QUANTIFICATION OF HUMAN UPPER EXTREMITY NERVES AND FASCICULAR ANATOMY

Abstract

Introduction

In this study we provide detailed quantification of upper extremity nerve and fascicular anatomy. The purpose is to provide values and trends in neural features useful for clinical applications and neural interface device design.

Methods

Nerve cross-sections were taken from 4 ulnar, 4 median, and 3 radial nerves from 5 arms of 3 human cadavers. Quantified nerve features included cross-sectional area, minor diameter, and major diameter. Fascicular features analyzed included count, perimeter, area, and position.

Results

Mean fascicular diameters were 0.57 ± 39, 0.6 ± 0.3, 0.5 ± 0.26 mm in the upper arm and 0.38 ± 0.18, 0.47 ± 0.18, 0.4 ± 0.27 mm in the forearm of ulnar, median, and radial nerves, respectively. Mean fascicular diameters were inversely proportional to fascicle count.

Conclusion

Detailed quantitative anatomy of upper extremity nerves is a resource for design of neural electrodes, guidance in extraneural procedures, and improved neurosurgical planning.

The goal of developing neuroprostheses and nerve repair techniques to restore function has continued to motivate investigation of intraneural morphology. Intraneural anatomy consists of epineurial loose collagenous tissue encasing fascicles, which are in turn enclosed in perineurial tissue that consists of several layers of living cells connected to each other by zona occludens or tight junctions. Although there are generalized topographical studies that map upper extremity nerve fascicles to their innervation points, quantitative data of both nerve and fascicular dimensions along the length of the entire arm in upper extremity nerves are limited. The purpose of this study was to quantify nerve and fascicular anatomy of the median, radial, and ulnar nerves along the arm to: (1) provide a range of summary values for use in electrode design and clinical procedures; and (2) evaluate trends in the quantified features.

METHODS

Nerve Histology

We dissected the upper extremity nerves of 5 arms from 3 embalmed cadavers. The exact composition of the embalming fluid was not known, but the removed nerves were preserved in 10% formalin. The ulnar, median, and radial nerves in each arm were identified and photographed in situ (Fig. 1A). Distances from the infraglenoid tubercle (axilla) to the medial epicondyle (elbow) and from the elbow to the radial styloid (wrist) were measured (Table 1). The position of the nerves at the axilla, elbow, and wrist was marked with dye before removal from the cadaver. From these 5 arms, 11 nerves were dissected and extracted, including 4 ulnar, 4 median, and 3 radial nerves. The branch points of origination for the anterior interosseous nerve (AIN) from the median nerve and posterior interosseous nerve (PIN) from the radial nerve were recorded (Table 1).

FIGURE 1.

Process overview. Extracted (median) nerve in the arm (A). The extracted nerve is laid out in relation to the shoulder, and each innervation point remains. The nerve was subdivided into the regions used in the analysis (B). The calculated convex hull of a representative cross-section (C) is shown by a connected line between each of the outer fascicles. The connected line segments originate from a point that represents the center of each outer fascicle in the convex hull. Fascicles not included in the convex hull yet are bisected by the line segments are also considered to be outer fascicles. The outer fascicles not included in the calculation of the convex hull are dashed. The inner fascicles are indicated by cross-hatched circles.

Table 1.

Gross anatomical measures

	Cadaver arms
Parameter	C1R	C2L	C2R	C3R	C3L
Gender	W	W	W	M	M
Axilla to elbow (cm)	20.5	20	20	28.6	28.4
AIN branch point (cm)	27.2	23	22	31.3	NR
Elbow to wrist (cm)	22.5	22	21	23.2	26.5
PIN branch point (cm)	19	20	20	NR	27.8

Measurements were determined from anatomical landmarks in cadavers C1–C3 in the left and right arms (L, R). Nerve branch measurements include the branch points of origination (cm) for the anterior interosseous nerve (AIN) from the median nerve and the posterior interosseous nerve (PIN) from the radial nerve. All subjects were >50 years of age. NR, measurement not recorded.

