Morton's neuroma is a mechanically induced degenerative neuropathy which has a strong predilection for the third common digital nerve in middle-aged women.1 Morton neuroma is not a nerve tumor; it represents perineural fibrosis as a result of mechanical pressure. Histological findings include neural degeneration, epineural and endovascular hyalinization, and perineural fibrosis.1
The condition is common to but not unique to the 3rd web space; some authors prefer to use the term interdigital neuroma instead of Morton's neuroma which was historically used to describe interdigital neuroma in the 3rd web space.2 The excessive motion between the third and fourth metatarsals, the tethered third common digital nerve in the third web space, the third and fourth metatarsal heads flanking the third common digital nerve, the stout third transverse intermetatarsal ligament overlying the third common digital nerve, and excessive weight bearing stress on the forefoot, particularly by wearing pointed and high-heeled shoes, can collectively produce micro-damage to the third common digital nerve. If allowed to continue for a long period of time, this can become manifested microscopically by nerve fibre degeneration and excessive intraneural and juxtaneural reparative fibrous tissue formation resulting in a significantly enlarged nerve. Such enlargement can cause further trauma, and therefore become even more symptomatic.
Ultrasound and magnetic resonance imaging are used for the diagnosis; various reports of specificity and sensitivity of these both investigations is reported. Sonography can reveal a Morton's neuroma in 85% of cases.3 Identification of the presumed plantar digital nerve in continuity with the mass improves diagnostic confidence. When non-surgical means fail to relieve patient's symptoms, surgical removal of this offending neuroma through a dorsal approach can produce dramatic relief of symptoms. In addition, when a painful recurring Morton's neuroma does not respond to conservative treatments, removal of this lesion through a plantar or dorsal approach can provide lasting relief in a good number of patients. There are however complications, persisting pain may be due to irritation at the cut end of the nerve, a stump neuroma. The type of approach has been blamed due to the poor results.1-12 In the current retrospective study we report the outcome of surgery using two different approaches.
In this pilot sequential study on the surgical outcome of interdigital (Morton's) neuroma performed between January 2005 and December 2007, 36 patients (Female: Male = 34:2) with a mean age 52.08 years (31–67 years) were included. Six patients had bilateral procedures (total 42 feet). The right side was affected in 9 patients and the left foot in 21 cases. One diabetic patient with peripheral neuropathy was excluded from the study. The frequent symptoms were metatarsalgia (pain and inflammation in the ball of foot) and difficulty in walking which has failed to respond to analgesia, anterior metatarsal support, shoe modification and local steroid injection in some occasions. Mulder clunk sign and tenderness in the concerned web space is the most frequent clinical sign. Mulder's clunk sign is positive when upon compression the metatarsal heads in the transverse direction and applying pressure to the affected web space a clunk is felt if Morton's neuroma is present. In this series, the 2nd web space was involved in 40% of the cases, the 3rd web space was involved in the rest. All these patients had sonographic confirmation of Morton's neuroma; ranging in size between 5 mm to 15 mm. In 15% of cases, there were fluid and bursa in the intermetatarsal area in addition to Morton's neuroma (Figure 1).
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Figure 1. Morton's neuroma shown in circled area.
Standing radiographs of the foot (anteroposterior /lateral/oblique) was taken to rule out bony pathology when required.
Interdigital (Morton's) neurectomy was performed for all these patients by one surgeon, and in all cases under general anaesthetics and tourniquet and histological confirmation of Morton's neuroma. Four patients underwent additional procedures (Chevron's osteotomy for hallux valgus).
The goals of surgery were the identification and excision of the digital nerve as proximal as interosseous muscles. The proximal end of the nerve was buried in the interosseous muscle. The digital bifurcation and branches were removed as much as possible.
Between January and December 2005, only plantar approach was used, and 20 neuromas were removed. The neurectomy was performed through plantar approach. A longitudinal incision was used when the neuroma was in one web space, a transverse incision was used when more than one web pace was explored through plantar approach (2 cases). The nerve trunk is identified; the nerve branches and the trunk were excised as far distal and as far proximal as possible.
Twenty-two neuromas (between Janauary and December 2006) were removed through a distal longitudinal dorsal approach, when more than one space was involved (4 cases) a “Y” shaped longitudinal incision was used to approach both spaces through the same approach. A laminar spreader is used to separate the metatarsals. The intermetatarsal ligament was completely released in this group. The transection level should be 1–2 cm proximal to the weight bearing pad of the forefoot at this more proximal level. The nerve stumps when possible in both groups were embedded in the small muscles of the foot. The tissue was sent for biopsy in both groups to confirm the diagnosis.
The procedure performed and patients were identified through the operative lists and theatre registers. Mean follow-up for our patients was 18 months (13–54 months). The follow-up protocol was to review the patient (s) 6 weeks, 12 weeks and 6 months or as required afterwards. A proforma was used for the analysis of results; the medical notes of all these patients were reviewed. The final outcome assessment was done via telephone contact. The second author analysed the results. The following parameters were analysed: The surgical approach, the duration for full relief of symptoms and for full post operative weight bearing, return to work, shoe wear and the complications in each group particularly scar concerns. Weight bearing as tolerated (on heel or lateral border of foot) was allowed the same day of surgery. Full weight bearing was defined as the ability to walk on the forefoot (toe-off) resuming the normal gait pattern. This information was obtained through the physiotherapy records, clinic follow up and by questioning the patient as to when they were able to walk properly.
Duration taken to full weight bearing, postoperative wound infection, hematoma, and scar problems were compared between patients having plantar approach and those having dorsal approach with Mann Whitney SPSS 15.0. A P value <0.05 was considered statistically significant.
