The insertion of an implant is an integral part of the enucleation procedure. There are different benefits and risks associated with each type of anophthalmic implant, of which there is no style that is most ideal for every case. Modification of surgical technique may be needed when using certain implants. The results of enucleation can be maximized by individualizing the choice of implant size and material for each patient.
Discussion
The ideal implant helps maintain the normal configuration of the orbital soft tissue, in addition to reducing the volume deficit created by removing the globe. Insuring the extraocular muscles are attached or anatomically approximated to the implant provides optimal prosthetic movement. Modern enucleation surgeons can choose among implants made from different materials in a variety of sizes and shapes. Porous implants can be fabricated from hydroxyapatite, porous polyethylene, and aluminum oxide. The tissue in-growth that occurs with these materials reduces the risk of implant migration. Insertion of a peg or post into porous implants allows coupling of the implant to the prosthesis, improving prosthetic movement.1
Commonly used nonporous implants are made from silicone or acrylic, and are significantly less expensive than their porous counterparts. Acrylic, silicone, aluminum oxide, and traditional hydroxyapatite implants are often wrapped in a covering material, to which the extraocular muscles can easily be attached. The muscles may be directly sutured to porous polyethylene and polymer-covered hydroxyapatite implants. When identical methods of muscle attachment are used, acrylic and silicone implants provide similar prosthetic movement, as seen in patients with porous implants who have not undergone insertion of a motility peg.2,3 Patients with pre-existing conjunctival shortage may develop inadequate fornices after enucleation. Placement of a primary dermal fat graft instead of an implant can help to expand these fornices.
Implant exposure remains one of the more common and serious complications of enucleation. Porous and nonporous implants wrapped in autogenous tissue or donor human sclera have very low exposure rates. Meticulous surgical technique may be needed to achieve similar success with porous implants that are either unwrapped or covered with an absorbable material. The extraocular muscles are often advanced anteriorly onto the front surface of these types of implants, and layered closure of Tenon's fascia and conjunctiva is performed. There is a growing body of evidence showing that insertion of uncovered porous implants in young children with retinoblastoma is associated with an unacceptably high rate of implant exposure.4 In young children, the surgeon should consider wrapping both porous and nonporous implants in autogenous or donor tissue.
The insertion of the motility peg or post into porous implants may also be associated with problems, including chronic discharge, implant infection, and implant exposure. Late exposures can occur after motility post insertion. One study has shown a 33% complication rate after post placement in porous polyethylene implants.5 Implant exposure appears to be less frequent in pegged hydroxyapatite implants.6
Most exposed nonporous implants will eventually extrude or require removal. While a minority of porous implant exposures will spontaneously heal, many of these implants can be salvaged by early repair of the defect, using a tissue patch graft covered by a vascularized conjunctival flap. Unfortunately, porous implant exposure or infection may occasionally also require implant removal.
Traditionally, many surgeons have placed a standard-sized implant in all patients, often using a smaller device in pediatric cases. A properly sized implant facilitates prosthetic fitting and transmission of movement to the prosthesis. An excessively small implant frequently results in a deep superior sulcus, requiring a larger prosthesis to fill the residual volume deficit. The weight of the prosthesis may then displace the lower lid and limit prosthetic movement. Implants that are too large often leave insufficient space to allow fitting and comfortable wear of a prosthesis.
There is a growing trend for surgeons to individualize the size of enucleation implants. This can be done empirically or by using sizing implants during surgery. Alternatively, implant size can be calculated prior to surgery by performing a preoperative ultrasound of the contralateral eye or determined during the procedure by measuring the volume of enucleated tissue.7 There can be significant variability in the size of the adult globe, measured to be between 7.0 and 9.0 ml.8 Most adult patients will accommodate implants between 20 and 22 mm in size.
While most enucleation implants are spherical in shape, they can be manufactured in a variety of configurations. Some implants have a posterior extension added to the implant, with the intent of filling the superior sulcus by displacing the orbital soft tissue upward. However, there is data that shows implant shape is much less important than implant size in reducing the sulcus deformity. The irregularly shaped Allen, Iowa, and Universal nonporous implants are felt to more effectively transmit movement to the prosthesis. Occasionally these implants will rotate after surgery, changing the orientation of their anterior surface and compromising prosthetic wear. A similar phenomenon can be seen when the anterior surface of porous implants are flattened to provide more room for the motility peg or perhaps provide improved coupling to the prosthesis.9 Prosthetic fit is not altered should a spherically shaped implant rotate after surgery.
Conclusion
Today's enucleation surgeons have a variety of implants at their disposal. No single implant is ideal for all patients. Nonporous spheres may be best for those individuals who are either at higher risk for developing implant exposure or considered to be poor candidates for subsequent motility peg or post insertion. Meticulous technique is needed when inserting porous implants that are either unwrapped or covered with absorbable materials. Covering porous implants with autogenous or human donor tissue appears to reduce the exposure rate, particularly in young children. Individualizing implant size is effective in minimizing the postopertative superior sulcus deformity that results from an orbital volume deficit.
References
- Guillinta P, Vasani SN, Granet DB, Kikkawa DO. Prosthetic motility in pegged versus unpegged integrated porous orbital implants. Ophthal Plast Reconstr Surg. 2003;19(2):119-122.
- Custer PL, Trinkaus KM, Fornoff J. Comparative motility of hydroxyapatite and alloplastic enucleation implants. Ophthalmology. 1999;106(3):513-516.
- Colen TP, Paridaens DA, Limij Hg, et al. Comparison of artificial eye amplitudes with acrylic and hydroxyapatite spherical enucleation implants. Ophthalmology. 2000;107(10):1889-1894.
- Custer PL, Trinkaus KM. Porous implant exposure: incidence, management, and morbidity. Ophthal Plast Reconstr Surg. 2007;23(1):1-7.
- Cheng MS, Liao SL, Lin LL. Late porous polyethylene implant exposure after motility coupling post placement. Am J Ophthalmol. 2004;138(3):420-424.
- Jordon DR, Klapper SR. A new titanium peg system for hydroxyapatite orbital implants. Ophthal Plast Reconstr Surg. 2000;16(5):380-387.
- Kaltreider SA, Lucarelli MJ. A simple algorithm for selection of implant size for enucleation and evisceration: a prospective study. Ophthal Plast Reconstr Surg. 2002;18(5):336-341.
- Custer PL, Trinkaus KM. Volumetric determination of enucleation implant size. Am J Ophthalmol. 1999;128(4):489-494.
- Custer PL. Postoperative rotation of hydroxyapatite enucleation implants. Arch Ophthalmol. 1999;117(11):1521-1523
Author Disclosure
Dr. Custer states that he has no financial relationship with the manufacturer or provider of any product or service discussed in this article or with the manufacturer or provider of any competing product or service.