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Supraglenoid tubercle

The supraglenoid tubercle is a small, roughened bony prominence situated on the superior margin of the glenoid cavity of the scapula, primarily serving as the attachment site for the long head of the biceps brachii muscle.^[1] This structure is part of the scapula's glenoid region, which forms the socket of the shoulder joint (glenohumeral joint), and it facilitates the muscle's role in shoulder flexion, elbow flexion, and forearm supination.^[2] In anatomical context, the supraglenoid tubercle lies immediately superior to the smooth, pear-shaped glenoid fossa and is distinct from the nearby infraglenoid tubercle, which attaches the long head of the triceps brachii.^[3] The biceps brachii's long head tendon originates from this tubercle, passes intra-articularly through the shoulder joint, pierces the capsule, and then runs within the bicipital groove of the humerus, contributing to the joint's stability while being susceptible to intra-articular pathologies.^[4] Its rough surface enhances tendon adhesion, preventing slippage during muscle contraction.^[5] Clinically, the supraglenoid tubercle is relevant in cases of avulsion fractures, which are uncommon in humans but can occur due to forceful contraction of the biceps, often in overhead-throwing athletes and associating with superior labrum anterior-posterior (SLAP) lesions of the glenoid labrum.^[6] Such injuries may present with shoulder pain, instability, or biceps tendonitis, typically managed conservatively or surgically to restore function and prevent chronic instability.^[7] In veterinary medicine, these fractures are more frequently reported in large animals like horses and dogs, highlighting species-specific biomechanical differences.^[8]

Gross anatomy

Location

The supraglenoid tubercle is a small, roughened bony prominence located on the superior aspect of the glenoid cavity of the scapula.^[1]^[5] It lies immediately superior to the glenoid fossa along the lateral border of the scapula, near its superior angle.^[2]^[9] In the standard anatomical position, the tubercle is positioned posterior to the coracoid process, which projects anteriorly from the superior aspect of the scapular body, and anterior to the supraspinatus fossa on the posterior surface of the scapula.^[2]^[10] This orientation contributes to the contour of the scapular neck, bridging the glenoid cavity and the superior scapular border.^[11]

Structure

The supraglenoid tubercle is a small bony prominence formed primarily of compact cortical bone externally, which provides strength and a roughened surface ideal for tendon attachment, while internally featuring trabecular bone that supports load distribution.^[12] Developmentally, the tubercle develops through endochondral ossification as part of the scapula's glenoid secondary ossification center, which emerges around 10-11 years of age. Failure of fusion may result in rare variants resembling a bipartite scapula, where the tubercle appears partially separated.^[13]

Attachments

Muscular origins

The supraglenoid tubercle serves as the primary site of origin for the long head of the biceps brachii muscle, where the tendon attaches.^[4]^[14] This tendon is intra-articular, coursing through the glenohumeral joint capsule after originating from the tubercle and the adjacent superior glenoid labrum, with a mean length of approximately 9 cm from its origin to the musculotendinous junction.^[4]^[15] Embryologically, the long head biceps tendon develops from mesodermal condensations in the glenohumeral interzonal mesenchyme around the 7th week of gestation, establishing its attachment to the forming supraglenoid tubercle.^[16]

Ligamentous relations

The superior glenohumeral ligament (SGHL) originates from the supraglenoid tubercle of the scapula, typically just anterior to the attachment of the long head of the biceps brachii tendon, and inserts onto the fovea capitis of the humerus, superior to the lesser tuberosity.^[17] This ligament often blends with the biceps tendon at its origin, contributing to the formation of the biceps reflection pulley, a sling-like structure that stabilizes the tendon as it traverses the rotator interval.^[18] The coracohumeral ligament (CHL) arises from the dorsolateral base of the coracoid process and extends superiorly to blend with the SGHL and the superior glenohumeral capsule, thereby reinforcing the sheath surrounding the long head of the biceps tendon near the supraglenoid tubercle.^[17] This reinforcement helps maintain the tendon's position within the proximal shoulder structures. In some anatomical variations, the coracoglenoid ligament—a distinct band related to the CHL—directly connects the coracoid process to the supraglenoid tubercle, further supporting the superior capsuloligamentous complex.^[19] The transverse humeral ligament spans the bicipital groove between the greater and lesser tuberosities of the humerus, forming a roof over the groove that contains and stabilizes the long head of the biceps tendon distally.^[20] Although not directly attached to the supraglenoid tubercle, it indirectly contributes to regional stability by preventing medial dislocation of the biceps tendon, which originates proximally at the tubercle.^[21] The supraglenoid tubercle is integrated into the glenohumeral joint capsule, which attaches along the glenoid rim beyond the tubercle and extends to the anatomical neck of the humerus, enclosing the tubercle's base with synovial membrane reflections that render the adjacent biceps tendon intra-articular yet extrasynovial.^[22]

