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Shoulder

The shoulder is a highly mobile complex of structures that connects the upper limb to the axial skeleton, primarily through the glenohumeral joint, which is the most versatile ball-and-socket articulation in the human body, enabling a wide range of motions such as flexion, extension, abduction, adduction, and rotation.^[1] This joint, formed by the head of the humerus fitting into the shallow glenoid cavity of the scapula, is supported by a loose capsule and reinforced by the fibrocartilaginous glenoid labrum, which deepens the socket for added stability despite the inherent trade-off of reduced bony congruence for enhanced mobility.^[1] The shoulder girdle also includes the sternoclavicular and acromioclavicular joints, along with the scapulothoracic articulation, forming a functional unit that allows coordinated movement between the clavicle, scapula, and proximal humerus.^[2] Key bones comprising the shoulder include the humerus (upper arm bone), scapula (shoulder blade), and clavicle (collarbone), which together provide the skeletal framework for attachment and leverage.^[3] Ligaments such as the glenohumeral ligaments, coracohumeral ligament, and coracoacromial ligament contribute to static stability, while the joint capsule and surrounding bursae facilitate smooth gliding during motion.^[1] Blood supply is primarily derived from the axillary artery and its branches, including the subscapular and anterior and posterior circumflex humeral arteries, ensuring nourishment to the region's muscles and joint structures.^[2] The shoulder's dynamic stability relies on a network of muscles, notably the rotator cuff group—comprising the supraspinatus, infraspinatus, teres minor, and subscapularis—which originate from the scapula and insert on the humerus to compress the humeral head into the glenoid and fine-tune movements.^[2] The deltoid muscle, a large triangular extrinsic muscle, overlies the joint and powers primary actions like abduction, while other scapulothoracic muscles such as the trapezius and serratus anterior enable scapular rotation and protraction essential for full arm elevation.^[3] Innervation is provided by branches of the brachial plexus, including the suprascapular, axillary, and subscapular nerves, coordinating the precise neuromuscular control required for everyday activities.^[2] Functionally, the shoulder permits exceptional range of motion—up to 180° of flexion, 150° of abduction, and 90° of external rotation—making it crucial for tasks involving reaching, lifting, and throwing, though this mobility predisposes it to common issues like instability, rotator cuff tears (affecting millions annually), and anterior dislocations, which account for 96% of shoulder dislocations.^[1]

Anatomy

Skeletal components

The shoulder girdle, also known as the pectoral girdle, consists of three primary bones: the clavicle, scapula, and proximal humerus, which together connect the upper limb to the axial skeleton and facilitate a wide range of arm movements.^[4] These bones form an incomplete ring that prioritizes mobility over stability, distinguishing the shoulder from more rigid girdles like the pelvic.^[5] The clavicle and scapula form the core of the girdle, while the proximal humerus articulates with the scapula to enable upper limb positioning.^[6] The clavicle is a long, slender, S-shaped bone that lies horizontally across the base of the neck, serving as the only direct bony link between the upper limb and the axial skeleton.^[7] Its medial sternal end is rounded and articulates with the manubrium of the sternum, while the lateral acromial end is flattened and connects to the acromion process of the scapula.^[4] The bone features subtle surface markings, such as the conoid tubercle and trapezoid line, but lacks significant articular surfaces beyond its ends.^[7] The scapula is a flat, triangular bone positioned on the posterior aspect of the thoracic wall, overlying ribs 2 through 7.^[8] It has three borders (superior, medial, and lateral), three angles (superior, inferior, and lateral), and two main surfaces: the convex posterior surface divided by the spine into supraspinous and infraspinous fossae, and the concave anterior subscapular fossa.^[4] Key articular surfaces include the acromion process, a flattened extension of the spine that projects laterally to form the acromioclavicular joint; the coracoid process, a curved, beak-like projection arising from the superior aspect of the scapular body, directed anterolaterally; and the glenoid cavity, a shallow, pear-shaped depression on the lateral angle that serves as the socket for the humeral head.^[8] The proximal humerus forms the upper portion of the humerus bone, extending from the shaft to include the head, neck, and tuberosities.^[9] Its primary articular surface is the humeral head, a smooth, hemispherical structure covered in hyaline cartilage, which articulates with the glenoid cavity to form a ball-and-socket configuration allowing approximately 25% contact area for enhanced range of motion.^[4] The anatomical neck separates the head from the greater and lesser tuberosities, while the surgical neck lies distal to these, marking a common fracture site.^[9] Ossification of these bones begins in utero and continues through adolescence, with multiple secondary centers contributing to their maturation. The clavicle is unique as the first bone to ossify via intramembranous ossification, starting with a primary center at the shaft's middle in weeks 5-6 of fetal development, followed by secondary centers at the ends around 18-22 years that fuse by 21-25 years.^[10] The scapula ossifies from a primary center in the body at 8 weeks in utero, with secondary centers including the coracoid process (one in the first year and another around 10-15 years, fusing by 15-18 years), acromion (14-20 years from multiple ossicles, fusing by approximately 22 years), glenoid (10-15 years), inferior angle (14-20 years), and medial border (around 20 years).^[11] For the proximal humerus, ossification initiates in the diaphysis at 8 weeks in utero, with the head at 1-6 months postnatally, greater tubercle at 1-3 years, and lesser tubercle at 3-5 years; these centers fuse by 16-20 years, with growth plates at the physes supporting longitudinal expansion.^[12] These skeletal components play crucial roles in supporting weight transmission and enabling mobility. The clavicle acts as a mechanical strut, transferring forces from the upper limb to the axial skeleton and bracing the shoulder against downward pressure during weight-bearing activities.^[7] The scapula provides a stable yet mobile base on the thoracic wall, distributing loads across the rib cage and positioning the glenoid for optimal humeral articulation during arm elevation and rotation.^[4] The proximal humerus, through its ball-and-socket interface, allows extensive multidirectional mobility while contributing to load-bearing via compressive forces across the joint.^[13] Overall, the girdle's design emphasizes flexibility for reaching and manipulation over robust weight support, contrasting with lower limb structures.^[13]

