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1
Department of Diagnostic Radiology, Wonkwang University School of Medicine and
Institute of Wonkwang Medical Science, 344-2 Shinyong-dong, Iksan, Jeonbuk,
570-711, South Korea.
2
Image Care of Troy, 451 Hoosick St., Troy, NY 12180.
Received November 10, 2000;
accepted after revision September 19, 2001.
Supported by a grant from Wonkwang University in 2000.
Abstract
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MATERIAL AND METHODS. MR imaging of 18 cases (17 patients, both knees in one patient) with the arcuate sign on conventional radiographs was retrospectively interpreted to evaluate the associated meniscal, ligamentous, and bony injuries. In 12 cases, MR findings were correlated with surgical results.
RESULTS. In all cases, avulsed bony fragments from the proximal pole of the fibula were attached to the fibular collateral ligament, the biceps femoris tendon, or both. Tear of the posterolateral capsule was seen in 12 cases (67%). Injury of the cruciate ligaments was noted in 16 cases (89%): injury to both the anterior cruciate ligament and posterior cruciate ligament was seen in nine cases (50%), injury to only the anterior cruciate ligament was seen in four, and injury to the posterior cruciate ligament only was noted in three. Bone bruises or gross fractures were seen in all cases: bone bruises on the anteromedial femoral condyle were noted in nine cases (50%) and were seen on the anteromedial tibial condyle in five cases (28%). Tear of the medial meniscus was seen in five cases (28%) and tear of the lateral meniscus in four cases (22%). Injury to the popliteus was seen in six cases (33%). Joint effusion was associated in all cases.
CONCLUSION. MR imaging is useful for evaluation of associated soft-tissue injuries in patients with the arcuate sign on conventional radiographs. Avulsion injury to the proximal fibula is an important indicator of the internal derangement of the knee and for predicting the mechanism of an injury with varus stress. Cruciate ligament tear and bone bruises on the anteromedial condyle of the femur and tibia are common associated findings.
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The fibular collateral ligament, also called the lateral collateral ligament, attaches to the posterior aspect of the lateral femoral condyle just above the groove for popliteus tendon proximally, and it runs posteriorly and inferiorly to insert on the head of the fibula along with the biceps femoris tendon distally [3,4,5]. Avulsion fractures involving the fibular collateral ligament usually occur at its distal attachment site on the head of the fibula as opposed to its proximal attachment site on the lateral femoral condyle [6, 7]. The finding of this fracture on conventional radiographs has been called the "arcuate" sign [6] (Figs. 1A, 1B, and 2A), and it is a marker of acute posterolateral rotatory instability [6].
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Isolated posterolateral instability of the knee is uncommon. Posterolateral instability of the knee occurs more commonly in association with tears of the posterior cruciate ligament, the anterior cruciate ligament, or both [7,8,9]. Confusion exists concerning the diagnosis and treatment of posterolateral instability because of the variety of ligamentous injuries associated with posterolateral injury. An early diagnosis is crucial to obtain optimal results, because an early surgical intervention might be needed in a patient whose knee has the acute complete injury of the posterolateral corner [1]. Patients in whom a posterolateral corner injury is combined with posterior cruciate ligament disruption are often treated surgically [9].
The purpose of this study was to evaluate associated knee injuries using MR imaging in patients who manifested the arcuate sign on conventional radiographs.
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The patients ranged from 19 to 64 years old (mean age, 38 ± 21 years) and included 10 men and seven women. Sixteen patients were involved in motor vehicle incidents, eight of whom were pedestrians struck by automobiles. MR images of all patients were obtained 1-40 days after trauma (mean interval, 8 ± 9.6 days) using a 1.0-T superconductive system (42 SP; Siemens Medical Systems, Erlangen, Germany). Sagittal, coronal, and axial images of all patients were obtained using a knee-joint coil. Parameters for images obtained in the sagittal and coronal views were TR range/TE range, 2500-2950/20-25 and 80-90; section thickness, 4 mm; gap, 0.4 mm; excitation, 1; matrix, 192/256 x 256; and field of view, 16 cm. Parameters for images obtained in the axial view were 300/15 and 275/12; flip angle, 25°; section thickness, 4 mm; gap, 0.4 mm; excitations, 2; matrix, 192 x 256; and field of view, 16 cm.
