Studies have shown that the lifetime risk of developing symptomatic knee osteoarthritis is 40% in men and 47% in women; and the risk can be increased to 60% when the patient's BMI reaches 30 kg/m2. In this population, knee replacement is the most common, effective, and cost-effective method in all treatments with reducing pain, improving joint function and quality of life. Postoperative cognitive dysfunction (POCD) is a common complication in the elderly, and its incidence can reach 25-40% in major non-cardiac surgery. As for POCD can prolong hospital stay, increase mortality and morbidity, and increase social and economic burden, it is especially important for the prevention and treatment of POCD. However, The mechanism of POCD is still unclear. Studies suggest that it may involve inflammation and oxidative stress in the central nervous system. But for the elderly, they are in a state of low inflammatory response. At the same time, surgery and tourniquets also can trigger or aggravate inflammatory response. Once the inflammatory factors released into the blood circulation, they can act on the central nervous system in a variety of ways. As a result,elderly patients are more likely to develop POCD. In addition, some studies have shown that pain can also increase cognitive burden. Aspirin, its pharmacodynamic effects includes anti-inflammatory and analgesic effects which may have a certain effect on the prevention of POCD.

Elderly patients undergoing knee replacement are more likely to develop POCD. The aim of this study is to determine the incidence of short-term POCD and long-term cognitive status in the elderly patients who use Aspirin atleast three months before knee replacement.And then, the study evaluates the effect of Aspirin on the incidence of postoperative delirium. In addition, it also shows the risk factors for POCD such as postoperative delirium, pain, C reactive protein and so on in the elderly patients who undergo knee replacement.

Inclusion Criteria:

• Age more than 60 years old undergoing elective knee replacement with general anesthesia
• ASA score grade I-III
• Sign that informed consent is willing to participate in the study
• Can communicate in verbal
• The time of using aspirin is at least 3 months

Exclusion Criteria:

• Central nervous system disease;
• Mental illness before or now;
• With using Antipsychotic treatment such as sedatives or antidepressants;
• With drug dependence;
• Severe visual, auditory, or motor impairment;
• Acute infection;
• Emergency surgery;
• Serious systemic diseases;
• Preoperative delirium and/ or cognitive dysfunction preoperative;
• History of hospitalization 3 months before surgery.
• Participating in other research

Patients who have major hemorrhoea perioperative, admitted to ICU postoperative and refuse participating the following up are screened out from the research.