Dislocation of the knee occurs when the tibia and femur are out of place in relation to each other. Although rare, dislocation of the knee may be the most serious acute knee injury and presents one of the few true orthopaedic emergencies. Immediate reduction and neurovascular assessment are important due to the high incidence of concomitant vascular injury and potential limb loss.
Structure and function
The knee joint is a hinge-type joint that allows flexion, extension and to slight degree, medial and lateral rotation. There are 4 major ligaments that provide stability for the joint: anterior cruciate, posterior cruciate, lateral collateral, and medial collateral ligaments. Dislocation of the knee typically injures both cruciate ligaments and one or both collateral ligaments.
The popliteal artery is the continuation of the femoral artery behind the knee. Five arteries are given off in the popliteal space, which provides collateral circulation to the knee joint. The popliteal artery is tethered across the joint due to being held firm above by the tendinous hiatus of adductor magnus and below by the tendinous arch of the soleus muscle. As a result, the artery is at increased risk for damage during knee dislocations. Up to 40% of knee dislocations have associated vascular injury (UptoDate).
The perineal nerve, a branch from the sciatic nerve, wraps around the neck of the fibula and provides sensory innervation to the dorsum of the foot and allows dorsiflexion of the ankle. Injury to the perineal nerve occurs in up to 25-35% of knee dislocations (Medscape).
Knee dislocations are rare and are generally due to trauma such as, motor vehicle accidents, falls, and other high energy injuries. There is no specific population at risk, although persons involved in high energy activities may tend to be of the younger generation.
Femoral-tibial dislocation is an infrequent and potentially limb threatening injury. The most common cause of injury is high energy trauma (i.e. motor vehicle accident, falls, auto-pedestrian accidents, industrial accidents, sports-related injuries) and often involves damage to multiple ligaments supporting the knee. Pain out of proportion to the injury, or absent or decreased pulses are suggestive of knee dislocation.
History: patients typically have a history of twisting with a grinding or tearing sensation and swelling. There are 5 types of dislocation, defined by displacement of tibia relative to the femur.
Posterior: due to an anterior-to-posterior blow to the proximal tibia
Anterior: due to hyperextension of the knee. Research has shown that approximately 30 degrees of hyperextension is needed to cause dislocation (Medscape).
Medial: due to a valgus force applied to the proximal tibia.
Lateral: due to a varus force applied to the proximal tibia.
Rotatory: generally due to an indirect rotatory force where the body rotates in the opposite direction of a planted food. Subdivisions of rotatory dislocation include: anteromedial, anterolateral, posteromedial, posterolateral.
Anterior and posterior dislocations comprise more than half of all knee dislocations and have a high incidence of concomitant popliteal artery injury.
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Physical: Pain and swelling may initially limit physical exam, however assessment of the direction of dislocation can still be determined. Hemarthrosis, due to a coexisting fracture or ligament tear, and ecchymosis are frequently seen. Up to half of dislocations self reduce prior to ED presentation making them difficult to diagnose, and as a result may be misdiagnosed as compartment syndrome or missed altogether. A finding of varus or valgus instability in full extension suggests a grossly unstable dislocation that has spontaneously reduced.
Examination of motor function, lower extremity sensory integrity, and all supporting ligaments is essential. Coexisting fractures may become displaced due to ligamentous stressing, making it important to obtain radiographs prior to testing the supporting ligaments. Assessment of dorsiflexion of the foot and sensation at the first webspace are required to rule out injury to the peroneal nerve.
Vascular assessment of distal and popliteal pulses is vital, as up to 60% of injuries may have concomitant injury to the popliteal artery. Any signs of vascular compromise (decreased or absent pulses, pale or dusky skin, parasthesias, paralysis, palpable thrills or audible bruits) require immediate consultation by a vascular surgeon.
The presence of normal pulses does not rule out popliteal vessel injury. In such cases, it is recommended to perform ankle-brachial or arterial-pressure indices (medscape). In a study of 38 patients, all of those with an ABI of <0.9 were found to have arterial injury (UptoDate).
Posterolateral rotatory dislocation, indicated by an anteromedial skin furrow, is irreducible by closed reduction and requires immediate open reduction.
Decreased or absent pulses, or an ABI of <0.9, requires emergent consultation by a vascular surgeon. Additionally, the presence of normal pulses does not rule out vessel injury.
Knee dislocation must be suspected in any patient with a history of trauma and gross deformity of the knee, complete laxity of supporting ligaments, pain out of proportion, or absent or decreased pulses.
