Diagnostic Imaging for Stress Fracture

Abstract

Introduction: Plain radiographs, triple phase technetium99 polyphosphonate bone scan, magnetic resonance imaging (MRI) and computed tomography (CT) have all been used to evaluate bone stress injury. It remains unclear which method is most useful. Multiple method diagnostic imaging is not uncommon and, in addition to the expense, likely exposes patients to unnecessary radiation, to minimal clinical advantage. Determination of the imaging modality of greatest value, and establishment of a reliable grading system, will assist prognostication and inform management decisions. Purpose: A number of imaging grading systems have been reported for tibial stress injury. In spite of the early existence of a grading system based on radiographic signs in cortical and trabecular bone (Savoca, 1971), plain x rays are considered to be of limited use in stress fracture diagnosis, having low sensitivity in the acute phase. Bone scintigraphy has traditionally been considered the gold standard for bone stress injury imaging. It is highly sensitive to the associated increased bone metabolism, but lacks specificity. Furthermore, bone scans can be negative when patients are symptomatic, and positive when patients are asymptomatic, often remaining positive long after a stress fracture ceases to be painful. A grading system for bone scans has been in existence for some considerable time (Zwas et al., 1987) and is frequently referred to in the literature. The extent to which the system is operationalized in clinical practice, however, is unknown. More recently MRI has been recognized as a sensitive and specific imaging tool for the assessment of bone stress injury. The first MRI grading system for tibial stress injuries was published by Fredericson and colleagues in 1995 and forms the basis of the grading categories utilized in the current report. Other MRI stress injury grading systems were subsequently published (Arendt and Griffiths, 1997; Yao et al., 1998; Kiuru et al., 2001; Ishibashi et al., 2002), with many similarities but some perplexing differences. In 2005, Gaeta and associates published a tibial stress injury grading system for CT. A lack of accord between grading systems is the basis for considerable confusion with respect to the most appropriate and practical method to evaluate tibial stress injury severity from diagnostic images. Our study objective was to identify the imaging modality that best predicts tibial stress injury clinical severity (pain) and time to recovery. In light of disappointing results from an original analysis of four imaging modalities using existing grading systems, we reassessed our MRI data using a novel system of grading more specific injury signs (periosteal and marrow edema, cortical abnormality, muscle edema) on T1 weighted and T2 weighted images, and analyzing the ability of those signs to predict clinical severity and time to recovery. Implementation: Fifty subjects with acute tibial stress injuries were examined on the same or subsequent days via standard AP and lateral radiographs, bone scans, MRI and CT, per standard procedures. A repeat MRI was obtained within 3 days of healing (asymptomatic with hopping). In the first analysis, all images were evaluated retrospectively by four independent graders. Images were blinded for patient identity and randomly ordered in batches (according to modality) for grading. A grading scale of 0 (indicating a negative finding) to 4 (presence of a fracture line) was developed and adapted where appropriate from Zwas and colleagues (1987), Fredericson and others (1995), and Gaeta and associates (2005). The relationships of image grade to clinical severity and time to healing were examined via correlation analyses. The predictive abilities of each imaging modality were examined via multiple regression analyses. Inter-observer reliability was evaluated via correlation, repeated measures ANOVA and Cronbach's alpha coefficient. In the second analysis, gra