Targeted Muscle Reinnervation to Lumbrical Muscles for Symptomatic Digital Stump Neuroma: Surgical Technique

Summary:

Targeted muscle reinnervation (TMR) represents a significant advancement in treating symptomatic stump neuromas. Unlike traditional passive procedures, TMR redirects axonal growth from sensory nerves to motor endplates, which may help prevent neuroma recurrence. This study detailed the surgical steps involved in applying TMR to the lumbrical muscles, supported by graphic, photographic, and video illustrations, aiming to ensure reproducibility.

Takeaways.

Question: How can the surgical steps of targeted muscle reinnervation (TMR) to lumbrical muscles be standardized and optimized to address the limitations of traditional neuroma treatments and improve management of digital stump neuromas?

Findings: This study provided a step-by-step, anatomically validated surgical technique for performing TMR to the lumbrical muscles, detailing exposure, nerve preparation, muscle selection, and coaptation steps.

Meaning: This technique offers surgeons a clear, reproducible approach for performing TMR in the hand, helping improve the management of symptomatic digital neuromas.

INTRODUCTION

Finger amputation inevitably results in neuroma formation, although these are often asymptomatic. The true incidence of painful stump neuroma is unknown, but has been reported as high as 7%.¹ Traditional surgical treatments, such as traction neurectomy, nerve implantation into adjacent tissue (eg, bone or muscle), nerve capping, and centrocentral anastomosis, often yield inconsistent results.² Targeted muscle reinnervation (TMR), originally developed to enhance intuitive prosthesis control, shows promising outcomes and is gaining acceptance among surgeons for addressing this challenging clinical issue.³ It involves coapting the neuropathic sensory nerve to a nearby expendable motor branch, offering viable motor endplates for axonal growth and reducing neuroma formation.^4,5 In cases of partial hand and finger amputations, cadaveric dissection studies have identified suitable expendable motor branches of intrinsic hand muscles as recipients, demonstrating consistent anatomy.⁶ Recent case reports have shown successful clinical outcomes in terms of residual and phantom limb pain relief.^7,8 This study aimed to provide a comprehensive, step-by-step description of TMR to the lumbrical muscles for the treatment of symptomatic digital stump neuromas.

SURGICAL TECHNIQUE

The patient is positioned supine under general anesthesia, without the use of muscle relaxants. An upper arm tourniquet is applied from the beginning of the procedure. The neuroma is marked on the skin preoperatively. A Bruner incision is planned with an extension into the palm toward the target lumbrical muscle.

As the entire surgery is performed under an arm tourniquet, the procedure starts with the dissection and stimulation of the chosen recipient nerve in the palm without exceeding the 30-minute tourniquet time, thus preventing ischemic muscle paralysis. The motor entry point (MEP) to the lumbrical muscle is found approximately 20 mm proximal to the proximal border of the A1 pulley (authors’ observation in cadaveric dissections, not published), which is consistent with the findings of Colonna et al,⁹ with the nerve located medially and dorsally to the muscle belly (Fig. 1). Using a handheld stimulator at 0.5 to 1 mA, we verify the recipient nerve and mark it with a vessel loop, following dissection of 1 cm proximal to the MEP. (See Video 1 [online], which demonstrates intraoperative electrical stimulation of the motor nerve to the second lumbrical muscle at 0.5 mA and shows the incision lines used for exposure.) (See Video 2 [online], which demonstrates TMR of the radiopalmar digital nerve of the small finger to the motor branch of the fourth lumbrical muscle. The video displays intraoperative electrical stimulation at 0.5 mA and mobilization of the lumbrical motor branch for coaptation.)

Fig. 1.

The second lumbrical muscle with its motor nerve (yellow vessel loop) radial to the flexor digitorum profundus of the middle finger is shown. The dashed line marks the proximal A1 pulley; the MEP lies about 20 mm proximally.

Video 1. This video demonstrates intraoperative stimulation of the motor branch to the second lumbrical muscle during targeted muscle reinnervation for treatment of painful digital neuromas. Electrical stimulation at 0.5 mA elicits visible contraction of the lumbrical, confirming accurate identification of the motor nerve.

Download video file ^{(1.1MB, mp4)}

Video 2. This video demonstrates the identification of the motor branch of the fourth lumbrical muscle for targeted muscle reinnervation in case of painful digital stump neuroma. Intraoperative stimulation at 0.5 mA confirms lumbrical activation before coaptation.

