Hydroxyapatite Ocular Implants

Hydroxyapatite ocular implants are porous implants that are derived from sea coral. The porous structure of these implants allows ingrowth of host fibrovascular tissue, which reduces the risk of migration, extrusion, and infection. In addition, the extraocular muscles can be more securely attached to the implants, allowing the implant to move synchronously with the other eye.

An understanding of the sequence of events after enucleation can reduce confusion when evaluating the post-operative eye. In addition, MRI and nuclear medicine may be requested to confirm vascularization of the implant, although this is no longer universally used.

Patient information and surgical demonstration videos for one type of ocular implant provide a good overview of the procedure.

The ocular implant is placed immediately after enucleation and a temporary ocular conformer is placed on top of the implant. This conformer stays in place for about 4 to 8 weeks after surgery, at which point an ocularist fits, shapes, and paints an ocular prosthesis to match the patient's other eye. The prosthesis sits on top of the implant like a large contact lens.

We can sometimes be a bit loose in terminology, but there is a distinction between implant and prosthesis.

Mechanical coupling between the implant and the prosthesis can be improved with a second procedure where a round-headed peg or screw is inserted into the implant (the back of the prosthesis is also modified to accommodate this peg). The hole for the peg is usually not drilled until 6 months after enucleation: Enough time to allow for implant vascularization.

Radiologists used to be called upon to confirm vascularization of the implant by bone scintigraphy or contrast-enhanced MRI prior to drilling the hole for the peg, but this is no longer routinely done. Bone scintigraphy can show increased activity within the implant starting 1 to 6 months after surgery, indicating fibrovascular ingrowth. Contrast-enhanced MRI can also reveal the extent of fibrovascular ingrowth.

The images above are from a patient 4 years post left eye enucleation. CT shows the implant with a high-attenuation prosthesis anterior to it. FDG/PET shows no activity in the implant. The MR images show the attachment of the extra-ocular muscles to the implant. Anterior to the implant is the prosthesis, which is low signal intensity on all pulse sequences. Both CT and MR show linear striations in the implant, which is presumably due to the channels in these implants, although I can't find a reference in this regard. Enhancement along the periphery of the implant indicates fibrovascular ingrowth. No peg was placed in this patient.

References

• Custer PL, Kennedy RH, Woog JJ, Kaltreider SA, Meyer DR. Orbital implants in enucleation surgery: a report by the American Academy of Ophthalmology. Ophthalmology. 2003 Oct;110(10):2054-61.
• De Potter P. Advances in imaging in oculoplastics. Curr Opin Ophthalmol. 2001 Oct;12(5):342-6.
• Domange-Testard A, Papathanassiou D, Menéroux B, Amans J, Liehn JC. SPECT-CT images of an ocular coralline hydroxyapatite implant visible on bone scintigraphy. Clin Nucl Med. 2007 Feb;32(2):132-4.
• Hamilton HE, Christianson MD, Williams JP, Thomas RA. Evaluation of vascularization of coralline hydroxyapatite ocular implants by magnetic resonance imaging. Clin Imaging. 1992 Oct-Dec;16(4):243-6.
• Shields CL, Shields JA, De Potter P, Singh AD. Problems with the hydroxyapatite orbital implant: experience with 250 consecutive cases. Br J Ophthalmol. 1994 Sep;78(9):702-6.