**Question:** Posteroinferior instability of glenohumeral joint is tested by

A. Applying pressure to the axillary nerve
B. Applying pressure to the subscapularis tendon
C. Applying pressure to the rotator cuff muscles
D. Applying pressure to the axillary artery

**Correct Answer:** B. Applying pressure to the subscapularis tendon

**Core Concept:** Glenohumeral joint is a ball-and-socket joint that allows a wide range of motion in the shoulder. Posteroinferior instability refers to the instability of the glenohumeral joint in the inferior and posterior direction. The subscapularis tendon plays a significant role in stabilizing the glenohumeral joint, particularly in maintaining the posterior stability.

**Why the Correct Answer is Right:** In the posteroinferior direction, the subscapularis tendon works in conjunction with other rotator cuff muscles (supraspinatus, infraspinatus, and teres minor) to provide rotational stability to the glenohumeral joint. The subscapularis tendon originates from the lesser tubercle of the humerus and inserts onto the lesser tubercle of the scapula. It acts as a dynamic stabilizer, pulling the humerus downward and posteriorly during arm abduction and external rotation. This action helps to prevent the humeral head from descending too far into the glenoid cavity, thus minimizing the risk of posteroinferior instability.

**Why Each Wrong Option is Incorrect:**

A. Applying pressure to the axillary nerve (option A) is not relevant to testing posteroinferior instability of the glenohumeral joint as the axillary nerve innervates the deltoid and teres major muscles, which are not directly involved in posteroinferior stability.

C. Applying pressure to the rotator cuff muscles (option C) is incorrect because the rotator cuff muscles primarily contribute to the stability of the joint during elevation and external rotation. They do not directly test posteroinferior instability.

D. Applying pressure to the axillary artery (option D) is irrelevant to assessing posteroinferior instability as the axillary artery is an anatomical landmark and not directly involved in stabilizing the glenohumeral joint.

**Clinical Pearl:**

In clinical practice, the subscapularis tendon is frequently tested during physical examination, along with the rotator cuff muscles, to evaluate the integrity of the rotator cuff and its contribution to stability. A positive test result indicates a possible tear or weakness in the subscapularis tendon, which may contribute to posteroinferior instability of the glenohumeral joint. A comprehensive shoulder examination should always include tests for subscapularis function, as a tear or weakness can lead to shoulder pain, instability, and limited range of motion.