MRI Features of Chronic Injuries of the Superior Peroneal Retinaculum

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1 Zehava Sadka Rosenberg 1 Jenny Bencardino 2 Donna Astion 3 Mark E. Schweitzer 1 Andrew Rokito 3 Steven Sheskier 3 Received July 31, 2002; accepted after revision June 26, Presented at the annual meeting of the Radiological Society of North America, Chicago, IL, November Radiology Department, Hospital for Joint Diseases, 301 E 17th St., New York, NY Address correspondence to Z. S. Rosenberg. 2 Huntington Hospital, North Shore Long Island Jewish Health Center, New Hyde Park, NY. 3 Orthopedic Department, St. Luke s-roosevelt Hospital, New York, NY AJR 2003;181: X/03/ American Roentgen Ray Society Original Report MRI Features of Chronic Injuries of the Superior Peroneal Retinaculum OBJECTIVE. The aims of this study were to assess, grade, and surgically correlate previously unreported MRI features of superior peroneal retinacular injuries in nine surgically proven cases and to record all soft-tissue and bony abnormalities associated with these injuries. CONCLUSION. MRI was found to be a useful tool for detecting and grading superior peroneal retinacular injuries and providing information, important for presurgical planning, regarding common concomitant soft-tissue and osseous abnormalities of the lateral collateral ligaments, peroneal tendons, and fibular groove. Superior peroneal retinacular injuries are frequently associated with MRI evidence of peroneal tendon dislocations and tears. Conversely, routine MRI studies may not depict dislocated peroneal tendon injuries, despite clinical history to that effect. T he superior peroneal retinaculum forms the posterolateral border of the peroneal tunnel and thus maintains the peroneal tendons in place within the fibular groove [1, 2] (Fig. 1). Congenital or traumatic absence, laxity, or tear of the superior peroneal retinaculum allows acute or chronic dislocation of the peroneal tendons [3, 4]. Treatment of acute and chronic superior peroneal retinacular injuries varies. Plaster immobilization is initially attempted, particularly in acute dislocations. Surgery is reserved for patients with painful, chronic, unstable dislocations and varies depending on the extent and classification of the disease. Injuries of the superior peroneal retinaculum, particularly in the acute phase, are often clinically mistaken for other causes of lateral ankle pain such as ligamentous injuries or peroneal tendon abnormalities [3, 4]. The concomitant presence of these latter entities with superior peroneal retinacular injuries further confounds the clinical diagnosis. Undetected superior peroneal retinacular injuries can lead to increased friction of the tendons as they slide in and out of the peroneal groove. This movement predisposes the tendons to longitudinal splits. Thus, a prompt diagnosis of superior peroneal retinacular injuries, before such tears occur, is important. MRI findings of the normal superior peroneal retinaculum have been described in the radiologic literature [5]. We reviewed nine cases of surgically proven superior peroneal retinacular injuries to assess the role of MRI in detecting and classifying these injuries and in detecting associated osseous and softtissue abnormalities. Materials and Methods A review of our teaching files revealed 34 cases of chronic superior peroneal retinacular injuries in 33 patients. Twenty-five patients were excluded from the study because of a lack of surgical correlation. Thus, nine patients (seven women and two men; age range, years; mean age, 38 years) were included in the study. Clinical indications for the MRI studies varied and included chronic peroneal tendon instability (n = 5), recurrent inversion injuries (n = 3), and lateral ankle pain (n = 1). MRI examinations of the ankle were performed with 1.5-T scanners using a transmit receive coil (Signa, General Electric Medical Systems, Milwaukee, WI). The patients were imaged in a supine position, feet first, with the ankle in mild plantar flexion. The MRI protocol included sagittal T1-weighted (TR range/te, /22) and STIR (4,000 5,000/21) sequences, axial T1- weighted ( /22) and fast spin-echo T2- weighted (TR range/te range, 3,000 3,500/80 90) AJR:181, December

