- Why it happens: Bacteria invade the joint (often via cut, surgery, or bloodstream).
- Red flags:
- 🔥 Fever + chills (even low-grade)
- 🌡️ Skin hot to the touch (like a radiator)
- 💢 Pain so severe you can’t bear weight
- Why it’s deadly: Untreated, it destroys cartilage in 48 hours and spreads to the bloodstream (sepsis).
- Action: ER within 6 hours. Requires IV antibiotics + joint drainage.
📉 Data point: 1 in 4 septic arthritis cases starts with “mild swelling”—delayed treatment = amputation risk (per Journal of Bone & Joint Surgery).
2. Gout or Pseudogout — Sudden & Brutal
- Why it happens:
- Gout: Uric acid crystals (from purine-rich foods/alcohol)
- Pseudogout: Calcium crystals (often in older adults)
- Red flags:
- ⚡ Pain that wakes you at 3 a.m. (like a “broken bone”)
- 🌶️ Skin bright red/purple (not just pink)
- 📈 Swelling peaks in 12–24 hours
- Why it’s tricky: Mimics infection—but antibiotics won’t help.
- Action: See doctor within 24 hours. Needs anti-inflammatory meds + crystal analysis.
💡 Pro tip: Pseudogout often follows knee injury; gout flares after red meat/alcohol binges.
3. Baker’s Cyst Rupture — The “Water Balloon” Effect
- Why it happens: Fluid from knee joint leaks into calf via a tear (often from arthritis or injury).
- Red flags:
- 💧 Swelling that “travels” down to calf (like a wave)
- 🦵 Calf tightness + bruising (even without knee pain)
- Why it’s confused: Feels like a blood clot—but no warmth/redness in calf.
- Action: Urgent ultrasound (within 48 hours) to rule out DVT.
4. Traumatic Injury (Even “Minor” Ones)
- Why it happens:
- Torn meniscus (from twisting)
- ACL tear (popping sound + instability)
- Fracture (even hairline cracks)
- Red flags:
- 🩸 Swelling within 2 hours (blood in joint = acute injury)
- 🤸 “Giving way” (knee buckles during walking)
- Why it’s missed: 30% of meniscus tears cause no pain—just swelling (per American Journal of Sports Medicine).
- Action: MRI within 72 hours. Delayed treatment = early arthritis.
5. Bursitis (“Housemaid’s Knee”) — The Kneeling Culprit
