This study is designed as a randomized controlled trial aiming to evaluate the feasibility and safety of ultrasound-guided mid-femoral lateral approach sciatic nerve paraneural sheath block, in order to provide a new sciatic nerve block approach for clinical practice that offers greater patient comfort and higher precision, thereby facilitating its use for specific patient populations and less experienced operators.

In clinical practice, the popliteal approach to sciatic nerve paraneural sheath block is the most commonly used. Studies have found that compared with extraneural injection, ultrasound-guided sciatic nerve paraneural sheath block via the popliteal approach has a faster onset and provides longer postoperative sensory and motor blockade. Paraneural sheath block aims to achieve efficient, rapid, and precise nerve blockade using a low volume of local anesthetic. However, the popliteal approach requires knee flexion for performance, necessitating the patient to be in a prone or lateral decubitus position. For patients unable to flex the knee or those with positioning difficulties due to conditions such as lower limb fractures, this method is less applicable. For the above reasons, the popliteal sciatic nerve block has certain limitations.

The mid-femoral lateral approach demonstrates unique advantages: First, the patient can maintain a supine position throughout the procedure, significantly enhancing comfort and compliance. Second, the sciatic nerve is relatively superficial at this level, offering good ultrasonographic visualization and being distant from major blood vessels and nerves. This makes it applicable even in obese patients with thick subcutaneous fat and suboptimal ultrasound images, theoretically enhancing procedural safety. Therefore, it is considered an effective solution for patients with positioning restrictions, such as those with lower limb fractures and severe pain, holding high potential for clinical promotion and research significance.

Although the anatomical feasibility of this approach has been preliminarily confirmed, its practical feasibility and safety require validation through clinical studies. In particular, whether a successful "paraneural sheath block" can be consistently achieved at the mid-femoral lateral level, similar to the classic popliteal approach, remains to be explored. While some literature suggests its theoretical feasibility, there is a lack of prospective, systematic clinical research evidence to support its practical application feasibility (e.g., block efficacy, procedural difficulty, success rate) and safety (e.g., complication rates, incidence of nerve injury).

This study is designed as a randomized controlled trial aiming to evaluate the feasibility and safety of ultrasound-guided mid-femoral lateral approach sciatic nerve paraneural sheath block. The goal is to provide a new sciatic nerve block option for clinical practice that offers greater patient comfort and more precise blockade, facilitating its use for specific patient populations and less experienced operators.

Inclusion Criteria:

1. Patients scheduled for elective unilateral surgery below the knee (e.g., internal fixation for ankle, metatarsal, or calcaneal fractures; hallux valgus correction; flatfoot reconstruction).
2. American Society of Anesthesiologists (ASA) physical status class I or II.
3. Aged 18 to 65 years.
4. Ability to understand the study procedure and provide written informed consent.

Exclusion Criteria:

1. Contraindications to peripheral nerve block:

   • Known allergy or history of toxicity to local anesthetics.
   • Pre-existing neuropathy in the operative limb.
   • Severe coagulopathy.
   • Infection at the intended needle puncture site.
   • Systemic infection.

2. Requirement for simultaneous surgery at a site other than the unilateral lower limb below the knee.

3. Previous surgery or significant scarring in the area of the planned needle insertion for the sciatic nerve block.

4. Body mass index (BMI) > 35 kg/m².

5. Patients receiving chronic pain therapy or with a history of opioid abuse.

6. Inability to communicate or cooperate with the study assessments (e.g., due to cognitive impairment, language barrier, or psychiatric disorder).

7. Pregnancy or lactation.