DOI: 10.1148/radiol.2443042063
(Radiology 2007;244:927-928.)
© RSNA, 2007
The Pronator Quadratus Sign1
Josh B. Moosikasuwan, MD
1 From the Department of Radiology, North Shore University Hospital, Manhasset, NY. Received December 6, 2004; revision requested January 1, 2005; revision received February 13; final version accepted March 3.
Address correspondence to the author, 2040 Matthews Ave, Bronx, NY 10462 (e-mail: moosikaj{at}yahoo.com).
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APPEARANCE
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The pronator quadratus sign (1–4), seen on the lateral radiograph of the forearm, is a displacement, anterior bowing, or obliteration of the thin lucent stripe of fat paralleling the distal radius and soft-tissue opacity of the pronator quadratus muscle (Figure).
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Figure a: Lateral radiographs of distal right forearm and wrist in two patients. (a) Normal: Fat plane (arrows) overlies pronator quadratus muscle (*). Maximum distance between fat plane and distal radius is 3 mm. (b) Pronator quadratus sign: Pronator quadratus muscle (*) along anterior surface of distal radius is swollen, volarly displacing overlying thin lucent stripe of fat (arrows). Comminuted fracture of distal radius with dorsal angulation is evident.
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Figure b: Lateral radiographs of distal right forearm and wrist in two patients. (a) Normal: Fat plane (arrows) overlies pronator quadratus muscle (*). Maximum distance between fat plane and distal radius is 3 mm. (b) Pronator quadratus sign: Pronator quadratus muscle (*) along anterior surface of distal radius is swollen, volarly displacing overlying thin lucent stripe of fat (arrows). Comminuted fracture of distal radius with dorsal angulation is evident.
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EXPLANATION
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The pronator quadratus muscle is a small quadrilateral muscle attached to the anterior aspects of the distal sixths of the radius and ulna. A well-defined fascia covers the muscle, preventing intermuscular communication and creating a distinct forearm space in which fluid can accumulate (4). A thin layer of fat overlies the fascial covering, between the pronator quadratus and flexor digitorum profundus muscles (1,4). This fat plane can be seen as a thin radiolucency on at least 90% of lateral radiographs of normal distal forearms (1,3). The thin radiolucency can either have a slight anterior convexity or be straight and parallel to the radius (1,3). In addition, the distal edge of the fat plane is in contact with the volar aspect of the radius (3).
The pronator quadratus sign is visible when fluid, such as blood, accumulates within this muscle (1–4). If swollen, the pronator quadratus muscle bulges anteriorly and can displace or even efface the lucent fat plane (1–4). The degree of displacement can be quantified by measuring the maximum distance between the fat plane and distal radius (3,5). Though the distance varies widely in normal forearms and increases with patient age, values greater than 7 mm in female and 10 mm in male subjects, regardless of age, are suspicious for an underlying abnormality (5).
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DISCUSSION
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The pronator quadratus sign is an important soft-tissue sign of fractures of the distal radius and ulna (1–7). With fractures, blood can accumulate within the pronator quadratus muscle. MacEwan (1) noted sensitivity and specificity of 98% and 94%, respectively, for the diagnosis of fracture in a review of 600 cases of lower forearm trauma at the level of the pronator quadratus attachment. However, subsequent studies have not confirmed this high degree of accuracy. For example, the pronator quadratus sign was present in only 51% of forearm fractures evident on wrist radiographs in a series of 1453 patients (5). In radiographically occult fractures detected on follow-up magnetic resonance images, the sensitivity and specificity of the pronator quadratus sign were even less, with values of 26% (13 of 50) and 70% (35 of 50), respectively (7). The absence of the sign, therefore, does not exclude a fracture.
There are a number of reasons for false-negative interpretations in the setting of trauma (1,2). The site of the fracture may not be at the same level as the pronator quadratus muscle or may involve the posterior cortex of the radius or ulna. In severe trauma, the fascia covering the muscle may be torn as well. Finally, the radiographs may be of poor quality: They may not be true laterals or may be overexposed. These problems may hinder differentiation of the fat plane from the soft-tissue opacity of the muscle.
Other trauma to the forearm not resulting in fracture (eg, wringer injury or muscle strain) and inflammatory and infectious conditions (eg, cellulitis, septic arthritis, or osteomyelitis) can result in swelling of the pronator quadratus muscle or soft-tissue edema outside the muscle and produce the sign as well (1,2). Nevertheless, in the setting of trauma, the presence of the pronator quadratus sign should prompt a careful investigation for fracture.
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FOOTNOTES
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Author stated no financial relationship to disclose.
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References
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- MacEwan DW. Changes due to trauma in the fat plane overlying the pronator quadratus muscle: a radiologic sign. Radiology 1964;82:879–886.[Medline]
- Zimmers TE. Fat plane radiological signs in wrist and elbow trauma. Am J Emerg Med 1984;2:526–532.[CrossRef][Medline]
- Sasaki Y, Sugioka Y. The pronator quadratus sign: its classification and diagnostic usefulness for injury and inflammation of the wrist. J Hand Surg [Br] 1989;14:80–83.[CrossRef][Medline]
- Sotereanos DG, McCarthy DM, Towers JD, et al. The pronator quadratus: a distinct forearm space? J Hand Surg [Am] 1995;20:496–499.
- Zammit-Maempel I, Bisset RA, Morris J, Forbes WS. The value of soft tissue signs in wrist trauma. Clin Radiol 1988;39:664–668.[CrossRef][Medline]
- Curtis DJ, Downey EF Jr, Brower AC, et al. Importance of soft-tissue evaluation in hand and wrist trauma: statistical evaluation. AJR Am J Roentgenol 1984;142:781–788.[Abstract/Free Full Text]
- Annamalai G, Raby N. Scaphoid and pronator fat stripes are unreliable soft tissue signs in the detection of radiographically occult fractures. Clin Radiol 2003;58:798–800.[CrossRef][Medline]
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