Radiologic Case Study
- October 2006 - Volume 29 · Issue 10:
A 57-year-old woman presented with left knee pain after falling off of a stool.
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Figure 1: Consecutive sagittal proton density MRI of the left knee. Figure 2: Consecutive coronal T2 MRI of the left knee.
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Answer to Radiologic Case Study
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Figure 1: Consecutive sagittal proton density MRI of the left knee shows a torn PCL (A,B) and ACL (C). Anterior translation of the tibia (D). Figure 2: Consecutive coronal T2 MRI of the left knee shows an intact lateral collateral ligament (A). Tears of the MCL and lateral meniscus can be seen, as well as lateral translation of the tibia (B-D).
The patient presented clinically with an anterolateral knee dislocation. She underwent an immediate closed reduction. To help guide further management, magnetic resonance imaging (MRI) was performed to assess the extent of ligamentous, meniscal, and osseous injuries, as well as the integrity of the popliteal vessels.
Magnetic resonance imaging demonstrates tears of the posterior cruciate ligament (PCL), anterior cruciate ligament (ACL), and medial collateral ligament (MCL), with considerable subcutaneous and myofascial edema (Figures 1-3). In addition, there is anterior and lateral translation of the tibia in relation to the femur (Figures 1 and 2). The popliteal artery is intact (Figure 3).
Figure 3: Axial T2 MRI of the left knee shows a torn MCL. The popliteal artery is intact. Considerable myofascial edema is present.
Knee dislocations, which refer to multiligamentous injury or multidirectional instability, are uncommon injuries. They account for <0.02% of all orthopedic injuries.1-4 Because many of these spontaneously reduce by presentation, this is likely an underestimation and the actual incidence is unknown. In addition, swelling and joint effusion may not be present due to capsular damage and fluid extravasation.5 The etiologies of knee dislocations have been identified as either high-velocity injuries such as motor vehicle accidents, or low-velocity injuries including falls and sports injuries. Generally defined as multiligamentous injury, there is some variability in the number of ligaments that must be torn to constitute knee dislocation. Although some sources define knee dislocations as rupture of at least 2 of the 4 major knee ligaments, multidirectional instability is likely the more accurate definition.6
A second example of a knee dislocation is seen in Figure 4: radiographs of an 18-year-old patient who injured and dislocated his knee while playing football. These initial radiographs demonstrate anatomic alignment of the tibia and femur, thus indicating spontaneous reduction. Magnetic resonance imaging demonstrates tears of the ACL, PCL, and lateral collateral ligament (Figure 5).
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Figure 4: AP (A) and cross-table lateral (B) radiographs of the left knee show anatomic alignment of the tibia and femur without fracture.
Figure 5: Select sagittal proton density MRI of the left knee shows ACL (A) and PCL (B) tears. Select coronal proton density MRI of the left knee with fat saturation shows a tear of the lateral collateral ligament (C) and an intact MCL (D).
Two main classification systems of knee dislocations exist. The first system is based on the position of the tibia in relation to the femur at presentation. Thus there are anterior, posterior, lateral, medial, or rotary dislocations. Rotary dislocations can be broken down into anteromedial, anterolateral, posteromedial, and posterolateral dislocations. This classification system helps in determining the mechanism of injury and potential associated injuries.6
Anterior knee dislocations are the most common. They usually are due to forced hyperextension as can occur during sports. These also have been noted to occur in morbidly obese patients during ambulation or when rising from a seated position. The posterior capsule and ACL are torn, and often the PCL as well.6 Injury to the popliteal artery is estimated to occur in about 39% of anterior knee dislocations.1
Posterior knee dislocations are the next most common type, often as a result of motor vehicle accidents when the tibia hits the dashboard. They also have been found to occur in morbidly obese patients when rising from a seated position or with a misstep. The PCL ruptures as the tibia is pushed posterior to the femur. The ACL is also commonly injured.6 The popliteal artery is injured in approximately 44% of posterior knee dislocations.1
Lateral and medial knee dislocations generally occur when the foot is planted and there is a varus or valgus force to the knee, respectively. Lateral dislocations account for 18% and medial dislocations account for 4% of knee dislocations.1 Both types involve tears of both collateral ligaments and at least one cruciate ligament. The popliteal artery is injured in only 6% of lateral dislocations.1 Neurologic injury occurs more often, due to the peroneal nerve being lateral to the fibular head.6
Twisting forces cause rotary dislocations. The most significant of these types is the posterolateral dislocation as it requires open reduction. Valgus stress combined with internal tibial rotation to a flexed, non-weight bearing knee causes the medial femoral condyle to buttonhole through the anteromedial capsule.5 Peroneal nerve palsy is often associated with this dislocation.7
The above classification system is limited in that with spontaneous knee displacements, the direction of displacement occasionally is unknown. Also, the ligaments that are injured are only suggested, and not definitively stated. Both cruciate ligaments are not always torn. In addition, the status of the collateral ligaments is not entirely addressed with anterior and posterior dislocations.7
