Implant types and chemical construction

Implant types and chemical construction

There are many different types of implants, classification ranging from shape (Spherical vs egg (oval) shaped), stock vs custom,[3] porous vs non porous, specific chemical make-up, and the presence of a peg or motility post. The most basic simplification can be to divide implant types into two main groups: nonintegrated (non-porous) and integrated (porous).[4]
[edit] Nonintegrated implants

Though there is evidence that ocular implants have been around for thousands of years (already cited reference 2) modern nonintegrated spherical intraconal implants came into existence around 1976 (not just glass eyes[3]).[5] Nonintegrated implants contain no unique apparatus for attachments to the extraocular muscles and do not allow in-growth of organic tissue into their inorganic substance. Such implants have no direct attachment to the ocular prosthesis.[4] Usually, these implants are covered with a material that permits fixation of the extraocular recti muscles, such as donor sclera or polyester gauze which improves implant motility, but does not allow for direct mechanical coupling between the implant and the artificial eye.[5] Non-integrated implants include the acrylic (PMMA[4]), glass, and silicone spheres.[6]

Polymethyl methacrylate (PMMA) (acrylic)

PMMA[4] is a transparent thermoplastic available for use as ocular prosthesis, replacement intraocular lenses when the original lens has been removed in the treatment of cataracts and has historically been used as hard contact lenses (see poly(methyl methacrylate)).

PMMA has a good degree of compatibility with human tissue, much more so than glass. Although various materials have been used to make nonintegrated implants in the past, polymethyl methacrylate (PMMA) is one of the implants of choice.[4]
[edit] Integrated implants (porous)

The porous nature of integrated implants allows fibrovascular ingrowth throughout the implant and thus also insertion of pegs or posts.[7] Because direct mechanical coupling is thought to improve artificial eye motility, attempts have been made to develop so-called ‘integrated implants’ that are directly connected to the artificial eye.[5] Historically, implants that directly attached to the prosthesis were unsuccessful because of chronic inflammation or infection arising from the exposed nonporous implant material.[7] This led to the development of quasi-integrated implants with a specially designed anterior surface that allegedly better transferred implant motility to the artificial eye through the closed conjunctiva and Tenon’s capsule.[5] In 1985, the problems associated with integrated implants were thought to be largely solved with the introduction of spherical implants made of porous calcium hydroxyapatite. This material allows for fibrovascular ingrowth within several months.[5] Porous enucleation implants currently are fabricated from a variety of materials including natural and synthetic hydroxyapatite, aluminum oxide, and polyethylene.

The surgeon can alter the contour of porous implants before insertion, and it is also possible to modify the contour in situ, although this is sometimes difficult.[7]

Hydroxyapetite (HA)

Hydroxyapatite implants are spherical and made in a variety of sizes and different materials (Coralline/ Synthetic/ Chinese).[6][7]

Since their introduction in 1989 when an implant made from hydroxyapatite received Food and Drug Administration approval, spherical hydroxyapatite implants have gained widespread popularity as an enucleation implant[5][7] and was at one point was the most commonly used orbital implant in the United States.[8][9] The porous nature of this material allows fibrovascular ingrowth throughout the implant and permits insertion of a coupling device (PEG) with reduced risk of inflammation or infection associated with earlier types of exposed integrated implants.[7]

hydroxyapatite is limited to preformed (stock[3]) spheres (for enucleation) or granules (for building up defects).[10]

One main disadvantage of HA is that it needs to be covered with exogenous material, such as sclera, polyethylene terephthalate, or vicryl mesh (which has the disadvantage of creating a rough implant tissue interface that can lead to technical difficulties in implantation and subsequent erosion of overlying tissue with the end stage being extrusion), as direct suturing is not possible for muscle attachment. Scleral covering carries with it the risk of transmission of infection, inflammation, and rejection.[8]

A recent study has shown that HA has a more rapid rate of fibrovascularization than Medpor.[8]

Porous polyethylene (PP)

MEDPOR is a high-density porous polyethylene (Medpor) [7] Implant manufactured from linear high-density polyethylene.[11][12] Development in polymer chemistry has allowed introduction of newer biocompatible material such as porous polyethylene (PP) to be introduced into the field of orbital implant surgery.[8] Porous polyethylene enucleation implants have been used since at least 1989.[7] It is available in dozens of prefabricated spherical and non-spherical shapes and in different sizes or plain blocks for individualized intraoperative customizing.[7] The material is firm but malleable and allows direct suturing of muscles to implant without wrapping or extra steps. Additionally, the smooth surface is less abrasive and irritating than other materials used for similar purposes.[10] Polyethylene also becomes vascularized, allowing placement of a titanium motility post that joins the implant to the prosthesis in the same way that the peg is used for hydroxyapatite implants.[7]