Joints and ligaments

The shoulder complex comprises three primary synovial joints: the sternoclavicular, acromioclavicular, and glenohumeral joints, which collectively enable a wide range of upper limb motion while relying on ligamentous reinforcements for stability.^[2] The sternoclavicular joint is a saddle-type synovial joint formed by the articulation of the medial clavicle with the manubrium of the sternum and the first costal cartilage, allowing motion in multiple planes including elevation, depression, protraction, retraction, and rotation, with approximately three degrees of freedom.^[2] It is reinforced by anterior and posterior sternoclavicular ligaments, as well as the interclavicular and costoclavicular ligaments, which provide anteroposterior and vertical stability to prevent excessive translation.^[2] The acromioclavicular joint is a plane-type synovial joint between the lateral clavicle and the acromion of the scapula, permitting gliding and rotational movements with limited degrees of freedom, primarily facilitating scapular motion relative to the clavicle.^[2] Its stability is maintained by the acromioclavicular ligaments, which resist horizontal shear, and the coracoclavicular ligaments—comprising the trapezoid (lateral) and conoid (medial) components—that anchor the coracoid process to the clavicle, providing crucial vertical and rotational restraint to counteract upward displacement of the clavicle.^[14]^[15] The glenohumeral joint, the main articulation of the shoulder, is a ball-and-socket synovial joint where the convex humeral head articulates with the shallow glenoid fossa of the scapula, offering extensive multiaxial mobility with three degrees of freedom in flexion-extension, abduction-adduction, and internal-external rotation, but at the cost of inherent instability due to the glenoid's shallow concavity.^[1] Key stabilizing ligaments include the superior glenohumeral ligament, a thin band that reinforces the anterosuperior capsule and limits inferior translation when the arm is at the side; the middle glenohumeral ligament, a thickened anterior capsular structure that prevents anterior humeral head subluxation during abduction; and the inferior glenohumeral ligament complex, the primary restraint against anterior and posterior dislocation in various positions of abduction and rotation, forming an anterior band, posterior band, and axillary pouch.^[1] Additionally, the coracoacromial ligament spans from the coracoid process to the acromion, forming the coracoacromial arch that overlies the humeral head to limit superior migration and protect the joint from impingement.^[14]^[15] The transverse humeral ligament bridges the bicipital groove of the humerus, composed of fibers from the subscapularis, supraspinatus, and coracohumeral ligaments, serving to contain the long head of the biceps tendon within the groove during shoulder motion.^[15] The glenohumeral joint capsule is a loose, fibrous sheath extending from the anatomical neck of the humerus to the glenoid rim, lined by synovium that allows permissive motion but necessitates additional stabilizers; it is reinforced inferiorly by the inferior glenohumeral ligament and superiorly by the coracohumeral ligament, which blends with the capsule to resist extension and external rotation.^[1] The glenoid labrum, a fibrocartilaginous ring attached to the glenoid rim, deepens the glenoid cavity by 50% and increases the articular surface area, enhancing joint congruence and resisting humeral head translation in multiple directions while serving as an attachment site for the glenohumeral ligaments and the long head of the biceps tendon.^[1]^[15]

Muscles and tendons

The shoulder's muscular anatomy is characterized by a complex arrangement of muscles that provide dynamic stability and mobility to the glenohumeral joint, with tendons and bursae facilitating smooth motion by connecting muscle bellies to bone and reducing friction, respectively. The primary muscle groups include the rotator cuff, which forms a musculotendinous cuff around the humeral head for joint compression and rotation; scapulothoracic muscles that stabilize and position the scapula against the thoracic wall; and larger primary movers like the deltoid and pectoralis major that generate powerful limb movements. These structures collectively enable the shoulder's wide range of motion while maintaining stability.^[16] The rotator cuff consists of four muscles originating from the scapula and inserting onto the humerus, collectively stabilizing the glenohumeral joint by depressing the humeral head during arm elevation. The supraspinatus originates from the supraspinous fossa of the scapula and inserts on the superior facet of the greater tubercle of the humerus; it initiates abduction of the arm up to approximately 15 degrees and is innervated by the suprascapular nerve. The infraspinatus arises from the infraspinous fossa and inserts on the middle facet of the greater tubercle, primarily externally rotating the humerus while also contributing to joint stability, with innervation from the suprascapular nerve. The teres minor originates from the lateral border of the scapula and inserts on the inferior facet of the greater tubercle, aiding in external rotation and adduction of the arm, innervated by the axillary nerve. Finally, the subscapularis, the largest rotator cuff muscle, originates from the subscapular fossa and inserts on the lesser tubercle of the humerus, facilitating internal rotation and adduction, with innervation from the upper and lower subscapular nerves. These muscles' tendons blend with the joint capsule to form a reinforcing collar that enhances glenohumeral congruence.^[16]^[17] Scapulothoracic muscles connect the scapula to the axial skeleton, enabling scapular rotation, elevation, and protraction essential for coordinated shoulder girdle motion. The trapezius, a broad superficial muscle, originates from the superior nuchal line, ligamentum nuchae, and spinous processes of C7 to T12, inserting on the lateral third of the clavicle, acromion, and scapular spine; its upper fibers elevate the scapula, while the middle and lower fibers retract and depress it, respectively. The rhomboid major and minor, deep to the trapezius, originate from the spinous processes of T2-T5 (major) and C7-T1 (minor), inserting along the medial border of the scapula from the spine to the inferior angle; they retract and elevate the scapula to position it for arm elevation. The serratus anterior originates from the lateral surfaces of ribs 1 through 8 or 9, inserting along the medial border of the scapula; it protracts and upwardly rotates the scapula, crucial for overhead reaching. The levator scapulae originates from the transverse processes of C1 to C4 and inserts on the superior angle and medial border of the scapula, primarily elevating the scapula to assist in shrugging motions. These muscles work in concert to maintain scapular stability against the thorax during dynamic activities.^[16] Among the primary movers, the deltoid is the most superficial and powerful abductor of the arm, originating from the lateral third of the clavicle (anterior fibers), acromion (lateral fibers), and scapular spine (posterior fibers), and inserting on the deltoid tuberosity of the humerus; the anterior portion flexes and medially rotates the arm, the lateral portion abducts it beyond 15 degrees, and the posterior portion extends and laterally rotates it, making it essential for a wide array of shoulder motions. The pectoralis major, a thick fan-shaped muscle, originates from the medial half of the clavicle (clavicular head), sternum, and costal cartilages of ribs 1 to 6 (sternocostal head), inserting on the lateral lip of the intertubercular groove of the humerus; it primarily flexes, adducts, and internally rotates the humerus, playing a key role in pushing and pulling actions such as in throwing or climbing. These muscles overpower the rotator cuff in force production but rely on it for joint centering during exertion.^[16]^[18]^[19] Key tendons in the shoulder include the long head of the biceps brachii, which originates from the supraglenoid tubercle of the scapula and traverses the glenohumeral joint within the rotator interval before entering the bicipital groove, providing anterior stability and acting as a humeral head depressor. The supraspinatus tendon extends from the muscle's belly across the superior aspect of the glenohumeral joint, passing beneath the coracoacromial arch to insert on the greater tubercle; this path makes it vulnerable to impingement but critical for initiating abduction while compressing the humeral head against the glenoid. These tendons integrate with surrounding musculature to transmit forces efficiently.^[16]^[17] Bursae in the shoulder are synovial sacs that minimize friction between tendons, muscles, and bony structures during motion. The subacromial bursa lies between the acromion and the supraspinatus tendon, cushioning the rotator cuff during elevation to prevent abrasive contact with the coracoacromial arch. The subdeltoid bursa, often continuous with the subacromial, is positioned deep to the deltoid muscle and superficial to the rotator cuff, facilitating gliding of the deltoid over the humeral head. The subscapular bursa, also known as the subcoracoid bursa, resides between the subscapularis tendon and the scapular neck or coracoid process, reducing friction during internal rotation and protecting the anterior capsule. These bursae are essential for preserving tendon integrity amid repetitive shoulder use.^[16]^[17]