The nerve samples were sectioned and embedded in 1-cm paraffin blocks along the nerve. Proximal ends were marked to preserve orientation for slicing. Paraffin-embedded blocks were sliced into 5-μm-thick sections. These sections were stained with toluidine blue or methylene blue and mounted onto slides for imaging. Images were digitized using a Zeiss AxioImager microscope (Carl Zeiss Microscopy GmbH, Jena, Germany) (Fig. 1C).

Quantitative Feature Analysis

The section lengths of each nerve (Table 1) were normalized to a scale of 0–1, with the axilla defined as 0, elbow as 0.5, and the wrist as 1. Each nerve was then divided into 9 sections (Fig. 1B). The 7 middle sections (sections 2–8) each contained one-eighth of the normalized arm, whereas sections 1 and 9, the most proximal and distal sections, mapping to the axilla and the wrist, each contained one-sixteenth of the normalized arm. Samples in each section were interpolated and compared across cadavers to represent 9 regions of the arm that were studied by Sunderland et al.¹

A total of 68, 65, and 58 samples were processed for the ulnar, median, and radial nerves, respectively (see Tables S1–S3 in Supplementary Material, available online). Nerves and fascicles were traced using a custom program built in MATLAB, version 2012 (The MathWorks, Inc., Natick, Massachusetts). The program imported each image and prompted the user to outline a border or mark a line of interest using the mouse. The x–y position of each click was recorded and then scaled to the actual size of the nerve section to calculate the distance, perimeter, or area in the cross-section. The x–y vertices for the nerve features and the location along the arm were saved to data files.

Nerve Measurements

The vertices outlining the external boundary of the epineurium (see Fig. S1A, available online) provided the cross-sectional nerve area in 191 images (68 ulnar, 65 median, and 58 radial). The areas were normalized to the maximum area along the entire length of the corresponding nerve. The user marked the height (minor axis) and width of the nerve (major axis).

Fascicular Measurements

The numbers of fascicles in each cross-section were counted. The vertices outlining the perineurium (see Fig. S1B, available online) were analyzed to find the fascicular area (A) and perimeter (P) in 3,339 fascicles. The circularity² of the fascicles was quantified as the squared ratio of circular-equivalent radius for the measured area by the circular-equivalent radius based on the measured perimeter:

$$\begin{gathered} \mathit{Circulari}ty={\left(\frac{\mathit{Area}\mathit{Based}\mathit{Radius}}{\mathit{Perimeter}\mathit{Based}\mathit{Radius}}\right)}^2 \\ ={\left(\frac{\sqrt{\left(\frac{A}{\pi }\right)}}{\left(\frac{P}{2\pi }\right)}\right)}^2=\frac{4\pi A}{P^2} \end{gathered}$$

The fascicular diameter was reported as the circle-equivalent diameter based on the measured area. The percentage change of fascicular diameter values based on measured perimeters compared with fascicular diameter values based on measured area was quantified. There was a diameter percentage change >1% in only 1% of fascicles due to differences in the area and perimeter calculation methods in the image processing software. The largest percentage change of 10.5% was calculated in the smallest fascicle diameter of 0.09 mm.

The outer fascicles were defined as fascicles located along the perimeter of the nerve (Fig. 1C). To determine the number of outer fascicles, a convex hull was approximated by the centroids of the fascicles. The fascicles used to generate the convex hull were considered outer fascicles (Fig. 1C, solid black lines). In addition, any circular boundary that intersected with the convex hull (Fig. 1C, thin black lines) was classified as an outer fascicle (Fig. 1C, dotted black lines). All remaining fascicles were defined as inner fascicles (Fig. 1C, cross-hatched fascicles).

Statistical Analysis

Nerve area and fascicular count were linearly interpolated using the interp1 MATLAB function for all nerve regions. Upper arm and forearm fascicle diameters were compared using a 2-sample t-test. Measured features were compared within each nerve, and features with significant correlations are reported. Analysis of covariance (ANCOVA) with Bonferroni pairwise testing was implemented to compare adjusted means of fascicular diameters across cadavers.

