The Biceps Femoris Short Head (BiFeS) block is a new fascial plane block technique targeting the sensory innervation of the posterolateral capsule of the knee while preserving motor function. The aim of this study is to evaluate the postoperative analgesic efficacy of the BiFeS block combined with the adductor canal block (ACB) in patients undergoing total knee arthroplasty, compared to ACB + iPACK block. The study is planned as a prospective, randomized, double-blind, parallel two-group trial. Patients will be randomized in a 1:1 ratio to receive ACB + iPACK (Group A) or ACB + BiFeS (Group B). Postoperative pain scores (VAS), opioid consumption, early mobilization, and quality of recovery (QoR-15) outcomes will be compared.

Total knee arthroplasty (TKA) is an effective surgical method for the treatment of advanced osteoarthritis, reducing pain and improving functional capacity. However, approximately 30-40% of cases experience severe postoperative pain, and 20-31% of patients develop persistent postoperative pain (1, 2). This not only delays early mobilization and rehabilitation but also negatively affects patient satisfaction and overall recovery.

Enhanced Recovery After Surgery (ERAS) protocols emphasize that effective postoperative analgesia is indispensable for early mobilization and functional recovery. Multimodal analgesic strategies, particularly combinations of motor-sparing regional anesthesia techniques, are a cornerstone of these protocols (3).

One of these regional anesthesia techniques, the femoral nerve block (FNB), provides strong analgesic efficacy but reduces quadriceps strength, increasing the risk of falls. Therefore, in recent years, "motor-sparing" approaches aimed at preserving motor function have gained prominence (4).

Recent high-level network meta-analyses have shown that combinations of motor-sparing regional techniques provide superior outcomes compared to single blocks. In a 2024 meta-analysis by Wang et al., including 30 randomized controlled trials (RCTs), the combination of continuous adductor canal block (cACB) + genicular nerve block (GNB) among motor-sparing approaches provided the lowest rest pain scores at 24 and 48 hours, while cACB + iPACK + GNB combination most effectively reduced pain on movement (5). In the same study, this combination also demonstrated the highest performance in functional recovery indicators, with the shortest Timed-Up-and-Go (TUG) time and greatest range of motion.

Similarly, a 2025 Bayesian network meta-analysis by Migliorini et al., analyzing 77 RCTs, reported the lowest visual analog scale (VAS) scores between postoperative days 1-3 with continuous periarticular analgesia/local infiltration analgesia (PCI/LIA); this was followed by continuous FNB/PCI and continuous ACB applications, respectively (6).

Both analyses emphasized that multimodal approaches, particularly combinations of ACB, iPACK, GNB, or LIA, reduce opioid consumption and support early mobilization.

Another large-scale Bayesian network meta-analysis published by Xue et al. in 2024 reported that the ACB + iPACK combination significantly reduced pain at rest and on movement at 48 hours compared to single blocks (ACB, FNB, GNB, iPACK), decreased opioid consumption, and accelerated early mobilization (7).

However, current techniques have several limitations. LIA is surgeon-dependent, requires a large local anesthetic volume (100-150 mL), and shows variable efficacy due to a lack of standardization in composition/concentration. High volumes also increase the risk of local anesthetic systemic toxicity (LAST). While FNB provides effective pain control, it is not compatible with motor-sparing goals due to quadriceps weakness. Continuous blocks offer analgesic advantages but add logistical burdens due to the need for catheters, pumps, and monitoring.

In this context, new regional techniques are needed that preserve motor function while targeting the sensory innervation of the posterolateral capsule. To address this, the Biceps Femoris Short Head (BiFeS) block, introduced by Kılıçaslan et al. in 2025, is an innovative fascial plane block technique targeting the nerve branches innervating the posterolateral capsule of the knee (8). Ultrasound-guided injection between the short head of the biceps femoris and semimembranosus muscles allows the spread of local anesthetic along the natural fascial plane between these two muscles. This plane constitutes an anatomical corridor including the terminal sensory branches of the common peroneal nerve, the popliteal extension of the posterior femoral cutaneous nerve, and lateral genicular nerve branches. The oblique orientation of muscle fibers facilitates proximal and distal spread of the local anesthetic, creating a broader sensory block encompassing the posterolateral capsule of the knee. Thus, the BiFeS block is expected to provide a wider sensory area than the iPACK block without affecting motor fibers.

Cadaveric studies and early clinical data suggest that the BiFeS block combined with ACB may contribute to multimodal approaches by enhancing postoperative analgesia while preserving motor-sparing efficacy (8). However, no randomized controlled trial has yet demonstrated the clinical effectiveness of this technique.

The hypothesis of this study is that the BiFeS block applied in combination with ACB provides superior postoperative analgesia compared to the iPACK block applied with ACB.

Inclusion Criteria:

• Scheduled for unilateral total knee arthroplasty
• Classified as ASA physical status I-III

Exclusion Criteria:

• Scheduled for revision arthroplasty or bilateral surgery
• Allergy to local anesthetics or contraindication to regional blocks
• Neurological disorders, peripheral neuropathy, or conditions affecting lower extremity muscle strength
• Coagulopathy or receiving anticoagulant therapy
• History of severe liver, kidney, or heart failure
• Chronic opioid use or history of substance abuse
• Patients experiencing intraoperative complications (e.g., excessive bleeding)
• Cases in which adequate sensory blockade is not achieved after the block