Topic: A prospective observational study to assess the feasibility of Ultrasound-Guided Crosswise Approach to Popliteal Sciatic block for below-knee surgeries in adult patients.

Introduction: With good knowledge of dermatomes, myotomes and osteotomes of lower limbs and the use of ultrasound for precise perineural deposition of local anaesthetics, Sciatic nerve block in the popliteal fossa is now a widely practiced regional anaesthesia technique to provide anaesthesia or analgesia for below-knee surgeries. It is either used alone or in combination with a femoral or saphenous nerve block. The ultrasound guided popliteal fossa nerve block can be performed in the prone, lateral and supine position. The prone or lateral position for popliteal sciatic nerve block (PSNB) is convenient for the anaesthesiologist. Not only giving such position is difficult and time consuming, it may also have detrimental effects especially in patients with comorbidities, obese pt, pt with spine injury and painful limbs as well as patient who is on mechanical ventilation. Hence, PSNB in the supine position has been suggested to provide greater patient comfort. The literature has described various supine PSNB approaches such as supine posterior, supine lateral, supine medial, gapped-supine, figure of four positions. However, lifting the leg or flexion at the hip and knee joints, is required to perform the block in these positions. This limb manipulation may result in severe pain or misalignment of non-stabilised fracture segments. Recently a novel ultrasound-guided distal transverse (crosswise) approach has been proposed to perform supine lateral PSNB in semiflexed limb position with minimum limb manipulation. In this ultrasound-guided crosswise approach to popliteal sciatic nerve (CAPS), the block is performed at or just proximal to the divergence of the sciatic nerve. Electrostimulation using peripheral nerve stimulator can also be used to identify & confirm the sciatic nerve components according to evoked motor responses. We in our institute perform sciatic nerve blocks via different approaches as a regular regional anaesthesia technique for below knee surgeries in adult patients.

In this prospective observational study, we aim to study the feasibility of Ultrasound guided popliteal-sciatic nerve block by distal transverse (crosswise) approach (CAPS) in patients posted for below-knee surgeries.

Research question:  Is Ultrasound guided Crosswise approach of popliteal-sciatic nerve block (CAPS), a feasible technique of sciatic nerve block in adult patients requiring below knee surgery?

Hypothesis:  USG guided CAPS block is a feasible technique of sciatic nerve block in adult patients requiring below knee surgery.

Aim: Assessing the feasibility of ultrasound guided crosswise approach to popliteal-sciatic nerve block((CAPS) for below knee surgeries in adult patients.

Primary objective: To analyse the feasibility of CAPS block in terms of:

I.        Success rate

II.      Patient comfort

III.     Operator satisfaction

Secondary objective:

·       To analyse block characteristics like Sciatic nerve visibility; number of needle passes; the sensory and motor block onset time, haemodynamic parameters, duration of post operative analgesia

·       Patient’s overall satisfaction.

Study design: Prospective observational study

Study place and Study area: Surgical/Orthopaedic operation theatres at LTMMC & LTMGH, Sion - Mumbai.

Study duration: After getting clearance from institutional ethics committee, the study will commence from November 2023 till May2025

Sample size:  81 adult patients.

Sampling method: Convenient consecutive consenting sample

Inclusion criteria:

-          Adult patients aged >18 years.

-          ASA I to IV

-          Mentally competent to give consent.

Exclusion criteria:

●      Patient refusing for peripheral nerve block

●      Patient not cooperative for USG assessment.

●      Patient with history of hypersensitivity/allergy to local anaesthetic

●      Patients with neurological deficits.

●      Patients with evidence of Infection at the site of injection.

Methodology: After obtaining approval from the Institutional ethics committee of LTMMC & LTMGH, Sion, we will proceed with the study. Consenting adult patients (age > 18 Yrs) satisfying inclusion criteria and posted for below knee surgeries will be prospectively recruited in the study. Informed consent will be obtained from the participants. Preoperative assessment and standard ‘nil per oral’(NPO) status of the patient before surgery is as per the standard institute protocol.  In the operation theatre, standard monitors are attached such as ECG, non-invasive blood pressure and pulse oximeter. Baseline vitals such as pulse rate, non-invasive systolic and diastolic blood pressure and saturation in room air are recorded. Supplemental Oxygen is given through appropriate oxygen delivery device if the room air SpO2 < 95%. Pre-procedure pain in the affected limb is assessed and recorded using the numeric rating scale (NRS) at rest as well as on movement of the affected limb. Ultra-sound guided Popliteal-Sciatic Nerve Block using Crosswise approach (CAPS) is administered using all aseptic precautions.

Block technique: USG machine placed on the opposite side of the limb to be blocked as per OT ergonomics.

Patient’s limb position: Patient is placed in supine position with the affected limb in semi flexed position. If required, a folded towel, soft pillow, or foam leg elevator may be placed underneath the calf muscles to create room for the probe placement.

Site of block: CAPS block is performed 5–10 cm proximal to the popliteal crease at or above the point of divergence of the components of sciatic nerve, where popliteal vessels may or may not be seen in the ultrasound imaging. A low-frequency curvilinear (Curved array) ultrasound probe is placed in a transverse plane perpendicular to the skin over the lateral aspect of the lower thigh. It is positioned on or below the intermuscular groove formed by the vastus lateralis and biceps femoris muscles. It corresponds to the upper border of the patella, just proximal to the level of the popliteal crease. The linear transducer is used in paediatric and thin-built adult patients.                   

