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Acute Knee Injuries: Causes and Complications
Because of the knee’s complexity, signs of injury are often subtle and variable. The authors discuss the many mechanisms and consequences you need to consider.
By Ben Fickenscher, MD, and Barry Knapp, MD
About a half-million people visit U.S. emergency departments each year to be evaluated for acute knee injuries—most commonly following sports-related accidents. Signs of these injuries are often easy to overlook, owing to the knee’s complexity, but to do so can put the patient at exteme risk. Soft-tissue injuries cause significant long-term joint dysfunction, while bony fractures are associated with complications such as nonunion, delayed union, osteoarthritis, avascular necrosis, fat embolism, and deep vein thrombosis. Neurovascular compromise and compartment syndromes can be life- and limb-threatening.
To best serve the patient with an acutely injured knee and to avoid short- and long-term complications, the emergency physician must be familiar with the anatomy and physiology of the joint, historical and physical exam findings unique to specific injuries, appropriate diagnostic strategies, and the appropriate management and follow-up plan for the various types of knee injuries.
DETERMINING THE ETIOLOGY
Evaluation of the injured knee must proceed carefully and systematically, as the types of injury are many and individual treatments and prognoses vary widely. A thorough, focused history enables the clinician to avoid missing subtle injuries and arrive at the correct diagnosis.
In evaluating acute knee pain or dysfunction, ask about a history of injury related to the lower back, hips, and ankles. Injuries to these sites often occur concurrently, complicate the clinical picture, and require an expanded diagnostic and therapeutic work-up. Also ask about infectious and inflammatory diseases or chronic conditions that may put the patient at risk for significant injury even in the face of seemingly minor trauma.
Most important to the interview is the mechanism of injury, especially the position of the leg at the time of injury and the direction and intensity of the forces acting on the body. Pain and swelling are important diagnostic clues to injury. However, a lack of pain or swelling at the time of the exam does not preclude the existence of significant internal damage.
Ask the patient about tearing or popping sensations, immediate deformity or swelling and its progression over time, and whether he could walk after the injury. Immediate deformity, hemarthrosis, instability, and inability to ambulate are predictive of intra-articular fractures, major ligamentous tears, and vascular injuries, while delayed swelling and pain are not. In fact, Noyes and colleagues found that 72% of patients presenting with acute hemarthrosis and an otherwise stable knee on exam had an anterior cruciate ligament (ACL) tear. In this study, 44% of all hemarthroses were secondary to a complete ACL rupture.
Other etiologies of hemarthrosis include intra-articular fractures, meniscal tears at the area of vascularity, and patellar dislocations. With major damage to the structures surrounding the joint, significant effusions may be inapparent at the time of the examination as the fluid disperses into the contiguous tissues. Intermittent or continuous locking of the knee, where the patient cannot fully extend the leg, suggests meniscal tears or debris, such as avulsed bone, interfering with the joint mechanism. Giving way, on the other hand, is a nonspecific historical element that may be due to instability, pain, or the anticipation of pain.
CONDUCTING THE PHYSICAL EXAM
Physical examination should begin with a rapid assessment of distal neurovascular integrity. While distal pulses and neurologic function do not rule out significant injury, they do allow time for a thorough and focused exam prior to consultation.
Assess the integrity of the popliteal artery not only by palpating the dorsalis pedis and posterior tibial pulses, but also by palpating the popliteal fossa for a strong impulse and the absence of a pulsatile hematoma. Two branches of the sciatic nerve pass through the knee as the tibial and peroneal nerves to supply the lower leg. Fractures about the knee, and especially traumatic knee dislocations, may injure these nerves, and their function must routinely be assessed.
The peroneal nerve supplies sensation to the first web space and the dorsal-lateral foot and provides motor function to the dorsal flexors of the foot. A palsy of the peroneal nerve will present as a foot drop with anesthesia or paresthesias along its course. The tibial nerve supplies sensation to the plantar foot and is responsible for motor function of the muscles of plantar-flexion. It is important to remember to examine both knees to determine normal versus abnormal anatomy, integrity, and function.
