Overview
The shoulder is a ball-and-socket joint. The humeral head is the ball, while the glenoid is a relatively flat socket. This design provides tremendous motion but less inherent stability than joints whose bones fit together more like a key in a lock. As a result, the surrounding soft tissues play a critical role in maintaining shoulder stability.
Bones
Humerus: The upper arm bone. The rounded upper end is called the humeral head.
Scapula: The scapula has three important parts:
- Glenoid – the socket that articulates with the humeral head. Both surfaces are covered with articular cartilage to form the shoulder joint.
- Body of the scapula – the large triangular portion commonly referred to as the shoulder blade.
- Acromion – the roof of the shoulder and an important structure in the impingement process. The more curved the acromion, the greater the tendency for rotator cuff irritation. Arthroscopic decompression removes a small amount of bone from its underside to increase clearance.
Capsule and Ligaments
All joints are surrounded by a strong envelope called the joint capsule. Unlike the knee, the major ligaments of the shoulder are thickenings of this capsule rather than separate structures. The most important is the anterior inferior glenohumeral ligament in the front lower quadrant, the primary restraint to anterior shoulder dislocation.
Labrum
The labrum is a ring of cartilage that surrounds the glenoid. Think of the shoulder as a bowling ball sitting on a relatively flat plate. The humeral head is the bowling ball, the glenoid is the plate, and the labrum is the rubber bumper around the edge that deepens the socket. It also provides the attachment for the shoulder capsule, stabilizing ligaments, and the long head of the biceps tendon.
Rotator Cuff
The rotator cuff consists of four muscles, although I think of them as three plus one. The supraspinatus, infraspinatus, and teres minor function together across the top and back of the shoulder. The subscapularis is different—it is the door to the shoulder. Most open shoulder procedures enter through this tendon before repairing it back to the bone. Most rotator cuff tears begin in the supraspinatus and enlarge progressively toward the back. The rotator cuff's primary role is to keep the humeral head centered on the glenoid while the larger muscles move the shoulder.
Deltoid
The deltoid covers all of these deeper structures and is the muscle visible beneath the skin. The rotator cuff lies much deeper and works together with the deltoid to move and stabilize the shoulder.
The Two-Room Concept
I think of the shoulder as a two-story house. The shoulder joint is the ground floor, and the subacromial space is the attic. The rotator cuff separates these two spaces. From inside the shoulder joint it forms the ceiling, and from the subacromial space it forms the floor. As long as the rotator cuff is intact, the shoulder joint and the subacromial space remain two separate rooms. However, if a rotator cuff tear is present, that barrier is breached, creating a direct connection between the two spaces.
Bursa
Within the subacromial space is a thin, cobweb-like structure called the bursa. It allows the rotator cuff to glide smoothly beneath the acromion. When inflamed or thickened, it can cause bursitis, pain, popping, and crepitus.