Function

Biceps brachii mechanics

The supraglenoid tubercle provides a fixed proximal anchor for the long head of the biceps brachii tendon, enabling the muscle to effectively generate torque for elbow flexion and forearm supination while maintaining shoulder joint stability during these movements. This attachment ensures that the tendon's intra-articular course across the glenohumeral joint allows coordinated action between the elbow and shoulder, preventing excessive humeral head migration and supporting overall upper limb function.^[4]^[3] In terms of force transmission, the biceps tendon exerts a compressive force on the humeral head via its origin at the tubercle during arm elevation, enhancing glenohumeral joint congruence. Biomechanical analyses indicate that biceps tension dissipates forces through the superior glenoid labrum, increasing contact pressures at the glenoid cartilage-bone interface, with peak tendon loads reaching approximately 100 N in dynamic overhead activities.^[23]^[24] The tubercle's position optimizes the length-tension relationship of the long head of the biceps brachii, positioning it to act as a humeral head depressor during abduction and limiting superior translation of the humeral head. This configuration allows maximal actin-myosin overlap near 90 degrees of abduction, where the muscle contributes to joint stability without significant torque production beyond this range.^[25] Comparatively, in humans the supraglenoid tubercle is relatively subtle, adapted for bipedal mobility with emphasis on reaching and manipulation, whereas in quadrupeds it is more prominent to accommodate weight-bearing demands during forelimb propulsion, often integrating with supraspinatus function for limb elevation.^[26]

Shoulder joint stability

The long head of the biceps tendon, originating from the supraglenoid tubercle, functions as a dynamic stabilizer of the glenohumeral joint by exerting tensile forces that resist anterior and superior translation of the humeral head during various arm motions, such as flexion and abduction.^[25] This stabilizing effect is particularly evident in synergy with the rotator cuff muscles, where the tendon's intra-articular course allows it to compress the humeral head against the glenoid fossa, enhancing overall joint congruence under dynamic loads.^[27] The supraglenoid tubercle contributes to shoulder stability through the origin of the long head of the biceps tendon, which is guided by the biceps pulley system formed by the superior glenohumeral ligament (SGHL), coracohumeral ligament (CHL), and the anterior-superior fibers of the subscapularis tendon; these structures collectively envelop and guide the tendon to prevent medial subluxation or dislocation.^[28]^[29] This pulley mechanism balances tendon forces during static shoulder positions, thereby reducing the risk of humeral head migration and associated instability. Additionally, the attachments at the tubercle provide a brief reference to surrounding ligamentous structures that reinforce this pulley without dominating the dynamic tendon role. Sensory feedback from mechanoreceptors in the long head of the biceps tendon and surrounding capsuloligamentous structures plays a key role in joint position sense, enabling reflexive neuromuscular control that further mitigates dislocation risk during everyday and athletic activities.^[30] This proprioceptive input helps coordinate muscle activation around the shoulder, promoting precise awareness of humeral head positioning relative to the glenoid. Degeneration of structures related to the supraglenoid tubercle, such as the long head of the biceps tendon, increases shoulder instability in older adults due to age-associated tendinosis and weakening of attachments; the incidence of acute anterior shoulder dislocations in patients over 60 years accounts for up to 20% of all such cases.^[31] These degenerative changes compromise the tendon's stabilizing capacity, leading to greater humeral head laxity and heightened vulnerability to traumatic or atraumatic dislocations.^[32]

