Knee dislocation is a relatively rare injury, representing about 0.2% of all orthopedic injuries. Because of association with neural and vascular injury, is important to be recognized. A missed injury could lead to loss of limb.
Motor vehicle collision or fall from height are usual causes. A dashboard injury resulting in axial load to flexed knee is often the cause.
Low energy trauma can lead to knee dislocation is seen in athletic injury.
Knee dislocation is associated with significant soft tissue disruption, vascular injury and neural injury.
Dislocation of the knee is mostly seen in young men. Over half of the cases are caused by motor-vehicle accidents. One third are caused by sports related injuries.
Dislocation is bilateral in 5% of patients.
Multiple trauma is associated with 14% and 44% of cases of knee dislocations.
[Read about knee anatomy]
Ligaments are frequently injured. Severity of injury would affect number of ligaments affected.
5-15% dislocations are associated with vascular injury. Reported incidence is 40-50% in anterior/posterior dislocations. The vessels are prone to injury due to tethering at the popliteal fossa at two points. Proximally, it is at the adductor hiatus and distally at fibrous tunnel at soleus muscle.
Common peroneal nerve injury occurs in 25% while tibial nerve injury is less common.
Fractures of tibia and femur are present in 16% of cases. Bony avulsion injuries of the PCL are not unusual. Marginal avulsion fractures of the lateral tibial plateau [Segond fracture] may be noted.
Anteromedial tibial plateau fractures in particular are associated with the presence of disruption of the PCL and posterolateral corner.
Classification of Knee Dislocation
This classification is based on direction of displacement of the tibia
- Most common type of dislocation
- Follows hyperextension injury
- Usually involves tear of PCL
- Arterial injury – Intimal tear due to traction
- Second most common
- Occurs due to axial load to flexed knee (dashboard injury) or fall on flexed knee
- Highest rate of vascular injury [complete tear of popliteal artery]
- Due to varus or valgus force
- Involves tears of both anterior and posterior cruciate ligaments
- Highest rate of peroneal nerve injury
- Varus or valgus force
- Associated with disrupted posterolateral corner or posterior cruciate ligament
- Posterolateral is most common rotational dislocation
- Usually irreducible due to buttonholding of femoral condyle through capsule
This classification is based on pattern of ligamentous involvement in knee dislocation. Four major ligament stabilizers are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL) and the posterolateral corner [PLC].
Posterolateral corner contains many dynamic and static stabilizers.
All the four are disrupted in about 11% of cases.
The most common pattern of injury is a bicruciate disruption with associated disruption of the MCL or posterolateral corner depending upon the direction of the deforming force.
Here is the classification
Multiligamentous injury with involvement of anterior cruciate ligament or posterior cruciate ligament.
Injury to anterior cruciate ligament and posterior cruciate ligament
Injury to ACL, PCL, and posteromedial corner or posterolateral corner (3 ligaments).
- KDIII M – (ACL, PCL, MCL) – Has highest rate of vascular injury
- KDIII L – (ACL, PCL, PLC, LCL)
Injury to ACL, PCL, PMC, and PLC (4 ligaments)
Multiligamentous injury with periarticular fracture
Clinical Presentation of Knee Dislocation
There would be history of motor vehicle injury or fall. Most often, the affected limb has a gross deformity of the knee with swelling and immobility
Up to 50% of knee dislocations are reduced by the time patient presents, therefore the dislocation may not be obvious.
Associated bony injuries may be present and would be confirmed by x-rays.
A careful vascular examination is required. The reported vascular damage incidence ranging from 7-64%. If vascular injury is identified or is probable, the patient should be immediately worked up for and taken to operating room and considered for surgical revascularization.
Coexistent peroneal nerve injury occurs in 25-35% of patients and must be ruled out.Decreased sensation at the first web space with impaired dorsiflexion of the foot is the most common finding]
The knee may present with subtle signs of trauma (swelling, effusion, abrasions)
If there is an obvious deformity, reduce it immediately, especially if pulses are absent.
A dimple sign indicates buttonholing of medial femoral condyle through medial capsule and the generally associated with irreducible posterolateral dislocation. This type is a contraindication to closed reduction due to risks of skin necrosis and needs immediate open reduction.
Ligaments should be assessed for stability.
Assess anterior and posterior cruciate ligaments, medial collateral ligament, lateral collateral ligament and posterolateral corner.
Clinical assessment of specific knee ligaments may be difficult because of pain.
- Recurvatum on passive elevation of the limb – disruption of the PCL and posterior capsule
- Gross laxity on varus or valgus testing with the knee in full extension – major capsular disruption.
- Abnormal Lachman Test – Cruciate ligament injury
- An increased degree of external rotation of the tibia in comparison with the normal knee is considered to indicate insufficiency of the posterolateral corner at 30° and laxity of the posterolateral corner at 90°.
