How To Address Stump Neuromas

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Given the challenges of stump neuromas in the foot, this author offers insights on the etiology, conservative management and surgical options such as nerve capping and transplantation.

   The stump neuroma is a natural and expected occurrence after nerve injury. When damaged, the proximal nerve segment attempts to regenerate, leading to a bulb-shaped thickening or stump. Trauma is a common cause for these injuries throughout the body.

   In the foot, however, iatrogenic etiologies are more frequent. Researchers have reported that up to 30 percent of stump neuromas elicit pain.¹ It is unclear why some are painful and others are not, but it most likely relates to the area and tissue into which the nerve regenerates.

   Interdigital neuromas typically have fairly standard histological characteristics. Myelinated fibers degenerate, leading to thickening and fibrosis of the epineurium and perineurium. Additionally, thickening and hyalinization takes place in the walls of the epineural and endoneural vessels.² This degenerative histology is in direct contrast to traumatically induced neuromas, which are more proliferative. The neural tissue demonstrates dense fibrous changes with tortuous irregular proliferation. [8]

   Nerves are remarkable in that they do go through some degree of repair and regeneration after injury. We are able to utilize this with surgical repair of transected nerves via direct end-to-end anastomosis, nerve grafting or by utilizing neural tunnels to guide the regenerating neurons to rejoin their distal segment. On the other hand, stump neuromas are characterized by disorganized architecture with nerve tissue growing out in no particular orientation.³

   Dellon described neural regeneration as an expected biological consequence after a peripheral nerve divides.⁴ Wallerian degeneration is the process that nerves go through to repair themselves after injury. Nerve growth factors produced from Schwann cells support this process. After injury, these neurotrophic factors diffuse outward, inducing nerve fibers to grow outward in many different directions beyond the border of transection. This haphazard regrowth leads to a bulb-shaped thickening that is otherwise known as a stump neuroma.

   This begs the question: if stump neuromas are a natural occurrence after nerve injury, why are they not all painful? Certainly, Morton’s neuroma excision is a fairly standard procedure performed by most foot and ankle surgeons. In fact, it is estimated that only 30 percent of stump neuromas become symptomatic after transection.¹ [9]

   The answer is not entirely clear but there are some theories. Most authors believe pain occurs when the haphazardly regenerating nerve fibers branch out into scar tissue or areas of adhesion.⁵ Weightbearing areas on the foot, locations prone to mechanical stimulation and areas that undergo repetitive trauma become problem spots as well. In short, the nerve fibers interact abnormally with surrounding connective tissue, leading to pain.

What You Should Know About The Etiology

   As I noted earlier, the formation of a terminal or stump neuroma is a natural occurrence after nerve injury. In the foot and ankle, nerve injuries generally result from either a traumatic event or as part of a surgical procedure, be it intentional or unintentional.⁶ The soft tissue envelope is relatively small and this places subcutaneous and deep nerves in danger of damage during traumatic events. The dorsal neural structures are at risk with laceration type injuries. The plantar structures are prone to damage with puncture wounds.

   Generally speaking, one should treat most traumatic nerve injuries acutely with end-to-end endoneural repair. However, this is not the standard of care with injuries distal to the arch of the foot. Frenette and Jackson looked at a small series of patients in whom the smaller distal nerves of the foot underwent primary repair. The authors reported only a 25 percent return to normal sensation.⁷ It is therefore not recommended to attempt repair on these patients due to the small nerve size and minimal sensory deficit acquired when patients undergo conservative treatment.

   Iatrogenic nerve injury is certainly a possible complication after foot and ankle surgery but is luckily uncommon when one employs good surgical techniques. Regardless, painful stump neuromas do occur and are more frequently involved with common digital nerve resection or resection of Morton’s neuroma.

   Logic would tell us that the large volume of Morton’s neuromas removed each year is the reason for this. However, there are studies that explain why there may be other factors that predispose the patient to painful stump neuromas after these procedures. [10]

   Amis and colleagues looked into this when they dissected second and third common digital nerves in fresh, frozen cadaveric feet.⁸ They found consistent, small, plantarly directed nerve branches along the course of these nerves that innervated the underlying skin. The authors hypothesized that these branches acted to anchor the nerves, preventing inadequate retraction deep into the foot after standard common digital nerve resection.

