*3.4. Landmarks*

With the TEL located at 0.00 mm in all the specimens, the division of the radial nerve into the superficial branch of the radial nerve (SBRN) and deep branch of the radial nerve (DBRN) occurred at a mean of 9.79 ± 12.2 mm distally. The arcade of Frohse (supinator arch) occurred at 42.3 ± 7.94 mm. The distal margin of the supinator, where the DBRN becomes the PIN, occurred at 78.6 ± 8.67 mm from the TEL.



\* Indicates significant difference in mean motor entry point distance between the listed muscle and the muscle below it (i.e., more distally) on the chart.

**Table 3.** Percentage distribution of each muscle among the branches of the radial nerve (e.g., muscular territories). Note: percentages for the brachioradialis (BR), the extensor carpi radialis brevis (ECRB) and the extensor digitorum (ED) sum to greater than 100% as certain specimens had dual innervation to these muscles.


**Figure 1.** Nerve to the brachioradialis (BR) arising from the superficial branch of a radial nerve (\*), with an additional smaller branch arising from the radial nerve proper (†). BR: brachioradialis; SBRN: superficial branch of the radial nerve; DBRN: deep branch of the radial nerve.

**Figure 2.** Nerve to extensor carpi radialis brevis (\*) arising from the superficial branch of the radial nerve. ECRB: extensor carpi radialis brevis; SBRN: superficial branch of radial nerve; DBRN: deep branch of radial nerve.

#### **4. Discussion**

### *4.1. Branching and Motor Entry Point Orders*

The order with which branches diverged from the main trunk of the radial nerve to serve the forearm extensors exhibited an overarching and relatively predictable pattern in most specimens. Deviation from the norm was notable in specific, consistent pairs (and one triplet) of muscles—often of the deep layer—whose branches typically exited near one another and thus were prone to swapping. This is evidenced by the statistically insignificant difference in the mean branch point distances between the ECRB and supinator; ED and ECU; ECU and APL and EDM; and the EI and EPL. There was similar variation in the MEP order, with statistically insignificant differences between the ECRB and supinator; ED and ECU; EDM and APL; and the EPL and EPB.

The overall MEP order found here is consistent with that described by Abrams et al., who used this metric to define the "order of innervation" [4]. Branovacki et al. produced a descriptive (i.e., not quantitative) account of the most common branch order in sixty specimens that agrees with the overall findings on the branching order here [5]. Lastly, although Mazurek and Shin did not explicitly report how they defined their "most commonly accepted order of innervation," their list does agree with this investigation's findings on the branching order [2].

Knowledge of the order in which muscles are innervated by the radial nerve, along with the length of individual branches, has important clinical implications. The shortest distance along a nerve trunk and the subsequent muscular branch (represented in this study by the MEP distance) is an important predictor of the order and timing of re-innervation (and thus the recovery of individual muscle function) after nerve injury [1,4]. Furthermore, the typical order in which nerves branch to serve muscles (here represented by branch distance) is an important factor to consider when localizing lesions in cases of nerve entrapment [2]. This information is particularly helpful when used in conjunction with knowledge of the locations of structures which are known to cause entrapment, such as the arcade of Frohse, as the relative location of these structures to individual motor branches can inform specific localization. Lastly, when understood together, branch order and MEP distance can aid in conceptualizing the overall course of the motor branches, which is important when dissecting in the vicinity of the nerve during surgery or planning for nerve transfers [8,10].

#### *4.2. Innervation of Brachialis by Radial Nerve*

This study noted the partial innervation of brachialis by the radial nerve in 60% of specimens, with the branch arising from the radial nerve at a mean of 66.4 ± 19.6 mm proximal to the TEL, in the plane between the brachialis and brachioradialis muscles. This is consistent with the findings of previous studies in Western populations that have documented rates of 50% [4], 65% [8] and 67% [7], and lower than the rates of 81.6% [11] and 100% [12] documented in Asian populations; Blackburn et al. hypothesized that part of this discrepancy may be due interracial differences [7]. As this branch from the radial nerve—present in potentially two-thirds of patients—travels in the plane between the brachialis and brachioradialis, care should be taken when surgically dissecting these two muscles apart (such as in an anterior approach to the humerus) to prevent the denervation of the lateral portion of the brachialis [7,11].

A survey of the six most commonly used preclinical anatomical texts at Tufts University School of Medicine—*Atlas of Human Anatomy*, 6th edition [13]; *Clinically Oriented Anatomy*, 7th edition [9]; *Essential Clinical Anatomy*, 5th edition [14]; *Grant's Atlas of Anatomy*, 13th edition [15]; *Gray's Anatomy for Students*, 3rd edition [16]; *Atlas of Anatomy*, 2nd edition [17]—yielded no mention of this common source of innervation. Given that the results of this study support the results of previous studies that the brachialis muscle is partially innervated by the radial nerve at a greater than 50% incidence rate, it can be argued that this should be regularly included in all textbooks and atlases, as well as in medical student anatomy curricula.

