Acromion

The acromion is a bony projection extending laterally and anteriorly from the spine of the scapula, forming the highest point of the shoulder and articulating with the lateral end of the clavicle to create the acromioclavicular joint.^[1] This structure is essential to the shoulder girdle, providing a stable platform for upper limb mobility while protecting underlying soft tissues.^[2] Anatomically, the acromion arises as an oblong continuation of the scapular spine above the supraspinous fossa, typically ossifying from multiple centers that fuse by early adulthood.^[1] It serves as a key attachment site for the deltoid and trapezius muscles, facilitating shoulder elevation and rotation, and forms part of the coracoacromial arch alongside the coracoid process and coracoacromial ligament to restrain superior migration of the humeral head during arm abduction.^[3]^[4] Functionally, it contributes to the overall stability of the glenohumeral joint by distributing forces across the shoulder complex and enabling a wide range of motion essential for daily activities and overhead movements.^[5] Clinically, the acromion's morphology plays a significant role in shoulder pathology, particularly subacromial impingement syndrome, where narrowing of the subacromial space compresses the rotator cuff tendons.^[6] According to Bigliani's classification, acromia are categorized as type I (flat), type II (curved), or type III (hooked), with type III shapes linked to increased risk of rotator cuff tears due to reduced subacromial clearance.^[7] Variations such as os acromiale—an unfused secondary ossification center—can also predispose individuals to impingement or instability, often requiring imaging for diagnosis and potential surgical intervention like acromioplasty.^[8]

Anatomy

Location and relations

The acromion is a bony projection that serves as the lateral extension of the scapular spine, extending anteriorly and laterally to arch over the glenohumeral joint.^[9]^[10] It articulates with the lateral end of the clavicle at the acromioclavicular joint and maintains close spatial proximity to the coracoid process of the scapula, collectively forming the superior boundary of the shoulder girdle.^[1]^[11] The acromion overhangs the glenoid cavity of the scapula and the head of the humerus, thereby defining the superior limit of the subacromial space between these structures.^[9]^[11] In typical adult anatomy, the acromion measures approximately 4.5 cm in length on average and presents a triangular or oblong configuration in its overall form.^[12]^[13]

Surfaces and borders

The acromion features distinct superior and inferior surfaces that contribute to its overall morphology as a flattened, triangular bony projection from the scapula. The superior surface is convex and rough in texture, positioned subcutaneously and readily palpable as the highest point of the shoulder.^[14] The inferior surface, in contrast, is smooth and concave, arching over the glenohumeral joint to form the superior boundary of the subacromial space.^[1] The borders of the acromion delineate its lateral extension and articulations. The lateral border is thick and rounded, often exhibiting small irregular tubercles along its edge.^[14] The medial border is thinner and concave, terminating in a small oval facet that facilitates the acromioclavicular joint.^[1] The anterior border curves forward, while the posterior border seamlessly blends with the spine of the scapula.^[9]

Attachments

The acromion serves as a key site for the origin of the deltoid muscle, which arises from its superior surface and lateral border, contributing to the muscle's broad coverage over the shoulder region.^[15] The deltoid's attachment here allows it to integrate with the scapular spine and clavicle origins, forming a unified muscular layer.^[16] The trapezius muscle inserts along the medial border of the acromion, specifically the medial aspect of the process and the superior lip of the adjacent scapular spine.^[17] This insertion point anchors the upper and middle fibers of the trapezius, linking it to the clavicle and scapular spine for posterior shoulder support.^[18] The coracoacromial ligament originates from the coracoid process of the scapula and attaches to the anterior aspect of the acromion, creating the coracoacromial arch that spans superior to the humeral head.^[19] This fibrous structure, triangular in shape, reinforces the superior aspect of the shoulder joint capsule.^[20] The acromioclavicular ligaments, including superior, inferior, anterior, and posterior components, directly connect the superior and lateral surfaces of the acromion to the distal clavicle, providing primary reinforcement for the acromioclavicular joint.^[5] These ligaments maintain articular stability between the acromion and clavicle.^[21] The supraspinatus tendon, originating from the supraspinous fossa of the scapula, passes inferior to the acromion and coracoacromial ligament en route to its insertion on the greater tubercle of the humerus.^[22] This positioning situates the tendon within the subacromial space formed by the acromion's inferior surface.^[19]

