💎 Crystal Arthropathies are a group of joint diseases caused by deposition of crystals in or around joints. They include Gout (monosodium urate), Pseudogout (calcium pyrophosphate dihydrate, CPPD), and rarer forms such as Calcium hydroxyapatite and Basic calcium phosphate deposition disease. These conditions mimic septic arthritis and must be recognised promptly.

🧬 Pathophysiology

Gout: Hyperuricaemia → monosodium urate crystal precipitation in joints → intense neutrophil-driven inflammation.
Pseudogout (CPPD): CPPD crystals deposit in cartilage → chondrocalcinosis → acute arthritis (often knees, wrists).
Hydroxyapatite/BCP: Tendon and periarticular deposition → calcific tendinitis, destructive arthropathy (e.g., Milwaukee shoulder).

📋 History Taking

Onset: Sudden (gout/pseudogout) vs chronic (hydroxyapatite, BCP).
Joints:
- 1st MTP (podagra) classic for gout.
- Knee, wrist, shoulder more common in pseudogout.
Associated symptoms: Fever, malaise, kidney stones (gout), recurrent attacks.
Risk factors:
- Gout – alcohol, red meat, obesity, diuretics, renal disease.
- Pseudogout – age, haemochromatosis, hyperparathyroidism, hypomagnesaemia.

💎 Crystal Arthropathies — Key Comparison


Condition	Crystal & Microscopy	Typical Joints	Notes / Risk Factors
🟥 Gout	Monosodium urate Needle-shaped, ⬇️ birefringent	1st MTP (podagra), ankle, knee	Hyperuricaemia, CKD, diuretics, alcohol, high purine diet
🟦 Pseudogout (CPPD)	Calcium pyrophosphate Rhomboid, ⬆️ birefringent	Knee (most common), wrist, shoulder	Elderly, haemochromatosis, hyperparathyroidism, hypomagnesaemia
🟩 BCP (Hydroxyapatite)	Calcium hydroxyapatite Not seen on light microscopy	Shoulder (Milwaukee shoulder), hip	Elderly, degenerative joints
🟨 Others (rare)	Xanthine, oxalate, cystine	Variable	Metabolic disorders, renal failure

🩺 Examination

Acute red, hot, swollen joint – may mimic septic arthritis.
Severe tenderness, limited range of motion.
Chronic gout – look for tophi in ears, elbows, fingers.
Systemic exam: assess for renal disease, metabolic disorders.

⚖️ Differential Diagnosis

Septic arthritis (must be excluded!)
Osteoarthritis (especially in chronic CPPD).
Rheumatoid arthritis or spondyloarthritis.

🔎 Investigations

Joint aspiration + polarised microscopy:
- Gout → negatively birefringent, needle-shaped urate crystals.
- Pseudogout → positively birefringent, rhomboid CPPD crystals.
Bloods: Uric acid (may be normal during attack), ESR/CRP raised, metabolic screen (calcium, phosphate, Mg, ferritin).
Imaging:
- Gout → “punched-out” erosions with overhanging edges.
- CPPD → chondrocalcinosis on X-ray (linear cartilage calcification).
- Hydroxyapatite → calcific deposits in tendons (shoulder).

💊 Management

Acute Gout:
- NSAIDs (ibuprofen, naproxen, indomethacin) unless contraindicated.
- Colchicine (500 mcg 2–3×/day until relief).
- Corticosteroids oral/intra-articular if resistant or contraindications.
Chronic Gout:
- Allopurinol (start 100 mg, titrate to urate <360 μmol/L).
- Febuxostat alternative if allopurinol not tolerated.
- Prophylaxis with colchicine during initiation to prevent flares.
- Lifestyle: weight loss, ↓ purine foods, ↓ alcohol.
Pseudogout (CPPD):
- NSAIDs or colchicine for flares.
- Steroid injections for single-joint attacks.
- Treat underlying cause (e.g., haemochromatosis, hyperparathyroidism).
Hydroxyapatite/BCP deposition:
- NSAIDs, local steroid injections.
- Physiotherapy to restore movement.
- Occasionally surgical removal of deposits.

🌟 Key points: • Crystal diagnosis = joint aspiration under polarised light. • Always rule out septic arthritis in an acutely inflamed joint. • Long-term management is preventative: urate-lowering therapy in gout, metabolic correction in pseudogout.

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Crystal arthropathy