A 58-year-old woman with primary osteoarthritis of the knee is compared with a 58-year-old woman with rheumatoid arthritis of the same joint. Which feature best distinguishes osteoarthritis from rheumatoid arthritis?

Explanation

## Distinguishing Features of OA vs RA **Key Point:** Osteoarthritis is characteristically **asymmetric and monoarticular or oligoarticular**, while rheumatoid arthritis is **symmetric and polyarticular**. This is the single most useful clinical discriminator. ### Comparative Table | Feature | Osteoarthritis | Rheumatoid Arthritis | | --- | --- | --- | | **Joint pattern** | Asymmetric, monoarticular/oligoarticular | Symmetric, polyarticular (hands, wrists, knees) | | **Morning stiffness** | <30 minutes | >1–2 hours | | **ESR/CRP** | Normal or mildly elevated | Markedly elevated | | **Rheumatoid factor** | Negative | Positive (70–80%) | | **Onset** | Insidious, gradual | Acute or subacute | | **Age of onset** | >45 years (degenerative) | 30–50 years (inflammatory) | | **Systemic symptoms** | Absent | Present (fever, malaise, weight loss) | ### Why Asymmetry Matters **High-Yield:** In OA, the disease process is **mechanical wear-and-tear** affecting joints unequally based on prior trauma, biomechanical stress, and local cartilage loss. A patient may have severe OA in the right knee (previous meniscectomy) but minimal changes in the left knee. In RA, the **systemic autoimmune process** triggers symmetric inflammation of synovial joints bilaterally. Both knees, both wrists, both hands are typically affected in a mirror-image pattern. **Clinical Pearl:** A patient presenting with pain in the right knee only, with normal inflammatory markers and negative rheumatoid factor, almost always has OA. Conversely, a patient with bilateral hand swelling, morning stiffness >1 hour, and elevated ESR/RF has RA until proven otherwise. ### Why Other Options Are Incorrect - **Morning stiffness >2 hours:** Seen in RA, not OA. OA stiffness is brief (<30 min) and improves with movement. - **Elevated ESR:** Non-specific; occurs in both, but is **marked in RA** and mild/normal in OA. - **Rheumatoid factor:** Specific for RA, not OA. However, 20–30% of RA patients are seronegative, so absence of RF does not exclude RA. [cite:Harrison 21e Ch 326]