Introduction: The incidence of rotational malalignment after femoral nailing has been reported to be at least 20%. If the deformity is recognised early, it can be corrected by changing the distal locking screw and rotating the bone prior to fracture union. It is common practice to use the same distal locking screw of the nail if this surgery is performed, however, there is a risk of the new drill hole "cutting out" into the old screw hole. The degree of rotational deformity that needs to be corrected to use the same distal locking hole without cut out of the screw has not been defined.
Method: Ten femora, five from cadavera and five synthetic ("Synbone"), were stabilised in a vice and then fitted with one distal transverse screw. The screw was then removed and a second distal transverse screw was inserted at the same level after variable amounts of rotation. The bone bridge between the drill holes was then measured and any cut out was noted.
Results: Both of the femora cut out when rotated 10 degrees, and one when rotated 15 degrees. The size of the bone bridge between drill holes in femora rotated by 20 degrees was 3 mm. This bone bridge was increased to 4mm when the femora were rotated by 25 degrees, and 8 and 9 mm when rotated by 30 degrees.
Conclusion: The amount of rotational deformity that needs to be corrected in order to use the same distal locking hole in a femoral nail is significant. In our study, this equates to a correction of at least 25 degrees, but this is not a definitive value in practice. Particular attention must be paid to the location and size of the distal locking screw when correcting malrotation after femoral nailing, to ensure an adequate bone bridge between the two holes.