The knee joint depends on ligaments and muscles for support (Fig...266z3). It is frequently subjected to injuries from traumatic forces while extended or in various stages of flexion. These traumatic forces include abduction, flexion, and internal rotation of the femur on the tibia; adduction, flexion, and external rotation of the femur on the tibia; hyperextension; and anteroposterior displacement. By far the most common are abduction, flexion, and internal rotation of the femur on the tibia, which produce injuries to the medial side of the knee. Injuries to the lateral side of the knee are produced by adduction, flexion, and external rotation. Such forces may result in a strain or rupture of the medial or lateral collateral ligaments, the anterior or posterior cruciate ligaments, the capsular structures, or a tear in the medial or lateral meniscus, singularly or in combination. Functional instability of the knee is determined by stress testing, which will demonstrate abnormal laxity when properly done.
FIG. 266-3. Ligaments of the right knee joint. The articular capsule and the patella have been removed. (From Spencer AP, Mason EB: Human Anatomy and Physiology. Menlo Park, Benjamin/Cummings, 1979, p 174. Used by permission.)
Initial stress testing is an abduction or valgus deformity ( Fig 26.6-4) applied to the knee, which is in approximately 30° of flexion, to determine the integrity of the medial capsular and ligamentous structures. The medial collateral ligament supplies the majority of restraint to valgus deformities of the knee in all stages of flexion. A varus or adduction force is then applied to the lateral aspect of the knee, again with approximately 30° of flexion, to ascertain the integrity of the lateral structures. The lateral collateral ligament, similar to the medial collateral ligament, is the major restraint to varus laxity on the knee at all positions of flexion. If there is a demonstrated laxity of greater than 1 cm without a firm end-point as compared to the other knee, there is a complete rupture of the medial or lateral collateral ligament. 10 If there is laxity with a firm end-point or a laxity of less than 1 cm, an incomplete or partial tear is present. If there is no demonstrated instability but there is pain, the patient has suffered a strain in the ligamentous structures tested. The patient who is unstable with the varus or valgus test performed with 30° of flexion should be brought into full extension, if possible, and similar maneuvers carried out. Medial instability in full extension indicates a severe lesion involving the cruciate ligaments and posterior capsule along with the medial ligaments. Lateral instability in extension likewise indicates a severe injury that may involve the posterolateral corner of the knee as well as the cruciate ligaments. Peroneal nerve injuries may also occur in lateral injuries.
FIG. 266-4. Valgus stress in full extension (A) and in 30° of flexion (B). (From Scott WN. Ligament and Extensor Mechanism Injuries of the Knee: Diagnosis and Treatment. St. Louis, Mosby-Year Book, 1991, p 91. Used with permission.)
Injury to the anterior cruciate ligament may be the most common ligamentous injury today.10 The mechanism of injury is usually noncontact; a deceleration, hyperextension, or marked internal rotation of the tibia on the femur results in an injury to the cruciate. This injury is often associated with a "pop" and swelling that develops within hours. This pop is considered pathognomonic for anterior cruciate injury. 11 There may be an associated medial meniscal tear as well. Such a mechanism of injury combined with the presence of a traumatic effusion is very suggestive of a disruption of the anterior cruciate ligament.
The diagnosis of the anterior cruciate ligament injury is ascertained by using the Lachman test ( Fig 266-5), the anterior drawer sign, (Fig.,266-6) and the pivot shift
(Flg.,266-7).12 Although the anterior drawer sign has been used for a long time, it is not very sensitive. The maneuver is done with 45° flexion at the hip and 90° flexion at the knee. The physician then attempts to forwardly displace the tibia from the femur. A displacement of greater than 6 mm as compared to the normal, opposite knee indicates that there has been an injury to the anterior cruciate ligament. There are false-negatives associated with this maneuver. The Lachman test is a much more sensitive test.10 The examiner places the knee in 20° of flexion by resting it on a pillow and stabilizes the femur above the knee with his or her nondominant hand. The dominant hand is placed behind the leg at the level of the tibial tubercle, and the examiner introduces an anterior force, attempting to displace the tibia forward. If a displacement of greater than 5 mm as compared to the opposite knee occurs, or if there is a soft, mushy end-point, then a tear in the anterior cruciate ligament has occurred. Although this examination is more sensitive than the anterior drawer and able to identify partial tears in the anterior cruciate ligament when the examiner is skilled, it is difficult on patients who have large legs. The pivot shift is the third maneuver by which the examiner can determine the integrity of the anterior cruciate ligament. The pivot shift is easily performed once the examiner is familiar with it, but it may be somewhat painful to the patient. While the patient is supine and relaxed, the examiner lifts the heel of the foot to approximately 45° of hip flexion with the knee fully extended. The opposite hand grasps the knee with the thumb behind the fibular head. The examiner then internally rotates the ankle and knee, applies a valgus force to the knee, and flexes the knee. If an anterior subluxation of the tibia is present, a sudden visible, audible, and palpable reduction of the subluxation occurs at about 20 to 40° of flexion. This indicates a deficit in the anterior cruciate ligament, which is required to stabilize the knee in this position. There are other tests described in the literature to determine the integrity of the anterior cruciate ligament, including the jerk test and dynamic extension testing.
