Extending the Window: A Systematic Review of Pharmacological Adjuncts for Single-Shot Adductor Canal Blocks in Total Knee Arthroplasty

Background/Objectives: Adductor canal blocks (ACBs) are widely used for postoperative analgesia following total knee arthroplasty (TKA). However, the duration of analgesia with a single-shot ACB is limited. Pharmacological adjuncts may enhance and prolong the duration of single-injection blocks, but their efficacy in this setting remains unclear. The aim of this study was to assess the analgesic effectiveness of adjuncts added to local anaesthetic for single-shot ACBs following TKA. Methods: An extensive systematic literature review was performed on Medline, Embase, CINAHL, Cochrane CENTRAL, and Web of Science. Adult patients undergoing primary TKA who received a single-shot ACB with an adjunct added to LA were eligible, with a single-shot ACB with LA alone as the comparator. The primary outcome was postoperative analgesic efficacy, assessed by pain scores (VAS/NRS), time to first rescue analgesia, total postoperative opioid consumption, or sensory block duration. Secondary outcomes included functional recovery measures and adverse events. Risk of bias was evaluated using the Cochrane RoB 2 tool, and the certainty of evidence for each adjunct–outcome combination was assessed using GRADE. Due to clinical heterogeneity, a meta-analysis was not feasible and findings were synthesised narratively. Results: Nine randomised controlled trials (RCTs) assessing the analgesic efficacy of adjuncts added to local anaesthetic in ACB following TKA were included in this review. Adjuncts included dexmedetomidine, dexamethasone, butorphanol, buprenorphine, and magnesium sulphate. Three RCTs demonstrated improvements in early postoperative analgesia with dexmedetomidine at doses of 0.5 µg/kg, while lower doses (0.25 µg/kg) did not. Dexamethasone also decreased early postoperative pain across two RCTs and showed the most evidence for significant prolongation in sensory blockade, with a dose of 4 mg needed to produce significant effects. Butorphanol and buprenorphine demonstrated a significant reduction in postoperative opioid consumption and improved pain, but evidence was limited to single trials. Findings for magnesium were inconsistent. No adjunct was associated with any serious side effect or adverse event. Conclusions: Pharmacological adjuncts added to single-shot ACBs following TKA generally improved early postoperative pain and reduced opioid consumption compared with LA alone, with the most consistent benefits observed for dexmedetomidine (0.5 µg/kg) and dexamethasone (≥4 mg). However, these effects appeared dose-dependent, were largely confined to the first 24 h after surgery, and were supported by moderate-to-low certainty evidence with limited functional outcome data. Further high-quality, adequately powered RCTs with standardised functional endpoints and longer follow-up are required to define optimal dosing, clarify safety, and determine whether improved analgesia translates into meaningful gains in rehabilitation and recovery.