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M01.03.032 Shoulder Joint
The shoulder joint, also known as the glenohumeral joint, is a ball-and-socket synovial joint that facilitates a wide range of movement but comes with the trade-off of less stability. This joint is formed by the articulation between the scapula and the humerus, and it is one of the most mobile joints in the human body.
Key Aspects of the Shoulder Joint
- Articulation: Scapula and humerus
- Type: Ball and socket synovial joint
- Function: High mobility, low stability
In this article, we will explore the anatomical structure, ligaments, movements, and clinical correlations of the shoulder joint.
Anatomical Structure
Articulating Surfaces
The shoulder joint is formed by:
- Head of the humerus: A large, round surface
- Glenoid fossa of the scapula: A shallow, cup-like cavity
These two surfaces are covered with hyaline cartilage, and the glenoid cavity is deepened by the glenoid labrum (a fibrocartilage rim) to provide better joint stability.
Table 1: Articulating Surfaces of the Shoulder Joint
| Structure | Description |
|---|---|
| Head of the Humerus | Large, round surface that fits into the glenoid fossa |
| Glenoid Fossa of Scapula | Shallow cavity, deepened by the glenoid labrum |
| Cartilage | Hyaline cartilage covering articulating surfaces |
Joint Capsule
- Fibrous Sheath: Encloses the joint from the humeral neck to the glenoid fossa.
- Laxity: This allows for greater mobility, especially in abduction.
- Synovial Membrane: Lines the capsule, producing synovial fluid to reduce friction.
Ligaments
Ligaments are crucial in stabilizing the shoulder joint. The main ligaments include:
- Glenohumeral Ligaments (Superior, Middle, Inferior): Reinforce the joint capsule and stabilize the anterior aspect of the joint.
- Coracohumeral Ligament: Supports the superior part of the joint capsule.
- Transverse Humeral Ligament: Holds the tendon of the long head of the biceps in the intertubercular groove.
- Coracoacromial Ligament: Forms the coracoacromial arch, preventing superior displacement of the humeral head.
Table 2: Ligaments of the Shoulder Joint
| Ligament | Function |
|---|---|
| Glenohumeral (Superior, Middle, Inferior) | Stabilizes the anterior joint and reinforces the capsule |
| Coracohumeral | Supports the superior part of the capsule |
| Transverse Humeral | Holds the biceps tendon in place |
| Coracoacromial | Prevents superior displacement of the humeral head |
Bursae
Bursae are fluid-filled sacs that reduce friction during movement. The main bursae in the shoulder include:
- Subacromial Bursa: Reduces friction beneath the deltoid and allows smooth motion of the rotator cuff tendons.
- Subscapular Bursa: Reduces friction between the subscapularis tendon and scapula.
Movements of the Shoulder Joint
The shoulder joint is extremely mobile, capable of a wide range of movements:
Major Movements:
- Flexion: Moving the upper limb forward in the sagittal plane
- Extension: Moving the upper limb backward in the sagittal plane
- Abduction: Moving the upper limb away from the body in the coronal plane
- Adduction: Bringing the upper limb towards the body in the coronal plane
- Internal Rotation: Rotating the arm towards the body
- External Rotation: Rotating the arm away from the body
- Circumduction: Moving the arm in a circular motion
Table 3: Muscles Involved in Shoulder Movements
| Movement | Muscles Involved |
|---|---|
| Flexion | Pectoralis major, anterior deltoid, coracobrachialis |
| Extension | Posterior deltoid, latissimus dorsi, teres major |
| Abduction | Supraspinatus (first 15°), deltoid (15°-90°), trapezius, serratus anterior (past 90°) |
| Adduction | Pectoralis major, latissimus dorsi, teres major |
| Internal Rotation | Subscapularis, pectoralis major, latissimus dorsi, teres major, anterior deltoid |
| External Rotation | Infraspinatus, teres minor |
Mobility vs. Stability
The shoulder joint is highly mobile but less stable, with several structures contributing to its mobility and stability.
Mobility Factors:
- Ball-and-socket joint: Offers 360° movement
- Shallow glenoid cavity: Allows extensive movement
- Lax joint capsule: Facilitates mobility
Stability Factors:
- Rotator Cuff Muscles: Surround the joint, compressing the humeral head into the glenoid cavity.
- Glenoid Labrum: Deepens the cavity and reduces the risk of dislocation.
- Ligaments: Reinforce the joint capsule and provide support.
- Biceps Tendon: Helps depress the humeral head, aiding stability.
Table 4: Factors Contributing to Mobility and Stability
| Factor | Contribution to Joint |
|---|---|
| Mobility Factors | High range of motion, shallow glenoid cavity |
| Rotator Cuff Muscles | Stabilize and compress humeral head into glenoid |
| Glenoid Labrum | Deepens glenoid cavity, reduces dislocation risk |
| Ligaments | Reinforce joint capsule |
| Biceps Tendon | Contributes to stability by depressing humeral head |
Blood Supply and Innervation
- Blood Supply: Anterior and posterior circumflex humeral arteries, branches of the axillary artery, and contributions from the suprascapular artery.
- Innervation: Sensory input from the axillary and suprascapular nerves.
Clinical Relevance
Common Injuries
- Dislocation of the Shoulder: Most commonly anterior (95%), but posterior (4%) and inferior (1%) dislocations also occur. Anterior dislocation is often caused by excessive extension and lateral rotation of the humerus. Associated injuries include Hill-Sachs and Bankart lesions.
- Rotator Cuff Tendonitis: Caused by overuse and strain, leading to inflammation and degeneration of tendons and bursae. Symptoms include the “painful arc” during abduction.
Key Points to Remember
- The shoulder joint is the most mobile but least stable joint in the human body.
- Injuries like dislocations and rotator cuff tendonitis are common.
- Ligaments, the rotator cuff, and the glenoid labrum are essential for maintaining stability.