The shoulder is a ball and socket joint, meaning it is created by two bones that fit together similarly to how a golf ball sits on a golf tee. The shoulder joint structure allows for a wide-range circular movement in all directions.
Three primary bones form the structure of the shoulder:
• Arm bone, or humerus
• Shoulder blade, or scapula
• Collar bone, or clavicle
The shoulder region includes two main joints formed from the bones mentioned above:
o Glenohumeral joint, or shoulder joint
o Acromioclavicular, or AC, joint
The shoulder joint, or glenohumeral joint, has an outer cup around the glenoid fossa made of cartilage called the labrum. A labrum is an extension of a bone to create more surface area for a joint. There is also a labrum in the hip joint. The purpose of the labrum is to provide stability. It has a rubbery texture and can become torn either through injury or degeneration over time.
Shoulder Joint Capsule
The shoulder joint itself is encased in a thin layer of connective tissue that wraps around the joint called a joint capsule. This capsule, whether in the shoulder or other joints, contains synovial fluid. In the healthy shoulder, this joint fluid lubricates the joint and contains nutrients that help stay healthy. In a shoulder with inflammation, osteoarthritis, or another source of inflammation, this capsule can become swollen beyond the joint’s capacity and cause pain.
Ligaments are structures in the body that connect bones. Their role is to help provide stability and structure to joints. Ligaments can become torn, stretched, loose, or degenerated, which leads to instability and pain. There are several critical ligaments in the shoulder, including:
- Acromioclavicular ligaments
- Help to stabilize the AC joint (affected in shoulder separations)
- Coracoclavicular ligaments
- Helps to support the front, inside portion of the shoulder (affected in shoulder separations)
- Glenohumeral ligaments: superior, middle, and inferior
- Wrap across the front of the shoulder joint to provide stability
Shoulder Tendons and Rotator Cuff
- Tendons connect muscles to bones. They are rubbery connections made from bundles of collagen fibers. A healthy tendon is like a shoelace with the bands taught and firm. When there is tearing or degeneration, the fibers become frayed, wavy, thickened, and loose. This is called “tendinopathy.” Tendinopathy can occur from previous tears that did not heal properly or with wear and tear over time. Often this condition causes pain with movement because the fibers are chronically inflamed. When inflammation occurs more suddenly, this is called “tendonitis.” In addition, injury can cause sudden tearing either of some fibers in the tendon, called partial tearing, or of the whole tendon, called full tearing. There are several tendons in the shoulder. One of the most common sites of injury is the rotator cuff, which is made of four different muscles and tendons.
Rotator cuff tendons:
- Crosses the front of the shoulder
- Cross the top of the shoulder
- Cross the back of the shoulder
- Teres minor
- Under the infraspinatus
- Biceps tendon
- The long head of the biceps tendon crosses over the top of the humerus and connects into the top portion of the labrum.