Good vascularization of the Bio-eye HA orbital implant is necessary for the implant to provide maximum stability in the orbit and to ensure full epithelialization of the motility peg hole. The hydroxyapatite orbital implant must be well vascularized before drilling the peg hole, and vascular ingrowth must be assessed prior to drilling. This ability to vascularize is the key element in the success of the Bio-eye HA orbital implant Shields 1992. If vascularization is not assessed by some means prior to drilling, the risk of infection is greatly, and unnecessarily, increased. If the implant is well vascularized, the implant surfaces that have been exposed inside the drill hole will re-epithelialize around the peg, completely lining the exposed surfaces inside the hole. This capability of the Bio-eye HA orbital implant allows it to become a direct motility implant while still remaining a buried implant, since no implant surfaces must remain exposed to achieve coupling between the artificial eye and the implant. It should be noted that excellent motility is often possible without the need of a motility/support peg Shields 1992. The degree of motility achieved by the undrilled hydroxyapatite implant should be evaluated before drilling is considered. If evaluation indicates that greater motility is desired, then a motility/support peg is recommended and a peg hole should be drilled. The preferred method of assessing vascular ingrowth is by means of a technetium-99m bone scan, performed approximately 6 months post-implantation Perry 1987, Ferrone 1992, however, an MRI scan with contrast agent may also give some indication of blood vessel ingrowth Shields 1991, Soll 1987, Shields 1992.
Grading Bone Scans
Grading of the technetium 99m bone scan has been established as follows and is most accurate when done by reference to a graph showing the uptake of the technetium 99m through the orbits and mid-face. If the orbital implant shows uptake greater darker than the uptake of the mid-facial bones, it is graded 4+. If the uptake of technetium 99m in the implant is equal to that of the mid-facial bones, it is graded 3+. If the uptake of the implant is greater than one-half the distance between the uptake of the normal orbit and the uptake of the mid-facial bones, it is graded 2+. Uptake greater than the normal orbit, but less than 2+, is graded 1+. If the technetium 99m bone scan shows an uptake of 2+ or better, the implant is sufficiently vascularized and the hole may be drilled Perry 1991. The time required for the implant to become vascularized is variable because the fate of the implant wrapping material is variable Soll 1987 and because of individual variation DePotter 1992. Porous hydroxyapatite will vascularize more quickly when it is not wrapped in any collagen material, such as sclera or fascia lata. Likewise, any openings that are cut into the wrapping material will increase the rate of vascularization Ferrone 1992, DePotter 1992 . Most hydroxyapatite orbital implants, including wrapped implants, will be vascularized within 6 months of implantation. Perry 1991a, Soll 1987a, Ferrone 1992, DePotter 1992.