In this post, we will delve into the spaces of the shoulder, their boundaries, and the structures that pass through them. Within this complex network of muscles and bones, various intermuscular spaces play vital roles. Moreover, they facilitate the passage of nerves and vessels. Among these spaces are the quadrangular space, upper triangular space, and lower triangular space. There are specific boundaries that delineate them. Additionally, there are distinct structures that traverse through theses spaces. Let’s explore each space in detail:

Quadrangular Space

The quadrangular space, located under the shoulder joint, is marked by several specific points. These include the subscapularis muscle in the front and the lower edge of the teres minor in the back. Furthermore, the upper border of the teres major defines its bottom edge. Medially, the long head of the triceps brachii forms its border. Finally, on the lateral side, the surgical neck of the humerus forms the border. Key structures traversing this space include the axillary nerve and the posterior circumflex humeral vessels.

Axillary nerve injury,
often resulting from shoulder dislocations or trauma, can lead to significant functional impairment. As a crucial contributor to shoulder joint innervation and Deltoid muscle function, damage to the Axillary nerve can manifest as weakness in glenohumeral abduction or loss of sensation in the regimental badge area. Patients may experience difficulty with shoulder abduction (Deltoid) and external rotation (Teres minor).

(picture showing regimental badge area)

Upper Triangular Space

Adjacent to the quadrangular space lies the upper triangular space, bounded superiorly by the inferior border of the teres minor, laterally by the medial border of the long head of the triceps brachii, and inferiorly by the superior border of the teres major. Within this space, the circumflex scapular vessels course their path.

Lower Triangular Space

Completing the trio of intermuscular spaces is the lower triangular space, defined by the lateral border of the long head of the triceps brachii medially, the medial border of the humerus laterally, and the lower border of the teres major superiorly. Notable structures passing through this space include the radial nerve and the profunda brachii vessels. The topics of scapular anastamosis, brachial plexus closely realated to this topic. you can read more on that for better understanding of this topic

Summary