Location and Structure
The muscle is located on the anterior side of the scapula and consists of superficial and deep layers. It occupies its own osseofascial compartment formed by the fusion of the subscapular fascia with the margins of the scapula.
- Origin (O): Costal surface of the scapula — subscapular fossa (fossa subscapularis).
- Insertion (I): The muscle fibers converge into a broad, flat tendon that passes through the subdeltoid space to insert on the lesser tubercle of the humerus (tuberculum minus humeri) and its crest (crista tuberculi minoris).
Topography and Neighboring Structures
The subscapularis muscle serves as a key anatomical landmark and contributes to the formation of several anatomical spaces:
- Posterior wall of the axilla — formed by the subscapularis muscle (along with its fascia), latissimus dorsi, teres major, and the long head of the triceps brachii.
- Subscapular fascial space — the muscle forms the posterior wall of this space (with the serratus anterior forming the anterior wall).
- Openings of the posterior axillary wall — the inferior margin of the m. subscapularis (along with the teres minor) serves as the superior border for two important vascular and neural passages:
- Quadrangular space (foramen quadrilaterum) — located more laterally, transmitting the axillary nerve (n. axillaris) and posterior humeral circumflex artery.
- Triangular space (foramen trilaterum) — located more medially, transmitting the circumflex scapular artery (a. circumflexa scapulae).
Blood Supply and Innervation
- Blood supply: Provided by a major branch, the subscapular artery (a. subscapularis). It arises near the inferior border of the muscle and gives off 2–3 subscapular branches (rr. subscapulares) directly to the muscle tissue before dividing into its terminal branches.
- Innervation: Supplied by the subscapular nerves (nn. subscapulares, segments C₅–C₆) and the thoracodorsal nerve (n. thoracodorsalis, segments C₆–C₈ from the posterior cord of the brachial plexus).
Function
As part of the musculoskeletal system and the rotator cuff group, the subscapularis muscle performs the following actions:
- Internally rotates the arm (medial rotation).
- Adducts the arm toward the trunk.
- Assists in lowering the raised arm and extending the shoulder.
- Reinforces the joint capsule — proximal to its insertion, the tendon blends closely with the anterior capsule of the glenohumeral joint.
Clinical Significance: Subscapular Bursa
A large subscapular bursa (bursa subtendinea m. subscapularis or bursa synovialis subscapularis) lies between the tendon of the subscapularis muscle and the neck of the scapula/shoulder joint capsule.
It is clinically significant because it frequently communicates with the joint cavity. In cases of purulent arthritis or periarthritis, this bursa is often involved in the inflammatory process. If it ruptures, pus can spread directly into the fascial compartment of the subscapularis muscle.