In just about every form of arthritis, the loss of bone or cartilage results in changes to the shape of a joint. In osteoarthritis, the damage to cartilage and bone interrupts the normal function of the joint. A healthy joint depends on a balance between an injury-and-repair process that occurs from the work of the chondroblasts and chondroclasts. In osteoarthritis, this balance is disrupted. The disease process causes cells called osteoblasts to form new bone and cartilage in places where it shouldn't, and this leads to pain and swelling.
Pitting and Fraying of Cartilage
The osteoarthritis disease process limits the ability of the cartilage's cells to release the amount of protein needed to repair and preserve cartilage in the joints. This means that attempts at repair become overwhelmed, and the normally firm cartilage begins to soften. Over time, it becomes frayed and cracked and also develops pits. This wearing away causes cartilage to lose some of its elasticity and protective surface. It also increases the likelihood that the joint structure will deteriorate over time, and, if an injury happens to such a joint, even greater damage occurs.
Deterioration of Cartilage
Eventually, the pits and cracks in the cartilage deepen further and cause large areas to deteriorate and wear away completely. This allows the 2 bone surfaces to rub together. With time, these surfaces become smooth and polished from the constant grating upon each other. The bones lose their ability to fit smoothly together, which keeps them from moving easily.
Loss of Bone Shape
As the cartilage continues to wear away, attempts at repair continue. The joint begins to lose its normal shape, and the space between the joint narrows. In addition, bony growths called osteophytes, or spurs, begin to form where the ligaments and joint capsule attach to the bone. Fluid-filled cysts also often form in the bone and near the joints. In addition, fragments of bone and cartilage may also break off from the damaged joint surface and float freely in the joint space.
The Impact of Osteoarthritis
Together, these changes lead to joint pain and a decrease in the ability to move a joint within its full range, known as a joint's range of motion. The breakdown of cartilage and bone may also result in inflammation. A joint with osteoarthritis is larger than a normal healthy joint, which leads to its swollen and enlarged look.