Place the patient supine on a stretcher with both legs exposed, so that the injured knee can be compared to the uninjured knee. First, evaluate the knee for swelling and distinguish local soft-tissue swelling from effusion. A small effusion may be difficult to identify on physical exam, but should be detected with careful inspection. By applying gentle, consistent, inferomedial pressure on the suprapatellar pouch, any fluid should be appreciated by acute fullness of the knee medially.
Next, it is important to identify the area (or areas) of maximal tenderness, the location of which will aid in arriving at the proper diagnosis. Pain along the tibial or femoral epiphysis might indicate a physis injury in a growing child or adolescent. Pain with palpation medially or laterally suggests collateral ligament strain, while pain along the joint line is suspicious for meniscal injury. Posterior pain and fullness may indicate popliteal artery pseudoaneurysm.
A traumatic Baker cyst, formed when effusion fluid is forced posteriorly into the popliteal fossa, may present similarly; however, it is imperative to rule out vascular injury. This is often done using color Doppler ultrasonography, magnetic resonance angiography, or computed tomography angiography (CTA).
If the patient experiences pain on palpation of the tissues on either side of the patella, be alert to the possibility of a patellar dislocation that reduced spontaneously prior to presentation.
TESTING FOR TROUBLE
Once areas of pain have been identified, assess the knee through range-of-motion and stress testing. Again, it is important to make determinations of normal versus abnormal by comparison with the patient’s uninjured knee. The knee should travel from slight hyperextension to about 135 degrees of flexion. Rotation at the knee is prevented at full extension, but it should rotate approximately 40° with the knee flexed to 90°. Range of motion should be demonstrated both actively and passively. The integrity of the extensor mechanism can be determined by having the patient extend the leg against gravity, with any weakness relative to the other knee requiring further investigation. Complete rupture of either the quadriceps tendon or the patellar tendon, evidenced by an inability to extend the leg against gravity, mandates immediate orthopedic consultation and surgical fixation within seven days.
The anterior drawer test, where the patient’s knee is flexed to 90° and the ability of the tibia to move anteriorly relative to the femur is assessed, is probably the best-known test for ACL injury but also the most unreliable in the acute setting. A recent review of the literature calculated this test to have a sensitivity of only 20% and a specificity of 88% in evaluating the integrity of the ACL.
The pivot shift or “jerk” test is also not recommended for ACL testing in the acute setting. In this maneuver, the tibia is subluxed with the knee in extension and felt to relocate with flexion. While sensitive for ACL injury when positive, the test causes significant pain to the patient and may worsen an acute injury.
Lachman’s test is an alternative test for ACL integrity that increases diagnostic accuracy to nearly 100% while remaining safe. The knee is flexed to 20° to 30° and the thigh is stabilized with one hand. The other hand is then used to pull the tibia anteriorly, and the distance of tibial travel is gauged. If the distance traveled is greater than that in the uninjured knee, or if the endpoint feels “soft,” the test is positive.
While the anterior drawer sign fails in diagnosing ACL injuries, the posterior drawer sign remains the gold standard for posterior cruciate ligament (PCL) testing. If the tibia moves 5 mm relative to the femur or a soft endpoint is felt, the test is considered positive for PCL injury. The PCL can also be tested by looking for the “posterior sag sign.” The patient is placed supine and encouraged to relax the muscles of the leg. The leg is then elevated passively at the ankle. If the tibia sags posteriorly, the test is positive.
Meniscal injuries can be assessed using the McMurray and Apley tests. For the McMurray test, grasp the leg at the ankle with one hand and hold the knee along the joint line with the other. Alternately rotate the leg internally and externally while it is flexed at the knee. If the patient feels pain or the examiner feels crepitance at the joint line, the test is positive. Positive findings with external rotation indicate medial meniscus pathology, and vice versa.
Apley’s test is performed with the patient prone. While the knee is flexed to 90º, apply downward pressure to the heel, rotating the leg internally and externally. Pain with these maneuvers that is relieved with traction and neutral positioning of the leg is suggestive of meniscal trauma.
