A Closer Look At Endoscopic Nerve Decompression For Morton’s ‘Neuroma’

Author(s): 
Damien Dauphinée, DPM, FACFAS, CWS-P

The interdigital neuroma is a common source of forefoot pain that podiatric foot and ankle surgeons see in their practices every day. The characteristic symptoms have been well known since Morton first described them in 1876.1 Even though Morton believed the problem was located at the fourth metatarsophalangeal articulation, his name has been associated with the pathology for over a century. Peripheral nerve surgeons have chosen to avoid the term “neuroma,” opting instead for “interdigital neuritis,” which better describes the problem.

   The plantar digital nerves are the terminal branches of the medial and lateral plantar nerves. The medial plantar nerve has four digital branches. The most medial branch represents the proper digital nerve to the medial aspect of the great toe. The next three branches represent the first, second and third common digital nerves. These three nerves are distributed to the medial and lateral aspects of the first, second and third interspaces respectively.

   The superficial branch of the lateral plantar nerve has two digital branches. The more lateral branch represents the proper digital nerve to the lateral aspect of the fifth toe and the other branch represents the fourth common digital nerve for the fourth interspace.

   Along their distal course, the interdigital nerves are accompanied by the digital arteries and veins as well as the lumbrical tendons. At the level of the lesser metatarsal heads, the neurovascular bundle and the lumbrical tendons enter the “metatarsal tunnel,” which is formed by the lesser metatarsophalangeal joint (MPJ) capsules, the deep transverse intermetatarsal ligament, and the transverse fibers of the plantar aponeurosis.2 The neurovascular bundle and the lumbrical tendons pass beneath the deep transverse intermetatarsal ligament. Just distal to the deep transverse intermetatarsal ligament, the interdigital nerves branch to the adjacent toes to provide sensation.

   Gauthier initially supported the theory that the interdigital neuroma is a form of entrapment neuropathy.3 He meticulously described the mechanism of the interdigital nerve entrapment as it passes under the deep transverse intermetatarsal ligament. During the last stages of the stance phase of the gait cycle, the loads transmit to the metatarsal heads and the toes dorsiflex. The interdigital nerve subsequently compresses between the plantar soft tissues and the unyielding, anterior edge of the transverse metatarsal ligament. Fashionable shoes that are tight in the forefoot area and have increased heel height further aggravate the compression.

   The diagnosis is typically a clinical one with pain upon palpation of the involved interspace with or without side-to-side pressure applied to the metatarsal heads. Sometimes, the physician will note a palpable click. Patients complain of burning pain that often radiates to the toes and occasionally to the calf. Some patients describe the classic “rolled up sock” sensation in the forefoot and many patients report that removing their shoes and rubbing the forefoot helps to relieve the pain.

What The Research Reveals About Endoscopic Release

Researchers have shown that an endoscopic release of the deep transverse intermetatarsal ligament is quite effective in relieving the pressure on the nerve.4 In a 1994 cadaveric study, Barrett and Pignetti, using an endoscopic approach, released the deep transverse intermetatarsal ligament, leaving the entrapment neuroma intact. In the prospective study, 17 patients with 19 neuromas had endoscopic decompression of the intermetatarsal nerve. Ten of 17 patients required no pain medications and four others needed fewer than three tablets of acetaminophen (500 mg)/hydrocodone (5 mg).4 Fifteen of 17 patients reported excellent results while one patient had a fair result and one had a poor outcome.

   Barrett and Walsh subsequently published a retrospective look at 69 patients who had decompression of 96 interspaces.5 Of the 96 interspaces released, 39 were second interspaces and 57 were third interspaces. The authors reported 86 percent excellent or good results, and 14 percent poor results. Of the interspaces with poor results, Barrett and Walsh noted five patients required further surgery and had a traditional neurectomy. The study adds that none of the patients who had interspace decompression developed a true amputation neuroma.

A Step-By-Step Guide To Surgical Technique

When it comes to endoscopic nerve decompression for Morton’s neuroma, I use a single portal approach with a second incision proximal to the interspace for the metatarsal retractor. The endoscopic surgical instrumentation set Edintrak (Instratek) contains the ideal metatarsal retractor along with a facial elevator instrument to bluntly dissect the tissue planes. The set also has an obturator and cannula for appropriate placement of the conduit (cannula) for the placement of the 2.7-mm, 30-degree endoscope. The dorsal portal incision allows for placement of a metatarsal retractor, which places tension across the deep transverse intermetatarsal ligament. This allows for easier visualization and transection.

   The surgeon makes a second incision transversely in the web space, which allows for placement of the obturator and cannula beneath the deep transverse intermetatarsal ligament. This configuration and the oval-shaped cannula eliminate the need for the third incision to allow for passage of the cannula or obturator out of the foot. After fully visualizing the deep transverse intermetatarsal ligament, one would transect the ligament using a curved hook blade.

   The surgeon can visualize separation of the transected edges of the deep transverse intermetatarsal ligament by placing further traction on the metatarsal retractor in the dorsal portal incision. One can usually identify and protect the lumbrical tendon during the procedure.

In Conclusion

In my experience, the endoscopic decompression of the intermetatarsal nerve technique is efficacious. Due to the minimal invasiveness of the technique and the preservation of the neurovascular structures, I have seen no vascular compromise to a digit in any patient in whom I have decompressed adjacent interspaces. The entire procedure takes approximately six to eight minutes and one can perform it bilaterally in under 15 minutes when symptoms exist on both sides.

   Surgeons can allow patients to bear weight immediately in a post-op shoe. Have patients bring a loose fitting athletic shoe with them to the first post-op appointment. One would dress the stab incisions with Telfa bandages and then the patient resumes normal bathing routines.

   Dr. Dauphinée is the President of the Texas Podiatric Medical Association. He is a Fellow of the American College of Foot and Ankle Surgeons, a Fellow of the American College of Certified Wound Specialists and a Fellow of the Association of Extremity Nerve Surgeons.

References
1. Morton TG. A peculiar and painful affection of the fourth metatarso-phalangeal articulation. Am J Med Sci. 1876; 71:37–9.
2. Morris MA. Morton’s metatarsalgia. Clin Orthop 1977; 127:203–7.
3. Gauthier G. Thomas Morton’s disease: a nerve entrapment syndrome. A new surgical technique. Clin Orthop. 1979; 142:90–2.
4. Barrett SL, Pignetti T. Endoscopic decompression for intermetatarsal nerve entrapment: the EDIN technique: preliminary study with cadaveric specimens; early clinical results.
J Foot Ankle Surg. 1994; 33(5):503.
5. Barrett SL, Walsh AS. Endoscopic decompression of intermetatarsal nerve entrapment: a retrospective study. J Am Podiatr Med Assoc. 2006; 96(1):19-23.

   Editor’s note: For related articles, see “A Guide To Endoscopic Decompression For Intermetatarsal Neuromas” in the September 2011 issue, “Point-Counterpoint: Nerve Decompression In Diabetic Patients: Should It Be Done?” in the June 2005 issue or “Nerve Decompression Study Offers Provocative Findings” in the November 2005 issue.

Comments

Microsurgical dissection with laser ablation of proximal and distal nerves works very well.

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