Anatomical and histological analysis of the scalene triangle in a Japanese population: implications for thoracic outlet syndrome diagnosis and management

Abstract

Background

Thoracic outlet syndrome (TOS) arises from compression of neurovascular structures within the scalene triangle, costoclavicular space, or pectoralis minor insertion. In Western populations, the average scalene triangle base width is 10.7 mm; however, corresponding anatomical data for Japanese individuals are lacking, and the association between triangle dimensions and body size remains unclear.

Methods

This study assessed 42 Japanese cadavers (21 males, 21 females) to measure the scalene triangle base width, transverse anterior scalene muscle insertion width, and clavicle length using a sliding caliper (Model No. 19976; Shinwa Measures, Japan). The scalene triangle was defined as the space between the anterior scalene muscle, the middle scalene muscle, and the first rib. Muscle insertions were histologically evaluated using hematoxylin-eosin, Elastica-Masson, and Safranin O staining after formalin fixation and paraffin embedding. Statistical analysis was performed with EZR (version 4.0.2).

Results

The average base width of the scalene triangle was 8.2 mm, which was narrower than that reported in Western populations. Males had significantly wider triangle bases and longer clavicles than did females (p < 0.01). A positive correlation was found between the base width and clavicle length (r = 0.45, p < 0.01), suggesting that body size may influence the dimensions of the scalene triangle. In larger individuals, however, an increase in triangle size may be offset by proportionally larger nerves and vessels, potentially maintaining similar spatial crowding. Histologically, the anterior scalene muscle consistently inserted onto the superior, posterior, and inferior surfaces of the first rib and was adjacent to the parietal pleura. Conversely, the middle scalene muscle is inserted only onto the superior surface of the first rib. No fibrocartilage was observed at either insertion site, indicating a fibrous enthesis.

Conclusions

This study presents the first detailed anatomical and histological analysis of the scalene triangle in Japanese individuals, revealing wider base widths in cadavers than those reported in Japanese TOS patients undergoing endoscopic surgery. This discrepancy suggests a possible link between triangle narrowing and TOS pathogenesis, with implications for diagnosis and treatment strategies. Additionally, the proximity of the anterior scalene muscle to the parietal pleura may increase the risk of pleural injury during surgery in this region.

Clinical trial registration

Not applicable.

Background

Thoracic outlet syndrome (TOS), first described by Peet et al. in 1956 [1], results from compression of neurovascular structures by muscles or bones in one of three anatomical regions: the scalene triangle, the costoclavicular space, or the subcoracoid space at the insertion of the pectoralis minor muscle [2, 3]. TOS is classified into three types: venous, arterial, and neurogenic [4]. Clinical symptoms include pain extending from the neck to the upper limbs, muscle weakness in the upper limbs, sensory disturbances, coldness in the fingers, and paleness of the fingertips [5].

Diagnosis is based on the patient’s clinical history and physical examination, including provocation tests such as the Wright, Roos, and Morley tests [6]. Additionally, various imaging and functional studies are used in combination, including plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), electrophysiological studies, ultrasonography, and angiography [6, 7].

Electrophysiological studies are useful for differentiating conditions such as carpal tunnel syndrome and cubital tunnel syndrome; however, diagnosing TOS based solely on these tests is challenging, as TOS often coexists with these peripheral nerve entrapment syndromes [8]. Plain radiography is employed to detect cervical ribs and assess abnormalities in the clavicle or first rib [6–8]. Ultrasonography allows for the evaluation of scalene muscle morphology and subclavian arterial flow, whereas CT angiography is used to assess stenosis of the subclavian vein and artery [9, 10]. MRI is effective for excluding cervical spine pathology and for assessing soft tissue abnormalities around the brachial plexus [6, 7]. However, due to the absence of standardized diagnostic criteria, diagnostic strategies vary considerably among institutions [6, 7, 11].

