Continuous lumbar plexus block


Lumbar plexus block (LPB) is often referred to as the psoas compartment block as successful lumbar plexus block can be achieved by LA placement within the fascial compartment of the psoas (major) muscle. The terminology is further confused by use of the term "anterior lumbar plexus block" for the more correct "fascia iliaca block"; the conventional lumbar plexus block being therefore coined "posterior lumbar plexus block".

Lumbar plexus block has received its share of bad press over the years – and for good reason. Lumbar plexus block is associated with relatively high risk of inadvertent intraneural, intravascular, and subarachnoid injection. There are several reasons why lumbar plexus block carries a higher risk for these complications. Specifically, lumbar plexus block is a deep block and requires a long (8-12 cm) needle. A long (8-12 cm) 22G needle has a high resistance to flow; therefore, with this needle the operator has limited ability to assess abnormally high injection resistance and consequent possible intraneural needle placement. This is compounded by the fact that the lumbosacral roots are relatively large and immobile and are therefore easily impaled. Second, because needle aspiration resistance is also high, the operator may not appreciate inadvertent intravascular needle placement – of particular relevance given the psoas muscle has relatively high vascularity. Finally, dural cuffs can extend beyond the vertebral column increasing the risk of subarachnoid injection.

For these reasons, we advise against performinglumbar plexus block with an 8-12 cm 22G needle. Always perform the block with an 8-12 cm 18G Tuohy needle, and consider whether the risks associated with a SS technique might outweigh the benefits. That said, continuous lumbar plexus block can be performed very safely when a few rules are followed. First, penetration of a dural cuff is unlikely when using an 18G Tuohy tip. Second, intravascular needle placement is easily detected when using an 18G needle (blood will almost always freely aspirate), and additional safety is afforded by fractionally injecting LA via the catheter. Third, intraneural needle placement is less likely with an 18G Tuohy needle, although due to a needle trajectory nearly perpendicular to the lumbar plexus, and the existence of relatively large immobile lumbar roots, the risk remains. Like all perineural catheter techniques, it is prudent to leave no more than 3 cm of catheter beyond the needle tip as being a block performed close to the neuraxis, it carries a risk of epidural (or subarachnoid) catheter misplacement.

Lumbar plexus block has been traditionally performed by use of the loss-of-resistance (LOR) or neurostimulation techniques. However, a fundamental problem with these techniques is their relatively high false negative rate in detecting needle passage beyond/deep to the lumbar plexus (no LOR or motor response). These neurolocalisation techniques are therefore associated with a relatively high risk of trauma to deep vessels (e.g. ovarian/testicular artery/vein), the ureter, and if the needle puncture site is inappropriately cephalad/lateral, the kidney. Ultrasound can minimise the risk of trauma to the aforementioned deep structures by providing an accurate estimate of lumbar plexus depth. While the lumbar plexus ultrasound anatomy has been well defined, 1, 2 there are currently only two significantly sized case series in which US has been used to assist lumbar plexus block placement. 3, 4 No RCTs have been published. There are several reports of real time ultrasound needle guidance forlumbar plexus block (all with concomitant neurostimulation), but for most practitioners, real-time needle guidance is at present too technically challenging. This is because the depth to the lumbar plexus (6-11 cm) 5 and the required needle orientation more parallel to the US beam renders shaft and needle tip visualization difficult. We therefore agree with Chin/Perlas, 6 Marhofer/Harrop-Griffiths and Ilfeld et al, 4 in that US is best limited to pre-needle puncture scanning to estimate the position of the relevant bony landmarks and  lumbar plexus depth.

Skill level: advanced: mainly due to the complication risk if improperly performed.

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

Common indications

Common – Revision total hip joint replacement (+/- combined with parasacral block).

Less common – Primary total hip joint replacement (+/- combined with parasacral block).

Even less common – Total knee joint replacement (usually combined with sciatic block) although continuous femoral block is a safer and equally effective alternative.

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


The lumbar plexus forms part of the lumbosacral plexus (Fig. 1A). It is formed by the ventral divisions of the first four lumbar nerves (L1-L4) and from contributions of the subcostal nerve (T12). The lumbar plexus forms lateral to the intervertebral foramina passing through the psoas muscle at the junction of its posterior third and anterior two thirds. The psoas muscle lies immediately anterior to the transverse process (TP) and lateral to the vertebral body; however, it extends a couple of cm lateral to the TP, therefore, needle advancement can still be lateral to the TP and still enter the muscle (Fig. 1B). The TP has been shown to be a reliable estimate of LP depth 5 and readily visualised with US 4. In a series of over 50 patients with a BMI range typical for patients undergoing lower limb joint replacement, Ilfeld et al 4 showed the LP lies consistently 2-3 cm deep to the TP. The TP depth itself however, was highly variable.

