What is a Morton's Neuroma?

Microscopically we see cellular and structural changes to the nerve and the surrounding soft tissue structures. The histopathology tells us it’s not just the nerve itself that we are seeing cellular changes in, but also the vascular structures, in particular the arteries where there is endarterial thickening with luminal narrowing. The schwann cells can show loss of cylinders from the nerve bundles or they may undergo proliferation depending on where the nerve is in terms of its injury. Adipose tissue can become necrotic, we can see areas of myxoid degeneration and there can be infiltration of inflammatory cells in the bursal tissue.

Despite all these findings, a neuroma will always show PERINURAL FIBROSIS – thickening. It’s not until we look under the microscope and see the cellular detail of the nerve and its adjacent surrounding structures that we can appreciate the pathogenesis theories, that I will explain a little later.

The key message is that Morton’s Neuroma originate through adaptive changes and footwear becomes very important in reducing development risk and managing symptoms.

Patient presentation

The presentation of a patient with Morton’s neuroma typically starts with the description of ill-defined plantar forefoot pain. Pain is exacerbated by tight shoes and the patient may describe anything from altered sensation, which occurs on and off throughout the day to more profound paraesthesia with and without higher intensity sharp shooting/lancinating type pain. It can often lead to intractable pain not responding to conservative treatment and interfering with patients walking activity.

Theories why Morton's Neuroma develop

Well, there are four main ones which we will explore, and the first three are interrelated.

The first theory is Chronic Trauma theory from chronic repetitive mechanical trauma to the nerve because of structural and/or mechanical effects during weight bearing that puts pressure on the nerve. This is underpinned by neurodynamics and the lack of glide or slide on the nerve between the third and fourth metatarsophalangeal joint (MTPJ) within its neighbouring tissue. We know the space between the 3rd and 4th metatarsal heads is the most common location for these lesions. Anatomically the third digital nerve has a double origin and is thicker, increasing vulnerability of the nerve to trauma. Also the third nerve is less mobile and tethered to the adjacent flexor digitorum brevis muscle belly, which restricts movement when the ankle and toes dorsiflex ⁽¹⁴⁾.

The second theory is that described by Bossley and Cairney in 1980 ⁽⁷⁾, suggesting the intermetatarsal bursa is responsible. The neurovascular bundle of the digit lies very close to the bursa at its most distal and inferior aspect and should there be an increase in the volume of the bursa, the nerve becomes adherent to the bursa. Bossley & Cairney found that bursae are consistently found intermetatarsal 2/3 and 3/4 but not 1/2 or 4/5. So, this may be why we are seeing neuroma more consistently in these interspaces, and it may be the inflammation of bursae that creates fibrosis of the underlying nerve.

The entrapment theory described by Gauthier in 1979 ⁽¹⁵⁾ identified the distal transverse intermetatarsal ligament at its most anterior edge as the site where the nerve is compressed or squeezed in late propulsion during the gait cycle. Continued irritation leads to thickening and with ongoing chronicity of compression injury we get proliferation of Schwann cells. So it is similar to the bursitis theory where chronic compression is the instigating element.

Ischaemic theory involves sclerosis of the arteries and narrowing of their lumen to contribute to ischaemia that then goes on to create nerve changes and alterations in Schwann cell function ⁽¹⁶⁾. Maybe, this is what we are seeing in systemic connective tissue conditions like rheumatoid arthritis where multiple neuroma are most common.

Assessment & examination

A simple forefoot squeeze to compress the metatarsal heads in the transverse plane or squeezing the nerve in the sagittal plane between your thumb and finger will reproduce symptoms. You can combine the two manoeuvres within the one as transverse compression (forefoot squeeze) and direct compression between your fingers to produce a ‘Mulders sign’ or palpable click.

In 27-84% of patients with neuroma, there may be reproduction of nerve pain such as shooting, lancinating or increased tingling. Sharp et al. ⁽¹⁷⁾ report a high positive predictive value of clinical assessment in the diagnosis of Morton’s neuroma.

