Biologic Behavior of Hydroxyapatite Used in Facial Augmentation

The recent finding that shrinkage of key areas of the facial skeleton contributes to the aging appearance of the face has prompted a search for the most appropriate bone-like implant material. Evidence that hydroxyapatite, in granular form, maintains volume in the long term supports its use in the correction of aging, in addition to its use in the correction of inherently deficient areas of the facial skeleton. The biologic response of hydroxyapatite needs to be fully understood for its use to be confidently recommended.

Samples of 'living' hydroxyapatite from the anterior maxilla, zygoma, and mandible of 17 patients were analyzed. These were obtained during revision procedures performed between 6 months and 15 years following original placement on the facial skeleton.

Histology showed that in every case, the individual granules were embedded within a mass of collagen that made up about half of the total implant volume. The collagen mass also contained fine elastin, fibroblasts, lymphocytes, occasional granulomas, and vessels. By 2 years, a new compact bone containing osteoblasts and osteocytes was present in all specimens in the deep (osseous) aspect. Bone progressively replaced the original collagen between the granules with a sharply defined transition at the interface.

This study confirmed a two-stage biologic change following onlay placement of hydroxyapatite granules on the facial skeleton, i.e., initial collagen formation with subsequent conversion to bone. This integrates the implant with the host bone which stabilizes the implant position and shape initially and in long term.

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