In determining meniscal pathology, the best diagnostic accuracy is reached through employing multiple techniques rather than relying on a single test. Fowler found the sensitivity and specificity of joint line tenderness, the Apley test, and the McMurray test to be 85% and 29%, 16% and 80%, and 28% and 95%, respectively. However, he also found that a combination of these maneuvers were highly predictive of findings during subsequent arthroscopy.
Finally, assess the collateral ligaments with valgus and varus stress testing with the patient supine and the knee held in both full extension and at 30 degrees of flexion. The testing is deemed positive when the joint line of the injured knee opens more than does that of the uninjured knee under the same stress. When held in full extension, the cruciate ligaments provide resistance to valgus or varus forces, and a positive test is indicative of a complete collateral ligament tear. At 30°, however, the cruciate support is removed. Positive stress testing at 30° and negative testing at full extension suggest a partial tear.
GETTING THE RIGHT RADIOGRAPHS
If done in a consistent, complete, and methodical manner, the physical exam of the acutely injured knee should provide the examiner with reliable knowledge of the integrity of the knee and its surrounding structures and may identify the exact nature of the injury. Suspicion of significant injury should prompt radiographic evaluation to confirm suspected bony injury and to rule out associated avulsions, effusions, or dislocations.
When bony fracture is suspected or significant injury prevents reliable physical examination of the knee, plain radiographs are indicated. The standard series consists of anteroposterior (AP), lateral, and oblique views. It is important to look not only for fracture lines, but also to assess the alignment of the structures, search for the presence of small avulsion fractures, and evaluate any effusions or fluid levels.
On the lateral radiograph, a fat-fluid level indicates an intra-articular fracture. The oblique view may reveal tibial plateau fractures and obliquely oriented femoral condyle fractures not seen on AP or lateral films.
Other views are available and should be requested depending on the clinical scenario. A tunnel view assesses the intercondylar notch and may reveal tibial spine fractures or loose fragments within this space. An axial view, also known as a “sunrise” or “skyline” view, is important to investigate suspected patellar subluxation or to identify a vertical patellar fracture.
While not sensitive for ligamentous injury, plain radiographs may reveal small avulsion fragments suggestive of damage to the support structures of the knee. A classic finding is the lateral capsular sign, or Segond fracture, which is an avulsion at the lateral tibial condyle that indicates injury to the ACL. Avulsion at the medial tibial plateau suggests PCL or medial meniscus injury, while fragments noted at the superior aspect of the patella are a clue to quadriceps tendon rupture.
Other plain film diagnostic views exist, but none are particularly useful for the acutely injured knee. These should be ordered in consultation with the orthopedist. If clinical suspicion for significant underlying soft-tissue pathology is high, a negative standard radiographic series should prompt either immobilization with orthopedic follow-up or immediate consultation and advanced imaging or management. If the concern for substantial injury is low, less aggressive treatment with rest, ice, elevation, and partial weight bearing and referral to primary care is probably sufficient.
When plain films fail to identify suspected fractures or to adequately delineate known fractures, further imaging may be warranted. Computed tomography is especially useful in assessing the degree of articular depression present in tibial plateau fractures and is used by the orthopedist in preoperative planning. Magnetic resonance imaging is more sensitive at identifying meniscal and ligamentous injury, but it is largely reserved for orthopedic follow-up and preoperative planning.
Arthrocentesis is another modality that is useful in the acute setting, being both diagnostic and therapeutic in cases of traumatic effusion and acute pain. Fat globules in a bloody effusion are diagnostic of intra-articular fracture and should prompt appropriate imaging, treatment, and consultation. While the decompression of the joint by removal of a tense effusion in itself often relieves pain significantly, injection of anesthetics at the end of the procedure enhances patient comfort and thereby aids in further physical examination. Bupivacaine is often used, but morphine (5 mg diluted to 30 ml in normal saline) has been shown to be more effective and provides up to 24 hours of relief.