Neurovascular structures

The neurovascular structures of the shoulder region primarily involve the brachial plexus for innervation, the axillary artery and its tributaries for arterial supply, corresponding venous drainage, lymphatic pathways to the axillary nodes, and specific intermuscular spaces in the axilla facilitating neurovascular passage. These elements ensure motor control, sensory feedback, and nourishment to the shoulder girdle and upper limb. The brachial plexus, formed by the anterior primary rami of the spinal nerves C5 through T1, serves as the primary neural network supplying the upper extremity, including the shoulder.^[20] It organizes into roots, trunks, divisions, cords, and terminal branches, traversing the axilla to distribute motor and sensory fibers. Key branches relevant to the shoulder include the suprascapular nerve, originating from the upper trunk (C5-C6), which provides motor innervation to the supraspinatus and infraspinatus muscles; the axillary nerve, arising from the posterior cord (C5-C6), which innervates the deltoid and teres minor muscles while also contributing sensory branches; and the musculocutaneous nerve, emerging from the lateral cord (C5-C7), which supplies the coracobrachialis, biceps brachii, and brachialis muscles.^[21]^[22]^[23] Arterial supply to the shoulder begins with the subclavian artery, which continues as the axillary artery after crossing the first rib, supplying the region via branches such as the superior thoracic, thoracoacromial, lateral thoracic, subscapular, anterior and posterior humeral circumflex arteries.^[24] The axillary artery then transitions to the brachial artery distal to the teres major muscle, providing further vascularization to the proximal arm. Venous drainage parallels this pathway, with superficial veins including the cephalic vein (lateral aspect, draining into the axillary vein) and basilic vein (medial aspect, joining the brachial veins to form the axillary vein), which ultimately converges with the subclavian vein.^[25]^[26] Lymphatic drainage from the shoulder and upper limb converges on the axillary lymph nodes, which are grouped into five levels based on their relation to the pectoralis minor muscle: lateral, anterior (pectoral), posterior (subscapular), central, and apical. These nodes receive lymph from the upper limb, lateral chest wall, and breast, filtering it before it proceeds to the subclavian lymphatic trunk and thoracic duct or right lymphatic duct.^[27] The axilla contains critical neurovascular elements passing through defined intermuscular spaces. The quadrangular space, bounded superiorly by the teres minor, inferiorly by the teres major, medially by the long head of the triceps, and laterally by the humerus, transmits the axillary nerve and posterior humeral circumflex artery to the posterior shoulder.^[28] The upper triangular space, formed by the teres minor superiorly, teres major inferiorly, and long head of the triceps medially, allows passage of the circumflex scapular artery. The lower triangular space (triangular interval), bounded by the teres major superiorly, long head of the triceps medially, and humerus laterally, conveys the radial nerve and profunda brachii artery.^[28]^[29] Sensory innervation of the shoulder derives from dermatomes C3 through C5 and peripheral nerves of the brachial plexus. The C4 dermatome covers the superior shoulder and infraclavicular region, primarily via supraclavicular nerves (C3-C4); the C5 dermatome supplies the lateral shoulder through contributions from the axillary nerve (C5-C6). Additional sensory input comes from the lateral pectoral nerve (C5-C7) for the anterior chest wall and the intercostobrachial nerve (T2) for the medial upper arm.^[30]^[30]