FIG. 266-5. The Lachman test is performed with the knee flexed between 15° and 30°. (From Scott WN. Ligament and Extensor Mechanism Injuries of the Knee: Diagnosis and Treatment. St. Louis, Mosby-Year Book, 1991, p 94. Used with permission.)
FIG. 266-6. Anterior drawer test. (From Scott WN. Ligament and Extensor Mechanism Injuries of the Knee: Diagnosis and Treatment. St. Louis, Mosby-Year Book, 1991, p 95. Used with permission.)
FIG. 266-7. In the pivot shift of Galway and Macintosh, the test is done with the knee in full extension with application of a valgus and internal rotation stress. The "clunk" of reduction is felt in the first 20° to 30° of flexion. (From Scott WN. Ligament and Extensor Mechanism Injuries of the Knee: Diagnosis and Treatment. St. Louis, Mosby-Year Book, 1991, p 95. Used with permission.)
The posterior cruciate ligament can also suffer an isolated injury or be injured in combination with other ligamentous structures of the knee. In contrast to anterior cruciate injuries, isolated posterior cruciate injuries are seen much less frequently. The posterior cruciate ligament provides initial resistance to posterior translation at all angles of flexion of the knee. The mechanism of injury then is usually an anterior to posterior force applied to the tibia or lower leg. Posterior cruciate injuries are seen in association with other ligamentous injuries when a serious injury has occurred to the knee. A deficit in this ligament is determined by the posterior drawer test (Fig.266:8). The knee is examined with flexion at the hip and at the knee as described for the anterior drawer sign. The physician applies a posterior force to the tibial tubercle. If there is displacement posteriorly, then the examiner can diagnose an injury to this ligament. The physician might also notice a posterior sag or drop back of the tibial tubercle due to loss of integrity of the posterior cruciate when observing the knee with 45° flexion at the hip and 90° flexion at the knee. This test can be misleading, however, if there is a straight anterior instability resulting in a subluxation of the knee forward. This abnormal position would give the physician the false impression of too much posterior play when performing the posterior drawer test because the knee would be reduced to its normal anatomic alignment from the forwardly subluxed position.
FIG. 266-8. Posterior drawer test. (From Scott WN. Ligament and Extensor Mechanism Injuries of the Knee: Diagnosis and Treatment. St. Louis, Mosby-Year Book, 1991, p 97. Used with permission.)
Combined instabilities of the knee are often seen, especially in athletes. Anteromedial and anterolateral instability are the two that occur most frequently. They result from external rotation and abduction or adduction forces placed on the knee. Virtually any combination of medial and lateral instabilities of the knee can occur, however.
One knee injury that is especially difficult to detect is injury to the posterolateral structures. Posterolateral instability usually involves a tear of the popliteus-arcuate complex, which may occur in combination with lateral ligament injury and possible anterior or posterior cruciate ligament injury. Isolated injuries to the popliteus-arcuate complex can occur themselves but are rare. Isolated posterolateral instability is demonstrated by testing at 0 to 30° of flexion for maximal posterior translation, and at 90° of flexion for maximal external rotation as compared to the normal opposite knee. Further testing to determine the integrity of the lateral collateral ligament and anterior or posterior cruciates must be done as well.
Most ligamentous injuries of the knee present with hemarthroses. In fact, approximately 75 percent of all hemarthroses are due to disruption of the anterior cruciate ligament.10 Serious ligament injuries, however, may present with minimal pain and no hemarthrosis due to complete disruption of the ligamentous and capsular fibers, allowing leakage of the blood into the soft tissue spaces. Hemarthrosis can also be due to osteochondral fractures or fractures that extend into the joint line or peripheral meniscal tears. Traumatic hemarthroses usually occur within minutes to hours of injury, in contrast to chronic effusions of the knee due to synovial inflammation, which occur one to two days after strenuous use of the joint.
