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1 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.
2 Department of Radiology, Ohio State University Medical Center, S-207 Rhodes
Hall, 450 W. Tenth Ave., Columbus, OH 43210.
3 Department of Radiology, Veterans Administration Medical Center, 3350 La Jolla
Village Dr., San Diego, CA 92161.
4 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.
Received December 6, 2001;
accepted after revision July 30, 2002.
Address correspondence to G.-S. Huang.
Abstract
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MATERIALS AND 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. All 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. An avulsion fracture of the fibular head generally involves the styloid process and causes injury of some of the major stabilizers in the posterolateral corner. Avulsion fractures are strongly associated with disruption of the posterior cruciate ligament.
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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 10-15°. The following sequences were obtained in all patients: spin-echo proton density—weighted (TR range/TE range, 1800-2200/20-30) and T2-weighted (1800-2200/80-90) 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 192-256 x 256 matrix. In one patient, an additional short tau inversion recovery (STIR) (2500-3000/35-40; inversion time: range, 100-150 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. A 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. Arthroscopy 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.
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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 postero-superior 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).
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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.
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A 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.
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All 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. A 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. All 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.
Almost 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.
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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 external 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 arcuate 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]. Avulsion fractures of the fibular head generally involve the styloid process and are uncommon, occurring in 0.6% of the 13 patients in our study.
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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. An avulsion of the popliteofibular, arcuate, and fabellofibular ligaments involves the apex of 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 general, 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.
Avulsion 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]. Anatomically, 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.
An 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].
A concomitant injury of either the anterior or posterior cruciate ligament in patients with an injury involving structures in the posterolateral aspect of the knee has been stressed in the literature [4, 13, 15,16,17]. 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. Accordingly, 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.
Additionally, 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. Associated 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.
An 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. Another 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.
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.
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