Anatomy & Function
Dr. Pamela Mehta, MD
Shoulder Joint Anatomy
Your shoulder is a large and complex joint that allows you to perform many movements with your arm. It has both dexterity and strength, allowing you to move your hands into any position they require, while also supporting heavy pushing and pulling movements.
The shoulder is a “ball and socket” joint, meaning it can move through many angles, as the “ball” (the head of your humerus) moves within the socket (the shoulder blade). While we call it the shoulder joint, it’s actually made up of four joints that work together to help your shoulder movements.
To allow for complex movements and heavy lifting, your shoulder joint contains many muscles and has cartilage on its surface to allow your bones to move easily against each other. To keep your shoulder stable, the joint also contains ligaments and tendons.
Your shoulder joint is commonly injured during sporting activities, household chores, and work-related incidents. It can also suffer from inflammation, which gives you pain in the shoulder that can prevent restful sleep.
Your Shoulder joint includes three bones:
- The humerus is the bone of your upper arm, connecting your shoulder to your elbow. The top of your humerus is rounded to allow for a wide range of motion.
- The scapula (shoulder blade) is the bone that sits at the back of the shoulder. It is triangular and connects to both the humerus (the joint’s “ball”) in the glenoid cavity (the joint’s “socket”), and the clavicle bone, to support and strengthen the joint.
- The clavicle (collarbone) is a thin bone that connects the scapula to the sternum (breastbone). This bone also helps to stabilize the shoulder joint when you lift your arms, shrug, move the shoulder back and forward, and reach behind your back.
Joints of the Shoulder
While we refer to the “shoulder joint”, it’s actually a collection of four joints.
- The Glenohumeral joint (GH joint) is the main joint of the shoulder and is the “ball and socket” joint between the scapula and humerus.
- The Acromioclavicular joint (AC joint) is the joint between the acromion of the scapula and your clavicle.
- The Sternoclavicular joint (SC joint) is the joint between the clavicle and sternum.
The Scapulothoracic joint is the joint between the scapula and the ribs at the back of your chest.
Your shoulder joints contain articular cartilage, also known as chondral cartilage, which lines the surface of the shoulder bones within the joint. Articular cartilage is made of strong tissue which has a smooth surface. When the bones of your shoulder joint move against each other, the cartilage allows them to glide freely and prevents friction.
Shoulder arthritis happens when the cartilage is injured or worn down over time and the bones begin to rub against each other, causing pain and inflammation.
Your Shoulder has many ligaments that help to stabilize the shoulder and keep the shoulder bones in position while the shoulder joint moves.
- Glenohumeral ligaments – Attaching between the humerus and the scapula, these three ligaments (superior, middle, and inferior glenohumeral ligaments) keep the glenohumeral joint stable as it moves.
- Coracohumeral ligament – A strong and broad ligament that joins the scapula and the humerus. This ligament helps to strengthen the upper part of the glenohumeral joint.
- Transverse humeral ligament – Attaching in two places to the humerus, this broad ligament keeps the biceps tendon in place.
- Coracoacromial ligament – Attaches to the coracoid process and acromion of the scapula, this ligament protects the top of your humerus.
The shoulder joint requires muscles that can perform strong and complex movements of your arm. Some of your shoulder muscles include:
The Rotator Cuff. A group of four muscles – subscapularis, infraspinatus, teres minor, and supraspinatus. These muscles stabilize the glenohumeral joint and help you perform a wide range of motions and activities including heavy lifting. Rotator cuff tear is a common condition that can cause shoulder pain.
Deltoid. The largest muscle in your shoulder, this muscle is triangular and covers the glenohumeral joint. The deltoid muscle connects all three of your shoulder bones and is important in protecting your shoulder joint during heavy lifting.
Trapezius. This muscle, along with the levator scapulae and rhomboid muscles, connect the base of your skull and spine to the clavicle and scapula. They help to support your posture.
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