How is periosteal reactive bone formation oriented?

• A. Perpendicular to the cortex, thickest at the cortical surface and thinnest at the leading edge
• B. Parallel to the cortex, with uniform thickness
• C. On the inner medullary surface
• D. Absent in slow-growing lesions

Answer: (A)

Periosteal reactive bone formation reflects how fast the bone is responding to a stimulus. When the process is rapid, the periosteum is lifted quickly and new bone tends to be laid down as spicules that extend perpendicular to the cortical surface. These perpendicular rays radiate away from the cortex, and they’re thickest closest to where they begin at the cortical surface, thinning toward the leading edge as the reaction travels outward. This pattern contrasts with slower processes, which often produce layers parallel to the cortex (onion-skin appearance) rather than radiating perpendicular spicules.