DECISION RULES
Multiple, well-constructed studies strongly support forgoing radiographic evaluation of many acute knee injuries. These studies focus instead on validated clinical decision rules formulated in an effort to reduce radiation exposure and cost and to save time. The two accepted decision rules guiding radiographic evaluation of the acutely injured knee are the Pittsburgh and the Ottawa knee rules (see box on page 25). Both rules were born from the observation that fewer than 7% of patients presenting with an acute knee injury have sustained a fracture. In recognition of the fact that missed fractures risk significant patient morbidity and medical liability, these rules were calibrated toward high sensitivity at the expense of specificity. The Pittsburgh Knee Rules have a sensitivity of 99% and specificity of 60% and have been shown to reduce the need for radiographs by 52%. The Ottawa Knee Rules have a sensitivity of 98% and specificity of 49% while decreasing radiograph use by 28%.
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Of the two sets of rules, the Ottawa rules have been studied the most often. Perry and Stiell found in their review of the published literature that the Ottawa rules, when properly applied, resulted in fewer radiographs, less time spent in the emergency department, and a mean cost savings of between $34 and $55 per patient. They also did not result in misdiagnosis. The false negative rate of both rules approaches 1%. Therefore, using these rules to avoid radiographic investigation does not obviate the need for appropriate immobilization of the joint and routine orthopedic follow-up of all worrisome knee injuries. TYPES OF KNEE INJURY
Ligamentous, meniscal, extensor mechanism, and other soft-tissue injuries of the knee far surpass bony fracture in incidence. These injuries, however, are variable in their presentation, severity, and treatment. Additionally, they are most often not identifiable on radiographs routinely available to the emergency physician. It is important, therefore, to be familiar with these injuries and to formulate a consistent, appropriate treatment and referral strategy.
While none of the injuries, in isolation, requires emergent repair, some do warrant surgical treatment within two weeks for optimal functional result. It is thus prudent to approach treatment of all significant soft-tissue injuries of the knee in similar fashion. Unless contraindications exist, all patients should be prescribed rest, ice, elevation, and a nonsteroidal anti-inflammatory drug. The knee should be placed in an immobilizer and the patient should be advised to keep off the leg, if possible. Finally, all patients should have reliable and timely primary care or orthopedic follow-up.
While soft-tissue injuries of the knee are amenable to conservative treatment and outpatient follow-up, bony fractures and dislocations must be discovered on initial presentation and managed immediately. Significant morbidity and medicolegal liability are possible if these injuries are missed. The management plan of these injuries should most often be made in concert with an orthopedic consultation while the patient is in the emergency department. Fractures of the knee include distal femur fractures, tibial spine and plateau fractures, and patellar fractures.
DISTAL FEMUR FRACTURES
Distal femur fractures account for approximately 4% of all femur fractures. Fracture location may be described as supracondylar, intercondylar, and condylar. Of these, intercondylar and condylar fractures are considered intra-articular. These fractures are most common in high-velocity or high-energy injuries such as those sustained in motor vehicle collisions and falls from a height. Patients will typically present with pain and swelling above the patella and an acute hemarthrosis, and in many cases they cannot put weight on the injured leg.
These fractures are usually readily apparent on AP and lateral radiographic views of the knee, but they will occasionally require an oblique view to see an obliquely oriented fracture line. Given the extreme forces required for such injuries, the hip and tibia should also routinely be imaged to rule out concomitant fractures.
It is important to remember with these (and all fractures of the knee) that any violation of the skin overlying a fracture constitutes an open fracture and mandates irrigation, antibiotics, and urgent orthopedic evaluation and treatment. Routine management in the emergency department includes posterior splinting, traction if the fracture is displaced, and analgesia in anticipation of repair in the operating room. Complications of distal femur fractures include deep vein thrombosis, fat embolism, delayed union, malunion, angulation deformity, and osteoarthritis. Accurate identification, proper management, and early orthopedic consultation can minimize these risks.
TIBIAL SPINE FRACTURES
The intercondylar eminence, commonly called the tibial spine, is centrally located on the surface of the proximal tibia. It is formed by the medial and lateral tubercles, the former being larger and seated more anteriorly. The ACL and anterior horns of the menisci attach anteriorly on the anterior intercondylar fossa and medial tubercle, while the PCL and posterior horns of the menisci attach posteriorly in the posterior intercondylar fossa and to the lateral tubercle.
