Intramuscular Innervation of the Supraspinatus Muscle Assessed Using Sihler's Staining: Potential Application in Myofascial Pain Syndrome

Abstract

Despite the positive effects of botulinum neurotoxin (BoNT) injection into the neural arborized area, there is no anatomical evidence in the literature regarding the neural arborization of the supraspinatus muscle. The present study aimed to define the intramuscular neural arborized pattern of the supraspinatus muscle using the modified Sihler's staining method to facilitate the establishment of safe and effective injection sites in patients with myofascial pain in the supraspinatus muscle. Seventeen supraspinatus muscles from 15 embalmed cadavers were dissected. Precise suprascapular nerve entry locations were also observed. Intramuscular neural arborization was visualized by Sihler's staining. The supraspinatus muscle was divided into four portions named A, B, C, and D. The nerve entry points were observed in 88.2% (15 of 17 cases) of section B and 76.5% (13 of 17 cases) of section C of the supraspinatus muscle, respectively. The concentration of intramuscular neural arborization was highest in section B of the supraspinatus muscle, which was the center of the supraspinatus muscle. When the clinician performs a trigger point and a BoNT injection into the supraspinatus muscle, injection within the medial 25-75% of the supraspinatus muscle will lead to optimal results when using small amounts of BoNT and prevent undesirable paralysis.

Keywords: botulinum neurotoxin; myofascial pain syndrome; shoulder pain; supraspinatus muscle.

Figure 1

Illustration of the general supraspinatus muscle and the location of the nerve entry points of the supraspinatus muscle. The supraspinatus muscle is equally divided into four portions (A–D) between the medial border of the scapula and the greater tubercle of humerus, each representing 25% of the total length. Black dots indicate suprascapular nerve entry points of the supraspinatus muscle. MBS, medial border of the scapula; GTH, greater tubercle of the humerus.

Figure 2

Proportion of intramuscular innervation distribution of the suprascapular nerve to the supraspinatus muscle (above), and corresponding illustration (below). Violet-stained nerve structures are mostly observed in section B, followed by sections C, A, and D. MBS, medial border of the scapula; GTH, greater tubercle of the humerus.

Figure 2

Proportion of intramuscular innervation distribution of the suprascapular nerve to the supraspinatus muscle (above), and corresponding illustration (below). Violet-stained nerve structures are mostly observed in section B, followed by sections C, A, and D. MBS, medial border of the scapula; GTH, greater tubercle of the humerus.

Figure 3

Injection guidelines based on surface landmarks. Ultrasonography scanning was performed from the MBS to GTH (A). Location of the injection sites using two fingers from the MBS (B). The panoramic view of ultrasonography image showing the four divisions of the SS. The location of 1/4 of the SS can be easily predicted with two fingers (C). HR: humerus; TP, trapezius; yellow arrowhead, right next to the second finger; dotted black arrow, ultrasonography scanning site; red circles, injection points; Sup, superior; Lat, lateral.