This is an international, multi-center, randomized, double blind, placebo controlled study of AMG 785 in skeletally mature adults with a fresh unilateral tibial diaphyseal fracture status post a definitive fracture fixation with an intramedullary (IM) nail. The primary hypothesis for this study is that AMG 785 compared with placebo is effective in reducing radiographic healing time of fresh tibial diaphyseal fractures.