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13 Articles in Volume 11, Issue #7
Fibromyalgia: Practical Approaches To Diagnosis and Treatment
Juvenile Fibromyalgia: Diagnostic Challenges and Treatment Options
Aqua Therapy Helpful in Treatment Of Systemic Lupus Erythematosus
Axial Neck Pain, Radiculopathy, and Myelopathy: Recognition and Treatment
Early Treatment of TMD May Prevent Chronic Pain and Disability
Identifying Psychological Factors That Influence Surgical Outcomes
Managing Morton’s Entrapment
Premedicated Mask May Hold Promise for Migraine Patients
Mother With Low Back Pain
The Hip Replacement Patient
Evidence-based Medicine: Losing the Patient’s Voice?
What Is Going Wrong With Research?
Risk for Sedation and Car Accidents

Managing Morton’s Entrapment

Often misunderstood, Morton’s entrapment is best treated by peripheral nerve decompression rather than surgical nerve resection.

Of the most common foot conditions treated by clinicians today, none is probably more misunderstood than Morton’s entrapment. First referred to as “Morton’s neuroma” in 1958,1 this condition is nothing more than an entrapment of a peripheral nerve.2 However, confusing and erroneous nomenclature has perpetuated a number of misguided but commonly employed treatments, which, in the authors’ opinions, are still considered within the “standard of care.”3 In fact, in 2009, the American College of Foot and Ankle Surgeons, noted that “Morton’s intermetatarsal neuroma is a compression neuropathy of the common digital nerve.”4

Dellon poignantly states that “there is no other human chronic nerve compression syndrome in which the treatment recommended is resection of the nerve with expected loss of its function.”5 As a true nerve entrapment no different in pathology from carpal tunnel syndrome, Morton’s entrapment treatment should be oriented toward decompression. Indeed, the treatment success rate of peripheral nerve decompression in Morton’s entrapment is higher than with surgical resection, has a much lower complication rate, and precludes serious complications associated with nerve excision.

The article will review diagnosis and treatment options, including less-invasive decompression options, for management of Morton’s entrapment and recurrent neuromas.

Entrapment or Neuroma?
In Morton’s entrapment, the common plantar digital nerve, also sometimes referred to as the intermetatarsal nerve, gets compressed from forefoot plantar pressure in the late midstance and propulsive phases of gait against the distal margin of the transverse intermetatarsal ligament (TIML) (Figure 1).

Figure 1. The etiology of Morton’s entrapment, with the intermetatarsal nerve impinged by the intermetatarsal ligament. Note that this figure demonstrates the second interspace, which is much less frequent than third interspace involvement.

In peripheral nerve physiology, it is well understood that the term neuroma implies a specific pathophysiology in response to a true nerve injury6 and not degenerative, as seen in Morton’s entrapment, which is histologically compression neuropathy.7,8 Although it has been generally accepted and reported in the medical literature that there is a very high success rate, usually reported at about 85%, with resection of the Morton’s entrapment, Womack and Richardson9 recently published compelling data in direct contradistinction to these success rates. In their series of 120 patients who underwent resection for “Morton’s neuroma,” only 50% had a good or excellent result, 10% had a fair result, and 40% had a poor outcome.

Over the past decade, other treatments, such as alcohol sclerosing, radio-frequency ablation, and cryoablation, have been popularized as successful treatments.10-18 Although there have been overall equivalent success rates in or higher than the 80% success rate range reported with these treatments compared with surgical resection, it is undeniable that what all of these treatments have in common is attempted destruction of the peripheral nerve with some level of peripheral nerve injury in a nerve that simply has an area of focal compression. Recently, in 2011, Espinosa et al reported that their group had only a 22% (7 of 32 patients) success rate with ultrasound-guided alcohol sclerosing injections for the treatment of Morton’s neuroma (see Table 1).19-21

Table 1. Studies That Evaluate Efficacy of Alcohol Injections as Treatment of Morton’s Entrapment/Neuroma

Diagnosis
The diagnosis of Morton’s entrapment is highly dependent on the patient’s medical history and physical examination. Common symptom descriptions include some or all of the following: “It feels like my sock is wadded up under my foot,” “cramping,” “numbness,” “burning,” “radiating sensations into the adjacent toes,” “the inability to walk barefoot on a hard floor,” and “tingling.” Usually, patients with Morton’s entrapment demonstrate pain with plantar palpation of the interspace between the metatarsal heads.