The tibial spine may fracture under forces that cause knee twisting, hyperflexion, hyperextension, or varus-valgus stress to the joint. Patients present with pain and swelling of the knee, often with acute tense hemarthrosis. They will be unable to bear weight on the affected leg. The exam may suggest ACL rupture, with a positive anterior drawer sign and Lachman test. This is especially true in children and adolescents whose physeal attachments are weak relative to their collateral ligaments. For this reason, a fracture of the tibial spine should be considered in any child or adolescent who presents with a history and examination consistent with an ACL injury.
A tunnel view may be added to the standard AP and lateral radiographic series to increase diagnostic accuracy. Tibial spine fractures are not infrequently associated with other ligamentous and intra-articular injury, and these must be searched for on both the physical exam and radiographic imaging. (A caveat on radiographic evaluation: a fabella, an inconsistent sesamoid bone that rests in the lateral head of the gastrocnemius muscle, may occasionally be mistaken for an avulsion of the tibial spine.) When managed appropriately, tibial spine fractures are associated with low complication rates that are largely related to long-term ACL function. The three types of tibial spine fractures are listed in the table below.
TIBIAL PLATEAU FRACTURES
Tibial plateau fractures are intra-articular fractures that involve the tibial condyles proximal to the tibial tuberosity. These represent 1% of all fractures and 8% of fractures in the elderly. They are most often caused by a valgus force applied to an axially loaded knee and require high-energy forces. A classic example of a common mechanism for tibial plateau fractures is the car bumper injury, where the lateral plateau is broken when the bumper of a moving vehicle strikes a pedestrian’s leg from the side, driving the femoral condyles forcefully into the tibial plateau.
Lateral plateau fractures constitute up to 70% of all plateau fractures, likely secondary to the relative protection from varus stress afforded to the medial aspect of the knee by the contralateral leg. In as many as one-third of injuries, both plateaus are broken. While the general rule for plateau fractures is high-impact, high-energy forces, the elderly may sustain significant injury in the face of seemingly incidental trauma or during routine daily activities. Thus, it is important to suspect a tibial plateau fracture in the elderly patient who presents with an acute knee hemarthrosis.
Most often, patients present with intense pain following significant trauma, acute hemarthrosis with decreased range of motion, an inability to bear weight, and joint line tenderness to palpation.
When you suspect injury to the plateau, assess the neurovascular integrity of the distal extremity thoroughly. Vascular complications are very common as a result of plateau fractures, due to injury to the popliteal artery as it courses through the fossa. Lateral tibial condylar fractures may cause a peroneal nerve palsy, which most often represents a temporary neurapraxia. Palsies are also associated with anterior tibial artery injury.
Plateau fractures also carry up to a 25% risk of concomitant ligamentous injury. Most fractures can be diagnosed by reviewing the standard radiographic knee series including an oblique view. These films should also be closely investigated for avulsion fragments suggesting ligamentous damage. As mentioned previously, the Segond fracture, or lateral capsular sign, is an avulsion of the lateral plateau denoting ACL rupture. In the instance of a highly suspected plateau injury without concrete radiographic evidence, a cross-table lateral view might be obtained to look for a fat-fluid level. This finding is almost diagnostic of an occult fracture.
Several classification systems are used to guide treatment of plateau fractures, but no consensus exists. One often-cited system is the revised Hohl classification, which first separates nondisplaced or minimally displaced fractures from displaced fractures, then separates the displaced fractures by the amount and nature of displacement or plateau depression.
No matter the classification system used by the orthopedic surgeon, it is most important that the emergency physician recognize the fracture and initiate the proper consultation. Management strategies for tibial plateau fractures range from compression dressings to closed reduction and casting or skeletal traction, to open reduction and internal fixation. Conservative therapy, generally reserved for stable nondisplaced fractures, consists of a knee immobilizer after successful reduction, non-weight bearing, and rest, ice, and elevation. Early casting has been associated with residual knee stiffness and is not recommended.
