Botulinum neurotoxin has become a popular treatment in recent years due to its proven safety and effectiveness in reducing muscle tone from aesthetics to rehabilitations in focal spasticity of the upper limb, leading to an improvement in active upper limb function [1-9]. Several clinical trials have included the shoulder muscles, and they have shown positive results in reducing shoulder spasticity and improving shoulder function [10-16]. In fact, an observational study of chronic post-stroke spasticity has also demonstrated that botulinum toxin injections can reduce pain and improve shoulder function [17]. However, despite these findings, no specific injection points have been suggested for the teres minor muscle so far.

To achieve maximum effectiveness, administering botulinum neurotoxin injections requires precise dosage and location. Studies have shown that injections near the neural arborized region, which has a high density of neuromuscular junctions, are especially effective [18-24]. However, it can be challenging to locate small nerves accurately through visual inspection. To overcome this issue, researchers have used a technique called Sihler staining, which highlights nerves while making muscle fibers transparent [25-34]. The objective of the current study is to utilize Sihler staining to better understand the distribution of intramuscular nerves in the teres minor muscle.

This study was conducted in compliance with the Act on Dissection and Preservation of Corpses of the Republic of Korea (Act number: 14885) and approved by the Institutional Cadaver Research Committee of the College of Medicine, the Catholic University of Korea (MC22SISI0056).

Before beginning the dissection, consent and approval were obtained from the families of the cadaver subjects. The study utilized a modified version of Sihler’s method to reveal the intramuscular nerve patterns in the teres minor muscle, using 6 cadavers. The cadavers were of Korean origin, 3 males and 3 females, age ranging from 66 to 78 years were used in the study. The twelve teres minor muscles were dissected and aligned according to anatomical structure (as depicted in Fig. 1) and stained to detect intramuscular neural distribution. The teres minor muscle was extracted from the 2/3 point of the axillary border of the scapula (0/5), where the muscle originates ant the insertion point of the greater tubercle of the humerus (5/5) (as illustrated in Fig. 2). The arborization patterns in the muscles were analyzed in relation to the vertical length of the muscle, which was divided into 5 parts. This division was based on previous studies that found botulinum neurotoxin to spread a few centimeters from the injection site [17-19].

The teres minor muscle originates from the back surface of the axillary border of the scapula, covering about two-thirds of its length. Additionally, it originates from two aponeurotic laminae that separate it from the infraspinatus and teres major muscles. The main function of the teres minor muscle is lateral rotation, and it also contributes to shoulder adduction in conjunction with the infraspinatus muscle. However, the two muscles are distinguished by their nerve supply, with the teres minor muscle being supplied by the axillary nerve and the infraspinatus muscle by the suprascapular nerve. If either muscle becomes stiff, it can restrict movement of the shoulder joint, leading to limitations in movement [35].

Chemical neurolysis with phenol, alcohol, and botulinum neurotoxin injections is a potential treatment for muscle stiffness [36]. Botulinum neurotoxin works by blocking the release of acetylcholine from nerve endings at the neuromuscular junction, resulting in selective effects on motor nerves without impacting sensory nerves [27, 37-39]. Compared to phenol and alcohol, botulinum neurotoxin has lower side effects, making it a popular option for managing spasticity. To achieve the maximum effect, it is recommended that these injections be administered close to areas where neuromuscular junctions are most densely distributed.

The muscles located at the back of the shoulder include the posterior deltoid, infraspinatus, and teres minor muscles, which can cause limited internal rotation due to muscle stiffness [40]. The findings of this study on the injection point may help to improve shoulder movement.

The significance of using neuromuscular junction-targeted botulinum neurotoxin injections has been confirmed in clinical studies in the biceps brachii muscle and iliopsoas muscle. Neuromuscular junction-oriented injections result in much greater volume reduction than control injections [18, 19].

In this study, the origin and insertion point of the teres minor muscle were selected as the reference point for injections, as they are easily identifiable, especially in patients with limited upper extremity movement that may make it challenging to locate other landmarks.

There are limited studies that have investigated the anatomy of the teres minor muscle. Our study identified the location of the intramuscular neural dense area of the nerve branch that connects to the teres minor muscle. This area was found to be located between 1/5 and 3/5 from the reference point and these areas should considered in clinical fields (Fig. 5). This finding could serve as a helpful guideline for injections targeting the teres minor muscle.