Figure 2. Dramatic hammering of the second digit after treatment with injectable corticosteroid for treatment of proposed “neuroma.” Note the dorsal angulation of the second proximal phalanx, indicative of plantar plate rupture.

The examiner needs to be cautious, as patients frequently are diagnosed with nerve pathology of the forefoot when in fact there may be a global condition involving the whole foot, specifically the metatarsal head or plantar plate adjacent to the interspace. This is especially true when the patient has had previous corticosteroid injections. Such repeated injections can result in weakening and possible rupture of the plantar plate, which in turn causes hammering of the digit, as seen in Figure 2. Such an occurrence is reported by the patient as a sudden change in digit placement that is accompanied by pain, edema, and possible bruising to the ball of the foot just proximal to the affected digit.

There is a reported decreased success rate of treatment with the second interspace compared with the third interspace19 with both alcohol sclerosing injections and endoscopic decompression.22,23 This is likely due to the hypermobility of the first ray, which allows for overloading of the second metatarsal head during gait. As the second metatarsal head becomes overloaded, the pressure in the second intermetatarsal space simultaneously increases. It has been shown that with a gastrocnemius equinus, there is significant overloading of the forefoot during gait, which can act as an exogenous source of nerve compression.24-31 Successful treatment of forefoot nerve entrapment has been well documented via surgical treatment to address the equinus with an endoscopic gastrocnemius recession.32

Physicial Examination
The Mulder’s sign is a well-recognized clinical test in which the examiner compresses the forefoot from medial to lateral while pressing on the plantar aspect of the affected interspace—with a positive sign being pain accompanied by a “click” or “pop.” Gauthier, who published excellent results on decompression in 1979,33 was given the honor of a clinical test being named after him, the “Gauthier’s test,” in which the interspace is compressed from dorsal to plantar and the foot is compressed medial to lateral simultaneously. Finally, a Bratkowski test is considered positive if the patient is shown to have pain with plantar palpation of the affected interspace while the toes are being dorsiflected or hyperextended and if a nodular mass or thickening is revealed.5

Magnetic resonance imaging and diagnostic ultrasound studies can be helpful in the diagnosis of forefoot nerve entrapment but are not definitive.34-38 Diagnostic ultrasonography of the forefoot is highly dependent on technician and equipment, but is very effective in ruling out other forefoot pathology, such as plantar plate rupture (which in many cases is caused by steroid injections as part of the initial treatment) and the presence of any bursal structures.39

There is a correlation between failed nerve resection for Morton’s entrapment, resulting in a recurrent Morton’s neuroma and tarsal tunnel syndrome. Wolfort and Dellon found that of the 13 cases they treated for recurrent Morton’s neuroma, 54% had coexistent tarsal tunnel syndrome.40 This begs the question: Did the initial neurectomy fail because there was a more proximal entrapment at the medial ankle of the tibial nerve, leading to an inaccurate diagnosis? Consequently, it is recommended that all patients presenting with symptoms of forefoot nerve entrapment have an examination of the entire lower extremity peripheral nerve anatomy—specifically the tarsal tunnel.

Diagnostic lidocaine blocks are extremely beneficial in assisting in making an accurate diagnosis and can help the practitioner decide on a course of treatment, specifically when two adjacent interspaces are symptomatic. This technique requires skill in administration and interpretation of results. For example, the fact that a small amount of lidocaine placed just in the area of the common plantar digital nerve eliminates all of the patient’s pain does not mean that all of the symptoms are coming from that perceived nerve entrapment. However, this could mean that the afferent signal from pathology of other adjacent structures in the area has been blocked. Regardless, the use of selective diagnostic nerve blocks in evaluating the cause of forefoot pain is invaluable and highly predictive. It also allows the practitioner to separate symptoms and pathology when two interspaces are affected.

Table 2. Studies That Evaluate Efficacy of Corticosteroid Injection as Treatment of Morton’s Entrapment/Neuroma

Conservative Care
It has generally been recommended that prior to surgical treatment for Morton’s entrapment, all methods of “conservative” care must be exhausted, including but not limited to corticosteroid injections, application of offloading pads, sclerosing injections with alcohol or phenol, and radiofrequency ablation. This ideology is probably correct if the practitioner still erroneously believes that this condition is something other than a peripheral nerve entrapment. However, as previously delineated, this condition is nothing more than a peripheral nerve entrapment, and except for offloading and modification of shoe gear, the other types of “conservative” treatments actually impair ultimate surgical outcome and allow for a continuance of axonal degeneration at the level of the focal entrapment.