#### *4.3. Muscular Innervation by Superficial Branch of Radial Nerve*

The superficial branch of the radial nerve was found to provide sole innervation to ECRB in 25.7% of specimens in this study (Figure 2). The breakdown of the innervation source for the ECRB was remarkably consistent with that found by Abrams et al., who documented innervation rates of 25% by SBRN, 30% by radial nerve and 45% by what the authors termed PIN (which we would refer to as the DBRN as it has not yet reached the distal border of supinator; see below) [4]. These findings are noteworthy given that there exists a commonly held notion that the SBRN is a purely sensory nerve and serves no motor function. As above, none of the six textbooks surveyed discussed this common variant, and many reinforced the teaching that SBRN is sensory-only.

Furthermore, the current study sample also included one specimen with what appeared to be the primary innervation of the brachioradialis by SBRN (with an additional, much less substantial branch from the radial nerve, Figure 1 above). A search of the existing literature revealed no other studies that have reported such an anomaly. A limitation to this study is that it cannot confirm that this nerve from SBRN carried motor fibers, although its bulky appearance and the lack of other significant nerve supply was quite suggestive of this.

Knowledge of the potential for innervation of either the ECRB or brachioradialis muscles by the SBRN would be of significant importance in many surgical settings. Given the SBRN's very superficial location under the skin and lack of robust protection, it is susceptible to iatrogenic injury during various surgical approaches, including the commonly performed radial forearm free flap for the reconstruction of tissue defects, where overlying tissue is harvested while the SBRN is ideally left intact [18,19]. It is well known that injury to the distal portion of the nerve can result in intractable pain or neuroma formation [20], but there has been less focus on the potential for proximal injury, let alone the possible consequence of muscular denervation that such injury could entail. Although the SBRN

is more proximally protected thanks to the overlying mobile wad, it is still susceptible to injury or entrapment due to surgical dissection, post-surgical edema or scarring, or—in a case of parathyroid autotransplantation followed post-operatively by one of the authors (F.K.S.)—delayed transplanted gland hypertrophy. The knowledge that SBRN has a motor component in nearly a third of the studied specimens should prompt heightened concern for the preservation of the nerve if at all possible.

#### *4.4. Inconsistency of Nomenclature and Territories*

Through the course of reviewing existing literature, it became apparent that a disparity exists in the naming conventions for the DBRN and its successor, PIN, between clinicians and anatomists. Most notably, literature authored by physicians typically did not distinguish DBRN from PIN at all. Instead, the nerve that remains after the exit of SBRN from the radial nerve was referred to exclusively as the PIN by multiple authors [2,4,5]. This is in contrast to anatomical textbooks; for instance, *Clinically Oriented Anatomy* references the division of the radial nerve into the SBRN and DBRN, and describes the course of the latter as follows:

"the deep branch of the radial nerve, after it pierces the supinator, runs in the fascial plane between superficial and deep extensor muscles in close proximity to the posterior interosseous artery; it is usually referred to as the posterior interosseous nerve" [9].

*Terminologia Anatomica* agrees, recognizing a "deep branch" of the radial nerve that terminates in the "posterior interosseous nerve" [21]. It is unclear as to why clinicians deviate from this paradigm and combine DBRN into PIN; regardless, it leads to lack of clarity and represents a discrepancy between basic science and clinical terminology that would ideally be reconciled.

#### **5. Conclusions**

Although the radial nerve does exhibit a noteworthy degree of variability in the posterior forearm, in terms of both branch territories and order of innervation, this investigation identified a pattern that was overall quite predictable. The results here serve to provide a more cohesive view of the morphologic parameters and patterning of the radial nerve and its motor branches in the distal arm and forearm than has been offered in the existing literature and to bring to further light some notable variations have often been overlooked. Given that injury to, or disease of, the radial nerve can present significant functional limitations that negatively affect quality of life [22], it is imperative that the relevant clinicians understand the nerve's anatomy and its potential for variation in thorough detail. These findings will ideally help to inform clinicians in their approach to the examination and diagnosis of radial nerve pathology, as well as in the safe performance of surgical dissection in the nerve's vicinity.

**Author Contributions:** Conceptualization, F.K.S., R.S.L.; methodology, F.K.S., R.S.L.; software, J.J.S.; formal analysis, F.K.S., R.S.L., J.J.S.; data curation, J.J.S.; writing—original draft preparation, F.K.S.; writing—review and editing, R.S.L., J.J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to acknowledge Anne Agur for her assistance in revising this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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*Article*
