n avulsion fracture of the head of the fibula has been described as an important

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1 Guo-Shu Huang 1 Joseph S. Yu 2 Muhammad Munshi 3 Wing P. Chan 4 Chian-Her Lee 5 Cheng-Yu Chen 1 Donald Resnick 3 Received December 6, 2001; accepted after revision July 30, Department of Radiology, Tri-Service General Hospital, National Defense Medical Center, No. 325, Section 2, Cheng-Kung Rd., Neihu, Taipei 114, Taiwan, Republic of China. ddress correspondence to G.-S. Huang. 2 Department of Radiology, Ohio State University Medical Center, S-207 Rhodes Hall, 450 W. Tenth ve., Columbus, OH Department of Radiology, Veterans dministration Medical Center, 3350 La Jolla Village Dr., San Diego, C Department of Radiology, Taipei Medical University- Municipal Wan Fang Hospital, 111 Hsing-Long Rd., Section 3, Taipei 116, Taiwan, Republic of China. 5 Department of Orthopedic Surgery, Tri-Service General Hospital, National Defense Medical Center, Taipei 114, Taiwan, Republic of China. JR 2003;180: X/03/ merican Roentgen Ray Society vulsion Fracture of the Head of the Fibula (the rcuate Sign): MR Imaging Findings Predictive of Injuries to the Posterolateral Ligaments and Posterior Cruciate Ligament OBJECTIVE. The purpose of this study was to describe the significance of an avulsion fracture of the head of the fibula ( arcuate sign) and its association with injuries of the knee on MR imaging. MTERILS ND METHODS. We conducted a retrospective search of 2318 patients who underwent conventional radiography and MR imaging after an episode of knee trauma. Patients were included in this study if they had an avulsion fracture of the head of the fibula revealed on conventional radiography and underwent arthroscopy. Thirteen patients, all of whom were men, satisfied the inclusion criteria. Ten patients underwent further explorative surgery. The clinical, radiographic, MR imaging, and surgical findings were then reviewed. RESULTS. The avulsion fracture of the styloid process of the fibular head was apparently related to injuries of the arcuate complex in all 13 patients. Radiographically, the bony fragment was horizontally oriented and similar in size in most patients, ranging from 8 to 10 mm in length and from 2 to 5 mm in width. On MR imaging, the fibular avulsion was identified in 11 of the 13 patients. The other two patients had marrow edema in the fibular styloid process, although the avulsion fracture was not evident. ll patients had injuries of the posterior cruciate ligament (six tibial avulsions, seven midsubstance tears). No patient had a tear of the anterior cruciate ligament. Disruption of the lateral collateral ligament was evident in seven patients, and one patient had a tear of the popliteal tendon. During surgery, six patients had disruption of the arcuate complex, but this disruption could not be identified on the MR images. CONCLUSION. n avulsion fracture of the fibular head generally involves the styloid process and causes injury of some of the major stabilizers in the posterolateral corner. vulsion fractures are strongly associated with disruption of the posterior cruciate ligament. n avulsion fracture of the head of the fibula has been described as an important indicator of posterolateral instability of the knee. The arcuate sign is used to describe an avulsed bone fragment related to the insertion site of the arcuate complex, which consists of the fabellofibular, popliteofibular, and arcuate ligaments [1]. The mechanism of this injury, which leads to posterolateral instability, is most commonly a direct blow against the anteromedial tibia with the knee in extension. Recognition of posterolateral instability of the knee is important clinically because it may affect the treatment outcome of the associated instability of the anterior posteriorcruciate ligament [2]. Unrecognized and untreated posterolateral instability may result in a failure of reconstruction of either the anterior or posterior cruciate ligament [3, 4]. Therefore, identifying the presence of an avulsion fracture of the fibular head and recognizing any associated injuries are of great clinical importance. The aim of this study was to evaluate the pattern and significance of an avulsion fracture of the fibular head by analysis of the associated intraarticular and extraarticular injuries of the knee. Materials and Methods We performed a retrospective search for patients who underwent conventional radiography and MR imaging after an episode of knee trauma between February 1995 and July Patients who had radiographic findings of an avulsion fracture of the fibular head (arcuate sign) and who underwent arthroscopy were enrolled in this study. The avulsed osseous fragment from the fibular head was documented and characterized on anteroposterior and lateral projections by consensus of two radiologists who were experienced in musculoskeletal imaging. meticulous evaluation was performed to exclude JR:180, February

