Influence of sensory block duration on rebound pain after outpatient orthopaedic foot surgery under popliteal sciatic nerve block: an observational study

Rebound pain after surgery under peripheral nerve block refers to the transition from adequate analgesia to severe acute postoperative pain following sensory block regression [1, 2]. This can be a significant clinical issue in outpatient surgeries. Non-modifiable risk factors include younger age, female sex and bone surgery, while dexamethasone is a modifiable independent factor that reduces the incidence [1-5]. Identifying other potentially modifiable factors associated with rebound pain could help guide preventive or therapeutic strategies. Evidence regarding the impact of block duration remains limited [1, 6, 7]. In our clinical practice, we perform outpatient open foot surgeries under sciatic popliteal block combined with intravenous dexamethasone and have observed a trend suggesting that longer sensory block duration correlates with reduced postoperative pain. We evaluated the incidence of rebound pain after outpatient orthopaedic open foot surgery using a popliteal sciatic nerve block and intravenous dexamethasone and explored the link between sensory block duration and rebound pain. This single-centre prospective, cohort study was approved by the research ethics committee of Hospital Universitario Doctor Peset, València, Spain and written consent was obtained from all patients. Adult patients (ASA physical status 1–3) scheduled for elective outpatient orthopaedic open foot surgery under popliteal sciatic nerve block were included. Patients with type 1 diabetes or undergoing chronic pain treatment were not studied. Patients received 4–8 mg intravenous dexamethasone before the nerve block, performed by trained anaesthetists who selected the volume and type of local anaesthetic (0.5% levobupivacaine or a 50:50 mepivacaine/levobupivacaine mixture) according to their clinical practice. The postoperative analgesic protocol included paracetamol and dexketoprofen, with tramadol for rescue. The primary outcome measures were incidence of rebound pain (numerical rating scale (NRS) ≥ 7 after sensory block regression) and sensory block duration (time from block establishment to pain onset). Patients were followed up by telephone at 24 h, 48 h and 72 h post-surgery. Statistical analysis included univariate comparison between patients with and without rebound pain, Spearman's ρ to assess correlation and multivariate logistic regression for the primary outcome (incidence of rebound pain). A priori variables considered for inclusion in the model were age and sex, along with any other recorded data associated with rebound pain (cut-off p ≤ 0.2). A sample size of 100 patients was calculated to ensure adequate power for the regression model, considering a maximum of four independent variables and a frequency of 40%. From February 2023 to May 2024, 101 patients were included of whom 37 (37%) developed rebound pain. Mean (SD) sensory block duration for nerve blocks using 0.5% levobupivacaine was significantly longer than those using a mixture (35.5 (8.9) vs. 25.5 (8.3) h, p < 0.01). Comparisons between patients with and without rebound pain are shown in Table 1. Sensory block duration was the only variable significantly associated with rebound pain and correlated moderately with the highest self-reported pain score after block regression (Spearman's ρ = -0.409, p < 0.01). Multivariate logistic regression analysis included sensory block duration, age and analgesic protocol completion (sex was excluded as most patients were female and type of local anaesthetic was excluded due to its association with sensory block duration). Only sensory block duration was independently associated with a reduced incidence of rebound pain (OR 0.929, 95%CI 0.884–0.976, p < 0.01). The overall model fit was good (Hosmer-Lemeshow test p = 0.753), although the explanatory power of the variables was low (Nagelkerke's R2 = 0.179). Despite prophylactic dexamethasone, 36.6% of patients developed rebound pain and sensory block duration was identified as a protective factor, with longer durations independently associated with a reduced incidence of rebound pain. However, the association had a low odds ratio and the regression model explained only a small portion of the variation in rebound pain, which may limit the clinical significance. When the type of local anaesthetic used was introduced in the logistic regression model in place of block duration, no independent association was found with the incidence of rebound pain (0.5% levobupivacaine OR 0.549, 95%CI 0.23–1.30, p = 0.173). This suggests that, whilst anaesthetic influenced block duration, the duration itself was the main predictor of rebound pain. Limitations of our study include the observational design; lack of a control group without dexamethasone; and limited generalisability to other surgeries or blocks. In conclusion, a longer sensory block duration was independently associated with a reduced incidence of rebound pain after ambulatory open foot surgery under sciatic popliteal nerve block with prophylactic dexamethasone. Extending sensory block duration was protective and may represent another modifiable anaesthetic factor, along with dexamethasone, to reduce rebound pain. The study was registered at ClinicalTrials.gov (NCT06309381). The data that support the findings of this study are available from the corresponding author, upon reasonable request. No statistical code is available. No external funding and no competing interests declared.