ASRA Pain Medicine Update

Ultrasound Applications for Management of Morton’s Neuroma

Aug 30, 2022

Ultrasonography in Pain Medicine SIG

Pranab Kumar MD, FRCA, FRCPC, FIPP, CIPS
Department of Anesthesiology and Pain Medicine, University Hospital Network, Toronto Western Hospital, Ontario, Canada

Vanja Srbljak MD, FCAI, FFPMRCA
Department of Anesthesiology and Pain Medicine, University Hospital of North Midlands, Stoke-on-Trent, United Kingdom


Introduction

Morton neuroma (MN) is painful enlargement of the common plantar digital nerve in the intermetatarsal space (IMS), first described by Civinini in 1835.1 In 1876, Thomas George Morton reported it as a “painful affection of the foot” and treated his patients with considerable success at the Philadelphia Polyclinic by amputating the affected toe.2

MN is frequently located in the third IMS (66% of the cases), but it can also affect the second (32% of cases) or the fourth space (2% of cases) (Figure 1). The exact prevalence of MN is unknown, although results of one study suggest that it may be the most common mechanical neuropathy after carpal tunnel syndrome.3

The most accepted etiology is an entrapment of the nerve beneath the deep transverse metatarsal ligament (DTML) supported by microscopic changes demonstrating a thickened nerve with histologic findings of perineural fibrosis of the nerve distal to the ligament with a normal appearance proximally.

Clinical Presentation

Patients present with forefoot burning pain commonly on the plantar side, often reported as “walking on pebbles” and occasionally associated with numbness of the toes. Pain is elicited during weight-bearing activities where the respective IMS is loaded, as well as when wearing high-heeled shoes, while rest and removal of shoes brings relief.

History and clinical examination usually raise the suspicion of MN. The most sensitive clinical tests for the diagnosis of MN are the thumb index finger squeeze test, Mulder’s click test, and foot squeeze test, in that order, or a sonographic “Mulders Sign” as the lesion escapes the IMS.4 The interphalangeal divergence “Vulcan salute sign” on a conventional weight bearing dorso-plantar view radiographs is highly specific for Morton’s neuroma.5 In a metanalysis, Bignotti et al. demonstrated that ultrasound (US) due to high soft tissue resolution was as sensitive as MRI for identification of Morton’s neuroma.6

The forefoot disorders that mimic MN include metatarsal stress injuries, synovitis, plantar plate tears, metatarso-phalangeal joint laxity, and Freiberg’s disease.3,6

Diagnosis

US evaluation is an inexpensive diagnostic tool that is dynamic, non-invasive, and portable. MN typically appears as a well-circumscribed hypoechoic mobile lesion with direct pressure, (compared to the muscle) within the normal fatty tissue of the IMS. A small proportion (<15%) of MN appear anechoic or with a mixed echotexture.6 Identification of MN requires dorsal sagittal view of the foot with concurrent digital application of pressure at the web space. This forces the tissues of the space to spread, separating the layers superior (mobile under pressure) and deep (static under pressure) to the DTML (Figure 2). Other approaches (eg, transverse or plantar) can also be employed, but dorsal approach is mostly described in the literature for diagnostic purposes.

Treatment

The treatment of MN traditionally starts with conservative measures including the use of orthotic devices (wide toe box shoes, metatarsal pads, insole), analgesics, and gastrocnemius stretching. After failed conservative treatment, interventions offered include local injections with cortisone, alcohol, hyaluronic acid, and onabotulinumtoxinA (Botox®).7

Ultrasonography-guided alcohol injection (USGAI) of MN represents a popular alternative to steroid injections.8,9 A 20% alcohol solution has proven to be the minimum concentration for neural inhibition, while concentrations greater than 50% are likely more toxic for the neuroma and may result in a more complete ablation of the lesion.9,10

Radiofrequency (RF) and cryoablation have claimed to represent alternative options to percutaneous release of DTML or surgical excision of the neuroma. Despite the complications associated with neurectomy (ie, wound infection, hypersensitive scars, keloid, painful stump neuroma) and less than ideal long-term results, operative excision continues to represent the gold standard of treatment.11

Description of US-Guided Approach12

The patient is positioned supine, and the foot position and needle entry will vary in the dorsal, transverse, or plantar approaches accordingly. Appropriate sterilization of the area is undertaken, and an US transducer is enclosed in a sterile probe cover.

A transverse approach is described here where the patient is supine with the foot positioned on the examination table to restrict movement and allow stable needle placement through the inter-web space.                                                                           

The procedure starts with a dynamic scan with the use of a linear 7–12 MHz US probe to confirm the presence of MN and select the optimal access point. We perform a short axis scan from plantar aspect to be able to see the metatarsal heads from second to fifth metatarsal.  Then we focus in between the third and fourth metatarsal heads.  We then squeeze the metatarsal heads so that we can see the neuroma popping out between the metatarsal head, and the depth of the neuroma is noted.  The probe is rotated to a long axis (Figure 3A) and adjusted to lie between the third and fourth metatarsals to visualize the neuroma at the same depth (Figure 3B).  A 25 g, 1.5 inch hypodermic needle in the gap is inserted between two metatarsals while maintaining the US probe in the plantar aspect. By visualizing the needle all the way, we make sure not to damage the nerve (Figure 3C). Since the neuroma is seen better in the short axis, a biplanar view may be undertaken before injection especially if the neuroma is small. Hydro location with normal saline serves to make the neuroma distinct (Figure 4) and the intralesional injectate is then administered. A wide variety of corticosteroid – local anesthetic combinations have been reported in the literature. In our institution, 20 mg of Depo-Medrone (or equivalent), in a total volume of 2.5 ml of a long-acting local anesthetic, typically 0.5% bupivacaine, are used. Our preferred alcohol concentration for chemical neurolysis is 70% or higher, and, during the intralesional infilatration, it is paramount that the alcohol leak be contained within the neuroma to limit damage to other tissues.9

