Surgery for Morton Neuroma


Background

Morton neuroma, also referred to as interdigital neuroma or interdigital neuritis, is a common condition that involves enlargement of the interdigital nerve of the foot. [1(See the image below.) It derives its common name from Thomas Morton, who, in 1876, first described interdigital nerve compression [2 and theorized that the nerve was being compressed between the metatarsal heads.

Plantar view showing the relationships between thePlantar view showing the relationships between the metatarsal heads, the intermetatarsal ligament, and the neuroma.

Interdigital neuritis is a mechanically induced nerve irritation due to intrinsic and extrinsic biomechanical factors that results in a combination of pain, paresthesias, and numbness in the forefoot. A greater understanding of forefoot anatomy and biomechanics has created an environment that facilitates further delineation of the pathophysiology of interdigital neuroma.

Surgical procedures for treating Morton neuroma have historically been aimed either at directly dealing with the pathologic nerve or at altering the local biomechanical environment in which the nerve exists. The evolution of surgical care has resulted in some basic principles and goals, which are the foundation for the current surgical options. There is a growing body of literature that elaborates on many aspects of this seemingly simple problem.

This article outlines some current controversies regarding Morton neuroma; the fact that the eponym is likely a misnomer is a reflection of these controversies. The histology, use of adjunctive imaging modalities, and choice of surgical procedure all involve some variance of opinion. Current and future work will be addressing these issues.

Anatomy

The medial plantar nerve has four digital branches that begin at the medial aspect of the great toe and then form the common digital nerves of the first, second, and third web spaces. The lateral plantar nerve supplies the fourth web space and, in 27% of patients, contributes to the third web space. This increased thickness of the third digital nerve was hypothesized to predispose it to mechanical irritation; however, studies have shown no correlation between those patients with this anatomic variant and those with symptoms of Morton neuroma.

The common digital nerves lie plantar to the intermetatarsal ligament and give off small branches to the plantar skin approximately 1-2 cm proximal to the bifurcation of the nerve. These branches also must be resected at the time of surgery because they may prevent nerve stump retraction after neurectomy and lead to a recurrent neuroma. [2]

Pathophysiology

The term neuroma, as applied to this condition, may be a misnomer. Morton neuroma may be best described as a clinical syndrome stemming from a constellation of factors related to the local anatomy and the forces applied on the forefoot with ambulation and shoe wear. [3 Interestingly, it is not uncommon for the histopathology to be interpreted as relatively normal, even in light of classic preoperative findings and dramatic postoperative results. Clearly, the histopathology is not that of a typical neuroma. [4 Some or all of the following may be observed:

  • Sclerosis of the endoneurium
  • Hyalinization of the walls of endoneurial vessels
  • Thickened perineurium
  • Demyelinization of nerve fibers

Etiology

Anatomic studies have revealed a wealth of information and helped to resolve some misconceptions regarding the etiology of this disease. The intermetatarsal space is narrower in the second and third interspaces than in the first and fourth interspaces, and this difference correlates with the clinical presentation pattern. [5 The composition of the tarsometatarsal articulation allows relative hypermobility between the second and third metatarsals, which contributes to mechanical irritation of the nerve.

Narrow toe-box footwear can exacerbate the compression between the metatarsal heads, and hyperextension of the toes in high-heeled shoes tethers the nerve beneath the ligament and may expose it to more biomechanical stresses with gait. [4 Less commonly, metatarsophalangeal (MTP) joint pathology, inflamed bursa, or lipomas can create compression at the level of the common digital nerve.

Epidemiology

Morton neuromas are more common in women, with a female-to-male ratio of 4:1. They tend to occur in the fifth decade of life and frequently are exacerbated by constrictive footwear. [4Symptoms are typically unilateral, with the third interspace most often involved, followed by the second and the fourth interspaces. [4Simultaneous neuromas occurring in the same foot are uncommon. [6]

Prognosis

In general, review of the orthopedic literature from the past several decades supports an estimate of 75-95% satisfactory results after surgical intervention, regardless of technique or approach. Nonsurgical management probably results in a 20-30% success rate.