RESULTS

Nerve Characterization

Nerve maps with representative cross-sections for each region were constructed for the ulnar, median, and radial nerves (see Figs. S2–S4, available online). The nerve area and nerve diameters are shown in Tables 2 and 3. The median nerve from 1 cadaver (number C1R) was excluded in the area analysis because the sample at the elbow location was not available. In all other samples, the elbow section represented the largest value and would most likely have been the largest in our data. Hence, including normalized data from this arm would incorrectly skew the data. The minor and major diameters (Table 3) and the area of the nerves (Table 2) followed the same trends for each nerve. Proximally (region 1), the ulnar nerve was the smallest nerve with a minor diameter of 3.19 mm, a major diameter of 3.91 mm, and an area of 10.16 mm². The radial nerve was the largest nerve in the proximal location (region 1) with a minor diameter of 3.42 ± 0.29 mm, a major diameter of 7.82 ± 0.81 mm, and an area of 24.6 ± 1.46 mm.² In the distal regions (regions 6–9), the radial nerve was the smallest nerve with a minor diameter of 0.93 ± 0.3 mm, a major diameter of 3.17 ± 0.52 mm, and an area of 2.52 ± 0.17 mm² at region 6. The area of the median nerve was statistically larger (P <0.05) than the ulnar nerve in regions 2, 3, 7, and 8, using a 1-tailed 2-sample t-test. The largest standard deviation of area was 7.76 mm² in the ulnar nerve and 18.87 mm² in the median nerve, both at the elbow (region 5).

Table 2.

Mean nerve area

Regions	Ulnar nerve area (mm2)	Median nerve area (mm2)	Radial nerve area (mm2)
1	10.16*	18.7*	24.6 ± 1.46
2	8.39 ± 2.2	15.7 ± 6.78	13.94 ± 6.49
3	8.65 ± 1.26	14.96 ± 4.66	9.46 ± 4.02
4	11.48 ± 6.27	18.85 ± 9.84	7.9 ± 1.3
5	14.88 ± 7.76	27.59 ± 18.87	10.21 ± 3.01
6	8.62 ± 4.88	10.94 ± 2.77	2.52 ± 0.17
7	5.34 ± 1.75	8.27 ± 2.18	1.59*
8	6.28 ± 1.01	9.92 ± 2.72	0.4*
9	9.53*	15.96 ± 9.78	—

Nerve area (mm²) in ulnar, median, and radial nerves across 9 regions.

^*One cadaver arm and only 1 sample.

Table 3.

Major and minor diameters

	Ulnar nerve		Median nerve		Radial nerve
Regions	DMinor (mm)	DMajor (mm)	DMinor (mm)	DMajor (mm)	DMinor (mm)	DMajor (mm)
1	3.19*	3.91*	3.93*	5.95*	3.42 ± 0.29	7.82 ± 0.81
2	2.58 ± 0.65	4.01 ± 0.16	3.24 ± 0.71	5.67 ± 1.21	3.28 ± 1.05	5.19 ± 1.23
3	2.6 ± 0.27	4.04 ± 0.33	2.99 ± 0.66	6.27 ± 0.85	2.82 ± 0.4	4.56 ± 1.43
4	2.91 ± 0.91	4.72 ± 1.48	3.35 ± 1.39	7.07 ± 1.29	2.29 ± 0.36	5.03 ± 1.08
5	3.24 ± 0.90	5.68 ± 1.33	3.81 ± 1.45	8.56 ± 2.78	2.72 ± 0.49	4.64 ± 0.55
6	2.33 ± 0.79	4.87 ± 1.49	2.73 ± 0.78	5.26 ± 0.58	0.93 ± 0.3	3.17 ± 0.52
7	1.93 ± 0.34	3.34 ± 0.52	2.3 ± 0.27	4.47 ± 0.71	0.79*	2.22*
8	2.01 ± 0.40	3.86 ± 0.86	2.42 ± 0.31	5.25 ± 0.83	0.39*	1.08*
9	2.42*	4.89*	3.16 ± 1.17	6.18 ± 1.72	—	—

Major diameters (mm) and minor diameters (mm) in the ulnar, median, and radial nerves across all 9 regions.

^*One cadaver arm and 1 sample.

