This prospective, randomized, double-blind clinical trial aims to compare the effects of two different posterior knee analgesia techniques-iPACK block (Interspace Between the Popliteal Artery and the Capsule of the posterior Knee) and the Biceps Femoris Short Head (BiFeS) block-when combined with an adductor canal block (ACB) in patients undergoing elective unilateral total knee arthroplasty (TKA). Effective postoperative analgesia is essential for early mobilization, enhanced recovery, and patient satisfaction after TKA. Although ACB provides good anterior knee analgesia while preserving quadriceps strength, additional posterior knee analgesia is often required.

The iPACK block is a commonly used technique that targets the posterior knee capsule, whereas the newer BiFeS block aims to provide more selective posterolateral sensory blockade with potentially reduced risk of motor involvement. However, comparative clinical data between these two techniques are limited.

The primary objective of this study is to evaluate the difference between the iPACK and BiFeS blocks in terms of Quality of Recovery-15 (QoR-15) scores at 24 hours postoperatively. Secondary objectives include comparisons of postoperative pain scores, opioid consumption, time to first analgesic request, quadriceps and anterior tibialis muscle strength, functional test performance, time to mobilization, active range of motion, and the incidence of adverse events.

A total of 74 adult patients scheduled for elective unilateral TKA under spinal anesthesia will be randomized in a 1:1 ratio into two groups: ACB + iPACK block or ACB + BiFeS block. All blocks will be performed under ultrasound guidance by an experienced anesthesiologist. Outcome assessments will be conducted by blinded investigators. The study aims to provide high-quality clinical evidence regarding the effectiveness of the BiFeS block compared to the widely used iPACK technique.

This study is a prospective, randomized, double-blind, parallel-group, single-center clinical trial designed to compare the effects of two posterior knee analgesia techniques-iPACK block and Biceps Femoris Short Head (BiFeS) block-when combined with an adductor canal block (ACB) on postoperative recovery outcomes following elective total knee arthroplasty (TKA). Postoperative pain after TKA is often severe and can limit early mobilization, delay rehabilitation, and negatively affect recovery. Multimodal analgesia strategies, including regional anesthesia techniques, play an essential role in optimizing postoperative outcomes.

The adductor canal block is widely used because it provides effective anterior knee analgesia while largely preserving quadriceps motor strength. However, it does not adequately address posterior knee pain. The iPACK block is frequently used to supplement ACB, providing analgesia to the posterior knee capsule by targeting the interspace between the popliteal artery and the posterior capsule. The BiFeS block is a newer regional anesthesia technique that involves injection between the femur and the short head of the biceps femoris muscle, targeting terminal sensory branches supplying the posterolateral knee. Cadaveric and early clinical data suggest that the BiFeS block may provide more selective posterior analgesia with limited spread to motor fibers, but comparative clinical data are scarce.

The primary objective of this study is to determine whether the combination of ACB + BiFeS block results in superior quality of recovery compared to ACB + iPACK block, as measured by the QoR-15 score at 24 hours postoperatively. Secondary outcomes include QoR-15 at 48 hours; pain scores at rest and during movement; total opioid consumption at 24 and 48 hours; time to first opioid request; quadriceps and anterior tibialis muscle strength; functional performance assessed using the Timed Up and Go (TUG) and Five Times Sit-to-Stand (FTSST) tests; active knee range of motion; time to first mobilization; and the incidence of adverse events such as nausea, vomiting, pruritus, sedation, neurological symptoms, local anesthetic systemic toxicity, and postoperative falls.

Eligible participants are adults aged 18-75 years undergoing elective primary unilateral TKA under spinal anesthesia, with an ASA physical status of I-III. Exclusion criteria include allergy to study medications, coagulopathy, anticoagulant therapy, severe cardiac, hepatic, or renal dysfunction, pre-existing neuropathic pain, infection at the injection site, neuromuscular disorders of the lower limb, pregnancy, or inability to cooperate with postoperative assessments. Written informed consent will be obtained from all participants. The study has received ethics approval from the Atatürk University Clinical Research Ethics Committee.

Participants will be randomized in a 1:1 ratio using computer-generated block randomization. Both patients and outcome assessors will remain blinded to group assignments. All block procedures will be performed under ultrasound guidance by an experienced anesthesiologist not involved in postoperative assessments. Standardized perioperative management will include spinal anesthesia with hyperbaric bupivacaine, perioperative non-opioid analgesics (paracetamol, dexketoprofen), antiemetic prophylaxis, and postoperative patient-controlled analgesia with fentanyl.

The sample size was calculated based on the minimal clinically important difference (MCID) for the QoR-15 score, assuming an effect size of 8 points, standard deviation of 10, power of 90%, and α = 0.05, resulting in 33 patients per group. Accounting for a 10% dropout rate, 74 patients will be enrolled.

Data will be analyzed using the intention-to-treat principle. Continuous variables will be compared with independent-samples t-tests or Mann-Whitney U tests based on distribution. Mixed-effects models will be used for repeated pain measurements. Categorical variables will be compared using chi-square or Fisher's exact tests. ANCOVA may be applied for adjusted analyses of QoR-15 outcomes.

This trial is expected to contribute important clinical evidence regarding the analgesic effectiveness, functional recovery benefits, and safety profile of the BiFeS block compared with the widely used iPACK block, helping guide postoperative analgesia strategies in total knee arthroplasty.

Inclusion Criteria:

• Age 18-75 years
• Scheduled for elective primary unilateral total knee arthroplasty (TKA)
• Planned spinal anesthesia
• ASA physical status I-III
• Ability to understand study procedures and provide written informed consent

Exclusion Criteria:

• Known allergy or hypersensitivity to local anesthetics or other study medications
• Coagulopathy or current anticoagulant therapy
• Severe hepatic, renal, or cardiac failure
• History of neuropathic pain
• Infection at the site of proposed nerve block
• Pre-existing neuromuscular disorders in the lower extremities
• Inability to cooperate or refusal to participate in the study
• Pregnancy

Individual participant data (IPD) will not be shared because the study involves sensitive patient health information, and data confidentiality will be strictly maintained according to institutional ethical guidelines.

Participants and outcome assessors will remain blinded to group allocation. The anesthesiologist performing the block procedures will not participate in postoperative assessments.