The Anatomy of the Shoulder
The Shoulder – A Complex Joint
The shoulder joint is a “ball-and-socket” joint composed of three bones:
- Clavicle (collarbone)
- Scapula (shoulder blade)
- Humerus (upper arm bone)
Most ball and socket joints are constructed of a large socket into which the ball fits nicely which is a very stable structure.
However, the shoulder joint is made up of a ball which is larger than the socket. This is not a very stable type of joint and tends to be easily injured. Muscles and tendons anchor the shoulder which adds to its stability. Because of its structure, the shoulder has a greater range of motion that any other joint in the human body.
Getting Into the Detail:
- The “ball” portion of the shoulder joint, or glenohumeral joint, is the rounded head of the upper arm bone, called the humerus.
- The “socket” portion is the shallow, dish-like part of the scapula (shoulder blade) that is called the glenoid.
- The surfaces of the head of the humerus and the glenoid are covered in a smooth tissue called articular cartilage so that the two bones can glide smoothly against one another.
Other bony prominences that play a part in the function and stability of the shoulder joint include: the acromion, which sits at the top of the shoulder blade; the coracoid process that is a hook-like projection located on the anterior side of the upper portion of the shoulder blade; and the clavicle, or collar bone.
The humeral head is typically larger than the glenoid so a band of fibrous cartilage, called the labrum, surrounds the glenoid to create a deeper socket for the humeral head to articulate with, which provides greater stability to the shoulder joint. The tendon of the long head of the biceps muscle attaches to the labrum at the top of the glenoid. The shoulder joint capsule is a soft membrane that encompasses the joint and is reinforced by the ligaments that run between the humerus and the glenoid. The purpose of the joint capsule is to contain the normal lubricating joint fluid and provide joint stability by allowing only certain motions of the shoulder.
The major muscles involved with movement of the shoulder are the four rotator cuff muscles and the deltoid. These muscles allow the upper arm to rotate in and out, move forward, out to the side, and behind the back. Tendons are the bands of fibrous connective tissue that attach these muscles to the humerus. A few of the rotator cuff muscles and tendons run between the shoulder joint and the acromion of the scapula. A fluid-filled bursa sac exists between the rotator cuff and the acromion to help protect the muscle and tendons from rubbing against the bone.