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Quadrangular space

The quadrangular space, also known as the quadrilateral space, is a diamond-shaped anatomical compartment located in the posterior aspect of the axilla within the shoulder region, serving as a conduit for neurovascular structures exiting the axilla to supply the posterolateral shoulder.^[1]^[2] It is bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, medially by the long head of the triceps brachii muscle, and laterally by the surgical neck of the humerus, forming a tetrahedral space approximately 2–3 cm in diameter.^[1]^[2] The primary contents include the axillary nerve, which arises from the posterior cord of the brachial plexus (roots C5–C6) and divides within the space into anterior and posterior branches to innervate the deltoid and teres minor muscles as well as the superolateral brachial cutaneous nerve, and the posterior humeral circumflex artery (PHCA), a branch of the third part of the axillary artery that provides approximately 64% of the blood supply to the humeral head via its anterior and posterior branches.^[1]^[2] Clinically, the quadrangular space is significant due to its association with quadrangular space syndrome (QSS), a rare compressive neuropathy or vasculopathy where fibrous bands, hypertrophy of surrounding muscles, or repetitive overhead activities lead to entrapment of the axillary nerve and/or PHCA, resulting in symptoms such as posterior shoulder pain, deltoid weakness, paresthesia over the lateral arm, and potential vascular compromise like PHCA thrombosis or occlusion.^[1]^[2] This condition is most prevalent among overhead athletes, such as volleyball players and swimmers, and diagnosis often involves MRI or angiography, with treatment ranging from conservative management to surgical decompression.^[2] Anatomical variations, including tortuosity of the PHCA or early branching of the axillary nerve, can predispose individuals to QSS by altering the space's dimensions.^[1]

Anatomy

Overview

The quadrangular space, also known as the quadrilateral space, is one of three key anatomical spaces in the posterior aspect of the axilla, alongside the triangular space and triangular interval, and is characterized by its quadrilateral shape resembling a four-sided foramen.^[3]^[1] This space facilitates the passage of structures from the axilla to the posterior compartment of the upper arm.^[4] Positioned near the surgical neck of the humerus, the quadrangular space lies in the posterior shoulder region, acting as a conduit that connects the anterior axillary compartment with the posterior shoulder compartments.^[1] In adults, it typically measures approximately 2 cm in height and width, though dimensions vary individually, with reported mean depths of about 1.2-1.5 cm based on cadaveric studies.^[5]^[6] Functionally, the quadrangular space serves as a critical passageway for neurovascular structures that supply the posterior shoulder and deltoid region, including the axillary nerve and posterior humeral circumflex artery.^[1] This arrangement supports the innervation and vascularization essential for shoulder mobility and stability.^[4]

Boundaries

The quadrangular space, also known as the quadrilateral space, is defined by four distinct anatomical boundaries that enclose this region in the posterior aspect of the shoulder. The superior border is formed by the inferior margin of the teres minor muscle, which arises from the lateral border of the scapula and inserts onto the greater tubercle of the humerus, contributing to the space's upper limit.^[7] The inferior border consists of the superior margin of the teres major muscle, originating from the inferior angle of the scapula and inserting into the medial lip of the intertubercular sulcus of the humerus, providing the lower demarcation.^[7] Medially, the boundary is established by the long head of the triceps brachii muscle, which originates from the infraglenoid tubercle of the scapula and extends along the posterior humerus, with its lateral margin forming the medial wall of the space.^[1] Laterally, the medial aspect of the surgical neck of the humerus serves as the boundary, representing the proximal portion of the humeral shaft just distal to the anatomical neck, completing the enclosure.^[1] These structures intersect to create a diamond-shaped aperture, with the teres minor and teres major muscles converging laterally toward the humerus, thereby delineating a confined quadrangular region that facilitates the passage of neurovascular elements from the axilla to the posterior arm.^[7] This configuration underscores the space's role as a critical anatomical compartment, bounded by muscular and osseous elements, though fibrous bands may be present as anatomical variants.^[1]