Block Procedure: The standard block procedure is followed. Site & side of surgery is confirmed. Skin around the site of the regional block scrubbed, painted & draped. Landmarks of the site block are as follows: from the superior border of patella a line drawn perpendicular to the tendon of biceps femoris. At approximately 3-inch distance proximal to this line along the tendon of biceps femoris, local infiltration (using 2% lignocaine) is given in skin and subcutaneous tissue anterior to the tendon for local anaesthesia. USG probe is placed at this level. The structures and sciatic nerve are identified. Movement of the probe proximally brings tibial and common peroneal nerves together to form the sciatic nerve at a variable point, above the popliteal crease. At this level, A 21-gauge, 100-mm echogenic nerve block needle is inserted perpendicular to the skin from lateral to medial direction using an out-of-plane technique. After placing the needle tip in the perineural space at or above the divergence, negative aspiration is checked and hydro-dissection performed. Once the space is confirmed, 20 ml of local anaesthesia solution (0.5% ropivacaine + 4mg Dexamethasone) is administered after confirming negative aspiration for blood. A “doughnut sign” can be seen due to circumferential spread of local anaesthesia around the nerve(s). Upon tracing the SCN distally, the local anaesthesia spread around the TN, and CPN is visualised to verify the correct anatomic placement of the block.  

Depending upon the need of surgery, the anaesthesiologist will administer additional saphenous nerve block (Adductor canal Block) by standard technique with the patient in the same supine position with leg extended in neutral or externally rotated. Using a high-frequency linear ultrasound probe placed on the anterior aspect of the patient’s thigh at the mid-thigh level. The femur bone is identified and the probe moved medially until the boat-shaped sartorius muscle and femoral vessels lying beneath it are visualised. A 21G insulated needle is inserted via in-plane technique, 10 ml of 0.375% ropivacaine is deposited on either side of the femoral artery after negative aspiration. Details of additional saphenous block given will be noted.

Block assessment: Sensory block and motor block onset are assessed after two minutes following administration of block and every 30 seconds thereafter, by pin prick test and movements at the ankle, respectively, and the time of block onset is noted. Sensory function is assessed as sensation to pinprick with a 23-gauge dental needle in the tibial (plantar surface of the foot), and common peroneal (dorsal surface of the foot) territories. Motor function is tested for both tibial (plantar flexion) and common peroneal (dorsiflexion) components.

The sensory and motor blockades are graded on a 4-point scale.  Successful Block (primary outcome) - The block is considered as successful if anaesthesia and paresis (a score of 3 for both sensory and motor nerves) are achieved within 40 minutes. Sensory block onset - defined as time from completion of block administration to loss of pin prick& touch sensation and motor block onset – defined as the time from completion of block administration to loss of movements at the ankle joint.

Surgery is allowed to start after adequate sensory and motor blockade is achieved. In patients with failure to achieve adequate surgical anaesthesia after 40 minutes of administration of block, it was considered as a block failure and converted to general anaesthesia.

Sciatic nerve visibility will be assessed by the operator using a 6-point visibility scale, where a score of 0 indicates that no nerve/muscle was identified, and a score of 5 indicates complete visibility with multiple fascicles/perimysium identifiable. [Table 2]. This will be recorded.

No. of needle passes to administer the drug at the desired site (perineural sheath of sciatic nerve) will be noted

Haemodynamic parameters including heart rate, blood pressure and peripheral oxygen saturation is recorded at regular time intervals (every 15 minutes). The time required for first rescue analgesia and any complications is also noted.

After shifting the patient to post anaesthesia care unit, assessment of pain is done every 30 minutes (till the patient stays in PACU) using numeric rating scale (NRS,0–10 scale, with 0 being no pain and 10 being worst pain). When the NRS score is > 3, patient receives a rescue analgesic as per the PACU protocol. The time of first rescue analgesia given to the patient is noted. Duration of postoperative analgesia is defined as the time interval between completion of block administration and first rescue analgesia (NRS >3). Twelve hours postoperatively, overall patient satisfaction is assessed based on three point Lickert’s scale (1, most satisfactory; 2, satisfactory; 3, not satisfactory).

Operator rating for the technique with respect to overall ease of administering the block will also be noted on 3 -point scale where 1 being – easy; 2 - fair; and 3 being – difficult.

All patients receive pre-medication in the form of injection Midazolam or Dexmedetomidine infusion before administering the block as per the discretion of the anaesthesiologist and patients’ condition. The drug & dose used for sedation is noted.

THE FOLLOWING PARAMETERS TO BE NOTED:

1. Demographic parameters, Diagnosis, Types of surgery
2. Baseline hemodynamic parameters Heart rate (HR), non-invasive systolic (SBP) and diastolic (DBP) blood pressure and SpO2
3. Base-line Pain assessment using NRS (Numeric rating scale)
4. Peri-operative sedation (as per the anaesthesiologist’s discretion)
5. Time of administering the Block (‘zero time”)
6. Sciatic nerve visibility score
7. No. of needle passes
8. Sensory & Motor assessment score (after 2 minutes of block administration and every 30 seconds thereafter
9. Sensory Block Onset Time
10. Motor Block Onset Time
11. Surgery start time
12. Hemodynamic parameters (HR, SBP, DBP, SpO2) - every 15 minutes
13. The time required for first rescue analgesia
14. Post operative Pain (Numeric rating scale) in PACU (every 30 minutes)
15. Duration of post-operative analgesia
16. Operator rating with respect to ease of block administration (3-point scale)
17. Overall patient satisfaction (after 12 hours of surgery) - three point Lickert’s scale
18. Complications (if any)