/anatomical involvement as follows:

  • KD I - Multiligamentous rupture with either cruciate intact

  • KD II - Bicruciate rupture with both collaterals intact (rare)

  • KD IIIM - Bicruciate and medial collateral ligament (MCL) rupture

  • KD IIIL - Bicruciate and lateral collateral ligament (LCL) rupture

  • KD IV - Panligament rupture

  • KD V - Knee dislocation with periarticular fracture

  • C (added to above) - Arterial injury included

  • N (added to above) - Nerve injury included


See the list below:

  • Most often, the affected limb has a gross deformity of the knee with swelling and immobility, but up to 50% of knee dislocations are reduced by the time of ED presentation and may not be obvious.

  • Many knee dislocations have associated fractures; thus, it is important to obtain radiographs prior to ligamentous stressing to avoid promoting fracture displacement. In the absence of coexistent fracture, a thorough examination of all ligamentous structures is imperative, especially in patients with head injuries or in those who are intoxicated and may not be able to communicate symptoms adequately. The finding of varus or valgus instability in full extension of the knee is suggestive of a spontaneously reduced yet grossly unstable dislocation. In addition, pain out of proportion, or absent or decreased pulses are red flags of such an injury.

  • A careful vascular examination is required. The popliteal artery may be damaged in all variants of knee dislocation/subluxation, with reported incidence ranging from 7-64%.[10]

  • In cases presenting with "hard signs" of arterial injury, immediate surgical revascularization is indicated, and there should be no delay to the operating room (ie, waiting for arteriography). Hard signs of vascular injury include the absence of pulses, expanding or pulsatile hematomas, palpable thrills or audible bruits, and/or the history of pulsating hemorrhage.

  • In cases that do not present with "hard" arterial findings, it is advised to perform ankle-brachial or arterial-pressure indices since the presence of normal pulses does not rule out the presence of clinically significant vascular injury.[11, 12] Rose et al reported 15 of 173 limbs (9%) having normal pulses in the setting of major arterial injury.[12] ABI/API measurements of less than 0.90 are shown to have a 95% sensitivity and 97% specificity for arterial injury of consequence.[13, 14]

  • Cases in which there are no hard findings but the ABI/API is less than 0.90 should receive immediate vascular surgical consultation and further vascular imaging and perfusion surveillance. The traditional approach to universally pursue arteriography is increasingly being replaced with other less risky and less costly options: duplex ultrasonography (100% sensitivity and 97% specificity for clinically significant arterial injury[15] ), or CT angiography (95-100% sensitivity and 97-98% specificity for clinically significant arterial injury[16, 17] ). Ongoing debate surrounds the appropriate application of these imaging options; thus, the decision regarding modality choice should be made in conjunction with the consulting vascular surgeon.

  • Regardless of the imaging pursued, all knee dislocations not requiring immediate surgical revascularization should be admitted for serial perfusion checks as delayed intimal flap thromboses, arteriovenous (AV) fistulas, and pseudoaneurysms of significance certainly occur and may need subsequent intervention/repair.

  • Coexistent peroneal nerve injury occurs in 25-35% of patients and must be ruled out. This injury most commonly manifests with decreased sensation at the first webspace with impaired dorsiflexion of the foot.


The knee is a very stable joint generally requiring high-energy trauma to produce dislocation. At least 3 major ligaments typically rupture for dislocation to occur. Common mechanisms of injury include the following:

  • Motor vehicle collisions

  • Auto-pedestrian impact

  • Industrial injuries

  • Falls

  • Athletic injuries





Imaging Studies


Plain radiographs are recommended post reduction and prior to any provocative ligamentous stressing.[8]

Briefly, the ankle-brachial index compares the Doppler pressure of an arm to a leg to screen for lower limb ischemia. This straightforward measurement is performed by recording the highest Doppler sound of the brachial pulse and comparing it to the highest Doppler sound of the posterior tibial or dorsalis pedis artery. The ankle Doppler pressure is then divided by the brachial Doppler pressure to calculate the index. Indexes less than 0.9 indicate an abnormal result and should prompt further vascular imaging/assessment.

