Objective To evaluate the effect of different approaches to thoracolumbar interfascial plane (TLIP) block on the analgesic efficacy after lumbar spinal fusion.Methods Seventy-five patients of both sexes,of American Society of Anesthesiologists physical status Ⅰ or Ⅱ,aged 40-70 yr,with body mass index ≤40 kg/m2,undergoing posterior median approach to lumbar spinal fusion,were divided into 3 groups (n =25 each) using a random number table method:patient-controlled intravenous analgesia (PCIA) group (group P),medial approach to TLIP combined with PCIA group (group MP) and lateral approach to TLIP combined with PCIA group (group LP).TLIP block was performed by advancing the block needle in a lateral to medial direction and injecting 0.5％ ropivacaine 20 ml locally between the bilateral multifidus and longissimus muscles in group MP.TLIP block was performed by advancing the block needle in a medial to lateral direction and injecting 0.5％ ropivacaine 20 ml locally between the bilateral longissimus and iliocostalis muscles in group LP.The identification rate and distinction score between multifidus and iliocostalis muscles obtained from ultrasonic images were recorded before block in MP and LP groups.All the patients received PCIA until 48 h after surgery.PCIA solution contained sufentanil 100 μg and tropisetron 10 mg diluted to 100 ml with 0.9％ normal saline.The PCA pump was set up with a 2 ml bolus dose,a 15-min lockout interval and background infusion at a rate of 1 ml/h.When visual analogue scale score at rest ＞4 after surgery,parecoxib 40 mg was intravenously injected for rescue analgesia.The development of adverse reactions such as nausea and vomiting was recorded within 48 h after surgery.Results The identification rate and distinction score of the iliocostalis was significantly higher than that of the multifidus (P＜0.05).Compared with group P,the intraoperative consumption of propofol and remifentanil was significantly decreased,the pressing times of PCA,amount of drugs consumed in PCA and consumption of parecoxib were decreased within 48 h after surgery (P＜0.05),and no significant change was found in the incidence of nausea or vomiting in MP and LP groups (P＞0.05).There was no significant difference in the parameters mentioned above between group MP and group LP (P＞0.05).Conclusion Ultrasound-guided medial and lateral approaches to TLIP block is helpful in achieving low-dose opioid anesthesia mode and more helpful in enhancing the efficacy of PCIA when used for lumbar spinal fusion.