A 62-year-old woman from Delhi presents with a 3-year history of progressive pain in both knees, worse with climbing stairs and prolonged standing. On examination, she has mild valgus deformity of the right knee, crepitus on flexion-extension, and restricted range of motion. X-ray of both knees shows joint space narrowing, osteophytes at the margins, and subchondral sclerosis. She denies any history of trauma, inflammatory symptoms, or morning stiffness. What is the most likely diagnosis and what is the primary pathological hallmark of this condition?

Explanation

## Diagnosis: Primary Osteoarthritis (OA) **Key Point:** The clinical presentation—bilateral knee involvement, mechanical pain (worse with activity, relieved by rest), absence of morning stiffness or systemic symptoms, and radiological findings of joint space narrowing, osteophytes, and subchondral sclerosis—is pathognomonic for osteoarthritis. ### Pathological Hallmarks of OA **High-Yield:** The primary pathological feature is **irreversible loss of hyaline articular cartilage** due to: 1. Cartilage matrix degradation (collagen type II and proteoglycan loss) 2. Chondrocyte dysfunction and apoptosis 3. Failure of cartilage repair mechanisms ### Radiological Features (Kellgren-Lawrence Grade 2–3) | Finding | Pathology | |---------|----------| | Joint space narrowing | Cartilage loss | | Osteophytes (bone spurs) | Marginal bone proliferation | | Subchondral sclerosis | Increased bone density beneath cartilage | | Subchondral cysts | Synovial fluid intrusion into bone | **Clinical Pearl:** The absence of morning stiffness (which typically lasts <30 minutes in OA vs. >1 hour in rheumatoid arthritis) and the mechanical nature of pain (activity-related, not inflammatory) distinguish OA from inflammatory arthropathies. **Mnemonic: LOSS** — Loss of cartilage, Osteophytes, Sclerosis, Space narrowing (radiological triad). ### Why OA, Not Other Conditions? - **Not RA:** No systemic inflammatory markers expected; no morning stiffness; no symmetrical small joint involvement; no pannus on imaging - **Not spondyloarthropathy:** No axial involvement, enthesitis, or HLA-B27 association mentioned - **Not gout:** No acute inflammatory episodes, tophi, or crystal arthropathy history [cite:Robbins 10e Ch 26]