PA32.5-7 | Arthritis (RA, OA, Gout) & Bone Morphology — Summary & Reflection

REFLECT

Before moving to the summary, take 5 minutes with these reflective questions. Write a one-sentence answer to each in your margin or notebook:

1. Mechanism vs Buzzword: You can recall 'pannus' as a RA term — but can you explain how a pannus forms from the cytokine cascade you read about? Trace it: HLA-DR4 → T cell → cytokine → synoviocyte → pannus. If any step is hazy, re-read the immunopathogenesis block.

1. The practical examiner's question: You are shown a slide with a large mass of giant cells. The examiner asks: 'Is this a giant-cell tumour or a granuloma?' What features would you use to distinguish them morphologically?

1. Clinical reasoning: A patient with gout has been started on allopurinol and three weeks later presents with a new acute attack. Is the drug failing? What explanation would you give the patient, and what should have been prescribed alongside it?

1. Self-assessment: Which of the six bone lesion patterns are you least confident identifying under the microscope? Plan to pull up that photomicrograph from Robbins or a digital atlas before your next practical session.

KEY TAKEAWAYS

Core takeaways from this SDL:

Rheumatoid Arthritis:
• Autoimmune; HLA-DR4 + citrullinated peptides → CD4+ T cells → TNF-α/IL-1/IL-6 cascade → pannus formation → cartilage and bone destruction
• Symmetric small-joint involvement; DIP joints SPARED; morning stiffness >1 hour
• Serology: RF (IgM anti-IgG Fc) + anti-CCP (specific); X-ray: osteopenia + erosions, NO osteophytes
• Extra-articular: rheumatoid nodules (fibrinoid necrosis + palisading macrophages), AA amyloidosis, vasculitis

Osteoarthritis:
• Degenerative; chondrocyte MMP activation → collagen/aggrecan loss → fibrillation → eburnation
• Asymmetric weight-bearing joints + DIP (Heberden's) / PIP (Bouchard's) nodes
• X-ray: osteophytes + subchondral sclerosis + cysts; NO erosions, NO osteopenia
• Micro: fibrillation, chondrocyte clusters (brood capsules), eburnation

Gouty Arthritis:
• Hyperuricemia → MSU crystal deposition → NLRP3 inflammasome → IL-1β → neutrophilic arthritis
• Needle-shaped, NEGATIVELY birefringent crystals; podagra (first MTP); radiolucent renal stones
• Tophus: MSU crystals + foreign-body giant cell granulomatous reaction
• Complications: urate nephropathy, nephrolithiasis, acute uric acid nephropathy

Bone Morphology Practical — Six Patterns:
1. Osteomyelitis → empty lacunae + sequestrum (dead bone inside involucrum)
2. Paget disease → mosaic cement lines (jigsaw pattern)
3. Osteosarcoma → malignant osteoid from pleomorphic cells (metaphysis, 10–20 yr)
4. GCT → osteoclast-type giant cells + similar stromal nuclei (epiphysis, 20–40 yr)
5. Ewing sarcoma → small round blue cells + CD99+ + t(11;22) (diaphysis, <20 yr)
6. Bone metastases → cohesive epithelial cells in bone; lytic (lung/kidney) or blastic (prostate)

Pattern-recognition shortcut: Location by age — Metaphysis/child = osteosarcoma; Epiphysis/young adult = GCT; Diaphysis/child = Ewing; Any bone/>40 yr = metastasis first.