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Original Report |
1
Department of Radiology, Harvard Medical School and Beth Israel Deaconess
Medical Center, 330 Brookline Ave., Boston, MA 02215.
2
Department of Radiology, Temple University Hospital, 3401 N. Broad St,
Philadelphia, PA 19106.
3
Present address: Department of Radiology, Florida Hospital, 601 E. Rawlings
St., Orlando, FL 32806.
Received July 12, 2001;
accepted after revision October 10, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Seattle, April-May 2001.
Abstract
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CONCLUSION. A midsternal stripe thicker than 3 mm should raise one's suspicion of the presence of sternal dehiscence. However, this sign is rarely observed in patients with this complication and does not add incremental value to the finding of sternal wire displacement in establishing the diagnosis of dehiscence.
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The purpose of this study was threefold: first, to compare the frequency of the midsternal stripe sign identified in a large group of patients with proven dehiscence with that identified in a control group; second, to identify a threshold value of stripe thickness that could be used to distinguish between patients with and without dehiscence; and third, to determine whether the midsternal stripe sign adds any incremental value to the radiographic diagnosis of dehiscence using the alternative sign of sternal wire displacement.
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For each patient in the study cohort, a chest radiograph from the date of diagnosis of sternal dehiscence was compared with the initial postoperative radiograph obtained after median sternotomy (range of time interval, 3-69 days; mean time interval, 14 days). Each radiograph was retrospectively reviewed by two radiologists, and findings were determined by consensus. Radiographs were assessed for the presence of a midsternal stripe, which was defined as a midline vertical radiolucency overlying the sternum [8], and sternal wire displacement, which was defined as one or more wires misaligned in relation to others in the vertical row [5] (Fig. 1). To assess for displacement, we drew a vertical line through the center of the sutures on the first postoperative radiograph and then used that line as a reference to draw a vertical line on the subsequent postoperative radiographs. Displacement was considered present when a change in alignment of more than 3 mm involving one or more wires was found between the initial and subsequent postoperative radiographs. To determine the first date that sternal wire displacement was radiographically apparent, we reviewed additional chest radiographs obtained during the time interval between the first postoperative radiograph and the date of diagnosis of dehiscence.
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In images in which a midsternal stripe sign was visualized, we measured the maximal transverse width of the stripe using a ruler and then recorded values to the nearest millimeter. Measurements were obtained by consensus agreement by the two radiologists. We also reviewed serial radiographs between the date of surgery and the date of diagnosis of dehiscence to determine the first date that the stripe was present. If the stripe was present on earlier radiographs, we obtained serial measurements of the stripe to allow us to assess whether there had been progressive widening.
We retrospectively reviewed a series of postoperative radiographs for a group of control patients, composed of consecutive median sternotomy patients with no clinical evidence of sternal dehiscence or mediastinitis. Analysis of serial postoperative radiographs (mean time interval from preoperative radiograph to last postoperative radiograph, 8 days) in this group was performed in the same fashion as for the study cohort. We did not attempt to have blinded reviewers because of obvious differences in the number of radiographs in the two groups (the control group had a shorter hospital stay and fewer radiographs). Statistical analysis was performed using Fisher's exact test [11]. A p value of less than 0.05 was considered statistically significant.
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A midsternal stripe (Figs. 2 and 3) was identified in seven (20%) of 35 patients with sternal dehiscence and in two (4%) of 50 control group patients. This difference was not statistically significant. The mean width of the stripe was 15 mm in diameter (median, 4 mm; range, 2-37 mm) for the study cohort and 2 mm for the control group. The stripe measured less than 3 mm in diameter in both of the control group patients and in three of the study cohort patients. There were no cases in which the midsternal stripe became progressively wider on serial chest radiographs.
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Sternal wire displacement (Fig. 4) was observed in 26 (74%) of 35 patients with sternal dehiscence but was not identified in any of the control group patients (p < 0.0001). Sternal wire displacement was evident in six (86%) of seven patients with sternal dehiscence whose radiographs displayed a midsternal stripe. In the one case in which a stripe was not accompanied by sternal wire displacement, the stripe measured only 2 mm in diameter.
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Sternal wire abnormalities preceded the clinical diagnosis in 16 (46%) of 35 patients. The midsternal stripe sign preceded the clinical diagnosis of dehiscence in four (11%) of 35 patients but was accompanied by sternal wire displacement in all these patients. In three of the four patients, sternal wire abnormalities preceded visualization of the midsternal stripe by 3 days. In these three patients, the stripe was visible 1 day before the clinical diagnosis was made and sternal wire displacement was visible 4 days before the clinical diagnosis. In the remaining patient, sternal wire displacement and the midsternal stripe sign were first radiographically apparent at the same time.
