Which of the following is the PRIMARY mechanism of action of the copper intrauterine device (Cu-IUD)?

Explanation

## Copper IUD: Mechanism of Action ### Primary Mechanism: Spermicidal Effect **Key Point:** The copper IUD works **primarily as a spermicide**, not as a mechanical barrier or abortifacient. Copper ions are toxic to sperm, reducing their motility and viability in the female reproductive tract [cite:Park 26e Ch 7]. ### Detailed Mechanism 1. **Copper ion release:** The copper wire or surface of the IUD releases Cu^2+^ ions into the uterine cavity and fallopian tubes. 2. **Sperm toxicity:** Copper ions impair: - Sperm flagellar motility (reduced swimming ability) - Sperm viability (increased apoptosis) - Sperm capacitation (reduced ability to fertilize) 3. **Inflammatory response:** The copper also triggers a local inflammatory response with increased leucocytes and macrophages in the endometrium, which further inhibit sperm function. ### Secondary Mechanisms **Clinical Pearl:** While the primary mechanism is spermicidal, the Cu-IUD also has secondary effects: - **Endometrial changes:** Atrophy and thinning of the endometrium (but NOT to the point of preventing implantation in most cases) - **Tubal effects:** Reduced ovum transport (minor contribution) - **NOT a primary abortifacient:** Modern evidence does not support that the Cu-IUD works by preventing implantation as a primary mechanism ### Efficacy Data **High-Yield:** The Cu-IUD is **>99% effective** (0.8 per 100 woman-years failure rate), making it one of the most effective reversible contraceptives. This high efficacy is achieved primarily through sperm immobilization **before** fertilization occurs. ### Comparison: Cu-IUD vs. Levonorgestrel IUD (LNG-IUD) | Feature | Cu-IUD | LNG-IUD | | --- | --- | --- | | Primary mechanism | Spermicidal (copper toxicity) | Ovulation suppression + endometrial atrophy | | Copper ion release | Yes | No | | Systemic hormone absorption | Minimal (<5% systemic) | Moderate (20% systemic) | | Menstrual bleeding | Often increased | Decreased/amenorrhea | | Failure rate | 0.8 per 100 WY | 0.2 per 100 WY | | Duration | 10 years (some 12 years) | 5–7 years | **Mnemonic:** **CU-IUD = Copper Undermines sperm** (spermicidal action is primary) ### Why NOT Mechanical Blockade? **Warning:** A common misconception is that the IUD works by blocking the cervical canal. This is **incorrect**: - The IUD sits in the uterine cavity, not the cervix - Sperm readily pass through the cervix and enter the uterine cavity - The contraceptive effect occurs in the uterus and tubes, not at the cervix