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Division of Plastic and Reconstructive Surgery, University of Western Ontario, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, Canada
Division of Plastic and Reconstructive Surgery, University of Western Ontario, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, Canada
Surgical Mechatronics Research Laboratory, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, CanadaDepartment of Mechanical and Materials Engineering, Western University, London, Ontario, Canada
Surgical Mechatronics Research Laboratory, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, CanadaDepartment of Mechatronics Systems Engineering, Western University, London, Ontario, Canada
Surgical Mechatronics Research Laboratory, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, CanadaDepartment of Mechatronics Systems Engineering, Western University, London, Ontario, CanadaSchool of Biomedical Engineering, Western University, London, Ontario, Canada
Corresponding author: Joshua Gillis, MD, Division of Plastic and Reconstructive Surgery, University of Western Ontario, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, 268 Grosvenor St, Room D0-215, London, Ontario N6A 4L6, Canada.
Division of Plastic and Reconstructive Surgery, University of Western Ontario, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, CanadaSurgical Mechatronics Research Laboratory, Roth McFarlane Hand and Upper Limb Center, St. Joseph’s Health Care, London, Ontario, Canada
The motor branch of the ulnar nerve contains fascicles that innervate the intrinsic musculature of the hand. This cadaveric study aimed to describe the organization and consistency of the internal topography of the motor branch of the ulnar nerve.
Methods
Five fresh-frozen cadaveric specimens with an average age of 74 years (range, 65–88 years) were dissected. The ulnar nerve was exposed and transfixed to the underlying tissues to maintain its orientation throughout the dissection. The dorsal cutaneous branch (DCB) and the volar sensory branch were identified and reflected to expose the motor branch. The fascicles to the first dorsal interosseus (FDI), flexor pollicis brevis, and abductor digiti minimi (ADM) were identified. Internal neurolysis was performed distal to proximal to identify the interfascicular arrangement of these fascicles within the motor branch. The organization of these fascicles was noted, and the branch points of the DCB, FDI, and ADM were measured relative to the pisiform using a handheld electronic caliper.
Results
The internal topography of the motor branch was consistent among all specimens. Proximal to the pisiform, the arrangement from radial to ulnar was as follows: volar sensory branch, flexor pollicis brevis, FDI/intrinsic muscles, ADM, and DCB. The position of these branches remained consistent as the deep motor branch curved radially within the palm and traveled to the terminal musculature. The locations of the average branch points of the FDI, ADM, and DCB with respect to the pisiform were as follows: FDI, 4.6 cm distal (range, 4.1–4.9 cm), 4.5 cm radial (range, 4.1–4.9 cm); ADM, 0.65 cm distal (range, 0.3–1.1 cm), 0.7 cm radial (range, 0.3–1.1 cm), DCB, 7.7 cm proximal (range, 4.2–10.1 cm), and 0.4 cm ulnar (range, 0.3–0.8 cm).
Conclusions
The internal topography of the ulnar nerve motor branch was consistent among the specimens studied. The topography of the motor branches was maintained as the motor branch turns radially within the palm.
Clinical relevance
This study provides further understanding of the internal topography of the ulnar nerve motor branch at the wrist level.
The ulnar nerve is a continuation of the C8-T1 infraclavicular component of the brachial plexus and is primarily responsible for innervation of the intrinsic musculature of the hand.
classic work in 1945 first described the internal topography of this nerve, demonstrating consistent organization of the volar sensory branch (VSB), motor branch, and dorsal sensory branch (DCB) within the nerve from radial to ulnar. Jabaley et al
later corroborated this finding in an evaluation of 4 ulnar nerves using microdissection techniques and documented the location of intraneural plexuses to validate the safety of intraneural dissection. Other authors have described the branching patterns of the cutaneous fascicles of the ulnar nerve and detailed their location with respect to common surgical incisions.
demonstrated that the abductor digiti quinti was the first branch from the motor branch that coursed radial to the pisiform.
