Pautler B, Marchese C, Swancutt M, Olinger A, Beutel B
Kansas City University, Kansas City, Missouri, USA

INTRODUCTION
The ulnar nerve originates from the brachial plexus and travels distally to innervate various muscles and cutaneous regions of the upper extremity. Injuries to the ulnar nerve can be particularly debilitating for patients due to loss of function in several intrinsic muscles of the hand. One treatment option for high ulnar nerve lesions is nerve transfer from the anterior interosseus nerve (AIN) to the deep motor branch of the ulnar nerve. The transfer requires a working knowledge of internal fascicular relationships. This study aimed to completely map the peripheral branch patterns and internal topography of the ulnar nerve.

METHODS
Fifty-four embalmed cadaveric upper extremities were dissected from the level of the forearm to the hand to expose the ulnar nerve and its peripheral branches, including the dorsal cutaneous (DCB), volar sensory (VSB), and motor branches. The motor branch underwent additional internal neurolysis to record the topographic orientation of the abductor digiti minimi (ADM) and flexor pollicis brevis (FPB) fascicles, as well as a combined fascicle containing branches to several intrinsic muscles including all interossei, the third and fourth lumbricals, and the adductor pollicis muscle. This combined branch was collectively referred to and identified by its terminal branch, the first dorsal interosseus muscle (FDI). Utilizing a standardized grid system, the branch point location of each terminal branch was measured relative to the hook of the hamate. After 10 minutes wrapped in water-soaked gauze to rehydrate the tissues, diameters of the VSB, DCB, and motor fascicles were also measured 10 centimeters proximal to the hypothenar base. All measurements were taken using electronic calipers.

RESULTS
The mean branch point coordinates relative to the hook of the hamate for the FDI were 22.8 mm radial and 32.1 mm distal, 13.6 mm radial and 25.1 mm distal for the FPB, 7.7 mm ulnar and 5.2 mm proximal for the ADM, and 15.7 mm ulnar and 95.3 mm proximal for the DCB. Male donors had significantly more radial FDI (p<0.01) and FPB (p=0.04), and more ulnar (p=0.04) and proximal (p<0.01) DCB branch points. There were no differences in branch location with respect to laterality (p=0.26-0.84). The diameters of the peripheral branches were consistent between the specimens, with a mean of 2.3 mm, 1.9 mm, and 1.6 mm for the VSB, motor branch, and DCB, respectively. Proximal to the hook of the hamate, the internal topography of the ulnar nerve also showed consistency. From radial to ulnar, the fascicular arrangement was VSB, FPB, FDI/intrinsics, ADM, and DCB. After passing the hook of the hamate and coursing radially into the palm, this arrangement remained consistent with the FPB fascicle proximal to the FDI/intrinsics fascicle in 17 of 20 specimens. The remaining three specimens exhibited a 90° rotation with the FPB fascicle located volar to the FDI fascicle.

CONCLUSION
This study provides a complete characterization and mapping of the distal ulnar nerve and its peripheral branches. The reliable topography of the fascicles within the ulnar nerve, specifically its deep motor branch, may assist surgeons with AIN transfer placement and lead to improved patient outcomes.