Function

Kinematic movements

The shoulder complex enables a wide array of movements through the coordinated action of its primary joints: the glenohumeral (GH), acromioclavicular (AC), and sternoclavicular (SC) joints, along with the scapulothoracic articulation.^[31] Kinematic movements at the shoulder primarily occur in three planes: sagittal (flexion and extension), frontal (abduction and adduction), and transverse (internal and external rotation), with circumduction representing a circular combination of these motions.^[32] Flexion involves forward elevation of the arm in the sagittal plane, typically driven by anterior deltoid and pectoralis major as prime movers.^[1] Extension moves the arm posteriorly, countering flexion, while abduction raises the arm laterally away from the body's midline, and adduction brings it toward the midline.^[32] Internal rotation turns the arm medially (palm facing inward), and external rotation turns it laterally (palm facing outward); circumduction combines these to trace a conical path, enhancing overall mobility.^[1] A key feature of shoulder kinematics is the scapulohumeral rhythm, which describes the synchronized contribution of GH and scapulothoracic motions to achieve full arm elevation. During abduction, this rhythm follows an approximate 2:1 ratio, where for every 2 degrees of GH motion, the scapula contributes 1 degree of upward rotation via the scapulothoracic interface, allowing total elevation up to 180 degrees.^[33] Specifically, the initial 30 degrees of abduction occur primarily at the GH joint (about 30 degrees GH to 0 degrees scapular), followed by the 2:1 ratio for the remaining 150 degrees (120 degrees GH and 60 degrees scapular upward rotation).^[34] This coordination prevents impingement and optimizes the GH joint's shallow socket for greater range. Active range of motion (AROM), achieved through voluntary muscle contraction, is generally less than passive range of motion (PROM), where an external force moves the joint without resistance. Typical AROM values include 150-180 degrees for flexion, 45-60 degrees for extension, 150-180 degrees for abduction (full elevation), 30 degrees for adduction, 70 degrees for internal rotation, and 90 degrees for external rotation.^[35] PROM often exceeds these by 10-20 degrees, such as up to 180-190 degrees in flexion, as the joint capsule and soft tissues are stretched without muscular limitation; however, individual variations exist based on age and flexibility. Accessory motions, or arthrokinematics, facilitate these osteokinematic movements through subtle joint surface interactions at the GH joint. Superior and inferior glides allow the humeral head to translate vertically relative to the glenoid fossa, with inferior glide essential for full abduction to accommodate the greater tuberosity's clearance under the acromion.^[36] Anterior and posterior translations provide mediolateral stability, where the humeral head rolls and slides posteriorly during abduction and anteriorly during adduction, preventing excessive shear forces.^[37] These glides and translations, typically ranging 0.2-0.6 cm (2-6 mm) in healthy joints, are critical for smooth, pain-free motion but can be restricted by capsular tightness.^[38]

Biomechanical roles

The shoulder complex plays a critical role in load-bearing by distributing forces across its joints and soft tissues, enabling the upper limb to support and manipulate objects while maintaining structural integrity. Central to this are force couples, where opposing muscle groups generate balanced torques for efficient movement and stability. For instance, the deltoid and upper trapezius form a force couple during arm elevation, with the deltoid providing upward pull on the humerus and the trapezius elevating and rotating the scapula to facilitate smooth glenohumeral motion.^[39] Complementing this, the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) create a compressive force couple that centers the humeral head within the glenoid fossa, countering the deltoid's superior shear and preventing superior migration during elevation.^[39]^[40] Stability in the shoulder arises from the interplay of static and dynamic mechanisms, ensuring joint congruence under varying loads. Static stabilizers, including the glenohumeral ligaments and glenoid labrum, provide passive restraint by deepening the glenoid socket and limiting excessive translation, with the labrum increasing the glenoid's depth by up to 50%.^[39] Dynamic stabilizers, primarily the rotator cuff and long head of the biceps, actively generate concavity-compression to balance destabilizing forces from larger muscles like the deltoid and pectoralis major, maintaining humeral head centering throughout the range of motion.^[39] This dual system allows the shoulder's inherent laxity—essential for its wide mobility—to coexist with robust load-bearing capacity. In overhead activities, such as reaching or lifting, the glenohumeral joint manages significant compressive forces, approximately 0.5 times body weight for tasks like lifting to shoulder height, which are distributed across the articular cartilage and subchondral bone to optimize stress absorption and minimize peak pressures.^[41] The rotator cuff's compression enhances this distribution by increasing joint contact area, reducing shear and eccentric loading on the glenoid.^[39] The shoulder also excels in propulsion during dynamic tasks like throwing or pushing, where it serves as a key link in the kinetic chain for energy transfer. Energy generated from the lower extremities and trunk is sequentially transmitted through the shoulder via coordinated muscle activation, with the deltoid and rotator cuff optimizing torque and angular velocity to propel the arm forward while the rotator cable structure distributes tensile forces like a suspension bridge.^[42]^[39] This efficient transfer maximizes velocity in throwing motions, where peak internal rotation speeds exceed 7000 degrees per second, without compromising joint stability.^[42]

Development

Embryonic formation

The embryonic formation of the shoulder, or pectoral girdle, begins with the development of the upper limb bud from interactions between ectodermal and mesodermal tissues during the fourth week of gestation. The upper limb bud emerges around Carnegie stage 13-14 (approximately days 28-32 post-fertilization), positioned lateral to the lower cervical and upper thoracic somites, marking the initial outgrowth that will give rise to the humerus, scapula, and clavicle. This process is driven by signaling from the overlying ectoderm, which thickens to form the apical ectodermal ridge (AER) by the end of week 4, a structure essential for regulating proximal-distal axis growth through the release of fibroblast growth factors (FGFs).^[43]^[44] The core mesodermal components contributing to shoulder structures originate from distinct embryonic layers. Somitic mesoderm, derived from paraxial mesoderm adjacent to the neural tube, differentiates into myotomes that provide myogenic precursor cells migrating into the limb bud to form the shoulder's musculature, including muscles attaching to the scapula and humerus; sclerotomal derivatives from the same somites contribute to the medial border of the scapula and associated vertebral elements. Meanwhile, lateral plate mesoderm, specifically its somatic layer, migrates into the limb bud to form the cartilaginous precursors of the appendicular skeleton, including the bulk of the scapula body, coracoid process, glenoid fossa, and humerus shaft, while also supporting the formation of the pectoral girdle through ventral expansion and lateral positioning by Carnegie stage 17 (week 5). The clavicle arises uniquely from a combination of lateral plate mesoderm at somite levels 1-14 and contributions from cephalic mesoderm in its rostral portion, enabling its early intramembranous ossification distinct from the endochondral processes of other girdle elements.^[43]^[44]^[45] Genetic regulation orchestrates the precise patterning of these structures, with Hox genes and FGF signaling playing pivotal roles. Hox genes, such as Hoxb5, Hoxc6, and Hoxa5, pre-pattern somitic and lateral plate mesoderm to specify scapular blade identity and positioning, where disruptions like Hox5 compound mutations lead to rostral shifts in scapula location. FGF signaling, particularly FGF10 from the lateral plate mesoderm and FGF8 from the AER, promotes proliferation and differentiation of mesenchymal progenitors for proximal elements like the humerus head and scapula glenoid, with Tbx5 upstream activation ensuring coordinated girdle formation—null mutations in Fgf10 result in posterior scapula defects. By week 5, these molecular cues establish the foundational anlagen, with chondrification initiating in the humerus and scapula medial border at Carnegie stage 17, setting the stage for joint interzone formation by stage 19.^[46]^[43]^[47]