2 Rosenberg et al. sequences, and coronal T1-weighted images (TR range/te, /22). Proton density axial images (TR range/te range, 1,100 1,500/30 40) were occasionally obtained. The field of view was cm. The slice thickness was 3 4 mm with an interslice gap of 1 mm. The image matrix was for T1-weighted images and for T2-weighted images. The MRI studies were retrospectively reviewed by two radiologists experienced in musculoskeletal imaging to confirm the presence and type of superior peroneal retinacular injury and to assess peroneal tendon abnormalities such as dislocations and tears, the appearance of the fibular groove, and the presence of lateral collateral ligament injuries. The initial MRI interpretation was not changed during the second retrospective interpretation. The radiologists were familiar with the clinical history but had no knowledge of the surgical results during the initial and retrospective interpretations of the MRI examination. The superior peroneal retinaculum was defined as normal [5] when it depicted normal morphology and normal attachment at its origin from the distal fibula (Fig. 2). Superior peroneal retinacular injuries were graded by consensus using Oden s classification [4] (Fig. 3). Type I injuries consist of elevation or stripping off of the periosteal attachment of the superior peroneal retinaculum to the lateral malleolus at the level of the fibular groove. The stripped-off periosteum and superior peroneal retinaculum form a pouchlike configuration lateral AT SPR IPR to the distal fibula into which the peroneal tendons can dislocate. In type II injuries, there is a tear of the superior peroneal retinaculum at its attachment to the distal fibula. Type III injuries consist of a distal fibular avulsion fracture at the attachment of the retinaculum to the lateral malleolus. Type IV injuries are characterized by a tear of the retinaculum at its posterior attachment. The presence of the following soft-tissue and osseous variants and abnormal conditions within the peroneal tunnel were also recorded: peroneal tendon subluxation or dislocation, torn peroneal tendons, peroneus quartus muscle, lateral collateral ligament injury, and abnormal shape of the fibular groove [6 8]. Longitudinal splits of the peroneal tendons were defined on the basis of previously established MRI criteria [6, 7]. The peroneal tendons were defined as subluxed when they were partially out of the fibular groove and defined as dislocated when they were lateral and anterior to the groove. Lateral collateral ligament tears were defined as thickening, attenuation, or discontinuity of the anterior and posterior tibiofibular, anterior and posterior talofibular, and calcaneofibular ligaments. The peroneus quartus muscle was identified as an accessory musculotendinous unit located posteromedial to the peroneus brevis and longus tendons [8]. The fibular groove, present approximately 1 cm above the tip of the distal fibula, was defined as convex when the posterior cortex of the fibula bulged outward, concave when the posterior fibular surface Fig. 1. Drawing of normal superior peroneal retinaculum. Retinaculum originates from distal fibula. Most common insertion sites include lateral wall of calcaneus and aponeurosis of Achilles tendon. AT = Achilles tendon, = peroneus longus, SPR = superior peroneal retinaculum, IPR = inferior peroneal retinaculum, = peroneus brevis. had a shallow-to-deep depression, flat when neither concavity or convexity was found but the surface of the groove was smooth, and irregular when small osseous ridges arose from the surface of the groove. Results Soft-Tissue Abnormalities Superior peroneal retinacular injuries. Type I injuries, depicted as a linear low-signal structure forming a pouch lateral to the distal fibula at the level of the lateral malleolus, were seen in seven MRI examinations (78%) (Figs. 4 and 5). In two of these patients, the pouch was collapsed against the lateral wall of the fibula, depicted as a lowsignal linear structure lateral to the fibular cortex (Fig. 6). A type II injury (tear of the superior peroneal retinaculum from the lateral malleolus) was noted in one case (11%) (Fig. 7). Poor definition and thickening of the retinaculum at its insertion to the fibula were interpreted as a tear. A type III injury was noted in one case (11%). This injury was depicted as a disruption of the retinaculum at the point of its insertion to the fibula (Fig. 8). A focal cortical defect of the distal fibular tip at the insertion site, associated with bone marrow edema, was also noted. The avulsed fragment was not noted. Peroneal