The open intervertebral disk space visible on AP sacroiliac joints radiographs is located between which vertebrae?

The open intervertebral disk space visible on AP sacroiliac joints radiographs is located between which vertebrae?

• A. L4-L5
• B. L6-S1
• C. L5-S1
• D. L3-L4

Answer: (C)

On AP sacroiliac joint radiographs, the area of interest sits at the lumbosacral junction where the last lumbar vertebra meets the first sacral segment. The disk space between these two—L5 and S1—is the open intervertebral space that is typically visible on this projection. The reason this space is the one you see clearly is that the radiograph is oriented to capture the sacrum and SI joints with minimal angulation, so the L5-S1 level lies right at the exposure field and remains unfused or uncollapsed in many patients, whereas higher spaces may be obscured by the pelvic bones. Also, there isn’t a vertebra beyond L5 in standard anatomy, so L6-S1 isn’t a normal junction. Therefore, the open disk space seen on this view is between L5 and S1.

On AP sacroiliac joint radiographs, the area of interest sits at the lumbosacral junction where the last lumbar vertebra meets the first sacral segment. The disk space between these two—L5 and S1—is the open intervertebral space that is typically visible on this projection. The reason this space is the one you see clearly is that the radiograph is oriented to capture the sacrum and SI joints with minimal angulation, so the L5-S1 level lies right at the exposure field and remains unfused or uncollapsed in many patients, whereas higher spaces may be obscured by the pelvic bones. Also, there isn’t a vertebra beyond L5 in standard anatomy, so L6-S1 isn’t a normal junction. Therefore, the open disk space seen on this view is between L5 and S1.