Anatomy of the Shoulder: Joints, Rotator Cuff and Capsule

Your shoulder is four joints working together, not one. The ball of the upper arm bone sits in a shallow socket on the shoulder blade. The collarbone links the shoulder blade to the breastbone, and the shoulder blade glides over the ribs. Four rotator cuff muscles hold the ball centered. A loose sleeve of tissue (the capsule) and a rim of cartilage (the labrum) add stability. This design gives the shoulder more range than any other joint in the body, at the cost of stability.

Most shoulder problems start in one of these parts, and many are shaped by how well the shoulder blade and upper back move underneath it. This page explains each part in plain words and what it means for the pain you feel.

Key takeaways

Four joints make up the shoulder complex: the main ball-and-socket joint, the AC joint, the SC joint and the shoulder blade gliding on the ribs.
The socket is shallow: only part of the ball touches the socket at any time, so muscles and soft tissue do much of the work of keeping it in place.
The rotator cuff is four muscles: supraspinatus, infraspinatus, teres minor and subscapularis, which keep the ball centered as the arm moves.
The capsule controls range: when it thickens and tightens, as in frozen shoulder, movement is lost in every direction.
Shoulder joint pain is usually felt in the upper arm: pain from the rotator cuff and bursa is commonly felt on the outer side of the upper arm rather than on top of the shoulder.

The bones

Upper arm bone (humerus): its rounded top is the ball of the shoulder.
Shoulder blade (scapula): carries the socket (glenoid), the bony roof over the top of the shoulder (acromion), and a hooked bump at the front (coracoid process) that anchors muscles and ligaments.
Collarbone (clavicle): a strut from the breastbone to the acromion that holds the shoulder out to the side.

The four joints of the shoulder complex

1. The main shoulder joint (glenohumeral joint)

This is the ball-and-socket joint most people mean by “the shoulder”. The socket is about a quarter to a third the size of the ball, often compared to a golf ball on a tee. It moves in every direction: forward, back, out to the side and rotating. Dislocations, labral tears, frozen shoulder and shoulder arthritis all start here.

2. The AC joint (acromioclavicular joint)

A small joint on top of the shoulder where the end of the collarbone meets the acromion. You can usually feel it as a small bump. It is often injured in a fall onto the point of the shoulder (an AC joint separation) and is a common site of arthritis from middle age. Pain is usually felt right on top of the shoulder, and reaching across the body tends to aggravate it.

3. The SC joint (sternoclavicular joint)

Where the inner end of the collarbone meets the breastbone, at the base of the front of the neck. It is the only true bony joint connecting the arm to the trunk. It moves a little every time you shrug or reach. SC joint problems are less common and are felt at the front, near the base of the neck.

4. The shoulder blade on the ribs (scapulothoracic joint)

Not a joint in the usual sense, because there is no cartilage or capsule. The shoulder blade slides over the back of the ribcage on layers of muscle. As you raise your arm, the shoulder blade rotates upward and tips back so the socket follows the ball. If it moves poorly, the main shoulder joint works from a less steady base.

The rotator cuff

Four muscles start on the shoulder blade and attach by tendons to the top of the upper arm bone. Their tendons blend together into a cuff around the ball.

Supraspinatus: runs along the top of the shoulder blade, under the acromion, to the top of the ball. It helps lift the arm out to the side and is the tendon most often involved in tendon pain and tears.
Infraspinatus: covers the back of the shoulder blade and turns the arm outward.
Teres minor: a smaller muscle below infraspinatus that also turns the arm outward.
Subscapularis: lies on the front of the shoulder blade, between it and the ribs, and turns the arm inward.

The cuff’s main job is to keep the ball centered in the socket while the larger muscles, such as the deltoid, move the arm. Rotator cuff tendon problems become more common with age. Many people over 60 have a cuff tear on a scan without any pain.

The shoulder trades bony stability for range, and asks its muscles to pay the difference.

The capsule, labrum and ligaments

Capsule: a loose sleeve of tissue around the main shoulder joint, slack enough to let the arm move freely. In frozen shoulder (adhesive capsulitis), the capsule becomes inflamed, then thickens and shrinks, and movement is lost in every direction, especially turning the arm outward.
Labrum: a rim of firm cartilage around the socket that deepens it and anchors the capsule and the biceps tendon. It can tear in a dislocation (a Bankart lesion at the front), or at the top where the biceps attaches (a SLAP tear).
Glenohumeral ligaments: thickened bands in the front of the capsule that tighten at the ends of range to stop the ball sliding too far.
Coracoclavicular ligaments: strong bands from the coracoid to the collarbone that hold the collarbone down. They are torn in more severe AC joint separations.

The bursa and the biceps tendon

A thin fluid-filled sac (the subacromial bursa) sits between the rotator cuff and the acromion and lets the tendon glide. When it becomes irritated (shoulder bursitis), raising the arm between about 60 and 120 degrees often hurts most. The long head of the biceps tendon runs in a groove at the front of the upper arm, through the joint, to the top of the socket. It can be a source of pain at the front of the shoulder.

How the shoulder moves

Lifting the arm fully overhead to about 180 degrees needs all four joints. Roughly two-thirds of the movement happens at the main shoulder joint and one-third comes from the shoulder blade rotating on the ribs. The collarbone lifts and rotates at both of its joints. Near the top, the upper back straightens a little. If any one of these is stiff or painful, the others have to make up the difference, and reaching overhead may feel blocked, pinched or weak.

How the shoulder connects to the neck and upper back

The muscles that steady the shoulder blade, including the upper trapezius, levator scapulae and rhomboids, attach to the neck and upper back. So neck pain can change how the shoulder blade sits, and a stiff shoulder can overload the neck muscles. The nerves that supply the rotator cuff, the deltoid and the skin over the shoulder all come from the C5 and C6 levels of the neck. That is why neck problems can cause pain over the shoulder or weakness when lifting the arm, even when the shoulder joint is healthy.

Which problems start where

Rotator cuff and bursa: rotator cuff tendinopathy, tears and bursitis. Usually an ache on the outer upper arm, worse lifting the arm or lying on that side.
Capsule: frozen shoulder. Pain then stiffness in every direction, often lasting one to three years in total.
Labrum and ligaments: instability after a dislocation, or catching and clicking with deep shoulder pain.
Joint surfaces: shoulder arthritis, with gradual stiffness, grinding and aching.
AC joint: pain on top of the shoulder, often after a fall or with age-related arthritis.
Biceps tendon: pain at the front of the shoulder, worse lifting with the palm up.

What it means for you: pain on top of the shoulder and around the shoulder blade often comes from the neck or the AC joint. Pain on the outer upper arm more often comes from the shoulder joint, cuff or bursa. Pain that changes as you move your arm points toward the shoulder. Pain that changes as you move your head points toward the neck.

Get it checked if

Your shoulder looks out of place: after a fall or injury, with a visible change in shape or inability to move the arm, go to the emergency department.
You suddenly cannot lift your arm: after an injury or a heavy lift, see a doctor within days. Some acute rotator cuff tears are best assessed early.
Left shoulder pain comes with chest symptoms: chest pain, breathlessness, sweating or nausea needs your local emergency number now.
The joint is hot, red and swollen with fever: see a doctor today.
Night pain does not ease with any position: especially with unexplained weight loss or a history of cancer, get it checked soon.