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Comments Regarding the Spur Sign

200 Alta Vista Drive, Sedona, AZ 86351-6302 e-mail: jhharris{at}myexcel.com

Editor:

I read the description of the spur sign (1), in the June 2005 issue of Radiology, with considerable interest. The author chose to describe what I believe to be one of the easiest musculoskeletal signs to recognize. However, the injury it represents, the Letournel both-column ("floating acetabulum") fracture, is probably the least well understood acetabular fracture because of its complex pathologic characteristics and the inherent ambiguity in the Letournel classification of acetabular fractures. The author, a resident or fellow, is to be commended for undertaking this very difficult topic. However, the article does contain some serious errors that I wish to call to your attention.

The author is correct in that the spur sign indicates the both-column fracture of Letournel (2). The inherent ambiguity of the Letournel system is that the Letournel fracture types—transverse, T-shaped and its varieties, transverse and posterior fractures, anterior column or wall with hemitransverse—all involve the anterior and posterior columns (ie, both columns). The distinction between this group of fractures and the both-column (floating acetabulum) fracture is that in the latter, fractures occur both anteriorly and posteriorly to the acetabulum, separating the acetabulum from the axial skeleton. This distinctly different fracture pattern has been called the "floating acetabulum" by Tile (3), as well as by Harris et al (4). In the acetabular classifications of both Tile and Harris et al, the floating acetabulum injury is a separate category from the other acetabular fractures named above that involve both columns.

Regarding figure 2, the iliac fracture inferior to the arcuate line and adjacent to the sacroiliac joint does involve the posterior column. However, the lateral aspect of the fracture indicated by the dashed line is not the anterior column of the acetabulum but merely the lateral (or external) wall of the iliac wing. It is impossible to depict a both-column (floating acetabulum) fracture on a single computed tomographic (CT) image.

Figure 3a, as drawn, is inaccurate in that it does not show the transverse fracture of the posterior column that separates the acetabulum from the sacroiliac joint (5).

The author has alluded to the distinction between the floating acetabulum and the other Letournel fracture types that involve both the anterior and posterior columns. However, it is essential that this distinction be unequivocal to the readers. The sentence, "When the two acetabular columns are simultaneously fractured, the connection between the axial skeleton and the acetabular articular surface provided by the sacroiliac joint is severed, and the acetabulum is set free," is imprecise in that there is no mention of the anterior column fracture separating the acetabulum from the axial skeleton interiorly. This must happen for the entire acetabulum to become "floating"—that is, detached both anteriorly and posteriorly.

Finally, the conclusion statement should have read: "In conclusion, the spur sign is indicative of a both-column (floating acetabulum) acetabular fracture," rather than as printed, "fracture to both the anterior and posterior acetabular columns," for the reasons I have mentioned.

	References

TOP References References

 

1. Johnson TS. The spur sign. Radiology 2005;235:1023–1024.[Free Full Text]
2. Letournel E. Acetabular fractures: classification and management. Clin Orthop Relat Res 1980;151:81–106.
3. Tile M. Fractures of the acetabulum: classification. In: Tile M, ed. Fractures of the pelvis and acetabulum. Media, Pa: Williams & Wilkins, 1985; 259–304.
4. Harris JH Jr, Coupe KJ, Lee JS, Trotscher T. Acetabular fractures revisited. II. A new CT-based classification. AJR Am J Roentgenol 2004;182:1367–1375.
5. Vrahas MS, Tile M. Fractures of the acetabulum. In: Bucholz RW, Heckman JD, eds. Rockwood and Green's fractures in adults. 5th ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 2002; 1527.

Response

Mallinckrodt Institute of Radiology, Washington University Medical Center, 510 S Kingshighway Blvd, St. Louis, MO 63110

I thank Dr Harris for his interest in my article (1).

I agree with Dr Harris regarding the difficulty with and ambiguity of the acetabular fracture classification system. He has meticulously evaluated these issues in his recent articles (2,3) and has devised a diagnostic algorithm that is simple and extremely useful. I did not include his findings in my article because the final revision of "The Spur Sign" was accepted prior to the publication of his articles. However, the purpose of my article was to point out the importance of the spur sign as pathognomonic of both-column fractures rather than to discuss the complex anatomy of the floating acetabulum.

Regarding figure 2, I agree with Dr Harris that it is difficult to depict an associated both-column fracture on a single image. But Brandser and Marsh (4) were able to classify many acetabular fractures into three broad categories (transverse type, column type, or wall type, based on the Letournel classification [5]) simply by noting the orientation of the main fracture line on transverse CT images. For example, a fracture with a horizontally oriented fracture line (figure 2 in the article) would be a column-type fracture, which divides the acetabulum in a coronal plane into anterior and posterior fragments. Brandser and Marsh also noted that a vertically oriented fracture line suggests a transverse-type fracture, which splits the acetabulum into top and bottom fragments.

The caption can be clarified to: "Medial displacement (curly arrow) of the anterior fracture fragment exposes a spur fragment, which originates from a shard of bone that remains attached to the sacroiliac joint." The dashed line indicates a horizontally oriented fracture line, which, according to Brandser and Marsh, would suggest a column type of fracture. These two findings on a single transverse CT image support the diagnosis of an associated both-column type (Letournel classification) or floating acetabulum type (Harris classification) acetabular fracture. Figure 3a is a drawing of a normal acetabulum that shows the acetabular columns according to Letournel.

Regarding the conclusion, it should have read "indicative of an associated both-column (Letournel classification) or floating acetabulum (Harris classification) type fracture of the acetabulum."

	References

TOP References References

 

1. Johnson TS. The spur sign. Radiology 2005;235:1023–1024.[Free Full Text]
2. Harris JH Jr, Lee JS, Coupe KJ, Trotscher T. Acetabular fractures revisited. I. Redefinition of the Letournel anterior column. AJR Am J Roentgenol 2004;182:1363–1366.
3. Harris JH Jr, Coupe KJ, Lee JS, Trotscher T. Acetabular fractures revisited. II. A new CT-based classification. AJR Am J Roentgenol 2004;182:1367–1375.
4. Brandser E, Marsh JL. Acetabular fractures: easier classification with a systematic approach. AJR Am J Roentgenol 1998;171:1217–1228.[Free Full Text]
5. Letournel E. Acetabular fractures: classification and management. Clin Orthop Relat Res 1980;151:81–106.

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