Thyl Snoeck, PhD, Haute Ecole Paul Henri Spaak, Department of Anatomy, Morphology and Biomechanics, Avenue Charles Schaller, 91, 1160 Brussels, Belgium. E:firstname.lastname@example.org
The axillary arch (AA) is a muscular anatomical variation in the fossa axillaris that has been extensively studied in cadaveric specimens. Within these dissections, different innervations of the AA have been proposed, but this has never been explored in vivo. Knowledge of the innervation of the AA is required in order to better understand its function (e.g. predisposition for certain sports and/or activities, understanding shoulder injuries in overhead sports). Here, we report on the use of surface stimulodetection electromyography (SSEMG) to resolve the innervation of the AA in 20 subjects (12 women, eight men – mean age of 21.3 ± 2.7 years) with a uni- or bilateral AA. SSEMG of each muscle [M. latissimus dorsi (MLD) and M. pectoralis major] was performed with a four-channel electrostimulation measuring system in order to determine the innervation of the AA. The results showed co-contraction of the MLD in 85% of the subjects after AA stimulation. In the remaining subjects, no specific localized response was observed due to non-specific nerve stimulation, inherent to the proximity of the brachial plexus in these individuals. Our findings demonstrate that SSEMG exploration offers a practical and reliable tool for investigating anatomical aspects of muscle innervation in vivo. Using this approach, we conclude that the AA receives the same innervation as the MLD (the N. thoracodorsalis), and may be considered a muscular extension of the latter.
There are two different approaches, both based on cadaveric dissection, to interpret the origin of the muscular variation: first, an embryological; and second, a functional approach.
1 Certain authors (e.g. Ruge, 1905; Baulac & Meininger, 1981) believe this muscle to be an embryological remnant of the M. panniculus carnosus (Besana-Ciani & Greenall, 2005), based on the fact that it regressed and created a muscular-tendinous bridge in the fossa axillaris. Because the AA is of pectoral embryological origin, it would therefore be innervated by the Nn. pectoralis medialis or lateralis.
2 To others (e.g. Le Bouedec et al. 1993), the common innervation of the AA represents a functional unity that can be interpreted as a fusion of the AA with the MLD or the AA with the MPM.
This study aims to determine the innervation of the AA by means of analysis of surface stimulodetection electromyography (SSEMG). This approach should be able to confirm or contradict the findings from cadaveric-based studies, and contribute to a possible explanation of muscular co-contraction (MPM and AA or MLD and AA). The volume and the function of the AA depends on this co-contraction and can lead to a possible TOS (Hafner et al. 2010).
Stimulation of the AA muscle and, hence, of the nerve’s axonal endings, implies a reaction of all muscular fibers innervated by it. The contraction of either muscle (MLD or MPM) was examined during AA stimulation to determine the common innervation of functional units by a single axonal ending (Brichet et al. 1980).
Materials and methods
A group of 239 Belgian physical therapy students, with a mean age of 21.3 ± 2.7 years, was examined. Twenty subjects (8.4%) presented with a uni- (N = 12) or bilateral (N = 8) AA after visual screening by two independent physical therapy experts (Fig. 1a). Nine subjects (six women, three men) with an AA and 11 (six women, five men) without an AA were retained for further investigation. The study protocol was approved by the local Bioethics Committee, and an informed, written consent was obtained from all participants.
The subjects were placed in lateral decubitus, with the arm of interest in a relaxed position at an abduction angle of 90°.
Surface stimulodetection electromyography was performed by means of a four-channel electrostimulation measuring system (Neuropack EP/EMG, Nihon Kohden America, CA, USA). Two sets of surface recording electrodes were placed on each muscle (MLD and MPM). The cathode was positioned on the muscle’s motor point, whereas the anode was positioned between the motor point and the tendon (Fig. 1b). The stimulating electrode was placed on the muscular belly of the AA. The stimulation current was a square wave pulse of 1 ms duration and of variable amplitude.
The electrodes on the MPM and MLD registered the depolarization of the muscular fibers. Contraction of the fibers occurred in response to stimulation of the AA. The SSEMG response was recorded and checked both visually and palpably.
All the stimulated AAs showed a visible, palpable and oscilloscopic response. The clinical data were consistent with the electrical activity recorded by the oscilloscope. In 17 arches (85%), an associated contraction of the MLD was detected without response from the MPM (Fig. 2).
In three arches (15%) only a plexus response, i.e. one that affected the entire plexus brachialis, was noticed. Here, specific stimulation failed to produce a localized response. A reduction of the stimulation intensity failed to show a more specific response. This type of activity reflects non-specific nerve stimulation, inherent to the proximity of the plexus in these individuals.
Axillary arch innervation has been extensively described, based on the analysis of cadaveric specimens (Takafuji et al. 1991; Omar et al. 2010; Wang et al. 2011). The present in vivo study constitutes a novel approach, using SSEMG to demonstrate functional muscle innervation. Nevertheless, exploration of this anatomical region with SSEMG revealed some restrictions. For example, in 15% (three out of 20) of the examined subjects, the SSEMG methodology was not able to discern AA innervation after selective and individual stimulation. This could be attributed to possible anatomical variations (e.g. proximity of muscular and nerve structures) of the plexus brachialis (Guy et al. 2011; Wang et al. 2011). Differences in depolarization, distinctive to muscle and nerve fibers, may also have played a role.
Despite these limitations, in 85% (17 out of 20) of subjects, SSEMG showed clear and coherent results concerning the innervation of the AA.
Based on Testut’s model (Testut, 1884), in which muscular variation shares a common innervation with the primary muscle, the positive SSEMG responses in 17 subjects confirm the AA as an extension of the MLD.
No results were found that would validate the AA as an anatomical variation of the MPM. Moreover, these results are not compatible with the embryological model, either because the AA is not pectoral in origin or, because this model is incorrect. This study clearly shows that the AA shares its innervation with the MLD through the N. thoracodorsalis. With this knowledge one can speculate whether sports activities involving the MLD may also impact on the AA and may modify the kinematic movement of the shoulder (Clarys et al. 2008). Moreover, certain activities involving the MLD (including certain sports and manual labor) can increase the volume of the AA and could therefore provoke a TOS (Provyn et al. 2011).
The results obtained from this study reveal a different outcome to that predicted by cadaveric investigations. However, the in vivo functional SSEMG explorations we report provide a realistic functional approach to anatomy permitting us to determine, in an in vivo situation, the functional entity of a muscle in specific open chain activity. Moreover, the use of electrically elicited contraction provides a very specific method to provoke tension in targeted muscles. SSEMG is routinely used in the clinical setting to investigate neuromuscular integrity, which in some cases extends to the diagnosis of aberrant innervations of skeletal muscle groups (e.g. to examine the possibility of substitution by collateral pathways or to assess the integrity of neuromuscular connections). Cadaveric dissection provides only a theoretical overview of functional anatomy, and cannot reliably assess factors such as hypoplasia or neurological functioning. Thus, the outcome of functional assays may not always conform to anatomical predictions based on cadaveric evidence (Clarys et al. 2008).
Anatomical cadaveric dissection and functional evaluation (e.g. using SSEMG) should therefore be considered as complementary approaches in order to understand kinematic and postural functioning. We believe that, in this setting, SSEMG is a recommended and pertinent method to explore the innervation of muscular anatomical variations.