Triangular interval

The triangular interval (also known as the lower triangular space or triangular interval of the axilla) is an anatomical space in the posterior aspect of the upper arm, located inferior to the axilla. It serves as a passageway for neurovascular structures between the axilla and the posterior compartment of the arm.^[1] The boundaries of the triangular interval are formed superiorly by the teres major muscle, medially by the long head of the triceps brachii, and laterally by the humerus (or the lateral head of the triceps brachii). It contains the radial nerve and the profunda brachii artery (deep brachial artery), along with accompanying veins.^[2] Clinically, the triangular interval is relevant in cases of nerve compression, such as triangular interval syndrome, where entrapment of the radial nerve can lead to pain, paresthesia, and weakness in the posterior arm and forearm.^[1]

Anatomical Overview

Location and Position

The triangular interval is defined as an intermuscular space located in the posterior aspect of the arm, positioned immediately inferior to the axilla and serving as a conduit connecting the axilla to the posterior compartment of the arm.^[3]^[4] This placement situates it within the upper limb's shoulder region, facilitating the transition of structures from the axillary space into the brachial area. Relative to the humerus, the triangular interval is oriented lateral to the long head of the triceps brachii and medial to the humeral shaft, contributing to its role in the posterior arm's compartmental organization.^[2]^[4] In relation to the scapular region, it lies below the inferior border of the teres major muscle and outside the formal boundaries of the axilla, marking a distinct extension beyond the axillary confines into the arm proper.^[3]^[2] Clinically, this interval functions as a critical passageway for neurovascular structures transitioning from the axillary to the brachial regions, underscoring its importance in upper limb anatomy and potential sites of compression or injury.^[4]

Boundaries

The triangular interval, also known as the lower triangular space, is defined by three distinct borders that form a stable anatomical aperture in the posterior aspect of the arm, facilitating communication between the axilla and the posterior compartment of the arm.^[4] The superior border is formed by the inferior margin of the teres major muscle, which provides a muscular superior limit to the space.^[1] Laterally, the space is bounded by the medial aspect of the shaft of the humerus, serving as a bony lateral constraint that contributes to the interval's structural integrity and triangular configuration.^[3] Medially, the long head of the triceps brachii muscle delineates the boundary, with occasional involvement of the lateral head of the triceps in some descriptions, further emphasizing the muscular enclosure.^[2] These borders converge to create a triangular-shaped passageway, with the teres major arching over the superior aspect, the long head of the triceps forming the medial muscular wall, and the humerus providing a rigid lateral edge that tapers the space inferiorly, ensuring a relatively fixed and stable aperture despite arm movements.^[4] This configuration underscores the interval's role as a defined conduit, through which structures such as the radial nerve and profunda brachii artery traverse from the axilla into the posterior arm.^[1] Anatomical variations in the triangular interval are infrequent but can include fibrous bands or septa bridging the teres major and triceps brachii muscles, which may alter the sharpness of the borders and potentially narrow the space.^[1] Additionally, variations in muscle attachments, such as atypical insertions of the teres major or triceps heads, can modify the precise delineation of the boundaries, though these do not typically disrupt the overall triangular geometry.^[2]

Neurovascular Contents

Arterial Structures

The primary artery traversing the triangular interval is the profunda brachii artery (also known as the deep brachial artery), which arises as the first and largest branch from the posterior aspect of the brachial artery in the axilla, typically at or just distal to the inferior border of the teres major muscle. This vessel, accompanied by the radial nerve and venae comitantes, descends posteriorly through the triangular interval between the long and lateral heads of the triceps brachii muscle to enter the posterior compartment of the arm. Once in the posterior compartment, it courses laterally along the spiral groove of the humerus, providing essential vascular supply to the region.^[1] Key branches of the profunda brachii artery arise within or immediately adjacent to the triangular interval, including the deltoid branch, which extends superiorly to supply the deltoid muscle and anastomose with the posterior circumflex humeral artery, and the middle collateral artery, which descends to contribute to the arterial network around the elbow joint. These branches ensure targeted perfusion to the deltoid and proximal elbow structures, supporting the musculature and connective tissues in the posterior arm. The profunda brachii artery plays a critical role in the vascular supply to the posterior arm compartment, nourishing the triceps brachii, anconeus, and portions of the humerus shaft, while facilitating anastomotic connections that maintain collateral circulation. Compression of this artery within the triangular interval, such as from anomalous muscular fascicles or iatrogenic factors like surgical plating, can compromise distal perfusion and lead to ischemic changes in the supplied tissues.