The quadrangular space serves as a conduit for key neurovascular structures transitioning from the axilla to the posterior aspect of the shoulder, primarily the axillary nerve and the posterior humeral circumflex artery (PHCA), which are typically accompanied by the posterior humeral circumflex vein.^[1]^[8] These elements are the sole major contents of the space, with no significant lymphatic vessels or other neurovascular bundles routinely present.^[1]^[2] The axillary nerve originates from the posterior cord of the brachial plexus, primarily derived from the C5 and C6 spinal roots, with occasional contributions from C4.^[2]^[1] It enters the quadrangular space from the axilla, positioned superiorly relative to the PHCA, and exits posteriorly toward the scapular region to supply the posterolateral shoulder girdle.^[1] Upon passage through the space, the nerve divides into two main branches: the anterior branch, which innervates the anterior and middle portions of the deltoid muscle and provides articular branches to the glenohumeral joint; and the posterior branch, which supplies the posterior deltoid and teres minor muscles while terminating as the superior lateral cutaneous nerve of the arm for sensory innervation to the skin overlying the inferior deltoid (often termed the regimental badge area).^[1]^[2] The PHCA arises as a branch from the third part of the axillary artery, distal to the pectoralis minor tendon.^[8] It courses alongside the axillary nerve and posterior humeral circumflex vein through the quadrangular space, curving posteriorly around the surgical neck of the humerus to reach the posterior shoulder.^[8] Within or immediately after the space, the artery bifurcates into anterior and posterior branches, which anastomose with the anterior humeral circumflex artery and other vessels like the profunda brachii to form collateral circulation.^[8] The PHCA provides essential vascular supply to the deltoid, teres minor, and teres major muscles, as well as the glenohumeral joint and approximately 64% of the humeral head's blood flow, perfusing its superior, inferior, and lateral aspects.^[8]^[1]

Relations

Adjacent Structures

The quadrangular space is bordered superiorly by the teres minor muscle and inferiorly by the teres major muscle, which act as dynamic boundaries that can alter the space's dimensions during arm abduction and rotation due to their contraction and relaxation.^[9] Anteriorly, it lies in proximity to the subscapularis muscle, anterior to which the axillary nerve passes before entering the space, while inferiorly, the latissimus dorsi muscle adjoins via its insertion near the teres major, contributing to the posterior axillary fold.^[4] Osseously, the space is situated adjacent to the glenohumeral joint capsule medially and the proximal humerus laterally at the surgical neck, where the humeral head may approximate the space during shoulder abduction, influencing its patency.^[1] In terms of neurovascular relations, the radial nerve passes inferior to the quadrangular space through the adjacent triangular interval, separated by the long head of the triceps brachii, while the circumflex scapular artery, a medial branch of the subscapular artery, courses nearby via the superior triangular space.^[9] Soft tissues surrounding the space include the overlying deltoid fascia, which receives innervation and vascular supply from structures emerging from the space, and nearby insertions of the infraspinatus tendon on the greater tubercle of the humerus, contributing to the posterior shoulder's fascial continuity.^[4]

Comparison with Other Spaces

The quadrangular space, upper triangular space, and lower triangular space (also known as the triangular interval) are three intermuscular compartments in the posterior aspect of the axilla and shoulder region, collectively facilitating the passage of neurovascular structures from the axilla to the upper arm and scapular regions.^[9]^[1] The upper triangular space is bounded superiorly by the teres minor muscle, laterally by the long head of the triceps brachii, and inferiorly by the teres major muscle; it primarily transmits the circumflex scapular artery to supply the scapular musculature.^[9] In contrast, the lower triangular space is delineated superiorly by the teres major, medially by the long head of the triceps brachii, and laterally by the humerus; it conveys the radial nerve and the profunda brachii artery, supporting innervation and vascularization of the posterior arm compartment for functions such as elbow extension.^[9] Key distinctions among these spaces lie in their positions, shapes, and functional emphases: the quadrangular space occupies a more superior and lateral position relative to the others, adopting a quadrilateral configuration that channels the axillary nerve and posterior humeral circumflex artery toward shoulder girdle innervation and deltoid/posterior shoulder vascularization, whereas the triangular spaces maintain a triangular outline and direct flow toward scapular circulation (upper) or arm extensors (lower).^[9]^[1] All three spaces share muscular borders, such as the teres major and teres minor, which contribute to compartmentalizing the posterior axilla but underscore their varied roles in regional anatomy.^[9] Clinically, accurate differentiation of these spaces is essential, as misidentification during surgical interventions in the posterior shoulder—such as for rotator cuff repairs or axillary nerve decompression—can lead to inadvertent neurovascular injury or suboptimal outcomes.^[1]

