The comparison of spread of methylene blue after the Pericapsular Nerve Group block and a double injection selectively targeting the articular branches to the anterior hip capsule in human cadavers

Abstract

Objectives

In this study, the spread of methylene blue was compared between an ultrasound‐guided Pericapsular Nerve Group (PENG) block and a double injection technique, where the approach towards the inferomedial acetabulum was added to the latter.

Methods

The two techniques were performed in 11 fresh frozen cadavers. The spread was measured after anatomical dissection in which the supplying femoral and obturator nerves were identified.

Results and Conclusion

Our study demonstrates adequate staining of the iliac bone with comparable distal and medial spread in both techniques, indicating that the PENG block with a single injection is adequate in blocking the hip capsule with 10 mL local anesthetics. Staining of the femoral nerve occurred in 2/6 specimens after the PENG block, and staining of the obturator nerve in 1 specimen in each group.

INTRODUCTION

Based on an anatomic cadaveric study in which the innervation of the anterior hip capsule was explored, a novel ultrasound‐guided approach was developed and published in 2018: the Pericapsular Nerve Group (PENG) block. The anterior hip capsule is innervated by branches of the femoral nerve (FN), obturator nerve (ON), and accessory obturator nerve (AON). The branches of the FN and AON are found between the anterior inferior iliac spine (AIIS) and the iliopubic eminence (IPE), and the ON is located near the inferomedial part of the acetabulum. ¹ In the PENG block, the IPE and AIIS are visualized, and the needle tip is placed between the psoas tendon and the pubic ramus posteriorly where the local anesthetic solution was injected. In the original PENG study, the technique was performed on 5 patients with a hip fracture, with all 5 patients experiencing adequate analgesia with rest pain scores of 1–3 after 30 min. ² Since then, more studies have been published on the effect of the PENG block for multiple implications regarding pain originating from the hip joint, being a widely implemented block in clinical practice nowadays. ³ , ⁴ Medial and caudal spread is important and requires volume. ⁴ Using higher volumes in the PENG block is related to increasing incidence of motor weakness in clinical studies and staining of the femoral nerve in anatomical studies, due to aberrant spread. ³ , ⁴ , ⁵ , ⁶

A technique in which the articular branches are more selectively targeted at multiple sites has the potential of leading to adequate spread with a lower total volume. This might reduce the possibility of motor weakness due to extensive spread. A double injection technique was described in chemical hip denervation for inoperable hip fractures, targeting the articular branches of the femoral nerve between the AIIS and IPE below the psoas tendon and targeting the articular branches from the obturator nerve at the inferomedial acetabulum. ⁷

The objective of this study is to visualize the spread of 10 mL of methylene blue (MB) in cadavers, comparing a single‐shot PENG block to a low‐volume PENG block with the addition of an injection targeting the inferomedial acetabulum. We expect to provide important insight with this study if the spread is sufficient in the PENG block with 10 mL and if a double technique might benefit in more selective targeting of the articular branches with less spread to the supplying nerves. Hereby we hope to contribute to optimizing the technique for hip blocks.

METHODS

For this study, fresh frozen human cadavers (−20°C) were used. The human cadavers at the department of Medical Imaging, Anatomy of the Radboudumc, are obtained according to the Dutch body donation program for science and education (Wet op de lijkbezorging, Art. 18 §1, Art. 19, Art. 67–70, April 1991, https://wetten.overheid.nl/BWBR0005009/2022‐01‐01). Body donations of humans aged 18 years and older with a valid handwritten statement are accepted. The study with the use of the cadavers was approved by the Medical Ethics Review Committee region Arnhem‐Nijmegen (Commissie Mensgebonden Onderzoek [CMO] region Arnhem‐Nijmegen file No. 2017‐3941).

