What is the result of the breakdown of joint cartilage and underlying bone called?

The breakdown of joint cartilage and the underlying bone is specifically referred to as osteoarthritis. This condition is characterized by the degeneration of cartilage, which affects movement and can cause pain due to the friction between bones as the protective cartilage diminishes. Osteoarthritis typically occurs with age or due to joint injury and is often seen in weight-bearing joints like the knees and hips.

In contrast, arthritis is a broader term that encompasses various disorders causing inflammation in the joints, and rheumatism refers to a range of conditions affecting the musculoskeletal system. Osteoporosis, on the other hand, relates to a decrease in bone density, which leads to weak and brittle bones, but does not specifically involve the breakdown of cartilage in the joints. Therefore, osteoarthritis is the most precise term for the condition described in the question.