Scapula Anatomy and Function
The scapulae, commonly referred to as the shoulder blades, are two large, triangular bones located on the upper back. They play a crucial role in the mobility and stability of the shoulder joint.
Anatomical Location
- The scapulae are situated on the posterior (back) side of the rib cage, one on each side of the body. They articulate with the clavicle (collarbone) at the acromioclavicular joint and with the humerus (upper arm bone) at the glenohumeral joint.
Shape and Structure
- Triangular Shape: The scapula has a roughly triangular shape, with three main borders (superior, medial, and lateral) and three main angles (superior, inferior, and lateral).
- Flat Bone: The scapula is a flat bone, and its broad surface allows attachment of various muscles that facilitate the movement of the arm.
Key Parts of the Scapula
- Spine of the Scapula: This prominent ridge runs across the posterior surface of the scapula. The spine divides the back of the scapula into two regions: the supraspinous fossa (above the spine) and the infraspinous fossa (below the spine). It also extends laterally to form the acromion process.
- Acromion Process: The acromion is a bony prominence at the top of the shoulder. It forms the highest point of the scapula and articulates with the clavicle to form the acromioclavicular joint.
- Coracoid Process: A smaller bony projection that extends anteriorly (forward) from the scapula. It serves as an attachment point for several muscles and ligaments, including the coracobrachialis muscle and the pectoralis minor muscle.
- Glenoid Cavity: A shallow, cup-like depression that forms the socket of the ball-and-socket shoulder joint, where the head of the humerus (upper arm bone) fits. It allows a wide range of motion for the arm.
- Supraspinous Fossa: The area above the spine, which serves as the attachment site for the supraspinatus muscle.
- Infraspinous Fossa: The area below the spine, which provides attachment for the infraspinatus muscle.
- Subscapular Fossa: Located on the anterior side of the scapula, it provides attachment for the subscapularis muscle.
Muscle Attachments
- The scapula is a key site for muscle attachments that allow for shoulder movement. Some of the major muscles that attach to the scapula include:
- Supraspinatus, Infraspinatus, Subscapularis, and Teres Minor (Rotator Cuff Muscles)
- Deltoid (attaches to the acromion)
- Trapezius (attaches along the spine and the acromion)
- Rhomboids (attaches along the medial border)
- Levator Scapulae (attaches to the superior angle)
Function
- The primary function of the scapula is to provide a stable base for the upper limb to move efficiently. It also serves as an anchor for muscles that control arm and shoulder movements. The scapula helps in lifting, pushing, pulling, and stabilizing the shoulder.
Articulations
- Sternoclavicular Joint: The medial end of the clavicle articulates with the sternum at the sternoclavicular joint.
- Acromioclavicular Joint: The lateral end of the clavicle articulates with the acromion process of the scapula at the acromioclavicular joint.
- Glenohumeral Joint: The head of the humerus fits into the glenoid cavity of the scapula to form the ball-and-socket shoulder joint.
Clinical Relevance
- Scapular Fractures: Though rare, fractures of the scapula can occur due to trauma, such as in high-impact accidents or falls.
- Scapular Winging: This condition occurs when the scapula protrudes outward, typically due to a weakness or paralysis of the serratus anterior muscle, often caused by nerve injury.
In summary, the scapula is a complex bone that serves as a critical part of the shoulder’s structure, providing attachment sites for muscles and contributing to the shoulder’s mobility and function.