2 Huang et al. an osseous fragment that arose from the lateral and posterior rims of the tibial plateau. n arcuate sign was identified in 13 of 2318 patients. ll were men, ranging in age from 18 to 35 years (average age, 26 years). In each instance, the knee injury was the result of a motor vehicle collision. None of the patients had sustained a complete knee dislocation. No patient had a second traumatic episode of the same knee during the interval between the initial injury and the MR imaging examination. The MR imaging examinations were performed on the same day that the radiographs were obtained and within 2 28 weeks of the injury (mean, 13 weeks). Each study was performed using a 1.5-T MR scanner (Vista; Picker, Cleveland, OH) and a dedicated extremity coil, with the knee externally rotated The following sequences were obtained in all patients: spin-echo proton density weighted (TR range/te range, /20 30) and T2-weighted ( /80 90) sequences in both the coronal and sagittal planes and an axial gradient-recalled echo sequence (flip angle, 20 ). The parameters included a 14- to 16-cm field of view, a 3- to 4-mm slice thickness, a 0.5-mm interslice gap, 1 excitation, and a matrix. In one patient, an additional short tau inversion recovery (STIR) ( /35 40; inversion time: range, msec) sequence was acquired in the coronal plane. The MR images were reviewed by the same two musculoskeletal radiologists who had evaluated the radiographs and were unaware of the clinical and surgical findings. consensus was reached for every interpretation. The MR images were evaluated for abnormalities in bones, ligaments, menisci, cartilage, tendons, muscles, and soft tissues in and around the knee joint. rthroscopy was performed in all 13 patients, and 10 patients underwent explorative surgery for ligament reconstruction. The clinical, radiographic, MR imaging, and surgical findings were then reviewed. Results The incidence of an avulsion fracture of the fibular head was 0.6%, and the avulsions involved the apex of the fibular head (fibular styloid process) in all 13 patients. In eight of the 13 patients, the avulsed bone fragments of the fibular styloid process were identified on the anteroposterior (Figs. 1, 2, 3, and 4) and lateral (Figs. 1B and 2B) radiographs. In the other five patients, the avulsed osseous fragments were identified only on the anteroposterior radiographs but not on the lateral radiographs. The bone fragment was oriented in a horizontal direction in 10 patients (Figs. 1, 2, and 3), but in the remaining three patients, the fracture fragments appeared tilted (Fig. 4). Displacement of the bone fragment varied from 2 to 12 mm (average, 5 mm). The size of the fragment ranged from 8 to 10 mm in length and from 2 to 5 mm in width. Six patients had an avulsion of the posterior rim of the tibial plateau at the attachment of the posterior cruciate ligament (Fig. 1B). One patient had a patellar fracture, and one patient had an osteochondral fracture of the medial femoral condyle. On the MR images of 11 patients, the avulsed fibular fragment originated from either the attachment of the popliteofibular ligament or the attachment of the popliteofibular, arcuate, and fabellofibular ligaments at the posterosuperior apex of the styloid process of the fibular head (Figs. 1C, 1D, 2C, 3B, and 3C). These findings were confirmed at the time of explorative surgery in 10 patients. The typical location of the avulsed osseous fragment was adjacent to the posterolateral rim of the tibial plateau in both the sagittal and coronal planes (Figs. 1C, 1D, 2C, 3B, and 3C). In the remaining two patients, the avulsed bone fragment could not be identified on any of the MR images. However, in these two patients, marrow edema in the fibular styloid process and the surrounding soft tissue swelling in the region of the arcuate complex were noted (Figs. 4B and 4C). On MR imaging, all 13 patients had an injury of the posterior cruciate ligament that was confirmed by arthroscopy. Six patients had a tibial avulsion at the