Anophthalmic Orbital Implants: Current Concepts and Controversies

Current Porous Orbital Implants

Perry introduced coralline (sea coral) hydroxyapatite (HA) spheres (Bio-Eye^® Orbital Implants [Integrated Orbital Implants, Inc., San Diego, CA]) in the mid 1980s, launching a new era of porous orbital implants.^[1] By 1992, coralline HA implants were the most frequently used implants after primary enucleation.^[2] The interconnecting porous structure (Figures 1A and 1B) of the HA implant allowed host fibrovascular ingrowth, potentially reducing the risk of migration, extrusion, and infection.^[3] The HA implant also allowed secure attachment to the extraocular muscles, which in turn lead to improved implant motility.^[1,4] By drilling into the HA implant and placing a peg (which is subsequently coupled to the posterior prosthetic surface), a wide range of artificial eye movements as well as fine darting eye movements (commonly seen during close conversational speech) were seen. The increased prosthetic movement gave a more lifelike quality to the artificial eye.

Although HA implants represented a significant advance in anophthalmic surgery, experience with HA over the last decade has expanded our understanding of the limitations of HA. Reported complications are not uncommon and include implant exposure, conjunctival thinning, socket discharge, pyogenic granuloma formation, implant infection, and persistent pain or discomfort.^[5,6,7] Implant exposure problems continue to deter some surgeons from using HA implants, but these complications may be more closely related to surgical implantation technique, implant wrap selection, host factors, as well as properties related to HA spheres.^[3,8,9]

The introduction of HA as an orbital implant significantly raised the costs associated with enucleation, evisceration, and secondary orbital implant procedure. Additional expenses associated with HA placement include the cost of the implant (approximately $600 more than a traditional polymethylmethacrylate or silicone sphere implant, which typically cost $15-$50/sphere), an implant wrap material, assessment of implant vascularization with a confirmatory magnetic resonance imaging (MRI) study, a secondary drilling procedure with peg placement, and prosthesis modification. In the search for porous orbital implants with a reduced complication profile and diminished surgical and postoperative costs, numerous alternative implant materials were introduced around the world.

Synthetic porous polyethylene (Medpor^® [Porex Surgical, Inc., Newnan, GA]) implants were introduced over a decade ago for use in the orbit and have been widely accepted as an alternative to the Bio-Eye^® HA.^[10,11] Porous polyethylene implants have pores similar to HA implants, but they are less uniform in size and more irregular in shape (Figures 2A and 2B). Although less biocompatible than HA implants,^[12] porous polyethylene implants are typically well tolerated by orbital soft tissue. They have a smoother surface than HA implants, which permits easier implantation and potentially less irritation of the overlying conjunctiva following placement. These implants have a high tensile strength, yet are malleable, which allows sculpting of the anterior surface of the implant. They may be used with or without a wrapping material, and the extraocular muscles can be sutured directly onto the implant, although most surgeons find this difficult without predrilled holes. Porous polyethylene implants are available in spherical, egg, conical, and mounded shapes (Medpor Quad^™ Motility Implant [Porex Surgical, Inc., Newnan, GA])^[10,13,14] The anterior surface can also be manufactured with a smooth, nonporous surface to prevent abrasion of the overlying tissue (e.g., the Smooth Surface Tunnel Sphere [SST^TM] [Porex Surgical, Inc., Newnan, GA]) while retaining a larger pore size posteriorly to facilitate fibrovascular ingrowth. A standard spherical Medpor^® implant costs approximately $200 (USD) less than the Bio-Eye^® HA sphere, depending on the quantity ordered.

Synthetic hydroxyapatite implants (Figures 3A and 3B) (FCI, Issy-Les-Moulineaux, Cedex, France) are currently in their third generation (FCI₃).^[8] The FCI₃ synthetic HA implant has an identical chemical composition to that of the Bio-Eye^®, although scanning electron microscopy (SEM) has revealed decreased pore uniformity and interconnectivity as well as the presence of blind pouches.^[15] Central implant fibrovascularization in a rabbit model still appears to occur in a similar manner in both the Bio-Eye^® and FCI₃ implants.^[16] The synthetic FCI₃ implant has gained popularity in many parts of the world over the past 10 years; however, it is not yet available in the United States. The problems and complications associated with the synthetic FCI₃ implant are similar to those of the Bio-Eye^®.^[8] The FCI₃ implant remains significantly cheaper than the Bio-Eye^® (approximately $480 USD versus $650 USD).

Other forms of HA implants in use around the world include the Chinese HA and the Brazilian HA implants.^[17,18] Although less expensive than the Bio-Eye^®, these implants have impurities or poor porous structure that offer little advantage over the other available models. Other implants of different materials and designs continue to surface, some of which are of little added value,^[19] while others have been in use only for a short time and their advantages and disadvantages are not yet apparent.^[20]

Aluminum oxide (Al₂O₃) is a ceramic implant biomaterial that has been used in orthopedic surgery and dentistry for more than 30 years. It is a porous, inert substance, and has been suggested as a standard reference material in studies of implant biocompatibility (Figure 4A).^[21] Spherical- and egg-shaped aluminum oxide implants (Bio-ceramic orbital implants, FCI Ophthalmics, Marshfield Hills, MA) are currently available for use, with a mounded prototype in the planning stages. The bioceramic implant permits host fibrovascular ingrowth similar to the Bio-Eye^®.^[22] Human fibroblasts and osteoblasts (invitro) proliferate more rapidly on aluminum oxide than on HA, suggesting that aluminum oxide is a more biocompatible substance than HA.^[12,21] The bioceramic implant is lightweight and has a uniform pore structure and excellent pore interconnectivity (Figure 4B).^[15] The microcrystalline structure is smoother than the rough-surfaced Bio-Eye^® (Figure 5). In our experience, anophthalmic sockets reconstructed with aluminum oxide implants appear to have less postoperative tissue inflammation than sockets in which hydroxyapatite implants have been placed.^[23] Problems (e.g., exposure) encountered with aluminum oxide implant use are similar to those seen with the Bio-Eye^® orbital implants, but they appear to occur less often.^[23,24] As with the other available porous orbital implants, aluminum oxide is less expensive than the Bio-Eye^® ($450 versus $650 USD).

© 2005 Comprehensive Ophthalmology Update, LLC

Cite this: Anophthalmic Orbital Implants: Current Concepts and Controversies - Medscape - Nov 01, 2005.