Cancer can indeed spread to the joints, but this usually happens indirectly through the spread of cancer to the bones near or within the joint rather than the joint tissue itself being a primary site of metastasis. When cancer cells travel from their original location to other parts of the body, a process called metastasis, they often target the bones, which can then affect the joints due to their close anatomical relationship.
The bones are among the most common sites for cancer metastasis after the lungs and liver. Cancers such as breast, prostate, lung, kidney, and thyroid frequently spread to the skeleton. When cancer cells invade bone tissue, they disrupt the normal balance of bone remodeling, which involves bone breakdown by cells called osteoclasts and bone formation by osteoblasts. This disruption can lead to two main types of bone metastases: osteolytic, where bone is destroyed, and osteoblastic, where abnormal new bone forms. Sometimes, both processes occur simultaneously, especially in breast cancer.
Bone metastases can cause significant damage to the bone structure, leading to pain, fractures, and impaired function. Since joints are formed by the ends of bones, when the bones around a joint are affected by metastatic cancer, the joint can become painful, swollen, and less mobile. This can mimic joint disease symptoms, but the underlying cause is the cancer in the bone rather than the joint tissue itself.
The spread of cancer to bone involves complex biological interactions. For example, breast cancer cells can hijack certain immune cells in the bone marrow to obtain iron, which helps them survive and grow in the bone environment. This process also disrupts normal blood cell production in the bone marrow. Such adaptations allow cancer cells to thrive in the bone and contribute to the progression of metastatic bone disease.
Patients with bone metastases often experience bone pain that can be dull, aching, or sharp if a fracture or nerve compression occurs. The pain may worsen with movement and can be severe enough to disrupt sleep. In addition to pain, bone metastases can weaken bones, leading to pathological fractures—breaks that occur with minimal trauma because the bone is compromised by cancer.
The joints themselves are less commonly the direct site of cancer spread because joint tissues like cartilage have limited blood supply, making them less accessible to circulating cancer cells. However, cancer can invade the joint space if it extensively involves the adjacent bone or soft tissues. This can cause joint swelling, inflammation, and reduced range of motion.
In summary, while cancer does not commonly spread directly to the joint tissues, it frequently metastasizes to bones near joints, which can secondarily affect joint function and cause symptoms similar to joint disease. The involvement of bone metastases near joints is a significant clinical concern because it leads to pain, fractures, and disability, requiring careful management to maintain quality of life.