Nevertheless, these reports do not include a detailed description of the internal interfascicular topography within the ulnar nerve motor branch at the proximal wrist level. A thorough understanding of this organization may provide insight into patient outcomes and assist clinicians when assessing traumatic injuries. This is also important in the context of nerve transfer surgery for those with high ulnar nerve lesions. In these patients, the use of an end-to-end or end-to-side nerve transfer from the anterior interosseous nerve (AIN) branch to the pronator quadratus into the motor branch of the ulnar nerve can improve intrinsic muscle recovery.
Knowing the internal fascicular topography of the motor branch of the ulnar nerve may help delineate the appropriate placement of this nerve transfer to better target the specific muscles, such as the dorsal interossei versus the abductor digiti minimi (ADM). This allows the surgeon to select the site of the epineurotomy or neurotomy depending on the transfer chosen.
This study aimed to describe the internal fascicular organization of the ulnar nerve motor branch in cadaveric specimens.
Materials and Methods
Ethics approval was obtained from the institutional review board of Western University prior to the commencement of this study, and all protocols were according to the Declaration of Helsinki of 1975 (revised in 2000). Five fresh-frozen upper limb cadaveric specimens, sourced from the Life Legacy donation program, were amputated at the proximal forearm. Prior to inclusion, the specimens were verified to be free from prior surgical intervention or traumatic injuries that would alter soft tissue structures by reviewing available medical records and inspecting for the evidence of cutaneous scars.
All dissections and measurements were performed using ×3.5 loupe magnification and began with the removal of the volar skin superficial to the palmar fascia. A longitudinal incision was made over the course of the flexor carpi ulnaris, and the ulnar nerve was identified proximally in the forearm. The ulnar nerve was dissected proximal to distal, using pins to transfix it to the underlying soft tissues to ensure its position was not altered or rotated. The DCB was identified and transfixed as it was encountered. The incision was then carried into the Guyon canal, and the skin radial to this incision was elevated and removed to expose the long digital flexors as they entered the carpal tunnel. The transverse carpal ligament was incised longitudinally and the median nerve, flexor digitorum superficialis, and flexor digitorum profundus were transected proximally and reflected distally.
The VSB was traced to the common digital nerves and then transected distally. These branches were transposed radially to allow for full visualization of the motor branch because it is the more radial structure at the wrist level. The motor branch was traced as it curved radially, once again transfixing the branches using pins to ensure relative positions were preserved (Fig. 1). The nerve was followed to its terminal branches of the deep head of flexor pollicis brevis (FPB), first dorsal interosseus (FDI), and ADM. With these terminal branches identified, interfascicular dissection was performed in a retrograde fashion to the DCB using microsurgical instruments and marked with pins. During the dissection, the motor branches to the FDI and individual lumbricals and interossei were kept as a group as they coalesced. The course of each fascicle was verified by 2 independent reviewers during the dissection. Internal neurolysis was employed every 5 to 10 mm, with the fascicles traced visually between these areas (Fig. 2).
Figure 1Anatomy of the ulnar nerve deep motor branch. Superficial structures have been removed to visualize the ulnar nerve motor branch. A Overview of the ulnar motor branch within the palm with fascicles to the intrinsic musculature, with terminal branches in the FDI and the deep head of FPB. B Magnified view demonstrating the FDI, FPB, and adductor pollicis with associated branches of the ulnar motor nerve.
Figure 2Interfascicular dissection of the deep motor branch. The individual fascicles of the motor branch were dissected in a retrograde fashion using loupe magnification to determine the internal topography of the motor branch within the ulnar nerve. A Motor branch to the FPB muscle (white arrows, most radial). B Motor fascicles to the lumbricals, interossei, and adductor pollicis muscle (white arrow, between 2 pins). C Motor branch to the ADM muscle (white arrows, most ulnar).