musculotendinous unit. In four patients (46%), three of whom had clinical history of chronic peroneal tendon dislocation, the peroneal tendons were in their normal location within the fibular groove (Fig. 6). An abnormal position of the peroneus brevis was noted in five cases (56%), all of them with type I superior peroneal retinacular injuries (Figs. 4, 5, and 9). Subluxation was noted in two of the five cases, and dislocation was noted in three of the five cases. The peroneus longus was dislocated in two cases with either peroneus brevis subluxation (n = 1) or peroneus brevis dislocation (n = 1). In the other seven cases, the peroneus longus was in normal position (Figs. 5, 6, and 9). Two of the four patients in whom the peroneal tendons were in the normal location had a type I superior peroneal retinacular injury (Fig. 6). In both cases, however, the pouch was not elevated away from the fibula but, rather, collapsed against the fibular lateral cortex. The two other cases were noted in one patient with a type II injury and another patient with a type III injury (Fig. 8). Longitudinal tears of the peroneus brevis tendon were identified in five cases (56%) (Figs. 5 and 9). In four (80%) of these five 1552 AJR:181, December 2003

3 MRI Features of the Superior Peroneal Retinaculum cases, the peroneal tendons were either subluxed or dislocated. No tears of the peroneus longus tendon were identified. A peroneus quartus muscle (Fig. 9) was found in two patients (22%). Lateral collateral ligament injuries. Seven (78%) of the nine patients had MRI evidence of injuries to the lateral collateral ligament. In all seven, there was a thickened or discontinuous anterior talofibular ligament. Associated calcaneofibular ligament thickening or tearing was found in four of these seven cases. Osseous Changes Associated with the Superior Peroneal Retinacular Injuries Six patients (67%) had anatomic variants of the fibular groove known to predispose to instability and tears of the peroneal tendons such as flat (n = 5) (Figs. 4 and 5) and convex (n = 1) posterior fibular surfaces. Irregular cortical surface (Fig. 5) was noted in one flat and one convex groove (Fig. 8). Only three patients (33%) had concave fibular grooves (Figs. 6 and 9). Surgical Results The superior peroneal retinaculum was explored in all nine patients. Surgery confirmed the MRI diagnosis of type I superior peroneal retinacular injury in seven of those nine patients and the diagnosis of type III tear in the eighth patient. Thus, there were eight true-positive findings. Fig. 3. Schematic representation of Oden s surgical classification of superior peroneal retinacular injuries [4]. Normal indicates that superior peroneal retinaculum originates from distal fibula. Small fibrous ridge is noted. Type I indicates that superior peroneal retinaculum is stripped off distal fibula, forming pouch lateral to bone. Peroneal tendons can sublux or dislocate into this pouch. Type II indicates that superior peroneal retinaculum is avulsed of its distal fibular insertion. Type III indicates that superior peroneal retinaculum, along with small avulsion fragment, is avulsed off distal fibula. Type IV indicates superior peroneal retinaculum is torn off at its posterior attachment. = peroneus brevis tendon, = peroneus longus tendon, SPR = superior peroneal retinaculum. Fig. 2. MRI of normal superior peroneal retinaculum in 30-year-old man. Axial T1-weighted image (TR/TE, 700/ 22) depicts superior peroneal retinaculum (thick white arrows) blending with fibrous ridge (thin white arrow) at fibular malleolar attachment site and traversing posteriorly toward aponeurosis of Achilles tendon. Flat fibular groove (straight black arrows) accommodates peroneal tendons (curved arrow). Fibula Normal Fibrous ridge SPR Type I Pouch Type II Type III Type IV AJR:181, December