Variations

Morphological types

The acromion exhibits morphological variations in its shape, which are classified primarily using the Bigliani system to assess potential implications for shoulder pathology. This classification, based on the inferior surface contour observed in cadaveric and imaging studies, divides the acromion into three main types: Type I (flat), Type II (curved), and Type III (hooked). A Type IV (convex) acromion, with an upturned undersurface, was later described and is associated with a lower risk of impingement compared to Type III.^[7] Type I features a flat undersurface with an approximate anterior slope angle of 13°, representing about 17% of cases; Type II has a smooth, curved undersurface with a slope around 30°, comprising roughly 43%; and Type III displays a hooked or beaked anterior inferior extension with a slope near 27°, accounting for approximately 39-40%.^[23] The hooked Type III acromion is particularly associated with an elevated risk of rotator cuff tears, with studies reporting its presence in up to 70% of cases involving full-thickness tears due to reduced subacromial space and increased impingement on the supraspinatus tendon.^[24] Morphological types are typically evaluated using magnetic resonance imaging (MRI) or plain radiographs, such as true anteroposterior views, to determine the acromial index (ratio of coracoacromial distance to humeral head diameter) and critical shoulder angle (angle between the glenoid plane and line connecting the inferior glenoid to acromion tip). These metrics help quantify slope and overhang, with MRI offering superior soft-tissue visualization for precise classification.^[25] Prevalence of acromial morphological types varies across populations, reflecting ethnic differences; for instance, Type III hooked acromia occur more frequently in Black South African cohorts compared to White counterparts.^[26]

Os acromiale

Os acromiale refers to the failure of osseous union between one or more of the acromion's secondary ossification centers, resulting in a mobile accessory bone connected by fibrocartilage or synovium, typically involving the preacromion, mesacromion, or metacromion segments.^[8] This developmental anomaly arises from incomplete fusion, which normally occurs between ages 15 and 18, though it may persist up to age 25 in some cases.^[8] The acromion develops from four primary ossification centers originating from the scapular spine: the preacromion forming the distal tip, the mesoacromion the middle portion, the metaacromion the proximal base, and the basiacromion the attachment to the scapular spine.^[8] The prevalence of os acromiale varies widely, ranging from 1% to 15% in the general population, with higher rates observed in individuals of African descent, such as 18.2% in South African cadaveric studies compared to 7.7% in European samples.^[8] It is bilateral in approximately 60% of affected individuals and more common in males.^[27] Os acromiale is classified into three main types based on the unfused segment:

Type I (preacromiale): Involves the distal tip (preacromion), representing the smallest and least common variant.
Type II (mesoacromiale): Involves the middle segment (mesacromion), the most prevalent type, accounting for the majority of cases.
Type III (metaacromiale): Involves the proximal base (metacromion), which is rarer and often larger.^[8]

Radiographic identification is straightforward in adults, with the unfused segment appearing as a distinct ossicle separated by a pseudoarthrosis, best visualized on axillary lateral X-rays showing a "double-density" sign or cortical discontinuity.^[8] Advanced imaging such as MRI or CT may reveal associated degenerative changes like sclerosis, cysts, or hypertrophy at the nonunion site, while SPECT-CT can assess viability in younger patients.^[8] The condition is frequently asymptomatic and discovered incidentally, but symptomatic cases may present with subacromial pain, tenderness, reduced range of motion, or weakness, particularly following trauma or in overhead athletes due to micromotion at the pseudojoint.^[8]^[27] Management of os acromiale prioritizes conservative approaches for asymptomatic or mildly symptomatic cases, including physical therapy, nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroid injections, with observation often sufficient as symptoms may resolve spontaneously over 6 months or more.^[8] For persistent symptoms, surgical intervention is indicated, with options tailored to fragment size and patient factors: excision of small fragments (e.g., preacromiale) via open or arthroscopic techniques to prevent impingement, or internal fixation using tension band wiring, screws, or plates for larger meso- or metaacromiale fragments to achieve fusion.^[8] Arthroscopic excision is preferred in active patients for reduced morbidity, while fusion yields high union rates (up to 90%) but carries risks of hardware failure or deltoid weakness.^[8]