Long-term postoperative outcomes have been quantified. [7891011]

Coughlin et al monitored 66 patients (71 feet, 74 neuromas) over an average of 5.8 years. [7A dorsal approach was used, and a neurectomy was performed 3 cm proximal to the intermetatarsal ligament. The results were as follows:

  • Histology was available on 72 of the 74 specimens, and all were neuromas
  • Rates were excellent in 61% (no problems, no or only mild pain), good in 24% (few problems, mild pain at most, willingness to have surgery again), fair in 8% (moderate pain, some difficulty walking, reservations about surgery outcome), and poor in 8% (continued pain and difficulty walking, regrets about having surgery)
  • Thirty percent had no shoe-wear restrictions, 53% had mild restrictions, and 17% had major restrictions
  • Sixty-five percent of the feet were pain-free, and 4% were severely painful
  • Complications included one deep infection requiring incision and drainage; six superficial infections resolved with antibiotics; intractable plantar keratosis in lesser metatarsal heads in 15% of the feet, one of which had MTP joint instability; and continued major shoe-wear restriction believed to be related to neuroma surgery in 8%

A study by Nery et al retrospectively followed 168 consecutive patients at a median of 7.1 years in whom nonoperative management had failed. [8All patients underwent magnetic resonance imaging (MRI) to rule out other pathology and, after neurectomy, had a pathology-proven neuroma. In contrast to the approach followed by Coughlin et al, the surgical procedure involved a transverse plantar approach and preservation of the deep transverse incision. The results were as follows:

  • A good result (no pain or paresthesia) occurred in 89%, a fair result (mild pain or paresthesia without recurrence) in 7%, and a poor result (complications or persistent symptoms) in 4%
  • In all patients with a poor result, a recurrent neuroma was found on a second surgery and resected; all were subsequently asymptomatic

The study authors concluded that the transverse plantar incision approach for Morton neuroma resection is comparable in outcomes to other approaches. They also stated that MRI is not needed for diagnosis but was required in these cases by insurance providers.

Akermark et al followed 55 patients with a longitudinal plantar incision for at least 2 years and found an 86% rate of overall satisfaction. [10 The complication rate in this study was low (5%), and may have been related to meticulous care taken to keep scar formation between the metatarsal heads.

Colgrove et al compared transposition with resection in a randomized trial. [9 At the 36- to 48-month follow-up, they found the resection group to have 86% excellent results and 14% fair results; meanwhile, the transposition group had 96% excellent results and 4% good results. The authors concluded that resection is unnecessary for symptom relief. To date, however, no repeat study has been conducted to confirm their results.

Kasparek et al reported good-to-excellent results in 75 (77%) feet at a mean of 15.3 years after surgery in their retrospective series. [11]

Reichert et al assessed the long-term results of dorsal neurectomy in 41 patients treated for Morton neuroma (average follow-up, 7.4 years; range, 5-12). [12 Mean American Orthopedic Foot and Ankle Society (AOFAS) score rose from 39.4 ± 7.84 preoperatively to 83.4 ± 12.1 postoperatively; mean visual analogue scale (VAS) score fell from 7.04 ± 1.4 preoperatively to 1.4 ± 0.8 postoperatively. Results were reported as very good in 31 patients, good in six, satisfactory in one, and poor in three. Single neuromas that were larger than 3 mm and were resected within 12 months of symptom onset were associated with the best results.

Kundert et al prospectively studied clinical outcomes and complications in 44 patients (51 feet, 56 neuromas) who underwent excision of Morton neuroma via a plantar longitudinal approach. [13 Mean VAS score fell from 8 preoperatively to 0.4 at final follow-up (mean, 54 months; range, 12-99). The overall complication rate was 7.1%, and the incidence of scar problems (eg, delayed wound healing, hypertrophic scar formation, and inclusion cyst) was 5.2%.