Postnatal maturation

The postnatal maturation of the shoulder involves progressive ossification, growth plate development, and adaptations influenced by hormonal changes across infancy, childhood, adolescence, and into adulthood. The clavicle, a key component of the shoulder girdle, undergoes intramembranous ossification that begins prenatally around 5-6 weeks of gestation, making it the first bone to ossify in the human skeleton, though secondary ossification at the ends continues postnatally with full fusion into early adulthood (around 21-25 years).^[48] In contrast, the scapula's primary ossification center forms prenatally in the body during the 8th week of gestation, but secondary centers emerge postnatally: the coracoid process develops two centers between 12 and 18 months of age, while the subcoracoid secondary center appears around 8-10 years.^[49] For the proximal humerus, the primary diaphyseal center ossifies prenatally in the 8th week, with the secondary epiphyseal center for the humeral head appearing by 6 months postnatally, followed by centers for the greater tuberosity at 3 years and lesser tuberosity at 5 years, contributing to the shoulder's articular surface by age 1-2.^[50] Growth plates, or physes, in the shoulder region facilitate longitudinal bone growth during childhood and fuse during adolescence under hormonal regulation. The proximal humeral physis, a major growth contributor, typically fuses between ages 14 and 18 (earlier in females, 14-17 years, vs. males 16-18 years), while scapular secondary centers like the acromion (appearing 14-20 years, fusing by ~22 years) and coracoid (fusing by 16-17 years) complete maturation by early adulthood.^[51] This fusion process is primarily driven by sex hormones, with estrogen accelerating epiphyseal closure in both sexes by promoting chondrocyte senescence and apoptosis in the growth plate, leading to the invasion of osteoblasts and eventual bony bridging.^[52] Growth hormone and insulin-like growth factor-1 also support earlier phases of endochondral ossification but yield to estrogen's dominant role in maturation timing, which varies by sex—earlier in females due to higher estrogen levels.^[53] Puberty introduces significant remodeling to the shoulder, enhancing its biomechanical capacity for upper limb function. In males, the pubertal surge in testosterone drives substantial increases in muscle mass around the rotator cuff and deltoid, bolstering shoulder stability and strength by adolescence.^[54] Conversely, females often experience heightened joint laxity during puberty, particularly in the glenohumeral joint, attributed to estrogen-mediated relaxation of ligaments and capsules, which can peak post-menarche and increase susceptibility to instability.^[55] These changes coincide with rapid skeletal growth, where the shoulder broadens and the glenoid deepens slightly, optimizing load distribution. In adulthood and senescence, the shoulder undergoes adaptive declines that reduce functional range. By the elderly years (typically over 60), shoulder range of motion diminishes, with losses in abduction, forward elevation, and internal rotation averaging 10-20 degrees compared to younger adults, primarily due to progressive capsular tightening and collagen cross-linking in the glenohumeral capsule.^[56] This fibrosis, often exacerbated by reduced estrogen in postmenopausal women, leads to adhesive capsulitis-like changes, restricting motion without acute injury and reflecting cumulative degenerative effects on soft tissues.^[57]

Clinical significance

Injuries and fractures

The shoulder is highly susceptible to traumatic injuries due to its wide range of motion and structural complexity, with fractures and dislocations being among the most common orthopedic emergencies. Proximal humerus fractures account for 5-6% of all adult fractures and frequently result from low-energy mechanisms such as falls in elderly patients with osteoporotic bone.^[58] Clavicle fractures, often from direct impact or falls onto the shoulder, represent another prevalent injury, while glenohumeral dislocations and rotator cuff tears can arise from high-force trauma or repetitive stress. These injuries often lead to pain, instability, and impaired function, necessitating prompt evaluation and management to prevent complications like nonunion or chronic instability. Proximal humerus fractures are classified using the Neer system, which categorizes them based on the displacement of the four major segments: the humeral head, greater tuberosity, lesser tuberosity, and shaft, with a part considered displaced if separation exceeds 1 cm or angulation surpasses 45 degrees.^[59] This system guides treatment by assessing the number of displaced fragments, from one-part (nondisplaced) to four-part (highly unstable) fractures. The primary mechanism in the elderly is a ground-level fall onto an outstretched hand, leading to axial loading and potential avascular necrosis of the humeral head in complex cases.^[60] Clavicle fractures are typically divided into midshaft (the most common type, comprising about 80% of cases) and distal types, with the latter accounting for 10-30% and often involving the acromioclavicular joint. Midshaft fractures result from direct trauma or indirect forces during falls, while distal fractures may stem from higher-energy impacts affecting the lateral clavicle. Healing rates for conservatively managed clavicle fractures generally range from 8-12 weeks, with midshaft injuries showing union in most cases, though distal fractures have a higher risk of nonunion (up to 44% in unstable patterns) due to poorer blood supply and soft tissue attachments.^[61]^[62] Glenohumeral dislocations occur when the humeral head displaces from the glenoid fossa, with anterior dislocations comprising over 95% of cases, often from an abduction-external rotation force such as a fall on an outstretched arm. Posterior dislocations, rarer at about 2-4%, typically arise from axial loading with internal rotation, like during seizures or electric shocks, while inferior (luxatio erecta) dislocations involve hyperabduction and are the least common, at less than 1%. Associated Hill-Sachs lesions, which are posterolateral humeral head compression fractures from impact against the glenoid rim, occur in 54-85% of anterior dislocations and contribute to recurrent instability if engaging with the glenoid track.^[63]^[64] Rotator cuff tears involve disruption of the supraspinatus, infraspinatus, teres minor, or subscapularis tendons, classified as acute (traumatic, often in younger patients from sudden overload) or degenerative (gradual wear in older individuals due to age-related tendon changes and repetitive microtrauma). Acute tears typically follow high-energy events like dislocations, presenting with sudden weakness, whereas degenerative tears progress insidiously and are linked to subacromial impingement syndromes, where tendon compression under the acromion leads to inflammation and partial-to-full thickness damage. Impingement often exacerbates tears by narrowing the subacromial space, with prevalence increasing after age 40.^[65]^[66] Treatment for shoulder injuries varies by type and severity, with conservative approaches favored for stable fractures and tears, while surgical intervention is indicated for displaced or complex cases. Conservative management commonly involves sling immobilization for 3-6 weeks to allow healing, combined with physical therapy to restore range of motion and strength, achieving good outcomes in nondisplaced proximal humerus or midshaft clavicle fractures. Surgical options include open reduction and internal fixation (ORIF) using plates or screws for unstable fractures like displaced proximal humerus or distal clavicle injuries, and arthroscopic techniques for rotator cuff repairs or dislocation stabilizations, which offer smaller incisions, faster recovery, and lower infection rates compared to open methods, though with potentially longer operative times.^[67]^[68]