Neural Structures

The radial nerve, originating from the posterior cord of the brachial plexus with contributions from spinal roots C5 to T1, serves as the primary neural structure traversing the triangular interval as it descends from the axilla into the posterior compartment of the arm.^[4]^[5] This nerve accompanies the profunda brachii artery through the interval.^[6] The radial nerve enters the triangular interval posterior to the axillary artery and passes lateral to the long head of the triceps brachii, continuing distally along the spiral groove of the humerus before piercing the lateral intermuscular septum to reach the anterior compartment.^[4]^[5]^[6] Near or within the triangular interval, the radial nerve gives rise to key sensory branches, including the posterior cutaneous nerve of the arm, which emerges in the axilla or immediately proximal to the interval, and the lower lateral cutaneous nerve of the arm, which arises in the proximal spiral groove.^[5]^[4] Functionally, the radial nerve provides motor innervation to the triceps brachii muscle (facilitating elbow extension) and, via its continuation as the posterior interosseous nerve, to the extensor muscles of the posterior forearm, including the extensor carpi radialis longus, extensor digitorum, and extensor carpi ulnaris (enabling wrist and finger extension).^[4]^[5] It also supplies sensory innervation to the skin of the posterior arm through the posterior cutaneous nerve of the arm and to the posterolateral aspect of the hand via its superficial branch.^[6]^[5]

Relations to Adjacent Spaces

Distinction from Quadrangular Space

The triangular interval and quadrangular space are two distinct intermuscular spaces in the posterior aspect of the shoulder and upper arm, often confused due to their proximity and partial shared boundaries, but they differ fundamentally in position, borders, and transmitted structures. The quadrangular space is situated within the axilla at approximately the mid-axillary level, superior to the teres major muscle, whereas the triangular interval lies inferior to the teres major, marking the transition from the axilla to the posterior compartment of the arm.^[3]^[2] In terms of borders, the quadrangular space forms a four-sided passageway bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, medially by the long head of the triceps brachii, and laterally by the surgical neck of the humerus; notably, the triangular interval lacks the superior teres minor boundary and is instead defined superiorly by the teres major muscle, medially by the long head of the triceps brachii, and laterally by the shaft of the humerus, resulting in a triangular configuration.^[3]^[2] This shared medial border with the long head of the triceps underscores their anatomical adjacency without overlap. The exclusion of the teres minor from the triangular interval's superior limit positions it distinctly lower, facilitating passage from the axilla into the arm's posterior compartment.^[3] The contents of these spaces further highlight their differences, with the quadrangular space transmitting the axillary nerve and the posterior circumflex humeral artery, which supply the deltoid and teres minor muscles, in contrast to the triangular interval, which conveys the radial nerve and the profunda brachii artery (along with its accompanying veins), essential for posterior arm innervation and vascularization.^[2]^[3] These divergent neurovascular elements reflect their roles in separate pathways: the quadrangular space supports shoulder girdle structures, while the triangular interval extends into the brachial region's extensor compartment. Clinically, these distinctions are critical for diagnosing entrapment neuropathies, as compression in the quadrangular space is associated with axillary nerve dysfunction—manifesting as deltoid weakness and sensory loss over the lateral shoulder (known as quadrilateral space syndrome)—whereas issues in the triangular interval involve radial nerve entrapment, leading to posterior arm pain, wrist drop, and finger extension deficits (triangular interval syndrome).^[2]^[3] Accurate differentiation aids in targeted imaging and surgical approaches, preventing misattribution of symptoms between these adjacent yet independent spaces.

Distinction from Triangular Space

The triangular interval and the upper triangular space are both intermuscular gaps in the shoulder girdle facilitating the passage of neurovascular structures, but they differ markedly in position and function.^[2] The triangular interval is located inferiorly, below the teres major muscle in the posterior compartment of the arm, serving as a conduit between the axilla and the posterior arm. In contrast, the upper triangular space is situated more superiorly, between the teres minor and subscapularis muscles near the scapula, within the axillary region.^[7] This positional variance places the triangular interval distal to the axilla, while the upper triangular space remains proximal, adjacent to the posterior scapular region.^[8] Their boundaries further highlight these differences, with the upper triangular space enclosed superiorly by the teres minor, inferiorly by the teres major, and laterally by the long head of the triceps brachii.^[2] The triangular interval, however, shares the teres major as its superior boundary but diverges by being bounded laterally by the lateral head of the triceps brachii or the humeral shaft and medially by the long head of the triceps brachii, resulting in a more elongated, interval-like configuration rather than a compact triangular enclosure.^[3] This substitution of the teres minor with the lateral triceps head in the triangular interval's borders underscores its lower, arm-centric orientation compared to the scapular-focused upper triangular space.^[7] The structures transmitted through these spaces also diverge significantly, reflecting their anatomical roles. The upper triangular space primarily accommodates the circumflex scapular artery and vein, supplying the scapular region.^[8] Conversely, the triangular interval conveys the radial nerve and the profunda brachii artery (deep brachial artery), which support the posterior arm and forearm innervation and vascularization.^[1] These content disparities emphasize the upper triangular space's role in scapular circulation versus the triangular interval's involvement in brachial neurovascular distribution.^[2] Terminologically, the term "triangular space" conventionally denotes the upper structure, while "triangular interval" is employed to specify the lower one, avoiding ambiguity in anatomical descriptions.^[3] This nomenclature helps distinguish the two in clinical and educational contexts, with the "interval" suffix highlighting the lower space's more open, passageway-like nature.^[7]

