This is a prospective, randomized study from pre-surgery to 6 weeks post-surgery designed to detect changes in coagulability following elective total knee and hip arthroplasty. Patients will be randomized to one of the two standard prophylaxis treatments used at Sinai Hospital:

1. Arixtra (2.5 mg SQ/QD) subcutaneous injection daily for 2 weeks followed by aspirin 325 mg for 5 weeks, or
2. Pneumatic compression stockings (MCS, Active Care, medical compression systems Inc., Israel) for 2 weeks with concomitant Aspirin 325 mg daily for 5 weeks.

Inclusion Criteria:

1. Men or women age ≥ 18 years old
2. Total knee or total hip arthroplasty planned in the 60 days following study enrollment

Exclusion Criteria:

1. Pre-existing anti-coagulant therapy (Heparin, Lovenox, Arixtra, Warfarin, Xarelto, and all other non-mentioned anticoagulants) on admission
2. History of bleeding diathesis
3. Known active malignancy
4. Prothrombin time >1.5 x control
5. Platelet count < 100,000/mm3, hematocrit < 30%, creatinine clearance <30 mL/min
6. Immunosuppression, e.g., human immunodeficiency virus (HIV) infection, Chronic hepatitis B virus (HBV) or hepatitis C virus (HCV) infection, Bacterial endocarditis
7. Major surgery within past 3 months of enrollment
8. Patients requiring bilateral arthroplasty
9. Body weight <50 kg (venous thromboembolism prophylaxis only)
10. Unable to return to site for follow-up visit