Articular surfaces and capsule
The articulation is formed by two main bony structures. On one side is the large, sphere-shaped head of the humerus. On the other side, it articulates with the glenoid cavity of the scapula. Since the surface area of the glenoid cavity is significantly smaller than the size of the humeral head, a special cartilaginous structure—the glenoid labrum (labrum glenoidale)—runs along its edge. It increases the congruence (matching) of the surfaces and makes the socket deeper.
The joint capsule is described as very thin and loose. It originates along the margin of the glenoid labrum and attaches to the humerus, enclosing its anatomical neck. The capsule has anatomically predetermined weak spots where it is least durable—these are the areas beneath the coracoid process and in the axilla.
Ligamentous apparatus and "shoulder arch"
Despite its high mobility, the joint is reinforced by several important ligaments that restrict excessive displacement and stabilize the capsule:
- Coracohumeral ligament (lig. coracohumerale) — a strong band extending from the coracoid process of the scapula to the humerus.
- Glenohumeral ligaments (ligg. glenohumeralia) — a complex consisting of three bundles (superior, middle, and inferior) blending into the capsule.
- Coracoacromial ligament (lig. coracoacromiale) — does not directly connect the bones of the joint, but plays a crucial biomechanical role. It stretches over the joint, forming the so-called "shoulder arch," which protects the articulation from above.
Biomechanics and auxiliary structures
By its shape and biomechanics, this is a multiaxial (ball-and-socket) joint. This configuration allows movements in all possible planes. However, the flip side of this freedom is marked joint instability.
To ensure smooth gliding and reduce tissue friction in the joint region, synovial bursae (bursae) are present. Among these, the most significant are the subdeltoid and subscapular bursae.
In real life, movements rarely occur in isolation within this articulation alone. It exhibits a high degree of combined function and works in close coordination with the scapula and clavicle, ensuring the complex function of the entire upper limb.
Briefly: answers to topical questions
Which muscles form the rotator cuff of the shoulder?
The rotator cuff of the shoulder is formed by four muscles whose tendons blend into the capsule of the shoulder joint.
- Subscapularis muscle (m. subscapularis).
- Teres minor muscle (m. teres minor).
- Infraspinatus muscle (m. infraspinatus).
- Supraspinatus muscle (m. supraspinatus).
Their primary function is the dynamic stabilization of the shoulder joint and rotation of the humeral head.
What anatomical structures form the "shoulder arch"?
The "shoulder arch" is formed by three anatomical structures:
- Acromion of the scapula.
- Coracoid process of the scapula.
- Coracoacromial ligament (lig. coracoacromiale).
The arch is located above the shoulder joint and limits upward abduction of the arm in the joint to the horizontal level.
Which tendon passes directly inside the cavity of the shoulder joint?
The tendon of the long head of the biceps brachii (m. biceps brachii, caput longum) passes directly inside the cavity of the shoulder joint. It originates from the supraglenoid tubercle and the superior glenoid labrum, passes through the entire joint cavity within the rotator interval, and lies in the intertubercular sulcus on the anterior surface of the humeral head. The tendon is tightly enclosed by the intertubercular synovial sheath.
What specific types of movements are possible in the shoulder joint?
The shoulder joint is a multiaxial (ball-and-socket) joint, allowing movements around all three axes.
- Flexion (flexio) — up to 180° and extension (extensio) — up to 45° (around the frontal axis).
- Abduction (abductio) — up to 180° and adduction (adductio) (around the sagittal axis).
- Internal rotation (rotatio) — up to 90° and external rotation — up to 90° (around the vertical axis).
The wide range of motion is due to the shape of the articular surfaces and the loose capsule.
Which synovial bursae are located in the region of the shoulder joint?
Several synovial bursae are located in the region of the shoulder joint to facilitate the gliding of tendons and muscles.
- Subscapular bursa (bursa subtendinea m. subscapularis) — located beneath the tendon of the subscapularis muscle, often communicating with the joint cavity.
- Subacromial bursa (bursa subacromialis) — situated beneath the acromion.
- Subdeltoid bursa (bursa subdeltoidea) — lies above the greater tubercle of the humerus beneath the deltoid muscle.
- Subcoracoid bursa (bursa m. coracobrachialis) — located more superficially than the subscapular bursa.
From which arteries is the blood supply to the shoulder joint derived?
The blood supply to the shoulder joint is derived from a well-developed periarticular vascular network formed by branches of the axillary artery and the thyrocervical trunk.
- Anterior circumflex humeral artery (a. circumflexa humeri anterior).
- Posterior circumflex humeral artery (a. circumflexa humeri posterior).
- Deltoid branch of the thoracoacromial artery (r. deltoideus a. thoracoacromialis).
- Suprascapular artery (a. suprascapularis) — participates in the blood supply to the posterosuperior parts of the capsule.
Which nerves provide innervation to the capsule of the shoulder joint?
Innervation of the shoulder joint capsule is provided by branches of the brachial plexus in accordance with Hilton's law.
- Axillary nerve (n. axillaris) — the main source, innervating the lateral and inferior parts.
- Suprascapular nerve (n. suprascapularis) — innervates the posterosuperior parts of the capsule.
- Lateral pectoral nerve (n. pectoralis lateralis) — innervates the anterior part of the capsule.
- Subscapular nerve (n. subscapularis) — innervates the anterior and inferior parts of the capsule.
To which anatomical structures does the articular capsule of the shoulder joint attach?
The articular capsule of the shoulder joint attaches to the scapula and humerus, forming a free cavity.
- On the scapula — it is fixed along the bony margin of the glenoid cavity and the edge of the glenoid labrum (labrum glenoidale).
- On the humerus — superiorly, it attaches to the anatomical neck (collum anatomicum), leaving the greater and lesser tubercles outside the joint cavity. Inferiorly and medially, the attachment line extends significantly lower, reaching the level of the surgical neck (collum chirurgicum), where it forms the axillary recess (recessus axillaris).
Imagine a billiard ball (the humeral head) resting on a flat saucer (the glenoid cavity). To keep the ball from rolling off immediately, a soft rim is attached along the edge of the saucer (the glenoid labrum — labrum glenoidale).
Class and exam questions
What type of joint is articulatio humeri?
This is a multiaxial ball-and-socket joint.
What forms the "arch of the shoulder"?
The arch of the shoulder is formed by the coracoacromial ligament (lig. coracoacromiale), which spans over the joint.
Where are the weak spots of the shoulder joint capsule located?
They are located below the coracoid process and in the axillary region.
What is the function of the glenoid labrum?
The glenoid labrum (labrum glenoidale) surrounds the glenoid cavity of the scapula, deepening it and increasing the contact area with the humeral head.
What else to study on this topic
- Topography of the glenoid labrum (labrum glenoidale)
- Line of capsule attachment to the anatomical neck
- Anatomy and location of synovial bursae (subdeltoid, subscapular)
- Weak spots of the capsule: clinical significance
- Biomechanics of combined movements with the scapula and clavicle
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