Neuromuscular compartmentation of the subscapularis muscle and its clinical implication for botulinum neurotoxin injection

Abstract

In this study, using immunohistochemistry with fresh cadavers, deliberate histological profiling was performed to determine which fibers are predominant within each compartment. To verify the fascial compartmentation of the SSC and elucidate its histological components of type I and II fibers using macroscopic, histological observation and cadaveric simulation for providing an anatomical reference of efficient injection of the BoNT into the SSC. Seven fixed and three fresh cadavers (six males and four females; mean age, 82.5 years) were used in this study. The dissected specimens revealed a distinct fascia demarcating the SSC into the superior and inferior compartments. The Sihler's staining revealed that the upper and lower subscapular nerves (USN and LSN) innervated the SSC, with two territories distributed by each nerve, mostly corresponding to the superior and inferior compartments of the muscle, although there were some tiny communicating twigs between the USN and LSN. The immunohistochemical stain revealed the density of each type of fiber. Compared with the whole muscle area, the densities of the slow-twitch type I fibers were 22.26 ± 3.11% (mean ± SD) in the superior and 81.15 ± 0.76% in the inferior compartments, and the densities of the fast-twitch type II fiber were 77.74% ± 3.11% in the superior and 18.85 ± 0.76% in the inferior compartments. The compartments had different proportions of slow-fast muscle fibers, corresponding to the functional differences between the superior compartment as an early-onset internal rotator and the inferior compartment as a durable stabilizer of the glenohumeral joint.

Figure 1

Fascial septum of the subscapularis (SSC) in the cadaver. The red dotted line indicates the fascial septum separating the superior compartment from the inferior compartment (A). The superior and inferior compartments are clearly divided by the fascial septum (B). In the sagittal section view, the fascial septum divides the superior and inferior compartments (C). The red arrowheads indicate the fascial septum. The blue box indicates the histological evaluation. Sc superior compartment of the SSC, Ic inferior compartment of the SSC, S superior, M medial.

Figure 2

Ultrasound-guided image (A) of an injection into the subscapularis muscle (B) and sectional image (C) of the corresponding injected site. The ultrasound image demonstrates the needle placement (arrowheads). The blue dye is targeted to the superior compartment. The compartments are shown to be separated by the fascial septum. Red dotted line and red arrowhead indicate the fascial septum. LT lesser tubercle, SSC subscapularis muscle, Sc superior compartment of the SSC, Ic inferior compartment of the SSC; IS infraspinatus, A anterior, M medial, S superior, P posterior.

Figure 3

Neuromuscular compartmentation of the subscapularis muscle (SSC). Illustration (A) and Sihler’s stained specimen (B) of typical innervation pattern is shown (A). Main branches of the upper subscapular nerve (USN) and lower subscapular nerve (LSN) are identified to be compartmentalized; however, the tiny branches of the USN and LSN are widely distributed in the SSC. Red and blue lines indicate the USN and LSN, respectively.

Figure 4

Analysis of the muscle component histological profile of the subscapularis muscle. The 40 × magnified image of the superior compartment (row A, red box), transitional area around the fascial septum (row B, blue box), and inferior compartment (row C, green box) of the SSC are observed by the Masson’s trichrome (column 1) and immunohistochemistry (columns 2 and 3). Columns 2 and 3 indicate the fast and slow type muscles, respectively.

Figure 5

Regional immunoactivity ratio between the type I and type II muscles in the two regions (superior and inferior compartments) of the subscapularis muscle (SSC) presented in Fig. 4. Compared with the whole muscle area, the densities of the type I (slow-twitch) fibers were 22.26 ± 3.11% (mean ± standard deviation) in the superior and 81.15 ± 0.76% in the inferior compartments, and the densities of the type II (fast-twitch) fibers were 77.74% ± 3.11% in the superior and 18.85 ± 0.76% in the inferior compartments.