Delirium is an acute confusional state, it is a sign of acute encephalopathy, also known as acute brain failure, acute brain dysfunction or mental state changes.Postoperative delirium can cause post-traumatic stress disorder, affects patients' quality of life, extend the length of hospital stay, increased hospitalization cost, and is closely relative to short-term and long-term mortality after surgery. Exogenous 40 Hz stimulation can improve cognitive functioning. Therefore, the aim of this study was to investigate the effect of exogenous 40Hz stimulation on the incidence of postoperative delirium in elderly patients undergoing hip and knee replacement.

Inclusion Criteria:

1.65-85 years old, gender not limited 2.General anesthesia unilateral total hip replacement surgery or unilateral total knee replacement 3.ASA classification I-III 4.No drugs for mental and central nervous system diseases were used before surgery 5.Preoperative MMSE score illiterate > 17 points, primary > 20 points, junior high school and above > 24 points 6.Obtaining informed consent

Exclusion Criteria:

1. Severe liver and kidney dysfunction
2. here were mental and nervous system diseases before operation
3. Have a history of alcoholism, long-term use of sedative or analgesic
4. A history of a delirium and other reasons are reluctant to cooperate
5. Massive blood loss and blood transfusion during operation, prolonged operation time