Scapula

The scapula, commonly known as the shoulder blade, is a flat, triangular-shaped bone that provides a vital part of the shoulder girdle. It serves as an attachment site for several muscles that facilitate the shoulder’s complex movements. Structurally, it features a body, two processes (the acromion and the coracoid process), and a glenoid cavity, which articulates with the humerus to form the shoulder joint.

Location

The scapula is located on the posterior side of the ribcage, between the second and seventh ribs. It lies on the upper back part of the body, symmetrically on each side, and is positioned over the dorsal surface of the rib cage.

Anatomy

Body of the Scapula

• Dorsal Surface: The dorsal surface is divided into two parts by the scapular spine, a prominent ridge running diagonally across the back of the scapula. The area above the spine is called the supraspinous fossa, and the area below is the infraspinous fossa. These fossae serve as attachment sites for the supraspinatus and infraspinatus muscles, respectively.
• Costal Surface (Anterior Surface): This surface faces the rib cage and contains the subscapular fossa, a broad concavity that accommodates the subscapularis muscle.

Borders of the Scapula

• Medial (Vertebral) Border: This is the edge closest to the spine, running parallel to the vertebral column.
• Lateral (Axillary) Border: This border is closest to the armpit and extends from the glenoid cavity to the inferior angle of the scapula.
• Superior Border: The shortest and sharpest of the three borders, extending between the superior angle and the coracoid process.

Angles of the Scapula

• Superior Angle: The junction between the superior and medial borders.
• Inferior Angle: Where the medial and lateral borders converge; important in movements of the scapula.
• Lateral Angle: The thickest part of the scapula, housing the glenoid cavity, which articulates with the humerus to form the shoulder joint.

Processes of the Scapula

• Acromion: A bony projection from the scapular spine that forms the highest point of the shoulder; it articulates with the clavicle to form part of the acromioclavicular joint.
• Coracoid Process: A hook-like structure projecting anteriorly from the superior border; serves as an attachment point for several muscles and ligaments.

Glenoid Cavity

A shallow, pear-shaped articular surface on the lateral angle of the scapula. It articulates with the head of the humerus to form the shoulder joint. Surrounding the cavity is the glenoid labrum, a fibrocartilaginous rim that deepens the socket and aids in joint stability.

Other Features

• Scapular Notch: A notch located on the superior border that serves as a passageway for the suprascapular nerve.
• Spine of the Scapula: A prominent ridge of bone on the dorsal surface that divides the back of the scapula into the supraspinous and infraspinous fossae.

Overall, the scapula’s anatomy is intricate, with each part serving specific roles in shoulder mobility and stability. Its connections with the clavicle, humerus, and numerous muscles and ligaments make it central to the function of the upper limb.

Attachments

The scapula serves as an attachment site for numerous muscles that contribute to its motion and the overall function of the shoulder. Here are the major muscles attached to the scapula:

Muscles Attaching to the Anterior (Costal) Surface

• Subscapularis: Originates from the subscapular fossa and inserts into the lesser tubercle of the humerus; part of the rotator cuff muscle group.

Muscles Attaching to the Posterior Surface

• Supraspinatus: Originates from the supraspinous fossa and inserts into the greater tubercle of the humerus; also part of the rotator cuff.
• Infraspinatus: Arises from the infraspinous fossa and attaches to the greater tubercle of the humerus; another rotator cuff muscle.
• Teres Minor: Originates from the middle part of the lateral border and inserts into the greater tubercle of the humerus; completes the rotator cuff group.
• Teres Major: Attaches from the lower third of the lateral border to the medial lip of the bicipital groove of the humerus; not part of the rotator cuff but assists in arm movement.

Muscles Attaching to the Lateral Border

• Triceps Brachii (long head): Extends from the infraglenoid tubercle at the lateral angle of the scapula to the olecranon of the ulna, contributing to elbow extension.

Muscles Attaching to the Superior Border

• Omohyoid (inferior belly): Attaches indirectly to the scapula via connective tissue to the superior border; involved in depressing the hyoid bone.

