Shoulder (Glenohumeral) Joint
The glenohumeral joint is one of the joints associated with the shoulder girdle that allow a full range of movement of the upper limb. For completeness, the other joints are the sternoclavicular, acromioclavicular, and scapulothoracic joints.
Compared to the hip joint, the glenohumeral joint is built for mobility and is therefore inherently less stable than the hip joint. This compromise between mobility and stability does mean that the shoulder joint is prone to injury, since much of what stability the shoulder joint does have relies on the ligaments and muscles that surround the joint.
|Articulation surfaces||Glenoid fossa (scapula), head of the humerus|
|Type||Synovial joint (not directly joined; the bones share synovial cavity that is closed with articular capsule which connects the bones)|
|Ligaments||Coracohumeral, superior glenohumeral, middle glenohumeral, inferior glenohumeral, transverse humeral ligaments|
|Rotator cuff muscles||Supraspinatus, Infraspinatus, Teres minor, Subscapularis (SITS)|
Flexion: pectoralis major, deltoid (prime); coracobrachialis, biceps brachii (accessory)
Extension: deltoid (prime); teres major, latissimus dorsi, long head of biceps (accessory)
Abduction: deltoid (prime); supraspinatus (accessory)
Adduction: pectoralis major, latissimus dorsi (prime); teres major, long head of triceps brachii (accessory)
Medial rotation: subscapularis (prime); pectoralis major, deltoid, teres major (accessory)
Lateral rotation: infraspinatus (prime); teres major, deltoid (accessory)
|Clinical importance||Degenerative tendonitis of the rotator cuff muscles, calcific scapulohumeral bursitis, subacromial bursitis, adhesive capsulitis|
To understand this tendency for injury, we will first look at the bony anatomy that creates the mobility at the shoulder, and then we will discuss how the glenohumeral joint is reinforced by both the ligaments and muscles that cross the joint, with particular focus on the rotator cuff muscles . Lastly, we will apply what we learn about the joint to common injuries involving the glenohumeral joint.
- Bony Anatomy
- Joint Capsule, Bursae, and Ligaments
- The Rotator Cuff Muscles
- Other Muscles and Their Movements
- Glenohumeral Joint Injuries
- Related diagrams and images
The glenohumeral joint is simply the articulation between the laterally projecting glenoid fossa (depression), or glenoid cavity, of the scapula and the head of the humerus. When compared to its counterpart in the hip, the acetabulum, the glenoid fossa is quite shallow and accepts only about 1/3 of the head of the humerus.
Even though the glenoid fossa is deepened and enlarged slightly by the glenoid labrum, a ring of fibrocartilage attaching to the outside margin of the fossa, the shallowness of the glenoid fossa creates a joint that is mobile, yet less stable than the hip joint.
Joint Capsule, Bursae, and Ligaments
As a synovial joint, the articulation between the glenoid fossa and the humeral head has a number of features characteristic of these types of joints. One is a capsule that surrounds the joint cavity and encloses synovial fluid that lubricates the inside of the joint and the articular surfaces. Medially, it attaches to the outer margin of the glenoid fossa (outside of the labrum) and laterally to the anatomical neck of the humerus which lies just distal to the head of the humerus. Inferiorly, the capsule is looser, and tends to lie in folds when the arm is fully adducted. These folds allow for abduction of the arm, during which they become taught.
The joint capsule is protected, specifically from wear, by bursae which are fluid filled sacs that provide cushioning between structures. Associated with the glenohumeral joint are a number of bursae that protect the joint capsule from excessive wear from overlying structures. The most common of these bursae is the subacromial. This bursa lies between the acromion process of the scapula and an underlying rotator cuff muscle, the supraspinatus. Since the supraspinatus tendon blends into the superior aspect of the joint capsule, this bursa protects the joint capsule and the muscle tendon from wear from the acromion process during abduction of the arm.
The joint capsule acquires some strength from thickenings that occur superiorly, anteriorly, and laterally. These capsule thickenings form five ligaments. From superior to inferior, they are the:
- Coracohumeral Ligament – runs from the coracoid process of the scapula to the greater tubercle of the humerus
- Superior, Middle, and Inferior Glenohumeral Ligaments – pass from the margin of the glenoid fossa to the lesser tubercle and anatomical neck of the humerus
- Transverse Humeral Ligament – lies between the greater and lesser tubercles of the humerus, holding the tendon of the long head of biceps brachii in the intertubercular groove
In spite of these ligamentous thickenings of the capsule, the structural integrity of the joint (the ability of a structure to hold together under a load) is derived from the strength of the rotator cuff muscles that cross the joint superiorly, posteriorly, and anteriorly.
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The Rotator Cuff Muscles
These muscles are essential to the healthy functioning of the glenohumeral joint. They are a set of four muscles that run from the scapula to the humerus. As their tendons approach the humerus, they blend with the capsule, reinforcing it further. This group of muscles may also be referred to as the SITS muscles: the supraspinatus, infraspinatus, teres minor, and subscapularis.
Three of these muscles, the supraspinatus, infraspinatus, and teres minor, all originate from the posterior aspect of the scapula and insert onto the greater tubercle of the humerus. The fourth arises from the anterior surface of the scapula and inserts onto the lesser tubercle of the humerus.
- Supraspinatus – origin: supraspinous fossa above the spine of the scapula, insertion: superior facet on the greater tubercle, action: initial 15° of arm abduction.
