What happens to the capsule when left behind after an explant?

Leaving a capsule behind is preferred if it is thin and pliable. If there are calcific components, or silicone granulomatous material, or if the capsule is deforming the breast tissue, then removal of the capsule may be warranted.  In general, saline (a relatively innocuous substance) leaking is not going to cause issues as your body will likely absorb this. A number of scientific studies have demonstrated the safety of silicone implants; yet, many patients continue to have anxiety about their use.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

dear socalanonymommy,
in my opinion you shouldn't be worried. There is small chance that the capsule or silicone which remained in capsule will ever cause you any health issues. Your body will break down the capsule and the quantities of silicone in it is very low.
Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery 
Member, Am. Society of Plastic Surgery

Whether you need a capsulectomy depends upon whether there is something wrong with the capsule (such as capsular contracture), and whether there is something wrong with the implant (such as rupture).  If you did not have any of these problems, then capsule removal is probably not needed as the body will break down and remodel the scar tissue on its own.  The amount of silicone left behind from unruptured implants of the types FDA-approved in the United States during the last 10 years is miniscule.  Any foreign substance can make you sick if it gets infected, but silicone gel is a bio-inert substance that is very unlikely to make you sick on its own.  It doesn't sound like you have anything to worry about from this, but it would be a good idea to speak with your plastic surgeon if you have concerns.