It was noticed that the plantar group has a trend towards delayed full weight bearing (mean of 3 weeks, range 2–8 weeks). The patients in this group continued to rely on heel walking. While full weight bearing was prompt in the dorsal group with mean of two weeks (range 1–6 weeks). All patients were able to walk full weight bearing in all for over a mile with trainers; this was possible after a mean of 3 weeks period from surgery in the plantar but a mean of 2 weeks in the dorsal group. Overall, returning to work, driving and recreational activities were earlier in the dorsal group. Patients were allowed driving when able to full weight bear (Figure 2).
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Figure 2. The weight-bearing ability following different approaches.
One patient had postoperative haematoma in the plantar group, and this resolved spontaneously. Superficial infection occurred in 4 (3 from the plantar and 1 from the dorsal group). Among the plantar group two patients continued with painful scar for 3–4 months. Seven patients (feet) had reduced sensation in the concerned web space after surgery (4 among the dorsal and 3 among the plantar group). In two patients in the plantar group, callosity persisted until the last follow-up; one patient among the dorsal group had scar sensitivity for 5 months.
One patient in the plantar approach had recurrence and required further surgery through the same approach. Adhesions around the stump were excised. This patient had thin plantar fat pad. While major activity restrictions following surgery were uncommon, those with wound problems needed to use shoes with high toe box to prevent pressure problems (4 patients), until the symptoms improved. Although there was subjective numbness in twenty feet, the pattern of numbness was quite variable and it was bothersome in only seven feet.
Postoperative wound infection, hematoma and scar problems were significantly higher (P <0.05) in patients having plantar approach as compared to those having dorsal approach (Mann Whitney SPSS 15.0). The duration taken to full weight bearing was significantly better in the dorsal group (P <0.05) using the same statistical tools. This may signify better decompression of the nerve in the dorsal group because of the intermetatarsal ligament release.
Morton's neuroma was first described by Thomas Morton in 1876, as a non-neoplasic injury represented by perineural fibrosis of the plantar digital nerve. Morton's neuroma is also called interdigital perineural fibrosis; it most often affects the 3rd interspace; 60% of our patients had Morton's neuroma in the 3rd web space.
The philosophy of two different surgical approaches for interdigital (Morton's) neuroma is because of the different belief in the aetiology. Some believe that taut transverse metatarsal ligament play a critical role in compressing the interdigital nerve.1 The advocates of plantar approach believe that this approach is safe, they do not believe that there is need to release the deep transverse ligament; they believe that the transverse intermetatarsal ligament is not contributing to neuroma formation and that its release predisposes to metatarsalgia by diastases of metatarsal bones.2 The dorsal approach allows for the option of transverse ligament release strategy. The dorsal approach was first described by McElvenny in 1943.4 The advantages of the dorsal approach as compared to plantar approach for excision of interdigital (Morton's neuroma) is claimed to be as follows: (1) The ability to release the intermetatarsal ligament; (2) The dorsal incision being in the non weight bearing surface of the foot, allows for early rehabilitation; (3) It is thought that the plantar cutanous nerves are easier to find and excise through dorsal approach as compared to plantar approach where dissecting the nerves in plantar fatty tissue can be difficult.1
The concern associated with dorsal approach is the metatarsal splaying because of the intermetatarsal ligament release. This in our series and others however was not a problem.7
In a study, the overall satisfaction using dorsal approach for excision of Morton's neuroma was rated excellent or good in 85% of the patients, and 65% of patients were pain-free at the time of final follow-up at 5 years. The patients who had had either bilateral neuroma excision or excisions of adjacent neuromas in the same foot in a staged fashion had a slightly lower level of satisfaction, but this difference was not significant.6 At a mean follow-up of 18 months and apart from one recurrence in the plantar group the overall results were satisfactory; the dorsal approach however rated marginally better.
Recurrence of symptoms following interdigital neurectomy is believed to be due to wrong diagnosis, wrong interspace, failure to divide the transverse metatarsal ligament, too distal transection of common plantar digital nerve,6 an association of tarsal tunnel syndrome7 and incomplete removal. In a study on 34 patients (thirty-seven feet) who had a re-operation for pain that persisted after excision of a plantar interdigital (Morton's) neuroma, it was found that in 67 per cent of cases, the recurrent pain in these patients had probably resulted from an incomplete initial excision of neuroma.4 In our recurrent case (incidence of 3%), there was adhesions around the proximal stump of the nerve on exploration; this was the result of inadequate resection, therefore we recommend burying the stump in the small muscles of the foot when possible. The failure rate of neurectomy is reported to be as high as 14% to 21%.
Various incidence rates of recurrence of neuroma are reported, it is however accepted that dorsal approach is better in this regard. The results of surgical treatment for recurrent neuroma, however, are better when approached through plantar aspect of the foot.1,2,4
Proper identification and excision of the nerve trunk help in reducing the incidence of recurrence. In our study and others, we found that dorsal approach to neurectomy was better than plantar approach in regards to postoperative weight-bearing, early return to work was faster and shorter hospitals stay in the plantar group.4 In this series and others, painful scars were more common in the plantar group; we had two patients in the plantar group with painful scar. There are other reports of long-term good outcome using dorsal approach for Morton's neurectomy.2
In conclusion, our results are in favour of dorsal approach for Morton's neurectomy; both rehabilitation and long term results are better.
Acknowledgment: We thank the clinical effectiveness department for their help, especially Mr. C Taylor, Mr J Birtwistle and Mr V Brown for helping in collecting the data and its analysis.
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