ScapulaEdit
The scapula, commonly known as the shoulder blade, is a large, flat, triangular bone that forms the posterior aspect of the pectoral girdle. It sits on the upper back wall of the thorax, gliding over the convexities of the rib cage and providing a broad surface for muscle attachment. The scapula plays a central role in the function of the upper limb by acting as a stable platform for the rotator cuff and related shoulder muscles, while also permitting a wide range of arm movements through its coordinated motion with the clavicle and humerus.
Although it does not form a direct, fixed connection with the spine, the scapula interacts with surrounding bones and muscles to enable complex shoulder mechanics. Its movements are coordinated with the humerus at the glenohumeral joint and with the clavicle at the acromioclavicular joint, creating the functional unit known as the shoulder girdle. The glenoid cavity (glenoid fossa) of the scapula articulates with the head of the humerus, forming the ball-and-socket joint that provides most of the shoulder’s mobility, while the acromion and coracoid process serve as important landmarks and articulation sites for ligaments and muscles.
Anatomy
Shape and surfaces
The scapula is composed of a body that forms a broad, concave costal (anterior) surface and a convex dorsal (posterior) surface. The anterior surface projects forward toward the rib cage, while the posterior surface features the prominent scapular spine that divides the concave supraspinous fossa from the infraspinous fossa. The superior border is short, and the medial (vertebral) border runs parallel to the spine of the thoracic cage, with the lateral (axillary) border forming the lateral contour of the chest wall.
Borders and angles
Key borders and angles include the superior border, the medial (vertebral) border, and the lateral (axillary) border, along with the superior and inferior angles. These features serve as attachment points for several muscles and define the scapula’s relationship to surrounding structures.
Processes
Two prominent processes extend from the scapula: - The acromion, which forms the tip of the shoulder and provides attachment for the deltoid muscle and ligaments; it also communicates with the clavicle at the acromioclavicular joint. - The coracoid process, a hook-like projection that serves as an attachment site for several muscles, including the biceps brachii and the coracobrachialis.
Articulations
- Glenoid cavity/fossa: the shallow socket on the lateral aspect of the scapula that articulates with the head of the humerus to form the glenohumeral joint.
- Acromion: a continuation of the scapular spine that articulates with the clavicle at the AC joint.
- Scapulothoracic interface: while not a true synovial joint, the scapula slides over the thoracic wall and participates in the functional scapulothoracic movement essential to arm elevation.
Muscular attachments
The scapula provides extensive attachment surfaces for muscles that move and stabilize the upper limb. Major examples include: - Serratus anterior, which anchors the scapula to the thoracic wall and facilitates protraction. - The rhomboids and levator scapulae, which elevate and retract the scapula. - The trapezius, which moves the scapula in multiple directions. - The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), which originate or insert around the scapula and stabilize the glenohumeral joint. - The pectoral muscles in the chest wall also interact with the scapula through their distal attachments.
Nerve and vascular supply
Blood supply to the scapular region comes from branches of the subclavian and axillary arteries, including the circumflex scapular artery and suprascapular vessels, with contributions from the dorsal scapular circulation. Innervation of the surrounding muscles arises from several nerves, notably the suprascapular nerve, the dorsal scapular nerve, the axillary nerve, and the subscapular nerves, all delivering motor and, in many cases, sensory inputs to maintain shoulder function.
Development and variation
The scapula develops from embryonic mesenchyme and undergoes ossification in a combination of centers, with growth and fusion completing during late adolescence. Variation in shape, size, and the exact contours of the scapular borders and processes occurs across individuals and populations, but the core plan remains consistent.
Function and biomechanics
The scapula serves as both a lever and a stabilizer for the upper limb. Its motion is tightly coupled with the movement of the humerus in what is described as scapulohumeral rhythm. Proper coordination between the scapula and the humerus expands the range of arm elevation and rotation, while preventing impingement and maintaining joint stability. Muscular balance around the scapula—especially between the serratus anterior, the trapezius, and the rotator cuff—supports efficient shoulder mechanics during lifting, pushing, and throwing activities.
The scapula also participates in a functional, dynamic relationship with the thorax. The scapulothoracic articulation—though not a true joint—allows the scapula to rotate, tilt, and protract/retract in response to arm movement, posture, and breathing mechanics. This coordination has psychological and ergonomic implications, as sustained poor posture or muscular imbalance can influence scapular position and overall shoulder function.
Clinical significance
Scapular structure and motion are central to the assessment and management of shoulder disorders. Common issues include: - Scapular dyskinesis: abnormal movement patterns of the scapula that can contribute to pain and dysfunction in the shoulder complex. - Winged scapula: a condition often arising from injury to the long thoracic nerve or other stabilizing structures, resulting in medial rotation and protrusion of the scapula. - Fractures and contusions: traumatic injuries to the scapula can require imaging and, in some cases, surgical management. - Impingement and rotator cuff pathology: scapular positioning and motion can influence the likelihood of impingement or cuff degeneration.
Management typically emphasizes restoring balanced scapular motion, improving posture, and strengthening the periscapular muscles, sometimes in coordination with harnessing the clavicle and humerus to preserve functional shoulder motion. See discussions of physical therapy approaches and the role of targeted exercises in rehabilitation for conditions involving the scapula and shoulder girdle.
Evolution and comparative anatomy
In many vertebrates, the scapula is adapted to their mode of locomotion and limb use. Comparative anatomy highlights variations in scapular shape and articulation that reflect different functional demands, from weight-bearing forelimbs in some quadrupeds to highly mobile forelimbs in brachiators and other mammals.