Snapping Scapula Syndrome (SSS) is a condition characterized by a grinding, popping, or crackling sound, known as crepitus, that is felt or heard with movement of the shoulder blade. This sound occurs when the scapula, or shoulder blade, moves over the rib cage and is generally a sign that the smooth gliding mechanism is being disrupted. While the sound itself may be painless, the friction that causes it can lead to inflammation and significant discomfort. The causes of this friction are varied, stemming from either fixed structural problems or dynamic issues related to muscle function and soft tissues.
Understanding the Scapulothoracic Joint
The scapulothoracic joint is a functional articulation where the scapula glides across the curved surface of the posterior rib cage. This articulation is not a true anatomical joint like the knee or elbow. It lacks a joint capsule or ligaments and is stabilized entirely by surrounding muscles. Smooth movement is protected by fluid-filled sacs called bursae, which act as cushions to reduce friction between the bones and soft tissues.
The two main bursae involved are the infraserratus bursa, located between the serratus anterior muscle and the chest wall, and the supraserratus bursa. The characteristic snapping sound occurs when the scapula or an associated soft tissue structure physically catches or rubs against the ribs, disrupting this normally silent gliding motion.
Causes Related to Bony and Structural Abnormalities
Snapping Scapula Syndrome can be the result of fixed, mechanical irregularities that permanently alter the space between the shoulder blade and the rib cage. These structural abnormalities are often present from birth (congenital) or develop due to trauma or disease. The physical presence of an irregularity acts like a stone in a shoe, causing friction as the scapula slides past it.
Bony Irregularities
One of the most frequently identified irregularities is the osteochondroma, a benign bone tumor that projects from the bone surface, often from the anterior surface of the scapula or a rib. These bony projections create a direct physical obstruction that the overlying soft tissue must rub against during shoulder movement. Another common anatomical variation is Luschka’s tubercle, which is a localized bony or fibrocartilaginous prominence found at the superior-medial corner of the scapula.
Alignment and Trauma
Abnormal shape or curvature of the scapula itself can also predispose an individual to SSS, such as an unusually angled medial border. Furthermore, conditions affecting the spine, such as excessive thoracic kyphosis (a pronounced forward rounding of the upper back) or scoliosis, can change the alignment of the chest wall. This altered rib cage shape forces the scapula to articulate in an unusual path, increasing the likelihood of painful rubbing. Prior injuries, specifically fractures of the ribs or the scapula that heal in a poor position (malunion), can also leave a fixed bump that causes the snapping sensation.
Causes Related to Muscle Function and Soft Tissues
More commonly, Snapping Scapula Syndrome results from dynamic issues involving the muscles and soft tissues that control the shoulder blade. These problems relate to movement patterns, inflammation, or changes in tissue texture, rather than fixed bone shape. Any issue that causes the scapula to move incorrectly over the ribs can compress or irritate the tissues in between.
Scapular Dyskinesis
Scapular dyskinesis describes poor control or mistimed movement of the shoulder blade, often due to weakness or imbalance in the periscapular muscles. Muscles like the serratus anterior and the rhomboids are responsible for keeping the scapula flush against the ribs and guiding its rotation. When these muscles are weak, the scapula may wing out or tilt abnormally, pushing its edges closer to the chest wall and causing the friction.
Bursitis and Tissue Changes
Repetitive activity, especially overhead movements common in sports, can lead to inflammation and thickening of the scapulothoracic bursa, a condition called bursitis. An inflamed bursa swells, taking up more space, and may roll or pop audibly as the scapula slides over it, failing in its role as a friction reducer. Chronic irritation from muscle imbalance or trauma can also lead to the formation of scar tissue or fibrous nodules within the muscle layers beneath the scapula. These firm, non-pliable tissue areas then get painfully caught or compressed between the bone surfaces during arm movement.