Other diagnoses to consider include fractures of the femur, patella, tibia, and fibula.
Plain radiograph: anteroposterior and lateral radiographs should be obtained post reduction for confirmation and to assess for any ligament damage. Radiographs are recommended prior to any ligamentous stressing, but should only be done prereduction in the presence of good peripheral pulses.
Use algorithm (UptoDate) for post reduction management and to determine if there is a need for vascular imaging.
Ankle-brachial indices: is the ratio of the blood pressure in the lower leg to the blood pressure in the arm, measured by Doppler ultrasonography, and is used to screen for lower limb ischemia. An index of <0.9 warrants immediate consult of a vascular surgeon.
Duplex/Ultrasonography: reliable, non-invasive test for vascular assessment.
CT Angiography: requires additional contrast dye, but is reliable, non-invasive assessment of extremity vascular trauma.
Direct Arteriography: carries risk of arterial injury and requires specialist.
MRI: to identify ligamentous injury, joint capsule, meniscus, and articular cartilage integrity. If multiple ligamentous injuries are detected on MRI, but not previously suspected, do immediate imaging of popliteal artery.
Risk factors and prevention
There is not much one can do to prevent or avoid a knee dislocation, other than to avoid trauma or falls. In general, care should be taken when engaging in high energy activities or impact sports, such as downhill skiing. Additionally, proper safety equipment should be used in industrial settings to avoid falls and injury.
The goals of treatment are to restore function and mobility of the knee joint.
Splinting of the extremity and rapid transport to an ED should be done immediately. If there is evidence of vascular damage (absent pulses, bruits, hemorrhage, etc.) a reduction should be performed without delay.
The vast majority of knee dislocations are reducible by closed reduction. An exception being posterolateral dislocations, which requires open reduction due to buttonholing of the medial femoral condyle through the medial capsule and/or MCL. Posterolateral dislocation is indicated by an anteromedial skin furrow, which deepens with longitudinal traction and is due to internal pressure from medially displaced femoral condyle.
Application of longitudinal traction to the knee joint is sufficient to reduce the majority of cases. If longitudinal traction alone does not work, application of force in the reverse direction of the dislocation is indicated. For example, for a posterior dislocation, the clinician lifts the tibia anteriorly while pushing the femur in the posterior direction.
Assessment of vascular compromise should be done immediately post-reduction. Any signs of vascular injury require immediate consultation of a vascular surgeon. In the absence of specific signs of vascular injury, an ABI should be measured. Readings of <0.9 warrant further vascular workup with CT angiography or duplex ultrasonography.
To reduce recurrence of dislocation, a splint should be applied with the knee in 20 degrees of flexion, and ice and elevation of the knee.
Follow up care:
Admit the patient to the hospital to monitor extremity vascular function. Regular assessment of neurovascular function should be done every 3-4 hours for at least 24 hours.
AP and lateral radiographs should be done one week after discharge to confirm reduction. Orthopaedic follow up is recommended to assess ligament injury and the need for surgical reconstruction.
Following treatment and prompt reduction, 60-70% of patients recover with a painless, stable knee. Approximately 15% of patients will return to reasonable function, and the remaining 15% have a chronically unstable knee (Medscape). However, even with a full recovery, many patients are unable to return their pre-injury function level. Patients who have reconstruction of injured ligaments tend to have better functional results.
The most dangerous complication is injury to the popliteal artery. Delay in treatment of vascular compromise increases the risk of warm ischemia and above the knee amputation. The majority of cases with ischemia present for greater than 8 hours will require amputation. A popliteal artery lesion or thrombosis can also form, but may not be present for up to a few weeks post injury.
Short term complications: peroneal nerve injury, compartment syndrome, DVT
Possible associated fractures include: tibial plateau, tibial shaft, proximal fibula
Other complications: ligamentous injury, pseudoaneurysm, chronic instability, arthrosis, stiffness, chronic pain
Depending on the recovery and severity of the injury, patients who suffer from dislocation of the knee may not be able to return to their preceding activities or occupation. This can have a significant impact on their psychosocial health as well as economic status.
Knee dislocation is often confused with subluxation of the knee, which is a partial dislocation due to ligamentous injury and the knee feels as if it has “given out”.
Knee dislocation, tibiofemoral dislocation, popliteal artery injury.
On examination, recognize signs of vascular compromise and ligamentous instability; correctly perform reduction of the knee; be able to interpret vascular imaging, tests, and radiographs correctly; apply splint post reduction.