Weightbearing observation may show separation of the toes between which the neuroma is present that can be described as ‘V-sign’. This most likely represents a ‘large lesion’. However, you need to rule out a plantar plate tear which causes such deviation and other soft tissue tumours.

What other tests can we use to help the diagnosis?

Ultrasound still remains the modality of first choice for confirming a clinical suspicion of Morton’s neuroma. There are a few studies that showed that ultrasound provides better accuracy or is as good as MRI. Typically, it shows a neuroma as an oval hypoechoic mass orientated parallel to long axis of metatarsals. Ultrasound is best able to pick up lesions 5mm or greater and you can also check for inflammatory changes and effusions.

Non operative treatment

1. Modified activities and ‘don’t do what hurts until it doesn’t hurt to do it anymore’

2. Change of shoe style (wide toe box, lace up)

Saygi et al. (12) showed comparative results of two conservative treatments, steroid injections versus shoe modifications, in a randomised clinical trial with 12 months follow-up. At 6 months, 74 % of patients reported good improvement in the steroid group versus 27% in the shoe modification group. After 12 months this increased to 82% in the steroid group and 63% in the shoe group. Findings indicated that both cortisone and footwear modifications produced positive outcomes. From a clinical practise perspective, this is the only study which replicates what occurs in the clinical setting for conservative management and hence has most relevance for a health professional.

Three special features to consider in footwear to prevent aggravation of neuroma on the basis of the pathogenesis theories include:

3. Functional orthoses (metatarsal and arch support)

A recent randomised clinical trial by de Oliveira et al. ⁽¹³⁾ investigated the effectiveness of customised foot orthoses with metatarsal and arch support compared to flat insoles for relieving symptoms in Morton’s neuroma. They found that patients using foot orthoses had reduced pain when walking compared to the control group (flat insoles).

When used as part of a treatment plan, the FRANKIE4 Support Footbeds are much like a soft pre-fabricated (off the shelf) support insole, a great base for podiatrists and physiotherapists or other skilled people to customise (if needed). Our Support footbeds have soft contoured support that our customers report to be very comfortable according to verified buyer reviews. Supportive additions such as metatarsal dome padding can be added in the forefoot area in a position specific to wearers needs.

4. Pain medications (e.g. anti-inflammatories)

• Cortisone injections
• Serial local anaesthetic injections
• Sclerosing agents
• Extra Corporeal Shock wave therapy

Surgery

Studies indicate a range of satisfaction outcomes after surgical management ranging from 60% to 96% in uncontrolled trials ^{(11, 18, 19)}. It is worth asking the question why some patients are NOT happy after having a neurectomy.

If we look at little deeper into some studies, such as the one by Lee et al. ^(18), which is a 10-year follow up of neurectomy, we identify why patient outcomes may not be as good as they could be. Although the visual analogue scale improved in all patients and 61% reported good satisfaction, there was a great deal of ‘numbness’ of the toes reported by 11 out of the 13 patients and this appeared to affect the satisfaction results.

We as surgeons expect the interdigital area to be NUMB, because we are removing a nerve whose role is to provide sensation/feeling. This needs to be communicated to the patient as something to expect. This is an important consideration that needs to be discussed prior to surgery to prepare the patient and make them aware that altered sensation in the webspace can be a consequence of removing the nerve. As I see it, it’s not a complication as such, because we know we are removing a structure that allows you to feel sensation and a consequence is altered sensation/numbness.

Again, I want to focus on the relatively poor outcome of another paper, this one by Kasparek and Schneider ⁽²⁰⁾, which helps in improving our understanding of surgical outcomes. In this retrospective review, although just short of 80% reported good to excellent outcomes after surgery, 12% and 8.2% reported fair and poor results respectively, such that patients reported similar pain after their surgery compared to before their surgery.

