Popliteal plexus block: does opioid sparing translate into better recovery?

Effective postoperative pain management after total knee arthroplasty (TKA) involves more than reducing early pain scores. Modern perioperative analgesia should provide adequate pain relief while minimizing opioid-related adverse effects, preserving motor function for early physiotherapy, and supporting functional recovery. These goals do not always coincide [1]. As opioid-sparing multimodal analgesia and enhanced recovery after surgery (ERAS) protocols have become widely adopted, early ambulation and efficient rehabilitation have become important clinical goals [1]. Therefore, regional analgesic techniques should be evaluated not only for analgesic efficacy but also for their effects on motor function and recovery.

The evolution of regional anesthesia for TKA reflects this trade-off. Femoral nerve block, once the reference standard for postoperative analgesia, provided effective anteromedial knee analgesia but substantially impaired quadriceps function, increasing perioperative fall risk and delaying early mobilization [1]. These limitations prompted a shift toward the adductor canal block and femoral triangle block, which preserve quadriceps function while providing analgesia to the medial and anterior knee compartments [1]. However, both techniques share an anatomical limitation. The posterior knee capsule is innervated primarily by articular branches outside the saphenous nerve territory and may remain inadequately covered. Residual posterior knee pain can limit early rehabilitation.

The popliteal plexus block (PPB) was developed to address this anatomical gap. The popliteal plexus is formed mainly by genicular branches from the tibial nerve and the posterior branch of the obturator nerve, contributing to sensory innervation of the intra-articular and posterior knee region [2,3]. The PPB targets the plexus rather than the tibial nerve trunk and is intended to provide posterior knee analgesia with minimal motor impairment. In healthy volunteers, injectate volumes of 10 to 30 mL did not impair ankle plantar flexion or dorsiflexion strength or produce clinically relevant changes in motor nerve conduction [4]. This approach differs from both tibial nerve block and infiltration between the popliteal artery and capsule of the posterior knee (IPACK). The tibial nerve block provides posterior capsular coverage but may cause plantar flexor paresis. The PPB and IPACK differ primarily in their intended anatomical targets and injection planes; however, the effects of both techniques depend on the extent and pattern of injectate spread within the relevant anatomical planes. Whether the motor-sparing profile of the PPB improves functional recovery remains uncertain.

In this issue of the Korean Journal of Pain, Lee and colleagues report a systematic review and meta-analysis of PPB efficacy in adults undergoing lower extremity surgery [5]. Eight randomized controlled trials (RCTs) enrolling 684 participants were included: six in TKA and two in arthroscopic anterior cruciate ligament (ACL) reconstruction. Five trials administering the PPB as an adjunct to an established regional block (adductor canal block, continuous adductor canal block, femoral triangle block, or continuous femoral nerve block) were pooled quantitatively. Three trials provided head-to-head comparisons and were reported descriptively. In the IPACK and tibial nerve block trials, both groups received the same background block, the femoral triangle block and adductor canal block, respectively. The fascia iliaca block trial compared the two regional techniques without an additional background block under spinal anesthesia [6–8]. As an adjunctive strategy, the PPB significantly reduced 24-hour opioid consumption (standardized mean difference [SMD] –1.32, 95% CI –2.29 to –0.34, P = 0.008, five studies), pain scores at 12 hours (mean difference [MD] –1.32, 95% CI –2.25 to –0.39, P = 0.005, four studies) and 24 hours (MD –0.52, 95% CI –1.02 to –0.02, P = 0.042, four studies), and the incidence of postoperative nausea and vomiting (PONV: RR 0.21, 95% CI 0.08 to 0.55, P = 0.002, three studies) compared with the background block alone [5].

