OA vs RA vs JIA: The Inflammatory Split

The Inflammatory Split: Three Questions, One Diagnosis

Where does it hurt, how long is the morning stiffness, and what do the film and blood say? Run the three-question reflex on every joint case and the arthritis vignettes write themselves.

The first question is distribution, and the two hand maps are mirror opposites. Osteoarthritis lives in the DIP joints (Heberden nodes), the PIP joints (Bouchard nodes), the first CMC joint, the knees, the hips, and the first MTP joint, and it spares the MCP and wrist. Rheumatoid arthritis lives in the MCP, PIP, and wrist joints symmetrically, plus the MTP joints of the feet, and it spares the DIP joints. When the boards describe a square thumb base or a DIP bump, they are handing you OA; when they describe boggy symmetric wrist and knuckle swelling, they are handing you RA.

The second question is the stiffness clock. OA stiffens for under 30 minutes in the morning, a brief gelling that burns off with movement, and its pain is worse with use and better with rest. Inflammatory arthritis stiffens for more than an hour and loosens as the synovium thaws through activity, with fatigue riding along. Children with JIA run the same clock: over an hour of morning stiffness and a limp after naps.

The third question is the film and the blood panel. OA shows joint space narrowing, osteophytes, subchondral sclerosis (eburnation), and subchondral cysts, with a silent serology. RA shows periarticular osteopenia and marginal erosions at the bare areas, with rheumatoid factor and anti-CCP, the antibody that is far more specific and predicts erosive disease. Erosions are the rheumatoid stamp; osteophytes are the OA one.

Flip between the two diseases and hold the split.

Paint the Pattern: Distribution Is the First Discriminator

Tap the joints on the body map and read what each region announces. Four patterns, one body: the diagnosis narrows before a single lab returns.

The distribution is the loudest feature in every arthritis stem, and symmetry is the part students miss. Symmetry is an inflammatory flag: RA paints both hands, both wrists, both feet; OA is happy to live in one first CMC or one knee. The four patterns boards reuse: OA paints the DIP joints, first CMC joints, knees, hips, and first MTP; RA paints the MCP, PIP, and wrist joints symmetrically; oligoarticular JIA paints exactly one knee in a young child; polyarticular JIA paints the adult RA map in a child. When the pattern is asymmetric and one joint is involved, think crystal disease, trauma, or infection before anything autoimmune.

Keep the stiffness clock running while you paint. The same map can look mechanical in a 66-year-old with 10 minutes of gelling and inflammatory in a 44-year-old with 90 minutes of stiffness; the clock decides which disease the map belongs to. On boards, the vignette hands you both the map and the clock, and the answer falls out of the combination.

Tap each region and read what it announces.

Osteoarthritis: The Wear-and-Tear Joint

The joints that carry load and pinch cartilage wear down; the pain follows use, and the morning stiffness is a short gelling, never an hour. Learn the joints it loves, the film it makes, and the management ladder that starts with weight loss and exercise.

OA is mechanical, not autoimmune. In the hands it makes the Heberden nodes at the DIP joints and Bouchard nodes at the PIP joints, and arthritis of the first carpometacarpal joint gives the square hand with pain on pinch and grip. In the lower body it hits the knees, hips, and first MTP joints: knee pain on stairs and squatting with medial compartment narrowing, hip pain in the groin and anterior thigh with a limp and loss of internal rotation, and the bunion joint that boards love to name. OA spares the MCP and wrist, and that one fact answers more questions than any lab: if the MCP or wrist is swollen, the story is rheumatoid, not wear and tear.

The pain rule is mechanical: worse with use, better with rest, with a brief morning gelling of under 30 minutes. Risk accumulates with age, obesity, female sex, prior trauma, and repetitive occupational load, and obesity is the modifiable heavyweight because each kilogram multiplies into several kilograms of force across the knee with every step. The film reads like a sentence: joint space narrowing (lost cartilage), osteophytes (the bone stabilizing a loose joint), subchondral sclerosis or eburnation (polished bone), and subchondral cysts (geodes where fluid is forced into bone). What is absent matters as much: no marginal erosions, no periarticular osteopenia, no positive serology.

Management is a mechanical ladder with no disease-modifying rung: weight loss and exercise first (the quadriceps unloads the knee), then acetaminophen and topical NSAIDs, then oral NSAIDs and intra-articular steroids for flares, and total joint arthroplasty for refractory pain. There is no OA equivalent of methotrexate, and the boards test that asymmetry: the arthritis with a disease-modifying drug is RA, the one without is OA.

Read the stamps and name the disease.

Rheumatoid Arthritis: The Symmetric Story

Symmetric MCP, PIP, and wrist synovitis with more than an hour of morning stiffness, a blood panel that names the disease, and a DMARD ladder that starts at diagnosis because erosions do not wait.

RA is the autoimmune arthritis of middle age: symmetric synovitis of the MCP, PIP, and wrist joints (and the MTP joints of the feet), sparing the DIP joints, with more than an hour of morning stiffness, fatigue, and low-grade systemic inflammation. Years of untreated synovitis write the deformities boards photograph in words: ulnar drift, swan-neck (PIP hyperextension with DIP flexion), boutonniere (PIP flexion with DIP hyperextension), and MCP subluxation. The serology pairs rheumatoid factor (IgM against the Fc of IgG, sensitive but leaky) with anti-CCP, which is far more specific and predicts erosive disease. The film shows periarticular osteopenia first, then marginal erosions at the bare areas, and a large share of erosive damage accumulates within the first two years, which is why the clock starts at diagnosis.

