How to Facilitate the Selection of Recipient Fascicles of the Ulnar Nerve during Nerve Transfers? The Vasa Nervorum as an Anatomical Landmark

Abstract

Introduction

Multiple nerve transfers have been described for the treatment of ulnar nerve injuries. We highlight the transfer of the anterior interosseous nerve to the motor fascicle of the ulnar nerve in the forearm. We emphasize the importance of correctly matching the function of the fascicles to optimize outcomes. The objective proposed is to identify the presence and topography of the vasa nervorum between the motor and sensory fascicles of the ulnar nerve, to validate it as an anatomical landmark in nerve transfer surgery.

Materials and Methods

A descriptive, observational, cross-sectional study was carried out on cadaveric material. Twenty upper limbs fixed with formaldehyde were used; they were injected with colored latex intravascularly and dissected. Two samples of ulnar nerves were collected for histological study. The presence or absence of the vasa nervorum and their topography between the motor and sensory fascicles of the ulnar nerve were identified.

Results

The presence of the vasa nervorum of the ulnar nerve between its motor and sensory fascicles was confirmed by anatomical dissection and histological study in 100% of the cases.

Conclusion

the vasa nervorum between the sensory and motor fascicles of the ulnar nerve is a constant structure that constitutes a valid reper for interfascicular dissection in nerve transfer surgery.

 

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Keywords

ulnar nerve - nerve transfer - vasa nervorum - anatomy - microsurgery

Introduction

The ulnar nerve provides motor innervation to most of the intrinsic muscles of the hand. Its injury causes a deficit in fine pincers and strength, mainly due to alteration of the first and second fingers, a deficit in the global grip of strength due to imbalance of digital flexion and extension, mainly of the fourth and fifth fingers, and a significant sensory deficit because it corresponds to a support sector which predisposes to injuries.[1] Ulnar nerve injuries classified as Sunderland grade 4 or 5,[2] in early stages, may be amenable to surgical treatment using various techniques: direct neurorrhaphy, nerve graft interposition, or nerve transfers.[1] Nerve transfers are a relatively recent advance in peripheral nerve surgery,[1] for the restoration of both motor and sensory deficits. In high-lying lesions, distal nerve transfers can provide a source of regenerative axons near the target organ, shortening reinnervation times. They have the advantage over other treatments, such as tendon transpositions, of preserving the original muscle-tendon relationship, respecting the muscle's biomechanics.

An important concept to keep in mind is avoiding aberrant reinnervation, for which knowledge of fascicular anatomy is essential, ensuring that specific fascicles are selected for proper reinnervation.[1] Several authors[3] [4] [5] describe a fascicular map with a consistent distribution pattern, which would allow us to understand its topography.

Regarding the anatomy of the ulnar nerve, at the level of the anterior antebrachial region, its fascicles are grouped in an orderly fashion, with the most lateral fascicles corresponding to the sensory branch, the central fascicles to the motor branch, and the most medial fascicles to its dorsal cutaneous (sensory) branch.[3] [4] [5] [6] ([Fig. 1]). Distally, it continues its course through Guyon's canal, in the carpus, where it divides into its two terminal branches, a superficial sensory branch and a deep motor branch.[7]

Fig. 1 Mapping diagram of the fascicular distribution of the ulnar nerve in the studied sections. A) Longitudinal section; B) Cross section. In red: Motor fascicles, DCB (Dorsal Cutaneous Branch), MB (Motor Branch Fascicles), SB (Sensory Branch Fascicles).

Most of the motor axons of the ulnar nerve travel through the motor branch, which is also made up of a large number of axons responsible for proprioceptive sensitivity.[8] The motor fasciculus corresponds to 30-35% of the cross-sectional area of the nerve[3] [4] and can be identified, by interfascicular dissection, as an independent anatomical structure 90 mm from the radial styloid process.[4] [5] We emphasize that this identification was described by performing an intraneural dissection from proximal to distal of the ulnar nerve (a maneuver that is attempted to avoid injuring fascicles).