Duplex ultrasonography is a reliable, noninvasive, low-risk, low-cost option. Duplex ultrasonography appears to be an excellent modality for vascular injury assessment.[18, 19, 15] Fry et al reported 100% sensitivity and 97% specificity for clinically significant arterial injury.[15] This modality only incurs about 10% of the cost of arteriography with little to no risk profile.[20]

CT angiography is another reliable alternative to arteriography without the risk of direct arterial injury. It does require additional contrast beyond that used for chest/abdomen/pelvis body CTs that are often also indicated in these types of trauma cases; thus, it may have added risk of nephropathy or contrast reactions over arteriography, which uses less contrast. Inaba et al reported 100% sensitivity and 100% specificity for lower extremity arterial injury of significance.[16] Soto et al reported 95% sensitivity and 98.7% specificity.[17, 8]

Direct arteriography is the criterion standard but carries risk of arterial injury from direct catheterization of the artery while also requiring specialist involvement to perform (ie, interventional radiologist or vascular surgeon).



Prehospital Care

Prehospital personnel should splint the extremity and provide rapid transport to a medical facility.

Perform field reduction for patients with evidence of vascular compromise.

Emergency Department Care


Do not delay reduction in limbs with obvious vascular impairment. Only patients with good peripheral pulses should undergo prereduction radiographs. Reduction is straightforward and often easily accomplished in the ED. After adequate sedation, longitudinal traction will relocate the majority of knee dislocations. Prereduction and postreduction photos of a lateral knee dislocation are shown in the images below.

Lateral knee dislocation (before reduction). Lateral knee dislocation (before reduction).
Lateral knee dislocation after reduction. Lateral knee dislocation after reduction.


Posterolateral dislocations are particularly difficult and often require operative reduction. This is especially true when the medial femoral condyle button-holes through the medial aspect of the joint capsule and/or MCL — an occurrence that is often accompanied by a "dimple sign" overlying the medial aspect of the knee.

After reduction, splint the lower extremity in approximately 20 degrees of flexion to avoid postreduction re-dislocation, apply ice, and keep the knee elevated. Postreduction radiographs should be obtained, preferably before further ligamentous stressing/assessment.

Postreduction hard signs of arterial injury should prompt emergent vascular surgical intervention that should not be delayed for arteriography. In this setting, arteriograms may indeed be contributory to the surgical decision matrix but can be performed in the operating room by the vascular surgeon with less contrast administration than traditional arteriography tends to use.

All reduced knee dislocations without hard signs of arterial injury should be assessed with ABI/API measurements. Any reading of less than 0.90 should prompt further imaging (ie, arteriography vs CT angiography vs duplex sonography), which should be decided upon in conjunction with the vascular consult.

All knee dislocations, regardless of emergent revascularization needs, should be admitted for serial perfusion checks.


Always consult both orthopedic and vascular surgeons. Many patients have significant vascular injury requiring surgical revascularization, and all patients will at least require admission for serial vascular checks and further surgical stabilization consideration.



Medication Summary

NSAIDs, analgesics, and anxiolytics are used to treat the pain associated with dislocations.


Class Summary

Pain control is essential to quality patient care. It ensures patient comfort, promotes pulmonary toilet, and aids physical therapy regimens. Many analgesics have sedating properties that benefit patients with injuries.

Fentanyl citrate (Duragesic, Sublimaze)

Narcotic analgesic with greater potency and much shorter half-life than morphine sulfate. Excellent choice for pain management and sedation with its short duration time (30-60 min) and ease of titration. Easily and quickly reversed by naloxone. After initial dose, subsequent doses should not be titrated more frequently than q3h or q6h.

Meperidine (Demerol)

Narcotic analgesic with multiple actions similar to those of morphine. May produce less constipation, smooth muscle spasm, and depression of cough reflex than similar analgesic doses of morphine.

Oxycodone and acetaminophen (Percocet)

Drug combination indicated for relief of moderately severe to severe pain. DOC for aspirin-hypersensitive patients.

Acetaminophen and codeine (Tylenol #3)

Drug combination indicated for treatment of mild to moderately severe pain.

Hydrocodone bitartrate and acetaminophen (Vicodin ES)

Drug combination indicated for relief of moderately severe to severe pain.

