Previous study demonstrated that neuroinflammation induced by surgery is the main cause of the postoperative cognitive dysfunction. As an agonist of NMDA receptor, ketamine is proved to be an anti-inflammation agent. In present study investigators hypothesized that low dose ketamine would prevent the cognition decline after orthopedic surgery in aged patients.

Inclusion Criteria:

• Elder than 60
• Easily communicated with Chinese
• Selected to kneel or hip replacement
• Agree to take part into this clinical trail

Exclusion Criteria:

• Existing cerebral disease， or have a history of neurological and psychiatric disease including AD, stroke and psychosis
• Existing cognitive impairment as evidenced by Mini-Mental State Examination scores below 24
• Several audition or vision disorder
• Preoperative systolic Blood pressure over 190mmHg or diastolic pressure >100mmHg
• Existing a history of hyperthyroidism
• Patient with glaucoma
• Unwillingness to comply with the protocol or procedures