Introduction
History of the Procedure
In 1876, Thomas Morton first described interdigital nerve compression.1 Morton theorized that the nerve was being compressed between the metatarsal heads. Surgical procedures 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 for Morton neuroma has resulted in some basic principles and goals, which are the foundation for the current surgical options. There is an increased body of literature that elaborates on many aspects of this seemingly simple problem.
Problem
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 to further delineate the pathophysiology of interdigital neuroma.
Frequency
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.2 Symptoms are typically unilateral, with the third interspace most often involved followed by the second and the fourth interspaces.2 Simultaneous neuromas occurring in the same foot are rare.3
Etiology
See Pathophysiology.
Pathophysiology
The term neuroma may be a misnomer for the condition. 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. 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,2 but 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
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, which correlates with the clinical presentation pattern.4 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.2 Less commonly, metatarsophalangeal (MTP) joint pathology, inflamed bursa, or lipomas can create compression at the level of the common digital nerve.
Presentation
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.
A provocative examination 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.5
Indications
The indication for surgery is significant pain or dysfunction that is not tolerated by the patient and that fails to respond to conservative management.
Relevant Anatomy
In addition to the discussion above, key anatomic concepts must be considered when choosing the appropriate surgical treatment of an interdigital neuroma. The medial plantar nerve has 4 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.1
Contraindications
Once the diagnosis is made and conservative management has failed, few contraindications to 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.
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References
Weinfeld SB, Myerson MS. Interdigital Neuritis: Diagnosis and Treatment. J Am Acad Orthop Surg. Nov 1996;4(6):328-335. [Medline].
Coughlin MJ, Mann RA. Surgery of the Foot and Ankle. 7th ed. Mosby-Year Book;1999.
Thompson FM, Deland JT. Occurrence of two interdigital neuromas in one foot. Foot Ankle. Jan 1993;14(1):15-7. [Medline].
Levitsky KA, Alman BA, Jevsevar DS. Digital nerves of the foot: anatomic variations and implications regarding the pathogenesis of interdigital neuroma. Foot Ankle. May 1993;14(4):208-14. [Medline].
Mulder J. The Causative Mechanism in Morton's Metatarsalgia. J Bone Joint Surg Br. 1951;33:94-95.
Lee MJ, Kim S, Huh YM, Song HT, Lee SA, Lee JW, et al. Morton neuroma: evaluated with ultrasonography and MR imaging. Korean J Radiol. Mar-Apr 2007;8(2):148-55. [Medline].
Zanetti M, Weishaupt D. MR imaging of the forefoot: Morton neuroma and differential diagnoses. Semin Musculoskelet Radiol. Sep 2005;9(3):175-86. [Medline].
Giannini S, Bacchini P, Ceccarelli F, Vannini F. Interdigital neuroma: clinical examination and histopathologic results in 63 cases treated with excision. Foot Ankle Int. Feb 2004;25(2):79-84. [Medline].
Read JW, Noakes JB, Kerr D. Morton''s metatarsalgia: sonographic findings and correlated histopathology. Foot Ankle Int. Mar 1999;20(3):153-61. [Medline].
Iagnocco A, Coari G, Palombi G. Sonography in the study of metatarsalgia. J Rheumatol. Jun 2001;28(6):1338-40. [Medline].
Timins ME. MR imaging of the foot and ankle. Foot Ankle Clin. Mar 2000;5(1):83-101, vi. [Medline].
Terk MR, Kwong PK, Suthar M. Morton neuroma: evaluation with MR imaging performed with contrast enhancement and fat suppression. Radiology. Oct 1993;189(1):239-41. [Medline].
Saygi B, Yildirim Y, Saygi EK, Kara H, Esemenli T. Morton neuroma: comparative results of two conservative methods. Foot Ankle Int. Jul 2005;26(7):556-9. [Medline].
Valente M, Crucil M, Alecci V. Operative treatment of interdigital Morton's neuroma. Chir Organi Mov. Mar 14 2008;[Medline].
Coughlin MJ, Pinsonneault T. Operative treatment of interdigital neuroma. A long-term follow-up study. J Bone Joint Surg Am. Sep 2001;83-A(9):1321-8. [Medline].
Benedetti RS, Baxter DE, Davis PF. Clinical results of simultaneous adjacent interdigital neurectomy in the foot. Foot Ankle Int. May 1996;17(5):264-8. [Medline].
Further Reading
Keywords
interdigital neuroma, interdigital neuritis, forefoot pain, forefoot paresthesias, forefoot numbness, foot pain, Inge retractor, neurectomy, plantar keratosis
