Shoulder joint
The shoulder joint is the most mobile joint in the human body. The spherical shape allows movement in three planes: flexion-extension, abduction-adduction and rotation. However, this mobility comes at the expense of stability—the shoulder is also the most commonly dislocated joint.
Bone structures
- The head of the humerus is a hemisphere with a diameter of about 5 cm
- Glenoid cavity of the scapula (glenoid) - a shallow oval fossa covering only 1/3 of the surface of the head
- Labrum (labrum) - a cartilaginous ring along the edge of the cavity, deepening it by 50%
This ratio has been compared to a golf ball on a football court - the head is much larger than the tee.
Rotator cuff
The main stabilizer of the shoulder joint is the rotator cuff (rotator cuff), consisting of four muscles:
| Muscle | Function |
|---|---|
| Supraspinatus | Beginning of arm abduction |
| Infraspinatus (infraspinatus) | External rotation |
| Small round (teres minor) | External rotation |
| Subscapularis (subscapularis) | Internal Rotation |
The tendons of these muscles cover the head of the humerus from above, behind and in front, holding it in the glenoid cavity. Torn rotator cuff is one of the most common causes of shoulder pain in people over 50 years of age.
Bundles and bags
- Coracoacromial ligament forms a “roof” over the joint
- Articular-brachial ligaments (upper, middle, lower) strengthen the capsule in front
- Subacromial bursa - ensures the sliding of tendons under the acromion
Biomechanics
Full abduction of the arm (180°) includes scapulohumeral rhythm: for every 2° of movement in the joint, the scapula rotates 1°. Thus, out of 180° of abduction, only 120° is due to the shoulder joint, and 60° is due to scapular rotation.
Clinical significance
- Shoulder dislocation - the head comes out of the socket (in 95% of cases - forward)
- Impingment syndrome - pinching of the cuff tendons under the acromion
- Adhesive capsulitis (“frozen shoulder”) - inflammation and wrinkling of the capsule
Understanding the anatomy of the shoulder joint is critical for orthopedic surgeons during arthroscopic surgery.
The Humio](/) app provides an interactive 3D model of the shoulder joint - rotate, zoom and view the rotator cuff, labrum and ligaments from all sides.