Understanding the Anatomy of Knee Arthritis

The knee joint is formed by the femur, tibia and patella, cushioned by articular cartilage, menisci, and lubricated by synovial fluid within the synovium. Osteoarthritis is the progressive breakdown of that cartilage plus underlying (subchondral) bone changes — primary OA has no single known cause beyond age-related wear, while secondary OA follows a specific injury or condition.

The basic structure of the knee

The knee joint connects the femur (thighbone), tibia (shinbone) and patella (kneecap). Smooth articular cartilage covers the ends of these bones, allowing them to glide against each other with minimal friction. Two C-shaped menisci act as shock absorbers between the femur and tibia, while the synovium — a thin membrane lining the joint — produces synovial fluid that lubricates and nourishes the cartilage.

What actually happens in osteoarthritis

Knee osteoarthritis is a degenerative joint disease involving progressive loss of articular cartilage, and it is now understood to affect the entire joint — not just cartilage, but also subchondral bone, the joint capsule, the synovial membrane, the menisci, ligaments, and the surrounding muscles and tendons.

Clinically, primary knee osteoarthritis refers to cartilage degeneration without a specific identifiable cause, generally attributed to age-related wear; secondary knee osteoarthritis results from cartilage degeneration due to a known cause, such as a prior injury.

Who is affected, and how common it is

Depending on the data source, roughly 13% of women and 10% of men aged 60 and older have symptomatic knee osteoarthritis, and among people older than 70, prevalence rises to as high as 40%. Not everyone with radiographic (X-ray) evidence of osteoarthritis experiences symptoms, which is why diagnosis combines imaging with reported symptoms.

Scientific sources and experts

• Knee Osteoarthritis — StatPearls, NCBI Bookshelf (NIH) — https://www.ncbi.nlm.nih.gov/books/NBK507884/

• NIAMS (National Institute of Arthritis and Musculoskeletal and Skin Diseases), patient/educational joint-anatomy resource — https://www.niams.nih.gov/health-topics/educational-resources/health-lesson-learning-about-joints

FAQ

What parts of the knee are affected by osteoarthritis?

All of them to some degree — cartilage, subchondral bone, the synovial membrane, menisci, ligaments, and surrounding muscles and tendons, though cartilage loss is the defining feature.

What causes cartilage breakdown in the knee?

Primary osteoarthritis has no single specific cause and is linked to age-related wear; secondary osteoarthritis follows a known cause such as a previous injury.

Is knee osteoarthritis the same as rheumatoid arthritis?

No — osteoarthritis is a degenerative, wear-related joint condition, while rheumatoid arthritis is an autoimmune inflammatory disease. Their morning-stiffness patterns typically differ too — see our related article on morning knee stiffness.

How common is knee osteoarthritis by age?

Roughly 13% of women and 10% of men over 60 have symptomatic knee osteoarthritis, rising to as high as 40% of people over 70.