Diagnostic Imaging

Plain radiographs are useful in the initial evaluation of an athlete in whom a stress fracture is suspected. In more advanced or established cases, cortical or medullary fracture lines, regional osteopenia, sclerosis, and callus formation may be noted. Unfortunately, radiographs are initially negative in up to 70% of stress fractures and may not show evidence of injury for 2 to 4 weeks after symptoms begin [28]. In two of the largest series of medial malleolar stress fractures, Shelbourne and colleagues [15] and Orava and colleagues [20] have noted only two of six and three of eight patients, respectively, who had positive initial radiographs. Other authors have reported positive initial radiographs in less than half of their patients who had a suspected medial malleolus stress fracture [22,27]. A characteristic cystic lesion associated with medial malleolar stress fractures caused by microfracture and bone resorption has been described [21,27].

Fig. 4. STIR MR image of the distal right fibula. Note abnormally increased intramedullary signal (arrows). (From Slawson SH, Arendt E, Engebretsen L, et al. Fibular stress fracture in a 20-year-old woman. Orthopedics 1994;17(4):375-9; with permission.)

Because of the high incidence of false-negative radiographs early in the course of stress fractures, additional diagnostic imaging is often indicted. Radionuclide bone scanning has traditionally been the test of choice in this situation but is being supplanted by MRI. An increased uptake observed on a bone scan correlates with increased bone activity caused by fatigue failure and confirms the diagnosis of stress fracture [21]. Despite being sensitive, bone scanning is not specific and can yield false-positive rates between 13% and 24% [28]. Additionally, it can be difficult to localize the precise anatomic location of injury.

MRI has numerous practical advantages over radionuclide scanning. MRI provides precise anatomic resolution and can differentiate between a stress reaction and stress fracture; and it is noninvasive, multiplanar, and uses no radiation. MRI also is more sensitive and specific, provides greater information,

Fig. 5. T1-weighted MRI of a medial malleolus stress fracture. (Copyright 1995 by the American Orthopaedic Foot and Ankle Society (AOFAS), originally published in Foot and Ankle International, January 1995, Vol. 16(1) page 50 and reproduced here with permission.)

and is able to detect preradiographic bony changes. It has the disadvantages of greater cost and lacks the fine specialized resolution of CT scanning (Figs. 4 and 5) [28,29].

MRI has been used to detect both medial malleolar [27] and distal fibular [29] stress fractures. Okada and colleagues [27] were the first to report the MRI findings of medial malleolar stress fracture as a vertical, linear decreased T1-weighted signal at the plafond-malleolus junction. The early changes detectable on MRI show on short tau inversion recovery (STIR) sequences as increased signal representing edema and hemorrhage. Later, T2-weighted images demonstrate a linear area of decreased signal within a region of increased bone marrow signal. The corresponding T1-weighted images show a low signal line surrounded by an area of intermediate signal [28-30].

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