Among the subtypes of TOS, neurogenic TOS (nTOS) is considered the most prevalent. Historically, it has been stated that more than 90% of TOS cases are neurogenic; however, this figure is often based on outdated literature or subjective clinical impressions and lacks robust empirical evidence [12]. Illig et al. conducted a prospective registry of patients referred for suspected TOS and found that 82% presented with symptoms consistent with nTOS, while 80% were ultimately diagnosed with moderate to severe nTOS. The annual incidence of nTOS in that region (with a population of approximately 3 million) was estimated to be 2–3 per 100,000 individuals [12], a more realistic value compared to earlier inflated estimates of 3–80 per 1,000.

nTOS is caused by compression of the brachial plexus, typically due to chronic impingement by the anterior scalene muscle or by abnormal fibrous bands or ligaments. In several cases, no distinct imaging abnormalities are observed, which complicates the diagnostic process [4]. However, recent advances in high-resolution ultrasonography and MRI have enabled improved visualization of anatomical anomalies such as cervical ribs or fibrous bands, thereby enhancing diagnostic accuracy [13]. Additionally, a recent systematic review and meta-analysis reported the prevalence and anatomical characteristics of the subclavius posticus muscle, highlighting its potential role in nTOS. Recognition of such anatomical variants is important in both diagnosis and surgical planning for TOS [14].

Treatment of TOS generally begins with conservative approaches, including pharmacotherapy with analgesics and anti-inflammatory drugs, physical therapy, and nerve block injections [8, 15, 16]. When these treatments are ineffective, surgical intervention is considered. Common procedures include first rib resection and scalene muscle release [7, 9, 10, 17, 18], although these techniques are associated with a relatively high incidence of complications [15].

Recently, endoscopic approaches have been increasingly reported, allowing surgeons to perform the procedure under direct visualization using a large monitor [9, 19]. The most frequent complications of TOS surgery are pleural and lung injuries, particularly during first rib resection, with incidence rates reported between 14% and 34% [15, 20].

At our institution, we perform endoscope-assisted transaxillary first rib resection to reduce surgical complications. This approach has successfully reduced the incidence of pleural and pulmonary injuries to 6% [9]. However, such injuries still occur, typically during detachment of the anterior scalene muscle [9, 19].

These findings underscore the challenges in diagnosing TOS and evaluating its epidemiology and treatment outcomes. Surgical complications remain common, with reported rates ranging from 16 to 34% [14, 19]. Even with endoscopic techniques, pleural and lung injuries cannot be entirely avoided [9, 19].

The thoracic outlet is a critical anatomical region through which neurovascular structures pass from the neck to the upper limb. In clinical anatomy, the thoracic outlet typically refers to the scalene triangle, costoclavicular space, and subcoracoid space as described above. However, in theoretical anatomical literature, the thoracic outlet is sometimes defined more narrowly as the lateral thoracic aperture, bordered by the first thoracic vertebra, the first rib, and the manubrium of the sternum. This distinction should be acknowledged to clarify terminology across different anatomical and clinical contexts. The scalene triangle is a narrow triangular space defined anteriorly, posteriorly, and inferiorly by the anterior scalene muscle, the middle scalene muscle, and the first rib, respectively. The anterior scalene muscle typically inserts on the scalene tubercle of the first rib, while the middle scalene muscle inserts more posteriorly and laterally. These muscular attachments are fundamental in defining the boundaries of the scalene triangle and are consistently described in standard anatomical texts such as Gray’s Anatomy and Clinically Oriented Anatomy [21, 22]. This compact anatomical space is traversed by the brachial plexus (C5–T1 nerve roots) and the subclavian artery [2, 3], In contrast the subclavian vein lies anterior to the anterior scalene muscle and does not pass through the triangle. The size of the costoclavicular space, located between the clavicle and the first rib, changes depending on the position of the upper limb and scapula [23]. Additionally, the space beneath the insertion of the pectoralis minor at the coracoid process also serves as a passage for the neurovascular bundle [2]. Furthermore, the anatomical structure of the thoracic outlet reflects musculoskeletal development during embryogenesis, and abnormal differentiation of muscles, ribs, or nerves derived from the sixth cervical and first thoracic somites is considered a contributing factor in the development of TOS [3].

Anatomical studies of the thoracic outlet have evaluated the size of the scalene triangle by measuring its base width, height, and volume [24, 25]. Dahlstrom et al. reported that the average base width of the scalene triangle was 10.7 mm [24]. Moreover, previous studies have shown that the volume of the scalene triangle is significantly smaller in patients with TOS than in formalin-fixed donor cadavers, suggesting that a reduced triangle volume may be a risk factor for the development of TOS [9, 19, 25]. However, it remains unclear whether this narrowing is due to congenital anatomical variation or secondary pathological changes such as muscle hypertrophy or fibrosis.