This distribution corresponds to the funicular topography by zones described by Zancolli[9] for the ulnar nerve. The distribution of the fascicles in each zone of the ulnar nerve, along with their mapping and vascularization, is described. A pre-collateral zone is identified, located along the arm, characterized by multiple anastomoses and a fascicular distribution organized in complex plexuses. A collateral zone, at the level of the elbow, is described, where the fascicles corresponding to the motor branches for the flexor carpi ulnaris and the flexor digitorum profundus muscles are distinguished. A preterminal zone, located in the distal three-quarters of the forearm, is subdivided into a proximal sector (Zone A), where the fascicles of the dorsal cutaneous branch can be identified, and a distal sector (Zone B), where the funicular distribution allows the separation into two groups: motor and sensory, corresponding to the fascicles of the terminal branches.[9]

One of the methods to restore intrinsic muscle function in ulnar nerve injury is the transfer of the anterior interosseous nerve to the motor branch of the ulnar nerve in the forearm.[1] [6] [10] [11] Currently, the methods used to identify the recipient fascicle of this transfer are the retrograde interfascicular dissection from distal to proximal, from its division to the anastomosis site (which generates morbidity and possible fascicle injury),[12] or the division using a cleavage plane generated by mechanical dissection which can be imprecise.[6] [13]

It would be important to have a constant anatomical reference that would allow for the precise identification of the motor and sensory fascicles of the ulnar nerve. Some authors describe the presence of a longitudinal vessel of the vasa nervorum between both fascicles of the ulnar nerve.[6] [9] Therefore, the objective of this study is to demonstrate the variability in the presence or absence of this blood vessel and its topography between both fascicles, to validate it as an anatomical reference during nerve transfer surgery.

 

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Materials and Methods

This study consisted of a descriptive, observational, cross-sectional cadaveric study conducted from June to December 2022 in the Department of Anatomy of the Faculty of Medicine of the University of the Republic, Montevideo, Uruguay.

Twenty cadaveric upper limbs fixed with 15% formaldehyde (Montevideo Solution) were included.[14] These upper limbs were acquired through the Department of Anatomy of the Faculty of Medicine of Montevideo, Uruguay, and acquired through the faculty's cadaveric donation program. Adult cadavers of both sexes were included in the study. Cadavers with limbs that had undergone previous surgical interventions were excluded.

For intravascular injection with latex, the brachial artery was dissected and identified in the middle third of the arm and cannulated using an abbocath® 14 angiocatheter. Intravascular lavage was performed with 20 cc of water. The prevulcanized latex was then mixed with red acrylic ink, the latter in a ratio of 2 cc of acrylic ink to 10 cc of prevulcanized latex, to obtain 20 cc of colored latex. This was injected under manual pressure with a 20 cc syringe into the brachial artery distally, after ligating the vessel proximally, taking care not to exert excessive pressure to avoid rupturing small vessels.

The dissection performed replicated the technique described for the transfer of the anterior interosseous nerve to the motor branch of the ulnar nerve. The dissection was performed under optical magnification (3.5x). The approach was made to Guyon's canal,[6] [13] extending proximally over the wrist in a zigzag fashion, and then over the course of the ulnar nerve in the forearm ([Fig. 2]). The palmar carpal ligament, the palmaris minor muscle (if present), and the antebrachial fascia were exposed and sectioned ([Fig. 3]).

Fig. 2 Incision performed following Mackinnon's technique for Supercharge with Taleisnik's incision.

Fig. 3 Anterior view of the forearm after skin reflection and removal of the fascia. 1: Flexor carpi ulnaris muscle. 2: Ulnar nerve. 3: Dorsal branch of the ulnar nerve. 4: Ulnar vessels. 5: Flexor digitorum superficialis muscle. 6: Palmar carpal ligament. *: Deep branch of the ulnar nerve. +: Superficial branch of the ulnar nerve.

