Clinical History: A 35 year-old female with history of prior ankle sprain presents with lateral pain and bruising. T2-weighted axial images are provided at a level just above the tip of the fibula (1a) and 1 cm above this site (1b). What are the findings” What is your diagnosis?
1. Dislocation of the peroneus longus tendon with Type I injury of the superior peroneal retinaculum.
2. Chronic partial tear of the anterior talofibular ligament.
The most common ankle injury is a lateral ligamentous sprain. An important injury that may have a similar clinical presentation and is often misdiagnosed as “ankle sprain” is that of peroneal tendon dislocation with injury of the superior peroneal retinaculum. This may occur in isolation or in conjunction with anterolateral ankle instability. Distinguishing between injuries of these structures and other causes of pain in this region is important in planning appropriate treatment, as peroneal retinacular injuries often require operative repair. MRI is optimally suited for evaluating injured lateral ankle soft tissues and for diagnosing lateral ankle pathologies that may have similar clinical presentations.
Anatomy and Function
The peroneal tendons are positioned posterolaterally and function as evertors and plantar flexors as well as dynamic stabilizers of the foot and ankle. The peroneus longus muscle originates from the upper fibula and courses along the lateral aspect of the ankle before turning medially beneath the cuboid. It courses through the deep plantar aspect of the midfoot, supporting the transverse arch, before inserting onto the medial cuneiform and the base of the first metatarsal. The peroneus brevis originates from the lower fibular shaft and inserts onto the base of the fifth metatarsal. The tendons share a common tendon sheath above the level of the tip of the fibula and are held in place by the superior and inferior peroneal retinacula. The superior peroneal retinaculum (SPR) functions as the primary restraint to peroneal tendon subluxation and is also a secondary restraint to anterolateral ankle instability. It is formed from a confluence of the common peroneal sheath and the superficial fascia of the leg. Anteriorly it attaches to and blends together with the lateral fibular periosteum. Posteriorly it has variable attachments to the Achilles tendon and the calcaneus. The SPR creates a fibro-osseous tunnel for the peroneal tendons contained within their common tendon sheath. More distally, at the level of the peroneal tubercle, the tendons have separate tendon sheaths and separate fibro-osseous tunnels formed by the inferior peroneal retinaculum (3a).
Normal MRI Anatomy
On MRI, the normal peroneal tendons typically appear dark on all pulse sequences. On axial sequences at the level of the ankle, the peroneal tendons are found posterior to the lateral malleolus within the peroneal or retromalleolar groove, which is sometimes deepened by a small fibrous ridge at the lateral fibular margin. The groove may alternatively be flat or convex, which can predispose to subluxation. At this level the peroneus brevis tendon is positioned anteromedial to the peroneus longus and is crescentic in cross-section (Figures 4a&4a). The peroneus brevis myotendinous junction is lower in position than that of the peroneus longus, and may be seen at the level of the tibiotalar joint (Figure 4b). The SPR is seen as a thin dark band posterolateral to the tendons at the level of the lateral malleolus, attaching to the periosteum at the posterolateral margin of the fibula. The uninjured periosteum is thin and indistinct from the bone cortex. Distal to the lateral malleolus, the peroneus brevis courses anterior to the peroneus longus (Figures 4a&4d). The peroneal tubercle is variable in size and projects laterally from the anterior process of the calcaneus, separating the positions of the peroneus brevis and longus tendons (Figure 4c). Normal variant anatomy in this region may include a peroneus quartus muscle, a low-lying peroneus brevis muscle belly, or an os peroneum.4
Mechanism of Injury
Injury of the superior peroneal retinaculum (SPR) occurs with peroneal dislocation through forceful ankle dorsiflexion and concomitant reflex peroneal muscle contraction. This injury has been described in conjunction with numerous sports activities, particularly snow skiing. The injury can occur when ski tips suddenly become lodged in the snow and the skier’s forward momentum causes passive ankle dorsiflexion. Some controversy exists about the associated role of ankle inversion or eversion.
Patients with chronic injury and recurrent tendon subluxation may present with inability to recall a specific traumatic episode. Nontraumatic dislocations can be seen congenitally, particularly with calcaneovalgus feet, or acquired, such as in patients with neuromuscular disease.1 Heel valgus may predispose to injury.
In the most common form of SPR injury (Oden classification2 Type I), the SPR is not torn, but becomes detached from the lateral malleolus together with stripping and elevation of the periosteum to which it is attached, forming a false pouch. In a type II injury, the SPR is torn near the lateral fibular margin. In a type III SPR injury, there is also an associated avulsion fracture, which may be detected radiographically as a small fleck of bone detached from the lateral fibular margin. Type IV injury involves a tear of the posterior portion of the SPR. The vast majority of injuries are Type I, without an actual tear of the retinaculum. In one study of 73 cases,3 the authors did not find any retinacular tears. With any of these types of injury, the incompetent retinaculum can no longer restrain the tendons and recurrent subluxations or dislocations may occur. Concomitant lateral ligamentous injuries are frequently seen and may cause ankle instability.