Disorders and pain syndromes

Shoulder disorders and pain syndromes encompass a range of non-traumatic conditions that lead to chronic pain and functional limitations in the glenohumeral joint, often involving inflammation, degeneration, or fibrosis of surrounding structures. These pathologies, distinct from acute injuries, commonly affect middle-aged adults and can significantly impair daily activities due to restricted motion and persistent discomfort. Common examples include rotator cuff tendinopathies, adhesive capsulitis, and arthritic changes, which may overlap or mimic one another in presentation. Referred pain from distant sources further complicates diagnosis and management. Rotator cuff tendinopathy refers to degenerative changes in the tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles, often resulting in pain exacerbated by overhead activities and nocturnal discomfort. Subacromial impingement syndrome, a frequent subtype, arises from mechanical compression of the rotator cuff tendons and subacromial bursa within the narrowed subacromial space, leading to inflammation, edema, and progressive tendon weakening. Calcific tendinitis, another variant, involves the deposition of calcium phosphate crystals in the rotator cuff substance, typically 1-2 cm proximal to the tendon insertion, and affects 2.5-7.5% of adults, predominantly women in their 40s and 50s. This condition progresses through precalcific remodeling, formative calcific deposition, resorptive inflammation (causing acute severe pain), and postcalcific healing phases, with up to 20% of cases remaining asymptomatic. Diagnosis relies on clinical history, physical examination for painful arc (60-120 degrees abduction), and imaging such as ultrasound or radiography to confirm tendon thickening or calcific deposits. Adhesive capsulitis, commonly known as frozen shoulder, is characterized by idiopathic fibrosis and thickening of the glenohumeral joint capsule, resulting in progressive stiffness and pain that limits both active and passive range of motion. The condition unfolds in three stages: the freezing phase (2-9 months), marked by diffuse, worsening pain and gradual stiffness; the frozen phase (4-12 months), with reduced pain but persistent severe limitation; and the thawing phase, involving slow recovery of mobility over 5-26 months, though full resolution occurs in only about 50% of cases. Prevalence is 2-5%, peaking around age 55 and slightly more common in women (1.4:1 ratio). Key risk factors include diabetes mellitus, which increases incidence 3-10-fold and prolongs duration due to advanced glycation end-products promoting fibrosis, as well as thyroid disorders, autoimmune diseases, and prolonged immobilization. Other associations encompass longer diabetes duration, hyperlipidemia, and poor sleep quality, with diabetic patients experiencing greater motion deficits and slower recovery. Osteoarthritis of the glenohumeral joint involves progressive articular cartilage loss, subchondral bone sclerosis, and osteophyte formation, primarily affecting the posterior glenoid and central humeral head, leading to eccentric joint space narrowing and a characteristic "goat's beard" osteophyte on the humeral head. This degenerative process, prevalent in 16-20% of individuals over 65, manifests as posterior or nocturnal pain, stiffness, and reduced function, particularly in external rotation and overhead tasks, with risk factors including advanced age, female sex, obesity, and prior trauma. In contrast, rheumatoid arthritis presents with uniform joint space narrowing due to even cartilage erosion across the joint surface, alongside marginal bone erosions at the glenohumeral margins from chronic synovitis. Erosions in rheumatoid arthritis occur later in large joints like the shoulder, contributing to instability and deformity, and are driven by inflammatory cytokines such as TNF-α and IL-1. Referred pain to the shoulder arises from convergent neural inputs where somatic or visceral afferents synapse on the same second-order neurons, often mimicking primary shoulder pathology. Cervical spine sources, particularly C5-C6 radiculopathy or facet joint irritation, commonly refer pain to the posterior shoulder, scapula, or trapezius due to shared dermatomes and brachial plexus overlap, as demonstrated by provocative discography and facet stimulation studies. Visceral origins include cardiac ischemia, pulmonary embolism, or diaphragmatic irritation, which project pain to the ipsilateral shoulder via phrenic (C3-C5) or vagal pathways, presenting as acute or pleuritic discomfort without local tenderness. Management of these shoulder pain syndromes emphasizes conservative approaches to alleviate symptoms and restore function, tailored to the underlying pathology. Physical therapy, including stretching, strengthening exercises, and modalities like ultrasound, improves range of motion and function comparably to other interventions in the short term (1-3 months), with sustained benefits up to 12 months in conditions like impingement and capsulitis. Intra-articular or subacromial corticosteroid injections provide rapid pain relief and enhanced mobility in early stages of frozen shoulder or tendinopathy, outperforming placebo but showing equivalent long-term outcomes to physical therapy at 6-12 months. Pharmacologic options, such as non-steroidal anti-inflammatory drugs (e.g., ibuprofen) and analgesics, target inflammation and nociception, offering short-term symptom control when combined with rehabilitation, though evidence supports their use as adjuncts rather than monotherapy. For refractory cases, advanced therapies like platelet-rich plasma injections may promote tendon healing in tendinopathies, but multidisciplinary evaluation is essential to address comorbidities like diabetes.