site of attachment of this ligament (Fig. 1D), whereas seven patients had a midsubstance tear of this ligament (Figs. 2D and 3D). The anterior cruciate ligament was intact in every knee. Disruption of the lateral collateral ligament was identified in seven patients and was best depicted on the coronal MR images (Figs. 3C and 4C). None of the patients had a tear of the tendon of the biceps femoris muscle. No evidence of an avulsed bone fragment originating from the site of attachment of the lateral collateral ligament or the tendon of the biceps femoris muscle was revealed on MR imaging or at explorative surgery. popliteal tendon tear was identified on MR imaging in one patient (Fig. 1E). This finding was confirmed at the time of exploratory surgery. Six of 10 patients who underwent surgical exploration proved to have disruption of the arcuate complex, but on MR imaging, the integrity of the arcuate, popliteofibular, and fabellofibular ligaments could not be accurately assessed. ll patients had an injury of the medial collateral ligament. The medial collateral ligament appeared acutely abnormal in five patients, showing abnormal signal intensity and thickening. The other eight patients had evidence of chronic injury consisting of fibrous thickening of the medial collateral ligament. meniscal tear involving the medial meniscus in five patients (three peripheral tears of the posterior horn and two complex tears of the posterior body and posterior horn) and the lateral meniscus in six patients (two complex tears of the posterior horn and one complex tear of the posterior body and posterior horn, two peripheral tears of the posterior horn) was noted. ll tears were confirmed arthroscopically. Regions of marrow edema were identified in 13 locations in five patients. These areas included the anterior lateral tibial plateau in three patients, medial tibial plateau in two patients, lateral femoral condyle in two patients, medial femoral condyle in one patient, patella in one patient, posterior tibial plateau in one patient, and fibular head in three patients. The most common clinical complaints were pain in the posterolateral aspect of the knee and a sensation of the knee giving way during walking or running. Findings of a posterior drawer test were positive in all patients. The findings of the external rotation recurvatum test were positive in eight patients, and the findings of the adduction stress test were positive in seven patients. lmost identical radiographic and MR imaging findings with respect to the avulsion fracture of the fibular styloid process in three patients who did not undergo open surgery supported the concept that the avulsed bone fragment originated from the attachment of the arcuate complex. Discussion Injuries that affect the ligaments in the posterolateral aspect of the knee are important because they may lead to posterolateral instability, characterized clinically by posterior subluxation and external rotation of the tibial plateau relative to the femur [5]. Posterolateral instability is considerably less common than anterolateral instability [6]. The most common mechanism of injury is a direct blow to an extended knee with an anteromedially directed force to the proximal tibia while the tibia is externally rotated [5, 6]. Unnoted and untreated posterolateral injuries of the knee may result in chronic instability and failure of the reconstruction of the cruciate ligament. This type of injury is often subtle, with a paucity of clinical signs and is frequently overlooked at the time of the initial clinical examination [7]. The posterolateral structures of the knee are complex and are composed of both ligaments and tendons that act as static restraints to posterior translation, varus angulation, and exter- 382 JR:180, February 2003

3 MR Imaging of an vulsion Fracture of the Fibular Head B D C E Fig year-old man with acute posterolateral instability of left knee after motor vehicle collision. and B, nteroposterior () and lateral (B) radiographs reveal avulsion fracture of styloid process of fibular head (long arrow) and posterior cruciate ligament avulsion fracture at posterior tibial plateau (short arrow). C and D, Sagittal spin-echo proton density weighted image (TR/TE, 1800/20) (C) and coronal short tau inversion recovery (STIR) image (D) (3000/40; inversion time, 100 msec) reveal avulsed bone fragment (long arrow) with marrow edema in corresponding site of attachment of popliteofibular ligament adjacent to popliteal tendon (arrowhead, C). Note avulsion fracture of posterior tibial plateau at attachment of posterior cruciate ligament (short arrow, D) in coronal STIR image. E, Coronal STIR image (3000/40; inversion time, 100 msec) obtained posterior to D reveals tear of popliteal tendon (arrowheads) at musculotendinous junction with hemorrhage and edema in popliteal muscle and surrounding tissues. nal rotation of the knee [8]. These structures include the lateral collateral ligament, fabellofibular ligament, popliteofibular ligament (the fibular origin of the popliteus muscle), arcuate ligament, and popliteal tendon, in addition to the tendon of the biceps femoris muscle. However, one biomechanical study considered the popliteofibular ligament to be the primary static stabilizer to the posterolateral corner of the knee [8]. The fibular styloid process is the site of attachment for the arcuate, fabellofibular, and popliteofibular ligaments, often termed the ar- JR:180, February 2003 cuate complex [9, 10] (Figs. 5 and 6). The fabellofibular ligament inserts in the lateral base of the apex of the fibular head (fibular styloid process). The arcuate ligament inserts in the fibular styloid process deep relative to the fabellofibular ligament. The popliteofibular ligament inserts in the upper facet of the apex of the fibular head, just medial and posterior to the insertions of the arcuate and fabellofibular ligaments. Identification of an avulsion fracture of the fibular head is important because it may reflect an injury to these ligaments, although interstitial tears of these ligaments are seen more commonly than bone avulsions [1]. vulsion fractures of the fibular head generally involve the styloid process and are uncommon, occurring in 0.6% of the 13 patients in our study. The arcuate sign has been described as a small avulsion of bone from the fibular head that is pulled away by injury to the arcuate complex [1]. Several different types of avulsion fractures of the fibular head occur, however, related to the attachment sites of the many ligaments that attach to the fibular head. n avulsion of the popliteofibular, arcuate, and fabellofibular ligaments involves the apex of 383

4 Huang et al. B C D Fig year-old man with chronic posterolateral instability of right knee after motor vehicle collision. and B, nteroposterior () and lateral (B) radiographs reveal avulsed bone fragment of fibular styloid process (arrow). Transverse orientation and posterior location of avulsed bone fragment differ from that of Segond fracture. C, Sagittal spin-echo proton density weighted MR image (TR/TE, 1800/20) reveals avulsed osseous fragment of styloid process of fibular head (arrow) adjacent to popliteal tendon (arrowhead). D, Sagittal spin-echo proton density weighted MR image (1800/20) obtained medial to C reveals disruption of posterior cruciate ligament (arrow). the fibular head (fibular styloid process). This avulsion has a characteristic appearance with an elliptical fragment of bone with its long axis oriented horizontally on an anteroposterior radiograph of the knee. The avulsion may not be evident on a lateral radiograph because the bone fragment often is superimposed on the cortex of the posterior tibial plateau. In gen- 384 eral, the bone fragment is well depicted on an anteroposterior radiograph obtained with slight internal rotation of the knee and on a lateral radiograph obtained with slight external rotation of the knee. On MR imaging, the avulsed bone fragments were identified in 11 of 13 patients on either the sagittal or coronal images. In the remaining two patients, marrow edema was seen in the fibular head without identification of an avulsion fracture. Because of the importance of an avulsion fracture of the fibular head in predicting posterolateral instability, it is necessary to carefully examine the fibular head in all MR images in patients with acute injuries. vulsion fractures of the fibular head have been ascribed to an injury of the lateral collateral ligament and tendon of the biceps femoris muscle in most reported cases [11, 12]. natomically, the lateral collateral ligament and tendon of the long head of the biceps femoris muscle are attached to the lateral margin of the fibular head [9] (Figs. 5 and 6). The popliteofibular, arcuate, and fabellofibular ligaments are attached to the fibular styloid process [9] (Figs. 5 and 6). In our series, the avulsed bone fragment was surgically confirmed to originate from the posterosuperior aspect or fibular styloid