At present, the base width reported by Dahlstrom and Olinger [24], serves as a benchmark for evaluating the size of the scalene triangle and remains one of the few objective anatomical parameters included in diagnostic assessments for TOS [9, 19, 24]. However, their findings are based on cadavers of Western origin. Anatomical data on the scalene triangle in Asian populations, particularly in Japanese individuals with diverse body types, are still limited, and the variation in triangle shape with body build remains unclear.

Additionally, injection of botulinum toxin into the anterior scalene muscle has been reported to alleviate symptoms in patients with nTOS, possibly due to muscle relaxation and the consequent reduction in compression within the scalene triangle [26].

However, detailed anatomical studies on the anterior scalene muscle, particularly its spatial relationship with the parietal pleura, remain limited. Therefore, this study aimed to comprehensively investigate the anatomy and histology of the scalene triangle and its related structures in Japanese cadavers. Specifically, we measured the base width of the scalene triangle as a structural parameter, the lateral width of the anterior scalene muscle insertion as a muscle-specific characteristic, and the clavicle length as an indicator of body size. Furthermore, we analyzed the insertion sites of the anterior scalene muscle and middle scalene muscle on the first rib, examined their spatial relationship with the pleura, evaluated the tissue characteristics at these attachment sites, and explored potential anatomical mechanisms underlying the development of TOS and the occurrence of intraoperative complications.

Methods

Subjects

This anatomical and histological study was conducted at the Faculty of Medicine, Yamagata University, between April 1, 2022, and March 31, 2023. All cadavers used in this study were of Japanese origin and had been donated to our institution under the “Act on Body Donation for Medical and Dental Education,” established in 1983, by individuals who had provided written informed consent during their lifetime for the donation of their bodies for anatomical education and research. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Yamagata University (Approval No. 2021 − 211).

Overall, 42 cadavers (21 males and 21 females) were included. None showed evidence of trauma, neoplasms, prior surgery in the region of the scalene triangle, or a history of TOS. Cadavers with macroscopic evidence of severe muscle atrophy were excluded. The average age at death was 84.1 ± 10.8 years. The cadavers were fixed in 10% formalin, preserved in 30% ethanol, and stored in cloths containing phenol.

Tissue shrinkage caused by formalin fixation may have affected the anatomical measurements, which is one of the limitations of the present study.

Measurement of the scalene triangle

Medical students were exposed to the scalene triangle under the guidance of anatomy instructors, as part of a practical anatomy lecture. At the stage where the epidermis of the entire cadaver was removed, the distance from the central part of the sternoclavicular joint to the central part of the acromioclavicular joint was measured as the clavicle length using a sliding digital caliper (Model No. 19976; Shinwa Measures, Japan) (Fig. 1A). Next, the sternocleidomastoid muscle, omohyoid muscle, and clavicle were removed, exposing the anterior scalene muscle and middle scalene muscle, first rib, subclavian artery, and brachial plexus. We defined the interspace formed by the anterior scalene muscle, middle scalene muscle, and first rib as the scalene triangle (Fig. 1B). No cervical ribs or marked variations in the subclavian vein were observed. Anatomical variations of the brachial plexus in relation to the scalene muscles were evaluated and are described in the Results section. The anterior scalene muscle and middle scalene muscle were exposed from the first rib insertion point to the posterior of the first rib.

Fig. 1.

Neck anatomy. The clavicle length (A) is defined as the distance from the center of the acromial end of the clavicle (acromioclavicular joint) to the center of the sternal end (sternoclavicular joint). The solid black line indicates the base width of the scalene triangle, measured as the distance from the lateral insertion of the anterior scalene muscle to the medial insertion of the middle scalene muscle on the posterior aspect of the first rib. The broken black line indicates the transverse width of the insertion of the anterior scalene muscle. Panel (C) shows the scalene triangle, delineated by the anterior scalene muscle, middle scalene muscle, and the first rib, indicated by white lines

The distance from the lateral insertion of the anterior scalene muscle to the medial insertion of the middle scalene muscle on the posterior aspect of the first rib was defined as the base width of the scalene triangle. The distance from the medial to lateral ends of the anterior scalene muscle insertion on the first rib was defined as the transverse width of the anterior scalene muscle insertion (Fig. 1C). The base width of the scalene triangle and the insertion width of the anterior scalene muscle were measured using a sliding digital caliper, in the same manner as for clavicle length.