PP has been shown to have a good outcome, and in 2004, it was the most commonly used orbital implant in the United States.[8][13] Porous polyethylene fulfills several criteria for a successful implant, including little propensity to migrate and restoration of defect in an anatomic fashion; it is readily available, cost-effective, and can be easily modified or custom-fit for each defect.[10] The PP implant does not require to be covered and therefore avoids some of the problems associated with hydroxyapatite implants.[8]

Bioceramic

Bioceramic prosthetics are made of aluminum oxide (Al2O3). Aluminum oxide is a synthetic ceramic biomaterial that has been used for more than 35 years in the orthopedic and dental fields for a variety of prosthetic applications because of its low friction, durability, stability, and inertness.[14] Aluminum oxide ocular implants can be obtained in spherical and non-spherical (egg-shaped) shapes and in different sizes [7] for use in the anophthalmic socket The bioceramic implant is available in spherical and egg-shaped models for use in the anophthalmic socket. It received US Food and Drug Administration approval in April 2000 and was approved by Health and Welfare, Canada, in February 2001.[14]

Aluminum oxide has previously been shown to be more biocompatible than HA in cell culture studies and has been suggested as the standard reference material when biocompatibility studies are required to investigate new products. The rate of exposure previously associated with the bioceramic implant (2%) was less than most reports on the HA or porous polyethylene implant (0% to 50%).[14]

Conical orbital implant (COI) and multipurpose conical orbital implant (MCOI)

The safe and effective sphere (still popular and easy to use) was supplemented with the pyramid or COI implant.[10] The COI has unique design elements that have been incorporated into an overall conical shape, including a flat anterior surface, superior projection and preformed channels for the rectus muscles.[15] 5-0 Vicryl suture needles can be passed with slight difficulty straight through the implant to be tied on the anterior surface. In addition, this implant features a slightly recessed slot for the superior rectus and a protrusion to fill the superior fornix.[10]

The newest model is the multipurpose conical orbital implant, which was designed to address the issues of the postoperative anophthalmic orbit being at risk for the development of socket abnormalities including enophthalmos, retraction of the upper eyelid, deepening of the superior sulcus, backward tilt of the prothesis, and stretching of the lower eyelid.1 after evisceration or enucleation, These problems are generally thought to be secondary to orbital volume deficiencies which is also addressed by MCOIs. The conical shape of the multipurpose conical porous polyethylene orbital implant (MCOI) (Porex Medical) more closely matches the anatomic shape of the orbit than a spherical implant. The wider anterior portion, combined with the narrower and longer posterior portion, allows for a more complete and natural replacement of the lost orbital volume. This shape reduces the risk of superior sulcus deformity and puts more volume within the muscle cone.[16][17] Muscles can be placed at any location the surgeon desires with these implants. This is advantageous for cases of damaged or lost muscles after trauma, and the remaining muscles are transposed to improve postoperative motility. And in anticipation of future peg placement there is a 6-mm diameter flattened surface, which eliminates the need to shave a flat anterior surface prior to peg placement.[10]

Both implants (COI and MCOI) are composed of interconnecting channels that allow ingrowth of host connective tissue. Complete implant vascularization reduces the risk of infection, extrusion, and other complications associated with nonintegrated implants. And both implants produce superior motility and postoperative cosmesis.[10]

Pegged (motility post) implants

In hydroxyapatite implants a secondary procedure can insert an externalized, round-headed peg or screw into the implant. The prosthesis is modified to accommodate the peg, creating a ball-and-socket joint:[7] after fibrovascular ingrowth is completed, a small hole can be drilled into the anterior surface of the implant. After conjunctivalization of this hole, it can be fitted with a peg with a rounded top that fits into a corresponding dimple at the posterior surface of the artificial eye. This peg thus directly transfers implant motility to the artificial eye.[5] However, the motility peg is mounted in only a minority of patients. This may partially be the result of problems associated with peg placement, whereas hydroxyapatite implants are assumed to yield superior artificial eye motility even without the peg.[5]

Polyethylene also becomes vascularized, allowing placement of a titanium motility post that joins the implant to the prosthesis in the same way that the peg is used for hydroxyapatite implants.[7]