With all branches identified and transfixed to the underlying soft tissues, the branch points were measured relative to the pisiform using calibrated electronic calipers (accuracy, 0.05 mm; resolution, 0.01 mm). A 2-dimensional coordinate system was designed to normalize the description of branch location for specimens of different sides. The center of the pisiform was set as the origin, as it was easily palpable, and the y-axis was defined in line with the long axis of the limb, with distal denoted as positive and proximal as negative. The x-axis was defined perpendicular to the long axis with the radial direction denoted as positive and the ulnar side as negative. Pins affixing the nerve were then removed ensuring that nerve position was maintained.
Results
The average age of specimens was 73 years (range, 65–88 years). Four left-handed and 1 right-handed specimen were included.
The internal topography of the motor branches for all specimens was consistent (Fig. 3).
Proximal to the pisiform, the arrangement from radial to ulnar was as follows: VSB, FPB, FDI/interossei and lumbrical branches, ADM, and DCB. After the DCB and VSB left the main trunk, the orientation of the muscular branches remained consistent as the nerve coursed radially through the Guyon canal deep into the tendinous arch of the hypothenar muscles, around the hook of the hamate, and into the palm. In the palm, the arrangement of the muscular branches proximal to distal was FPB, FDI, and ADM.
Figure 3Pictorial representation of the internal topography of the ulnar nerve distal to the DCB takeoff. As demonstrated across all specimens examined, the ulnar nerve has reproducible internal topography of motor fascicles that is maintained throughout its course. Proximal to the wrist from radial-to-ulnar the branches are volar sensory, motor branch to the FPB, motor branch to interossei/adductor pollicis/lumbricals, motor branch to ADM, dorsal sensory branch. The organization of the motor branch is persistent after it dives deep within the hand and courses radially, with the ulnar most branches residing distal and radial branches becoming proximal during its transverse orientation. Cutaneous anatomy adapted based on the existing anatomic texts.
Two palmar cutaneous branches arise from the sensory component at approximately 6 cm and 2.5 cm proximal to the wrist crease, and a third branch is intimately associated with the motor branch of ADM.
The average branch point of the DCB was 7.7 cm proximal to the pisiform (range, 4.2–10.1 cm) and 0.4 cm ulnar to the pisiform (range, 0.3–0.8 cm). The average branch point of the ADM was 0.65 cm distal to the pisiform (range, 0.3–1.1 cm) and 0.7 cm radial to the pisiform (range, 0.3–1.1 cm). The average branch point of the FDI was 4.6 cm distal to the pisiform (range, 4.1–4.9 cm) and 4.5 cm radial to the pisiform (range, 4.1–4.9 cm). Data for all specimens are shown in Table 1.
Table 1Relative Position of Fascicles Within the Motor Branch
The position is relative to pisiform, with directions defined as follows: y axis = proximal (positive)/distal (negative) and x axis = radial (positive)/ulnar (negative). All measurements are in centimeters.
Specimen
Branch to FDI
Branch Point to ADM
DCB
X
Y
X
Y
X
Y
1
4.9
4.6
1.1
0.6
0.4
−8.1
2
4.6
4.9
0.75
0.4
0.4
−7.7
3
4.2
4.7
0.4
1
0.3
−10.1
4
3.9
4.9
0.3
1
0.3
−8.3
5
4.7
4.1
0.7
0
0.8
−4.2
Avg
4.46
4.64
0.65
0.6
0.44
−7.68
∗ The position is relative to pisiform, with directions defined as follows: y axis = proximal (positive)/distal (negative) and x axis = radial (positive)/ulnar (negative). All measurements are in centimeters.
An understanding of the anatomy of this nerve is important to accurately diagnose and effectively manage these injuries. Many other authors have previously described the course and branching patterns of the ulnar nerve, but the internal topography of the motor branch in the forearm has not been elucidated.
