SHOULDER OR ARM PAIN? ⠀ TRIANGULAR INTERVAL SYNDROME (TIS) ⠀ The Triangular Interval (also known as the lateral triangular space, lower triangular space and triceps hiatus) is a space connecting the axilla to the posterior compartment of the arm. It is one of the three intermuscular spaces found in the axillary space. The other two spaces are Quadrangular Space and Triangular Space. ⠀ Triangular Interval (4) Borders ▪️superior: teres major ▪️lateral: lateral head of the triceps or the humerus ▪️medial: long head of the triceps Contents ▪️profunda brachii artery ▪️radial nerve ⠀ Quadrangular Space (5) Borders ▪️medial: long head of triceps ▪️lateral: humeral shaft ▪️superior: teres minor ▪️inferior: teres major Contents ▪️axillary nerve ▪️posterior humeral circumflex artery ⠀ Triangular Space (3) Borders ▪️inferior: teres major ▪️lateral: long head of triceps ▪️superior: lower border of teres minor Contents ▪️scapular circumflex artery ⠀ Triangular Interval Syndrome (TIS) was described as a differential diagnosis for radicular pain in the upper extremity. It is a condition where the radial nerve is entrapped in the triangular interval resulting in upper extremity radicular pain. ⠀ The radial nerve and profunda brachii pass through the triangular interval and are hence vulnerable. The triangular interval has a potential for compromise secondary alterations in thickness of the teres major and triceps. It is described based on cadaveric studies that fibrous bands were commonly present between the teres major and triceps. When these bands were present, rotation of the shoulder caused a reduction in cross sectional area of the space. ⠀ Hypertrophy of the teres major can occur secondary to weight training and potentially compromise the triangular interval with resultant entrapment of the radial nerve. ⠀ Shoulder dysfunctions have a potential for shortening and hypertrophy of the teres major. ⠀ The triceps brachii has a potential to entrap the radial nerve in the triangular interval secondary to hypertrophy. The presence of a fibrous arch in the long head and lateral head further complicates the situation.