Each nerve area was normalized to its maximum cross-sectional area along its normalized length. The maximum nerve area in the ulnar nerve was 14.88 ± 7.76 mm² (Table 2) at the elbow. Three of 4 ulnar nerves had an increase in nerve area at the elbow, region 5, and the nerve that did not increase in region 5 (C3L) had an increase in region 4 (Fig. 2A). The largest increase in normalized ulnar area was 0.46 ± 0.05 from regions 4–5, and the largest decrease was −0.47 ± 0.1 from regions 5–6. The median nerve (Fig. 2B) exhibited its largest nerve area at region 4, with a value of 21.4 mm² (C2L) and 12.62 mm² (C3R), and in region 5, with a value of 48.8 mm² (C2R). In all median nerves, the largest increase in normalized area was 0.41 ± 0.18 and occurred at its maximum nerve area region. All median nerves had the largest normalized area decrease of −0.48 ± 0.2 at region 6. Measurements in the radial nerve were taken from the axilla along the nerve in the arm and along the posterior interosseous branch after the posterior interosseous branching point. The cross-sectional nerve area in the radial nerve decreased as the nerve traveled from the axilla. The minimum nerve area of 0.4 mm² for the radial nerve occurred at region 8, which is where the motor branch of the radial nerve terminates proximal to the wrist.

FIGURE 2.

Nerve area. Normalized nerve area of cross-sections taken along the arm in each nerve (each arm is represented by a different shape). The line is a smoothed fit of the mean interpolated values at each region demonstrating the trends in the ulnar (A), median (B), and radial (C) nerves. The black vertical lines indicate the elbow location.

The nerve ratios were determined for all nerve cross-sections by dividing the length of the minor diameter by the length of the major diameter. We found a wide range of nerve ratios, from nearly flat at 0.2, to mostly round at 0.9. The mean ratios for ulnar, median, and radial nerves were 0.6 ± 0.13, 0.51 ± 0.13, and 0.51 ± 0.18. There was no statistical difference between the general cross-section populations of the nerves (see Fig. S5, available online), indicating a general flat configuration for all nerves. A regression line was fit to the ulnar nerve ratios as a function of distance from the axilla in each cadaver arm (r ≤ −0.67, P ≤ 0.09). The radial nerve was highly variable and had no significant correlation along the arm. In the median nerve, the lowest nerve ratio occurred at the elbow in all cadavers.

Fascicular Characterization

The total numbers of fascicles were counted in each nerve cross-section and interpolated at each region location (Fig. 3; and Tables S1–S3, available online). In the ulnar and median nerves, the fascicle count was higher in the forearm (region 7, ulnar 20.7 ± 1.4 and median 18.6 ± 3.3) compared with the upper arm (region 2, ulnar 14.1 ± 2.7 and median 14 ± 2.6), whereas in the radial, the number of fascicles decreased from 22.7 ± 9.9 in region 1 to 3 in region 6 at the forearm. The radial nerve near its termination in the forearm had the smallest fascicle count of 2. The next lowest fascicle count of 3 was found in subject C2R near the elbow in the ulnar nerve. The highest fascicle count in all nerves was found in the median nerve from C2L at region 6 with a value of 42 fascicles. The median nerves had a localized peak of 23.2 ± 3.8 in fascicle count, which occurred in the elbow region (region 5). This peak was bordered by regional decreases with fascicle counts of 18 ± 1.9 in the distal upper arm (region 4) and 19 ± 2.4 (region 6) in the proximal forearm.

FIGURE 3.

Fascicle counts. Fascicle counts of cross-sections taken along the arm (each arm is represented by a different shape). The line is a smoothed fit of the mean interpolated values at each region, demonstrating the trends in the ulnar (A), median (B), and radial (C) nerves. The black vertical lines indicate the elbow location.

Ninety-seven percent of all fascicles measured had a circularity value >0.75 (see Fig. S6, available online). Seventy percent of all fascicles in the upper extremity nerves had a circularity of >0.9, and 94% had a circularity ratio of >0.8. Fascicles were generally circular, as would be expected by the persistent internal fascicular pressure.³

The fascicular size distribution for each region is shown in Figure 4. The total numbers of fascicles across all regions were 1,283, 1,373, and 683 in the ulnar, median, and radial nerves, respectively. The fascicle diameter was almost always <1 mm (>97% of the fascicles). The 3 largest fascicular diameters were from the ulnar nerve, with values of 2.64 mm (C2L), 3.36 mm (C2R), and 4.22 mm (C2R). Standard boxplot values⁴ were used with whisker lengths of 1.5, and values were considered to be outliers if they were not within 2.7σ from the median value. Outliers (Fig. 4, open circles) comprised only 1.55%, 1.45%, and 2.64% of ulnar, median, and radial fascicles, respectively.