I feel that inadequate resection (that is, the nerve truck is not cut away proximal enough) may be what is responsible for the poor outcome reported in this paper, and the high rate of second or revision surgery to remove what should have been removed in the first place. This is also supported by the histopathology reports after the second revisional surgery that indicated it was a neuroma. So, the neuroma in fact had not been removed in its entirety.

Does size matter?

Yes. You do need to consider size. A large neuroma is not likely to respond to non-operative treatment. Reducing intermetatarsal compressive/shear loads with an orthosis is not going to be enough with a large lesion. Mobilization or massage is not going to cut it. Even with sclerosing agents and cortisone, there is a lot of shrinking to be done to get a desirable effect with a large, thick, dense, fibrotic lesion. Size is important. Ask the radiologist to provide a measurement when ordering your ultrasound. Correlate this knowledge of size with the physical exam and degree of symptomology and see if it all make sense.

However, don’t underestimate the pain derived from small lesions. Don’t forget that pain is subjective. Small lesions are equally able to cause pain as much as larger lesions. So while a large lesion is not necessarily more painful than a small one, I feel that a large lesion is more recalcitrant to non-operative treatment than a smaller one.

Pain reported by the patient, the history and results of physical examination, irrespective of whether ultrasound shows pathology or not is still meaningful. The absence of frank pathology is meaningful. It’s likely to be something else such as a ‘neuritis’ or bursitis or adhesive capsulitis. If that’s the case you still need to intervene depending on the impact on quality of life and overall daily pain, because we treat human beings, not scans.

Consider the non-operative treatment I have discussed, as in my view, these non-operative treatments are going to be effective with small or no lesions. Small or no pathology is GOOD so communicate this to the patient because size does matter. You may be having another discussion with your patient if there was a 12mm lesion on musculoskeletal ultrasound, about prognosis with non-operative treatment and the need for further possible surgical intervention because of the size.

Redd et al. ⁽²¹⁾ showed using ultrasound that neuromas can be present but not necessarily painful, and that neuromas >5mm are more likely to cause symptoms. So 5mm is the number to keep in mind, and the number that makes a neuroma a neuroma.

Why are some patients not satisfied following neurectomy despite a successful procedure?

Consequences & complications of surgery

Let’s look at the consequence of neurectomy and also some of the complications. These are very different things. Consequences are an expected outcome because we are removing a nerve that is typically part of the human body (albeit it’s a pathological one).

Consequence of removing a neuroma includes: Persistent numbness of lesser digits, including scar sensitivity; Sensory deficit – post-op numbness of varying degree typically in the web space. The clinical outcome was not influenced by sensory deficits in our patients. Most of them reported that they did not have limitations in daily life and that they had been informed about it in detail before surgery. This shows that detailed information before surgery is important for the patient’s postoperative satisfaction ⁽²⁰⁾.

1. I warn my patients that when they walk barefoot that they are likely to feel a lump/swelling under the foot and that this is normal and resolves after 3 months.
2. Tenderness in involved or adjacent interspace is common in 60% of patients (often not recognised by patient).
3. Shoe wear restrictions may still apply, so warn patients that they may not be able to get back to their narrower higher heeled footwear.
   
   

Complications, which we don’t want or expect to happen, include:

1. CRPS: consider peri-operative 1g vit C. There is evidence to suggest that prophylactic use of Vit C reduces this risk.
2. Severe instability of the MTPJ which can lead to deviated digits/hammertoes.
3. Regrowth/stump neuroma.
4. Peripheral neuropathy.

Conclusion

DIAGNOSIS remains key. You want to know what you are dealing with in respect to size and number of lesions. Size matters in terms of whether or not we are likely to get a successful outcome with non-operative treatment. The bigger the lesion, the less likely it is to respond and the more likely to need surgery.

Musculoskeletal ultrasound is the modality of choice for diagnosis, however remember you need to correlate your physical exam with patient history to make sense out of it.

Treat the patient not the scan. Small lesions can be as painful as large ones, and are more likely to respond to non-operative treatment than large ones.