To identify the anterior interosseous nerve, the forearm flexor muscles are radially folded back to expose the pronator quadratus muscle. The anterior interosseous nerve and its accompanying blood vessels are observed along its proximal border. Proximally, the anterior interosseous nerve is released from the flexor digitorum profundus muscle. Distally, the course of the anterior interosseous nerve is dissected through the pronator quadratus muscle and sectioned at the midpoint of the muscle (where the anterior interosseous nerve begins to branch).[13] The anterior interosseous nerve is then shifted toward the ulnar nerve, and the transfer point is selected to avoid excessive tension on the anterior interosseous nerve.

Once the coaptation point is selected, the interfascicular dissection of the ulnar nerve is performed. The fascicles of the dorsal cutaneous branch are separated. The presence of the vasa nervorum is identified along the ulnar nerve by searching for it at the coaptation point of the transfer. The interfascicular dissection is then performed in the plane in which it is located. Both fascicles are separated at this point and identified by vascular loops.

In a second step, the nerve is exposed at the level of Guyon's canal, identifying its bifurcation into its sensory and motor branches. A retrograde interfascicular dissection is performed, coinciding with the dissection plane made at the forearm level. It is confirmed that this vessel is located between both fascicles (motor and sensory) of the ulnar nerve.

For histological processing, two 1-cm-long samples of the ulnar nerve were collected from two of the treated limbs. The samples were taken 6–7 cm proximal to the pisiform bone before dissection. The previously fixed nerves were sectioned, with a transverse section of each nerve sample every 2–3 mm. The samples were then passed through alcohols of increasing concentration, followed by paraffin embedding and the preparation of histological blocks. These blocks were subsequently sectioned into 4-micron slices, mounted on slides, and stained with Hematoxylin-Eosin for histological analysis.

Finally, photographic documentation and data collection were carried out in a table in Windows Excel 2021.

 

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Results

The presence of the vasa nervorum was identified in 100% (20 of 20) of the dissected upper limbs, and it was topographically located in the division plane of both fascicles of the ulnar nerve in 100% (20 of 20) of the dissected upper limbs, macroscopically ([Figs. 4] and [5]). In all cases, there was a correlation between the plane selected by the topography of the vasa nervorum and the division plane of the sensory and motor branches by retrograde dissection.

Fig. 4 Anterior view of the ulnar nerve. *: Vessel of the vasa nervorum.

Fig. 5 Interfascicular dissection performed, exposing the relationship of the vasa nervorum between the motor and sensory fascicles of the ulnar nerve. 1: Motor fascicle of the ulnar nerve. 2: Sensory fascicle of the ulnar nerve. *: Vasa nervorum.

Microscopically, the presence of the vasa nervorum was identified between both fascicles of the ulnar nerve in both histologically processed samples ([Fig. 6]). In one of the histological sections, this artery had dimensions of 230 × 200 microns.

Fig. 6 Section stained with hematoxylin-eosin of the ulnar nerve. *: Fascicular groups. A: Large-caliber artery of the vasa nervorum. V: Pathway through which the artery could be observed through translucent tissues.

 

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Discussion

Currently, the surgical technique described for the transfer of the anterior interosseous nerve to the motor branch of the ulnar nerve proposes two methods. It can be performed through retrograde dissection, which allows for the identification of an exact fascicular separation plane but adds greater morbidity. Other authors propose seeking a natural cleavage plane between the motor and sensory fascicles through mechanical dissection,[6] relying solely on theoretical knowledge of fascicular distribution, without a specific anatomical landmark to precisely determine the separation plane between the fascicles.

Performing interfascicular dissection without a defined plane could result in an inaccurate dissection, either excluding part of the fascicles directed toward the motor branch from the transfer or including fascicles intended for the sensory branch, which could lead to suboptimal outcomes.

For this reason, we prioritize the importance of recognizing a specific and constant repertoire that determines a plane of separation between both fascicles during surgery, avoiding recruiting axons toward fascicles of the sensory branch and utilizing as many fascicles of the motor branch as possible. This allows for optimized results.

Some authors describe the presence of a longitudinal vessel of the vasa nervorum between both fascicles of the ulnar nerve.[6] [9] Our study proposes to validate the vasa nervorum of the ulnar nerve as an anatomical resource for interfascicular dissection.

In all cases, we were able to find the presence of a vasa nervorum vessel running longitudinally in the plane between both ulnar nerve bundles, both macroscopically and microscopically.

