The sternoclavicular joint (SC joint) is the joint between the sternum (breastbone) and the clavicle (collarbone). Pain in this joint can be caused by various conditions such as injury, inflammation, or degenerative changes.

Here are some possible causes of sternoclavicular joint pain:

1. Injury: A direct blow to the chest or shoulder, a fall on an outstretched arm, or a sudden twisting of the arm can cause injury to the SC joint, leading to pain and swelling.

2. Arthritis: Arthritis of the SC joint can result from wear and tear, infection, or autoimmune diseases like rheumatoid arthritis. This can lead to pain, stiffness, and swelling in the joint.

3. Infection: In rare cases, the SC joint can become infected, leading to pain, redness, and swelling in the area. This is usually associated with fever and other signs of infection.

4. Tumors: Tumors in the chest can sometimes spread to the SC joint, causing pain and swelling.

5. Other conditions: Other conditions that can cause sternoclavicular joint pain include osteoporosis, osteomyelitis, and costochondritis.

Treatment for sternoclavicular joint pain depends on the underlying cause. Rest, ice, and over-the-counter pain medications can help relieve pain and inflammation. In some cases, physical therapy or corticosteroid injections may be necessary. Surgery may be required in rare cases where conservative treatment fails to provide relief or in cases of severe injury or degenerative changes. It's important to consult with a healthcare professional for an accurate diagnosis and appropriate treatment plan.