process rather than the lateral margin of the fibular head. This location was also confirmed on the radiographs and MR images in all of our patients. On the basis of our radiographic and MR imaging findings and surgical results, we speculate that this pattern of avulsion injury is related to the arcuate complex and is more common than an avulsion injury related to the lateral collateral ligament and tendon of the biceps femoris muscle. n avulsion fracture of the fibular head may be associated with other ligamentous and musculotendinous injuries in the posterolateral corner of the knee [11, 12]. In this setting, the lateral collateral ligament, tendon of the biceps femoris muscle, and the popliteal musculotendinous region may also be injured. This information is important for establishing an appropriate treatment plan. In our series, six patients had disruption of the arcuate complex, and one patient had a tear of the popliteal tendon. MR imaging has been used for the evaluation of the integrity of the posterolateral corner of the knee [13]. The lateral collateral ligament, tendon of the biceps femoris muscle, and popliteal muscle and tendon are larger structures, and their injuries can be accurately assessed on routine MR images [13]. However, the popliteofibular, arcuate, and fabellofibular ligaments are not easily recognized and evaluated on routine MR images because of their relatively smaller size and oblique orientation. Therefore, some investigators have considered the oblique coronal imaging plane as the optimal imaging plane for visualizing these structures [14]. concomitant injury of either the anterior or posterior cruciate ligament in patients with an injury involving structures in the posterolat- JR:180, February 2003

5 MR Imaging of an vulsion Fracture of the Fibular Head eral aspect of the knee has been stressed in the literature [4, 13, 15 17]. When a patient sustains an injury to both the arcuate complex and a cruciate ligament, it may compromise the function and stability of the knee to a greater extent than an isolated injury to either structure alone [16]. In our series, the avulsion fracture of the fibular head (arcuate sign) was associated with a tear of the posterior cruciate ligament in all patients. The exact mechanism for this injury is uncertain. Because all our patients were injured in a motor vehicle collision, knee trauma characterized by a hyperextension injury combined with a varus force presumably contributed to the coexistence of an injury to the posterior cruciate ligament and an avulsion fracture of the fibular head. ccordingly, an associated injury of a cruciate ligament, particularly the posterior cruciate ligament, should always be suspected when an avulsion fracture of the fibular head is recognized on the routine radiographs, particularly in patients involved in a motor vehicle collision. Therefore, we recommend that when this avulsion fracture is detected radiographically, a subsequent MR imaging examination should be performed to specifically evaluate the posterior cruciate ligament. dditionally, associated injuries of the medial compartment, menisci, and bones were frequent. We found that all our patients had an associated injury of the medial collateral ligament. ssociated meniscal tears were divided almost equally with regard to the medial and lateral menisci. In addition to avulsion fractures of the fibular head and posterior tibial plateau, the bone abnormalities depicted on the conventional radiographs and MR images in our patients reflected the pattern of injury that is consistent with a hyperextension varus force. The associated injuries were commonly seen in the anterior tibial plateau and anterior femoral condyle. Radiographically, the arcuate sign may resemble a Segond fracture, which is associated with anterolateral rotatory instability and produced by an avulsion of the capsular ligament and fibers of the iliotibial band [18]. The more lateral and posterior location of the arcuate sign fracture should obviate any mistake, however. Lateral radiographs obtained with slight external rotation are helpful for confirming the fibular site of injury in those cases with inadequate information derived from the routine anteroposterior and lateral radiographs. On MR imaging, these two types of avulsion fractures can be differentiated from one another. JR:180, February 2003 B C D Fig year-old man with chronic posterolateral instability of right knee after motor vehicle collision., nteroposterior radiograph