The fresh frozen human cadavers were used a month prior to the study in a trauma masterclass for final‐year residents and consultants. All cadavers have had an external fixation for a fractured pelvis (made with a small incision at the lower edge of the body of the pubic bone), which was removed after the course. The external fixation was placed either supra‐acetabular in the anterior inferior iliac spine 10°–20° cranial and 20°–30° medial or on the iliac crest. Afterwards they were frozen and defrosted again for educational purposes on the day of the study. All cadavers were screened by the two practitioners who performed the infiltrations (RS and KO) for feasibility. Both performers are anesthesiologists and pain specialists with skilled training in ultrasound‐guided locoregional and pain interventions and experience in the used technique. Assessment yielded 12 operable specimens. They were infiltrated with 10 mL dye (0.125% solution of methylene blue; 1.25 mg/mL).

Seven specimens were infiltrated using the single injection technique via the technique of the PEricapsular Nerve Group (PENG) block. One cadaver (single injection technique) was excluded directly because of backflow of dye through an incision on the upper thigh. Five specimens were infiltrated using the double injection technique: 6 mL via a PENG block and 4 mL via an anterior approach targeting the articular branches of the obturator nerve at the inferomedial acetabulum. The blocks were performed simultaneously on 12 available specimens. The PENG block was performed in less time compared to the double technique, leading to an uneven distribution.

For the PENG block, the iliopubic eminence, anterior inferior iliac spine, and psoas tendon were identified by ultrasound. The needle was introduced in plane with the target just lateral from the psoas tendon, and the dye was administered and visualized in the fascial plane between the psoas tendon and ilium.

For the second technique, the head of the femur was identified and followed caudomedially until the inferomedial acetabulum was identified. The needle was inserted in plane until contact with the bone, where the dye was injected. The techniques were performed with ultrasound with the Toshiba Aplio MX using a 3.5 MHz curvilinear probe via in‐plane technique. A 20‐gauge, 10‐cm RF needle was used for all interventions.

The dissection was performed using a standardized protocol based on images of the Sobotta Atlas of Anatomy, Volume 1, IV–IV 25th edition, Paulsen, Friedrich, ISBN 978‐3‐437‐44130‐1 Elsevier Clinic and surgical educational platform of Incision Academy (https://academy.incision.care/). The protocol was reviewed and adapted by an expert in anatomy (DD).

The dissection was performed within 2 to 3 h after the injection. A 20‐cm vertical incision from the Poupart ligament was performed downwards to flip the skin laterally. Thereafter the fascia was removed. This part of the dissections was performed by surgical residents under the direct supervision of one senior trauma surgeon (ET). All underlying muscles were dissected layer by layer at the origin or insertion and flipped to the side until the iliac bone was reached. The last part of the dissection, including the identification of the obturator and femoral nerves, was performed by the senior trauma surgeon.

Outcome measures are the spread of the dye and staining of the supplying femoral and obturator nerve. The distance of the spread of the dye was measured from a horizontal and vertical line crossing in the middle of the distal insertion of the m. iliopsoas in centimeters. The femoral nerve and obturator nerve were identified and analyzed for staining with the dye. Figure 1 shows the identification of the femoral nerve in a specimen.

FIGURE 1.

Ultrasound images with the anatomical position of the ultrasound probe. The images were made in a living subject to depict the used techniques in this study. Because of the ectomorph body type of the subject, the images were made with a linear probe. (1) Technique according to the Pericapsular Nerve Group block, identification of the anterior inferior iliac spine (AIIS), iliopubic eminence (IPE), the psoas tendon (PT), femoral nerve (FN) and femoral artery (FA). (2A) Identification of the head of the femur (FH) and the superomedial acetabulum (SMA). (2B) Medial positioning of the probe, identification of the inferomedial acetabulum (IMA).

The spread of the dye was evaluated, measured and photographed by two independent anesthesiology residents (EH and LB). The total horizontal and vertical spread was measured, and compared, as was the cranial, distal, medial, and lateral spread. The presence of staining of the iliac bone from the iliopubic eminence to the anterior inferior iliac spine was considered to be adequate.

RESULTS

We analyzed 6 cadavers after the single injection technique and 5 cadavers after the double injection technique. Table 1 shows the characteristics of the used cadavers.

TABLE 1.

Characteristics and spread per specimen.