Tibial plateau fractures have an overall favorable prognosis. Predictors of morbidity include high-energy injuries, a large degree of articular depression, high degree of comminution, and open fractures. Anticipated complications include loss of reduction, compartment syndrome, and deep vein thrombosis. Half of patients develop osteoarthritis within 10 years of injury.
PATELLAR FRACTURES
The patella, the largest sesamoid bone in the body, is embedded in the quadriceps tendon and is important to the extensor mechanism of the leg. Patellar fractures account for only about 1% of all fractures, but the majority are intra-articular and must be managed properly to avoid knee joint dysfunction. The history of injury is generally a fall or other direct blow to the patella or forceful flexion of the leg against a contracted quadriceps muscle. Patients present with tenderness to palpation over the patella and often have acute hemarthrosis. If fracture fragments are displaced, there may be a visible or palpable defect.
It is important to assess the patient’s ability to actively and fully extend the leg. In the setting of significant effusion, hemarthrosis, or pain, it may be necessary to perform arthrocentesis with injection of a local anesthetic to allow for accurate examination. While the inability to fully extend the leg against gravity is a sensitive indicator of extensor mechanism disruption and possible patellar fracture, the patient may still be able to extend the leg if the extensor retinaculum remains intact.
Any breaks in the skin around the knee joint should be assumed to represent an open fracture until proven otherwise. The leg should be examined in the position of injury. If any doubt exists regarding an open fracture, a saline load test can be performed. In this test, saline is instilled into the joint via arthrocentesis. Any fluid leakage from local wounds is diagnostic of an open joint. Remember the potential for concomitant injuries and specifically search for these during the physical and radiographic exam. Common associated injuries include femoral neck fractures, hip dislocations, and acetabular fractures.
Patellar fractures should be identifiable on AP, lateral, and sunrise radiographs. False positives may occur in the setting of the bipartite or tripartite patella. These represent a developmental variant resulting from the fusion of two or more ossification centers and can be mistaken for fracture lines. When in doubt, get images of the contralateral patella, since the vast majority of bipartite and tripartite patellas are bilateral.
The management of patellar fractures has evolved significantly over time, and various treatment strategies are still being debated in the literature. Nonoperative therapy, open reduction and internal fixation, and sub-total to total patellectomy are all common options. Conservative treatment consists of extension splinting for four to six weeks with immediate partial weight-bearing. The only absolute indication for operative management is the open fracture. These patients should all have their tetanus status addressed and receive intravenous antibiotics while awaiting surgical debridement and fixation.
Given the continued debate regarding treatment of other fracture types, it is advisable to obtain close orthopedic consultation for all patellar fractures. Complications of patellar fractures include avascular necrosis of small peripheral fragments, persistent patellofemoral pain, and osteoarthritis. These complications can be minimized through accurate diagnosis, appropriate management, and early orthopedic consultation.
The three type of patellar fractures are listed in the table below.
DIAGNOSING DISLOCATIONS
Patellar dislocations occur most commonly in children and adolescents. The majority of dislocations are lateral and extra-articular, but intra-articular and superior dislocations do occur. The mechanism for injury is generally either a direct blow to the anteromedial patella or forced internal rotation of the femur on a planted, externally rotated leg with the knee flexed. The degree of injury ranges from subluxation to frank dislocation, which in turn predicts the amount of retinacular injury, from strain to tear.
Risk factors for patellar dislocation include genu valgum, or the “knock-knee” deformity, relatively weak quadriceps, a history of previous dislocation, and a large Q angle, formed by the intersection of a line drawn from the tibial tubercle to the center of the patella with one drawn from the center of the patella to the anterior superior iliac spine. These lines should be drawn with the leg extended and the quadriceps muscles contracted. The angle should be less than 108 in men and less than 158 in women.
Patients presenting with patellar dislocations often have a history of their knee “giving out,” associated with pain and swelling. They may or may not be able to bear weight and frequently are not able to flex the knee. A significant dislocation should be obvious on careful examination of the knee. If an osteochondral fracture is present in association with the dislocation, there may be acute hemarthrosis.
