3-PHASE BONE-SCINTIGRAPHY OF HYDROXYAPATITE OCULAR IMPLANTS

Hydroxyapatite ocular implants are replicas of lamellar bone tissue de rived from the exoskeleton of a reef-building coral by a hydrothermal chemical exchange reaction. Attached to the eye muscles, they act as a passive framework for fibrovascular ingrowth and can be drilled to ho ld the visible part of the artificial eye and allow synchronous eye mo vement. Fibrovascular ingrowth has to be confirmed by bone scintigraph y before the drilling procedure. This study monitored the vascular ing rowth into the implant in ten patients over 12 months to establish a c linically feasible imaging protocol. Tracer accumulation was monitored visually and quantitatively in dynamic and single-photon emission tom ography (SPET) scans after the intravenous administration of 600 MBq o f Tc-99m-DPD. The implants showed no tracer accumulation in the arteri al or blood pool phase, Accordingly, dynamic scintigraphy can be omitt ed from the imaging protocol. Delayed tracer accumulation appeared no earlier than 2 and no later than 6 months after surgery. Planar scinti graphy is not recommended as high-resolution SPET is necessary to sepa rate the implant from the surrounding bone, We conclude that imaging c an be confined to high-resolution SPET 3 h after tracer injection, no earlier than 3 months after surgery. The vascularized hydroxyapatite o rbital implant is an important in vivo model for bone-seeking agents t o study their uptake kinetics independently of any soft tissue and bon e disease, Our results provide evidence that in normal bones the chemi cal adsorption of Tc-99m-DPD into the crystalline structure of hydroxy apatite is the only quantitatively relevant uptake mechanism.