## Combined TBSA Calculation: Lower Limbs + Back **Key Point:** The Rule of Nines uses a systematic division of body surface. Lower limbs and back of trunk are the largest individual regions, each representing 18% TBSA. ### Calculation Breakdown | Region | TBSA % | | --- | --- | | Both lower limbs (thigh + leg + foot each) | 18% + 18% = 36% | | Back and buttocks | 18% | | **Combined Total** | **54%** | **High-Yield:** This combined area (54%) represents more than half the body surface and is critical for rapid triage in burn units. Injuries affecting these regions carry significant fluid loss and mortality risk. **Clinical Pearl:** In severe burns involving the lower limbs and back, fluid resuscitation requirements are proportionally higher. The Parkland formula uses TBSA % to calculate IV fluid needs: 4 mL × body weight (kg) × TBSA (%). **Mnemonic:** "18 + 18 + 18 = 54" — Two lower limbs (18% each) plus back (18%) equals 54% of total body surface. [cite:Park 26e Ch 12]
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