Embryological origins

The acromion, as part of the lateral extension of the scapula, derives primarily from the lateral plate mesoderm during early embryonic development, with the chondrogenic anlage forming around Carnegie stage 18 (approximately 6-7 weeks gestation).^[28] This somatic layer of the lateral plate mesoderm contributes to the bulk of the scapular body excluding the medial border, which originates from somitic mesoderm, establishing the foundational cartilage template for the acromion as an outgrowth of the scapular blade.^[28] Osteogenesis of the scapula, including the acromion base, initiates from the primary ossification center in the scapular body around the 8th gestational week, while the acromion proper develops through secondary ossification centers that appear later.^[29] The acromion ossifies via four secondary centers—pre-acromion, meso-acromion, meta-acromion, and basi-acromion—which emerge around the time of puberty (typically 12-15 years) and progressively fuse to the scapular spine and each other, completing by approximately age 22, though fusion can extend to 25 years in some cases.^[30] These centers arise sequentially from the cartilaginous precursor, with the basi-acromion fusing first by about age 12, followed by the others; incomplete fusion of these centers can result in os acromiale, a developmental variant.^[30] Genetic regulation of this process involves Hox genes, such as Hoxc6, which pattern the proximal limb structures including the acromion, alongside FGF signaling pathways that orchestrate limb bud outgrowth and scapular specification from the lateral plate mesoderm.^[31] Hox genes establish rostrocaudal identity in the emerging shoulder girdle, while FGF ligands like FGF10 promote mesenchymal proliferation essential for acromial formation.^[32]^[33] Congenital anomalies of the acromion are rare and often linked to broader scapular dysplasias, such as Sprengel's deformity, where failure of caudal migration of the scapula during weeks 5-12 of gestation results in an elevated, hypoplastic scapula that may include acromial absence or malformation.^[34] This syndrome, the most common congenital shoulder anomaly, arises from disruptions in the descent of the scapular anlage from its initial position at C4-C5 levels, potentially involving genetic factors like Emx2 mutations that impair lateral plate mesoderm-derived structures.^[35] Other dysplasias, such as those in cleidocranial dysplasia, can indirectly affect acromial development through altered ossification timing, though isolated acromial agenesis remains exceptionally uncommon.^[36]

Postnatal changes

The acromion completes its postnatal ossification process through the fusion of multiple secondary ossification centers, which typically integrate fully with the scapular spine by early adulthood, between the ages of 18 and 25 years. This fusion marks the end of skeletal maturation for the acromion, transitioning it from a cartilaginous structure to a fully ossified bony process capable of supporting shoulder biomechanics. Variations in fusion timing can occur, with fusion typically completing between ages 18 and 25, though some imaging studies in adolescents suggest earlier completion around 14-16 years; incomplete fusion beyond age 25 may indicate conditions like os acromiale.^[37]^[38] With advancing age, degenerative changes become prominent in the acromion, particularly the formation of anterior enthesophytes or spurs at sites of ligament and tendon attachment, often beginning after age 50 due to chronic mechanical stress and osteoarthritis-related remodeling. These spurs arise from repetitive microtrauma and degenerative processes at the acromioclavicular joint, with prevalence increasing significantly in older populations, reaching up to 57% in individuals aged 85 years and above. Such formations may be linked to differences in occupational or activity-related loading on the shoulder.^[39]^[40]^[41] In conditions like osteoporosis, the acromion experiences bone density reduction through resorption, which compromises its structural integrity and elevates fracture risk, especially under minimal trauma. This age-related demineralization correlates with overall skeletal fragility, though acromial thickness and volumetric density may vary individually without always reaching statistical significance in affected patients.^[42]^[43]

