Teres minor muscle

Origin and insertion

Teres minor originates from the scapula, via two heads that are separated by a groove for the circumflex scapular artery. The attachment site spans the upper two-thirds of the lateral border of scapula. The fibers quickly fuse into a single muscle belly which courses superolaterally towards the proximal humerus. 

Teres minor inserts onto the greater tubercle of humerus. Specifically, the superior fibers of the muscle give off a short tendon that inserts to the inferior facet of the greater tubercle of humerus. While the inferior fibers insert adjacent to this, directly onto the proximal shaft of humerus, just distal to the tendon of the superior part and proximal to the origin of the lateral head of triceps brachii. 

Relations

Teres minor sits superficial to the long head of triceps brachii, being squeezed between the trapezius and deltoid muscles. Trapezius partially covers it’s medial surface, while deltoid covers it’s superolateral aspect. Inferior to the muscle is teres major, while infraspinatus is located superiorly. Like the tendons of supraspinatus, infraspinatus, subscapularis and long head of triceps brachii, the tendon of teres minor blends with the glenohumeral joint capsule as it crosses the shoulder joint, in this case with it’s inferoposterior surface. This relation provides reinforcement and mechanical support to the joint. 

The teres minor muscle comprises the borders of the two intermuscular spaces of the axilla;

• Quadrangular space is found below the shoulder joint. Teres minor comprises its superior border, while teres major bounds it inferiorly, long head of triceps medially, and humeral shaft laterally. The function of this space is to provide passageway for the axillary nerve and posterior circumflex humeral artery, from the anterior to posterior shoulder regions.
• Triangular space is located medially to the quadrangular space. Teres minor bounds it superiorly, while teres major and long head of triceps form it’s inferior and lateral borders, respectively. This space is traversed by the circumflex scapular artery.

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Innervation

Teres minor shares its innervation with the deltoid muscle; this is supplied by the axillary nerve (C5, C6), which stems from the posterior cord of brachial plexus.

Blood supply

Blood supply to teres minor comes from the circumflex scapular artery and posterior circumflex humeral artery. The former is a branch of the subscapular artery, while the latter stems from the axillary artery.

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Function

Rotator cuff muscles create a functional musculotendinous unit that both stabilize and produce movements on the shoulder joint. The main purpose of this entire muscular apparatus is to provide a wide range of arm motion while keeping the head of humerus centralized within the glenoid cavity.

All four muscles are firmly attached around the joint in such a way that they form a sleeve. Individually, each muscle has its own pulling axis that results in a certain movement (prime mover), while altogether they create a concavity compression. This is a stabilizing mechanism in which compression of the humerus into the concavity of glenoid fossa prevents its dislocation by translating forces. Teres minor provides support from the posterior aspect of the joint, preventing the humerus from dislocating posteriorly.

Concerning movements, when the arm is in the anatomical position, teres minor externally (laterally) rotates the arm. When the arm is elevated, teres minor can have multiple functions;

• Arm adduction: When working in synergy with teres major, latissimus dorsi and pectoralis major, teres minor adducts the arm from the abducted position (lowers it). The function of this action is to facilitate over-head upper limb movements such as pulling and throwing.
• Arm external rotation: Working together with infraspinatus, teres minor pulls the greater tubercle posteriorly, resulting in external (lateral) rotation of the arm. This action supports the shoulder and at the same time loads the force in the preload phase of overhead movements (e.g. throwing). After the overhead activity has been performed, teres minor contracts eccentrically to smoothen the free fall of the upper extremity.

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