reveals avulsed bone fragment of fibular styloid process (arrow). B, Sagittal spin-echo proton density weighted MR image (TR/TE, 1800/20) reveals avulsed osseous fragment (arrow) adjacent to popliteal tendon (arrowhead). C, Coronal spin-echo proton density weighted MR image (1800/20) reveals disruption of lateral collateral ligament (arrowhead) and avulsed osseous fragment (arrow) in corresponding course of popliteofibular ligament. D, Sagittal spin-echo proton density weighted MR image (1800/20) obtained medial to B reveals tear of posterior cruciate ligament (arrow). n important limitation of our study is the delay between injury and MR imaging, which ranged from 2 to 28 weeks (average, 13 weeks). This delay reduced the incidence of bone marrow edema, which was identified in the fibula in only three of 13 patients. It may also have reduced the sensitivity for detecting acute ligamentous injury. nother limitation is the awareness of the radiologists who reviewed the MR images that all patients had avulsion fractures of the fibular styloid on radiography, which may have increased the sensitivity for the detection of fibular styloid fractures with MR imaging. Finally, the absence of coronal oblique imaging, which has been reported to improve visualization of the posterolateral ligaments, may have reduced the sensitivity for detection of ligamentous injury. 385

6 Huang et al. Popliteofibular Medial B C Fig year-old man with acute posterolateral instability of left knee after motor vechicle collision., nteroposterior radiograph shows avulsion fracture of styloid process of fibular head with superior displacement and tilting of bone fragment (arrow). B, Sagittal spin-echo T2-weighted MR image (TR/TE, 1800/80) reveals edema in fibular head (arrow) and soft tissues around popliteal tendon (arrowhead) and arcuate complex in posterolateral corner. Bone fragment is not well depicted. C, Coronal spin-echo proton density weighted MR image (1800/20) reveals disruption of lateral collateral ligament (arrowhead) and edema in surrounding tissues. Note tear of posterior cruciate ligament (arrow). Lateral Fabellofibular and arcuate ligaments Lateral collateral ligament and dlb Fig. 5. Diagram of posterior aspect of fibula shows insertions of ligaments and tendons in fibula. Popliteofibular ligament inserts in upper facet of apex of fibular head, just medial to insertions of fabellofibular and arcuate ligaments. Lateral collateral ligament and direct arm of tendon of long head of biceps femoris muscle (dlb) are attached to lateral margin of fibular head. In conclusion, an avulsion fracture of the fibular head (arcuate sign) generally involves the fibular styloid process, which is produced by an avulsion at the site of attachment of the arcuate complex. This particular type of avulsion fracture has a characteristic radiographic appearance and important associated MR imaging findings, which include tears of the posterior cruciate ligament, collateral ligaments, menisci, or combinations of these. Whenever the arcuate sign is detected radiographically, we believe that subsequent MR imaging should be required. References Tibia 1. Shindell R, Walsh WM, Connolly JF. vulsion fracture of the fibula: the arcuate sign of posterolateral Fibula Popliteofibular Lateral collateral ligament dlb Fabellofibular and arcuate ligaments Fig. 6. Cross-sectional diagram through proximal tibia and fibula shows insertions of ligaments and tendons in fibula. Direct arm of tendon of long head of biceps femoris muscle (dlb) is attached to lateral margin of fibular head. Direct arm of tendon attaches to fibular head just lateral and posterior to attachment site of lateral collateral ligament. Fabellofibular and arcuate ligaments insert in apex of fibular head (fibular styloid process). ttachment of fabellofibular ligament is just superficial to that of arcuate ligament. Popliteofibular ligament inserts in upper facet of apex of fibular head, just medial and posterior to insertions of arcuate and fabellofibular ligaments. knee instability. Nebra Med J 1984;69: Veltri DM, Warren RF. Posterolateral instability of the knee. J Bone Joint Surg m 1994;76: O Brien SJ, Warren RF, Pavlov H, Panariello R, Wickiewicz TL. Reconstruction of the chronically insufficient anterior cruciate ligament with the central third of the patellar ligament. J Bone Joint Surg m 1991;73: Fleming RE, Blatz DJ, McCarroll JR. Posterior 386 JR:180, February 2003

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