Clinical Significance

Pathologies

The quadrilateral space syndrome (QSS), also known as quadrangular space syndrome, is a rare neurovascular entrapment disorder characterized by compression of the axillary nerve and posterior circumflex humeral artery within the quadrangular space.^[10]^[11] This condition primarily manifests as an entrapment neuropathy or vascular compression, often resulting from fibrous bands, traumatic injuries, or repetitive overhead arm motions in activities such as volleyball or weightlifting.^[12]^[13] Symptoms of QSS typically include posterior shoulder pain that worsens with overhead activities, weakness in the deltoid and teres minor muscles leading to impaired external rotation, and paresthesia along the lateral arm in the distribution of the axillary nerve.^[10]^[11] Vascular involvement may present as ischemia of the deltoid muscle or, less commonly, aneurysm formation in the posterior circumflex humeral artery, potentially causing distal embolic events.^[14]^[15] Etiologies of QSS encompass traumatic factors such as humeral fractures or shoulder dislocations, which can lead to scar tissue formation and space narrowing, as well as non-traumatic causes including hypertrophied subscapularis or teres minor muscles and idiopathic fibrosis. A January 2025 case series proposed classifying neurogenic QSS as "dynamic" (e.g., due to muscle hypertrophy from repetitive motion) or "static" (e.g., fixed structures like fibrous bands or paralabral cysts), based on clinical history, MRI, and exam findings in 4 patients.^[10]^[13]^[16] The incidence is notably higher among athletes engaged in overhead sports, affecting individuals typically aged 20-40 years, often the dominant shoulder due to repetitive microtrauma.^[10]^[11] A 2023 systematic review of 15 studies on QSS highlighted that surgical decompression yields success rates exceeding 80% in persistent cases, with one included series reporting improvement in 16 of 18 patients following neurolysis.^[11]

Diagnosis and Management

Diagnosis of quadrangular space syndrome (QSS) begins with a thorough clinical examination, focusing on patients presenting with posterior shoulder pain exacerbated by overhead activities or abduction and external rotation, along with weakness in external rotation and possible nondermatomal paresthesias in the axillary nerve distribution.^[10] Point tenderness over the quadrilateral space is a key finding, and provocative testing in the throwing position may reproduce symptoms; weakness in abduction and external rotation can indicate axillary nerve involvement, while atrophy of the teres minor or deltoid may be evident on inspection.^[13] Electromyography (EMG) and nerve conduction studies are useful to confirm axillary nerve dysfunction, though they have a high false-negative rate and are not diagnostic alone.^[12] Imaging plays a central role in multimodal diagnosis, with magnetic resonance imaging (MRI) as the initial modality of choice to visualize nerve compression, teres minor atrophy, or associated soft tissue abnormalities such as fibrous bands or paralabral cysts.^[10] Ultrasound provides dynamic assessment of vascular flow in the posterior humeral circumflex artery (PHCA), detecting stenosis or occlusion, and can measure axillary nerve cross-sectional area for thickening; a 2021 case report demonstrated its efficacy in confirming QSS when MRI and angiography were inconclusive, with normalized nerve dimensions post-treatment.^[17] Angiography or magnetic resonance angiography is indicated for suspected vascular involvement, revealing PHCA compression or aneurysm during provocative arm positioning.^[18] Guidelines from 2021-2022 studies emphasize combining these modalities for accurate diagnosis, avoiding reliance on a single test due to the syndrome's rarity and overlap with other shoulder pathologies.^[19] Management of QSS is initially conservative for mild to moderate cases, prioritizing nonoperative approaches to alleviate compression and restore function. Nonsteroidal anti-inflammatory drugs (NSAIDs), activity modification to avoid overhead motions, and physical therapy—including scapular stabilization exercises, rotator cuff strengthening, posterior capsule stretching, and manual soft tissue mobilization—are recommended for 3-6 months.^[12] Ultrasound-guided steroid or lidocaine injections into the quadrilateral space can provide diagnostic confirmation and symptomatic relief, with some cases resolving after nerve blocks alone.^[17] In a 2025 case series of 4 patients with neurogenic QSS, conservative rehabilitation resolved symptoms in 25% (1/4 cases).^[16] For refractory QSS persisting beyond 6 months or with significant neurovascular compromise, surgical intervention is indicated, typically via a posterior scapular approach for decompression, involving resection of fibrous bands, anomalous muscles, or vascular aneurysms.^[13] Anterior delto-pectoral approaches offer an alternative with potentially simpler access, as highlighted in a 2023 systematic review of 15 studies up to March 2022, which graded posterior approaches as more direct for neurolysis but noted both yield good outcomes in athletes.^[20] In the 2025 case series, surgical decompression in 3/4 cases led to immediate strength improvement, with all patients returning to prior performance levels.^[16] Postoperative rehabilitation focuses on gradual range-of-motion restoration and strengthening to prevent recurrence. Prognosis is favorable with early intervention, with most patients achieving symptom resolution and return to prior activity levels through conservative measures; surgical decompression reports success rates over 80% in resolving pain and weakness, though delays beyond 12-18 months may lead to persistent neuropathy or muscle atrophy.^[10] Complications are rare but include incomplete relief if underlying causes like emboli are not addressed, underscoring the need for comprehensive preoperative evaluation.^[18]

