Shoulder Joint — Articular Surfaces and Type
Type
The shoulder joint is a ball-and-socket type of synovial joint (glenohumeral joint).
It is the most mobile joint in the body and consequently one of the least stable. It is the most commonly dislocated joint.
Articular Surfaces
The joint is formed by articulation of:
- Head of the humerus — large, rounded (ball)
- Glenoid cavity of the scapula — small, shallow (socket)
The head of the humerus is approximately four times the size of the glenoid cavity. This disproportion permits great mobility at the cost of stability.
The joint is also known as the glenohumeral articulation.
Glenoid Labrum
The glenoid cavity is deepened by a fibrocartilaginous rim called the glenoid labrum (Latin: lip).
- Attached to the margins of the glenoid cavity
- Increases the depth of the cavity, contributing to joint stability
- The long head of biceps brachii tendon blends with the labrum superiorly
Orientation of the Glenoid Cavity
The scapula does not lie in the coronal plane. The glenoid cavity faces forwards and laterally, so the plane of the joint lies obliquely at approximately 45° to the sagittal plane.
Movements of the shoulder joint are described in relation to this oblique plane, not the standard anatomical planes.