Bucknall et al performed a prospective study aimed at determining  pre- and postoperative outcomes and satisfaction scores in 99 consecutive patients (78 women, 21 men; 111 feet) who underwent excision of Morton neuroma. [14They documented statistically significant differences between mean pre- and postoperative scores on the Manchester-Oxford Foot Questionnaire (MOXFQ) and the physical component of the Short Form (SF)-12.

In this study, 10 patients were dissatisfied, describing results as poor or very poor; however, most were satisfied, describing results as good or excellent. [14At follow-up, 63 of the 99 patients were pain-free; eight had worse scores than they had before the procedure. No statistically significant difference in outcome was found between single-site surgery and multiple-site surgery. Revision surgery was associated with significantly worse MOXFQ scores.



History

Patients typically present with pain and paresthesias localized to one interspace of the forefoot with radiation to the involved toes. The symptoms may wax and wane over many years and are exacerbated with compressive shoewear. Relief is common when the shoes are removed and the area is massaged. [15]

Physical Examination

A provocative examination (see the image below) involves manually compressing the forefoot and simultaneously palpating the affected web space between the fingers of the other hand. The compression may result in the Mulder sign, which is a painful and palpable click that reproduces the symptoms. [16]

This image demonstrates the key provocative physicThis image demonstrates the key provocative physical examination maneuver: simultaneous forefoot compression coupled with intermetatarsal space compression.

Imaging Studies

A large body of literature addresses the use of imaging modalities to aid in the diagnosis of Morton neuroma. [17The condition is most commonly diagnosed strictly on the basis of the history and physical examination findings. In equivocal cases, ultrasonography (US), magnetic resonance imaging (MRI), or both may be helpful. [18192021]

Standing radiographs are helpful for evaluation of overall osseous structure and joint abnormalities.

The skill of the technologists and physicians performing and interpreting MRI or US plays a key role in determining just how valuable these modalities can be in clinical practice.

On US, a Morton neuroma typically appears as a hypoechoic oblong mass oriented along the long axis of the foot. Prospective studies in symptomatic patients reveal that US can reliably detect these neuromas. [1722Furthermore, it can help identify contributing pathologic lesions, such as lipomas or bursae. [23Nonetheless, in a small series, 26 (54%) of 48 asymptomatic patients had greater than 5 mm enlargement of an interdigital nerve on US. [21This finding emphasizes the importance of the history and clinical examination and the risk of false positives.

US may be used to determine the size of a Morton neuroma, but its reliability appears to depend, to a large degree, on observer experience. [24It may also be used to identify posttreatment changes (eg, ill-defined borders, reduced or absent pain with the application of transducer pressure) after laser therapy for Morton neuroma. [25]

MRI technology and applications continue to evolve and can provide reliable information regarding the pathoanatomy of the forefoot. [26Characteristically, a Morton neuroma shows low intensity on T1- and T2-weighted MRI sequences, because of its high degree of fibrous content. In contrast, an intermetatarsal bursa is associated with increased intensity on T2-weighted images, because of its fluid content. [27MRI with gadolinium contrast enhancement and with fat suppression probably provides the most reliable images for diagnosis. [28]

Other Tests

Electrodiagnostic testing has been described to aid in diagnosis, but its use is not common, and its utility has yet to be determined. [2930]

Procedures

An injection of local anesthetic with sterile technique may be useful in the diagnostic evaluation of Morton neuroma. [7The temporary resolution of pain, paresthesias, or both in response to the injection may confirm the location of the pathology. A positive response to the injection tends to be predictive of satisfactory postoperative outcomes.



Approach Considerations

The indication for surgery is significant pain or dysfunction that is not tolerated by the patient and that fails to respond to conservative management.

Once the diagnosis is made and conservative management has failed, few contraindications for surgical intervention exist. Local soft-tissue factors, such as infections or vascular insufficiency, are likely to be the only absolute contraindications. Patient psychosocial issues may be relative contraindications and must be addressed prior to surgical procedures of any kind.

