Introduction: We have calculated the amount of antero-posterior diameter expansion by tibial intramedullary nails with distal anterior bend which were designed to prevent proximal posterior cortical fracture by the distal tip of a nail on insertion in the surgical treatment of tibial fracture.

Materials and Methods: Russell-Taylor® and AIM™ tibial nails were compared in respect to the amount of anteroposterior diameter expansion by the distal anterior bend of these nails. AIM™ tibial nails have shorter length but larger angle of distal anterior bend than Russell-Taylor® tibial nails. As Fig.1. shows, if we suppose that the length and angle of distal anterior bend of nail be L and _, respectively and the length of distal tapered portion be T, the amount of anteroposterior diameter expansion (E) by the distal anterior bend portion of the nail is [(L-T) _ sin_]. So, intramedullary nail with distal anterior bend passes down the medullary canal with an actual diameter (AD) of the sum of given diameter of the nail (D) and [(L-T) _ sin_] on anteroposterior plane.

Results: The amount of anteroposterior diameter expansion of Russell-Taylor® and AIM™ tibial nail was 2.81 mm and 3.26 mm more than the given nail diameter because the length and angle of distal anterior bend of Russell-Taylor® and AIM™ tibial nails were 64 mm, 3° and 47.5 mm, 5°, respectively and about 10 mm of distal tip of both nails are tapered to facilitate passage in the medullary canal.

Conclusion: On insertion of tibial nails with distal anterior bend, the anteroposterior diameter expansion effect by these nails should be carefully considered to prevent fracture of the isthmus. We think that the nail about 2 to 3 mm smaller than the final reamer used in diameter had better be used when you try to insert an intramedullary nail bent anteriorly at distal portion such as Russell-Taylor® and AIM™ tibial nails.