   Small communicating branches are also present in a certain percentage of the population between the second and third and/or third and fourth common digital nerves. These communicating branches travel transversely, deep to the metatarsals and are usually proximal to the actual Morton’s neuroma. When one does not identify this intraoperatively, the terminal nerve stump can become tethered once again and inadequate retraction deep into the foot can occur.⁹

Essential Diagnostic Insights

   Diagnosis of a stump neuroma is not always straightforward. A large part of the diagnosis draws from the clinical history and timing of symptoms. One should have heightened suspicions if the patient presents with classic nerve symptoms and a recent history of traumatic nerve injury or Morton’s neuroma surgery. Common complaints are burning pain, pins and needles sensations and/or electrical shooting or shocking.¹⁰ The pain is often localized to the area of nerve injury but may present more proximal.

   The timing of symptoms is also important. A classic stump neuroma does not become symptomatic for weeks to months after the initial insult. As I mentioned earlier, the terminal portion of the nerve goes through a certain amount of axonal regeneration after resection. However, this does take time and requires several weeks to have proliferated enough to become symptomatic.¹⁰

   When the symptoms become evident immediately or relatively soon after surgery, the surgeon needs to consider other diagnoses and rule them out. In the case of persistent pain after Morton’s neuroma resection, studies have shown that two-thirds of cases are due to incomplete excision of the nerve.¹⁰ Another potential cause is undiagnosed multiple neuromas, which reportedly occur 3.4 percent of the time.¹⁰ Lastly, one should consider that the wrong web space may have been diagnosed or the initial diagnosis was incorrect prior to surgery (see “What Common Conditions Can Be Misdiagnosed As Neuromas?” on page 74). [11]

   The location of pain on clinical exam is usually proximal to the metatarsal heads in the same web space that underwent surgery. It may also be under one of the adjacent metatarsal heads if a communicating branch has tethered the resected nerve in a lateral or medial direction.⁹ The plantar skin may be hypersensitive and the pain is often more intense than it was preoperatively. A positive Tinel’s sign is usually present and helps differentiate a nerve problem from a misdiagnosed condition.⁹ Performing diagnostic local anesthetic injections can help in making the correct diagnosis.

An Overview Of Initial Treatment Options And Injection Therapy

   Treatment for painful stump neuromas should begin with conservative management. Researchers have had varying success with physical therapy modalities such as mechanical desensitization, transcutaneous electrical nerve stimulation (TENS) and iontophoresis.¹¹ Additionally, offloading with specialty shoes or custom designed orthotics can be beneficial. Also consider medical therapy with medications like gabapentin (Neurontin, Pfizer) or duloxetine (Lyrica, Eli Lilly). One can also utilize topical nerve desensitizers such as capsaicin.

   Injection therapy can be a useful adjunct for treating these lesions. Agents such as alcohol, steroids, phenol, pepsin, formalin and hydrochloric acid have had varying success. Corticosteroids are widely accessible and are the most common injectable. [12]

   Of these injectables, sclerosing alcohol injections, popularized by Dockery, are the most intriguing.¹² Mozena and Clifford reported on 49 Morton’s neuromas that they injected multiple times with a 4% sclerosing alcohol solution.¹³ They reported a 74 percent success rate with a series of at least five injections given at weekly intervals. Although this technique has not been specifically studied for stump neuromas and the histological makeup is slightly different between the two entities, one might surmise a similar success rate.

Key Surgical Considerations

   There is nothing clinically available that stops nerve regeneration completely. Most surgical treatments are geared toward reducing the abnormal interaction regenerating nerves have with the surrounding connective tissue. When considering surgical intervention for stump neuromas, the first goal is complete excision of the abnormal stump proximally to the level of healthy nerve tissue. Using a sharp scalpel or scissors, one should make a single, uniform, 90-degree cut.

   Additionally, the surgeon should attempt to minimize scar formation at the operative and implantation site. One can achieve this with good surgical planning and meticulous dissection technique. Try to employ sharp dissection, creating full thickness flaps when possible. Avoid blunt dissection or creating multiple layers.¹¹

   The surgeon should relocate the nerve away from weightbearing areas and toward locations that will minimize motion on the distal segment. Ideally, one should attempt to discourage axon regrowth at the transected end. Although we cannot completely stop this process from occurring, there have been many studies exploring techniques to minimize this.

   Surgeons have utilized epineural sleeves for many years with good success. After transecting the nerve, fold the epineurium back on itself, exposing the underlying fasicles. Sharply transect the fasicles. Then fold back the epineurium distally, covering the exposed fasicles. Close with a minimally reactive 6-0 or smaller suture using a “purse string” stitch technique. The goal is to limit the axonal proliferation into the non-neural connective tissue.¹⁴

What The Research Reveals About Nerve Capping And Nerve Transplantation

   Nerve capping has also had varying success. Various silicone caps are most popular with success rates hovering at 70 percent.¹⁵ The surgical technique starts with an epineurial sleeve, which one caps with a silicone implant. This is an adjunct that again attempts to separate the regenerating nerve fibers from the surrounding connective tissue.