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Although the diagnosis is often evident from the physical examination findings, sternal dehiscence may be clinically occult in some patients [1,2,3,4,5,6,7]. Thus, there has been a longstanding interest in determining whether radiography could be used in identifying this complication [8,9,10]. Two major radiographic signs have been described: the midsternal stripe sign and sternal wire displacement. Our results show that visualization of the midsternal stripe sign does not contribute to the diagnosis of this complication. In contrast, sternal wire displacement is a very valuable radiographic sign of dehiscence and has the potential to play an important role in the diagnosis of this complication.
The midsternal stripe sign was first described by Escovitz et al. [8], who detected a midsternal radiolucency in 12 of 250 postoperative patients during a 2-year period and found that sternal dehiscence developed in four of these patients. A review by the same authors of the cases of 100 consecutive patients who had undergone median sternotomy revealed that sternal dehiscence did not develop in any patient who did not have a midsternal stripe. These authors concluded that the midsternal stripe sign may herald sternal dehiscence. Berkow and Demos [9] refuted this assertion on the basis of their identification of a midsternal radiolucency in 52 (30%) of 173 patients who underwent median sternotomy, only two of whom developed dehiscence. Both studies were limited by a small number of patients with dehiscence (n = 4), which precluded determination of whether a threshold value of stripe thickness might prove valuable for differentiating patients with dehiscence from those without this condition. Our study of 35 patients with sternal dehiscence provides the first data regarding the stripe sign in a large group of patients with this complication.
The relatively low prevalence (4%) of the midsternal stripe sign in the control group in our series is similar to the results reported by Escovitz et al. [8] but is significantly lower than the prevalences reported in other published series [4, 9, 12] (Table 1). The reason for this difference is unclear but may reflect variations in radiographic technique [10] among various institutions or in observer thresholds for the detection of thin stripes.
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In recognition of the fact that a thin stripe has been reported as a normal postoperative finding, we sought to identify a threshold value that could be used to differentiate patients with dehiscence from those without dehiscence. In this study and other published series (Table 1), a stripe thicker than 3 mm was observed only rarely in patients in the control groups. Both exceptions to this finding came from the series by Escovitz et al. [8], in which two patients with stripes of 4 mm and 6 mm, respectively, did not develop dehiscence. Thus, a stripe thicker than 3 mm is usually an abnormal finding and a stripe thicker than 6 mm is definitively so. Applying the strict criterion of 6 mm to our study population, radiographs of less than 10% of patients with dehiscence would have revealed a stripe sign of meaningful size, and all of these patients would also have shown sternal wire displacement.
It has been suggested that progressive widening of a midsternal stripe is a better indication of sternal dehiscence than the mere presence of a stripe sign [2, 3], although the prevalence of this finding in a group of patients with proven dehiscence has not been reported. It makes intuitive sense that progressive widening of the stripe should correlate with increasing degrees of sternal separation. Because we did not observe any patients with progressive widening on serial radiographs, we are unable to directly address this issue. Our data suggest that progressive widening is an uncommon radiographic manifestation of dehiscence, and thus, it is possible that our sample size of 35 patients with dehiscence may not have been sufficient for studying this variation of the midsternal stripe sign. However, we emphasize that our study cohort with proven dehiscence is by far the largest group to date in which this sign has been studied. A contributing factor to the lack of observance of this finding in our patients may be the fact that, in most patients, the stripe sign was first observed radiographically near the time of clinical diagnosis. Had the clinical diagnosis been further delayed, it is possible that a progressively widening stripe may have been visualized in the setting of worsening sternal separation.
In contrast to the stripe sign, sternal wire displacement is a highly valuable sign for detecting dehiscence. The high frequency of sternal wire displacement reported in association with dehiscence fits well with the proposed mechanism of this complication, in which sternal sutures are thought to pull or cut through the sternum rather than breaking [3,4,5, 10]. As the sternum separates, some sutures travel with the right side of the sternum and others migrate to the left side, producing a radiographic appearance that has been referred to as "wandering wires" [5] (Figs. 1, 3, and 4). Because the proposed mechanism of dehiscence involves wires pulling or cutting through the sternum rather than breaking, sternal wire fracture is not thought to play any major role in the development of dehiscence [3,4,5]. In a study that assessed the frequency of various sternal wire abnormalities in patients with dehiscence, Boiselle et al. [5] reported sternal wire fracture in only a small number of patients with sternal dehiscence, all of whom also showed other abnormalities such as sternal wire displacement. Thus, the identification of a sternal wire fracture should prompt one to look carefully for sternal wire displacement.
In summary, the midsternal stripe sign is an infrequent finding among patients with dehiscence and does not add incremental value to the finding of sternal wire displacement in establishing the diagnosis of this complication. When a midline radiolucent stripe wider than 3 mm is identified in the postoperative setting, one should carefully assess the sternal wires for evidence of displacement; displaced wires usually accompany the stripe sign in the setting of dehiscence.
Acknowledgments
We thank Charles White for his thoughtful review of this manuscript,
Michael Larson for assistance with photography, Max Rosen for assistance with
statistical analysis, and Patricia Wyatt for administrative assistance.
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