The Shoulder
Shoulder Anatomy
The connection of two bones is called a joint. The shoulder has 3 joints.
Glenohumeral Joint: This what we conceptualize as the “shoulder.” This ball-and-socket joint connects the head of the humerus (ball) with the glenoid (socket). Both the humeral head and glenoid are lined by a protective layer of cartilage that allows for smooth, painless motion during arm movement. Like the tires on a car, this cartilage wears away over time, a process called osteoarthritis. When the uncovered bones (i.e. without cartilage) rub against each other during routine shoulder motion, this can cause severe pain.
Acromioclavicular (AC) Joint: This joint is located above the glenohumeral joint and connects the acromion (shoulder blade) with the lateral end of the clavicle (collar bone). With repetitive overhead activities, the cartilage within this joint can wear down, leading to pain on top of the shoulder. A “shoulder separation” refers to an injury of this joint, whereby the bony ends are no longer opposed and held together.
Sternoclavicular (SC) Joint: This joint connects the medial end of the clavicle (collarbone) to the sternum. While a much less common source of injury, the soft tissues that hold these bones together can become torn or loose, resulting in feelings of instability in this region when raising the arm overhead.
Shoulder anatomy can be organized based upon the structures that most commonly cause disorders:
Bony anatomy
Passive soft tissue stabilizers (labrum and capsule)
Dynamic soft tissue stabilizers (rotator cuff and deltoid)
Bony Anatomy
Image adapted from Clinical Anatomy
Labrum: Think of the labrum as a fence along the perimeter of the glenoid socket. It’s a type of elastic soft tissue that deepens the socket to hold the humeral head in place. The anterior or posterior labrum is commonly injured during a shoulder dislocation. However, wear-and-tear of the superior labrum is commonly seen on MRI with aging and we have learned that it is uncommonly the true source of patients’ complaints.
Static Soft Tissue Stabilizers
Capsule: The capsule is a soft tissue covering that surrounds the entire glenohumeral joint and helps maintain joint stability. After trauma, injury, or even without precipitating cause, this capsule can become thickened and contracted, leading to shoulder stiffness. This process can cause a “frozen shoulder.”
Rotator Cuff: Perhaps the most important structure in the shoulder, the rotator cuff consists of four muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) that come together into a confluent tendon that attaches to the humeral head. The rotator cuff serves two critical roles: 1) it initiates shoulder movements that allow you to move your arm in space, and 2) it holds the humeral head against the glenoid to maintain joint stability.
Dynamic Soft Tissue Stabilizers
Deltoid: The deltoid is a large, powerful muscle that overlies and works in tandem with the rotator cuff to produce arm motion. When a patient has a nonfunctional rotator cuff, a reverse shoulder replacement can restore function by allowing the deltoid to take its place.
Disclaimer: The information provided on this website is intended for informational and educational purposes only. The opinions, views, and content presented here should not be construed as medical advice, diagnosis, or treatment. We welcome you to schedule a consultation with Dr. Brusalis for evaluation of your specific medical condition.