Plain radiographs in ligamentous injuries are typically normal or only reveal an effusion. An avulsion fracture at the site of attachment of the lateral capsular ligament on the laterial tibial condyle (Segond fracture) is a marker for anterior cruciate ligament rupture. 13 Cortical avulsion of the medial tibial plateau (very uncommon) is associated with tears of the posterior cruciate ligament and medial meniscus.14 Continued refinements in MRI have resulted in high-quality images of the ligamentous and meniscal structures of the knee resulting in an accuracy rate of close to 90 percent for meniscal and cruciate ligament disruption. 7 The ordering of this examination, however, is typically done by the patient's primary care provider or orthopedist in follow-up.
Stable injuries involving a single ligament with minor strain can be managed with a knee immobilizer, ice packs, elevation, nonsteroidal anti-inflammatory agents, and ambulation as soon as is comfortable for the patient.15 When knee immobilizers are placed, the patient must be instructed to perform daily range of motion exercises to avoid contracture and maintain mobility. These complications are more common in the elderly and can occur after only a few days of immobilization. While there is no universally accepted regimen for range of motion exercise, one regimen is to apply ice first to relieve pain, and then perform 10-20 knee flexion-extensions (no weights should be added) three or four times a day. These injuries should be referred to an orthopedic surgeon or the patient's primary care provider within the next few days to a week for follow-up examination. Complete rupture of an isolated ligament can generally be treated conservatively in the same fashion with quadriceps strengthening, range of motion exercises, and functional bracing being part of the follow-up care. 11 Professional athletes with single ligament ruptures or patients with more than one ligament torn and an unstable knee necessitate immediate orthopedic consultation so that definitive surgical management can be planned.
Arthrocentesis may be of therapeutic benefit in patients with large, tense effusions of the knee. Recurrence of the effusion following aspiration, however, is common. Arthrocentesis may also be of assistance diagnostically if the etiology of the effusion is not clearly due to trauma. The presence of blood and glistening fat globules is pathognomonic of lipohemarthrosis, which indicates intra-articular knee fracture. The major complication of arthrocentesis is septic arthritis.
Meniscal injuries of the knee occur by themselves or in combination with ligamentous injuries. For example, anterior cruciate injuries are commonly associated with meniscal injuries. Cutting, squatting, or twisting maneuvers may cause injury to the meniscus. The medial meniscus is approximately twice as likely as the lateral meniscus to be injured. Four-fifths of the tears involve the peripheral posterior aspect of the meniscus. 16 Many maneuvers have been described in the literature to determine whether a meniscus has been injured. Most of these tests, however, have an unacceptable specificity and sensitivity. Although the diagnosis of a meniscal tear is difficult to make in certain patients, a combination of a suggestive history and physical findings on examination should lead the emergency physician to consider the diagnosis. On questioning the patient, the physician should ask if the patient experiences locking of the knee joint on either flexion or extension that is painful and limits further activity. This sign clearly points to the diagnosis of a torn meniscus. Effusions that occur after activity; a sensation of popping, clicking, or snapping; a feeling of an unstable joint, especially with activity; or tenderness in the anterior joint space after excessive activity suggests the diagnosis of a meniscal tear. When performing a physical examination, a physician should attempt to identify atrophy of the quadriceps muscle due to disuse, and joint line tenderness, which is very suggestive. Various maneuvers, such as McMurray's test or the grind test, are useful but, as mentioned earlier, are positive only about 50 percent of the time. 17 If a tentative diagnosis of a meniscal tear is considered, referral to an orthopedic surgeon or the patient's primary care provider is warranted. Nonsteroidal anti-inflammatory agents and partial weight-bearing are advised pending follow-up. Definitive diagnosis can be made by MRI or arthroscopy with the latter having the advantage of allowing for definitive surgical treatment (usually partial meniscectomy or meniscal repair).
The patient who presents to the emergency department with a locked knee can experience a great deal of pain along with loss of mobility. Following conscious sedation, one can attempt to unlock the knee by positioning the patient with the leg hanging over the edge of the table with the knee in 90° or greater of flexion. After a period of relaxation, the physician can apply longitudinal traction to the knee with internal and external rotation in an attempt to unlock the joint. If this maneuver is unsuccessful, consultation with the orthopedic surgeon is recommended.
Knee dislocation (Fig 266-9) is a result of tremendous ligamentous disruption due to hyperextension, direct posterior force applied to the anterior tibia, force to the fibula or medial femur, force to the tibia or lateral femur, or rotatory force resulting in anterior, posterior lateral, medial, or rotatory dislocation. This injury typically occurs following sporting accidents and falls and posterior dislocation is most common. With posterior dislocation, there is complete disruption of the anterior and
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