FIGURE 4.

Fascicle diameter. Boxplots of fascicular distributions along the arm for the ulnar (A), median (B), and radial (C) nerves along the partitioned sections. The circles with diamonds mark the median value. The upper bound of the box represents the 75th percentile, whereas the lower bound represents the 25th percentile. The whiskers encompass extreme data not considered to be outliers, and the outliers are shown by open circles. Dagger symbol (†) indicates that 1 cadaver was sampled in that region.

Fascicular diameters were found to be larger in the upper arm region (regions 1–5) than they were in the forearm region (regions 6–9) of the arm in the ulnar, median, and radial nerves (P = 0.05). The maximum fascicular diameters from each cross-section in the upper arm were also larger than the forearm maximum fascicular diameters (P = 0.05), but there was no statistical difference in the minimum fascicle diameters for all cross-sections. The upper arm fascicles in the ulnar nerve had a mean diameter of 0.57 ± 0.39 mm, whereas in the forearm they were only 0.38 ± 0.18 mm. The median nerve fascicular diameters in the upper arm averaged 0.6 ± 0.3 mm compared with 0.47 ± 0.18 mm in the forearm. This pattern was found to be inverse to the fascicle count along the lengths of the ulnar and median nerves. An exponential fit was applied to the relationship between mean fascicle diameter and fascicle count for each cross-section (Fig. 5). The curve fit had an R² value of 0.91 (ulnar) and 0.6 (median), showing that mean fascicle diameter generally decreased with increasing number of fascicles.

FIGURE 5.

Relationship between fascicle count and fascicle diameter. Power fit of mean fascicle diameter compared with fascicle count in the ulnar (A) and median (B) nerves for all sections of the nerves.

A 1-way ANCOVA was employed to determine a statistically significant difference between arms on mean fascicle diameter while controlling for the number of fascicles in the cross-section (see Table S4, available online). Linear models that predicted the relationship between the number of fascicles in each cross-section and the mean fascicular diameter (see Table S4 online) had a low R² value in the radial nerve (R² = 0.21), with higher R² values in the median (R² = 0.52) and ulnar (R² = 0.66) nerves. The radial nerve samples included in this analysis contained samples from above the elbow. Nerve cross-sections with a fascicular count of ≤5 fascicles (3 data points) were not included in the ulnar ANCOVA analysis in order to approximate the ulnar as a linear model. The slopes in the linear models were not significant (see Table S4 online) in the ulnar (P = 0.15) and median (P = 0.7) nerves. There was a significant effect of subjects on the mean fascicular diameter after controlling for the number of fascicles in each cross-section. A pairwise Bonferroni correction test was used to determine which cadaver arms were significantly different. In the median nerve, 1 (C3L) of 4 arms was significantly different (P = 0.05) from 2 arms (C2L and C2R). Of the 4 ulnar nerves, the C2R arm was significantly different from the C3R arm.

The number of inner and outer fascicles was calculated in each cross-section using the convex hull method. The inner fascicle count in the nerve cross-sections followed a trend similar to fascicle count and was fit to a line with an R² = 0.89 (see Fig. S7, available online). The x-intercept was 6.08, and the slope was 0.58. Therefore, nerves with >6 fascicles were roughly split between inner and outer locations.

DISCUSSION

The gross morphology of peripheral and autonomic nerves has been reported extensively in the surgical literature, yet there is a lack of a more detailed internal morphology. Internal morphology, including nerve size, fascicle count, fascicle distribution, and fascicle size, are important for surgical planning of procedures, such as nerve repair and graft donor selection; for external procedures, such as microneurography, intraneural injection, or local anesthesia; and for the burgeoning field of neural interface design. Although non-invasive technologies, such as ultrasound and MRI, are available for nerve ratio and nerve area measurements in vivo, fascicular count and diameters are challenging to quantify in vivo. Sir Sydney Sunderland produced the most detailed account of the internal nerve morphology to date, but it was limited in its sampling along the length of the nerve,⁵ thereby requiring estimation of the internal nerve characteristics in the missing regions. Further, computational models of nerve behavior are increasingly valuable for understanding neural prosthesis interactions with the nerve.^6–9 These models could benefit from more accurate representative cross-sections along the length of the nerves. We have addressed these gaps here.

