Everything you need to know about the contraceptive implant

Desogestrel in combination with ethinyl estradiol may slightly increase the attributable risk of deep venous thrombosis, but this response has not been shown without estrogen.

Design features

The single-rod design means little discomfort for patients at insertion or removal, an unobtrusive implant, and almost no scarring. Insertion and removal are predictably brief. In US and European trials, which began 10 years ago, average insertion time was 1 minute, and removal time was 3 minutes. In contrast, Norplant required up to 10 minutes to insert and 1 hour to remove.⁸

Because only 1 rod is implanted, there is no risk of dislocating previously placed capsules.⁹ Nor is it necessary, as it was with Norplant, to create channels under the skin, which made implants difficult to palpate after insertion.

Finally, ethylene vinyl acetate, the plastic from which Implanon is made, is less likely than Norplant’s Silastic to form a fibrous sheath that can prolong removal.¹⁰

Pharmacology of Implanon

Implanon is a single nonbiodegradable rod of 40% ethylene vinyl acetate and 60% ENG (40 mm×2 mm) covered with a membrane of rate-controlling ethylene vinyl acetate 0.06 mm thick.

Bioavailability

The 68 mg of ENG contained in the rod are initially absorbed by the body at a rate of 60 μg per day, slowly declining to 30 μg per day after 2 years.

Peak serum concentrations (266 pg/mL) of ENG are achieved within 1 day after insertion, effectively suppressing ovulation (which requires 90 pg/mL ENG or more).^11,12

The steady release of ENG into the circulation avoids first-pass effects on the liver.

The bioavailability of ENG remains nearly 100% throughout 2 years of use. The elimination half-life of ENG is 25 hours, compared with 42 hours for Norplant’s LNG.

Rapid return to ovulation

After removal, serum ENG concentrations become undetectable within 1 week, and ovulation resumes in 94% of women within 3 to 6 weeks after the implant is removed.^11,12

Efficacy and safety

Liver enzyme-inducing drugs lower ENG levels

Like other contraceptive steroids, serum levels of ENG are reduced in women taking liver enzyme-inducing drugs such as rifampin, griseofulvin, phenylbutazone, phenytoin, and carbamazepine, as well as anti-HIV protease inhibitors. Pregnancies were reported among Australian women using Implanon along with some of these antiepileptic drugs.¹³

Equal efficacy in obese women?

The efficacy of the single-rod implant was studied in clinical trials involving 20,648 cycles of use.¹ Only 6 pregnancies occurred in this population—2 each in years 1, 2, and 3 of use. None of the women who weighed 154 lb (70 kg) or more became pregnant.¹²

However, questions remain as to whether the new implant will maintain its high efficacy in obese women, as it has not been studied in women weighing more than 130% of their ideal body weight. Serum levels of ENG are inversely related to body weight and diminish over time, but increased pregnancy rates in obese women have not been reported.

Potential for ectopic pregancy

Suspect ectopic pregnancy in the rare event that a woman becomes pregnant or experiences lower abdominal pain.¹ The reason: Pregnancies in women using contraceptive implants are more likely to be ectopic than are pregnancies in the general population. Ovulation is possible in the third year of use, but intrauterine pregnancies are very rare.¹

Limited metabolic effects

Published studies indicate that the metabolic effects of the ENG implant are unlikely to be clinically significant, including its effects on lipid and carbohydrate metabolism, liver function, hemostasis, blood pressure, and thyroid and adrenal function.^14-17

Adverse event rates

Overall, implants, including the ENG implant, appear to be safe. The rate of adverse events is comparable to rates in nonusers¹⁸ (death, neoplastic disease, cardiovascular events, anemia, hypertension, bone-density changes, diabetes, gall bladder disease, thrombocytopenia, and pelvic inflammatory disease).

Lactation

In a study comparing 42 lactating mother–infant pairs using the ENG implant, compared with 38 pairs using intrauterine devices, there were no significant differences in milk volume, milk constituents, timing and amount of supplementary food, or infant growth rates.¹⁹

Because it contains no estrogen, Implanon is a good choice for immediate postpartum contraception.

Insertion and removal

Although the ENG implant is designed to facilitate rapid, simple insertion and removal, clinicians require training in Implanon-specific technique.²⁰ Insertion takes an average of 1 to 2 minutes.²¹

The disposable trocar comes preloaded,²² and the needle tip has 2 cutting edges with different slopes. The extreme tip has a greater angle and is sharp to allow penetration through the skin. The second upper angle is smaller, and the corresponding edge is unsharpened to reduce the risk of placing the implant in muscle tissue.