Preparation of histological specimens

To analyze the histological characteristics of the insertions of the anterior scalene muscle and middle scalene muscle, both muscles were bilaterally dissected in 42 cadavers. Tissue samples were obtained from the distal insertion sites of the anterior scalene muscle and the middle scalene muscle. The first rib was sectioned at the costovertebral joint and the costochondral junction, including the adjacent brachial plexus and subclavian artery (Fig. 2A). Overall, 84 scalene triangle specimens were fixed in 10% formalin, defatted with alcohol, and decalcified using G-Chelate Quick (Product No. GCQ-1; Feather, Japan).

Fig. 2.

Anatomy of the scalene triangle and a variant with muscle fusion (left side). A Anatomy of the scalene triangle. The triangle is formed by the anterior scalene muscle (ASM), middle scalene muscle (MSM), and the first rib, and contains the brachial plexus (BP) and subclavian artery (SA). The ASM, SA, BP, and MSM are viewed from the medial aspect, with both muscles attached to the first rib. This specimen is excised as a single anatomical unit (B) Fusion of the scalene muscles. In this specimen, the ASM and MSM are macroscopically fused at their insertions on the first rib, resulting in the absence of an intermuscular space and a base width of 0 mm. This finding reflects a true anatomical variant rather than a measurement error. Abbreviations: ASM anterior scalene muscle, MSM middle scalene muscle, BP brachial plexus, SA subclavian artery

Thin longitudinal sections, parallel to the direction of the muscle fibers, were prepared from the insertion regions using a feather-trimming knife (Product No. 7-3254-01; Genostaff, Japan). These tissue samples were post-fixed in 4% paraformaldehyde, embedded in paraffin, and sectioned at a thickness of 4 μm along the longitudinal axis. The sections were deparaffinized with xylene, rehydrated with ethanol, and stained with hematoxylin and eosin, Elastica-Masson, and Safranin O.

Histological observations focused on the tissue architecture at the insertion sites of the anterior scalene muscle and middle scalene muscle, specifically evaluating the extent of muscle attachment, the presence or absence of fibrocartilage, and the presence or absence of collagen fibers.

Statistical analysis

Statistical analyses were performed using EZR version 4.0.2. The Shapiro–Wilk test was used to assess the normality of the data regarding the base width of the scalene triangle, the transverse width of the anterior scalene muscle insertion, and the length of the clavicle. When the data followed a normal distribution, parametric tests (t-tests) were used to compare sex differences as well as left–right differences for each parameter. The Spearman correlation coefficient (rs) was used to evaluate correlations between parameters. A p-value of < 0.05 was considered statistically significant.

Additionally, a post-hoc power analysis was conducted using G*Power version 3.1.9.7 (Heinrich Heine University Düsseldorf, Germany). Based on the observed effect size (Cohen’s d = 0.87) for the sex difference in the base width of the scalene triangle, the statistical power was calculated to be 0.79, indicating an adequate sample size for detecting this difference.

Results

Measurement of the scalene triangle

The base width of the scalene triangle, transverse width of the anterior scalene muscle insertion, and the clavicle length averaged 8.2 mm (range: 0–17.7 mm), 17.2 mm (range: 11.0–24.9 mm), and 150.0 mm (range: 123.8–178.5 mm), respectively. The mean base width of the scalene triangle was 10.1 mm (range: 0–17.7 mm) and 6.2 mm (range: 0–15.8 mm) in males and females, respectively, showing a statistically significant difference (p < 0.01). No significant sex difference was observed in the transverse width of the anterior scalene muscle insertion, which averaged 17.8 mm (range: 11.0–24.9 mm) and 16.5 mm (range: 11.7–21.6 mm) in males and females, respectively. The average clavicle length was 158.3 mm (range: 140.0–178.5 mm) and 141.8 mm (range: 123.8–164.6 mm) in males and females, respectively, also demonstrating a statistically significant difference (p < 0.01) (Table 1). A base width of 0 mm was observed in one specimen in which the anterior scalene muscle and middle scalene muscle were macroscopically fused at their insertions on the first rib, with no discernible intermuscular space (Fig. 2B).