Function

Biomechanical role

The acromion, in conjunction with the coracoid process and the coracoacromial ligament, forms the coracoacromial arch, which serves as a passive restraint to prevent superior migration of the humeral head during shoulder abduction.^[4] This arch creates an osseoligamentous canopy over the humeral head, distributing compressive forces and maintaining the subacromial space to facilitate smooth glenohumeral motion.^[4] Biomechanical studies demonstrate that disruption of this structure, such as through coracoacromial ligament release, results in increased superior translation of the humeral head by approximately 2 mm under load, underscoring its role in joint protection.^[44] The acromion enhances the mechanical efficiency of the deltoid muscle by extending its origin laterally, thereby increasing the moment arm for arm elevation and improving torque production during abduction and flexion.^[45] This anatomical adaptation allows the deltoid to generate greater leverage, with the middle deltoid portion achieving a peak moment arm of about 27 mm during mid-range abduction, contributing significantly to overhead arm raising.^[46] Such leverage is essential for counterbalancing the inferior pull of the humeral head and enabling efficient elevation without excessive muscle force.^[45] In the acromioclavicular (AC) joint, the acromion provides critical stability by articulating with the distal clavicle, facilitating the transmission of forces from the upper extremity to the trunk during dynamic activities.^[47] The joint's ligaments and the acromion's morphology resist anteroposterior and superior-inferior translations, typically limiting superior displacement to a few millimeters under compressive loads.^[47] This force distribution supports scapulothoracic rhythm and prevents excessive shear at the glenohumeral joint.^[47] During overhead shoulder motions such as abduction, the acromion undergoes kinematic adjustments through scapular posterior tilting, typically ranging from 5° to 10° upward relative to the thoracic plane, which elevates the anterior acromial edge and preserves subacromial clearance.^[48] This tilt, averaging around 6.7° from rest to 90° of elevation, integrates with upward rotation to optimize humeral head positioning and minimize impingement risks.^[48]

Muscle and ligament support

The acromion serves as a critical origin point for the deltoid muscle, particularly its middle fibers, which originate from the lateral aspect of the acromion and spine of the scapula. This attachment enables the deltoid to generate powerful shoulder abduction, initiating and sustaining arm elevation up to approximately 90 degrees in the coronal plane before contributions from other muscles become predominant.^[49] The mechanical advantage provided by the acromial origin enhances the deltoid's leverage, allowing efficient force transmission to the humerus for activities requiring overhead reaching or lifting.^[50] The upper fibers of the trapezius muscle insert onto the posterior aspect of the acromion and the superior lip of the spine of the scapula, contributing to scapular stabilization and motion. These fibers facilitate upward rotation of the scapula, a key component of full arm elevation beyond 90 degrees, by elevating the acromion and glenoid fossa in coordination with the serratus anterior.^[18] This rotational support is essential for maintaining glenohumeral alignment during overhead activities, preventing compensatory movements that could strain the shoulder joint.^[51] The coracoacromial ligament, spanning from the coracoid process to the anterior acromion, forms part of the coracoacromial arch and acts as a passive restraint limiting superior translation of the humeral head. By containing the humeral head within the glenohumeral joint, it helps prevent superior dislocations, particularly under loads that might otherwise displace the joint.^[4] This ligamentous structure provides a static barrier, complementing dynamic muscular stabilizers to maintain shoulder integrity during elevation and rotation.^[52] The inferior surface of the acromion contributes to the subacromial space, offering a gliding surface for the supraspinatus tendon as it courses beneath the arch during shoulder movements such as internal and external rotation. The subacromial bursa facilitates smooth tendon excursion over the acromion, reducing friction and protecting the tendon from abrasive contact with bone.^[53] This arrangement supports the supraspinatus's role in initiating abduction and stabilizing the humeral head, minimizing wear during repetitive overhead motions.^[54]

