AS correction changes femur height by?

• A. +2 mm per 5 mm
• B. +5 mm per 5 mm
• C. No change
• D. -2 mm per 5 mm

Answer: (D)

When you correct the AS region, it rebalances the pelvis and lower spine, which changes how the legs sit relative to the pelvis. The rule used in this approach is that for every 5 mm of AS correction, the femur height changes by 2 mm in the opposite direction—that is, it shortens by 2 mm. So the effect is a decrease in femur height of 2 mm per 5 mm of AS correction (a negative change).

For example, applying 10 mm of AS correction would be about a 4 mm decrease in femur height. This explains why the correct answer shows a negative change, and why positive changes or no change don’t fit the established relationship.