Where Shoulder Arthritis Happens

Shoulder arthritis most often develops in the glenohumeral joint, where your upper arm bone (humerus) meets your shoulder blade (scapula). This is the main joint that lets your arm rotate and move forward and backward. Cartilage lines both surfaces where these bones meet, and when that cartilage wears down or becomes inflamed, you feel pain and stiffness.

A second common site is the acromioclavicular joint, where your collarbone (clavicle) connects to the top of your shoulder blade. This joint handles the motion of shrugging and reaching across your body. Arthritis here often causes pain at the very top of the shoulder, especially when you cross your arm over your chest.

The shoulder is built differently from other joints like the knee or hip. It prioritizes range of motion over stability, which means the bones are held together more by muscles and tendons than by tight ligaments. This design makes the shoulder mobile but also means the cartilage can wear unevenly and the joint can become unstable as arthritis progresses.

Key Takeaways

  • The glenohumeral joint, where your upper arm bone meets your shoulder blade, is where most shoulder arthritis develops.
  • The acromioclavicular joint at the top of your shoulder is a second common site, and arthritis there causes pain when you reach across your body.
  • Cartilage that lines both bone surfaces wears down or becomes inflamed, reducing cushioning and causing the pain you feel.
  • The shoulder's design prioritizes movement over stability, which means arthritis can progress differently here than in weight-bearing joints.
  • Tendons and muscles around the joint can become irritated or damaged as arthritis worsens, adding to pain and limiting motion.

The Cartilage and How It Breaks Down

Cartilage is smooth, slippery tissue that covers the ends of your bones inside the joint. In a healthy shoulder, this cartilage is about 2 to 3 millimeters thick and lets the bones glide smoothly against each other with almost no friction. When you have arthritis, that cartilage gradually thins, cracks, or flakes away.

As cartilage wears down, the bone underneath becomes exposed. The body responds by growing extra bone (bone spurs) around the joint edges, trying to stabilize the joint. These spurs can pinch nearby tendons and nerves, which is why arthritis pain often gets worse when you move your arm in certain directions. The joint also produces less synovial fluid—the natural lubricant that keeps bones moving smoothly—so friction increases.

Osteoarthritis, the most common type of shoulder arthritis, develops this way over years or decades. Rheumatoid arthritis, an autoimmune condition, attacks the joint lining (synovium) directly, causing inflammation and cartilage damage that can happen much faster. Post-traumatic arthritis can develop after a shoulder injury or fracture, even years later, as the joint surfaces become uneven.

The Rotator Cuff and Surrounding Tendons

Four muscles and their tendons make up the rotator cuff—the supraspinatus, infraspinatus, teres minor, and subscapularis. These tendons attach your shoulder muscles to your upper arm bone and hold the joint in place while letting you rotate your arm. They do most of the work when you lift your arm overhead or reach behind your back.

Arthritis can damage these tendons in two ways. First, bone spurs from the arthritic joint can rub against the tendons as you move, causing irritation and small tears. Second, the inflammation from arthritis spreads to the tendons themselves, weakening them. A rotator cuff tear often develops alongside shoulder arthritis, and the two conditions together cause more pain and loss of strength than either alone.

When rotator cuff tendons are damaged, you may notice weakness when lifting your arm or pain that wakes you at night. Some people lose the ability to raise their arm overhead. Physical therapy can sometimes restore strength and reduce pain, but severe tears may need surgery.

The Bursa and Joint Lining

Between your rotator cuff tendons and the bone above them sits a small fluid-filled sac called the subacromial bursa. This bursa cushions the tendons and lets them slide smoothly under the bone when you move your arm. Arthritis and bone spurs can irritate this bursa, causing a condition called bursitis that adds swelling and pain on top of the arthritis itself.

The joint lining, called the synovium, produces the fluid that lubricates the joint. In arthritis, this lining becomes inflamed and thickened, producing excess fluid that causes swelling. In rheumatoid arthritis, the synovium is the primary target of the immune system, and inflammation here can be severe and rapid.

When the bursa and joint lining are both inflamed, your shoulder may feel warm to the touch and swell noticeably. This swelling limits how far you can move your arm and makes pain worse. Anti-inflammatory medications and ice can reduce this swelling temporarily, but the underlying arthritis continues to progress.

