Your shoulder is the most flexible joint in your body. It allows you to place and rotate your arm in many positions in front, above, to the side, and behind your body. This flexibility also makes your shoulder susceptible to instability and injury.

The shoulder is a ball-and-socket joint that has three main bones: the humerus (long arm bone), the clavicle (collarbone), and the scapula (also known as the shoulder blade). These bones are cushioned by a layer of cartilage. There are two main joints:

1.The acromioclavicular joint, which is located between the highest part of the scapula (also called the acromion) and the clavicle.

2.The glenohumeral joint, made up of the top, ball-shaped part of the humerus bone and the outer edge of the scapula. This joint is also referred to as the shoulder joint.

The ball at the top end of the arm bone fits into the small socket (glenoid) of the shoulder blade to form the shoulder joint (glenohumeral joint).

The socket of the glenoid is surrounded by a soft-tissue rim (labrum).

A smooth, durable surface (articular cartilage) on the head of the arm bone, and a thin inner lining (synovium) of the joint allows the smooth motion of the shoulder joint.