Clinical significance

Biceps tendonitis

Biceps tendonitis, also known as proximal biceps tendinitis or tendinopathy, refers to the inflammation and degenerative changes affecting the long head of the biceps brachii tendon as it originates from the supraglenoid tubercle. This condition arises primarily from overuse, subacromial impingement, or associated rotator cuff pathology, leading to irritation at the tendon's proximal attachment and within the bicipital groove. Symptoms typically manifest as anterior shoulder pain, often described as a deep, aching discomfort that intensifies with overhead activities, lifting, or resisted elbow flexion.^[33]^[34]^[32] Epidemiologically, biceps tendonitis is prevalent among overhead athletes such as throwers and swimmers, as well as manual laborers involving repetitive arm motions. It predominantly affects individuals over 40 years of age, with additional risk factors including smoking, which impairs tendon vascularity and healing. The condition is more common in men and those with occupational demands on the upper extremity.^[34]^[32]^[35] The pathophysiology involves repetitive microtears at the tendon's insertion on the supraglenoid tubercle, initiating tenosynovitis and progressive degeneration of the tendon substance. These microtears trigger an inflammatory response, resulting in synovial sheath effusion and fibrosis, while neovascularization emerges as a reparative process that contributes to further tendon thickening and pain sensitization. Magnetic resonance imaging (MRI) confirms these changes, revealing intrasubstance signal alterations, and peritendinous fluid, distinguishing active inflammation from chronic tendinosis.^[33]^[36]^[37] In differential diagnosis, biceps tendonitis is differentiated from superior labral anterior-posterior (SLAP) tears—often involving the biceps-labral anchor at the supraglenoid tubercle—by its characteristic pain localized to the bicipital groove rather than diffuse intra-articular shoulder discomfort or mechanical catching symptoms. Clinical tests like the Yergason or Speed maneuvers elicit groove tenderness specific to the tendon, aiding in this distinction.^[38]^[33]

Avulsion fractures

Avulsion fractures of the supraglenoid tubercle represent a rare traumatic injury to the shoulder, characterized by the separation of a bony fragment from the tubercle due to forceful pull from the long head of the biceps brachii tendon. These injuries typically arise from high-energy mechanisms involving sudden eccentric contraction of the biceps muscle, such as a fall on an outstretched arm or abrupt deceleration during overhead throwing activities in athletes. The tendon avulses the tubercle, often in conjunction with labral disruption, and comprise a small proportion of all proximal humerus or scapular fractures, with limited reported incidence in human cases.^[6]^[39] Such fractures are frequently associated with superior labrum anterior to posterior (SLAP) lesions, where the avulsion serves as a bony manifestation of the underlying labral pathology at the biceps anchor site. In overhead athletes, the repetitive or acute stress on the glenohumeral joint can lead to detachment at the supraglenoid origin, with studies indicating a notable overlap between tubercle avulsions and SLAP variants, though exact prevalence varies by population. Classification of these injuries often distinguishes between pure tendon avulsions without bone involvement and those with an attached bony fragment.^[6]^[39] Patients commonly experience acute, sharp pain in the shoulder at the time of injury, often accompanied by an audible snap and subsequent bruising or ecchymosis. A characteristic "Popeye" deformity may develop due to distal retraction of the biceps muscle belly, resulting in a visible bulge in the upper arm. Associated features include weakness in elbow flexion and forearm supination, along with restricted shoulder motion. Diagnosis relies on clinical examination, supplemented by imaging: plain radiographs can detect the displaced bony fragment (typically 1-3 cm proximally), while MRI provides detailed visualization of the avulsed tubercle, tendon integrity, and concomitant SLAP lesions or rotator cuff involvement.^[39]^[40] If left untreated, these fractures carry risks of nonunion or malunion of the bony fragment, particularly in displaced cases, leading to chronic shoulder instability, persistent pain, and functional deficits such as minimal or no long-term loss in elbow flexion or supination strength. Associated SLAP pathology may exacerbate impingement or instability, underscoring the need for early intervention to restore mechanics at the biceps attachment site.^[39]

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