Oxycodone and aspirin (Percodan)

Drug combination indicated for relief of moderately severe to severe pain.


Class Summary

Patients with painful injuries usually experience significant anxiety. Anxiolytics allow the clinician to administer a smaller analgesic dose to achieve the same effect.

Lorazepam (Ativan)

Sedative hypnotic in benzodiazepine class that has short onset of effect and relatively long half-life. By increasing action of GABA, a major inhibitory neurotransmitter, may depress all levels of CNS, including limbic and reticular formation. Excellent for patients who require sedation for longer than 24 h. Monitor BP after administering and adjust as necessary.

Nonsteroidal anti-inflammatory agents (NSAIDs)

Class Summary

These agents are used most commonly for the relief of mild to moderately severe pain. Although the effects of NSAIDs in the treatment of pain tend to be patient specific, ibuprofen is usually the DOC for initial therapy. Other options include flurbiprofen, ketoprofen, and naproxen.

Ibuprofen (Ibuprin, Advil, Motrin)

DOC for treatment of mild to moderately severe pain, if no contraindications. Inhibits inflammatory reactions and pain, probably by decreasing activity of enzyme cyclooxygenase, inhibiting prostaglandin synthesis.

Ketoprofen (Oruvail, Orudis, Actron)

Used for relief of mild to moderately severe pain and inflammation. Administer small dosages initially to patients with a small body size, the elderly, and those with renal or liver disease. Doses higher than 75 mg do not increase its therapeutic effects. Administer high doses with caution and closely observe the patient for response.

Flurbiprofen (Ansaid, Ocufen)

Has analgesic, antipyretic, and anti-inflammatory effects. May inhibit cyclooxygenase enzyme, inhibiting prostaglandin biosynthesis.

Naproxen (Anaprox, Naprelan, Naprosyn)

Used for relief of mild to moderately severe pain. Inhibits inflammatory reactions and pain by decreasing activity of enzyme cyclooxygenase, decreasing prostaglandin synthesis.



Further Inpatient Care

Historically, conventional arteriography was recommended for all cases of knee dislocation and, though it remains the criterion standard for popliteal artery evaluation, there is growing debate over its universal application.

Vascular assessment with the ankle-brachial index, duplex sonography, and/or CT angiography is changing this paradigm while an increasing number of popliteal injuries are being managed nonsurgically (generally those that show no significant thrombosis at 48-72 h). Many surgeons thus argue that arteriography should not be routine and that case-by-case utilization of other imaging modalities combined with vigilant observation is sufficient.

For the EM physician, it is important to recognize that vascular examination findings may be normal in the presence of significant popliteal artery injury[11, 12] and that some combination of further investigation/observation is warranted in all knee dislocations. This may be different for each institution and/or each surgeon and should be decided on in a case-by-case basis in conjunction with the vascular consult.

Time is of utmost concern, as vascular repair delayed more than 8 hours after injury carries an amputation rate of greater than 80%. In contrast, operative vascular repair within 8 hours of injury yields a limb-salvage rate of 80%.

The repair of coexistent popliteal vein injury is controversial. Fasciotomy is recommended after vascular repair, as severe swelling and development of compartment syndrome are common in the postoperative phase.

Operative repair of nerve injury remains controversial, as a poor prognosis is common with both operative and nonoperative care.

Operative ligamentous repair is recommended by most authors, as functional results are better than those of nonoperative care, but determining the ideal timing of this intervention is complex and is a decision best left to the orthopedist.


See the list below:

  • Patients considered for transfer should have undergone emergency reduction of the knee dislocation. Since time is crucial in salvaging the limb after a vascular injury, transfer should be initiated only if vascular consultation and/or evaluation are not available at the transferring institution or if an arteriogram has been performed and results are normal.


See the list below:

  • Popliteal artery injury

  • Popliteal vein injury

  • Peroneal nerve injury

  • Ligamentous injury

  • Compartment syndrome


See the list below:

  • When treated expeditiously and appropriately, 60-70% of patients will have a painless, stable knee. Of the remaining patients, one half will eventually have reasonable function, while the other half will have a chronically unstable and painful knee.

Patient Education

See the list below:

  • For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center. Also, see eMedicineHealth's patient education article Knee Dislocation.