Medical Therapy

The biomechanical goals of nonsurgical management are to reduce forefoot compression and to eliminate the effect of intermetatarsal ligament tension. In theory, this decreases the overall pressure on the nerve and allows it to function more normally. Shoe modifications are simple to understand but may be objectionable to some patients. Wide toe-box shoes are preferred to relieve pressure across the metatarsal heads. High-heeled and narrow shoes should be avoided. The use of a metatarsal pad orthotic device can help keep pressure off the nerve. [31]

The literature is conflicting regarding the success rate for nonsurgical management, but a reasonable estimate would be that 20-30% of patients would obtain either total resolution of symptoms or improvement to a satisfactory level with footwear modifications and restrictions. [32]

Corticosteroid or local anesthetic injections may be helpful, especially when coupled with the aforementioned shoe modifications. A dorsal injection is administered with 40 mg of methylprednisolone or a similar corticosteroid and 1% plain lidocaine or 0.5% plain bupivacaine in the same syringe. To avoid steroid-induced skin necrosis, do not inject into the superficial subcutaneous tissue. Ultrasound-guided injections are likely to yield better results than blind injections. [33]

Injections should not be used indiscriminately, because the injection itself is associated with mild risk. High-quality evidence supports the use of corticosteroid injections for short-term (3-month) symptom relief. [3435Symptom relief may last longer in smaller neuromas (< 5 mm). [35]

Alcohol sclerosing injections should be used with caution, in that they have not been shown to be reliably effective. [36373839]

Nonsteroidal anti-inflammatory drugs (NSAIDs) or antiseizure medications such as gabapentin or pregabalin (off label) have been used for symptom management, but their efficacy is not known. [4]

A study by Masala et al (N = 52) found percutaneous radiofrequency ablation (RFA) to be a safe, efficient, and minimally invasive technique for the treatment of symptomatic Morton neuroma. [40]

 

Surgical Therapy

Plantar vs dorsal approach

Current surgical care for Morton neuroma involves a few different options for either decompressing or resecting the nerve. [4142The first consideration relates to the choice of surgical approach: plantar or dorsal. The dorsal approach has the advantage of low wound complications, lower incidence of plantar scar formation, and immediate weightbearing. The plantar approach is more direct, requiring less dissection, and preserves the transverse metatarsal ligament, theoretically reducing metatarsal instability.

A randomized controlled trial comparing dorsal and plantar approaches showed similar clinical outcomes and rates of complications between the two, but with different types of complications occurring in each group. [43The plantar group most commonly had minor hypertrophic scar–related complications, whereas the dorsal group's complications were varied. A single retrospective review from the same author also found the two approaches to be comparable. [44]

A study by Habashy et al that compared plantar (n = 17) and dorsal (n = 20) approaches to neurectomy found no statistically significant differences between the dorsal- and plantar- approach groups with regard to outcomes and patient satisfaction. [45]

The plantar approach is through a 3- to 4-cm transverse incision centered over the neuroma on the distal metatarsal fat pad and away from the weightbearing area. [8This approach provides direct exposure of the nerve, which lies plantar to the intermetatarsal ligament. Alternatively, a longitudinal incision can be made in the interspace. Weightbearing is not recommended until the sutures are removed (2-3 weeks). (See the images below.)

Distal plantar transverse incision. Distal plantar transverse incision.
Plantar longitudinal incision. Plantar longitudinal incision.

The dorsal approach is through a similar-sized (3-cm) longitudinal incision centered on the metatarsal head interspace. [467A small Inge retractor or laminar spreader is used to widen the intermetatarsal space and apply tension to the transverse metatarsal ligament. The ligament is then transected. The neuroma can then be identified. This approach allows immediate postoperative weightbearing. (See the images below.)

Dorsal incision. Dorsal incision.
Superficial exposure. Superficial exposure.

Regardless of the approach, it is prudent to inspect the area carefully to look for concurrent pathology (eg, lipomas, osteophytes, or inflamed bursae). Care must also be taken to preserve the digital vascular structures.

Choice of definitive operation

After the decision is made as to which approach to use, the definitive surgical procedure must be performed—another somewhat controversial issue.

