Abstract
Anterograde intramedullary nail, which is currently the main stream, requires cutting the rotator cuff of the supraspinous muscle, and the application of a locking plate requires a large skin incision, being invasive. To minimize the invasiveness, retrograde intramedullary nail was developed by our collaborators. In this study, to evaluate the fixation stiffness and displacement of retrograde intramedullary nail, mechanical tests were performed for comparison with the conventional anterograde intramedullary nail and locking plate. The test implants were retro- and anterograde intramedullary nails and a locking plate. Two-part fracture was prepared by applying AO classification Type A3 osteotomy to a resin-made humerus model. As mechanical tests, valgus, torsion, and compression tests were performed. The displacements induced by valgus and compression decreased in the models with the locking plate, retrograde intramedullary nail, and anterograde intramedullary nail in this order, clarifying the superior fixation stability of the anterograde intramedullary nail. When the torques of internal and external rotations was averaged, the highest torque was detected in fixation with the locking plate. Retrograde intramedullary nail was equivalent to the other internal fixations in fixation against torsion of internal rotation, and superior to the locking plate in fixation against valgus and compression, suggesting that there is no problem with the fixation stability. Retrograde intramedullary nail is useful as an internal fixation method for the treatment of proximal humeral fracture with regard to reducing invasiveness.