To evaluate the associated meniscal, ligamentous, and bony injuries, all MR images were retrospectively interpreted in consensus by two musculoskeletal radiologists who were unaware of clinical data, previous reports, or surgical results. The radiologists used established criteria for diagnosing ligamentous injuries (partial or complete fiber disruption, abnormal orientation, or absence of ligament visualization) and meniscal tears. In addition, we used criteria reported by others for the diagnosis of meniscocapsular separation [10, 11], for popliteus injuries [12], for partial or complete tear of the cruciate ligaments [13,14,15], and for posterolateral capsular tear [16]. In 12 cases, MR findings were correlated with arthroscopic and surgical findings.
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Injuries to the cruciate ligaments were noted in 16 cases (89%) as follows: injury to both the anterior cruciate ligament and the posterior cruciate ligament in nine cases (50%), injury only to the anterior cruciate ligament in four, and injury only to the posterior cruciate ligament in three cases (Figs. 1D, 4B, and 4C). The anterior cruciate ligament was torn in 13 cases (72%): all of these cases had a tear from the tibia (six, avulsion with fracture fragments; one, a complete tear without fracture fragment; six, a partial tear at the tibial attachment without fracture) (Figs. 1D and 4B). The posterior cruciate ligament was torn in 12 cases (67%): eight had a mid third substance tear (seven complete tears and one partial tear) (Fig. 4C); two had a tear of the ligament from the femur; and two had an avulsion injury from the tibia. The deep lateral capsular ligament was torn in five cases (28%); the medial collateral ligament was torn in four (22%) (partial injury in all four); and the iliotibial band was torn in four (22%) (two, complete; one, partial; one, avulsion from the Gerdy tubercle).
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Bone bruises or gross fractures were seen in all cases. A total of 19 bone bruises were noted in the 17 patients: nine bruises were on the anteromedial femoral condyle (50%); five on the anteromedial tibial condyle (28%); two on the anterolateral tibial condyle; one on the posterolateral tibial condyle; one on the posterolateral femoral condyle; and one on the posteromedial femoral condyle (Figs. 2C and 2D). Two of nine cases of bone bruises on the medial femoral condyle also had bone bruises on the adjacent medial tibial condyle.
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Eight gross fractures were noted: four on the medial tibial condyle, three on the lateral tibial condyle, and one on both medial and lateral tibial condyles.
Meniscal injuries were seen in nine cases. The medial meniscus was torn in five cases (28%): three had a peripheral tear and two had a complex tear. The lateral meniscus was torn in one patient, who had a peripheral vertical tear. Three tears of the meniscal struts were seen in the lateral meniscus posterolaterally.
The popliteus musculotendinous unit was injured in six cases (33%); all of these cases also had an interstitial tear of the tendon and muscle belly at the myotendinous junction (Fig. 4A). Joint effusion was noted in all 18 cases.
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Surgical correlation was performed in 12 cases: arthroscopy in five cases, open exploration in three cases, and both arthroscopy and open exploration in four cases. Nine arthroscopic procedures confirmed associated injuries to the cruciate ligaments and menisci: a complete tear of the posterior cruciate ligament in seven cases (mid substance tear in six, femoral tear in one), anterior cruciate ligament injuries in six (tibial avulsion in three, distal complete tear in one, partial tear distally in two), a tear of the medial meniscus in five (peripheral in three, complex in two), and a tear of the lateral meniscus in three (meniscocapsular separation in two, peripheral tear in one). MR imaging accurately showed the cruciate ligament and meniscal injuries except in two cases of meniscocapsular separation. Seven open explorations of the knee confirmed the arcuate sign with an avulsion fracture from the fibular head and tear of the posterolateral capsule in seven cases and tear of the deep lateral capsular ligament in five cases. MR imaging showed posterolateral capsular tears as severe distortion of normally visible posterolateral structures with diffuse edema (Fig. 3A,3B) and showed tear of the deep lateral capsular ligament as disruption of the meniscotibial ligament with upward displacement of the lateral meniscus in the respective cases.