Table 1.

Comparison of measurements between men and women

Distance (mm)	Males	Females	p-value
Base width of the scalene triangle	10.1 ± 4.2	6.2 ± 4.2	< 0.01
Width of the anterior scalene muscle insertion	17.8 ± 3.0	16.5 ± 2.7	0.051
Clavicle length	158.3 ± 7.9	141.8 ± 10.0	< 0.01

Values are presented as mean ± standard deviation. Statistical analysis was performed using an unpaired t-test.

The width of the inferior border of the scalene triangle averaged 8.2 mm (range: 0–16.1 mm) and 8.1 mm (range: 0–17.7 mm) on the left and right sides, respectively. The width of the anterior scalene muscle insertion averaged 17.4 mm (range: 11.9–22.9 mm) and 16.9 mm (range: 11.0–24.9 mm) on the left and right sides, respectively. The clavicle length was 151.5 mm (range: 123.8–178.5 mm) and 148.6 mm (range: 125.7–173.2 mm) on the left and right sides, respectively. No statistically significant differences were observed between the left and right sides for any of the parameters (Table 2).

Table 2.

Left–right comparison of measurements

Distance (mm)	Left	Right	p-value
Base width of the scalene triangle	8.2 ± 4.7	8.1 ± 4.5	0.93
Width of the anterior scalene muscle insertion	17.4 ± 2.8	16.9 ± 3.0	0.39
Clavicle length	151.5 ± 11.7	148.6 ± 12.5	0.29

Values are presented as mean ± standard deviation. Statistical analysis was performed using a paired t-test

The correlation analysis revealed a moderate positive correlation between the base width of the scalene triangle and clavicle length (rs = 0.45, p < 0.01; Fig. 3). No significant association was observed between the base width of the scalene triangle and the transverse width of the anterior scalene muscle insertion, or between the clavicle length and the transverse width of the anterior scalene muscle insertion.

Fig. 3.

Correlation between the base width of the scalene triangle and clavicle length. A positive correlation is observed between the base width of the scalene triangle and the clavicle length, indicating that individuals with longer clavicles tend to have wider scalene triangles

Evaluation of the anterior scalene muscle and the middle scalene muscle

Anterior scalene muscle

In all specimens, the anterior scalene muscle inserted onto the superior, posterior, and inferior surfaces of the first rib and was in direct contact with the parietal pleura (Fig. 4A). In contrast, the subclavian artery passed deep to the anterior scalene muscle within the scalene triangle, continuing laterally toward the axilla (Fig. 4B). In some cadavers, the costocervical trunk originated at the point where the subclavian artery crossed posteriorly to the anterior scalene muscle (Fig. 4D).

Fig. 4.

Anatomy of the anterior scalene muscle (ASM) (left side). B Longitudinal section of the white-lined region shown in A. The ASM inserts on the superior, posterior, and inferior aspects of the first rib (white arrowheads). The subclavian artery (SA) passes deep to the ASM (D) Longitudinal section of the white-lined region shown in (C). Similar to (B), the ASM inserts on the superior, posterior, and inferior aspects of the first rib (white arrowhead). Unlike in (B), the subclavian artery (SA) shows branching at the point where it passes deep to the ASM. Abbreviations: ASM anterior scalene muscle, SA subclavian artery

Although the presence of a scalene tubercle on the superior surface of the first rib has been described in anatomical literature [21, 22, 27], no prominent tubercle or raised bony structure was observed at the insertion site of the anterior scalene muscle in any specimen. Anatomical variations in the course of the anterior scalene muscle relative to the brachial plexus were evaluated in 28 specimens in which the relevant structures were sufficiently preserved to allow detailed observation. Among these, variations were found in 14 specimens (50%). Unilateral variations were observed in 11 cadavers (left side in 4 and right side in 6), while bilateral variations were found in 3 cadavers.