Diagnostic approaches

Diagnosis of shoulder pathology begins with a thorough patient history, focusing on the onset, nature, location, and aggravating factors of pain or dysfunction, followed by a systematic physical examination to identify abnormalities in structure and function.^[69] The clinical examination typically proceeds in a stepwise manner: inspection, palpation, assessment of range of motion (ROM), and specialized provocative tests tailored to suspected conditions.^[69] Inspection involves observing the shoulder from anterior, posterior, and lateral views for signs of asymmetry, swelling, deformity, erythema, bruising, scars, or muscle atrophy, such as supraspinatus or infraspinatus wasting indicative of rotator cuff involvement.^[69] Palpation follows to assess tenderness over key landmarks including the sternoclavicular joint, clavicle, acromioclavicular (AC) joint, coracoid process, acromion, scapular spine, and biceps tendon groove, which may suggest joint pathology like osteoarthritis or tendinopathy.^[69] ROM evaluation starts with active movements—flexion, extension, abduction, adduction, and internal/external rotation—followed by passive testing to differentiate between joint restrictions (painful active motion) and neuromuscular issues (painless passive limitation).^[69] Normal active abduction typically reaches up to 180°, with mean values around 160° and minimal differences between dominant (≈159°) and non-dominant sides, with deviations signaling potential pathology.^[69]^[70] Special tests are performed to provoke specific symptoms and confirm suspected diagnoses, often with established sensitivity and specificity. For subacromial impingement, the Neer test involves stabilizing the scapula and passively flexing the internally rotated arm; pain reproduction indicates a positive result (sensitivity 72%, specificity 60%).^[69] The Hawkins-Kennedy test, with the arm at 90° flexion and elbow bent, entails rapid internal rotation; subacromial pain suggests impingement (sensitivity 80%, specificity 56%).^[69] Supraspinatus pathology is assessed via the empty can (Jobe's) test: the arm is abducted to 30° in the scapular plane with thumbs down, resisting downward pressure; pain or weakness is positive (sensitivity 81%, specificity 89%).^[69] For anterior instability, the apprehension test positions the shoulder at 90° abduction and external rotation with an anterior force on the humeral head; patient apprehension or pain confirms the diagnosis (sensitivity 81.8%).^[69] Imaging modalities are selected based on clinical suspicion to visualize bony and soft tissue structures. Plain X-rays, including anteroposterior, axillary, and scapular Y views, serve as the initial study for detecting fractures, arthritis, calcific tendinitis, or AC joint osteolysis, often sufficient for acute trauma evaluation.^[71] Ultrasound provides dynamic, real-time assessment of soft tissues like the rotator cuff and biceps tendon, excelling in identifying tears or effusions without radiation exposure, and is cost-effective for initial soft tissue evaluation.^[71] Magnetic resonance imaging (MRI) offers high-resolution multiplanar views of intra-articular structures, including the rotator cuff, labrum, and glenohumeral ligaments, making it ideal for detailed soft tissue pathology assessment.^[71] MR arthrography (MRA), involving intra-articular contrast injection, enhances visualization of labral tears and instability, providing superior sensitivity for subtle joint abnormalities compared to standard MRI.^[71] Diagnostic arthroscopy is a minimally invasive procedure employed when non-invasive methods are inconclusive, allowing direct visualization and potential intervention within the glenohumeral joint and subacromial space.^[72] Performed under anesthesia in beach chair or lateral decubitus positions, it uses a posterior portal for arthroscope insertion to systematically inspect structures via a standardized 14-point evaluation, including the glenoid rim, humeral head, and subacromial bursa.^[72] This technique is particularly valuable for confirming rotator cuff tears, labral pathology, biceps abnormalities, loose bodies, or adhesive capsulitis, often transitioning seamlessly to therapeutic procedures.^[72]

Comparative anatomy

In non-human mammals

In non-human mammals, the shoulder complex exhibits significant adaptations to quadrupedal locomotion, particularly in the scapula and clavicle. Quadrupedal species often feature an elongated scapula that facilitates protraction and retraction during stride cycles, enhancing step length and serving as a lever for forelimb swing.^[73] This morphology is evident in terrestrial cursorial mammals, where the scapula's length supports efficient propulsion.^[73] The clavicle is frequently reduced or absent in many quadrupeds, such as horses and dogs, allowing greater forelimb mobility under the body and reducing constraints from a fixed bony link to the sternum.^[74]^[73] Forelimb positioning varies markedly across mammalian groups to suit locomotor demands. In ungulates like horses, the forelimbs are positioned close to the median plane for weight-bearing support during high-speed running, with the scapula situated laterally on a narrow thorax to distribute forces efficiently.^[73] In contrast, primates exhibit forelimbs adapted for both weight-bearing in quadrupedal locomotion and grasping, with a more dorsally oriented scapula and retained clavicle providing stability for arboreal or terrestrial maneuvers.^[74]^[73] Muscle variations in the shoulder reflect these locomotor specializations. Cursorial runners, such as dogs, possess a well-developed trapezius muscle, with distinct cervical and thoracic portions showing heightened activity and excitation during trot and gallop to stabilize the scapula and facilitate stride.^[75] In carnivores, the acromion often fuses seamlessly with the scapular spine, supporting powerful deltoid and supraspinatus actions for predatory pursuits.^[74] Specialized examples highlight further diversity. In bats, the shoulder supports wing elongation through an adapted scapula and retained clavicle, enabling high-amplitude glenohumeral excursions for powered flight, with the humerus and elongated finger bones forming the wing framework.^[74]^[73] Whales exhibit flipper modifications where the shoulder joint remains mobile for pectoral oscillation, with the scapula and humerus encased in blubber. Although distal forelimb elements are reduced, the shoulder retains extensive musculature, such as the subscapularis and deltoideus, to facilitate flipper movements and contribute to hydrodynamic lift.^[76]^[77]

