If PI increases by 4 mm, what is the expected direction of femur height change?

• A. Increase
• B. Decrease
• C. No change
• D. Uncertain

Answer: (A)

Increasing pelvic incidence prompts the pelvis to tilt more anteriorly, which tends to deepen the backward curve of the lower spine (more lumbar lordosis) to keep the head balanced over the pelvis. To accommodate this sagittal shift, the hip joint often rotates toward extension, making the femur project more vertically in the upright posture. So a rise in PI by a small amount is associated with an upward change in the measured femur height, reflecting a positional adjustment rather than a change in bone length.