Is the canal flare index a reliable means of estimation of canal shape? Measurement of proximal femoral geometry by use of 3D models of the femur.

BACKGROUND

The canal flare index (CFI; the ratio of the diameter of the femoral canal at the isthmus in the anteroposterior (A-P) view to the diameter of the medullary canal 20 mm above the lesser trochanter) is often used as a canal characteristic. Clinically, however, CFI measurements are sometimes untrustworthy because of femoral rotation and, especially, greater anteversion among Japanese patients. Our objectives were to analyze femoral geometry, by use of 3D CAD models, to evaluate the effects of rotational error, and to seek an index less affected by rotation.

METHODS

Computed axial tomography (CAT) scan data from 60 femurs were used. By use of CAD software, 3D femoral models were created. The outside of the femur and the inside canal width 20 mm (P20) and 10 mm proximal (P10), and 10 mm (D10), 20 mm (D20), 30 mm (D30), and 40 mm (D40) distal from the center of the lesser trochanter, and at the isthmus were measured for different angles of femoral rotation. CFI, FFI (femoral flare index; the ratio of the extra-cortical diameters at the same levels as for the CFI), and other canal ratios (P20/D10, P20/D20, P20/D30, and P20/D40) were then calculated and the effect of rotational errors was investigated.

RESULTS

Mean CFI, FFI, P20/D10, P20/D20, P20/D30, and P20/D40 were 4.29, 2.08, 2.05, 2.49, 2.85, and 3.09 in the position without rotational error. CFI was not related to anteversion but had a negative correlation with isthmus canal width (only). In contrast FFI was almost constant at approximately 2.1 for different anteversion and age. With regard to the effect of rotational error, CFI changed by 1.31, FFI by 0.40, P20/D10 by 0.41, P20/D20 by 0.40, P20/D30 by 0.59, and P20/D40 by 0.80 for a variety of rotational angles.

CONCLUSIONS

Outside femoral shape was little different for any person; as a result, FFI was almost constant. In contrast, CFI was revealed to be affected by canal width at the isthmus only. With regard to the effect of rotation, P20/D20 was much less affected by rotation than CFI; it could, therefore, be an appropriate index for expressing proximal canal shape.