The shoulder blade, or scapula, is a flat bone on the upper back that serves as a foundation for the entire shoulder complex. Its name, derived from the Latin word for a small shovel, reflects its broad, triangular shape. The scapula is a fundamental part of the shoulder girdle, connecting the arm to the trunk of the body. Understanding its structure and function is important for comprehending the wide-ranging mobility of the human arm.
The Scapula’s Structure and Location
The scapula is a thin, triangular bone positioned on the back of the rib cage, typically spanning the area between the second and seventh ribs (T2-T7) along the spine. Unlike most bones that form a fixed joint with the body’s central axis, the scapula is primarily held in place by a complex network of muscles, allowing it to glide over the thoracic cage. This unique muscular attachment provides a stable yet highly mobile base for the arm.
The bone features a distinct ridge called the spine, which runs obliquely across the posterior surface. This spine terminates laterally in the acromion process, which forms the highest point of the shoulder and articulates with the collarbone. On the lateral side, the scapula also holds a shallow depression called the glenoid cavity, which is the socket portion of the main shoulder joint.
Essential Role in Arm Movement
The scapula acts as the anchor for the glenohumeral joint, which connects the upper arm bone (humerus) to the torso. The scapula’s movements are precisely coordinated with the arm’s motion, a relationship known as the scapulohumeral rhythm. For example, when the arm lifts overhead, the scapula must simultaneously rotate upward to prevent the arm bone from hitting the acromion process.
A large number of muscles attach to the scapula, including the four muscles of the rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis), which are responsible for rotating and stabilizing the arm. The trapezius, rhomboids, and serratus anterior muscles also attach here, controlling the scapula’s six distinct motions:
- Elevation
- Depression
- Protraction
- Retraction
- Upward rotation
- Downward rotation
The combined action of these muscles and the bone’s mobility provides the necessary power and stability for activities like throwing or reaching.
Common Causes of Scapular Pain
Pain around the shoulder blade is a frequent complaint, often stemming from everyday, non-traumatic causes. One common factor is poor or sustained posture, such as prolonged slouching while working at a desk or using a mobile phone. Maintaining a single position for too long strains the muscles that stabilize the scapula, leading to fatigue and discomfort.
Muscle strain is another frequent source of pain, often felt as trigger points in the muscles between the shoulder blade and the spine. Activities involving repetitive motion or overuse, such as painting or lifting heavy objects, can overload the trapezius and rhomboid muscles.
Pain in this area can also be referred from the neck or upper back (cervical or thoracic spine). Nerve irritation in these areas can manifest as aching or burning sensations near the scapula.