The histological study revealed the presence of a vasa nervorum artery that was larger than the other arteries observed. Given its size and peripheral position, we hypothesized that this artery would be visible to the naked eye through the surrounding translucent tissues. These microscopic findings are compatible with our macroscopic findings.

We are aware that some authors propose histological identification of motor bundles using the acetylcholinesterase technique, although this technique is very difficult in cadaveric studies, where the cadaveric sample lacks the presence of this neurotransmitter.[15]

In the proposed experimental model, we were able to test our hypothesis, providing evidence for the use of this vessel as a constant anatomical reference for intrafascicular dissection in clinical practice.

Based on our clinical experience, a potential limitation of this reference is that the procedure is generally performed under ischemia with a pneumatic cuff at the brachial level, which in some cases makes identification of the vasa nervorum difficult.

A question we must address is whether fascicle selection could modify the functional outcome. We propose that it could, given that imprecise selection leads some authors to obtain good results, while others have worse results with the same technique.

Authors like Mackinnon[13] presented good results in their work with nerve transfer from the end-to-side anterior interosseous nerve to the motor branch of the ulnar nerve in the forearm. In their work, all patients showed reinnervation of the intrinsic muscles of the ulnar nerve of the hand. There was also improvement in sensory function, grip strength, and complete flexion of the metacarpophalangeal joints of the fourth and fifth fingers. No patient reported any functional deficit when performing tasks in the prone position.[13]

We recognize that other authors propose other techniques for nerve transfer, such as Bertelli,[12] who states that when performing the supercharge end-to-side transfer technique, intrinsic muscle function would take a minimum of 12 months to recover. In his work, he performs an end-to-end transfer of the anterior interosseous nerve to the motor branch of the ulnar nerve at a more distal level, close to Guyon's canal. However, he argues that even with a more distal and end-to-end transfer, none of his patients achieved complete correction of the ulnar claw or hand muscle function.[12] In other works, Bertelli instead proposes other nerve transfers to recover motor branch function, such as transfer using the thenar branch of the median nerve.[16]

Regarding our work, we emphasize the importance of fascicular distribution when performing transfers. It may be argued that methods that do not prioritize the identification of motor branch fascicles in their technique may have worse results because the axon transfer does not respect fascicular distribution.

Regarding the transfer of the anterior interosseous nerve to the motor branch of the ulnar nerve, the methods currently described for identifying the recipient fascicle for this transfer are retrograde interfascicular dissection from distal to proximal, from its division to the anastomosis site (which generates morbidity and possible fascicle injury),[12] or division using a cleavage plane generated by mechanical dissection, which can be imprecise.[6] [13]

Therefore, we propose that, to properly assess this technique, it is essential to determine a reproducible anatomical model that systematizes the surgical technique so that the results are comparable.

In this way, we prioritize the relevance of the presented work. We propose the vasa nervorum as the proposed model for separating ulnar nerve fascicles for the transfer of the anterior interosseous nerve to the motor branch of the ulnar nerve in the forearm.

 

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Conclusions

Macroscopically, the existence of the vasa nervorum between the motor and sensory fascicles of the ulnar nerve was confirmed as a constant structure; therefore, it constitutes a valid, consistent, and reproducible anatomical landmark for interfascicular dissection aimed at nerve transfer. Microscopically, the presence of a single large-caliber vessel was confirmed, which, due to its size and location, would be visible macroscopically.

 

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Conflicto de Intereses

Los autores declaran no tener conflicto de intereses

Acknowledgement

The authors express their gratitude to all those who decided to donate their cadavers to anatomical research. They would like to thank Dr. Agustin Bellouta (Assistant in the Department of Pathological Anatomy at the Hospital de Clínicas “Dr. Manuel Quintela”) and Br. Santiago Vidal (Assistant in the Department of Histology at the Faculty of Medicine of the University of La Rioja (UdelaR) for their collaboration in the processing and documentation of histological samples.

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Publication History

Received: 04 May 2024

Accepted: 24 March 2025

Article published online:
21 July 2025