Nerve Morphology

Limited measures of nerve area have been reported. Our measurements of the ulnar and median nerves (Table 2) showed cross-sectional areas to be larger than in previous ultrasound studies.^10–13 We expect this is because ultrasound¹⁴ cannot detect the same epineurial edges as detected by optical microscopy. Previous qualitative cadaveric studies have reported the ulnar nerve area to be large at the elbow.¹⁵ Our findings are in agreement, because there was a localized increase in neural area at the elbow in both the ulnar and median nerves (Fig. 2A and B) using quantitative techniques. The radial nerve decreased in area along the arm (Fig. 2C), because its main motor branches terminate proximal to the wrist. Thomas et al. reported that nerve area increases as the nerve crosses joints, which is consistent with our study.¹⁶ The area of every nerve decreased in the forearm below the elbow, potentially due to increased branching in the forearm in both the ulnar and median nerves. An additional decrease in area in the median nerve mid-forearm was due to the AIN branch. Sunderland and colleagues⁵ measured the mean value location for AIN branch location to be 5.1 ± 1.5 cm distal to the elbow, resulting in reduced median nerve area after branching. In our measures, the branch was 3.6 ± 2.1 cm distal to the elbow. The median nerves again increased in area at the wrist before entering the carpal tunnel.

Miedany et al.¹⁷ reported a nerve flattening ratio (FR) in a population of 78 subjects (aged 29–67 years) very similar to our FR ratio in the ulnar nerve at the elbow. The sonographic median nerve FR values^10–13 reported from populations of at least 19 wrists in subjects aged 23–71 were within 1 standard deviation of our FR value at the wrist. The ulnar nerve became less circular with increased distances along the arm. The median nerve was the most flattened in the elbow region for all median nerves.

Fascicle Count

The fascicle count in each nerve cross-section along the distance of the arm was consistent but more complete than the trends described by Sunderland et al.⁵ Interestingly, there was substantial variability, up to a standard deviation of approximately 10, in the number of fascicles in a cross-section across the sections within a region and between subjects. This is consistent with the data from 20 arms reported by Sunder-land et al.⁵ The number of fascicles in the median nerve generally increased between the axilla and wrist, with a slight increase at the elbow (regions 4 and 5). In contrast, the radial nerve fascicle count generally decreased from the axilla to the wrist, but also with a slight increase at the elbow. In the ulnar nerve, it was generally consistent between the axilla (region 1) and elbow (regions 4 and 5); unlike the median and radial nerves, it decreased at the elbow (region 5) and then increased to a higher count and remained consistent from past the elbow (region 6) to the wrist (region 9). The low count of fascicles and large fascicular diameter recorded at the ulnar elbow is counterintuitive. Sunderland and Bradley posited that nerve cross-sections with many small fascicles are more resistant to stretching and compression.¹⁸ The median and radial nerves support this assertion, but the ulnar nerve does not. This could be a consequence of the ulnar nerve’s passage through the medial epicondyle at the elbow, unlike the median and radial nerves.

Fascicle Size

Fascicular diameter did not correlate with location or nerve size, but was inversely correlated with the fascicle count. Similarly, other groups have shown cross-sectional images at the elbow in the ulnar nerve consisting of a low fascicle count and a very large fascicle.^1,14,19 Combining the knowledge of fascicle count in locations of the arm (Fig. 3) with the relationship between count and size (Fig. 5) can guide nerve block injections to avoid regions with nerve patterns that are more susceptible to trauma, such as a unifascicular nerve with a large diameter.²⁰ The number of fascicles on the surface vs. central within the nerve affects the dissection and approach required for nerve repair, as well as graft choice when bridging a longer gap. Inconsistent nerve repair outcomes have prompted exploration of intraneural fascicular anatomy.^21–24 Given that fascicles have been shown to be spatially organized in rats,^25–27 nonhuman primates,²⁸ and humans,^1,29–34 optimal nerve repair would require an alignment as closely as possible.

