BP sacrum LOD is:

• A. P-A, M-L
• B. P-A, S-I
• C. P-A, I-S
• D. P-A only

Answer: (B)

BP sacrum means the sacral base is posterior. To correct this listing, the line of drive should move the base forward (P-A) and address the tilt that comes with the posterior base, which is a superior-to-inferior component (S-I). So the adjustment vector is a diagonal that combines P-A with S-I. This pairing directly corrects both the posterior base position and the associated tilt. A drive limited to P-A alone wouldn’t correct the tilt, and directions like medial-lateral or the opposite vertical orientation won’t appropriately address a base posterior listing.