Patients typically present with pain and swelling along the posterior aspect of the lateral malleolus.1 They may have felt a distinct pop at the time of acute injury or may report recurrent popping or snapping with activity, particularly when ascending or descending stairs. Tendon subluxation may be elicited during physical exam by dorsiflexing and internally rotating the ankle from a position of dorsiflexion and eversion. The clinical findings of SPR injury and associated chronic tendon subluxation can easily be mistaken for chronic lateral instability. The pain associated with peroneal tendon pathology is posterior to the lateral malleolus, in contradistinction to patients with lateral ligamentous sprains who have more anterolateral and/or inferolateral pain. However, the clinical diagnosis may be compromised by swelling or complicated by peroneal tendon pathology occurring concomitantly with ligamentous injuries and instability.
Although a small shell-like avulsion fracture off of the lateral malleolus may occasionally be found radiographically (type III SPR injury), this is absent in the majority of cases of peroneal tendon subluxation/dislocation. Ultrasound has been shown to be effective in real-time demonstration of recurrent peroneal subluxation or dislocation.4 MRI offers a complete assessment of the structures at risk, associated pathology, and diagnostic mimics.
The diagnosis of dislocation of the peroneal tendons is made when either or both tendons are not identified in their normal anatomic positions posterior to the lateral malleolus and the diagnosis of complete tendon rupture and retraction is excluded. Evaluating the tendons on serial images allows discrimination between tendon subluxation, dislocation, or tear. Either or both of the peroneal tendons may dislocate, and the determination of which tendon is dislocated must be made by following the tendons distally to their attachments. Sagittal images may be helpful for confirmation (Figure 5a, 5b).
With peroneal tendon dislocation, the periosteum is stripped and elevated together with the attached superior peroneal retinaculum, forming a false pouch lateral to the fibular margin. The tendons may reduce, but the SPR remains incompetent, allowing recurrent subluxation and/or dislocation. The resultant abnormal stress leads to tendon degeneration and tearing.
Identification of periosteal stripping and an abnormal pouch is particularly important in patients with recurrent episodes of tendon subluxation, who may have normally positioned tendons at the time of imaging. The SPR should be tightly affixed to the periosteum at the posterolateral margin of the distal fibula. Both peroneal tendons should lie medial to a vertical line drawn from the lateral margin of the distal fibula.
Other peroneal tendon pathology may also present with lateral ankle pain and swelling. Most commonly this involves a spectrum of tendon degeneration and tearing,8 more often involving the peroneus brevis tendon in a longitudinal fashion as it passes under the lateral malleolus. Longitudinal tearing of the peroneus brevis tendon, or “peroneal splits,”6 can be diagnosed when alteration of the peroneus brevis morphology is seen in the peroneal groove on careful sequential image inspection. The partially torn peroneus brevis split appears as an inverted U shape (Figure 8a). This can progress to complete separation into two components, with the peroneus longus tendon interposed between the split peroneus brevis tendon components. Different stages in the continuum of tendinosis, partial tear, and complete tear may coexist in the same tendon at adjacent levels. A rare normal variant bifurcated peroneal brevis tendon can be distinguished from a longitudinal tear by proximally identifying the presence of two separate myotendinous junctions.5 Also, care must be taken in not mistaking a normal mildly crescentic peroneus brevis for a partially torn U shaped tendon. The torn tendon is more notably distorted in shape, and often will be accompanied by adjacent segments of tendon degeneration and thickening as well as fluid in the tendon sheath.
Tendinosis less frequently involves the peroneus longus tendon. This may occur at the level of the lateral malleolus, particularly following peroneus brevis tear. Isolated peroneus longus tendon degeneration and tear typically occurs more distally at the midfoot9 where increased stresses are found as the tendon courses beneath the cuboid, or at the level of the peroneal tubercle, particularly when it is hypertrophied. Acute tears from sports-related injury or trauma are less common.
Tenosynovitis can occur alone or accompany tendon pathology and may be an alternative cause for pain along the course of the peroneal tendons. Other less frequent causes of pain or swelling at this location may include a ganglion cyst10 or a soft tissue mass such as giant cell tumor of tendon sheath.11
Since ankle inversion can be part of the underlying SPR injury mechanism and the clinical findings may not allow distinction between injuries of the SPR and the lateral ligaments, the lateral ankle ligaments must be carefully evaluated on MRI exams of this region. Indeed, Rosenberg et al.12 found a high percentage (78%) of patients with SPR injuries that had concomitant lateral ligamentous injuries.
Conservative treatment involves reduction of the displaced peroneal tendons and immobilization in a below knee cast for six weeks. However, with conservative therapy there is a high incidence of recurrence, particularly in young athletes. Surgical repair of the superior peroneal retinaculum is often necessary for definitive treatment in active patients. Several surgical techniques are in use including anatomic repair or reconstruction, with or without surgical deepening of the peroneal groove. The procedures are highly successful in preventing recurrence.13,14
Diagnosing the cause of persistent lateral pain following an ankle sprain may be clinically challenging. In such cases, peroneal tendon and superior peroneal retinaculum injuries are increasingly recognized as important etiologies that should not be missed. MRI provides detailed visualization of the anatomy of the lateral ankle region and distinguishes between entities which present with similar clinical findings, allowing accurate diagnosis and the choice of the optimal treatment strategy.
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