Embryological development and comparative anatomy of the brachioradialis muscle
During embryological development, the extensor forearm muscles differentiate before the flexors. In about 11-mm embryo length, a common extensor premuscle mass differentiates into three groups. The first group that develops into the extensors digitorum communis, carpi ulnaris, and digiti minimi, corresponds to the largest and most superficial group. The second group differentiates to BR, and extensors carpi radialis brevis and longus [7]. Therefore, alterations during this differentiation process from a common premuscular mass to distinct muscles could potentially give accessory muscles, such as the ABR.
Except for the embryological background of the typical BR, the muscle’s comparative anatomy is essential to better understand how an accessory muscle may develop. In some mammals, such as the Myrmecophagidae, the BR was considered to be composed of two heads, while recent studies have proved that the two heads have an independent origin, insertion, and innervation; therefore, it has been proposed that the BR typically coexists with the ABR [10].
Morphological variability of the accessory brachioradialis (ABR)
In the current study, the ABR was identified in 2.4%, with a bilateral presence in a male cadaver. The origin was adjacent to the typical BR, and the insertion onto the anterior surface of the proximal third of the radius. The innervation was provided by the RN that coursed posteriorly to the ABR. After a detailed literature search, only a few studies and case reports refer to the ABR variant. Recently, rare muscle variants of the upper limb have been studied to enhance the current knowledge of human morphological anatomy [18].
The ABR is considered a rare variant, with very few studies referring to its presence [2, 3, 5, 6, 11, 13]. Dawson [2] was the first to describe the ABR variant. The old studies conducted by Carrington et al. [1] and Gruber [3] recorded an incidence ranging between 0.5-1%, while the newer morphological study by Rodrigiuez-Niedefuhr et al. [11] identified a relatively higher incidence (2.8%). The ABR origin is relatively constant, proximal to the BR typical origin, while Rodriguez-Niedenfuhr et al. [11] identified, in a few cases, its origin from the humeral condylar ridge, distally to BR. Less frequent origin is considered when the ABR originates from the typical BR [2, 6, 11]. Tountas and Bergman [15] reported an ABR termed supinator longus accessorius or BR brevis (Gruber [3]) was described as arising adjacent to BR, inserting onto the radial tuberosity, and acting as a true supinator, or inserting into the supinator or the tendon of pronator teres or onto the ulna.
On the other hand, the ABR insertion is variable, including more frequently the radial tuberosity, along with the biceps brachii tendon, and less frequently the anterior surface of the radius, the supinator muscle, the pronator teres or rarely the sigmoid cavity of the ulna [2, 6, 11]. Hence, in the current study, the origin followed the current literature, while the insertion into the anterior surface of the radius is considered uncommon. Furthermore, in the current study, the ABR presence was bilaterally identified in a male cadaver. Such bilateral presence was also identified by Rodriguez-Niedenfuhr's [11] study, where they identified bilateral appearance in two cadavers. Lastly, its innervation has been studied only in the Rodriguez-Niedenfuhr et al. [11] study, which supported that the ABR was supplied by RN branches, similar to the current study.
Except from ABR, BR variants could also be detected, such as duplicated muscle bellies or tendons. Herma et al. [4] presented a report of a two-bellied BR coexisting with a duplicated RN superficial branch, predisposing to radial neuropathy. Nambi and Dhiwakar [9] identified a second aberrant BR muscular belly during the radial artery forearm flap. Except for the muscular part, the tendinous part could be presented with variant forms. Turkof et al. [16] studied the BR tendon split in 75 cadavers (150 upper limbs) and identified the split tendon in 9 upper limbs (6%). Interestingly, in 5 of them (3.33%), the RN superficial branch coursed between the two slips, indicating a possible site of compression [16]. Sanudo et al. [12], among 80 upper limbs, identified an BR uncommon insertion into the base of the third metacarpal bone.
Clinical implications of the brachioradialis accessorius
The BR is considered a possible site for RN compression, particularly when considering the muscle’s morphological variability [17]. In the current literature, few clinical cases refer to radial neuropathy due to the BR variants. Spinner and Spinner [13] intraoperatively identified the RN superficial branch running inferiorly to an ABR, causing progressive numbness in the right thumb dorsal aspect. Closely to Spinner and Spinner [13], Kravarski et al. [5], intraoperatively, identified another ABR form adjacent to the typical BR. Due to this close relationship between the ABR and RN superficial branch, the variant muscle could cause entrapment, the so-called Wartenberg’s syndrome, or cheiralgia paresthetica [5]. In the current study, the RN was coursing posteriorly to the atypical BR, and this course could potentially be an entrapment site. Closely to the current results, Rodriguez-Niedenfuhr et al. [11] observed a similar course of the RN posteriorly to the ABR. Muscle hypertrophy could precipitate this entrapment [4]. Nevertheless, surgeons should be aware of this variant while performing a radial forearm free flap to not confuse intraoperatively [5].
Limitations
The present study has a few limitations. Firstly, a higher sample would have been desirable (n = 83), particularly a mixed sample not exclusively derived from a specific (Poland, Central European) population, permitting us to generalize our findings. Wider populations and more extensive studies will enhance the current knowledge about the ABR variant. Also, the arterial supply and the relationship of the variant muscle with vascular structures were not possible to study in the two identified specimens.