The development of hammer digit syndrome from plantar plate rupture resulting from steroid injections cannot be considered “conservative” by any means. It has been demonstrated in the upper extremity, and our experience in the lower extremity is identical—that the use of corticosteroids decreases efficacy of decompression/neurolysis.41 As shown in Table 2,42-46 there is a wide range of success rates for the use of corticosteroid injections in the treatment of Morton’s entrapment. These results do not show any sequelae from injection, such as plantar plate rupture or fat pad atrophy, which already has been discussed, and do not deal with impairment of decompression outcomes when treatment failed and surgery was required. Rasmussen et al found that with a single steroid injection, 54% of patients had fewer than 3 months of relief, and 47% of patients ended up having a surgical excision.43 Although the use of a single injection cannot be recommended as a cure for symptoms in patients with Morton’s entrapment, the injection did not obviate successful surgery, concluded the authors.

Figure 3. Endoscopic transection of the intermetatarsal ligament.

Neurolysis vs. Neurectomy
Since 1979, when Gauthier described his excellent results with neurolysis in 206 patients with 304 “neuromas,” of whom 83% had good, 14% had fair, and only 3% had poor results, there have been numerous studies that support decompression/neurolysis over surgical resection.33 In addition to the reported success rates, there is the important question of neurologic and quality-of-life downgrade. For example, Pace published that in 78 patients treated by excision of the interdigital nerve, 82% reported excellent or good results, 10% fair, and 8% no improvement—however, 71% had restrictions with footwear.47

Both open and endoscopic decompression of the common plantar digital nerve have been successfully reported. Success rates may not be equivalent to resection, decompression did not produce neurologic downgrade, and there was much less postoperative morbidity.22,23,48-54

Endoscopic Decompression
Based on the work of Alexander,2 Gauthier,33 Dellon,51 and Diebold,54 endoscopic decompression of intermetatarsal nerve (EDIN) for the treatment of Morton’s entrapment was introduced by Barrett in 1993.23 Initial clinical results were excellent. Of 69 patients with 96 interspaces decompressed endoscopically, 86% had good or excellent results—14% of patients reported poor subjective rating.55 Although 14% of the series self-reported a poor result, no significant complications were encountered.

In 2010, a multinational, multisurgeon study was presented that documented a 95% success rate of third interspace decompressions, an 85% success rate of good or excellent results in the second interspace in 193 interspaces, and an overall success rate of 92%.22 Only 8% of the interspace decompressions were rated as poor. There were no revisions required in the isolated third interspace group, and in situations where both interspaces (second and third) were decompressed simultaneously, there was a 58% good, 33% fair, and an 8% poor rating22 (see Figures 3 to 5, pages 49 and 59).

Figure 4. Cannula view of the proximal margin of the intermetatarsal ligament.

 

Figure 5. Intraoperative photo showing the hook blade being passed interdigitally into the cannula alongside the scope.

The advantages of endoscopic decompression vs. open resection of the nerve include a much faster return to regular activity and less postoperative morbidity.52 Patients are allowed to be fully weight-bearing, if tolerated, immediately after EDIN, and can return to their regular shoes the next day.55

Treatment of Recurrent Morton’s Neuroma
In cases of failed neurectomy, the authors advocate two approaches: primarily radiofrequency ablation and secondarily mini-plantar approach neurectomy with transposition of the neuroma into the muscle of the quadratus plantae in the non–weight-bearing portion of the arch (see Figure 6, page 60).

Transposition of the stump of the common plantar digital nerve into innervated skeletal muscle provides the best environment for decreasing both pain and further neuroma formation.3,6 Although results of both of these procedures have been minimally reported,40 their success rates, in our experience, have been very high, in the 90th percentile.

Figure 6. Plantar view of the minimal incision used to access the intermetatarsal nerve transaction and implantation into the quadratus plantae. Markings on skin illustrate the location of the metatarsal heads and intermetatarsal space.

Surgical Technique: Radiofrequency Ablation of the Intermetatarsal Nerve
We recommend that there are two cases in which Morton’s entrapment should be treated with radio-frequency ablation: failure to reduce symptoms with decompression/neurolysis and previous damage to the intermetatarsal nerve incurred by iatrogenic means (eg, chemical injections, neurectomy) or secondary to a traumatic event. Proceeding straight to radiofrequency ablation before attempting decompression on a nontraumatized nerve is not recommended as initial surgical treatment.