Specifically, the C5 spinal nerve root pierced the anterior scalene muscle in 9 of 42 specimens (21%). In 4 of the 28 intact specimens (14%), the upper trunk of the brachial plexus (C5 and C6) passed through the anterior scalene muscle. In 1 specimen (4%), the C5 nerve root passed anterior to the anterior scalene muscle. Additionally, a muscular variant consistent with the scalenus minimus muscle, as previously reported [28], was identified in 7 of the 28 specimens (25%).

A schematic illustration is provided to clarify the spatial relationships between the anterior scalene muscle, the first rib, and the pleura (Fig. 5).

Fig. 5.

Schematic longitudinal section illustrating the relationship between the anterior scalene muscle (ASM), first rib (1st rib), and pleura. The red lines represent the ASM attaching to the superior, posterior, and inferior aspects of the 1 st rib. The parietal pleura is depicted adjacent to the inferior surface of the 1 st rib, and the visceral pleura and lung are located medially. Abbreviations:ASM anterior scalene muscle, 1st rib first rib

Histological examination confirmed that the muscle insertion extended across the superior, posterior, and inferior surfaces of the rib adjacent to the parietal pleura (Fig. 6A and B). Notably, more collagen fibers were observed in the inferior portion of the insertion than in the superior portion, suggesting a stronger mechanical attachment inferiorly (Fig. 6B). At the muscle-bone interface, fibrocartilaginous entheses, characterized by four layers—tendon, uncalcified fibrocartilage, calcified fibrocartilage, and bone—have been described in previous studies [29]. However, we did not observe this four-layered structure. Instead, the anterior scalene muscle exhibited a fibrous enthesis, in which the tendon inserts directly into the bone without intervening fibrocartilage (Fig. 6C and D).

Fig. 6.

Histological images of the anterior scalene muscle (ASM). Histological evaluation of the longitudinal section corresponding to Fig. 5B. Hematoxylin and Eosin staining (A), Elastica-Masson staining (B), and Safranin O staining (C) are shown. In A and B, the ASM is attached to the superior, posterior, and inferior surfaces of the first rib. More collagen fibers are observed in the inferior region than in the superior region, indicating stronger insertion at the inferior site (arrowhead). D shows a magnified view of C, where no fibrocartilage is observed at the insertion site, confirming that the structure is a fibrous enthesis Note: The parietal pleura could not be consistently identified in the histological sections. This may be due to detachment during specimen collection at the thoracic outlet, as well as during fixation and decalcification processes. Abbreviations:ASM anterior scalene muscle

Middle scalene muscle

The middle scalene muscle is inserted exclusively onto the superior surface of the first rib, with no extension to the posterior or inferior surfaces. The ventral ramus of the first thoracic spinal nerve (T1) passed between the middle scalene muscle and the parietal pleura (Fig. 7B). Unlike the anterior scalene muscle, which was in direct contact with the parietal pleura, the middle scalene muscle did not contact the pleura. Small blood vessels were also observed within the muscle tissue of the middle scalene muscle (Fig. 7D).

Fig. 7.

Anatomy of the middle scalene muscle (MSM) (left side). B Longitudinal section of the white-lined region in (A). In all specimens, the MSM was attached exclusively to the superior surface of the first rib. The ventral ramus of the first thoracic spinal nerve (T1) is visible inferior to the first rib. D Longitudinal section of the white-lined region in (C). As in (B), the MSM is attached to the superior surface of the first rib, and the T1 VR is observed posterior to the rib. In this section, the ventral rami of the cervical spinal nerves C5, C6, C7, and C8 are also identified, and small blood vessels are observed near the insertion site of the MSM. Abbreviations: MSM middle scalene muscle, T1 ventral ramus of the first thoracic spinal nerve, C5–C8 ventral rami of the fifth to eighth cervical spinal nerves

Histologically, the insertion of the middle scalene muscle was limited to the superior aspect of the first rib (Fig. 8A and B). Similar to the anterior scalene muscle, the attachment was classified as a fibrous enthesis, characterized by a direct insertion of the tendon into bone without intervening fibrocartilage (Fig. 8C) [29].

Fig. 8.