Effects of Tissue Processing and Other Factors

Neural histological processing introduces tissue shrinkage and could lead to neural quantitative values smaller than in vivo dimensions. Dyck et al.³⁵ reported that fixation reduced the fascicular area by 10%–43% in human cadaver nerves. Peripheral nerve fascicles in vivo are characterized as round,³⁶ and their diameters have been quantified based on their circular area.^37–39 The circularity index³⁵ describes values associated with shape distortion due to tissue shrinkage in tissue processing. A squared circularity index of <0.75 has been associated with 10% shrinkage in plastic-embedded sections.^40,41 Kundu et al.⁴² calculated a linear correction factor of 1.25 for fascicular diameters in immersion-fixed⁴³ and paraffin-embedded peripheral nerve porcine tissue. Other linear correction values reported for paraffin-embedded neural tissue, such as the brainstem and cerebrum, are in the range of 1.13–1.34.^44,45 The shrinkage change of the mean fascicle is within the variability across subjects. Therefore, we accept the circular assumption of fascicles and that our measures adequately represent the measures of the nerve. More importantly, the trends reported are valid and valuable.

Our measurements were obtained by manual evaluation of sections. Tan et al. examined the intra-user reliability in measurements of fascicular area and fascicle count and reported that, although there were differences in absolute values, the conclusions remained the same.²⁰ There have been reported differences across gender,²⁰ race,⁴⁶ age,⁴⁷ and body fat⁴⁸ in neural measurements. Our population is not large enough or stratified to separate on these variables. Although the absolute measures are likely to be affected by these variables, the ANCOVA supports the conclusion that the trends reported are consistent.

Implications for Neural Interface Design

Neural interfaces are designed to interact directly with nerves to restore various functions in neurologically impaired individuals.^49–52 Proper and safe design requires intimate knowledge of nerve anatomy. Important design parameters include the nerve flattening ratio,⁵³ fascicular counts, fascicular diameter distribution in each cross-section, and the location of fascicles.⁵⁴ These 3 parameters affect the electrical activation of axonal populations within nerves or the recording capabilities. Detailed neural anatomy is important to develop computational models. The sampling of cross-sections shown in Figures S3–S5 (available online) is a library of samples for model development. The trends and variability data can be used for developing Monte Carlo analyses in models important to electrode design and development of multipolar algorithms to improve selectivity of fascicles.^8,9

The number of exterior vs. interior fascicles can guide selection of electrode designs, such as spiral^55,56 or flat interface nerve electrodes (FINEs).^7,53 Penetrating electrodes, such as slowly penetrating interfascicular nerve electrodes (SPINEs),⁵⁷ longitudinal intrafascicular electrodes (LIFEs),^58,59 transverse intrafascicular multichannel electrodes (TIMEs),⁶⁰ and microelectrode arrays (MEAs),^61,62 may more easily selectively activate deeper fascicles and isolate targets from neighbors of varying diameters due to their localization inside the nerve.

Human anatomical data are especially important, because the nerve structures of preclinical animal models are significantly different, such that direct translation to the human requires additional careful assessment.^63,64 The nerve flattening ratio, fascicle count, and motor branch access are major factors in surgical planning of electrode placement.

In conclusion, many serial cross-sections from the axilla to the wrist of 4 ulnar, 4 median, and 3 radial nerves were histologically processed and quantified within 9 regions. There were several similarities and differences between each of the nerves. Several representative sections along the upper extremity and the quantified measurements from the ulnar, median, and radial nerves are available for researchers interested in designing neural technology and planning peripheral nerve clinical and surgical procedures.

Abbreviations

AIN

anterior interosseous nerve

ANCOVA

analysis of covariance

C1R

cadaver 1, right arm

C2L

cadaver 2, left arm

C2R

cadaver 2, right arm

C3R

cadaver 3, right arm

C3L

cadaver 3, left arm

CS

cross-section

FINE

flat interface nerve electrodes

FR

flattening ratio

LIFE

longitudinal intrafascicular electrodes

MEA

microelectrode array

NR

not recorded

PIN

posterior interosseous nerve

SPINE

slowly penetrating interfascicular nerve electrode

TIME

transverse intrafascicular multichannel electrodes