Articular surfaces and congruence
The joint is formed by the spherical humeral head (caput humeri) and the shallow glenoid cavity of the scapula (cavitas glenoidalis). There is a pronounced difference in shape and area between them (incongruence).
To smooth out this mismatch, the cartilaginous glenoid labrum (labrum glenoidale) is located along the rim of the cavity. It performs two important functions:
- Increases the volume of the glenoid cavity without restricting mobility.
- Cushions shocks and impacts during movements of the humeral head.
Despite the presence of the glenoid labrum, incongruence persists, which is why shoulder dislocations occur more frequently than in any other joint.
Articular capsule
The joint capsule (capsula articularis) is loose and relatively thin. The fibrous layer is distributed unevenly: there are thickened areas (due to interwoven ligaments) and prominent "weak spots".
Capsule attachment sites:
- On the scapula: attaches to the bony margin of the glenoid cavity.
- On the humerus: encompasses the head and attaches superiorly to the anatomical neck (collum anatomicum). The greater and lesser tubercles remain outside the joint cavity.
- Inferiorly and medially, the capsule extends significantly lower, attaching at the level of the surgical neck. Due to this low attachment, an axillary recess (recessus axillaris) is formed in the inferior part.
Ligamentous apparatus and coracoacromial arch
The intrinsic ligaments of the shoulder joint are developed unevenly:
- Coracohumeral ligament (lig. coracohumerale): the strongest and most pronounced. Originates from the lateral margin of the coracoid process and attaches to both tubercles of the humerus.
- Glenohumeral ligaments (ligg. glenohumeralia, Flood's ligaments): superior, middle, and inferior. Weakly developed.
The "weak spots" of the capsule are located between the ligaments. The most vulnerable anterior weak spot is localized between the middle and inferior glenohumeral ligaments.
The coracoacromial arch—a structure located above the joint—has special biomechanical significance. It is formed by the acromion, the coracoid process, and the dense coracoacromial ligament (lig. coracoacromiale). The arch physically restricts upward abduction of the shoulder to the horizontal level. Raising the arm higher is only possible through the rotation of the scapula.
Muscular reinforcement of the capsule
Reinforcement of the joint is primarily provided by the tendons of the surrounding muscles. This occurs either indirectly (holding the articular ends) or directly—by the tendons interweaving into the capsule.
The topography of rotator muscle attachments forms a "protective framework":
- Superiorly and laterally: supraspinatus muscle (m. supraspinatus), attaches to the upper part of the greater tubercle.
- Posteriorly: infraspinatus muscle (m. infraspinatus) and teres minor muscle (m. teres minor), attach to the greater tubercle below the supraspinatus.
- Anteriorly: subscapularis muscle (m. subscapularis), attaches with a broad tendon to the lesser tubercle.
Clinically, it is important that the joint is densely reinforced superiorly and posteriorly, while the muscular framework is absent inferiorly and medially. This is why the humeral head most commonly dislocates anteriorly and medially.
Synovial bursae and pathways of pus spread
A well-developed gliding apparatus of synovial bursae is located around the joint. In suppurative arthritis, they become pathways for the spread (extension) of infection into neighboring fascial spaces.
- Subscapular bursa (bursa subtendinea m. subscapularis): constantly communicates with the joint cavity. When ruptured, pus spreads into the osteofascial compartment of the subscapularis muscle.
- Subcoracoid bursa (bursa m. coracobrachialis): often merges with the subscapular bursa. Its rupture leads to fluid collections in the axillary fossa and the subtrapezial space.
- Intertubercular synovial sheath: a synovial outpouching tightly enclosing the tendon of the long head of the biceps. It ruptures rarely, but can cause pus to spread into the anterior fascial space of the arm, followed by propagation along the neurovascular bundles.
- In the subdeltoid space (above the joint), the subdeltoid and subacromial synovial bursae are also located.
Briefly: answers to topical questions
What types of movements occur in the shoulder joint?
Movements occur around multiple axes in the shoulder joint since it is a multiaxial ball-and-socket joint. The following movements are explicitly mentioned in sources:
- Abduction — limited upward to the horizontal level; elevation of the limb above the horizontal level is accomplished by the movement of the scapula.
- External rotation — mentioned as a movement whose strength may decrease in suprascapular neuropathy.
Which arteries form the arterial network of the shoulder joint?
The arterial network in the shoulder joint region is formed on the deep surface of the deltoid muscle. The anastomoses involve:
- Anterior circumflex humeral artery (a. circumflexa humeri anterior).
- Posterior circumflex humeral artery (a. circumflexa humeri posterior).
- Deltoid branch of the thoracoacromial artery (a. thoracoacromialis).
The anterior and posterior circumflex humeral arteries supply the shoulder joint and the deltoid muscle; these anastomoses provide collateral circulation.
Which nerves innervate the capsule of the shoulder joint?
The shoulder joint capsule is innervated by branches of several nerves of the brachial plexus (plexus brachialis). Innervation involves:
- Axillary nerve (n. axillaris) — the main source, innervating the lateral and inferior parts.
- Suprascapular nerve (n. suprascapularis) — innervates the posterosuperior parts.
- Lateral pectoral nerve (n. pectoralis lateralis) — innervates the anterior part.
- Subscapular nerve (n. subscapularis) — innervates the anterior and inferior parts.
To easily remember the localization of dislocations, keep the "weak zone" rule in mind: the joint is covered anteriorly, posteriorly, and superiorly by the tendons of the rotator muscles, while inferiorly and medially there is no "armor"—that is where the head pops out.
Class and exam questions
Why do humeral dislocations happen so often?
This is due to three factors: pronounced surface incongruence (the head is much larger than the cavity), a loose and thin capsule, and the lack of muscular reinforcement of the capsule inferiorly and medially.
Which structure limits the abduction of the arm above the horizontal level?
Coracoacromial arch. It is formed by the coracoid and acromial processes of the scapula, as well as the coracoacromial ligament stretched between them.
What is located in the intertubercular sulcus of the humerus?
It houses the tendon of the long head of the biceps brachii, surrounded by the intertubercular synovial sheath (an outpouching of the joint cavity).
Where does pus most commonly rupture in arthritis of the shoulder joint?
Via the subscapular synovial bursa, which has a direct connection to the joint. Pus enters the osteofascial compartment of the subscapularis muscle.
What else to study on this topic
- Palpation of external bony landmarks: the acromion and coracoid process
- Syntopy of the joint with the axillary neurovascular bundle
- Topography of the subdeltoid cellular space
- Biomechanics of scapular movements during arm abduction
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