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Avulsion fracture from the fibular head seen on radiographs in patients with injuries to the arcuate complex has been called the arcuate sign [6]. Radiographs may show a fragment of bone ranging from a faint fleck to a piece several millimeters in diameter [6, 19] (Figs. 1A, 1B, 2A, and 4D). The bone is pulled off by injury to the arcuate complex, which comprises the posterolateral corner of the knee. The arcuate sign signifies disruption of the arcuate complex and might also suggest the presence of acute posterolateral rotatory instability [6]. The mechanism of this injury is either a blow to the anteromedial tibia with the knee in extension, resulting in posterolateral subluxation, or a noncontact external rotation hyperextension injury [8, 9, 20,21,22].
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In our study, the pattern of associated bone bruises on MR imaging may suggest the mechanism of injury. When a pedestrian is struck by an automobile, bone bruises on the anteromedial femoral condyle with edematous and hemorrhagic swelling of overlying soft tissue may be caused by direct impaction of the anteromedial femoral condyle resulting in posterolateral subluxation of the femur or in hyperextension varus or external rotation hyperextension. Ross et al. [20] reported that bone bruises on the medial femoral condyle on MR imaging provided direct evidence that a hyperextension varus moment was involved in the mechanism of injury to their patients.
To our knowledge, no systematic studies of the arcuate sign have been reported. La Prade and Terry [21] reported three arcuate fractures among 71 consecutive posterolateral injuries. Baker et al. [8] reported seven tears of the fibular collateral ligament from the fibular styloid process occurring during a 10-year period in 17 knees of patients with posterolateral knee instability. The frequency of this type of fracture is probably low. Our 18 cases with arcuate sign were selected from 1,306 MR imaging knee examinations performed to evaluate injuries during 4 years after knee trauma occurred.
Our study suggests that avulsion fractures of the fibular head are infrequent. However, an alternative explanation is that these injuries are infrequently evaluated with MR imaging. Clinically, a patient may suffer only minimal pain and disability and may remain active immediately following an injury. Tenderness and swelling often are mild, but the patient may notice pain when standing as well as a sensation of knee instability in extension and hyper-extension [2, 7, 21, 22].
Physical findings in such injuries are usually subtle, and the adduction stress test at 30 degrees may reveal only a mild laxity [1, 2, 6, 8, 21, 22]. The external rotation recurvatum test is the most diagnostic, but these findings may be difficult to interpret. The test is performed by lifting the feet of the supine patient by the toes: findings are positive if the tibia of the injured leg will rotate externally and the knee will hyperextend. A posterior drawer test may show positive findings, but it should be performed both with and without internal tibial rotation to distinguish posterolateral rotatory subluxation from posterior cruciate ligament injury. Evaluation may be difficult without anesthesia, and, as in all knee examinations, the contralateral knee should be used for comparison [6, 21]. Baker et al. [8] have reported that posterolateral rotatory instability was diagnosed at the initial examination in 12 of the 17 knees in their study. Five knees in which posterolateral instability was not diagnosed on the initial examination were diagnosed on examination under anesthesia.
MR imaging can reveal injury to the posterolateral structures that might otherwise be missed during physical examination. Miller et al. [3] reported 30 cases of posterolateral injury involving at least one of the posterolateral structures among 481 knee MR imaging examinations. Posterolateral ligament injury was clinically suspected in only three of these 30 cases. In the series by Twaddle et al. [23], of 17 knee dislocations, MR imaging correctly showed 82% of the fibular collateral ligament injuries, although only 59% were suspected clinically.