Radiofrequency, as well as destructive procedures, including tumor and varicosity ablation, have been used to treat such nerve pathologies as arrhythmias and back pain. The focus of these treatments is to change the protein structure of cells by inducing a thermal injury to the cells in question, rendering them nonfunctional. Application of such a modality for the treatment of the aforementioned nerve pathology in the foot should be intuitive, not only because of ease of use, but also because it causes decreased invasiveness and trauma to the patient. It is important to note that appropriately placed diagnostic blocks should be used to aid the clinician in establishing an accurate diagnosis.

Figure 7. The radiofrequency needle is passed into the second intermetatarsal space.

Prior to starting the procedure, the area of maximum discomfort, as well as the course of the intermetatarsal nerve, is marked with a pen. Three specific depths also are marked on the dorsal skin, as seen in Figure 7. These depths will correlate with three treated points along the course of the nerve. Interdigitally, at the point of entry, a small wheal of 2% lidocaine is injected. Careful attention is given to avoid excessive fluid in this area. Excessive fluid can cause impedance of the radio frequency. It is important to note that the radiofrequency needle should be advanced in the same plane as the nerve being treated. In the case of the intermetatarsal nerve, its course is well established. The focused point of treatment will be at its course plantar to the intermetatarsal ligament.

After anesthesia is obtained, a 23 gauge radiofrequency needle is introduced through the skin (see Figure 7, page 60). During insertion of the needle, the anterior edge of the intermetatarsal ligament can be “strummed,” and it should be mentioned that plantar placement to this ligament is imperative for successful treatment. Appropriate positioning of the needle is confirmed by fluoroscopy (see Figures 8 and 9, page 61). Sensory stimulus is initiated first to ensure comfort of the patient as well as to reaffirm proper placement of the needle. Motor stimulus is then initiated to rule out ablation of a motor nerve.

 

Figure 8. Fluoroscopic image confirming position and depth of treatment. Freer was placed on correlating skin marking to ensure accurate treatment level.

 

Figure 9. Lateral fluoroscopic image illustrating plantar orientation of the radiofrequency needle.

Next, 0.5 cc of 0.5% marcaine is introduced through the cannulated radiofrequency needle and allowed to dissipate. A lesion is then created by initiating the radiofrequency device at 80°C for 90 seconds. This procedure is then repeated at all three levels. Finally, 0.5 cc of dexamethasone acetate 8 mg/mL is then introduced to the treated locations. The dressing consists of a bandage to the puncture site.

We have a series of eight patients who have undergone radiofrequency ablation of either the second or third intermetatarsal nerve. All nerves treated had either failed decompression/neurolysis or had been damaged by iatrogenic means. Six of the eight patients had complete resolution of symptoms. One patient had a decrease in pain from 10 to 6 as measured on a 10-point visual analog scale. One patient had no change in pain level, which was reported as 2 out of 10. Of the patients who did not achieve a full resolution of symptoms, neither received a second treatment.

This is recommended and should be discussed with the patient as a possibility prior to the initial treatment. Even though this reported group of patients is quite small, the results match or exceed the aforementioned reported neurectomy results. Further evaluation by a prospective study of a large group is needed to validate results.

Summary
After review of the pathophysiology and a comparison of surgical results for decompression/neurolysis vs. surgical excision of Morton’s entrapment (“neuroma”), we contend that there is compelling support for abandonment of treatments oriented toward resection of the nerve, or destruction of the nerve via chemical or thermal means as a primary treatment.

As a true focal nerve compression, decompression/neurolysis is a preferable technique in the treatment of Morton’s entrapment, not only because this treatment is oriented and aligned with the true pathology, but also because there is a significantly lower chance of serious posttreatment morbidity associated with nerve resection or other nerve-destructive procedures.

However, in those cases of recurrent Morton’s neuroma, in which the peripheral nerve has been iatrogenically injured, the authors have found high success rates with either radiofrequency ablation or mini-plantar neurectomy with muscular transposition.

Perceived conservative treatments, which do not allow for decompression or offloading of exogenous pressures that decreases focal nerve compression, cannot be considered “conservative,” as they only allow for continued axonal degeneration and symptoms for the patient. Corticosteroid injections can be temporizing measures for reduction of symptoms in patients but decrease the surgical outcome of peripheral nerve decompression. The downside of corticosteroid injections are the associated risk for fat pad atrophy and frequent rupture of the plantar plate of the metatarsalphalangeal joint with resultant hammer digit deformity developing subsequent to the treatment.

 

Last updated on: November 15, 2016
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