LP depth in relation to TP: median (quartiles)[range] = 2.5 (2-3) [0.2-4] cm.

TP depth in relation to skin: median (quartiles)[range] = 5 (4.5-5.5) [3.5-7.5] cm.

US estimates of TP depth were shown to closely predict that determined by needle depth.

The midpoint of the plexus appropriate for needle-skin puncture is consistently situated 1/5 of the distance from posterior superior iliac spine (PSIS) to the L3 spinous process (SP). 7 The inferior margin of the kidneys lie at approx. the L3 level (right slightly lower than left), and 4 cm cephalad of the highest point of the iliac crest. The hilum is 4-5 cm from the midline. Therefore, to avoid renal trauma, it is important to avoid excessive needle depth, but also a skin puncture site > 4 cm lateral to the midline and cephalad of L3 (or > 4cm above the iliac crest).



                               lumbar plexusgray1038

          Fig. 1. Lumbar plexus block: (A) Lumbar plexus anatomy and (B) Paravertebral muscles illustrating the lateral extent of the psoas muscle. 


1.    Ilioinguinal, iliohypogastric and genitofemoral nerves (T12-L2)

2.    Lateral femoral cutaneous nerve (L2-3)

3.    Femoral nerve (L2-4)

4.    Obturator nerve (L2-4)

Motor branches are distributed directly to psoas muscle, while the larger (distal) branches leave the muscle at various distances to run obliquely downward through the pelvic area to leave the pelvis under the inguinal ligament, with the exception of the obturator nerve which exits the pelvis through the obturator foramen.

Surface landmarks 7

Mark the posterior superior iliac spine (PSIS) and the L3 SP. Needle entry point is 1/5 of distance from PSIS to L3 (Fig. 2). Patients undergoing lower limb joint replacements often have an elevated BMI making surface landmark assessement (iliac crest, PSIS and SPs) imprecise. In these patients, the palpation estimated SP level is often lower than actual: the intercristal (="intercrestal") line bisects the L4 or L4/5 vertebral level; by palpation, it may bisect the L3 or L3/4 level. US can be used to more accurately identify the SP level either by counting vertebrae from the readily identified L5S1 level, or by US identification of the iliac crest.


Fig. 2. Surface landmarks for lumbar plexus block. Needle entry point is 1/5 of distance between poterior superior iliac spine and L3 spinous process.

Needle: 8-12 cm 18G Tuohy needle with multi-orifice catheter. Avoid using a fine (22G) 8-12 cm needle for this block.


1. Curvilinear probe

2. 8-12 cm Contiplex Tuohy catheter kit. Consider cutting catheter using a surgical blade to approximately 30 cm.

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

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

5. Tincture of benzoin (Friar's balsam)

6. Surgical blade (to cut catheter)

7. Drapes


Pre-needle puncture scanning and marking

1. First, with the probe aligned in the midline sagittal plane, identify the sacrum, which appears as a flat hyperechoic band with large anterior acoustic shadow.

2. Identify the L5S1 "gap" (intervertebral space between L5 and S1 vertebrae).

3. Identify the the SP of L5 then L4 and L3. Mark the L3 SP position

4. Sliding the probe laterally, note the approximate TP depth. Ultrasound is not used beyond this point.

5. Palpate the PSIS and mark its position.

6. Draw a line between the L3 SP and the PSIS.

7. Mark the planned needle-skin puncture point = 1/5 of distance from the PSIS to the L3 SP.

In thin patients, the lower pole of the kidney may be visible laterally at the L3 level.


Needling procedure

1. Infiltrate with lidocaine 1% the anticipated needle puncture site and needle tract down to the estimated level of the TP (Fig. 2).

2. Set nerve stimulator to 1.5 mA, pulse width 0.3 ms and 2 Hz.

3. With a 10 ml dextrose 5% filled syringe attached directly to the Tuohy needle, advance the needle to approximately 2-3 cm beyond the estimated TP depth, or until a quadriceps motor response is elicited (Fig. 3).


Fig. 3. Lumbar plexus block: tuohy needle advancement.

4. If bone is contacted, withdraw the needle and reangle either cephalad or caudad and readvance needle to 2-3 cm deep to the previously determined TP depth.

5. If the estimated LP depth is reached and no motor response is elicited, withdraw the needle and reangle 15-30 degrees in the direction of the PSIS-L3 line (either cephalad or caudad) and readvance needle again to the LP depth. Consider injecting 1-2 mL of dextrose, which will facilitate nerve stimulation.