Clinical Relevance

Triangular Interval Syndrome

Triangular interval syndrome (TIS) is a rare compressive neuropathy characterized by entrapment of the radial nerve, and occasionally the profunda brachii artery, within the triangular interval of the posterior arm, resulting in posterior arm pain and motor weakness.^[9]^[1] This condition arises when the radial nerve, which courses through the interval alongside the artery, becomes compressed against the humerus or by surrounding soft tissues, leading to adverse neural tension and impaired function.^[10] First described in case reports from the early 2000s, TIS primarily affects individuals with repetitive overhead arm activities or anatomical predispositions, though its overall incidence remains low due to the spacious nature of the interval under normal conditions.^[9] The etiology of TIS involves mechanical compression from fibrous bands connecting the triceps brachii and teres major muscles, severe hypertrophy of the teres major (as observed in elite bodybuilders), trauma-induced scarring, or anatomical variants such as accessory heads of the triceps that narrow the interval.^[9] Repetitive overhead motions in athletes, such as throwing or weightlifting, exacerbate this by increasing dynamic pressure on the neurovascular bundle, potentially leading to chronic inflammation and fibrosis around the nerve.^[9] Post-traumatic cases may stem from fractures or hematomas that alter the interval's boundaries, while idiopathic variants are less common but reported in non-athletic populations.^[10] Clinically, TIS manifests as radial-sided pain in the posterior arm and scapular region, radiating distally to the forearm and hand, often accompanied by weakness manifesting as wrist drop and difficulty with finger extension due to radial nerve motor branch involvement.^[9] Sensory disturbances include paresthesia or numbness in the posterior forearm and dorsal hand.^[10] These symptoms mimic cervical radiculopathy but lack upper cervical provocation, highlighting the need for targeted assessment.^[10] Diagnosis relies on a thorough clinical examination, including positive radial nerve bias tests that reproduce symptoms at the lateral scapular border, to differentiate TIS from other radial neuropathies.^[10] Electromyography (EMG) and nerve conduction studies confirm radial nerve dysfunction by demonstrating slowed conduction or denervation in affected muscles, while magnetic resonance imaging (MRI) visualizes compressive structures such as hypertrophied muscles or fibrous bands within the interval.^[9] Early identification is crucial in athletic populations, where TIS is more prevalent among throwers and overhead sport participants.

Surgical and Diagnostic Considerations

Surgical access to the triangular interval for decompression typically involves a posterior arm incision to expose the space, often utilizing a triceps-splitting approach that retracts the triceps heads to reveal the radial nerve and associated structures in the spiral groove distal to the interval.^[11] In cases of compression due to teres major hypertrophy or fibrous bands, release of the teres major tendon may be performed to alleviate entrapment, allowing direct visualization and neurolysis of the radial nerve within the bounded space formed by the teres major superiorly, long head of triceps medially, and humerus laterally.^[9] Diagnostic imaging plays a crucial role in evaluating potential compression in the triangular interval. Ultrasound provides dynamic assessment of nerve compression, detecting flattening, proximal thickening, and increased vascularity via Doppler, making it a first-line modality for identifying structural causes like fibrous bands.^[12] MRI excels in delineating soft tissue borders and anatomical variants, revealing nerve enlargement, T2 hyperintensity from edema, and secondary muscle denervation, while CT angiography is employed when vascular involvement of the profunda brachii artery is suspected to map any aberrant courses or compressions.^[12]^[9] Therapeutic management begins conservatively for mild cases of triangular interval-related nerve compression, incorporating physical therapy to improve mobility and reduce adverse neural tension, alongside nonsteroidal anti-inflammatory drugs (NSAIDs) for pain relief, which can lead to full recovery within 2-6 months.^[9] For refractory cases, surgical release of compressive elements yields favorable outcomes post-decompression.^[9] The triangular interval is encountered during humerus fracture repairs, particularly via posterior approaches, where preservation of the radial nerve is paramount to avoid iatrogenic injury, as the nerve traverses the interval before entering the spiral groove—achievable in up to 76% of shaft exposures with mobilization.^[11]