Continuous femoral block

Introduction: Continuous femoral block (CFB) combined with a sciatic nerve block (SS or continuous) has been shown to be the most effective analgesic technique for total knee joint replacement. It provides better analgesia than a SS femoral nerve block 1 and avoids side effects commonly associated with systemic or neuraxial opioids. 2 It is associated with fewer technical problems than continuous LA epidural analgesia (weakness of the contralateral leg, requirement for LA top ups etc). 3, 4 It has even been shown to facilitate mobilisation and early knee joint flexion cf. SS femoral nerve block alone. 1 Administration challenges relate primarily to quadriceps weakness (with potentially impaired mobilisation) and a secondary increased falls risk. 5 In our experience, motor block can be minimised with attention to the initial LA bolus and infusion regimen. Both the potential problems associated with motor block and the falls risk can be minimised by good patient/nursing and physiotherapy staff education e.g. nursing help when mobilising until patients appreciate the inevitable quadriceps weakness, and insistence on physiotherapy mobilisation by day 1 (with a walker if necessary). In our experience, the inevitable opioid related side effects when continuous femoral nerve block is withheld are a greater impediment to early patient mobilisation than continuous femoral block related quadriceps weakness.

Skill level: advanced: mainly due to the difficulty placing the needle tip under the femoral nerve using the out-of-plane needle-probe alignment technique.

Procedure time: 10-15 mins (10 mins for experienced practitioners)

Common indications

Strong – TKJR

Moderate – ACLR: 6 Most patients achieve satisfactory analgesia with a SS femoral and sciatic block, and many will achieve satisfactory analgesia with a SS femoral nerve block alone. 6 However, prediction of which patients will experience moderate-to-severe pain after resolution of a SS femoral block can be difficult. Furthermore, femoral catheter placement enables the replacement of long acting for short acting LA for the initial bolus, which is one way of minimising potentially problematic motor block.

Sedation: e.g. midazolam 2 mg + alfentanil 250-500 mcg

Anatomy: The femoral nerve is formed from the second, third and fourth lumbar roots of the lumbosacral plexus. It supplies major parts of the knee and the medial shin/ankle and foot (via its continuation as the saphenous nerve). In the inguinal region the femoral nerve lies immediately lateral to the femoral artery, often wedged between the artery and iliacus muscle posterolaterally (Fig. 0,1). The nerve lies deep to both the fascia lata and iliaca. Just distal to the inguinal crease, the nerve divides into anterior and posterior divisions. All branches of the femoral nerve supplying the knee (articular filaments from the nerve to vastus lateralis, intermedius, medialis and the saphenous nerve) arise from the posterior division. Therefore, the posterior aspect of the nerve, as proximal as possible represents the most logical site to place a catheter or LA for femoral block.


Fig. 0. Femoral nerve block: illustration of the left femoral nerve overlying the iliacus muscle. The convex up surface of this muscle is an important landmark for LA deposition/catheter placement during femoral block.

Surface landmarks: mark out inguinal crease

Needle: 4-5 cm (depending on patient weight: usually 5 cm; < 70 kg = 4 cm) 18G Tuohy needle with multi-orifice catheter.



1. High resolution (10-15 MHz) linear probe e.g. Sonosite L38

2. 4-5 cm Contiplex Tuohy catheter kit with catheter cut using a surgical blade to approximately 20-25 cm (cutting the catheter will make catheter placement less fiddly and will improve patient acceptance).

3. Medical cyanoacrylate (e.g. dermabond 0.5 mL vial)

4. Catheter anchoring device (e.g. Lockit)

5. Tincture of benzoin (Friar's balsam) or Skin-Prep®

6. Surgical blade (to cut catheter)

7. Drapes

Procedure: Out-of-Plane technique (Video 1 and 2)

1. First infiltrate under US guidance, with lidocaine 1%, the anticipated needle puncture site and needle tract down to just lateral/underneath the femoral nerve.

2. Needle entry point is approximately 3cm caudad of the probe and 2-3 cm lateral to femoral nerve.

3. Neurostimulation is rarely useful as motor responses are difficult to elicit when targeting the posterior aspect of the nerve.

4. Gently place the probe in the femoral crease and visualise the femoral vessels (medial to lateral: vein, artery) and then the hyperechoic femoral nerve ("VAN"). Sonographic visualisation of the nerve can be challenging – look for a fascicular appearance and a curved (convex up) demarcation between the iliacus muscle and nerve (Fig. 1).

                   out of plane ultrasound guided femoral catheter placement

Fig. 1. Figure 1A (ultrasound image of the right femoral region for ultrasound guided femoral nerve block) and 1B (needle-probe alignment during out-of-plane ultrasound guided femoral nerve catheter placement). FL=fascia lata. FI=fascia iliaca. FN=femoral nerve with its indistinct lateral margin. FA=femoral artery. I=iliacus muscle. A1=plunger path required to achieve the needle tip path (NT) depicted in Figure 1A. A1 is a mirror image of NT due to hinging of shaft around skin puncture stie. With needle puncture site 'A', an almost complete circle movement is required to access the posterior aspect of nerve. B=correct lateral needle puncture site required to perform an "oblique" needle approach, which facilitates needle tip access to the posterior aspect of the femoral nerve.

5. With a 10 ml dextrose or saline filled syringe attached directly to the Tuohy needle, puncture the skin approx. 2cm lateral to the nerve and approx 3cm caudad of probe. This needle puncture site will facilitate an "oblique" needle approach. Advance needle down to the fascia overlying the neurovascular structures (fascia lata and fascia iliaca) (Fig. 2).