Acute or chronic isolated posterolateral instability of the knee is uncommon. In most reports of posterolateral instability, a preponderance of patients have had combined ligamentous injuries [7,8,9, 20, 24]. Various ligamentous injuries were also noted in our study: anterior cruciate ligament injuries in 13 cases, posterior cruciate ligament in 12 cases, lateral capsular ligaments in five cases, medial collateral ligament in four cases, and iliotibial band in four. Baker et al. [8] noted 11 anterior cruciate ligament tears among 17 consecutive patients who had acute posterolateral rotatory instability. Baker et al. [9] also reported a series of 13 consecutive patients with acute combined posterior cruciate and posterolateral instabilities. Ross et al. [20] reported MR imaging findings of six posterolateral complex injuries, including two avulsion fractures of the proximal fibula. All had concomitant anterior cruciate ligament tears, and one patient also had partial posterior cruciate ligament injury.
The distribution of associated anterior cruciate ligament tears in our study is different from that of other studies [8, 20]. In our patients, all ligaments were torn at the tibial attachment area, rather than at the proximal or midportion of the ligament. Although it is not clear why most tears of the anterior cruciate ligament in our study were in the distal portion of the ligament, the reason could be related to the mechanism of the injuries, in that most patients were pedestrians who had been struck by automobiles. Therefore, in our study, the severity of stress applied and the precise vectors involved presumably were different from that of patients reported by the other authors. Another possible explanation may be related to the difference in the number of patients with avulsion fracture from the fibula: In the other studies [8, 20], cases with avulsion injury of the fibular collateral ligament from the fibula (having the arcuate sign) are few, whereas in our study, all cases have avulsion injury of the fibular head. The mechanisms of injury in our study may differ from those in the other studies.
In our study, associated bone bruises were found on the anteromedial aspect of the medial femoral condyles, the tibial condyles, or both. Ross et al. [20] reported that a bone contusion on the anteromedial femoral condyle was present in all patients who had complete posterolateral disruption. We agree with their premise that a bone bruise on the medial femoral condyle provides direct evidence that a hyperextension varus movement is involved in the mechanism of the injury in these patients. The associated finding of hemorrhagic edematous swelling of the overlying soft tissue of such anteromedial bone bruises may reflect direct impaction of the medial compartment, which causes varus stress with external rotation. In addition, contiguous bone bruises on the medial femoral condyle and the adjacent medial tibial condyle support the evidence of varus injury.
This study has some limitations. The number of cases is relatively small to generalize the significance of the arcuate sign. Also, not every case with the arcuate sign on radiographs had MR imaging of the knee joint during the study period. Some cases in this study could not be correlated with surgical findings. Although such limitations are introduced in a retrospective study, our study suggests that the arcuate sign is an indicator of ligament injury. The meaning of the arcuate sign as an indicator of ligament injury could be comparable to that of the Segond fracture, known as the lateral capsular sign [25, 26]. The Segond fracture is a well-known small avulsion fracture occurring at the proximal and lateral tibial condyle by the middle one third of the lateral capsular ligament and indicates major ligamentous injuries, particularly anterior cruciate ligament injury [26].
In conclusion, our data suggest that the arcuate sign on conventional radiographs is an important indicator of internal derangement of the knee and that this sign suggests that the mechanism of the injury is varus stress. MR imaging is useful for evaluating the associated soft-tissue injuries in patients with the arcuate sign. Cruciate ligament tears and bone bruises on the anteromedial condyle of the femur and tibia are common associated findings.
Acknowledgments
We acknowledge the editorial assistance of Bonnie Hami, Department of
Radiology, University Hospitals of Cleveland, Cleveland, OH.
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