Osteoarthritis
- Osteoarthritis (OA) is "wear-and-tear" arthritis: the cartilage cushion thins out, and the bone reacts.
- The four classic radiographic findings spell the story: non-uniform joint-space narrowing, osteophytes, subchondral sclerosis, and subchondral cysts.
- It loves the weight-bearing and high-mileage joints — hips, knees, the base of the thumb, and the small finger joints.
- It is a productive arthritis (bone is built up), which is the visual opposite of the erosive pattern you see in rheumatoid arthritis.
- The plain radiograph is the workhorse. You rarely need fancy imaging to make the call.
Think of a joint as a door hinge that's been swinging for sixty years. The smooth coating wears thin, the metal starts grinding, and little burrs build up around the edges. That, in a sentence, is osteoarthritis — and the beautiful thing is that the X-ray shows you every part of that story if you know where to look.
What's actually going wrong
The star of a healthy joint is articular cartilage — the slick, rubbery cap on the end of each bone that lets the two surfaces glide past each other like buttered glass. Cartilage is invisible on a radiograph (it's basically water-density soft tissue), so we don't see the cartilage itself. We see the gap it holds open between the bones — the joint space.
In OA, that cartilage gradually wears down. As it thins, the gap narrows. Then the bone underneath, no longer politely cushioned, starts to react: it hardens, it sprouts new bony spurs, and it sometimes develops fluid-filled pockets. Your job is to read those reactions off the film.
The four findings (your mnemonic isn't needed — just remember the hinge)
Radiologists describe OA with four cardinal findings. Walk through the hinge analogy and they fall out naturally.
| Finding | Plain-English translation | The hinge analogy |
|---|---|---|
| Joint-space narrowing | The cartilage gap shrinks, and crucially it's non-uniform — worst where the load is heaviest. | The coating wears thin where the door drags. |
| Osteophytes | New bony spurs growing at the joint margins. | Burrs building up around the edges. |
| Subchondral sclerosis | The bone just under the cartilage gets denser and whiter. | The metal hardens where it grinds. |
| Subchondral cysts | Small lucent (dark) pockets in that same bone, just beneath the surface. | Tiny pits worn into the stressed metal. |
That word non-uniform is doing heavy lifting. In OA the joint space narrows asymmetrically — in the knee, often just the inner (medial) compartment, so the joint looks lopsided. In rheumatoid arthritis the inflammation bathes the whole joint, so the space narrows uniformly. The pattern of the narrowing is half the diagnosis.
Productive, not erosive
Here's the single mental hook that keeps OA straight from its cousins. OA is a productive arthritis — the bone's response is to build (osteophytes, sclerosis, the whole "more bone" theme). The inflammatory arthritides like rheumatoid arthritis are erosive — they eat bone away and leave the surrounding bone looking thin and washed-out.
So when you put an OA film and an RA film side by side, OA looks like the bone has been doing extra credit — denser, spikier, busier — while RA looks like something has been nibbling at it. (This build-up-vs-tear-down split is worth its own deep dive in erosive vs productive patterns.)
A bony spur is not automatically arthritis, and "a little OA" on a report is the radiology equivalent of "a little gray hair" — extremely common with age and often not the cause of the patient's pain. Always tie the imaging to the symptoms before blaming OA for everything.
Where it shows up
OA is a geography lesson. It favors the joints that either carry your weight or get heavy daily use:
- Hips and knees — the big weight-bearers.
- First carpometacarpal joint — the base of the thumb, a classic, often making a "squared" look.
- Distal and proximal interphalangeal joints — the finger-tip and middle finger joints.
- Spine — the facet joints and disc spaces (technically spondylosis, OA's spinal cousin).
The distribution often clinches it. RA classically targets the knuckle (MCP) joints and the wrist while sparing the very tip joints; OA does the reverse, picking off the small finger joints closest to the nail. When someone hands you a hand film, ask "which rows of joints are involved?" before anything else.
A special flavor worth knowing: erosive (inflammatory) osteoarthritis, which hits the finger interphalangeal joints and produces a central erosion that gives a "gull-wing" appearance — a reminder that biology rarely respects our tidy categories.
How we image it
For the vast majority of OA, a plain radiograph is all you need, and for the knee a weight-bearing view is the standard — load the joint and the true narrowing reveals itself, much like a tire only looks flat once the car's parked on it. MRI gets involved when there's a question of something else — a cartilage defect, a meniscal tear, or marrow changes — not to diagnose garden-variety OA. CT mainly helps in complex anatomy like the spine or for surgical planning. If you ever see OA layered on top of thinned-out bone, it's worth remembering OA and metabolic bone disease can coexist and confuse the picture.
The one-line takeaway
If you remember nothing else: OA narrows the joint unevenly and makes more bone — spurs, sclerosis, and cysts — because it's a wear-and-tear, productive process. Spot the busy, lopsided joint, and you've got it.