Fig. 2. Femoral catheter placement: injection through needle prior to catheter advancement. "Oblique" needle-probe orientation is illustrated – approx. 30-45 degrees to the femoral nerve long axis.

6. Penetrate this fascia just lateral to the nerve. This is always associated with a "pop" when using an 18G Tuohy needle, but requires a short, sharp deliberate needle movement once the needle tip is against the fascia.

7. Angle the needle tip using a rotatory motion (Fig. 1A) until tissue displacement is observed just underneath the lateral aspect of the nerve. A useful landmark is the "convex up" curved demarcation between the femoral nerve above and iliacus muscle below (Fig. 3).


Fig. 3. LEFT Femoral nerve block: posterior surface of nerve indicated by the useful "convex up" curved demarcation between the femoral nerve above and iliacus muscle below.

8. Inject dextrose or saline aiming to observe injectate spread immediately lateral/under the femoral nerve. With this needle alignment and an 18G Tuohy tip, intraneural needle placement is very unlikely, so aim to get needle tip/injectate spread as close as possible to the posterior aspect of the femoral nerve.

9. Drop the probe, transfer the hand previously holding the probe to the needle hub. Disconnect syringe and advance the catheter with the pink catheter-advancing piece sited within needle hub. Advance catheter at least 5 cm beyond needle tip.

10. Withdraw needle over catheter and immediately stabilise catheter by pressing catheter against skin with finger at skin entry point.

11.  Withdraw catheter to 3 cm beyond needle tip.

12. Carefully apply medical cyanoacrylate (e.g. dermabond) to skin entry site (aids secural and minimises LA leakage)

13. Apply tincture of benzoin to 2cm radius (or Skin-Prep®) of skin around catheter puncture site (improves Lockit-skin adhesion).

14. Apply Lockit catheter fixation device

15. Protect catheter from torniquet using small gauze and paper tape.

16. Dress catheter after surgery as described in catheter fixation.

Local anesthetic regimen: Bolus 25 mL local via catheter e.g 12.5 ml ropivacaine 0.75% + 12.5 mL lidocaine 1% then postoperative infusion regimen 2-5 mL/hr + PRN 5-10 mL boluses e.g ropivacaine 0.2% (e.g. PainBuster with OnDemand elastomeric pump).

Specific Complications

1. Femoral nerve trauma/intraneural injection with consequent femoral neuropathy (numbness along anterior thigh). Thought to be a more common problem before the introduction of real time ultrasound needle guidance when using a 22G needle orientated perpendicular to the nerve (? intraneural needle placement/injection). In our experience, the problem is very rare when using an 18G Tuohy needle placed under real time ultrasound guidance.

Clinical PEARLS

1. If sonographic visualisation of the femoral nerve proves difficult (uncommon in experienced hands), use ultrasound to approximate the needle tip position (2-3 cm lateral and deep to femoral artery), drop the probe, and then aim for a sustained quads/patella motor response at between 0.2-0.5 mA.

2. Consider using the in-plane technique particularly if only a SS injection is planned (Video 3 + 4). Approach the posterior aspect of the femoral nerve from laterally aiming to position the needle tip immediately adjacent the posterolateral aspect of the nerve. With this approach it is important to position the probe as proximally as possible to ensure the block/catheter is placed before the nerve divides into its anterior and posterior divisions (compared to the out-of-plane approach, the catheter will advance medially cf. proximally). Further, leave the catheter < 2 cm beyond needle tip to ensure the catheter remains in proximity to the nerve.  

3. If there appear to be 2 arteries one on top of the other, probe placement is distal to the origin of the profunda femoris artery which arises from the femoral artery posterior surface. Move the probe proximally until only one artery is visible.

4. An alternative to aiming for the posterior aspect of the nerve is to aim for the anterior surface. Many anesthesiologists argue against this approach as it still requires penetration of the fascia lata and iliaca. The needle will therefore be inevitably advanced into the nerve which may result in blunt trauma.



Video 1. Out-of-plane femoral nerve catheter placement.

Video 2. Left sided out-of-plane femoral catheter placement (sonography). Aim to postion the needle tip under the lateral aspect of the nerve, where there is a higher proportion of sensory (cf. motor) fibres.

Video 3. In-plane femoral catheter placement. Similar to the out-of-plane apprach, aim to position the needle tip near the posterolateral aspect of the nerve.


Video 4. In-plane femoral catheter placement (sonography).



1.         Ilfeld BM, Le LT, Meyer RS, et al. Ambulatory continuous femoral nerve blocks decrease time to discharge readiness after tricompartment total knee arthroplasty: a randomized, triple-masked, placebo-controlled study. Anesthesiology 2008;108:703-13.

2.         Gehling M, Tryba M. Risks and side-effects of intrathecal morphine combined with spinal anaesthesia: a meta-analysis. Anaesthesia 2009;64:643-51.

3.         Barrington MJ, Olive D, Low K, et al. Continuous femoral nerve blockade or epidural analgesia after total knee replacement: a prospective randomized controlled trial. Anesth Analg 2005;101:1824-9.

4.         Zaric D, Boysen K, Christiansen C, et al. A comparison of epidural analgesia with combined continuous femoral-sciatic nerve blocks after total knee replacement. Anesth Analg 2006;102:1240-6.

5.         Ilfeld BM, Duke KB, Donohue MC. The association between lower extremity continuous peripheral nerve blocks and patient falls after knee and hip arthroplasty. Anesth Analg 2010;111:1552-4.

6.         Williams BA, Kentor ML, Vogt MT, et al. Reduction of verbal pain scores after anterior cruciate ligament reconstruction with 2-day continuous femoral nerve block: a randomized clinical trial. Anesthesiology 2006;104:315-27.