Shoulder Anatomy

Clavicle (Collarbone)

The collarbone is the curved bone that you can feel at the top of your chest. It connects the shoulder blade to the rib cage and helps keep your shoulder steady when you move your arm.

Scapula (Shoulder Blade)

The shoulder blade is a flat, triangular bone that rests on the back of your rib cage. It has a socket into which the upper arm bone fits. The top of the shoulder blade has a bump known as the acromion, which forms the top of your shoulder.

Humerus (Upper Arm Bone)

The humerus is the long bone in your upper arm. The top of the humerus is shaped like a ball, and it fits into the socket of the shoulder blade to make the ball-and-socket joint of the shoulder.

There are two main joints in the shoulder:

Glenohumeral Joint

This is the primary joint of the shoulder, where the ball of the humerus fits into the socket of the scapula. This joint allows for a lot of movement, letting you swing your arm in almost every direction. Because of this wide range of motion, the soft tissues surrounding the joint see a lot of stress and are commonly injured.

Acromioclavicular Joint (AC Joint)

This joint is where the collarbone meets the acromion of the shoulder blade. It helps you lift your arm above your head. Trauma to this joint can cause an AC Separation, or over time can develop AC joint arthritis.

The muscles around your shoulder give your arm strength and allow it to move. The most important group of muscles in the shoulder is known as the rotator cuff. The rotator cuff is made up of four muscles and their tendons, which are strong tissues that connect muscles to bones. These four muscles work together to help you lift and rotate your arm. The subscapularis is on the front of the shoulder, and helps turn your arm inward. The supraspinatus sits on top of the shoulder, and helps you lift your arm out to the side. The infraspinatus and teres minor are the remaining two rotator cuff muscles, and attach to the back of the shoulder to help with external rotation of your arm. When one of these tendons is torn, such as in a rotator cuff tear, it can cause significant pain, weakness, and difficulty sleeping.

The shoulder joint is surrounded and protected by soft tissues:

Cartilage

This is a type of tissue that covers the bones where they meet in the joint. It’s smooth and slippery, which allows the bones to move against each other without causing damage.

Labrum

The labrum is a ring of tough, fibrous tissue that surrounds the socket of the shoulder joint. This important structure deepens the socket, making it more of a “cup” for the ball at the top of the humerus. The labrum helps to stabilize the shoulder, keeping the ball and socket in place during movement. When there is a labrum tear, the shoulder can feel unstable, and dislocate easier.

Capsule

The shoulder joint is surrounded by a strong flexible tissue called the joint capsule. This capsule is filled with fluid that helps to lubricate the joint, allowing your shoulder to move smoothly. The joint capsule also plays a vital role in stability by limiting extreme movements that could cause dislocation.

Glenohumeral Ligaments

These are a group of three ligaments that connect the humerus to the shoulder blade. They help to strengthen the front of the joint capsule, providing stability and preventing the shoulder from dislocating.

Acromioclavicular (AC) Ligament

This ligament connects the collarbone to the shoulder blade at the acromioclavicular joint. It helps stabilize the top of the shoulder, supporting your ability to lift your arm above your head.

Coracoclavicular (CC) Ligaments

These two ligaments, known as the trapezoid and conoid ligaments, connect the collarbone to a small hook-like part of the shoulder blade called the coracoid process. The CC ligaments provide additional strength and stability to the acromioclavicular joint.

Biceps Tendon

The biceps tendon is a strong, cord-like structure that connects the biceps muscle in your upper arm to the shoulder. It actually has two parts: the long head and the short head. The long head of the biceps tendon attaches to the top of the shoulder socket and the labrum. It helps to stabilize the shoulder joint and also aids in arm movements like lifting and rotating. The short head of the biceps tendon attaches to a bump on the shoulder blade called the coracoid process.

Bursa

The shoulder has several small, fluid-filled sacs called bursae. Perhaps the most significant is the subacromial bursa, located between the rotator cuff and the acromion of the shoulder blade. Bursae help reduce friction and provide a smooth surface for muscles and tendons to glide over when the shoulder moves.

Each of these soft tissues in the shoulder plays an essential role in maintaining stability, providing strength, and allowing for a wide range of motion. If you have any shoulder pain or discomfort, understanding these structures can help you better communicate with your doctor about your symptoms.