History
Knee dislocations are described using either positional or anatomical classification systems. [8, 15] 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: medial, lateral, rotatory, posterior, and anterior [15] (see the image below).
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 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.
Medial, lateral, and rotatory dislocations require varus, valgus, or rotatory components of applied force. A lateral dislocation is illustrated in the image below.
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).
The anatomical classification system was developed by Schenck and modified by Wascher. [8] It describes the injury by its ligamentous/anatomical involvement as follows:
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KD I - Multiligamentous rupture with either cruciate intact
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KD II - Bicruciate rupture with both collaterals intact (rare)
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KD IIIM - Bicruciate and medial collateral ligament (MCL) rupture
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KD IIIL - Bicruciate and lateral collateral ligament (LCL) rupture
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KD IV - Panligament rupture
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KD V - Knee dislocation with periarticular fracture
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C (added to above) - Arterial injury included
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N (added to above) - Nerve injury included
Physical
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%. [24]
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. [19, 20] Rose et al reported 15 of 173 limbs (9%) having normal pulses in the setting of major arterial injury. [20] ABI/API measurements of less than 0.90 are shown to have a 95% sensitivity and 97% specificity for arterial injury of consequence. [25, 26]
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 [27] ), or CT angiography (95-100% sensitivity and 97-98% specificity for clinically significant arterial injury [28, 29] ). 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.
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Types of knee dislocation.
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Posterior knee dislocation.
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Lateral knee dislocation (before reduction).
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Open knee dislocation.
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Lateral knee dislocation after reduction.
