Only in extreme cases is surgery required for correction. Intraoperative fractures may occur. After surgery, bones with internal fixation devices in situ are at risk of periprosthetic fractures at the end of the implant, an area of relative mechanical stress. Post-operative femoral fractures are graded by the Vancouver classification. Venous thrombosis such as deep vein thrombosis and pulmonary embolism are relatively common following hip replacement surgery. Standard treatment with anticoagulants is for 7–10 days; however treatment for more than 21 days may be superior. Extended-duration anticoagulants (up to 35 days following surgery) may prevent VTE in people undergoing hip replacement surgery. Research from 2013 has on the other hand suggested that anticoagulants in otherwise healthy patients undergoing a so-called fast track protocol with hospital stays under five days, might only be necessary while in the hospital. Some physicians and patients may consider having an ultrasonography for deep vein thrombosis after hip replacement.