Variations and Development

Anatomical Variations

Anatomical variations of the quadrangular space primarily involve alterations in its muscular boundaries and neurovascular contents, which can influence the space's dimensions and susceptibility to compression. These variants are frequently documented in cadaveric dissections and contribute to the space's structural diversity, with implications for shoulder function and pathology.^[1] Muscular variants include anomalous fibrous bands that traverse the quadrangular space, often connecting the teres major to the long head of the triceps brachii; such bands were observed in 14 out of 16 cadaveric shoulders (87.5% prevalence) in a detailed dissection study.^[21] Additionally, an accessory subscapularis muscle may insert anterior to the axillary nerve, potentially narrowing the space or altering its boundaries.^[1] Reports of absent or accessory teres minor or teres major are less common, but variations in their size or attachments can modify the superior and inferior borders, as noted in anatomical reviews.^[22] Neurovascular variants encompass changes in the course and branching of the axillary nerve and posterior humeral circumflex artery (PHCA). The axillary nerve may divide into its anterior and posterior branches within the quadrangular space in 52% of cases, based on examination of 51 cadaveric specimens.^[1] In 12% of cases, this division occurs proximal to the space (high takeoff), increasing the nerve's exposure length within the axilla before entering the quadrangular space, as identified in a cadaveric study of 50 upper limbs.^[23] Rare branching anomalies of the axillary nerve, such as trifurcation, have been associated with altered innervation patterns in the region.^[24] For the PHCA, common variations include origin from the subscapular artery (rather than the third part of the axillary artery) or a tortuous course through the space, observed in multiple cadaveric analyses.^[8] Overall, anatomical studies indicate that such variants occur in a substantial proportion of individuals—up to 50% or more for specific features like nerve branching—though most remain asymptomatic.^[1] These changes can predispose to entrapment of neurovascular structures, as seen in quadrangular space syndrome.^[2] Detection typically relies on cadaveric dissection for research or advanced imaging techniques, including MRI to visualize muscle anomalies and nerve branching, musculoskeletal ultrasound for dynamic assessment, and arteriography for vascular variants.^[1]

