To evaluate the safety and efficacy of twice daily dosing and once daily dosing of YM150 in subjects undergoing primary elective knee replacement surgery

Inclusion Criteria:

• Subject is scheduled for elective primary knee arthroplasty
• Written Informed consent obtained

Exclusion Criteria:

• Subject has documented history of previous VTE
• Subject is considered to be at increased risk of VTE
• Subject has active bleeding or any condition associated with increased risk of bleeding
• Subject is planning to have surgery on the contralateral knee at the same time or within 6 weeks after enrollment into the study