Evolutionary adaptations

The pectoral girdle in early vertebrates originated as a supportive structure for the pectoral fins in primitive fish, primarily composed of dermal bones such as the cleithrum, supracleithrum, and posttemporal, which were juxtaposed to the base of the skull and derived from pharyngeal arch elements in jawless ancestors.^[78] This dermal framework provided anchorage for fin rays and musculature, enabling undulatory swimming, with fossil evidence from ostracoderms indicating a pharyngeal origin around the sixth branchial arch.^[78] In contrast, the transition to tetrapods marked a shift toward an endoskeletal composition, where cartilage-derived elements like the scapula and coracoid became dominant, replacing much of the dermal skeleton and detaching the girdle from direct cranial articulation to accommodate weight-bearing limbs.^[79] This endoskeletal emphasis, evident in early tetrapodomorphs like Eusthenopteron, facilitated the fins-to-limbs transition by allowing radial growth and increased appendicular flexibility.^[80] The move to terrestrial environments in amphibians involved structural integrations, including the persistence of the dermal clavicle alongside the endoskeletal scapula, providing ventral stability for laterally positioned limbs without full fusion but with enhanced bracing against gravitational loads. In anurans and other amphibians, the scapula and coracoid remained well-separated, supporting jumping and burrowing motions, while the clavicle and interclavicle formed a ventral shield.^[81] Reptiles further adapted the girdle for greater mobility, with reductions in dermal components like the cleithrum and refinements in scapular-coracoid articulation, enabling more efficient limb protraction and retraction suited to sprawling gaits and early predatory behaviors. These changes, seen in fossils like Seymouria, decoupled the girdle more fully from the axial skeleton, prioritizing dynamic range over rigid support.^[80] In primate evolution, the shoulder girdle underwent significant modifications for arboreal lifestyles, featuring an enlarged and oval-shaped glenoid cavity that enhanced overhead reaching and rapid limb excursion during brachiation and suspension.^[82] This glenoid expansion, prominent in hominoids, coupled with a broadened scapula, improved glenohumeral congruence for high-acceleration movements.^[82] Concurrently, the rotator cuff muscles—supraspinatus, infraspinatus, teres minor, and subscapularis—expanded in size and attachment area relative to body mass, providing superior stabilization against dislocating forces in overhead postures, as evidenced by comparative dissections across catarrhines.^[83] Fossil records illuminate these adaptations in hominins: Australopithecus species, such as A. afarensis, exhibit shoulder girdles with a more inferiorly oriented glenoid and reduced supraspinous fossa, adaptations that stabilized the arm during bipedal locomotion while retaining some arboreal capabilities, as seen in the 3.3-million-year-old Dikika specimen.^[84] In Homo erectus, dated to around 1.8 million years ago, the girdle shows further human-like features, including a shallower glenoid and expanded rotator cuff insertions, supporting precise manipulative tasks and overhead throwing essential for tool use and hunting, as inferred from Zhoukoudian fossils. These shifts reflect a progressive optimization for terrestrial bipedalism and technological innovation over millions of years.^[84]

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Most clinicians follow a stepwise approach summarised as Look, Feel, Move (active then passive), followed by special tests for specific pathology. Inspection ...
[67]
Radiologic evaluation of the painful shoulder in adults - UpToDate
Jul 22, 2024 · Shoulder radiographs are often the only imaging exam necessary for the evaluation of acute shoulder trauma, calcific tendonitis, arthritis, and ...
[68]
Diagnostic Shoulder Arthroscopy: Surgical Technique - PMC
Apr 11, 2019 · Discussion. Diagnostic shoulder arthroscopy is frequently used to evaluate and treat several shoulder conditions including rotator cuff tears, ...
[69]
Functional Analysis of the Primate Shoulder | International Journal of ...
Apr 13, 2010 · Quadrupedal walking entails muscle activities and joint forces that require a long scapula, the cranial margin of which has about the same ...
[70]
Comparative Anatomy of the Shoulder - Musculoskeletal Key
Oct 18, 2016 · In mammals without clavicles, the scapula has no bony attachments whatsoever. It becomes the sole support for limb and provides attachments ...
[71]
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### Summary on Trapezius Muscle in Quadrupedal Mammals (Dogs, Horses, Runners)
[72]
Functional anatomy of shoulder muscles in the Pacific white‐sided ...
Jul 8, 2023 · The backward swinging of the flipper occurs when the common tubercle of the humerus rotates medially (b). In contrast, forward swinging occurs ...
[73]
Gray Whale Adaptations: Flippers - Journey North
As whales adapted to life in water, the shoulder remained movable while the elbow and wrist stiffened. The forelimbs became rigid paddles (flippers). Two ...
[74]
Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle
Nov 1, 2023 · We propose that the pectoral girdle is established on the position of the sixth branchial arch in the jawless ancestor of jawed vertebrates, and ...
[75]
Multiple embryonic sources converge to form the pectoral girdle ...
Jul 26, 2024 · The morphological transformation of the pectoral/shoulder girdle is fundamental to the water-to-land transition in vertebrate evolution.
[76]
Vertebrate Evolution: Turning Heads - ScienceDirect.com
Sep 20, 2005 · The pectoral girdle evolved in primitive fish as a support for the pectoral fins, and was juxtaposed to the base of the skull.
[77]
Transformation of the pectoral girdle in the evolutionary origin of frogs
In all anurans, the scapula and coracoid are the two principal ossifications of the girdle, and they are well separated from each other. As the interclavicle ...
[78]
The morphology and evolutionary history of the glenohumeral joint ...
The glenoid cavity of suspensory primates exhibits an oval shape, which seems related to rapid limb motion with high acceleration increment when coupled with ...
[79]
Rotator cuff muscle size and the interpretation of scapular shape in ...
The current study examines the relative sizes of the rotator cuff muscles in a wider sample of primate taxa using published data on muscle mass and cross- ...
[80]
Fossil hominin shoulders support an African ape-like last ... - PNAS
We found that a sustained shift in scapular shape occurred during hominin evolution from an African ape-like ancestor to a modern human-like form.Missing: enlarged | Show results with:enlarged