The effect of C-arm malrotation on iliosacral screw placement

OBJECTIVES: We sought to determine whether inaccurate C-arm positioning could create images that lead to inaccurate interpretation of iliosacral screw positions. DESIGN: Cadaveric dissection study. SETTING: The learning institute of Zimmer Inc. in Warsaw, Indiana. METHODS: A laboratory investigation was performed using 3 nonpreserved cadaveric specimens. Several anatomic landmarks of the pelvis were outlined using radiographic markers and guide wires placed in several positions within the pelvis in each specimen. Using C-arm images we inserted the following: a "good" wire (GW), an out-the-front (OTF) wire, an out-the-back (OTB) wire placed into the sacral canal, an "in-out-in" (IOI) wire, and a wire in the S1 foramen (S1). The C-arm was then canted in 2-degree increments toward the head and then toward the feet starting from the optimum position. RESULTS: Properly positioned wires always appear to be contained within bone regardless of the amount of malrotation of the C-arm from the optimum inlet and outlet views. CONCLUSIONS: Improper malrotated fluoroscopic inlet and outlet views of the pelvis will distort the anatomic landmarks of the pelvis before improperly placed guide wires appear to be correctly placed. Properly placed guide wires will always appear correctly positioned regardless of the malrotation of the C-arm. "Malrotated" views, however, can be used to rule out certain incorrect screw positions.