Nerves and Blood Vessels in the Shoulder

Several nerves run through and around the shoulder joint. The suprascapular nerve, which controls the rotator cuff muscles, can be pinched by bone spurs or swelling. The axillary nerve, which controls the deltoid muscle on the outside of your shoulder, can also be compressed. When nerves are pinched, you may feel numbness, tingling, or weakness in addition to pain.

Blood vessels that feed the shoulder joint and surrounding tissues can also be affected by arthritis and swelling. Reduced blood flow slows healing and can make inflammation worse. This is one reason why shoulder arthritis can be slow to improve—the joint has a more limited blood supply than other joints like the knee.

Nerve and blood vessel involvement is why some people with shoulder arthritis develop frozen shoulder (adhesive capsulitis), where the joint becomes stiff and painful even when not moving. The lack of blood flow and the body's inflammatory response cause scar tissue to form inside the joint capsule, restricting motion further.

How Arthritis Changes the Joint Capsule

The joint capsule is a tough, fibrous sleeve that surrounds the entire shoulder joint and holds in the synovial fluid. In a healthy shoulder, this capsule is flexible and lets your arm move in many directions. Arthritis causes the capsule to thicken and tighten, especially on the front and underside of the joint.

As the capsule tightens, your range of motion shrinks. You may lose the ability to rotate your arm outward or reach behind your back. This stiffness often develops gradually, and many people don't notice it until they try to do something they've always done—like fastening a bra or reaching for a high shelf—and find they can't.

The tightening capsule also changes how the bones sit in the joint. Instead of the ball of your upper arm bone sitting centered in the socket, it may shift forward or upward. This misalignment puts more stress on the remaining cartilage and speeds up arthritis progression. Physical therapy that focuses on stretching and mobilizing the capsule can slow this process, though it requires consistent effort.

Bone Changes and Bone Spurs

As arthritis progresses, your body tries to stabilize the damaged joint by growing extra bone around the edges. These bone spurs, called osteophytes, are most common on the underside of the acromion (the bony roof of the shoulder) and along the edges of the socket. They develop over months or years and are visible on X-rays.

Bone spurs narrow the space where tendons move, a condition called subacromial impingement. When you lift your arm, the tendons get pinched between the spur and the bone below, causing sharp pain. Some people describe this as a catching or grinding sensation. The pain is often worse in certain positions—like reaching overhead or across your body—and better when your arm hangs at your side.

In advanced arthritis, bone spurs can become large enough to be felt under the skin at the top of your shoulder. They don't always cause pain, but they do limit motion and can eventually fuse parts of the joint together. In severe cases, the joint becomes so damaged that the bones rub directly against each other with no cartilage between them—a condition called bone-on-bone arthritis.

Frequently Asked Questions

Can shoulder arthritis affect both shoulders at the same time?

Yes. Rheumatoid arthritis almost always affects both shoulders symmetrically, often at the same stage. Osteoarthritis can affect one shoulder first, but many people develop it in both over time, especially if they've injured both shoulders or if they have a family history of arthritis.

Does arthritis in the shoulder spread to other joints?

Osteoarthritis doesn't spread from one joint to another, but if you have it in your shoulder, you may develop it in other joints as you age. Rheumatoid arthritis is a whole-body condition and often affects multiple joints simultaneously—hands, wrists, knees, and hips are common sites along with shoulders.

Why does shoulder arthritis pain get worse at night?

Pain often worsens at night because inflammation builds up during the day and you're more aware of pain when you're not distracted. Lying down can also put pressure on the shoulder joint or pinch tendons, depending on your sleeping position. Sleeping on the affected shoulder makes pain worse.

Can physical therapy reverse the cartilage damage from arthritis?

No, cartilage that has worn away cannot grow back. However, physical therapy can strengthen the muscles around the joint, improve your range of motion, reduce pain, and slow further damage. Stronger muscles take stress off the damaged cartilage and stabilize the joint better.

What's the difference between shoulder arthritis pain and a rotator cuff tear?

Arthritis pain is usually worse with certain movements and improves with rest, while rotator cuff tear pain is often constant and includes weakness—you may not be able to lift your arm at all. Many people have both conditions together, which makes diagnosis more complex. Imaging tests like MRI can show both cartilage damage and tendon tears.