Following the removal of the needle and prior to the application of pressure and bandaging, a post-injection scan is performed. The patient is advised to rest for at least two days and use appropriate orthotics. If pain remission is not satisfactory within 2 weeks, a second treatment of corticosteroid injections or alcohol neurolysis may be undertaken.8

Procedural complications include infection, damage to adjacent tissues, plantar fascia contraction, lipo-necrosis, flushing, post-treatment flare, and the possibility of treatment failure. Multiple corticosteroid injections should be avoided because of the risk of fat atrophy, adjacent joint capsule degeneration and rupture.7

Outcomes

It has been shown that US-guided procedures offer superior results to non-guided interventions, including more efficient pain reduction, fewer complications, and reduced referrals for further surgical management. The size of MN has been inversely correlated with the success since 84% of MN with a length >6.3 mm did not achieve pain remission after the first steroid injection.7 According to a recent meta-analysis, the treatment of MN by corticosteroid injections is less effective than USGAI in terms of permanent pain relief and patient satisfaction. Chemical neurolysis has been shown to provide short-term relief in up to 89% of patients but, in the long run, up to 30% of patients needed a repeat infiltration and 30% required surgical therapy.7,13 USGAI is gaining momentum in treatment of MN, with studies reporting an overall 71% satisfaction rate at 5 years.10 No robust long-term data exists on the efficacy of RF or cryoablation treatment of MN.

Visuals of procedure 


Conclusions

US is a novel tool, enabling diagnosis and guided interventional procedures for the management of MN without surgical treatment. Further research is needed to determine the long-term outcomes of various techniques to clearly define the long-term benefits and complications compared with open surgery. US is an operator-dependent imaging modality that requires a learning curve and, therefore, proves a limitation for those with lack of experience especially in the forefoot area where the structures are small in a constrained area.


References

1. Pasero G, Marson P. Filippo Civinini (1805-1844) and the discovery of plantar neuroma. Reumatismo 2006;58(4):319-22. https://doi.org/10.4081/reumatismo.2006.319

2. Morton T.S. VI. Metatarsalgia (Morton's painful affection of the foot), with an account of six cases cured by operation. Ann Surg 1893;17(6): 680-99.

3. Di Caprio F, Meringolo R, Eddine MS, et al. Morton's interdigital neuroma of the foot: a literature review. Foot Ankle Surg 2018;24(2): 92-8. https://doi.org/10.1016/j.fas.2017.01.007

4. Bhatia M, Thomson L. Morton's neuroma - current concepts review. J Clin Orthop Trauma 2020;11(3):406-9. https://doi.org/10.1016/j.jcot.2020.03.024

5. Galley J, Sutter R, Germann C, et al. The Vulcan salute sign: a non-sensitive but specific sign for Morton's neuroma on radiographs. Skeletal Radiol 2022;51(3):581-6. https://doi:10.1007/s00256-021-03851-3

6. Bignotti B, Signori A, Sormani MP, et al. Ultrasound versus magnetic resonance imaging for Morton neuroma: systematic review and meta-analysis. Eur Radiol 2015;25(8):2254-62. https://doi:10.1007/s00330-015-3633-3

7. Thomson L, Aujla RS, Divall P, et al. Non-surgical treatments for Morton's neuroma: a systematic review. Foot Ankle Surg 2020;26(7):736-43. https://doi:10.1016/j.fas.2019.09.0098  

8. Klontzas ME, Koltsakis E, Kakkos GA, et al. Ultrasound-guided treatment of Morton's neuroma. J Ultrason 2021;21(85):e134-38. http://doi:10.15557/JoU.2021.0022

9. Pasquali C, Vulcano E, Novario R, et al. Ultrasound-guided alcohol injection for Morton's neuroma. Foot Ankle Int 2015;36(1):55-9. https://doi:10.1177/1071100714551386

10. Gurdezi S, White T, Ramesh P. Alcohol injection for Morton's neuroma: a five-year follow-up. Foot Ankle Int 2013;34(8):1064-7. http://doi:10.1177/1071100713489555

11 Choi JY, Hong WH, Kim MJ, et al. Operative treatment options for Morton's neuroma other than neurectomy - a systematic review. Foot Ankle Surg 2022;28(4):450-9. https://doi:10.1016/j.fas.2021.10.011

12. Spinner DA, Eldon ER. MTP joint and Morton’s neuroma. In: Jankovic D, Peng P. Regional Nerve Blocks in Anesthesia and Pain Therapy: Imaging-guided and Traditional Techniques. 5th ed. Springer International Publishing; 2022.

13. Bucknall V, Rutherford D, MacDonald D, et al. Outcomes following excision of Morton's interdigital neuroma: a prospective study. Bone Joint J 2016;98-B(10):1376-81. http://doi:10.1302/0301-620X.98B10.37610
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