In 2011, Spiegel  investigated the facial determination of “female gender and feminization forehead cranioplasty,” showing that the prominence of the forehead at the glabella region and the position and shape of the eyebrows are the most significant for determining female gender. He pointed out that the upper third of the face is the most significant part of the face for determining female gender in both the frontal and profile views of the face [8, 9]. His study showed that the frontal view of the forehead modification was selected as most feminizing more often than the profile view .
In agreement with Spiegel’s  investigation, our study clearly supported the strong significance of forehead modification in permitting transgendered patients to be identified as women. Therefore, remodeling of the forehead for gender reassignment is an extensively satisfying procedure if several characteristic gender differences of the forehead are appreciated when the bony forehead is modified [6–9]. The male bossing all the way across the orbital rims, with a depression between the brows, can cause marked shadowing of the eyelids and eyes and is a factor in hooding of the lateral superior orbital rim. Thus, for male-to-female reassignment, bossing across the orbital rims and hooding of the lateral orbit must be reduced, whereas the concavity on the forehead above the area of the residual bossing must be filled [8, 13]. This procedure eliminates the masculine characteristic and gives a continuous female curvature of the forehead.
Despite the huge amount of literature devoted to alloplastic reconstruction of the cranial vault, only limited experience with feminizing surgery for the craniofacial area exists [12, 14, 19, 21]. For instance, Becking et al.  reported on transsexual patients who underwent “bony facial corrections,” but none of these patients had procedures on the upper third of the face. This article reports the largest volume of patients undergoing feminizing cranioplasty and supports the overall safety of the methods.
For augmentation of the cranial vault, hydroxyapatite cement is a welcome alternative to high-density polyethylene or methylmethacrylate as well as the traditional use of autogenous bone, especially for gender reassignment [19, 21, 22, 24]. The material maintains its intraoperative position after surgery [22, 23]. It is biocompatible and has good outcomes [21–23, 26–29, 31, 39, 40].
Our own animal studies on histologic examination of HAC showed fine fibrous septa containing newly formed small vessels [21, 22]. Sparsely distributed focal ossification areas characterized by an ossification matrix with osteocytes embedded in lacunae were seen. These observations support the osseointegration of the implant and showed in the animal study that although limited, there was new bone and vascularized formation within the HAC .
The high osteoconductive capacity of HAC makes it an excellent alternative for gender reassignment of the cranial vault . It is incorporated within the surrounding bony structures and permits secondary procedures [19, 22, 23].
Our own experimental studies [21, 22], performed to show the safety and osseointegration capacity of HAC with animal models, confirmed the osseous replacement without any toxic reactions or increases in serum calcium or phosphate levels, as shown by Friedman et al. . As the findings demonstrated, HAC is gradually reabsorbed and replaced by bone. The newly formed bone appeared to be effectively remodeled over time, producing lamellar bone nearly indistinguishable from normal, mature osseous tissue . This osteoconductive property prevents migration of the paste applied on the cranial vault and lowers the postoperative infection because of its vascular ingrowth [22, 23]. The capacity for osteoinduction was found to be dependent on the rate of the porosity and microstructure, with a mean pore diameter of 8 nm, depending on the amount of the solvent water, which enables the homogeneous osteoconversion. But our histologic examinations on Goettingen minipigs showed that the porosity and microstructure are dependent on the liquid substance. Mixing the HAC powder with sodium monophosphate intraoperatively decreases the microstructure of the resulting hydroxyapatite and limits the amount, tending to decrease the longer the material is exposed to a moist environment .
Even if the anterior wall of the frontal sinus needs to be set posteriorly and fixed with plates into a more reduced position for aesthetic reasons, HAC is used for additional contouring. It seals the bony fragments of the anterior frontal sinus wall after meticulous cleaning and checking of the sinus to ensure that the frontal nasal ducts are open and will be replaced by bony tissue over the time [39, 40].
Clinical and experimental studies have shown that HAC is a biocompatible substance that does not cause any chronic, inflammatory, allergenic, or toxic reaction [19, 21, 22, 26]. Therefore, it has a better resistance to bacterial infection, although inflammatory immunologic reactions have been reported by Moreira-Gonzalez et al.  and others . Moreira-Gonzalez et al.  analyzed post-tumor patients who had undergone cranioplasty using the various materials currently available. For the group of patients reconstructed with HAC, these authors reported an inflammatory immunologic reaction occurring some months to some years at least after implantation and resulting in erosion of the skin and exposure of the underlying material, requiring its removal. Whether the reported complications are related to subclinical infection, trauma, or the material itself remains unclear.
Since 2002, we have used a bone source for cranial vault reconstruction and recontouring, especially for augmentation of the forehead in nonsyndromic and non-post-tumor patients without seeing any postoperative complications  such as inflammatory reactions with swelling or mild redness over an 8 year period, which is in agreement with others [19, 21–24, 27–35, 39, 40]. Perhaps because no previous trauma or infections took place before frontal bone remodeling gender reassignment of the bony forehead in conjunction with HAC, augmentation has been a safe procedure without any major complications over the past several years in our hands. The operative time for recontouring of the cranial vault usually has been 1–1.5 h.
Since the first description of aesthetic forehead augmentation with methylmethacrylate by Gonzalles-Ulloa and Stevens , several materials including titanium, polyethelene, silicone, and rib bones have been used for cranioplasties but are no longer considered. By allowing long-lasting contouring of the noninfected bony forehead for gender reassignment, HAC avoids autogenous material. In contrast to established polymethylmethacrylate (PMMA)-based cements, application of the BoneSource cement enables ossification throughout an augmented volume. In addition, PMMA forms a capsule in the adjacent areas, is vulnerable to infections, and needs fixation . Due to the possibility that the adjacent tissue might be damaged because of the chemical thermogenic reactions that occur during the hardening process, its clinical application has been restricted . Whether the high-density polyethylene forms a capsule in the adjacent tissue or not, this material cannot fuse with the adjacent bone tissue. This eventually increases the incidence of infection and possibility of extrusion.