Embryological Origins

The upper limb bud emerges during the fourth week of gestation from the lateral plate mesoderm and contributions from somites at levels C4 to T2, with the apical ectodermal ridge (AER) directing proximal-to-distal outgrowth and patterning. This process establishes the foundational mesenchyme for skeletal and muscular elements, including the precursors to the shoulder region's boundaries. By the fifth week, myotomal cells from the hypaxial division of somites migrate into the limb bud, forming dorsoventral condensations that differentiate into flexor and extensor muscle groups.^[25] The muscular borders of the quadrangular space derive from specific myotomal segments. The teres minor and teres major originate from the C5-C6 dorsal scapular myotomes, emerging from the posterior condensation of the limb bud around the ninth gestational week (crown-rump length ~25 mm), initially as part of the deltoideus complex before separating.^[26] The long head of the triceps brachii arises from C7-C8 myotomes, with its lateral and long heads visible by the same stage, contributing to the medial boundary as muscle fibers elongate and attach to the scapula and humerus.^[27] The humerus develops from mesenchymal condensations in the limb bud core, undergoing chondrification by week 6 and primary ossification in the diaphysis by week 8, which defines the lateral boundary through elongation and modeling.^[25] Neurovascular contents form concurrently through remodeling of the brachial plexus and arterial axis. The axillary nerve arises from the posterior cord of the brachial plexus (C5-C6 roots), which develops from ventral rami entering the limb bud around week 5, branching to innervate the teres minor by week 8.^[28] The posterior humeral circumflex artery originates from branches of the subscapular artery, derived from the seventh intersegmental artery and the axial artery of the upper limb, vascularizing the region as the limb rotates and spaces define during weeks 6-8.^[8] The quadrangular space coalesces as these elements differentiate, with muscle attachments stabilizing and the humerus shaft elongating to separate compartments by the end of the third fetal month (~12 weeks), though rare anomalies like teres minor agenesis can disrupt this if myotomal migration fails early.^[26] This timeline aligns with the completion of basic upper limb myogenesis, transitioning to refinement in the second trimester.^[27]

History and Terminology

Etymology

The term "quadrangular space" originates from the Latin quadrangulus, meaning "four-angled" or "having four corners," which describes the roughly diamond-shaped or quadrilateral form created by the space's four anatomical boundaries: the teres minor superiorly, the long head of the triceps brachii medially, the teres major inferiorly, and the surgical neck of the humerus laterally.^[29]^[1] In anatomical nomenclature, "space" refers to an intermuscular passageway facilitating the transit of neurovascular structures, such as the axillary nerve and posterior humeral circumflex artery.^[1] Alternative terms include "quadrilateral space," a synonym emphasizing the four-sided enclosure formed by its muscular and osseous borders.^[1] The Latin designation foramen humerotricipitale translates to "humeral-tricipital foramen," underscoring the space's relation to the humerus bone and the long head of the triceps brachii muscle.^[6] It is also referred to as "Velpeau's space" in honor of the French surgeon and anatomist Alfred-Armand-Louis-Marie Velpeau (1795–1867), who contributed to its description in 19th-century surgical anatomy.^[6] This nomenclature evolved and became standardized during the 19th century in European and Anglo-American anatomy texts, promoting precise descriptions for surgical applications amid advancing medical education and operative techniques.

Key Historical Descriptions

The quadrangular space, also known as the quadrilateral space of Velpeau, received formal descriptions in French surgical anatomy during the early 19th century. French anatomist and surgeon Alfred-Armand-Louis-Marie Velpeau (1795–1867) provided one of the earliest detailed accounts in his works on operative medicine, delineating the space's boundaries in the context of axillary dissections for surgical procedures. This formalization emphasized its role as a neurovascular passageway, influencing subsequent European anatomical studies.^[30] In the mid-19th century, British anatomist Henry Gray elaborated on the relevant shoulder anatomy in early editions of Anatomy: Descriptive and Surgical, including its boundaries and contents, such as the axillary nerve and posterior circumflex humeral artery.^[31] This text marked a milestone in standardizing depictions for English-speaking medical education, shifting focus toward its clinical implications in shoulder pathology. By the 20th century, Indian anatomist B.D. Chaurasia refined these descriptions in his Handbook of General Anatomy (4th edition, 2010), integrating the space's boundaries and contents with enhanced clinical correlations for surgical and diagnostic contexts.^[32] The late 20th century saw increased attention to the quadrangular space in sports medicine, particularly following the 1983 description of quadrangular space syndrome (QSS) by Cahill and Palmer, who identified compression of its neurovascular elements in overhead-throwing athletes, leading to shoulder pain and dysfunction.^[33] This recognition spurred studies on its relevance in athletic injuries post-1980s. In the modern era, advancements in imaging have transformed understanding, with post-1990s adoption of CT and MRI enabling visualization of anatomical variants; for instance, a 1993 study highlighted MRI findings of fibrous bands causing nerve compression in QSS.^[34] Recent investigations, such as a 2021 narrative review, have used MRI to document variations like anterior axillary nerve courses, underscoring the space's heterogeneity and aiding non-invasive diagnostics.^[2] These milestones reflect a progression from descriptive cadaveric anatomy to imaging-driven insights into clinical variations.

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