Clinical significance

Injuries and fractures

Acromion fractures are rare injuries, accounting for approximately 8-16% of all scapular fractures. These isolated acromial fractures typically occur due to high-impact trauma, such as falls from height or motor vehicle collisions (MVCs), which exert direct force on the shoulder or indirect forces through the arm. They are frequently associated with other injuries to the shoulder girdle, including fractures of the clavicle or scapular body, as part of high-energy polytrauma. Fractures of the acromion are broadly classified as undisplaced or displaced based on the degree of separation at the fracture site. Displaced fractures may further involve disruption of the superior shoulder suspensory complex and can be subclassified using systems like Kuhn's, where type I fractures are minimally displaced, type II are displaced without subacromial space reduction, and type III involve displacement with subacromial narrowing. Age-related reductions in bone density can increase the risk of displacement in elderly patients. Diagnosis begins with clinical presentation of acute shoulder pain, swelling, and limited range of motion, often exacerbated by arm movement. Initial evaluation uses plain radiographs to identify the fracture, while computed tomography (CT) scans are crucial for precise assessment of displacement, fragment orientation, and associated injuries. Treatment depends on fracture characteristics and patient factors. Nondisplaced or minimally displaced fractures (<5 mm separation) are managed conservatively with sling immobilization for 4-6 weeks, followed by gradual mobilization and physical therapy to restore function. Displaced fractures (>5 mm) typically require surgical intervention via open reduction and internal fixation (ORIF) using cannulated screws, plates, or tension bands to achieve anatomical alignment, prevent subacromial impingement, and promote union, particularly when the superior shoulder suspensory complex is involved.

Pathological associations

The acromion is implicated in several non-traumatic shoulder pathologies, primarily through its morphological variations that alter the subacromial space and contribute to chronic overuse conditions. Subacromial impingement syndrome (SAIS), the most common disorder of the shoulder, arises when the hooked (Type III) acromion narrows the subacromial space, leading to repetitive compression of the rotator cuff tendons and bursa during overhead activities. This results in anterolateral shoulder pain, particularly with arm elevation, and accounts for 44–65% of all shoulder pain complaints presenting to clinicians.^[55] Diagnosis typically involves clinical tests like the Neer and Hawkins-Kennedy maneuvers, confirmed by imaging such as MRI to assess subacromial narrowing. Initial management is conservative, including physical therapy and corticosteroid injections, but refractory cases often require arthroscopic acromioplasty to resect the anteroinferior acromion and decompress the space, yielding good short-term pain relief in most patients.^[56] Rotator cuff tears frequently associate with acromial morphology, especially the Type III variant, where the hooked shape promotes tendon abrasion and degeneration over time. Studies indicate that approximately 70% of full-thickness rotator cuff tears occur in shoulders with a Type III acromion, reflecting the extrinsic mechanical stress on the supraspinatus tendon.^[57] The prevalence of these tears rises markedly after age 40, affecting 6.8–22.4% of individuals in this age group and escalating to over 50% by age 60 due to cumulative wear and intrinsic tendon aging.^[58] Diagnostic evaluation includes ultrasound or MRI to grade tear size and retraction, with therapeutic options ranging from conservative rehabilitation to arthroscopic repair, often combined with acromioplasty if impingement coexists. Os acromiale, an unfused acromial ossification center, can lead to pathological instability when the pre-acromion segment moves abnormally during deltoid contraction, mimicking or exacerbating impingement in symptomatic individuals. While most cases (up to 85–99% of the 1–15% population prevalence) remain asymptomatic, instability in symptomatic os acromiale contributes to subacromial impingement in 20–30% of diagnosed instances, presenting with localized anterolateral pain and positive impingement signs.^[8] Advanced imaging like CT distinguishes os acromiale from fractures and assesses mobility; treatment for symptomatic cases primarily involves surgical fixation of the mobile fragment to stabilize the acromion and alleviate impingement.^[59] In acromioclavicular (AC) joint osteoarthritis, anterior acromial osteophytes (spurs) commonly develop as part of degenerative changes, located inferiorly or anteriorly. These osteophytes narrow the AC joint space and correlate with superior migration of the humeral head, and are observed in approximately 30–50% of older adults with shoulder symptoms.^[60] Radiographic evaluation reveals joint space narrowing and osteophyte formation, with management focusing on anti-inflammatory measures or distal clavicle resection in severe cases to restore function and reduce pain from AC joint irritation.^[61]

