Intermetatarsal Fat Pad Sign: Radiographic Aid to Diagnosis of Occult Tarsometatarsal Joint Injuries

Abstract

AJR 2009; 192:W36–W37 0361–803X/09/1921–W36 © American Roentgen Ray Society Intermetatarsal Fat Pad Sign: Radiographic Aid to Diagnosis of Occult Tarsometatarsal Joint Injuries Lisfranc tarsometatarsal fracture–disloc­ ations are well­described injuries usually resulting from forced plantar flexion or direct blow or crush trauma to the forefoot. Although this is a relatively rare injury, timely recognition is essential because misdiagnosis can lead to chronic pain and instability in this patient population. Lisfranc injuries account for less than 1% of all fractures and have a reported incidence of one per 55,000 persons; however, the overall incidence may be more common than is generally recognized because misdiagnosis has been reported to occur in approximately 20% of all cases [1, 2]. Conventional radiographs, including weight­ bearing­view images, can reveal fractures of the metatarsal bases, and dis placement of the bases in advanced cases, but can be unreli­ able in the detection of subtle injuries and iso lated ligamentous injuries without bone frac tures [3]. Subtle malalignment of the tarsal–metatarsal junction can be difficult to diagnose radiographically. Norfray et al. [4] have even suggested that 1–2 mm of tarso­ metatarsal malalignment may be normal. The reason for this diagnostic dilemma lies in the fact that overlapping shadows, inappropriate positioning, and sub optimal exposure parame­ ters can make correct evaluation of osseous land marks difficult [4]. Despite its limitations in revealing the full extent of tarsometatarsal injury, radiography is often the first examination performed after the initial clinical examination. Mis diagnosis is highest in cases of isolated ligamentous injury with no visible fracture on the radiograph and in which metatarsal cuneiform offset may be subtle or absent. We have pro­ vided images of such a case, and we describe an interesting soft­tissue radiographic find­ ing that, to the best of our knowledge, has not been previously described. We have called this the “intermetatarsal fat pad sign.” Similar to the sail sign of olecranon and coronoid fossae fat pad displacement seen with intra­ articular injury to the elbow, we show that edema and hemorrhage from a Lisfranc ligamentous injury may also produce a radio­ graphically apparent displaced inter metatarsal fat pad (Fig. 1). We believe that awareness of the intermetatarsal fat pad sign may be of aid in the diagnosis of subtle radio graphic pre­ sentations of Lisfranc injury. Vincent Timpone Michael Tall Andrew Puckett David Grant U.S. Air Force Medical Center Travis Air Force Base, CA 94535