The cortical step sign as a tool for assessing and correcting rotational deformity in femoral shaft fractures.

OBJECTIVES: Rotational malalignment during femoral nailing is common. The difference in cortical width of the proximal and distal fracture fragments, the cortical step sign, is a commonly used yet poorly studied method of evaluating rotational alignment. This study aims to critically analyze the cortical step sign in cadaveric specimens using radiographic and direct measurements.
METHODS: One-centimeter segments from 20 cadaveric femora were harvested from the proximal, middle, and distal diaphyses. The medial and lateral cortical widths were measured in neutral and at 10 degrees , 20 degrees , and 30 degrees of internal rotation and external rotation directly from the gross specimens and indirectly using radiographs and cross-sectional imaging.
RESULTS: Anatomic, radiographic, and cross-sectional imaging measurements all demonstrated that cortical width changes with femoral rotation. Rotation (both internal rotation and external rotation) of the proximal and middle segments led to a decrease in medial cortical width and lateral cortical width in 70% to 100% of samples (up to 2.2 mm, or 20% of cortical width) indicating that the cortices are thickest directly medially and laterally in neutral rotation. In the distal femur, however, internal rotation and external rotation led to an increase in medial cortical width and lateral cortical width in 80% to 95% of cases (up to 1.75 mm), except in the case of the medial cortical width in internal rotation, which decreased in 80% of the specimens (up to 1.3 mm).
CONCLUSIONS: The cortical step sign, or incongruity of cortical widths on either side of a femur fracture, is indicative of rotational malreduction. Whether such malreduction is the result of internal rotation or external rotation, however, cannot be easily determined from this radiographic sign.