4 Rosenberg et al. One false-positive MRI result was detected in the ninth patient who underwent surgery. The MRI diagnosis was type II injury because of the thickening and indistinctness of the superior peroneal retinaculum at its attachment site to the fibular malleolus (Fig. 7). At surgery, the superior peroneal retinaculum was found to be intact, albeit edematous. Surgery correlated with the MRI findings of peroneal tendon tears and the presence of peroneus quartus in all patients. The shape of the fibular groove and the presence of ligamentous injuries were not mentioned in the surgical reports. Discussion Fig. 4. Type I superior peroneal retinacular injury in 44-year-old man after ankle sprain. Axial spin-echo proton density image (TR/TE, 1,183/30) shows pouch formed by stripped-off periosteum and superior peroneal retinaculum (thin solid arrows). Peroneus brevis and longus tendons (thick arrow) are dislocated into pouch. Fibular groove (open arrow) is flat. Fig. 5. Type I superior peroneal retinacular injury in 49-year-old woman with chronic dislocation of peroneus brevis tendon. Axial T1-weighted image (TR/TE, 900/22) depicts dislocation of peroneus brevis tendon (curved arrow) into pouch formed by elevated superior peroneal retinaculum (straight long arrow). Fibular groove (straight short arrows) is irregular and flat. Small fragment of peroneus brevis tendon remained in groove (open arrow) The superior peroneal retinaculum originates from the periosteum along the lateral margin of the distal fibula and fibular groove. Its insertion sites vary; the most common insertion is the aponeurosis of the Achilles tendon and the lateral calcaneous [1, 2] (Fig. 1). At its origin, the superior peroneal retinaculum is continuous with the superior extensor retinaculum and the apical fibers of the flexor retinaculum at its insertion. The superior peroneal retinaculum forms the lateral border of the peroneal tunnel in which the peroneal tendons traverse behind the distal fibula. A small fibrous ridge is occasionally seen originating from the distal fibula close to the origin of the superior peroneal retinaculum. This fibrous ridge, if present, increases the depth of the fibular groove. Traumatic injury to the superior peroneal retinaculum is produced by sudden dorsiflexion [3, 4]. A simultaneous forceful contraction of the peroneal muscles elevates the periosteal attachment of the superior peroneal retinaculum off the fibula. Inversion injuries and ankle instability may also produce increasing strain, laxity, and attenuation of the superior peroneal retinaculum. Thus, lateral collateral ligament tears are frequently associated with the injury. Skiing, soccer, ice skating, rugby, and gymnastics are among the many sports in which superior peroneal retinacular injuries have been described. Other causes for superior peroneal retinacular abnormalities include congenital foot deformities, presence of anomalous muscles within the fibular groove (peroneus quartus and low-lying belly of the peroneus brevis), and extensive bony injury such as distal tibial or calcaneal fractures. Ecchymosis, swelling, pain, and tenderness along the lateral malleolus are hallmarks of acute superior peroneal retinacular AJR:181, December 2003

5 MRI Features of the Superior Peroneal Retinaculum injuries. Clinically, acute superior peroneal retinacular injuries can be confused with isolated ankle sprains and instability. After chronic peroneal tendon dislocation ensues, the clinical diagnosis of superior peroneal retinacular injury is more easily established but again can be mistaken for chronic lateral ankle instability. Radiographs do not usually reveal the diagnosis of superior peroneal retinacular injury. A small linear ossification along the lateral margin of the distal fibula is characteristic of avulsion of the superior peroneal retinaculum and peroneal tendon dislocation [9] but is infrequently encountered. Sporadic reports of sonography, CT, and MRI diagnoses of peroneal tendon dislocations have been described, but little attention to the status of the superior peroneal retinaculum has been noted [6, 7, 10 12]. The normal MRI appearance of the superior peroneal retinaculum, however, has been described in detail. The superior peroneal retinaculum is optimally visualized on axial MRI (Fig. 2), on which it is depicted as a low-signal band originating laterally from the distal fibula in the region of the fibular groove. Occasionally a small fibrous ridge, depicted as a small triangular low-signal-intensity meniscuslike structure, is noted at or close to its origin. Nonoperative plaster immobilization is advocated in the treatment of patients with acute superior peroneal retinacular tears, although the results with this approach have been somewhat disappointing [3]. Surgical intervention is advised in patients with painful chronic dislocations when the tendons slip in and out of the fibular groove. Conservative treatment is preferable, however, if the dislocated tendons remain stable