- Infraspinatus – origin: infraspinous fossa below the spine of the scapula, insertion: middle facet on the greater tubercle, action: external rotation of the arm.
- Teres minor – origin: lateral aspect of the scapula, insertion: inferior facet on the greater tubercle, action: external rotation of the arm.
- Subscapularis – origin: subscapular fossa on the anterior surface of the scapula, insertion: lesser tubercle of the humerus, action: internal rotation of the arm.
While these muscles are involved in movement of the arm, they provide two other important functions at the shoulder.
- Humeral head stability: The relative weakness of the capsule itself, coupled with the bony structure of the glenohumeral joint, means that the rotator cuff muscles are largely responsible for keeping the head of the humerus positioned in the glenoid fossa. During any action or movement at the glenohumeral joint, it is the functioning of the rotator cuff muscles that properly aligns the head of the humerus in the glenoid fossa, allowing for a full range of movement.
- Reinforcement of the joint capsule: As mentioned previously, the tendons of the rotator cuff muscles, as they near their insertions onto the humerus, blend into the joint capsule providing strength to the superior, anterior, and posterior aspects of the capsule. It is no coincidence that dislocations of the glenohumeral joint typically occur in an antero-inferior direction where the capsule is weakest and not reinforced by the rotator cuff muscles.
Other Muscles and Their Movements
A summary of the muscles that cross the glenohumeral joint, and the movements that they create, are listed below for completeness. However, there is one other muscle that provides some integral support to the glenohumeral joint and is therefore worth specific mention; the biceps brachii muscle. The biceps brachii has two heads, the short head originates from the coracoid process of the scapula, and the long head originates from a tubercle on the superior aspect of the glenoid fossa, the supraglenoid tubercle.
The tendon of the long head passes over the head of the humerus to run in the intertubercular groove on the anterior aspect of the upper arm. This tendon actually runs within the joint capsule and will restrict superior movement of the humeral head in the glenoid fossa, thus aiding the rotator cuff muscles in the positioning of the humeral head. The biceps brachii continues down the humerus and inserts below the elbow joint onto the radial tubercle. Because it crosses the shoulder joint, the biceps brachii is a weak flexor of the arm, but is a more powerful flexor and supinator of the forearm.
Many other muscles cause movement of the glenohumeral joint. Below, the movements are listed along with the prime movers, and the accessory muscles involved in the movement.
- Deltoid (Prime mover)
- Teres major, latissimus dorsi , long head of biceps brachii (Accessory muscles)
- Deltoid (Prime mover)
- Supraspinatus (Accessory muscles)
- Pectoralis major, latissimius dorsi (Prime movers)
- Teres major, long head of triceps brachii (Accessory muscles)
- Subscapularis (Prime mover)
- Pectoralis major, deltoid, latissimus dorsi, teres major (Accessory muscles)
- Infraspinatus (Prime mover)
- Teres major, deltoid (Accessory muscles)
Glenohumeral Joint Injuries
As mentioned, the glenohumeral joint has great mobility; in fact, it has the most freedom of movement of all the joints in the body! This does mean however , that the bony surfaces of the joint itself and those surrounding the joint play a key role in injuries of the shoulder. For example, when the arm is abducted, the greater tubercle of the humerus will eventually restrict abduction by coming into contact with the acromion process. This restriction of movement is normally overcome by the upward rotation of the scapula that occurs in conjunction with glenohumeral abduction.
Abduction of the shoulder can also be increased by externally rotating the arm so that the greater tubercle is now moved out of the way of the acromion process. However, in many cases, externally rotating the arm is not an option (as in many sports activities), and muscle restriction can cause a decrease in the rotation of the scapula. Both of these mean that the limited space between the greater tubercle and the acromion process, that is filled with the subacromial bursa and the supraspinatus tendon, is vulnerable to repeated compression when the arm is abducted. This structural limitation is implicated in many common shoulder injuries, most often affecting one or more of the rotator cuff muscles.
Degenerative Tendonitis of the Rotator Cuff
Repetitive use of the upper limb in an abducted position, as occurs in many sports, is a common source of rotator cuff injury. The limited space between the greater tubercle and the acromion results in repeated compression of the rotator cuff, specifically the supraspinatus tendon, which can become inflamed and swollen. Repeated trauma of this sort, and the relatively poor blood supply to the supraspinatus tendon, can cause degenerative changes in the tendon that lead to calcium deposits and extreme pain. In these conditions, the tendon is much more susceptible to tearing, leading to pain and disability when attempts to abduct the arm are made.
Calcific Scapulohumeral Bursitis and Subacromial Bursitis
Inflammation and calcification of the subacromial bursa can also be a cause of pain and limited range of movement of the glenohumeral joint. Deposits of calcium in the supraspinatus tendon can cause increased impingement of the subacromial bursa during abduction causing damage, inflammation (known as subacromial bursitis), and eventually, calcification of the subacromial bursa.
Adhesive Capsulitis of the Glenohumeral Joint
Repeated inflammation of the joint capsule, rotator cuff, and subacromial bursa (as in the above conditions) can lead to thickening, scarring, and adhesion of the joint capsule, a condition commonly known as ‘frozen shoulder’. Inferiorly, the capsule is loose, and in full adduction lies in folds. Scarring and adhesion in the capsule in this region can restrict the range of abduction by not allowing the capsule to unfold. People with this condition have difficulty with abduction of the arm but may have up to a 45° range of movement by elevation and rotation of the scapula.