What is the degree of rotation for an AP oblique projection of the SI joints with the patient in an RPO position?

• A. 10-15 degrees
• B. 20-25 degrees
• C. 25-30 degrees
• D. 35-40 degrees

Answer: (C)

Opening the sacroiliac joints for an AP oblique view requires enough obliquity to bring the joint space into profile while keeping the surrounding bony margins clear. About 25–30 degrees of rotation accomplishes this balance. In an RPO position, this amount of tilt places the side of interest’s sacroiliac joint in a favorable orientation, reducing overlap from the ilium and sacrum and making the joint space and articular surfaces more visible. If you rotate less than 25 degrees, the joint becomes obscured by adjacent bones; rotate more than 30 degrees, and you risk distorting the joint anatomy and losing a true AP relationship. Hence, 25–30 degrees is the optimal range.