A second classification system developed by Schenck7 classifies knee dislocations (KD) based on ligamentous and associated injuries. This classification system is useful for surgical planning and comparing similar injuries. KDI dislocations involve one intact and one torn cruciate ligament. KDII injuries are rare, and involve both cruciate ligaments being torn with intact collateral ligaments. Both cruciate ligaments and either the MCL or lateral collateral ligament are torn in KDIIIM and KDIIIL injuries, respectively. KDIIIM dislocations are more common. KDIIIL dislocations have more disability and a worse prognosis. KDIV dislocations are injuries where both cruciate and both collateral ligaments are torn.8 Finally, KDV injuries are dislocations with associated periarticular fractures.9
In addition to the ligamentous damage in knee dislocations, vascular and neurologic complications should also be considered. The popliteal artery is tethered in the popliteal fossa by the adductor hiatus proximally and the fascial arch of the proximal soleus distally. Other than at the ends, the artery is not up against bone and thus is susceptible to injury, which occurs in approximately 20%-40% of knee dislocations.5 Popliteal artery injuries include transection, contusion, injury to the intimal and medial layers without loss of continuity, and thrombus formation.10 Compartment syndrome and deep vein thrombosis are other possible complications.5 Revascularization within 6-8 hours is recommended, as after 8 hours there is a higher chance of gangrene requiring amputation.1
If there is obvious vascular injury and limb-threatening ischemia, immediate vascular repair is necessary. If there are clinical signs of vascular injury without limb-threatening ischemia, selective arteriography is recommended. If there are no clinical signs of vascular injury, close monitoring without selective arteriography is recommended.6
Neurologic damage generally involves stretching or transecting the peroneal nerve. The peroneal nerve runs around the proximal fibula and is injured in approximately 33% of knee dislocations.5 Peroneal nerve injuries usually occur with lateral and posterolateral dislocations but can occur with any type of dislocation.6 It is generally advised to wait approximately 3 months before intervention, as there may be spontaneous recovery of nerve function. The prognosis for nerve recovery is generally poor.6
It is recommended that knee dislocations be reduced immediately with regular assessment of the neurovascular status. Closed reduction is often performed, except with posterolateral dislocations. Conservative treatment is usually reserved for patients who are elderly or sedentary, or have significant comorbidities. This consists of closed reduction and cast immobilization of the knee for 3-10 weeks. However, stiffness and dysfunction often result.5 Surgical treatment is the recommended route. Surgical timing, graft selection, and surgical technique remain controversial.4 The goal of surgery is anatomic repair and reconstruction of the injured ligaments and menisci (Figure 6).4
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Figure 6: AP (A) and lateral (B) radiographs of the right knee demonstrate repair of the ACL and PCL in a patient with KDIIIM injury. Although the MRI (not shown) suggested an MCL tear, the surgeon determined at the time of surgery that an MCL repair was not required.
Knee dislocations are relatively uncommon. However, due to potential neurovascular complications, it is important that the diagnosis is not missed. There are two main classification systems. The position classification system, defined by the relation of the tibia to the femur, helps identify the mechanism of injury and possible associated complications. The Schenck7 classification system defines the dislocation based on ligamentous and associated injuries, and thus is useful for surgical planning. It is well recognized that treatment should consist of immediate closed reduction with frequent neurovascular assessment, followed by surgical repair/reconstruction. The timing and surgical technique remain controversial.
- Green NE, Allen BL. Vascular injuries associated with dislocation of the knee. J Bone Joint Surg Am. 1977; 59:236-239.
- Hoover NW. Injuries of the popliteal artery associated with fractures and dislocations. Surg Clin North Am. 1961; 41:1099-1112.
- Kennedy JC. Complete dislocation of the knee joint. J Bone Joint Surg Am. 1963; 45:889-904.
- Rihn JA, Groff YJ, Harner CD, Cha PS. The acutely dislocated knee: evaluation and management. J Am Acad Orthop Surg. 2004; 12:334-346.
- Henrichs A. A Review of knee dislocations. J Athl Train. 2004; 39:365-369.
- Brautigan B, Johnson DL. The epidemiology of knee dislocations. Clin Sports Med. 2000; 19:387-397.
- Schenck RC Jr. The dislocated knee. Instr Course Lect. 1994; 43:127-136.
- Schenck RC Jr, Hunter RE, Ostrum RF, Perry CR. Knee dislocations. Instr Course Lect. 1999; 48:515-522.
- Helgeson MD, Lehman RA Jr, Murphy KP. Initial evaluation of the acute and chronic multiple ligament injured knee. J Knee Surg. 2005; 18:213-219.
- Reckling FW, Peltier LF. Acute knee dislocations and their complications. Clin Orthop Relat Res. 2004; 422:135-141.
Drs Bui, Ilaslan, and Sundaram are from the Department of Diagnostic Radiology and Dr Jones is from the Department of Orthopedic Surgery, Cleveland Clinic Foundation, Cleveland, Ohio.
Reprint requests: Kimmie Bui, MD, c/o Deborah Jones, Cleveland Clinic Foundation, Dept of Radiology, HB6, 9500 Euclid Ave, Cleveland, OH 44195.