outside the groove without motion. Surgical treatment regimes for symptomatic acute or recurrent superior peroneal retinacular injury and peroneal tendon dislocation include rerouting the tendons, reconstructing the superior peroneal retinaculum, and performing bony procedures such as fibular osteotomy and deepening of the fibular groove [3]. The surgical approach may differ on the basis of the grade of injury. Fibuloplasty, groove deepening, and retinacular reattachment through drill holes are usually performed. With type III injuries, in addition to retinacular repair, the avulsed bone is either removed or is anatomically reduced to provide a smooth cartilaginous surface. Eckert et al. [13] were the first to develop a surgical classification of superior peroneal tendon injuries on the basis of an exploration of AJR:181, December 2003 Fig. 6. Type I superior peroneal retinacular injury with collapsed pouch in 48-year-old man with posttraumatic clinically dislocatable peroneal tendons. Axial T1-weighted image (TR/TE, 900/22) depicts linear low signal lateral to distal fibular groove (white arrow) consistent with collapsed pouch. Peroneal tendons (black arrow) are in normal position. Fig. 7. False-positive finding of type II superior peroneal retinacular injury in 28-year-old woman with pain along lateral malleolus suspected clinically to have superior retinacular injury. Thickening and indistinctness of superior peroneal retinaculum are noted on axial T1-weighted image (TR/TE, 900/22) and were believed to be avulsion injury. At surgery, superior peroneal retinaculum was edematous but intact. 1555

6 Rosenberg et al. 73 cases. This classification was later modified by Oden [4] on the basis of his experience with treating patients with skiing injuries. Oden describes four types of superior peroneal retinacular injuries. In a type I injury, the superior peroneal retinaculum and periosteum are stripped off the distal fibula, forming a pouch lateral to the fibula into which the peroneal tendons may dislocate. In type II injuries, the superior peroneal retinaculum is torn off its attachment to the fibula. In type III injuries, a small bony fragment avulses off the distal fibula along with the superior peroneal retinaculum. In type IV injuries, the retinaculum is torn off its posterior attachment to the calcaneus. Type I injury, the most common in both series [4, 13], was noted in seven (78%) of our nine cases. This pattern of injury was easily detected on axial MRI as a pouch, bordered by the low-signal periosteum and superior peroneal retinaculum (Figs. 4, 5, and 9). The pouch was found lateral to the distal fibula at the level of the fibular groove. The peroneal tendons were dislocated or subluxed into the pouch in five of those seven cases. In the other two cases, the pouch was collapsed and the periosteum and superior peroneal retinaculum were depicted as a lowsignal line found against the lateral cortex of the distal fibula (Fig. 6). Interestingly, in both those cases the peroneal tendons were found in their normal position within the fibular groove despite a history of chronic peroneal tendon dislocation. Type III superior peroneal retinacular injury, characterized by a small avulsion fracture at the fibular attachment of the superior peroneal retinaculum, was noted in only one (11%) of our cases (Fig. 8). Although the avulsed fragment was not visualized, the lateral bony defect and marrow edema in the distal fibula at the level of the fibular groove were diagnostic of this category. This patient, as with the two described earlier, also had clinically dislocat- A able peroneal tendons but on MRI the tendons were in normal position. Surgery confirmed the MRI findings in eight of our nine patients (eight true-positive findings). Of these, seven patients had a type I retinacular injury and one patient had a type III injury. The MRI interpretation was false-positive in one patient encountered early in our study (Fig. 7). A type II injury of the superior peroneal retinaculum (a tear of the retinaculum at its fibular insertion) was diagnosed on the basis of the indistinct and thickened appearance of the retinaculum at its insertion to the fibula. At surgery, the retinaculum was intact, albeit mildly edematous. We have learned, since then, that frequently the outline of the superior peroneal retinaculum, particularly at its origin from the fibula, may be a bit thickened and poorly