One option is simply to divide the intermetatarsal ligament and thereby decompress the intermetatarsal space. This decompression may be coupled with neurolysis, depending on the surgeon's preference. Proponents of this option generally believe that decreasing the stresses on the nerve is the key to successful treatment and that if the nerve is preserved, some normal nerve function can return. Furthermore, if no nerve tissue is resected, there is no chance that a recurrent (stump) neuroma will form. A deep transverse metatarsal ligament release combined with a metatarsal-shortening osteotomy has been shown to yield better outcomes than release alone. [47]

A second option is to perform a neurectomy (see the images below). Proponents of this approach believe that the key to successful treatment is eliminating the pathologic nerve tissue and maintain that decompression would only decrease the ongoing mechanical stresses on that abnormally functioning nerve. It is very important to transect the nerve as far proximal to the bifurcation as possible. Multiple authors promote transecting the nerve 3 cm proximal to the proximal border of the intermetatarsal ligament; this allows proper resection of the plantar branches that may otherwise prevent nerve stump retraction away from the weightbearing surface. [748]

Deeper dissection. Deeper dissection.
Neuroma and adherent fibrofatty tissue. Neuroma and adherent fibrofatty tissue.

A third option is a transposition of the cut digital nerve. In this case, the digital nerve is transected sharply distal to the bifurcation. The common digital nerve is then dissected free and transposed into the interval between the transverse head of the adductor hallucis and the interossei muscles. [9]

Ratanshi et al retrospectively assessed neuroma excision with interpositional nerve grafting as a therapeutic option for Morton neuroma in eight patients (nine neuromas) in whom nonoperative therapy had failed. [49 Patients were followed for at least 1 year after the procedure. When last seen, all eight patients reported improvement in their pain, and no neuroma recurrences had developed. Seven of the eight regained sensation in the grafted hemitoe, and all eight resumed full weightbearing. There were no major complications associated with the procedure; however, one patient experienced wound dehiscence secondary to hematoma.

Postoperative Care

Postoperative care depends on the surgical approach. For the dorsal approach, the patient begins weightbearing as tolerated in a postoperative hard-soled shoe, with gradual introduction of appropriate footwear as tolerated once the wound has healed and swelling has resolved. For the plantar approach, weightbearing is restricted to the heel only until the incision heals, so as to prevent plantar wound complications.

Complications

An expected complication of the neurectomy is loss of cutaneous sensation of the interdigital skin. For most patients, this is not clinically significant. A variable amount of numbness may be present on the plantar skin, just proximal to the web space. Some authors believe that this can be more problematic and thus may be more likely to perform surgical decompression of the nerve (and not resect it) to avoid this complication. [2]

In the rare cases where simultaneous adjacent neurectomies have been performed, the toe is affected by a more dense numbness from the metatarsal head to the tip of the digit. One study showed that this increased numbness caused some awkwardness with nail care but no significant disability. [50]

Intractable plantar keratosis underneath adjacent metatarsal heads has been identified as a potential complication, likely as a result of weightbearing mechanics that were altered by releasing the intermetatarsal ligament. [7Wound complications are typically minor when they occur. Antibiotic treatment or debridement may be indicated.

Recurrent or persisting symptoms after surgical intervention may relate to a number of factors and can be difficult to treat. Patients who have had the decompression type of procedure may continue to have problems if the decompression was incomplete or if the nerve simply remains irritable. Those who have had neurectomy may develop a stump neuroma that may be even more painful than the original problem.

Surgical intervention may be indicated should the persisting symptoms be intolerable and fail to respond to conservative measures. [51In operating on a recurrent neuroma, the plantar approach provides more direct access to the nerve while avoiding the scarred tissue planes of the initial dorsal approach.

Long-Term Monitoring

Follow-up depends on the individual surgeon's preference. The preferred number of visits and their timing vary from one surgeon to another; however, in general, sutures are usually removed after 2-3 weeks, and suture removal is followed by one or two more visits at 3 and 6 months.

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