Glossary
Plain-language terms, explained simply
Words you might come across on this site or from your clinician, explained in everyday language — no medical background needed.
Basic terms
Osteoarthritis (OA)
The most common type of arthritis. It happens when the cartilage cushioning a joint wears down and the tissues around it become irritated, causing pain and stiffness.
Cartilage
A smooth, rubbery tissue covering the ends of your bones inside a joint. It lets bones glide against each other without rubbing directly.
Joint
The place where two bones meet and move against each other — like your knee, hip, shoulder, or the joints in your fingers.
Synovium and synovial fluid
A thin lining inside your joint that produces a slippery fluid. It lubricates the joint, the way oil lubricates a hinge.
Inflammation
The body's response to irritation or injury — it can cause swelling, warmth, and pain. In OA, low-level inflammation inside the joint can contribute to cartilage breaking down.
Chondrocyte
The living cells inside cartilage. They build and maintain the tissue around them, and they're sensitive to things like blood sugar and body-wide inflammation, not just to joint movement.
Metabolic syndrome
A cluster of conditions — like extra weight around the middle, high blood sugar, and high blood pressure — that often occur together and raise the risk of several health problems, osteoarthritis among them.
Obesity
Having a higher amount of body fat than is considered healthy. It's linked to osteoarthritis both by adding load to joints and through the effects of fat tissue on the whole body.
Insulin resistance
When the body's cells don't respond as well to insulin, the hormone that helps move sugar out of the blood. Over time this can raise blood sugar, and it's linked to several conditions, including some forms of osteoarthritis.
Bone spur (osteophyte)
A small, smooth growth of extra bone that can form at the edge of a joint affected by osteoarthritis, often visible on an X-ray.
Subchondral bone
The layer of bone directly underneath cartilage. In osteoarthritis, this bone can change shape and density too, not just the cartilage above it.
Joint space narrowing
A term doctors use when looking at an X-ray to describe a smaller gap between two bones in a joint — usually a sign that the cartilage between them has thinned.
Your joints, plainly explained
Osteoarthritis can affect any joint, but a few show up most often. Here's what each one looks like on the inside.
Knee
Your knee isn't one simple hinge — it's really three cushioned surfaces in one, called compartments: the inner compartment, the outer compartment, and the one behind your kneecap. Osteoarthritis can affect one, two, or all three unevenly, which is part of why knee OA can feel so different from person to person.
Hip
Your hip is a ball-and-socket joint: the rounded top of your thigh bone sits inside a cup-shaped socket in your pelvis, both cushioned with cartilage. Because it's a deep, fairly enclosed joint, hip OA tends to affect the whole joint surface together, rather than in separate compartments the way the knee can.
Shoulder
Your shoulder is also a ball-and-socket joint, but a shallower one than the hip — built more for a wide range of motion than for bearing weight. Shoulder OA is less common than knee or hip OA, but it's a specific focus of our own lab's research into how diet and metabolism affect joint health.
Hands
Your hands have many small joints — at the base of the thumb and along each finger. Osteoarthritis here can look different than in larger joints, sometimes forming firm, bony bumps near the fingertips or middle joints, alongside stiffness.
How OA changes a joint
However it starts, osteoarthritis follows a broadly similar path inside a joint: cartilage gradually thins, the cells that maintain it come under more stress, low-level inflammation builds up, and the bone underneath can change shape. What's different about metabolic OA is recognizing that things happening elsewhere in the body — blood sugar, body-wide inflammation, the signals fat tissue sends out — can drive that same process, not just mechanical wear on the joint itself.
For the fuller explanation, see What Is Metabolic OA, or browse specific, evidence-rated questions in our Explainers library.