Details of vascular Examination
It is important to rule out vascular injury both before and after reduction. Serial examinations may be mandatory.
The dorsalis pedis and posterior tibial pulses should be palpated though normal pulses do not indicate absence of arterial injury.
Distal capillary circulation findings are also not reliable as the collateral circulation can mask a complete popliteal artery occlusion
Measurement of Ankle-Brachial Index can help [ratio of the blood pressure at the ankle to the blood pressure in the upper arm]. It is used to supplement physical examination and assist in deciding whether angiography is required. At < 0.90, this index gives a positive predictive value of 100% for the presence of a vascular injury requiring surgical intervention.
- If ABI >0.9
- then monitor with serial examination (100% Negative Predictive Value)
- If ABI <0.9
- perform arterial duplex ultrasound or CT angiography
- if arterial injury confirmed then vascular surgery
If pulses are absent or diminished
- Perform reduction if not reduced yet
- Immediate surgical exploration if pulses are still absent following reduction
- ischemia time >8 hours can lead to amputation in as high as 86%
- If pulses present after reduction then measure ABI and work accordingly
Plain anteroposterior and lateral radiographs should be obtained in all cases of suspected dislocation.
Plain radiographs are recommended post reduction too.
Ankle Brachial Index
Ankle-brachial index compares the Doppler pressure of an arm to a leg to screen for lower limb ischemia. This 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 the patient should be considered for further vascular assessment.
It is a reliable, noninvasive and low-risk modality for vascular injury assessment. It is reported to have 100% sensitivity and 97% specificity for clinically significant arterial injury. Its low cost makes it quite an attractive investigation.
CT Angiography/MR Angiography
It is a reliable alternative to arteriography without the risk of direct arterial injury but requires additional contrast . It may have added risk of nephropathy or contrast reactions compared to arteriography, which uses less contrast].
MR Angiography can be used as an alternative to standard arteriography in the acute setting. But that does not provide adequate diagnostic information about the ligamentous injury and for that, conventional MRI is preferred.
It is the gold standard for detecting arterial injury and requires interventional radiologist or vascular surgeon to perform.
While some authors consider it to be mandatory while others think that it is necessary only in selected cases. In case of doubt it is safer to get arteriography done.
Presently indications for arteriography are
- Signs of impaired circulation such as ischemic color change
- Diminished or absent pulses on presentation
- Colour or temperature changes below the knee
- Reduced ankle-brachial index (< 0.90)
Treatment of Knee Dislocation
Knee dislocation is an orthopedic emergency. It should be identified and reduced immediately, especially when there is a vascular compromise.
After reduction the limb should be splinted in about 20 degrees of flexion to avoid post-reduction dislocation.
After reduction, splint the lower extremity in approximately 20 degrees of flexion to avoid postreduction re-dislocation, apply ice, and keep the knee elevated.
Obtain postreduction radiographs if there is no vascular compromise.
A patient with hard signs of arterial injury [absent pulses, cold limb, ischemia etc.] would require vascular intervention.
open fractures, open dislocation, irreducible dislocation, compartment syndrome are other indications of emergency surgery.
Obese and polytrauma patients can be considered for surgery too.
For vascular intervention,perform external fixation first, followed by vascular reconstruction. Perform fasciotomies after vascular repair or reconstruction.
Explore and repair capsular and ligamentous injury after vascularization surgery.
For persistant instability – bridging external fixator may be used. Reconstruction of cruciate ligaments may be delayed.
If there is no vascular injury, evaluate the pattern of ligamentous injury and treat accordingly. Assess with regular ankle brachial index measurements.
Any reading of less than 0.90 should prompt further imaging like arteriography vs CT angiography vs duplex sonography.
Prefer limited reconstruction of the capsule and collateral ligament.
Arhroscopic reconstruction of the cruciate ligaments 6-12 weeks [healing of capsule]
Any common peroneal nerve injury should go for delayed repair.
Post-operative rehabilitation with bracing and early movement reduces the risk of disabling stiffness.
- Stiffness (arthrofibrosis)
- Most common complication (38%)
- More common with delayed mobilization
- May necessitate a more rapid program of rehabilitation
- Manipulation under anaesthesia or arthrolysis in persistant cases betweeen 3-6 months
- Laxity and instability (37%)
- Peroneal nerve injury (25%)
- Most common in posterolateral dislocations
- Poor results with nerve exploration [acute or delayed repair both]
- Neurolysis and tendon transfers are the mainstay of treatment
- Transfer of the posterior tibial tendon (PTT) to the lateral cuneiform.
- Vascular compromise
- Claudication, skin changes, and muscle atrophy can occur
A return to normal function is uncommon. Some residual impairment of function is expected.
Joint stiffness and failure of the reconstruction is seen.
Risk of post-traumatic osteoarthritis of is 50%.
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