Some patients will come to the emergency department after a dislocation has been spontaneously, or actively, reduced. Evaluate acute dislocations with AP and lateral radiographs. Consider sunrise views, although this may be impossible due to loss of flexion at the knee. Look closely for osteochondral fragments, which may cause long-term complications and necessitate operative repair. If suspected but not noted on radiographs, an associated osteochondral fracture may be diagnosed via arthrocentesis. The presence of fat globules in the effusion indicates bony disruption.
Most patellar dislocations can be reduced without complication in the emergency department. An anteriomedially directed force is applied to the lateral margin of the displaced patella while the knee is gently extended. At times, the medial facet may lock onto the lateral femoral condyle, making reduction, as described above, difficult. In this instance, applying a posterior force on the lateral patella may unlock the facet and allow relocation.
All dislocations require post-reduction imaging. The patient’s knee should be immobilized in full extension and referred for orthopedic follow-up within seven days. Important adjuncts to immobilization are rest, ice, elevation, analgesia, and non-weight-bearing status. Patients should be advised that approximately 15% of patellar dislocations recur. Up to 50% of patients experience long-term pain or instability.
Tibiofemoral knee dislocation is a severe injury that is potentially limb-threatening. While it is relatively uncommon, it is also likely underdiagnosed. Maintain a high level of suspicion for tibial-femoral dislocations, especially after high-energy, multisystem trauma. Up to half of all dislocations reduce prior to the patient’s arrival in the emergency department and may be associated with other severe trauma that distracts from a thorough extremity examination. Two-thirds of knee dislocations occur during motor vehicle collisions, with the remaining caused by other high-impact mechanisms such as falls and sports injuries.
Tibiofemoral dislocations are described by the location of the tibia in relation to the femur. Anterior dislocations account for the majority and are caused by hyperextension of the knee with a posteriorly directed force on the distal femur. Posterior dislocations generally occur with a flexed knee and a posterior force on the proximal tibia, as in a dashboard injury. Dislocations may also be medial, lateral, or rotary.
DANGERS OF MISSING DISLOCATIONS
A dislocation causes major damage to the surrounding soft tissue, ligaments, and joint capsule. Up to 30% of all dislocations are open. The diagnosis is not difficult to make if the dislocation persists on presentation to the emergency department. However, dislocations disrupt the joint capsule and allow acute hemarthrosis to dissipate into the surrounding soft tissues, making the diagnosis of dislocations that have been reduced prior to presentation more problematic. The presence of varus and valgus instability of the knee with the leg in full extension indicates multiligamentous damage and should prompt further investigation, as should the presence of popliteal fossa ecchymosis. Also, inspect the knee for dislocation when there are concomitant injuries, especially fractures of the ipsilateral or contralateral leg, pelvis, ribs, and skull, and posterior hip dislocations.
The gravest complication resulting from a missed tibiofemoral dislocation is lower leg ischemia secondary to popliteal artery injury, which occurs in up to 33% of cases, most frequently during posterior dislocations. The popliteal artery is at significant risk for injury due to its relative immobility in the popliteal fossa. If blood flow via the popliteal artery is interrupted, there is not enough collateral circulation to adequately maintain perfusion to the leg. Delay in identification and treatment risks significant morbidity. The risk of compartment syndrome and amputation begins to rise after six hours of ischemia, with 86% of patients requiring amputation of the leg if treatment is delayed beyond eight hours.
The so-called “hard signs” of vascular trauma should be assessed: absence of a distal pulse; a pale, cold, or paralyzed leg; pulsatile bleeding; a bruit or palpable thrill; or an expanding or pulsatile hematoma. Several series have noted that at least two hard signs of arterial injury occur in up to 94% of popliteal artery disruptions; however, injuries continue to be missed and the absence of hard signs does not obviate the need for further investigation. In one large series, up to 10% of patients with confirmed vascular injury had normal pedal pulses on initial examination.
Nerve injury is relatively frequent during dislocations as well (see table above). Most commonly the peroneal nerve is affected, resulting in foot drop and loss of sensation of the dorsal foot. Unlike other injuries of the knee that most frequently cause temporary neurapraxias, the majority of peroneal injuries from traumatic tibiofemoral dislocation are permanent.
