Objective To investigate the effect of compound betamethasone adjuvant on break-through pain after unicompartmental knee arthroplasty under sciatic nerve combined with femoral nerve block.Methods A total of 100 patients underwent unicondylar knee arthroplasty,32 males and 68 females,aged 55-75 years,BMI 18.5-35.0 kg/m2,ASA physical status Ⅰ-Ⅲ,were divided into three groups according to random number table method:no adjuvant group(group C,n=34),dexamethasone adjuvant group(group D,n=33)and compound betamethasone adjuvant group(group B,n=33).The patients in the three groups received sciatic nerve block and 0.4％ropivacaine 15 ml before anesthesia in-duction,then femoral nerve block,0.4％ropivacaine 15 ml in group C,0.4％ropivacaine 15 ml in group D(containing dexamethasone 5 mg),and 0.4％ropivacaine 15 ml in group B(containing compound beca-methasone 4 mg).The occurrence of breakthrough pain,the number of effective analgesic pump compres-sions,opioid dosage,and the number of remedial analgesia cases were recorded.The ground movement dis-tance was recorded 0-24 hours,24-48 hours,and 48-72 hours after operation.The sleep quality scores and adverse events were also recorded.Results Compared with group C,the incidence rate of breakthrough pain was lower(P＜0.05),the number of effective analgesia pump compressions,the dosage of opioid,and the sleep quality score on the first night after operation were significantly decreased in group B(P＜0.05).Compared with group D,the incidence rate of breakthrough pain and breakthrough pain score were lower(P＜0.05),the number of effective analgesia pump compressions,the dosage of opioid,and the sleep quality score on the 1 st night after operation were significantly decreased in group B(P＜0.05).There was no significant difference in the ground movement distance of in different time periods and inci-dence of adverse events among the three groups.Conclusion Compound betamethasone adjuvant can reduce the incidence of breakthrough pain after unicompartmental knee arthroplasty under sciatic nerve com-bined with femoral nerve block,provide perfect analgesic effect,reduce postoperative opioid consumption,and improve the sleep quality of patients on the first night after surgery.