Spine of Scapula

The spine of the scapula is a prominent, horizontal ridge of bone located on the posterior surface of the scapula. It serves as a major anatomical landmark that separates the supraspinous and infraspinous fossae and provides attachment points for several muscles involved in shoulder movement and stabilization. It is easily palpable and plays an important role in forming the shoulder contour.

Location

The spine of the scapula is found on the posterior surface of the scapula, arising from the medial border and extending laterally and slightly superiorly toward the acromion. It is located approximately at the level of the T3 vertebra when the body is in anatomical position.

Structure

• Begins at the medial border of the scapula as a smooth triangular area.
• Projects laterally as a prominent bony ridge, increasing in height and thickness as it approaches the lateral border.
• Ends laterally at the acromion, which articulates with the clavicle at the acromioclavicular joint.
• The superior surface forms the supraspinous fossa, and the inferior surface forms the infraspinous fossa.

Function

• Muscle attachment: Serves as a stable anchor for muscles such as the trapezius and deltoid.
• Structural support: Reinforces the posterior scapula and provides leverage for upper limb movement.
• Anatomical division: Separates the posterior scapula into functional fossae for different muscle groups.

Physiological Role(s)

• Acts as a pulley system for the shoulder, enabling complex arm movements.
• Serves as a bony guide for muscle direction and alignment in the scapular plane.
• Helps transmit force between muscles of the upper back and shoulder girdle.

Relations

• Superiorly: Supraspinous fossa and supraspinatus muscle.
• Inferiorly: Infraspinous fossa and infraspinatus muscle.
• Anteriorly (deep): Subscapularis muscle lies on the anterior scapula.
• Lateral extension: Acromion process, which continues from the lateral end of the spine.

Muscle Attachments

• Trapezius: Inserts along the superior edge of the scapular spine.
• Deltoid: Originates from the inferior edge of the spine and lateral acromion.
• Supraspinatus and infraspinatus: Originate in the fossae created by the spine, though not directly attached to it.

Development

The spine of the scapula develops from ossification centers within the scapular body during fetal life. As growth proceeds, the spine becomes more defined and prominent. The acromion process develops as a secondary ossification center that fuses with the spine during adolescence. Final shaping is influenced by muscular forces acting on the scapula during growth and limb movement.

Clinical Significance

• Fractures: The scapular spine may fracture due to direct trauma, particularly in high-impact injuries like motor vehicle accidents or falls on the back.
• Scapular winging: Displacement of the scapula due to muscle paralysis may alter the position of the scapular spine, leading to abnormal shoulder mechanics.
• Surgical landmark: Used as a reference point during procedures involving scapular stabilization or posterior shoulder access.
• Muscular imbalance: Overuse or tightness in trapezius or deltoid can lead to scapular dysfunction or postural asymmetry.

Imaging

• X-ray: Scapular spine is visible on standard lateral and posterior shoulder radiographs, useful in detecting fractures or deformities.
• CT scan: Provides detailed bone morphology and is useful in complex scapular injuries or surgical planning.
• MRI: Evaluates surrounding muscle attachments, soft tissue injuries, and bone marrow edema in cases of chronic pain or overuse.

Anatomical Variations

• The size and prominence of the scapular spine may vary between individuals and between sexes, with males often having a more pronounced spine.
• In some cases, the acromion may be flat, curved, or hooked, affecting the lateral continuation of the scapular spine and influencing shoulder impingement risk.
• Fusion anomalies between the spine and acromion can be present congenitally or after injury.