This is a multi-center, randomized, double-blind, placebo/active-controlled study. 405 subjects undergoing orthopedic surgery under general anesthesia are planned to be enrolled and randomized into the HSK21542 group, morphine group, and placebo group.
18 years old ≤ age ≤74 years old, regardless of gender;
18 kg/m2≤BMI≤32 kg/m2，meet the weight range specified in the protocol；
American Society of Anesthesiologists (ASA) grade ⅰ-ⅲ;
Accept hip replacement under general anesthesia;
Patients with allergy to opioids or any component of the trial drug；
Patients with a history of severe cardiovascular and cerebrovascular diseases, ,and psychiatric disorders;
Last use of opioid or non-opioid analgesics less than 5 half-lives or the duration of drug efficacy prior to randomization; use of drugs with unclear half-lives that may affect analgesic efficacy within 7 days prior to randomization, or last use of drugs affecting analgesic efficacy less than 5 half-lives prior to randomization, as well as Chinese herbal medicines or proprietary Chinese medicines deemed by the investigator as potentially impacting the evaluation of efficacy and safety；
Continuous use of opioid analgesics for more than 7 days for any reason within 30 days prior to screening;