RA is systemic, and boards test the organs: rheumatoid nodules over extensor surfaces and in the lung, interstitial lung disease, pericarditis, scleritis, Felty syndrome (long-standing seropositive RA with splenomegaly and neutropenia, carrying serious infection risk), and C1-C2 atlantoaxial subluxation, which must be screened before any intubation. The treatment ladder is mechanism after mechanism: methotrexate plus folic acid at diagnosis, adding an anti-TNF agent (etanercept, infliximab, adalimumab) for inadequate response, then switching mechanism to abatacept (CTLA-4-Ig T-cell costimulation), rituximab (anti-CD20 B-cell depletion), or tocilizumab (anti-IL-6 receptor). Hydroxychloroquine and sulfasalazine serve mild disease, and hydroxychloroquine carries its own watch: baseline and annual retinal toxicity screening, because the maculopathy is irreversible. Steroids bridge while the DMARD takes hold; they never replace it, and methotrexate is teratogenic, so contraception is part of the prescription for women and men planning families.

Put the RA ladder in order.

JIA: The Pediatric Cast

Chronic arthritis in a child splits three ways: the one-knee girl, the symmetric small-joint teenager, and the fever-rash-ferritin storm. The uveitis is the silent partner in every subtype.

Juvenile idiopathic arthritis means chronic arthritis (more than 6 weeks) before age 16, and the first split is joint count. Oligoarticular JIA is four or fewer joints, most classically a single swollen knee in an ANA-positive girl between 1 and 6 years old, with a cool, mildly warm joint rather than a fiery one. Its danger is not the knee: chronic anterior uveitis that is bilateral, insidious, and asymptomatic, with no red eye, no pain, and no vision complaint until synechiae and vision loss are already there. That is why the highest-risk group gets serial slit-lamp examinations as often as every 3 months: the schedule, not the symptoms, protects the eyes.

Polyarticular JIA is five or more joints with the adult RA map in a child: symmetric MCP, PIP, wrist, and knee disease with hour-long morning stiffness. The RF-positive subset behaves like seropositive adult RA, often with anti-CCP and erosions, and is treated the same way: methotrexate first, then anti-TNF. Systemic-onset JIA (Still disease) is the storm: quotidian fevers spiking daily, often to 39 to 40 C in the evening and returning to baseline, a salmon-pink evanescent rash that comes and goes with the fever, hepatosplenomegaly, lymphadenopathy, and serositis, with ferritin often 10 times normal. The feared complication is macrophage activation syndrome: falling cell counts with a persistently high ferritin and persistent fever, treated as an emergency. Systemic disease responds to NSAIDs and glucocorticoids with IL-1 blockade (anakinra) for control. Chronic synovitis near growth plates also distorts growth itself: a lengthened limb (leg length discrepancy) or, with TMJ disease, micrognathia.

Score the JIA subtype from the clues.

The Inflammatory Discriminator

Distribution, stiffness clock, and labs: three questions, one tree, and every arthritis vignette on boards is one of its branches.

Run the fork in order. Who has the arthritis and where does it hurt? An adult with DIP, first CMC, knee, hip, or first MTP pain walks the mechanical branch; an adult with symmetric MCP, PIP, and wrist swelling walks the autoimmune branch; a child walks the pediatric branch. What does the stiffness clock and the film say? Under 30 minutes with osteophytes and eburnation is OA; more than an hour with anti-CCP, marginal erosions, and periarticular osteopenia is RA, and the treatment consequence is a DMARD, not an exercise plan. In the child: a single cool knee for weeks in an ANA-positive girl is oligoarticular JIA with slit-lamp screening; symmetric small joints in a teenager is polyarticular JIA; quotidian fevers with a salmon rash and ferritin through the roof is Still disease. And the trap door: one hot, red, immobile joint with fever in any child is septic arthritis until the tap says otherwise, and JIA waits its turn.

The discriminator also catches the overlap disease. OA and RA can coexist in an older adult, and the signal that a second disease has arrived is a pattern change: new symmetric MCP and wrist swelling, the stiffness clock crossing an hour, and rising inflammatory markers on top of the old DIP nodes and knee crepitus. Never assume new MCP and wrist disease is wear and tear; the DIP pattern does not protect the other joints.

Route each presentation through the tree.

The Extra-articular and Subtype Cast

RA visits organs, and JIA hides its worst complication in the eye. Hold the whole cast on one screen: SANTA, the slit-lamp schedule, and the one-line split of every subtype.

RA is a systemic disease, and the boards hand you the organ complications by name. Felty syndrome is the triad of long-standing seropositive RA, splenomegaly, and neutropenia, and its consequence is serious infection: fatigue and two recent infections requiring antibiotics in a rheumatoid patient is Felty until proven otherwise. Remember the cast as SANTA: Splenomegaly, Anemia, Neutropenia, Thrombocytopenia, Arthritis. The rest of the organ tour: subcutaneous and pulmonary nodules, interstitial lung disease, pericarditis, scleritis, and the cervical spine warning: C1-C2 atlantoaxial subluxation in long-standing disease, screened before intubation because neck extension during the airway can injure the cord.

JIA hides its worst complication in the eye. The stiffness clock runs the same in children as adults, and the subtype cast is short: oligoarticular is the one-knee ANA-positive girl whose asymptomatic uveitis needs slit-lamp exams as often as every 3 months; polyarticular is the adult RA map in a child, with the RF-positive subset behaving like seropositive adult RA; systemic Still is the quotidian fever, evanescent rash, and ferritin storm, watched for macrophage activation syndrome. One mnemonic holds the split together: OA gels, RA gloats, under 30 minutes of gelling versus more than an hour of stiffness that loosens with movement.

Tap each letter of the Felty cast.

Open each row of the cast.

Walkthrough: race the stiffness clock

Original practice scenarios, one at a time. Choose an answer, then open any option to work its reasoning.