Muscles Attaching to the Medial (Vertebral) Border

• Levator Scapulae: Originates from the upper cervical vertebrae and inserts into the superior angle of the scapula, elevating it.
• Rhomboid Minor: Arises from the lower cervical and upper thoracic vertebrae and attaches to the medial border, near the spine.
• Rhomboid Major: Extends from the upper thoracic vertebrae to the medial border below the rhomboid minor.

Muscles Attaching to the Spine of the Scapula

• Trapezius: This large muscle attaches to the spine and acromion of the scapula, the clavicle, and the neck vertebrae, functioning in the movement and stabilization of the scapula.
• Deltoid: Attaches to the lateral third of the clavicle, the acromion, and the spine of the scapula, extending to the deltoid tuberosity of the humerus and involved in arm abduction.

Muscles Attaching to the Coracoid Process

• Coracobrachialis: Originates from the coracoid process and inserts into the medial humerus; assists in flexing and adducting the arm.
• Biceps Brachii (short head): Also originates from the coracoid process and inserts into the radial tuberosity, involved in forearm supination and elbow flexion.
• Pectoralis Minor: Arises from the anterior surfaces of certain ribs and attaches to the coracoid process, acting to depress and protract the scapula.

These muscles work in concert to facilitate a wide range of movements and provide stability to the shoulder joint and upper limb.

Function

The scapula plays several critical roles in the function of the upper limb, acting as a central hub around which movements of the shoulder girdle revolve:

• Movement Facilitation: The scapula serves as the attachment site for multiple muscles that facilitate movements of the upper limb, including elevation, depression, protraction (abduction), retraction (adduction), and rotation. These movements are essential for positioning the hand and arm effectively in space, allowing for a wide range of activities from lifting to throwing.
• Stability: While the scapula enables a wide range of motion, it also provides stability for the shoulder joint. The muscles attached to the scapula, especially the rotator cuff muscles, help stabilize the head of the humerus within the glenoid cavity, preventing dislocation and contributing to the overall strength of the shoulder.
• Force Transmission: The scapula acts as a bridge between the trunk and the upper limb, allowing for the transmission of forces. When the upper limb is engaged in pushing, pulling, or lifting activities, the scapula helps to distribute forces between the arm and the body, aiding in efficient movement and reducing the risk of injury.
• Shock Absorption: The scapula also plays a role in absorbing shock during movements such as running or jumping. The mobility of the scapula allows it to adjust and move in response to impact, helping to dissipate forces and protect more vulnerable structures like the spine and ribcage.
• Supporting Upper Limb Function: By serving as an anchor point for muscles that extend to the hand and fingers, the scapula supports the fine motor functions of the upper limb. This support is crucial for intricate movements and tasks requiring precision and strength.

Overall, the scapula’s functions are integral to the complex mechanics of the shoulder and upper limb, enabling a balance between mobility and stability that is essential for everyday activities and athletic performance.

Clinical significance

Scapular Dyskinesis: This term refers to abnormal movement of the scapula during shoulder motion. Dyskinesis can result from muscle imbalances, injuries, or postural issues, leading to pain and reduced function. Understanding scapular mechanics is crucial for diagnosing and treating this condition.

Shoulder Impingement and Rotator Cuff Injuries: The position and movement of the scapula are closely related to the space available for rotator cuff tendons. Abnormal scapular positioning or movement can lead to impingement and subsequent tendonitis or tears of the rotator cuff.

Fractures: Although less common than other shoulder injuries, scapular fractures can occur and are typically the result of significant trauma. Treatment and recovery depend on understanding the scapula’s anatomy and its relationship with surrounding structures.

Rehabilitation and Physical Therapy: Effective rehabilitation from shoulder injuries often involves exercises aimed at strengthening scapular muscles and improving the coordination of scapular movement. This is essential for restoring normal shoulder function and preventing future injuries.

Surgical Considerations: In surgeries involving the shoulder, such as rotator cuff repair or shoulder replacement, the anatomy and function of the scapula are key factors in surgical planning and postoperative recovery.