Azouz S, Lucas H, Mahabir R, Noland SS. The Prevalence and Practice Patterns of Anterior Interosseous Nerve to Ulnar Motor Nerve Transfer Use: a Survey of Hand Surgeons, 2018. Available at: https://peripheralnerve.org/meeting/abstracts/2018/EP9.cgi. Accessed November 18, 2021.
In the main ulnar nerve within the forearm, the order of fascicles from radial to ulnar is as follows: VSB, FPB, FDI/interossei/lumbricals, ADM, and DCB. After the takeoff of the VSB and DCB, the topography of the motor branch was maintained as it travels radially and transversely toward the thenar musculature, resulting in radial fascicles residing proximally in the radioulnar plane and ulnar fascicles distally (Fig. 3). Our branching points are consistent with previous reports. The branching point of the DCB in this study at 7.7 cm proximal to the pisiform is comparable to values previously published (Sunderland
The transfer of the AIN to ulnar motor fascicle has been used in an end-to-end fashion for high ulnar nerve injuries and in a reverse or “supercharged” end-to-side (SETS) technique for compression neuropathy or to act as a “babysitter transfer” to preserve motor end plates until nerve recovery. Despite promising initial clinical results, the physiologic basis of the AIN-SETS technique is unclear.
One hypothesis is that the axons preferentially innervate the recipient fascicles in closer proximity to the coaptation. Alternative explanations include axon infiltration across the width of the recipient nerve before proceeding distally, implying that all distal targets would be affected similarly.
A disparity in the muscular reinnervation patterns of the AIN-SETS was noted by Head et al,
who reported on a cohort of 17 patients. The FDI demonstrated significantly more recovery than the ADM regarding strength and EMG amplitudes, despite the FDI being a more distal target than the ADM.
The results of this current study may serve as a basis for understanding this differential reinnervation. In the original description of the AIN-SETS transfer by Brown et al,
the AIN is coapted to the radial aspect of the ulnar nerve motor branch, where, based on the results of this study, the FDI and FPB are located. Both muscles would improve pinch strength with this placement of the coaptation and may explain the clinical results. Further studies are warranted to determine whether this will make a clinically relevant difference.
The limitations of this study are primarily related to the small number of cadaveric limbs evaluated, which may not capture all anatomic variants. Although consistent topography was observed in our specimens, variations in the interfascicular organization of the ulnar motor branch may exist. However, the clinical implications of targeting specific interfascicular areas is currently unclear, and future clinical studies are required to determine whether the anatomic placement of the AIN-SET will affect clinical nerve recovery and outcomes. Surgical dissection and the processing of cadaveric specimens may have caused alterations to anatomy, although attempts were made to minimize distortions of the structures by transfixing the nerve in its native position, as well as using fresh-frozen cadavers. The strengths of this study include the application of high magnification and microdissection for precise dissection of the individual fascicles, multistep verification of the course of individual fascicles, and the use of fresh-frozen specimens.
This study reports the internal topography of the ulnar nerve motor branch, which may improve the understanding of the reinnervation patterns observed in the AIN-SETS transfer. This information may also provide insight into the mechanism of fascicular regrowth in end-to-side transfers and assist in clinical decision-making in the application of this technique.
Acknowledgments
The authors thank Christine Zanutto for her expertise in medical illustration and Mohammad Haddara for his contribution to the laboratory. This study was supported by the Opportunities Fund of the Academic Health Sciences Centre Alternative Funding Plan of the Academic Medical Organization of Southwestern Ontario (AMOSO).
References
Sunderland S.
The intraneural topography of the radial, median and ulnar nerves.
Azouz S, Lucas H, Mahabir R, Noland SS. The Prevalence and Practice Patterns of Anterior Interosseous Nerve to Ulnar Motor Nerve Transfer Use: a Survey of Hand Surgeons, 2018. Available at: https://peripheralnerve.org/meeting/abstracts/2018/EP9.cgi. Accessed November 18, 2021.
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