Download video file ^{(1.6MB, mp4)}

Next, the incision is extended distally, the neuroma is isolated, and the affected digital nerve is freed up to the level of the branching of the dorsal digital nerve at the metacarpophalangeal joint. Excision of the entire digital nerve along with the neuroma, rather than performing only a proximal neurotomy, is preferred, because collateral sprouting from the contralateral digital nerve may result in aberrant reinnervation of the remaining neuroma (Fig. 2). The dorsal digital branch is preserved and, if additional length is needed, an intraneural neurolysis from distal to proximal is performed to gain an extra 4–5 mm (Fig. 2). Next, the nerve is turned over proximally for a tension-free epineural coaptation to the recipient nerve using 2 Nylon 9-0 stitches (Fig. 3) with the finger in full extension. A minimal to no size discrepancy is observed. Immediate finger mobilization without weight-bearing is allowed. The technique of TMR to the first lumbrical muscle is schematically depicted in Figure 4.

Fig. 2.

Dissection of the index stump shows the ulnopalmar digital nerve with a neuroma (white arrow) and the motor branch to the second lumbrical muscle (yellow vessel loop). The dorsal digital nerve branching (blue asterisk) and the intact radiopalmar digital nerve of the middle finger (blue arrow) are also indicated.

Fig. 3.

The neuroma and affected digital nerve are resected to healthy fascicles, enabling tension-free coaptation with the lumbrical motor branch while preserving the dorsal nerve.

Fig. 4.

Schematic illustration of TMR to the first lumbrical muscle for the treatment of an index stump neuroma on the radial side. The MEP of the lumbrical is shown in relation to the proximal border of the A1 pulley.

To date, 4 patients (3 men; median age 46 y, range 35–62 y) with traumatic finger amputations were treated with this technique. Neuropathic pain duration did not exceed 24 months at the time of surgery. Conservative treatment had failed in all cases, and 3 of the patients had undergone previous unsuccessful surgery for neuroma management.

At 1-year follow-up, all patients reported complete, permanent pain relief in the stump area. Mild residual pain in the palm during strong grip (NRS 3–4) was reported, distinct from the disabling neuropathic stump pain experienced preoperatively.

DISCUSSION

The implementation of TMR for symptomatic stump neuromas represents a promising advance in managing a difficult clinical problem.^3,7,8 Unlike traditional passive techniques, TMR offers an active solution by redirecting sensory axons toward motor endplates, potentially reducing neuroma recurrence and improving outcomes. However, as a relatively novel application, careful consideration of patient selection, timing, and comparison to established methods is essential.¹⁰ Compared with traditional nerve burial (in fat, muscle, or bone), TMR provides a physiological target for axonal regeneration.

In our technique, digital nerves—being purely sensory—were coapted to small, terminal motor branches of the lumbrical muscles. A single motor branch was transected, minimizing functional risk and likely preserving proprioceptive input, as the lumbricals typically receive multiple innervation sources.^6,9 The interossei muscles, if needed, can compensate for lumbrical function in fine finger positioning. The lumbrical muscles were selected over the interossei for their superficial course via a palmar approach, enabling safer dissection. The MEP is easier to locate at the proximal A1 pulley, and recent anatomical data show that lumbrical MEPs are more distal—closer to the digital nerves—than those of the interossei. This makes them well suited for TMR in this region. Additionally, lumbricals exhibit the highest mechanoreceptor density among intrinsic hand muscles, supporting their sensory receptiveness.⁹

We acknowledge that electromyographic confirmation of reinnervation was not performed, limiting the ability to verify functional integration. However, the goal in this setting is not motor restoration but prevention of neuroma formation. Pain relief remains the primary clinical endpoint.

Although all patients experienced resolution of stump neuroma pain, mild discomfort during strong grip persisted in some cases. This is likely related to excursion of the lumbrical muscle during activation of the flexor digitorum profundus tendons, which can transiently increase tension at the coaptation site. This mechanical pain is distinct from neuropathic stump pain and was of lower intensity, not interfering with daily function. Nevertheless, this potential outcome should be explicitly discussed with patients during preoperative counseling, and it should be considered when selecting candidates for this procedure.

Despite widespread enthusiasm for TMR and its application across various anatomical domains, prudent consideration is necessary regarding its comparative efficacy and superiority to conventional passive methods, such as simple nerve relocation to adjacent tissue. Comparative, larger studies with longer follow-up are needed to validate these findings.

DISCLOSURE

The authors have no financial interest to declare in relation to the content of this article.