Hydroxyapatite Implants - Zygomatic Bone Augumentation?

I think you are talking about implanting or layering hydroxyapatite on the malar bone to augment cheek bones.  This is an off lable use of the product but is being used by licensed physicians around the world.  It works, but it is not permanent.  The product(Radiesse) is injected as a bolus directly on top of the bone underlying the cheeks and is molded into place. It 'volumizes' that area to help the central face achieve that  more youthful inverted triangular appearance to the face.  Our faces over time start losing volume both in fat pads and bone loss as the profile of the face goes from the 'inverted triangular' appearance to a more squared off rectangular flatter appearance.  Restoring that shape gives a face a more refreshed, youthful appearance.

First, hydroxyapatite (HA) is a synthetic ceramic that does not change into bone. Bone may grow around it if it is placed into a defect. But as an augmentation material (sitting on top of bone), bone will not grow up into it but scar tissue will. Secondly,  HA has a history of being used as injected granules through an intraoral incision for cheek augmentation. It can produce some degree of cheek enhancement and if skillfully done still has a role for some types of cheek augmentation. It lacks the ability to shape it very well so it creates more of a mass effect as the material tends to spread out after injection. Think of it as an intermediate option between non-surgical injectable synthetic fillers and surgically placed synthetic implants. 

You are partly correct about
hydroxyapatite converting to bone over time. This, of course has major
advantages as there is not a foreign implant present and therefore not the risk
of the implant being the cause of problems in the future. I have considerable
experience with using the coral version of hydroxyapatite, based on over 500
patients to augment their cheeks as well as being involved in clinical research
with it.

The first thing to understand is that
not all hydroxyapatite products are the same. Corraline hydroxyapatite, is
hydroxyapatite obtained by purifying natural coral from the South Pacific and was
selected in the early days as it has a similar physical structure to human bone
as well as having the same mineral structure. Other hydroxyapatites are derived
from the bone of cattle. Synthetic versions of hydroxyapatite are also
available. The injectable filler called Radiesse, while having hydroxyapatite
in its name is different and is not comparable. So it is important to sure of
using the correct version of hydroxyapatite to obtain this permanent
correction. It has been shown that there is a 2 stage response of the body to
the placement of the granular version of coralline hydroxyapatite directly on
the surface of facial bone. The initial response is the ingrowth of fibrous
tissue into the pores in each granule as well as between the granules. After about 2 years the
persons bone grows in by a process of osteoconduction to replace the collagen.
So the definitive implant is formed by the patients bone.

To place the hydroxyapatite involves
surgery on the surface of the bone, to lift the bone’s membrane, and place the
granules in the pocket between the bone and the periosteum. It is not injected.

Two surgeons in the USA have major
experience with this product, Dr Steve Byrd in Dallas and Dr Henry Kawamoto in
Los Angeles.Coral was originally used for
reconstructive situations, especially an asymmetry following trauma and these
days is being used in the correction of aging changes of the cheeks where the
bone resorbs as a major part of the aging process.