Articular cartilage is the smooth, white tissue that covers the ends of bones inside a joint. It helps the knee move with low friction and spreads forces during walking, stairs, and sports. In the knee, it lines the femur (thighbone), tibia (shinbone), and the underside of the patella (kneecap). Clinicians talk about Articular cartilage when evaluating knee pain, swelling, mechanical symptoms, and arthritis.

Popliteal fossa is the shallow, diamond-shaped hollow at the back of the knee. It is an anatomical “passageway” that carries major nerves and blood vessels between the thigh and lower leg. Clinicians use it as a landmark during knee exams, ultrasound, and some procedures. It is also a common site for swelling, cysts, vascular problems, and nerve-related symptoms.

Baker’s cyst is a fluid-filled swelling that forms behind the knee. It is also called a popliteal cyst because it sits in the popliteal fossa (the hollow at the back of the knee). It commonly appears when the knee joint is irritated by arthritis or a meniscus injury. The term is used in orthopedics, sports medicine, radiology, and physical therapy to describe a specific pattern of knee-related swelling.

Pes anserine bursa is a small, fluid-filled sac on the inner (medial) side of the knee. It sits between the shinbone (tibia) and the tendons that attach there from three thigh muscles. Its plain purpose is to reduce friction so tissues can glide smoothly during movement. It is commonly discussed when evaluating inner-knee pain, especially “pes anserine bursitis.”

The Infrapatellar bursa is a small, fluid-filled sac near the front of the knee. It helps tissues glide smoothly where skin, tendon, and bone move against each other. It sits below the kneecap (patella), close to the patellar tendon and upper shinbone (tibia). It is commonly discussed when evaluating front-of-knee pain and swelling.

Prepatellar bursa is a small, fluid-filled sac located in front of the kneecap (patella). It helps tissues glide smoothly when the front of the knee moves or contacts surfaces. Clinicians most often discuss it when evaluating swelling or pain over the kneecap. It is commonly referenced in conditions like prepatellar bursitis (“housemaid’s knee”).

A Bursa is a small, fluid-filled sac that helps tissues glide smoothly over each other. It is most commonly discussed in orthopedics when pain comes from friction around a joint. In the knee, bursae sit near the kneecap, tendons, and prominent bony areas. When a bursa becomes irritated or infected, it can become painful and swollen.

The Joint capsule is the soft-tissue envelope that surrounds a synovial joint, including the knee. It helps hold the joint together while still allowing controlled motion. It contains the synovial lining that supports joint lubrication and nutrition. Clinicians talk about the Joint capsule when assessing swelling, instability, stiffness, or injuries around a joint.

Synovial membrane is a thin, specialized lining found inside many joints, including the knee. It produces synovial fluid, which helps joints move smoothly. It also plays a role in joint nutrition and inflammation. Clinicians discuss it often when evaluating swelling, arthritis, or unexplained knee pain.

Synovium is the thin, living lining inside many joints, including the knee. It makes synovial fluid, which helps joints move smoothly. It also helps regulate the joint environment by supporting cartilage nutrition and clearing debris. Clinicians commonly discuss Synovium when evaluating joint swelling, inflammation, or unexplained knee pain.