These results require careful interpretation. For both 24-hour opioid consumption and 12-hour pain intensity, the I² statistic approached 95%, consistent with substantial clinical diversity: differing surgical procedures, anesthetic modalities, background regional blocks, and non-standardized local anesthetic agents and volumes [5]. The two ACL reconstruction trials showed the largest standardized opioid-sparing effects (Hedges’ g, –2.71 and –2.33), whereas the three TKA trials ranged from –0.73 to –0.19 [5]. This difference may partly explain the observed heterogeneity, although no formal subgroup comparison was performed. GRADE certainty was rated as low for 24-hour opioid consumption and 12-hour pain intensity, incorporating both this inconsistency and concerns regarding blinding of outcome assessors in one included trial, and as moderate for 24-hour pain intensity [5]. An SMD of –1.32, while numerically large, is a dimensionless measure and cannot be directly translated into morphine milligram equivalents (MME). Future meta-analyses should report absolute MME reductions and opioid-free recovery rates. Sensitivity analyses confirmed a consistent opioid-sparing direction across all leave-one-out permutations (SMD range: –1.60 to –0.96), but exclusion of a single trial rendered the estimate borderline non-significant (P = 0.052) [5]. The pooled MD for 24-hour pain (approximately 0.52 points on a numeric rating scale) narrowly reached significance, with the estimate losing significance on removal of three of four contributing studies [5]. Clinical significance requires reference to a procedure-specific minimal clinically important difference (MCID).

PONV showed the most consistent result, with no observed heterogeneity and high certainty according to the authors’ GRADE assessment [5]. The reduction in PONV may partly reflect reduced opioid exposure, although this relationship was not directly evaluated in the meta-analysis. The head-to-head evidence requires separate consideration. Evidence that the PPB is effective as an adjunct does not establish superiority over other posterior knee analgesic techniques. Against IPACK combined with femoral triangle block [6], PPB demonstrated lower 24-hour opioid consumption (5.6 vs. 11.7 mg MME) and significantly reduced PONV incidence (RR 0.14, 95% CI 0.03 to 0.59). Against the tibial nerve block combined with the adductor canal block [7], the PPB was associated with lower opioid consumption than the tibial nerve block (1.6 vs. 2.0 mg MME), although the absolute difference was small and its clinical relevance is uncertain. No measurable difference was observed in 12-hour pain scores (MD 0.00, 95% CI –0.51 to 0.51). In the single fascia iliaca block comparison reported in the meta-analysis, the PPB was associated with higher 24-hour opioid consumption (8.8 vs. 6.8 mg MME) and greater 12-hour pain intensity (MD 1.50, 95% CI 0.03 to 2.97) [5]. Each comparison was based on a single trial in a different clinical setting. Together, these trials are insufficient to establish the PPB as the preferred technique.

Another limitation is the range of outcomes studied. The outcomes evaluated, including opioid consumption, pain scores, and PONV, provide useful information on analgesic efficacy but are insufficient to determine whether the PPB improves patient-centered functional recovery. Thus, the current evidence should be interpreted as insufficient to establish a functional benefit rather than as evidence that the PPB provides no functional benefit. Relevant recovery outcomes after TKA include independent ambulation, physiotherapy milestone attainment, preservation of quadriceps and plantar flexor strength, pain during functional movement, patient-reported quality of recovery (QoR), and discharge readiness. Reduced opioid consumption alone should not be assumed to improve mobilization. In a post hoc analysis of two randomized trials involving adults undergoing abdominal surgery, postoperative opioid consumption was not associated with mobilization time [9]. Sakai et al. [7] used rehabilitation goal attainment as the primary endpoint, directly assessing functional recovery. Sørensen et al. [10] assessed QoR-15 and early functional outcomes and found no advantage of 20 mL over 10 mL of bupivacaine. Future trials should assess time to first unassisted ambulation, validated QoR measures, opioid-free recovery, and motor strength in addition to conventional analgesic outcomes. PPB injection site, local anesthetic agent and volume, background regional block, and multimodal analgesia should also be standardized to improve comparisons across studies.

The meta-analysis by Lee and colleagues provides further evidence that adding the PPB to an established regional analgesic strategy can reduce early opioid consumption and postoperative pain [5]. PONV was the most consistent outcome. However, substantial heterogeneity, low certainty for key analgesic outcomes, and limited head-to-head evidence do not support the PPB as a preferred posterior knee analgesic technique for all patients. Whether these analgesic and opioid-sparing effects improve functional recovery while preserving motor function remains undetermined. Future trials should therefore include motor, functional, and patient-centered recovery outcomes in addition to conventional analgesic endpoints.