Measurement of mediolateral Cortical Thickness Index (CI). A represents the reference line perpendicular to the mechanical axis of the femur at the tip of the lesser trochanter. B represents a line 10 centimeters below and parallel to A. This line is limited by the periosteum of the femur. C represents the width of the medullary canal at the level of B. Mediolateral CI is then calculated as follows: (B − C)/C