Histological image of the middle scalene muscle (MSM). Histological evaluation of the longitudinal section corresponding to Fig. 7B. Hematoxylin and Eosin staining (A), Elastica-Masson staining (B), and Safranin O staining (C) are shown. The MSM is attached only to the superior surface of the first rib and is not adjacent to the parietal pleura located inferior to the rib. As shown in C, no fibrocartilage is observed at the insertion site of the MSM, indicating that the structure is a fibrous enthesis. Abbreviations:MSM middle scalene muscle

Discussion

Anatomical considerations

In this study, the average base width of the scalene triangle in Japanese cadavers was 8.2 mm, approximately 2 mm narrower than that reported by Dahlstrom and Olinger for Western populations [24]. While their study did not report sex-based differences, our findings showed that the base width was significantly greater in males than in females. A positive correlation between the base width of the scalene triangle and clavicle length suggests that the dimensions of the triangle are influenced by body size. We selected clavicle length as an indicator of body size because it is anatomically close to the thoracic outlet and directly reflects variation in this region. These discrepancies are likely due to anthropometric differences between Western and Japanese populations. These findings support the use of clavicle length as a surrogate for body size in anatomical studies. Previous research has demonstrated a strong correlation between clavicle length and height, with longer clavicles being associated with taller stature across various populations. Fontana et al. demonstrated that clavicle length is strongly correlated with patient height and varies with overall body size, supporting its relevance in morphological analysis of the thoracic outlet region [30].

Dahlstrom and Olinger did not provide information on body size or assess its relationship with triangle dimensions. Conversely, our study found significant sex differences in clavicle length, implying that body size differences contributed to the observed anatomical variations.

Intraoperative endoscopic measurements in Japanese patients with TOS revealed a mean scalene triangle base width of 6.4 mm [9], which is narrower than the 8.2 mm found in our cadaveric study. Although causality cannot be inferred from cadaver data, narrowing of the scalene triangle may be a risk factor for TOS in Japanese individuals. These findings may aid in the development of diagnostic criteria for TOS in Asian populations; however, body size should be taken into consideration. The width of the anterior scalene muscle insertion was not significantly different between sexes and showed no correlation with triangle base width or clavicle length, suggesting it is an independent anatomical feature. Macroscopic dissection revealed that the anterior scalene muscle inserted broadly on the superior, posterior, and inferior surfaces of the first rib and was adjacent to the parietal pleura. In contrast, the middle scalene muscle was inserted only on the superior surface and was not in contact with the pleura. These anatomical differences suggest that detaching the anterior scalene muscle during surgery may increase the risk of pleural injury. Moreover, when combined with high-resolution imaging techniques such as MRI and ultrasound, these findings may aid in preoperative evaluation and surgical planning for TOS.

Anatomical variations in the course of the anterior scalene muscle relative to the brachial plexus were observed in 14 of 28 specimens (50%), including cases in which the superior trunk (ventral rami of C5–C6) passed through the anterior scalene muscle—a variation previously reported as a significant risk factor for nTOS [31]. Additionally, a muscular variant consistent with the scalenus minimus muscle, which has been reported to occur in approximately 30% of individuals [28], was found in seven of 28 specimens (25%). These anatomical variations are considered important factors in understanding the pathogenesis and clinical diversity of TOS. The identification of these anatomical variations reinforces the clinical utility of preoperative imaging in identifying high-risk configurations. MRI and ultrasonography have been shown to detect fibrous bands, cervical ribs, and anomalous nerve paths, and our anatomical data may assist in the interpretation of such findings. Moreover, the narrower triangle dimensions observed in Japanese cadavers highlight the need for population-specific anatomical reference values for more accurate diagnosis and surgical planning. Age-related changes in anatomical structures, including those in regions near the thoracic outlet such as the scalene triangle and clavicle, have been previously suggested. Although the average age of cadavers in this study was 84.1 ± 10.8 years, we did not perform subgroup analyses based on age. Future investigations with broader and younger age distributions are warranted to assess whether age-related changes affect the dimensions or morphology of the scalene triangle and related structures.

