Knee Dislocation: Pearls and Pitfalls
- Apr 7th, 2015
- Manpreet Singh
In the U.S. there are about 1.3 million ED visits for knee trauma each year. Knee dislocation is an infrequent but serious limb-threatening injury that is traditionally associated with high-velocity injuries. The true frequency of knee dislocations is unclear as up to 50% of injuries self-reduce before presenting to the ED. The knee sits encapsulated by a dense supportive shell of ligaments and tendons; therefore, a significant blunt force is usually needed to disrupt the joint. However, there have been some rare reports of low-energy mechanisms resulting in dislocation; these were usually associated with a twisting / rotational action (often from athletic injury) and in obese individuals (1). Complications vary from ligamentous and meniscal injuries to more severe popliteal artery, popliteal vein, and peroneal nerve injuries. Common mechanisms of injury include motor vehicle accidents, falls, industrial accidents, or high-velocity sports (downhill skiing, football, etc.). The presentation of knee dislocations varies widely and is usually clinically obvious but may sometimes be subtle necessitating a high index of suspicion. A significant number (50%) of dislocations spontaneously reduce prior to ED evaluation and they are not uncommonly misdiagnosed as compartment syndrome or missed altogether (1-3).
The femoral artery gives rise to the popliteal artery. The collateral blood supply system of the lower leg breaks off from the popliteal artery below the knee joint. The popliteal artery is held down firmly at either end of the knee joint, proximally by the tendinous hiatus of the adductor magnus muscle and distally by the tendinous arch of the soleus muscle. As a result, the popliteal artery is anchored down like a bowstring, placing it at risk for injury during knee dislocations.
Knee dislocations may occur from MVAs, high impact sports, downhill skiing, or gym exercises (leg presses with heavy loads). Most dislocations will be clinically evident, presenting with a history of trauma and have an abnormal knee examination. Given the joint has a tendency to spontaneously reduce, it is important to determine the mechanism and position of the leg immediately following the injury. The patient may report a history of the knee “popping out” and then “popping in.”
The knee examination should focus on evaluating the 1.) appearance, 2.) integrity / stability of the knee joint, 3.) assessing distal perfusion, and 4.) evaluating for peroneal nerve injury.
1.) The degree of initial deformity cannot be used as a marker of the likelihood of injury to the popliteal vasculature (4). The physical exam may range from an obviously deformed knee with swelling and gross instability to a nearly normal exam (seen most commonly in obese patients). Hematoma leak into the adjacent soft tissue of the thigh or calf with a normal-appearing knee should raise suspicion for disruption of the joint capsule from a knee dislocation.
2.) Assess the ACL, PCL, MCL, LCL, and PLC. Usually, three or more major knee ligaments (3/4th) must rupture for the knee joint to dislocate, thus any knee exam with multiplanar instability should be a suspected dislocation that spontaneously reduced.
3.) Evaluating the vascular status of the limb is the most essential part of the physical examination. The circulatory status of the lower leg can be assessed by palpating distal as well as popliteal pulses, obtaining Ankle-Brachial Indices (ABIs), and performing screening duplex ultrasound (if available). Emergent vascular surgery consultation is warranted in patients with “hard” signs of vascular injury, these include:
- Absence of pulse
- Signs of limb ischemia
- Pale or dusky leg
- Paresthesias and paralysis
- Rapidly expanding hematoma
- Pulsatile bleeding
- Bruit or thrill over the wound
Delay of popliteal artery repair beyond 8 hours, in patients with popliteal artery injury, invariably leads to limb amputation (5). Physical examination alone is not reliable enough to rule out popliteal vessel injury. There is no physical exam finding that reliably rules out vascular injury. For example, the presence of warm skin over the lower extremity has been reported in the presence of complete popliteal arterial occlusion. Full dorsalis pedis and posterior tibial pulses following reduction do not exclude vessel injury as patients may still have occult intimal tear and are at risk of thrombus formation hours to days after event (5).
