MR Imaging of Ankle Impingement Lesions James Linklater, MBBS, FRANZCR Impingement is defined as a painful limitation of motion. Impingement lesions as identified on MR imaging of the ankle may relate to a range of soft tissue or bony pathologies that can be interpreted as predisposing to painful limitation of motion, accepting that the diagnosis of impingement remains clinical and not radiological. Typically, impingement lesions are classified according to their location and whether the underlying pathology is osseous or soft tissue in nature. Most commonly, impingement lesions relate to posttraumatic synovitis and intra-articular fibrous bands–scar tissue, capsular scarring, or bony prominences, the latter either developmental or acquired. Well-recognized sites of impingement around the ankle include the anterolateral, cen- troanterior, anteromedial, posteromedial, and posterior sites. This article briefly reviews the anatomy in these regions and focuses on common causes of impingement around the ankle; their pathogenesis, clinical features, and management; the approach to imaging of these lesions with MR imaging and their imaging features; and the relevant imaging differential diagnoses. ANTEROLATERAL IMPINGEMENT Anatomy of the Anterolateral Gutter The anterolateral gutter or recess of the ankle is the area bounded by the anterolateral border of the talar dome-body, the anterior border of the lateral malleolus, and the inferior margin of the anterior inferior tibiofibular ligament (AITFL) (Fig. 1). There is some variation in the degree of inferior extension of the inferior fascicle of the AITFL into the anterolateral gutter. The anterior talofibular ligament (ATFL) lies at the superficial margin of the anterolateral gutter. The calcaneofib- ular ligament lies at the inferior margin of the anterolateral gutter. The normal recess often contains a small amount of simple joint fluid. During ankle dorsiflexion, the anterolateral border of the talus protrudes somewhat into the anterolat- eral gutter, narrowing it and displacing any native joint fluid. Pathogenesis of Anterolateral Impingement Anterolateral soft tissue impingement most commonly occurs as a complication of a plantar flexion-inversion sprain, with associated tear of the ATFL. There is often an associated hemarthro- sis, with fibrinous debris in the joint and subsequently a posttraumatic synovitis in the anterolateral gutter. 1 In the weeks to months after the initial injury, the synovitis may impinge on the anterolateral talar dome during dorsiflexion, causing pain and restriction of dorsiflexion. Over time, the synovitis may coalesce and undergo hyalinized fibrosis, 2,3 marginating around the intra-articular inferior fascicle of the AITFL, the anterolateral talar dome, and the anterior margin of the lateral malleolus. The hyalinized fibrotic tissue may become triangular or meniscoid in shape. 4 During ankle dorsiflexion, the meniscoid lesion may impinge on the anterolateral margin of the talar dome, causing pain and limiting sports activity. 5 Repetitive abrasion may result in a chon- dral lesion on the anterolateral talar dome. 6 Syndesmotic ligament complex injuries may also be complicated by anterolateral impingement because of hypertrophic scar response and Castlereagh Sports Imaging, North Sydney Orthopaedic and Sports Medicine Centre, 286 Pacific Highway, Crows Nest, NSW 2065, Australia E-mail address: jameslinklater@casimaging.com.au KEYWORDS  Ankle  Ankle impingement  Ankle injuries  Ankle abnormalities  Ankle MR imaging  Dancer injuries  Athlete injuries Magn Reson Imaging Clin N Am 17 (2009) 775–800 doi:10.1016/j.mric.2009.06.006 1064-9689/09/$ – see front matter ª 2009 Published by Elsevier Inc. mri.theclinics.com