The goal of this clinical trial is to determine the appropriate Bispectral Index (BIS) range and propofol effect-site concentration required to maintain adequate anesthesia depth during total intravenous anesthesia (TIVA) combined with midazolam, dexmedetomidine, or esketamine in patients undergoing unilateral knee replacement surgery. The main questions it aims to answer are:

1. What are the appropriate BIS range and propofol effect-site concentration for maintaining adequate anesthesia depth when combined with each of the three medications (midazolam, dexmedetomidine, or esketamine) during TIVA?
2. How these combination regimens affect the quality of recovery and postoperative pain in patients？

Participants will:

1. Receive one of four anesthesia regimens during surgery: propofol alone, propofol combined with midazolam, dexmedetomidine, or esketamine.
2. Undergo a stepwise reduction of propofol effect-site concentration towards the end of surgery.
3. Have their consciousness assessed using the isolated forearm technique, while BIS, EEG signals, and vital signs are continuously monitored throughout the procedure.
4. Have their recovery quality and postoperative pain assessed after surgery.