   Krishnan and colleagues have described a more complicated technique of covering these damaged nerves with a pedicled regional flap or free flap.¹⁶ They found it to be effective and attractive for complex stump neuromas. Due to its complex nature and extensive recovery, one should consider this technique for the more advanced or failed therapies.¹⁶

   Nerve transplantation into local veins has garnered more attention recently in attempting to slow down distal axonal growth. Koch and co-workers looked at the histology of the distal nerve segment after transection and implantation into vein in a rat model.¹⁷ Typical findings occurred in the amputation neuromas of the control group. However, nerve stumps transposed into a vein lumen showed more highly organized endoneural architecture. The axons had a higher rate of myelination and a smaller stump formation in comparison to the control group. This technique both caps the stump and transposes it into a different environment.

   Transplanting the nerve into vein has proven clinically effective in upper extremity neuromas.¹⁸ The technique was also the focus of a series of lower extremity neuromas with satisfactory results. Eight patients underwent resection with transposition into vein with a mean follow-up of 17 months. Seven of the eight were satisfied and one recurred at two months. The surgical procedure involves either transecting the vein and placing the nerve into the open end, or creating a venotomy and placing the nerve through the open portion. Both are anchored with a transmural epineural stitch.¹⁹

   Authors have described the use of local bone or muscle as a transplant medium with good success in most anatomic locations throughout the body. Chindo and Miller compared the two after peroneal neuroma formation about the ankle.²⁰ Although both techniques yielded good results, implantation into bone was superior. Surgeons implanted the nerve into tibia or fibula by drilling a hole into the bone, feeding the distal segment of nerve into the opening and securing it with an epineural suture. The authors felt there was less tension on the nerve with this approach, yielding higher patient satisfaction.

   Implantation into local muscle has also provided excellent results in most anatomic locations and is probably the most widely used medium in the foot.²¹ Wolfort and Dellon resected 17 recurrent interdigital neuromas that occurred as a result of standard Morton’s neuroma excision. They used a plantar approach and implanted the resected stump into adjacent intrinsic muscles. They had good to excellent results in all patients with a mean follow up of 33.8 months. They implanted the nerve segment without tension into local muscle, using a 6-0 nylon epineurial stitch to secure the nerve to the muscle belly.

   This is an excellent option for plantar neuroma removal because one must dissect multiple soft tissue layers to access any of the osseous structures. Implantation of a plantar nerve into bone can cause undue trauma and lead to excess tension on the nerve as it stretches the relatively long distance.⁴

Other Pertinent Points

   By far the most common reason for stump neuroma resection is failed Morton’s neuroma surgery. Although we employ multiple techniques for limiting axonal regrowth at the stump as adjuncts to our surgery, possibly the simplest procedure is to relocate the nerve.

   In this instance, excise the stump and relocate the terminal end proximally into a non-weightbearing area of the foot. This creates less irritation on the segment. Reporting on a series of 37 feet, Johnson and colleagues noted complete pain relief or marked improvement in 67 percent, improvement with some pain in 9 percent and no improvement in 24 percent of patients.¹⁰ Most of these patients had a plantar approach to their nerve resection, which yielded satisfactory results.

In Conclusion

   Stump neuromas are a naturally occurring event after a nerve is injured or transected. Luckily, they rarely become symptomatic unless there is an abnormal interaction with the surrounding soft tissue. Conservative measures are usually adequate for treatment. However, when these fail, one should consider surgical intervention.

   When performing surgery, one should attempt to minimize scar tissue around the nerve, limit axonal regrowth and reduce stress on the nerve by redirecting it to areas with less tension. When removing nerves as a primary surgery, such as with Morton’s neuroma, avoid over-dissection to minimize scar tissue and allow the terminal end to retract deep into the arch of the foot.

Dr. Schroeder is a Fellow of the American College of Foot and Ankle Surgeons. He is board certified in foot and ankle surgery as well as reconstructive foot and ankle surgery. Dr. Schroeder is the Chief of Podiatric Surgery at Southwest Washington Medical Center in Vancouver, Wash. He is in private practice at the Vancouver Clinic in various locations in Vancouver, Wash.

For further reading, see “Is Injection Therapy The Best Solution For Foot Neuromas?” in the January 2002 issue of Podiatry Today.

References:

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