Diagnosis of persistent dislocations will be obvious on the standard radiographic series. If a reduced dislocation is suspected, evaluate the films closely for fractures and effusion. When arterial injury is suspected, contrast angiography remains the gold standard and perhaps the standard of care. Color-flow Doppler ultrasonography and CTA are becoming more readily available and accepted and are probably reasonable alternatives.
When the likelihood of vascular injury is low, obtain Doppler-assisted ankle-brachial indices (ABIs). Dr. Carrie Tibbles, associate professor of emergency medicine at Beth Israel Deaconess Medical Center, presented a diagnostic algorithm at the 2007 ACEP Scientific Assembly. Given hard signs of vascular injury, or an ABI of less than 0.9, she recommends immediate orthopedic consultation and on-table angiography in the operating suite. If only soft signs are present and the ABI is greater than or equal to 0.9, she advocates serial exams or CTA.
MANAGING TIBIOFEMORAL DISLOCATIONS
Once diagnosed, the tibiofemoral dislocation must be managed quickly and effectively. In the setting of a persistent dislocation, radiographic evaluation has no role prior to reduction. Evaluate the neurovascular status of the distal leg, administer procedural sedation and analgesia, and begin reduction without further delay.
During the reduction, longitudinal traction is applied on the ankle while the posteriorly displaced segment is lifted anteriorly. Take care to avoid placing unnecessary pressure on the popliteal fossa, to avoid further vascular complications. Posterolateral dislocations may not be amenable to reduction in the emergency department and may require emergent reduction under general anaesthesia in the operating suite.
After the reduction, again assess the neurovascular status of the leg. Any hard signs of vascular injury at any time, either before or after reduction, mandate emergent surgical consultation. Immobilize the reduced joint in slight flexion using a long-leg posterior splint. Circumferential casts or tight compression dressings should be avoided in the emergency department.
Surgical consult should be emergent for obvious vascular injury, vascular injury identified during post-reduction testing, or for irreducible or open dislocations. Acute complications are generally due to vascular sequelae and include distal ischemia and limb loss, compartment syndrome, deep vein thrombosis, arterial thrombosis, and the development of pseudoaneurysms. A compartment syndrome may develop within 24 to 48 hours after injury and requires emergent four-
compartment fasciotomy. All patients with acute traumatic tibiofemoral dislocations require surgical consultation to guide appropriate disposition.
WATCHWORDS: ACCURACY AND VIGILANCE
The possible etiologies for the pain and dysfunction of acute knee injuries are myriad. An understanding of the joint’s complex anatomy and a uniform and thorough approach to the history and examination are essential. Morbidity can be avoided through accurate diagnosis, appropriate management, routine orthopedic referral, and continuous vigilance for the possibility of neurovascular injury.
Suggested Reading
Bachmann LM, et al.: The accuracy of the Ottawa knee rule to rule out knee fractures: a systematic review. Ann Intern Med 140(2):121, 2004.
Browner BD, et al.: Skeletal Trauma: Basic Science, Management, and Reconstruction, 3rd ed, Elsevier Science, 2003, chapters 53-56.
Fowler PJ and Lubliner JA: The predictive value of five clinical signs in the evaluation of meniscal pathology. Arthroscopy 5(3):184, 1989.
Marx JA, et al. (eds): Rosen’s Emergency Medicine: Concepts and Clinical Practice, 6th ed, Elsevier Health Sciences, 2005, chapter 54.
Noyes FR, et al.: Arthroscopy in acute traumatic hemarthrosis of the knee: incidence of anterior cruciate tears and other injuries. J Bone Joint Surg Am 62(5):687, 1980.
Perry JJ and Stiell IG: Impact of clinical decision rules on clinical care of traumatic injuries to the foot and ankle, knee, cervical spine, and head. Injury 37(12):1157, 2006.
Seaberg DC, et al.: Multicenter comparison of two clinical decision rules for the use of radiography in acute, high-risk knee Injuries. Ann Emerg Med 32(1):8, 1998.
Tintinalli JE, et al.: Emergency Medicine: A Comprehensive Study Guide, 6th ed, McGraw-Hill Companies, 2003, section 23.
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