N, sex	Side	Spread iliac bone	Staining femoral nerve	Staining obturator nerve	Staining acc obt nerve	Spread medial in cm	Spread lateral in cm	Spread cranial in cm	Spread distal in cm	Spread horizontal	Spread vertical	Block	Body type
1, M	Left	Yes	No	No	NF	6	0		2	6	11	Double	Mesomorph
2, M	Left	Yes	No	NF	NF	5.5	1	9	5	6.5	14	Double	Endomorphe
3, M	Right	Yes	No	Yes	NF	5	2	6	2	7	8	Double	Mesomorphe
4, M	Left	Yes	No	No	NF	7	0	7	4	7	11	Double	Mesomorphe
5, M	Right	Yes	No	No	NF	6	0.5	5	4	6.5	9	Single	Endomorphe
6, F	Right	Yes	No	No	NF	3	0.5	1	4	3.5	5	Double	Ectomorphe, small
7, M	Right	Yes	Yes	No	NF	4.5	2.5	8	0	7	8	Single	Endomorphe
8, M	Left	Yes	No	Yes	NF	6	3	11	1	9	12	Single	Mesomorphe, tall
9, M	Right	Yes	Yes	No	NF	4.5	2.5	9	10	7	19	Single	Endomorphe
10, M	Right	Yes	No	No	NF	5	1	10	1.5	6	11.5	Single	Mesomorphe
11, M	Right	Yes	No	No	NF	4	1	4	2	5	6	Single	Endomorphe

Abbreviation: NF, not found at dissection.

All specimens had staining of the iliac bone between the iliopubic eminence and anterior inferior iliac spine. In the 6 specimens dissected after the single injection technique, there was staining of the obturator nerve and femoral nerve in two specimens. In the 5 dissected cadavers infiltrated via the double injection technique, one specimen showed staining of the obturator nerve. There was no staining of the femoral nerve found.

The total mean horizontal spread in all cadaveric specimens was 6.4 cm, and the total mean vertical spread was 10.4 cm. After the double injection technique, mean horizontal spread was 6, and mean vertical spread was 9.8 cm. After the single injection technique, this was respectively 6.8 and 10.9 cm.

From the middle of the distal part of the origin of the m. iliopsoas, mean medial spread was 5.1 and mean lateral spread was 1.3 cm. The mean medial spread was 5.3 cm after the double injection technique and 5 cm after the single injection technique. The lateral spread was 0.7 cm (double) and 1.8 cm (single). The mean cranial spread was 7.2 cm (6.4 cm after the double injection technique and 7.8 cm after the single injection technique). The mean distal spread was 3.2 cm (3.4 after the double technique and 3.1 after the single technique). Figure 2 shows an overview of the measured spread.

FIGURE 2.

Overview of the measured spread per technique and in total.

DISCUSSION

All injected cadavers had adequate staining of the targeted iliac bone between the iliopubic eminence and anterior inferior iliac spine, indicating correct effectuation of both techniques and that 10 mL is an adequate volume to achieve blockage of the articular branches to the hip capsule. The difference in medial and distal spread was minimal. Cranial and lateral spread was wider after the single technique compared to the double technique, resulting from the higher volume used in the single technique while spreading the volume in the double injection technique. The double technique shows a more contained vertical spread. This might be beneficial when extensive spread can lead to side effects, for example, when a neurolytic is used. This benefit must be weighed against the more invasive nature of a double injection, which might be technically more challenging.

The medial spread after the single technique was 5 cm, which was almost comparable to the measured spread after the double technique (5.3 cm), which targeted the inferomedial acetabulum, which is the most consistent landmark for the branches of the obturator nerve. Although the articular branches were not selectively dissected and identified, this anatomically suggests adequate spread towards the articular branches of the obturator nerve. ¹ This contradicts prior anatomical studies that do not show consistent staining of the articular branches of the obturator nerve after a single injection PENG block, even in higher volumes of 20–30 mL. ⁸ A recent CT scan study showed a mainly cranial, intramuscular spread with only a minor spread to the anterior hip capsule and without medial spread to the obturator foramen. ⁹ Our spread was both more caudal and medial in the iliopsoas plane compared to this study.

