Removing Hydroxyapatite Implant from Forehead?

 If it was injected in the skin, there is nothing to do except to wait for it to dissolve. This may take a year, but will go away.

At this point it would be best to remove the hydroxyapatite and go back to your pre-surgical state prior to the placing of any implant in the forehead.  It is important to make sure the implant is not infected.
Hydroxyapatite can be difficult to remove due to extensive scar tissue from this type of implant. It can have irregularities across the forehead that may be visible through the skin after the swelling subsides.  

Hydroxyapatite is much more porous and as such will have some vascular in-growth.
If HA was injected, removing it will be hard and bloody. If it was layered over the defect left by the removal of PMMA implant, it will not be easy to remove but could be contoured with a burr to lower the ridge. Your surgeon will know the answer.
Dr. P. Aldea

Hydroxyapatite comes in different forms. Radiesse is round particles of calcium hydroxyapatite (a natural mineral bone like material normally present in the body) floating in a gel. Because of its simple chemical mineral structure it is non-allergenic so no skin tests are needed and because of the particle size it can last 2 or more years after injection. It is most commonly injected to reduce skin wrinkles or folds of aging.
Hydroxyapatite is also available in paste and solid implant forms without gel for augmentation of bone. Unlike solid hydroxyapatite implants the paste is non-porous so bone and soft-tissue ingrowth does not occur. Over time the solid implant is partially absorbed and replaced by bone but it never goes away
If your doctor used the paste form it will be easier to remove than the solid form. The solid form has bone growth into over time making it harder to remove but it should still be doable with greater effort. If there is bone ingrowth it may have to be removed in the hospital using a burr because most office ambulatory surgery centers do not have the requisite bone removing equipment.

The hydroxyapatite is used for contouring and generally gets replaced by the body's own tissue. This might result in bone formation. If there is a ridge, you can have this burred down smooth.