Comparative anatomy

In mammals

The acromion in mammals is a laterally projecting bony process of the scapula that varies significantly in morphology and orientation across species, reflecting evolutionary adaptations to diverse locomotor demands such as brachiation, quadrupedalism, flight, and arboreal navigation. This structure typically arises from the scapular spine and provides key attachments for shoulder muscles, contributing to forelimb mobility while accommodating postural shifts from quadrupedal to orthograde positions in higher primates.^[62] In primates, the acromion closely resembles the human form, featuring a prominent lateral projection that supports a wide range of shoulder motions essential for suspensory and manipulative activities. This configuration is particularly adapted for brachiation, where the forelimbs bear the body's weight during overhead swinging. For instance, in gibbons (Hylobatidae), the acromion is notably narrow, which distinguishes it from the broader acromion in more knuckle-walking great apes like gorillas.^[63]^[64] Among quadrupedal mammals, the acromion shows a reduced lateral extension and a more dorsal or vertical alignment, prioritizing stability for weight-bearing on all fours rather than overhead elevation. In dogs (Canis lupus familiaris), for example, the acromion forms an enlarged distal end of the scapular spine, oriented primarily for attachment of the deltoideus muscle to facilitate propulsion and limb flexion during terrestrial gait, without the pronounced overhang seen in primates.^[65] This adaptation aligns with the absence or rudiment of the clavicle in many carnivores, allowing the scapula to function within a muscular sling for efficient ground-based locomotion.^[62] Bats (Chiroptera) exhibit one of the most specialized acromial forms, with a highly elongated process that integrates with extended clavicular elements to support the dynamic demands of powered flight. The acromion's cranial positioning provides mechanical advantage to flight muscles such as the deltoid, enabling expansive wing strokes required for sustained aerial maneuvering, while its offset from the glenoid joint maximizes humeral rotation.^[66] Functionally, the acromion diverges most notably in arboreal mammals, where it enlarges to accommodate greater overhead reach and pronograde-to-orthograde transitions, as seen in primates and tree-dwelling marsupials. This enlargement supports interconnected rotator cuff tendons that stabilize the shoulder during prehensile climbing and suspension, contrasting with the compact form in cursorial species and underscoring the acromion's role in facilitating vertical foraging strategies.^[62]

In non-mammalian vertebrates

In non-mammalian vertebrates, the acromion or its homologue represents an evolutionary adaptation of the pectoral girdle, transitioning from cartilaginous precursors in early vertebrates to more defined ossified structures in tetrapods. In ancestral fish-like forms, the pectoral girdle consists primarily of cartilaginous elements supporting fins, with no distinct acromial process; these precursors evolve into bony projections in tetrapods to facilitate limb articulation and axial integration.^[67] This ossification trend reflects increasing terrestrial demands, where the acromion-like features enhance stability and mobility in diverse locomotor modes across amphibians, reptiles, and birds.^[68] In amphibians, the acromion homologue is rudimentary and integrated into a simple dermal-endoskeletal girdle lacking prominent projections. The shoulder girdle forms a tripartite structure comprising a segmented coracoid (anterior procoracoid and posterior coracoid) and scapula, with dermal elements like the clavicle providing minimal reinforcement but no strong acromial extension.^[62] In anurans such as frogs, the scapula includes a thin processus acromialis that connects to the coracoid, forming the anterior margin of the glenoid foramen for basic humeral support, though overall ossification is reduced compared to more derived tetrapods.^[69] Among reptiles, particularly turtles, the acromion manifests as a ventrorostral process of the scapula, homologous to that in other amniotes and projecting anteriorly to articulate with the clavicle for girdle reinforcement. This structure develops from a shared mesenchymal primordium with the coracoid and scapula, differentiating late in ontogeny into a rod-like form that fuses seamlessly with the scapular blade.^[70] In turtles, it maintains a ligamentous connection to the plastron, enabling the shoulder girdle to move relative to the shell while supporting shell integration and limb retraction during locomotion.^[71] This adaptation underscores the acromion's role in accommodating the unique chelonian morphology, where enhanced ventral development aids aquatic and semi-terrestrial habits.^[72] In birds, the acromion process is well-developed on the scapula but modified for aerial locomotion, often extending cranially over the coracoid and articulating with the furcula's epicleideal process to contribute to the triosseal canal. This configuration, evolving from a caudal position in basal pennaraptorans to a cranial orientation in paravians, stabilizes the glenoid fossa and facilitates muscle attachments like the supracoracoideus for powered flight.^[73] The close fusion or association with the coracoid in modern birds reduces independent mobility but enhances overall flight efficiency, contrasting with the more versatile reptilian forms.^[74]