Spread to the femoral nerve is possible in intramuscular spread, aberrant spread along the border, through the intramuscular septum, or through the needle tract. ⁴ A recent cadaveric study estimated that the minimal effective volume of methylene blue for sparing the femoral nerve in the PENG block in 90% of patients was 13.2 mL. ¹⁰ We found staining of the femoral nerve in two specimens after a single injection. Staining of the supplying obturator nerve occurred in one cadaver in both techniques. In combination with the anatomical assumption that a lower volume injected laterally (6 mL compared to 10 mL) might lead to less aberrant spread, theoretically the double technique might have a lower incidence of motoric involvement due to blocking the supplying femoral nerve. We did not find staining of the femoral nerve in the double injection technique. Further studies are required to support this finding.

When using lower volumes, this beneficial effect might be negligible according to prior anatomical and clinical studies. ⁵ , ¹⁰ , ¹¹ The clinical effect of motor weakness might also be dose/concentration dependent. In techniques using a neurolytic fluid, for example, phenol or alcohol, spread towards the supplying nerves is even more undesirable due to the possible long‐term effects, and the most efficient technique should be used. ⁷

The used volumes of 6 and 4 mL in the double injection technique were adapted from the used technique by Ng et al., ⁷ where 5 and 3 mL were used. We adjusted these volumes to 6 and 4 mL to match the total of 10 mL we preferably use in the PENG block.

STRENGTHS AND LIMITATIONS

We wanted to perform a practical feasibility study to compare two often used and described techniques. The strength of our study is the comparison of two techniques, which are both considered to be adequately performed in all cadavers. The dissection and registration of the spread were done by individual researchers, limiting the risk of bias in the results. We only measured the spread and identified the femoral and obturator nerves to compare this in both techniques. We did not dissect and identify the articular branches to verify staining of these branches.

The limitation of our anatomical study is the small numbers of cadavers used in which anatomical differences in posture and origin of the insertion of the psoas tendon are present. Although we did not perform a power analysis, our results give an indication of the similarity in the spread in both techniques.

The spread of methylene blue in cadavers can't be translated directly to the spread of local anesthetics or neurolytic fluids in patients. The cadavers have been defrosted twice, which may have influenced the spread. They have been used in a surgical training intervention with an external fixation on the pelvis. Although the fixation was placed supra acetabularly, influence on the spread can't be ruled out. Further anatomical studies, when using, for example, dyed ropivacaine or aqueous phenol 6% in different volumes, and clinical dosage studies are needed to support our claims and to optimize clinical practice.

GENERAL CONCLUSION

Horizontal and medial spread was comparable in both techniques, indicating that the Pericapsular Nerve Group block is an adequate technique for blocking the hip capsule with 10 mL. Staining of the femoral nerve occurred in 2/6 specimens after the PENG block, and staining of the obturator nerve in 1 specimen in each group. No staining of the femoral nerve was found in the double injection technique. Additional anatomical and clinical research is needed to support these findings.

AUTHOR CONTRIBUTIONS

All authors have made significant scientific contributions to the work, are familiar with the content, and have approved the final version of this manuscript.

CONFLICT OF INTEREST STATEMENT

Kris Vissers is an editorial board member of Pain Practice and a co‐author of this article. The other authors do not have any conflicts of interest.

PATIENT CONSENT STATEMENT

Body donations of humans aged 18 years and older with a valid handwritten statement are accepted according to the Dutch body donation program for science and education (https://wetten.overheid.nl/BWBR0005009/2022‐01‐01).

Smits RJH, Tan ECTH, van den Bersselaar LR, de Bruijn A, Hendriksen E, Vissers KCP, et al. The comparison of spread of methylene blue after the Pericapsular Nerve Group block and a double injection selectively targeting the articular branches to the anterior hip capsule in human cadavers. Pain Pract. 2025;25:e70002. 10.1111/papr.70002

DATA AVAILABILITY STATEMENT

The data that supports the findings of this study are available in the supplementary material of this article.