The glenohumeral joint, also known as the shoulder joint, is a ball and socket type of synovial joint: the ball being the head of the humerus, and the socket being the glenoid cavity of the scapula.

The shoulder joint is the most mobile joint in our body, and it allows us to do everything from pull ups, to throwing a baseball, and hugging our loved ones!Let’s take a closer look at our articulating parts here.

So, the head of the humerus fits into this shallow glenoid cavity. The glenoid cavity is actually so shallow that it only covers one-third of the humeral head.

Having a shallow glenoid cavity allows for exceptional mobility as the humeral head can rotate freely to allow us an impressive range of motion.

At the same time, the contour of the glenoid cavity is lined by a fibrocartilaginous rim, called the glenoid labrum, which makes the cavity a little bit deeper, reducing the chance for dislocations.Like any respectable synovial joint, the glenohumeral joint is covered by a tough, but loose, joint capsule with an internal synovial lining.

The joint capsule extends from the anatomical neck of the humerus, to the margin of the glenoid. This capsule has two apertures, or passageways.

The first one is between the tubercles of the humerus, which provides passage for the tendon of the long head of the biceps brachii.

This tendon then passes in the intertubercular groove covered by a broad fibrous band, called the transverse humeral ligament, which runs from the lesser tubercle of the humerus, to the greater tubercle.

The second aperture is situated inferior to the coracoid process, and it allows communication between the subtendinous bursa of the subscapularis and the joint’s synovial cavity.

Speaking of which, there are actually two bursae around the glenohumeral joint, which are basically pockets of synovial fluid.

There is one between the capsule and the tendon of the subscapularis muscle called the subtendinous bursa of the subscapularis and another one right under the acromion called the subacromial bursa.

The role of bursae is to provide lubrication and reduce the friction when structures such as tendons, skin, muscles and bones rub against each other, especially the supraspinatus tendon, which rubs against the acromion as the joint moves.Now, the joint capsule is reinforced by numerous ligaments.

There are the glenohumeral ligaments, which are thickenings found within the joint capsule; the coracohumeral ligament, which runs from the scapula to the humerus; and the coraco-acromial ligament, which runs from the coracoid process to the acromial process of the scapula.

The glenohumeral ligaments are three bands, called the superior, middle, and inferior glenohumeral ligament, that radiate inferolaterally from the glenoid labrum to blend with the joint capsule, where it attaches to the anatomical neck of the humerus.

These ligaments reinforce the anterior part of the joint capsule. Next is the coracohumeral ligament, which runs from the coracoid process of the scapula, and then extends inferolaterally to attach to the greater tubercle of the humerus.

The coracohumeral ligament strengthens the upper part of the capsule. Finally, the coracoacromial ligament runs above the head of the humerus from the acromion and coracoid process on the scapula.

This creates an arch that overlies the head of the humerus to prevent it from slipping from the glenoid cavity superiorly.

Another major contributor to the stability of the shoulder joint are the rotator cuff muscles. This is a set of four muscles that form a musculotendinous cuff around the shoulder.

The rotator cuff muscles hold the humeral head in the glenoid cavity, and it also participates in arm movements, especially rotation - hence their name.

You can remember the names of these muscles with the SItS mnemonic. This stands for Supraspinatus, Infraspinatus, teres minor - hence the lower case “t” - and Subscapularis.

Notice that the inferior part of the joint capsule is not reinforced by rotator cuff muscles, so this is the weakest area of the joint, which is where most dislocations occur.

Now, the shoulder joint allows flexion and extension, which is moving the arm forward and backward in the sagittal plane; abduction and adduction, which is done in the coronal plane; medial rotation and lateral rotation; and finally, circumduction, which occurs when the arm is held straight at the elbow joint while the whole limb moves around the shoulder as if its drawing circles.The blood supply of the shoulder joint comes from the anterior and posterior circumflex humeral arteries, which are branches of the axillary artery, as well as from branches of the suprascapular artery, which is a branch of the thyrocervical trunk.Finally, regarding innervation: Remember Hilton’s law, which says the nerves that innervate the muscles crossing over a particular joint also innervate that joint.

So, for example, the deltoid muscle, triceps, and some rotator cuff muscles all cross the glenohumeral joint and act on it.

These are all innervated by the axillary nerve; therefore, the axillary nerve provides innervation to the glenohumeral joint via articular branches.

The rest of the innervation is accounted for by articular branches of the suprascapular nerve and the lateral pectoral nerve.

Before we wrap up, let’s take a quick break to identify the ligaments surrounding the glenohumeral joint. Alright, as a quick recap.

The glenohumeral joint, or the shoulder joint, is a ball and socket type of synovial joint between the head of the humerus and the glenoid cavity of the scapula.

The glenoid cavity only covers one-third of the humeral head, which allows for exceptional mobility. At the same time, the contour of the glenoid cavity is lined by a fibrocartilaginous rim, called the glenoid labrum, which makes the cavity a little bit deeper, reducing the chance for dislocations.

The bursae around the shoulder joint are the subacromial and the subtendinous bursa of the subscapularis, and they provide lubrication, reducing friction during arm movements.

The shoulder joint is surrounded by a joint capsule, which is reinforced by ligaments. There are the glenohumeral ligaments, which are thickenings found within the joint capsule; the coracohumeral ligament, which runs from the scapula to the humerus; and the coraco-acromial ligament, which runs from the coracoid process to the acromial process of the scapula.

The rotator cuff muscles also provide stability to the shoulder joint, and they can be remembered with the SItS mnemonic: Supraspinatus, Infraspinatus, teres minor, and Subscapularis.

The shoulder joint allows for flexion, extension, abduction, adduction, medial rotation and lateral rotation, and circumduction.