Pelleted HRT (oestrogen) is not commonly commercially available through the large pharmaceutical companies and are generally made by a compounding chemist.
They are popular in Australia as several of the leading endos like them and have a steady supply. They require a local anaesthetic, a small cut and then insertion of the pellet, about the size of a rice grain. followed by a stitch to close the wound.
They last from 4-12 months and give steady levels of E.
Disadvantages, the medic has to know how to put the pellet in and many family doctors don't. They can be extruded by physical exercise (very rare) if the wound has not healed. HRT cannot be stopped if so desired. They break down at the end of their life and E levels can rocket for a week or so that confuses the blood picture. They have to be available. You have a series of small scars where they are inserted.
Advantages, steady levels, don't have to remember to take E, cheap (in Australia).
They are usually only used when the E levels have been stabilised by other methods and any medical side effects have been dealt with and when the client is happy to be on E and has no intention of stopping.
I was on them for years and the stopped and went to patches as I could not find a medic to keep implanting them.