Lateral trochlear index. To calculate the lateral femoral condyle index, a perpendicular is drawn to the line denoting the Ca. The distance between this perpendicular and point 1 is divided by the distance between the perpendicular and point 2 and is expressed as a percentage. C1 indicates proximal circle in the femoral shaft; C2, distal circle in the femoral shaft; Ca, central axis of femur; 1, superior most aspect of anterior cartilage of the lateral condyle; 2, superior most aspect of posterior cartilage of the lateral condyle.