Managing Morton’s Entrapment
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 cyroablation, 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
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
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