differentiated from adjacent subcutaneous tissues. A number of soft-tissue and osseous abnormalities were noted in our patients with B Fig. 8. Type III superior peroneal retinacular injury in 48-year-old woman with history of chronic peroneal tendon instability. A, Axial T1-weighted image (TR/TE, 800/20) depicts focal defect in distal fibula (straight long arrow) consistent with avulsion fracture. Avulsed cortical fragment is not visualized. Superior peroneal retinaculum (open arrow) is thickened and disrupted. Peroneus brevis and peroneus longus tendons (curved arrows) are in normal position. Tear of peroneus brevis was suspected. B, Fast spin-echo T2-weighted image (4,500/96) depicts increased signal in distal fibula (asterisk) consistent with marrow edema. Fluid is also present in peroneal tendons sheath. Defect in distal fibula (long arrow) and thickened superior peroneal retinaculum (short arrow) are again seen AJR:181, December 2003

7 MRI Features of the Superior Peroneal Retinaculum superior peroneal retinacular injuries. Peroneal tendon dislocations (56%) and tears (56%) were common findings (Figs. 4, 5, and 9). Clinical evidence of chronic peroneal tendon dislocation was, however, present in three of the four cases in which the peroneal tendons were in the normal position on MRI. Dynamic MRI examination of the peroneal tendons may have been particularly informative in those cases [14]. Additionally, our results give credence to the hypothesis that peroneal tendon dislocation is predisposed to tearing because four of our five cases with MRI evidence of peroneal tendon tears had concomitant evidence of peroneal tendon dislocation. Not surprisingly, because of its proximity to the hard fibular surface, the peroneus brevis tendon was more commonly torn than the peroneus longus tendon. A high percentage of patients (78%) had lateral collateral ligament injuries, which was to be expected because superior peroneal retinacular injuries are often associated with inversion injuries of the ankle. Convex, flat, and irregular fibular grooves predispose peroneal tendon dislocation and tears [15]. Indeed, 67% of our cases exhibited such morphologic changes. Limitations of our study included the small patient population and the retrospective nature of the study. Additionally, our population consisted of only surgically proven type I and type III retinacular injuries. The ability of MRI to detect other categories of tears, therefore, remains to be determined. Another limitation of the study was the lack of routine dynamic or kinematic imaging of the peroneal tendons [14]. Regardless of our study s limitations, we believe our study indicates that MRI can provide useful information to both the radiologist and the clinician in the assessment of superior peroneal retinacular injury. Because superior peroneal retinacular injuries, particularly in the acute stage, can be confused with other causes of lateral ankle pain and instability, early MRI diagnosis of this condition can prevent the development of complications such as painful peroneal tendon instability and tearing. Additionally, MRI detection of osseous and soft-tissue abnormalities, concomitant with superior peroneal retinacular injuries, can play a crucial role in the surgical outcome of the retinacular repair. For example, repairing the retinaculum without addressing either the torn peroneal tendons or the presence of an anomalous peroneal muscle may result in suboptimal Fig. 9. MRI of type I superior peroneal retinacular injury in 37-year-old woman. Axial fat-suppressed proton density image (TR/TE, 3,280/32) depicts dislocation of fragmented peroneus brevis tendon (solid arrows) into pouch formed by stripped superior peroneal retinaculum. Peroneus longus tendon has migrated proximally (open arrow). Peroneus quartus tendon (arrowhead) is medial to peroneus longus tendon. postsurgical results. Similarly, correction of a morphologically abnormal fibular groove, easily detected on MRI, may be crucial for successful surgical outcome. In conclusion, MRI is a useful modality for detecting and grading superior peroneal retinacular injuries. Retinacular injuries are often associated with peroneal tendon dislocations, tears, and lateral collateral ligament abnormality. Conversely, superior peroneal retinacular injuries may be present in the face of normally positioned peroneal tendons. 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