6. On elicitation of an appropriate response, reduce the current ideally to < 0.7 mA. If responses cease above 0.7 mA, advance the needle slightly further and/or rotate the needle. If a sustained response is present at < 0.3 mA, withdraw needle slightly.

7. Inject all 10 mL of dextrose. If motor responses reappear, turn the stimulator off.

8. Disconnect syringe and advance the catheter with the catheter-advancing piece sited within needle hub. Advance catheter at least 5 cm beyond needle tip.

9. Withdraw needle over catheter.

10. Withdraw catheter to 3 cm beyond needle tip e.g. 6 cm needle depth = 9 cm catheter depth.

11.  Apply medical cyanoacrylate (e.g. dermabond) to skin entry site (aids secural and minimises LA leakage)

12. Apply tincture of benzoin (or "Skin-prep") to 2cm radius of skin around catheter puncture site (improves Lockit-skin adhesion).

13.  Apply Lockit catheter fixation device

14.Protect catheter from side supports/drapes using small gauze and paper tape.

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

Local anesthetic regimen: Fractionated 40 mL LA bolus via catheter e.g ropivacaine 0.5% then postoperative infusion regimen 5 mL/hr + PRN 5-10 mL boluses e.g ropivacaine 0.2% (e.g. On-Q C-Bloc elastomeric pump)

Specific Complications

1. Intraneural needle placement/injection.

2. Intravascular LA injection.

3. Puncture of a dural sleeve with consequent intrathecal injection.

See account at the start of this page under introduction. All can be minimised by use of an 8-12 cm 18G Tuohy needle. Never use a long (10 cm) 22G needle for lumbar plexus block – the risk of intravascular/intraneural/intrathecal injection is too high. Also, carefully evaluate whether the benefits of the SS technique justify the risks. 

Epidural spread (or spill) of LA is sometimes stated as a potential complication. However, in the absence of a catheter that has penetrated an intervertebral foramen (very uncommon), epidural spillage from an appropriately placed needle/catheter is rarely a significant clinical issue. To minimise the risk of intevertebral foramen penetration by the catheter, the needle bevel should be orientated laterally and the catheter left no further than 3 cm beyond the needle tip.  

Clinical PEARLS

1. An alternative technique for identifying the appropriate needle puncture point forlumbar plexus block is that described by Capdevilla et al: 5 on the intercristal line 2/3 of the distance between the midline sagittal and PSIS sagittal lines. This point will be lateral to the TP (but still within the lateral margin of the psoas muscle) in a significant proportion of patients.

2. In our experience, blockade of the sacral plexus occurs in only 30% of patients (? LA spread to sacral plexus or ?epidural spread from the lumbar plexus injection). For hip surgery, sacral plexus mediated pain can be managed with a SS parasacral block or intravenous opioid. For primary hip joint replacement, ongoing opioid requirement from sacral plexus mediated pain is uncommon beyond 4 hrs.

3. Catheter threading difficulty for continuous lumbar plexus block is uncommon using this approach. If it is encountered, withdraw the needle tip slightly (< 0.5 cm) while continuously attempting catheter advancement.




Video 1. Quadriceps twitches during lumbar plexus block





1. Karmakar MK, Ho AM, Li X, et al. Ultrasound-guided lumbar plexus block through the acoustic window of the lumbar ultrasound trident. Br J Anaesth 2008;100: 533-7.

2. Kirchmair L, Entner T, Wissel J, et al. A study of the paravertebral anatomy for ultrasound-guided posterior lumbar plexus block. Anesth Analg 2001;93: 477-81, 4th contents page.

3. Doi K, Sakura S, Hara K. A modified posterior approach to lumbar plexus block using a transverse ultrasound image and an approach from the lateral border of the transducer. Anaesth Intensive Care;38: 213-4.

4. Ilfeld BM, Loland VJ, Mariano ER. Prepuncture ultrasound imaging to predict transverse process and lumbar plexus depth for psoas compartment block and perineural catheter insertion: a prospective, observational study. Anesth Analg;110: 1725-8.

5. Capdevila X, Macaire P, Dadure C, et al. Continuous psoas compartment block for postoperative analgesia after total hip arthroplasty: new landmarks, technical guidelines, and clinical evaluation. Anesth Analg 2002;94: 1606-13, table of contents.

6. Chin KJ, Perlas A. Ultrasonography of the lumbar spine for neuraxial and lumbar plexus blocks. Curr Opin Anaesthesiol;24: 567-72.

7. Heller AR, Fuchs A, Rossel T, et al. Precision of traditional approaches for lumbar plexus block: impact and management of interindividual anatomic variability. Anesthesiology 2009;111: 525-32.