Histological considerations

Histological analysis revealed that the insertions of the anterior and middle scalene muscles consisted of fibrous entheses without fibrocartilage. The typical four-layer structure of fibrocartilaginous entheses (tendon, uncalcified fibrocartilage, calcified fibrocartilage, bone) [29] was not observed. In the anterior scalene muscle, the inferior portion of the insertion exhibited more collagen fibers than did the superior portion, suggesting a stronger mechanical attachment.

Fibrous entheses are generally found in diaphyseal bone regions where mechanical stress is relatively low, whereas fibrocartilaginous entheses are located near joints where strong compressive forces occur. These findings suggest that the anterior and middle scalene muscles attach in regions subject to less mechanical stress. This structural feature suggests that these muscles are primarily adapted to tensile rather than compressive stress, which may contribute to their susceptibility to chronic mechanical irritation in TOS. In addition, although age-related histological changes are known to occur in muscle attachment sites, this study did not include age-stratified histological evaluation. Future research should explore the potential influence of aging on the enthesis morphology and connective tissue composition of the scalene muscles.

Limitations

This study has some limitations. First, as it was conducted on cadavers, tissue properties might have been altered by formalin fixation, potentially affecting measurement accuracy. Second, all measurements were performed by a single observer, and interobserver reliability was not assessed, which may introduce observer bias. Third, although the sample size was determined based on sex balance and consistency with previous anatomical studies, a post-hoc power analysis confirmed its adequacy for detecting sex differences in the base width of the scalene triangle. Lastly, the base width ranged from 0 to 17.7 mm. Minimal values such as 0 mm may reflect true anatomical variations—as evidenced by the fusion of the anterior and middle scalene muscles in some specimens (Fig. 3)—but measurement error cannot be ruled out. Future studies should involve multiple examiners to enhance reproducibility and facilitate the assessment of interobserver reliability.

Fifth, this study did not assess the potential influence of age on anatomical or histological parameters. Although the average age of cadavers was 84.1 ± 10.8 years, age-related changes in the scalene triangle and muscle entheses may have influenced the findings. Future studies should incorporate age-stratified analysis to evaluate age-dependent anatomical and histological variation. Additionally, this study did not investigate the relationship between anatomical variations (such as the presence of the scalenus minimus or nerve piercings) and scalene triangle dimensions or histological findings. Moreover, the advanced age of the cadavers (mean, 84.1 years) may have influenced the soft tissue and bony structures, although an age-based subgroup analysis was not performed. The histological sectioning was perpendicular to the rib axis, which may have limited the ability to identify small bony landmarks, such as the scalene tubercle. Finally, comparison with clinical or imaging data from symptomatic patients was not included, limiting the direct applicability to clinical settings.

Conclusion

In Japanese cadavers, the average base width of the scalene triangle was 8.2 mm, narrower than that reported in Western populations. This width showed a positive correlation with clavicle length and significant sex differences, suggesting an association with body size. The anterior scalene muscle consistently inserted on the superior, posterior, and inferior aspects of the first rib, in contact with the parietal pleura, indicating a potential risk for pleural injury during surgery. Both scalene muscles exhibited fibrous entheses without fibrocartilage. These findings highlight the importance of anatomical assessment in TOS surgery and suggest that single-lung ventilation may help prevent iatrogenic injuries.

Abbreviations

TOS

Thoracic outlet syndrome

CT

Computed tomography

MRI

Magnetic resonance imaging

Authors’ contributions

T.N. conceived and designed the study, performed anatomical measurements and dissections, prepared specimens, and drafted the manuscript. H.S. and Mi.T. critically reviewed and revised the manuscript. Ma.T. and N.H. contributed to coordinating the body donation program and assisted with specimen preparation. All authors read and approved the final version of the manuscript.

Funding

This research received no external funding.

Data availability

The data supporting the findings of this study are available within the manuscript.

Declarations

Ethics approval and consent to participate

This study was approved by the Ethical Review Committee of the Faculty of Medicine, Yamagata University (Approval Number: 2021 − 211). All cadavers were donated under the “Act on Body Donation for Medical and Dental Education,” enacted in 1983, by individuals who had provided written consent during their lifetime for the use of their bodies in anatomical education and research. The study was conducted in accordance with the guidelines of the Ethics Committee of Yamagata University and the principles outlined in the Declaration of Helsinki.

Not applicable (cadavers were donated with prior written consent for research and education, as stated above).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.