Ultrasound-Guided Peripheral Nerve Blocks

Why Ultrasound?

• Superior success rates with fewer needle passes
• Less time to perform the block
• Real-time visualization of the needle, nerve, and spread of local anesthetic
• Ability to negate anatomic variability (particularly important in obese patients, where landmark-based failure rates are higher)
• Allows variable, often smaller volumes of local anesthetic - typically 10-30 mL, guided by what you observe rather than a fixed protocol

Ultrasound Basics for Nerve Imaging

Echogenicity

• Hypoechoic (dark/black): fluid, fat, structures through which sound passes easily
• Hyperechoic (bright/white): bone, fascial layers, connective tissue, needle

Nerve Appearance

Transducer Selection

Needle Approach: In-Plane vs Out-of-Plane

In-Plane (Long-Axis Needle Approach)

• Needle inserted at the side of the probe and advanced parallel to the ultrasound beam
• Entire needle shaft and tip visible as a bright echogenic line throughout advancement
• Preferred for most blocks - safer in experienced hands because you always see the tip
• Requires a longer skin-to-target path along the beam; takes more practice to stay perfectly in-plane
• Downside: longer needle path, more difficult to maneuver around structures

Out-of-Plane (Short-Axis Needle Approach)

• Needle crosses the ultrasound beam perpendicular to it
• Only the needle tip appears as a bright dot on screen (not the shaft)
• Easier for novices to get started; shorter path to target
• Risk: you may be visualizing the shaft rather than the true tip - advance incrementally and use "jiggling" to confirm position
• Better for superficial targets where the needle path is short

Needle Visibility Tips

1. Echogenic needles - textured/etched shafts with retroreflective surfaces; commercially available and most effective
2. Decrease the angle between needle and transducer to bring more of the shaft into the imaging plane
3. Heel-toe the probe (tilt toward the needle insertion site) to bring the beam more perpendicular to the needle shaft
4. Inject a small amount of saline or local anesthetic to confirm tip position by watching fluid spread
5. Jiggle the needle - small movements help identify the tip as a moving bright spot

Recognizing a Successful Injection

1. Distribute circumferentially around the nerve - the nerve appears to "float" or be surrounded by hypoechoic fluid
2. Clarify the nerve border - the spread separates the nerve from adjacent structures (commonly an artery wrapped in the same fascial sheath)
3. Track along the nerve path distally

Continuous Nerve Block Catheters

Universal Complications

Key Safety Rules (Applicable to All Blocks)

1. Aspirate before every injection - check for blood
2. Dose incrementally - inject in 3-5 mL aliquots with frequent aspiration
3. Dose on IBW in obese patients, not actual body weight (LAST risk)
4. Use the minimal effective volume - circumferential spread is the goal, not large volume flooding
5. If paresthesia is elicited or motor response at <0.5 mA on stimulator, withdraw 1-2 mm before injecting (suggests intimate needle-nerve contact)
6. Monitor the patient for at least 30 minutes post-block for immediate complications
7. Document pre-existing neurologic deficits before the block

Practical Workflow for Any Ultrasound-Guided Block

1. Survey scan (no needle) → identify nerve in short axis, map vessels, depth
2. Select transducer frequency based on depth
3. Plan needle approach (in-plane preferred)
4. Sterile prep: probe cover, sterile gel, sterile field
5. Infiltrate skin at insertion site with local anesthetic
6. Advance needle under real-time guidance to nerve vicinity
7. Confirm position: aspirate → inject 1 mL → watch for circumferential spread
8. Inject in increments (total 10-30 mL depending on block)
9. Remove needle, apply dressing
10. Monitor 30 min; document block onset time

Adductor Canal Block (ACB)

Overview and Clinical Rationale

Anatomy of the Adductor Canal

Contents - what you are blocking:

1. Saphenous nerve - terminal sensory branch of the femoral nerve; supplies the medial leg and infrapatellar branches to the knee joint. Located anterolateral to the femoral artery inside the canal.
2. Nerve to vastus medialis (NVM) - mixed nerve (sensory + motor); major contributor to anterior knee joint pain after TKA. Lies in a distinct fascial sheath, often slightly outside the true canal.
3. Posterior division of the obturator nerve - variable contribution.

The "true" ACB location debate: The NVM often lies just outside the adductor canal proper. A block placed too distally within the canal may miss the NVM entirely. The optimal injection point is midway between the ASIS and the patella, where the medial border of sartorius converges with the medial border of adductor longus - here you are most likely to cover both the saphenous nerve and NVM simultaneously.

Patient Positioning

Ultrasound-Guided Technique

Step 1 - Transducer setup

• High-frequency linear transducer (typically 10-15 MHz)
• Placed in a transverse (short-axis) orientation on the anteromedial thigh
• Start scanning at the junction of the middle and distal thirds of the thigh (midpoint between ASIS and superior patellar pole)

Step 2 - Identify the key landmarks on ultrasound

• Sartorius - superficial, triangular muscle forming the roof
• Superficial femoral artery (SFA) - round, pulsatile, non-compressible; the key landmark
• Superficial femoral vein (SFV) - posterior to the artery, compressible
• Saphenous nerve (SN) - small hyperechoic dot anterolateral to the SFA, just under the sartorius
• Vastoadductor membrane - a double echogenic line forming the canal roof; its inner border marks the true canal
• Nerve to vastus medialis - typically lateral to the SFA, may be in a separate fascial plane

Step 3 - Needle insertion and injection

• Insert block needle 2-3 cm lateral to the transducer, in-plane approach
• Advance deep to the sartorius muscle, into the triangular fascial space anterior/lateral to the SFA
• Target: the space between the SFA and the vastoadductor membrane
• Aspirate before injecting to confirm no intravascular placement
• Inject 10-20 mL of local anesthetic (15-20 mL typical for a full ACB)
  • Keep to 10-15 mL if concerned about spread - higher volumes may reach the femoral nerve proximally and cause quadriceps paresis
• Goal: watch local anesthetic spread circumferentially around the saphenous nerve and lateral to the artery, separating the nerve from adjacent structures

Continuous ACB Catheter

• Catheter threaded through the introducer needle under direct ultrasound visualization
• Confirm tip position by injecting a small volume and watching spread
• Infusion of local anesthetic postoperatively (e.g., 0.1-0.2% ropivacaine at 5-10 mL/hr)
• Continuous ACB > femoral block for mobilization on POD 1

Indications

ACB vs Femoral Nerve Block

Complications and Cautions

• Vascular injury / arterial pseudoaneurysm - rare; the SFA runs through the canal, so always aspirate and use real-time guidance
• Intramuscular injection - spread into the sartorius or adductor muscles risks severe myonecrosis - ensure proper fascial plane placement
• Unexpected thigh weakness - ACB is considered motor-sparing, but weakness can still occur if local anesthetic spreads proximally to the femoral nerve; always evaluate and implement fall-prevention protocols
  • Patient education: no unsupported ambulation
  • Physical therapy supervision for first ambulation
• Block failure - historically 30%+ with landmark techniques; ultrasound guidance dramatically reduces this

Nerve Stimulator Option

Recent Evidence

• A 2024 Bayesian network meta-analysis (J Clin Anesth, PMID 38692069) examined analgesic strategies for TKA and ranked nerve block combinations.
• A 2025 systematic review in BJA (PMID 39753403) specifically reviewed motor-sparing regional anaesthesia for TKA - directly relevant to ACB vs femoral block comparisons. Worth reviewing for current best practice in your unit.