4.) Peroneal nerve injury (“foot drop”) is the most common neurologic complication associated with knee dislocation. Complete nerve palsy in the acute setting portends a poor prognosis for recovery (6). The peroneal nerve provides ankle dorsiflexion, toe extension and sensation to first dorsal web space (deep peroneal nerve).
- Plain X-rays: Should be obtained if patient has good peripheral pulses. Used to confirm the clinical findings and document associated fractures (femur and tibial).
- Check for asymmetric / irregular joint space
- Check for avulsion fxs
- Segond sign – lateral tibial condyle avulsion fx
- Standard angiography: Standard of care modality.
- CT angiography: Provides accurate non-invasive assessment of vascular injury and is most commonly used in place of standard angiography.
- Color flow Doppler ultrasonography: Gaining acceptance as an alternative to standard angiography in select groups of low-risk patients.
- MRI: Helps determine extent of injury, will identify ligamentous injury, joint capsule, meniscus, and articular cartilage integrity. Rarely used in acute management of knee dislocations.
Classification is based on the position that the tibia is displaced relative to the femur. There are five general types of knee dislocations in order of frequency (7):
- Anterior: most common dislocation (50-60%) and occurs from hyperextension of the knee resulting in tearing of the posterior structures. This injury drives the distal femur posterior to the proximal tibia.
- Posterior: most commonly associated with popliteal artery injury. Usually results from direct blow to the proximal tibia displacing it posterior to the distal femur.
- Lateral or Medial
- Rotatory: There are 2 types of rotator dislocations with posterolateral being the most important, although rare. Posterolateral dislocations cannot be reduced by closed reduction. These result from the body rotating in opposite direction of the remainder of the lower leg.
The first step in management involves immediate reduction of obviously dislocated knee, especially if neurovascular compromise exists, without radiographs. Neurovascular status should be documented before and after attempting reduction. Prior to reduction evaluate the patient for signs of a posterolateral dislocation (ie “dimple sign”), as these dislocations are not amenable to closed reduction. An anteromedial skin furrow, or “dimple sign” at the medial joint line, is suggestive of a posterolateral dislocation, which are irreducible. Attempts at closed reduction may compromise the thin veil of skin overlying the prominent femoral condyle in posterolateral dislocations leading to skin necrosis.
The initial approach to reducing all knee dislocations is to apply longitudinal traction to the extremity. This is usually all that is required to reduce a knee. Anterior knee dislocations may require additional lifting of the distal femur, whereas posterior dislocations may require lifting of proximal tibia to complete reduction. After reduction, the knee should be immobilized in a long leg posterior splint with the knee in 15-20 degrees of flexion.
Vascular examination at bedside following a knee dislocation is of limited accuracy. Controversy exists regarding the necessity of angiography in all patients with knee dislocations to screen for the presence of popliteal artery injury. All patients with weak/absent pulses or hard signs for vascular injury should have angiography performed along with emergent vascular surgery consultation. It is recommended that patients with ABI < 0.9, asymmetric pulses, or abnormal doppler ultrasound obtain angiography (or comparable vascular study such as a CTA) in consultation with vascular surgeon to evaluate integrity of popliteal artery (8). Some studies have shown that vascular injury can be reliably excluded in patients with low-energy knee dislocations, a normal physical examination with Doppler US, and ABI > 0.9. However, these patients should receive close monitoring with serial examinations (9-11).
- Popliteal artery injury
- Compartment syndrome of leg
- Common peroneal nerve injury
- Associated fracture or ligamentous injury
- Early osteoarthritis
- Chronic pain
- Assume all patients with suspected knee dislocation have vascular injury until proven otherwise.
- Obese individuals may have knee dislocations with minor mechanisms, even higher index of suspicion in these individuals.
- Nearly half of all knee dislocations spontaneously reduce prior to ED presentation.
- Posterior knee dislocations are more commonly associated with popliteal artery injury.
- A normal physical examination alone does not reliably exclude vascular injury.
- Delay of popliteal artery repair beyond 8 hours invariably leads to limb amputation, act quickly!
- Posterolateral dislocations are irreducible by closed reduction.
- Peroneal nerve injury is the most commonly associated neurologic problem in knee dislocations.
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