Signs of arthritis in the knee include progressive or sudden pain, inflammation, weakness and stiffness in the knee joint, according to the American Academy of Orthopaedic Surgeons' OrthoInfo website. The affected individual may also have a buckling feeling in the knee or difficulty straightening and bending the joint.
Individuals with arthritis in the knee may notice warmth or redness around the joint, according to OrthoInfo. When loose fragments of cartilage interfere with the joint's motion, the knee may also lock or stick, and individuals may notice a creaking, grinding or snapping sound during movement. Some individuals may also notice that pain and other symptoms flare up in rainy weather, in the morning, after sitting or after vigorous activity. Individuals who notice any degree of knee pain or other unusual symptoms should make an appointment with a doctor for diagnosis. Common diagnostic tests include a physical examination, blood tests and imaging tests.
Although there are more than 100 different types of arthritis, the major forms that affect the knee joint are osteoarthritis, rheumatoid arthritis and post-traumatic arthritis, reports OrthoInfo. Osteoarthritis is the most common form of arthritis in the knee, normally affecting individuals over age 50 and producing pain that worsens over time. Rheumatoid arthritis generally affects both knee joints and includes pain, stiffness and swelling, while post-traumatic arthritis develops after an injury.