Hydroxylapatite granules and blocks as an extracranial augmenting material in rhesus monkeys

This study evaluates the use of the nonporous hydroxylapatite (HA) granules and blocks as an extracranial augmentation material in monkeys. Augmentation was performed in 12 Rhesus monkeys killed at three, six, and 12 months postoperatively. Clinical evaluation revealed that the HA granules become stable within three to four weeks, but there is loss of some augmented height due to settling of the material within the subpericranial pocket. The HA blocks maintained the augmented cranial contours, but 25% of them exhibited mobility. Histologic evaluation revealed no evidence of inflammation or bone resorption with either the HA granular or block form. The HA granules were separated from each other and from the bone interface by a layer of collagen fibers, and the blocks were also surrounded by a fibrous capsule. There was no evidence of bonding between the HA granules, and the bone-HA interface was separated by an intervening fibrous layer in all the HA granular specimens except for one in the one year group which showed evidence of bone formation in part of the HA bone interface. It was concluded that when used as an extracranial augmentation material, the nonporous HA granules were more stable than blocks; however, the blocks maintain a better contour.