teres major

Teres major is often called 'lat's little helper' because it works synergistically with latissimus dorsi to internally rotate, adduct, and extend the arm, and both muscles insert near each other on the humerus (latissimus dorsi inserts on the floor of the intertubercular groove, teres major on the medial lip). Unlike teres minor (a rotator cuff muscle that externally rotates the arm and is innervated by the axillary nerve), teres major is NOT part of the rotator cuff and has a distinct innervation and action.

Anatomically, teres major forms the lower border of the triangular and quadrangular spaces in the axilla: - Triangular space: bounded by teres major (inferior), teres minor (superior), and long head of triceps (lateral) — transmits the circumflex scapular artery. - Quadrangular space: bounded by teres major (inferior), teres minor (superior), long head of triceps (medial), and surgical neck of humerus (lateral) — transmits the axillary nerve and posterior circumflex humeral artery/vein. Quadrangular space syndrome can cause axillary nerve compression, leading to deltoid weakness and sensory loss over the lateral shoulder.

Clinically, teres major is relevant in shoulder dysfunction and mimics rotator cuff pathology on exam due to overlapping actions with latissimus dorsi. It is tested during shoulder internal rotation/adduction maneuvers (e.g., 'lift-off test' variations). It is also a landmark for identifying the boundaries of the axillary spaces on anatomy and imaging exams.

High-yield board points: - Innervation: lower subscapular nerve (C5-C6) — differentiates it from teres minor (axillary nerve). - Not part of rotator cuff (SITS muscles: supraspinatus, infraspinatus, teres minor, subscapularis). - Key landmark for triangular and quadrangular spaces. - Function: adduction, extension, internal rotation of the arm.