## Most Common Sites of Joint Involvement in RA **Key Point:** The **proximal interphalangeal (PIP) joints** are the most commonly involved joints in rheumatoid arthritis, followed by the metacarpophalangeal (MCP) joints and wrists. DIP joints are characteristically **spared**. ### Pattern of Joint Involvement | Joint Site | Frequency | Characteristic Finding | |---|---|---| | **PIP joints** | Most common (~95%) | Boutonnière deformity, fusiform swelling | | **MCP joints** | Very common (>90%) | Ulnar deviation, volar subluxation | | **Wrist** | Very common (>80%) | Radial deviation, carpal subluxation | | **DIP joints** | Rare (<5%) | Typically spared in RA | | **Knees / Ankles** | Common | Secondary systemic involvement | | **MTP joints (feet)** | Common (>90%) | Early involvement, metatarsalgia | **High-Yield:** DIP joint involvement is a **red flag** that should prompt reconsideration of the diagnosis — osteoarthritis or psoriatic arthritis should be considered instead of RA. ### Clinical Pearl The classic hand deformities in RA arise primarily from PIP and MCP involvement: - **Swan-neck deformity** = PIP hyperextension + DIP flexion (PIP-driven) - **Boutonnière deformity** = PIP flexion + DIP hyperextension (PIP-driven) - **Ulnar deviation** = MCP ulnar drift **Mnemonic: "PIP first, MCP second, DIP never"** — captures the hierarchy of small-joint involvement in RA. ### Why This Matters The predilection for PIP joints (and MCP joints) is so characteristic that: 1. It is embedded in the **2010 ACR/EULAR diagnostic criteria** for RA (small-joint involvement scores highest) 2. Symmetric PIP swelling with morning stiffness >1 hour is highly suggestive of RA 3. Sparing of DIP joints helps differentiate RA from osteoarthritis and psoriatic arthritis [cite: Harrison's Principles of Internal Medicine, 21e, Ch 326; Kelley & Firestein's Textbook of Rheumatology, 10e]
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