The Dorr classification recognizes 3 types of bone in the proximal femur. Ratios of bone dimensions in the femoral diaphysis can be helpful in preoperative planning of a hip arthroplasty.
Study: AP radiograph
Parameters:
(1) external diameter of femur in cm at a point 10 cm below the mid-point of the lesser trochanter
(2) internal diameter of femur (diaphyseal diameter) in cm at 10 cm below the mid-point of the lesser trochanter
(2) diameter of the intramedullary canal at the calcar (the calcar femorale is located below the lesser trochanter)
cortical thickness index =
= ((external diameter at 10 cm) - (internal diameter at 10 cm)) / (external diameter at 10 cm)
where:
• The cortical thickness index involves 2 times the cortical thickness.
canal-to-calcar ratio =
= (internal diameter at 10 cm) / (internal diameter at 3 cm)
Interpretation:
• The cortical thickness index in Dorr Type A femur (mean 0.58) is greater than for Type B (mean 0.50) which is greater than for Type C (mean 0.42).
• The canal-to-calcar ratio is greater in Type C (mean 0.64) than type A (mean 0.57), with type B overlapping with the other 2 types.
• Both ratios are helpful in preoperative planning as improperly selected stems can fracture the femur during surgery.
