Pmma Orbital Implant

[Paz Warsager]

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Purpose: To evaluate the outcome of primary orbital polymethylmethacrylate (PMMA) implant following the primary enucleation for retinoblastoma. Methods: Retrospective study of 321 retinoblastoma patients who underwent unilateral enucleation and PMMA implant for retinoblastoma by myoconjunctival technique. Outcome measures included implant centration and extrusion. Results: The mean age at the time of enucleation of patients with retinoblastoma was 35 months (median, 30 months; range,

SILICONE ORBITAL IMPLANT

Silicone orbital implant is very similar to PMMA implant and it differs only for its flexibility and a less smooth surface. These features guarantee a minor trauma on cover tissues and also a poor sliding of the implant. For these reasons the silicone implant is suitable to be contained in scleral shell after evisceration of the bulb.

Due to its low cost and good versatility, it is still chosen today over more expensive and perfected implants. It is well tolerated and expulsion is unlikely. In evisceration offers excellent motility, otherwise only achievable with porous implants.

THE SEARCH for the ideal eye implant for anophthalmic sockets continues notwithstanding the availability of orbital implants for years. Size, shape, composition, and cost determine the ideal type of implant. It must approximate the normal eye, resemble the globe in shape, occupy the excess orbital volume, and compensate for the missing viscera. It must also be made of material that the body would not reject. In the Philippines, the solid acrylic implant is commonly used because of its low cost. A five-year survey of cases at the Plastic and Lacrimal Clinic of the Philippine General Hospital reported that the most common anophthalmic socket problems were socket contraction and implant extrusion (Pagkatipunan PN and Mangubat LR, Anophthalmic socket in the plastic and lacrimal clinic, Department of Ophthalmology and Visual Sciences-Philippine General Hospital, 1996). Surgical technique, infection, and type of implant used were the usual causes. A five-year survey of 2,775 cases found 110 (3.9%) socket problems with 11 cases of implant extrusion (Tecson JV and Mangubat LR, Plastic and lacrimal clinic: A fiveyear survey, Department of Ophthalmology and Visual Sciences-Philippine General Hospital, 2001).

Integrated implants such as hydroxyapatite (HA) and porous polyethylene (PPE) are preferred because they allow tissue ingrowth and are associated with low incidence of migration. But these cannot be used for immunocompromised patients or those who have infections because the pores may serve as nidus for infection. Nonintegrated implants such as solid silicone and polymethylmethacrylate (PMMA) have been widely used because of their inert properties and low cost. However, these are associated with a high incidence of extrusion and migration. To date, no randomized clinical trial has been conducted to evaluate the stability of the various nonintegrated implants. We hypothesized that for most cases of migration of noninegrated implants, weight may be a cause. A hollow implant would be expected to be more stable compared with a solid implant of the same size as it would be subjected to less gravitational force.

We conducted this study to evaluate the suitability of an experimental hollow PMMA implant for the anophthalmic socket. We determined its physical properties (water absorption capacity, bulk density, hardness strength) and identified complications (infection, wound dehiscence, and implant extrusion and migration) vis--vis solid acrylic spheres.

METHODOLOGY

Nito-Seiki Manufacturing Corporation (Baesa, Quezon City, Philippines) molded the hollow implant (Figure 1). Medical-grade PMMA powder was used to form halfspheres by plastic injection technique. Pairs of half-spheres were fused ultrasonically to form 20-mm hollow PMMA spheres. The physical properties of the hollow PMMA implant were tested by the Department of Science and Technology (Taguig, Metro Manila, Philippines). The following parameters were measured: water absorption, bulk density, hardness. The ultra structure of the implants was observed under scanning electron microscopy. The study complied with the protocol established in the Declaration of Helsinki and was approved by the bioethics committee of the University of the PhilippinesPhilippine General Hospital (UP-PGH). Study subjects seen consecutively at the Department of Ophthalmology and Visual Sciences of the PGH were selected based on the following inclusion criteria: male or female 18 years old or older with clinically diagnosed nonseeing eyes secondary to traumatic globe injuries, ruptured corneal ulcers, or staphyloma who were advised to undergo evisceration or enucleation with orbital implantation or reimplantation for extruded orbital implants. Patients were excluded from the study if their normal eye had an axial length of less than 21mm determined by A-scan or an intraoperative determination showed the socket would have difficulty accommodating a 20-mm implant. Subjects were randomly assigned into the control group, which received the solid acrylic implant, or the treatment group, which received the hollow PMMA implant.

All participants were interviewed and underwent a complete eye examination consisting of visual acuity assessment, gross orbital exam, extraocular muscle function, intraocular pressure (IOP) determination, funduscopy, and slit-lamp examination. The clinical diagnosis was recorded. The mean axial length (average of 3 measurements) of the fellow normal eye measured by A-scan was used to estimate the length of the affected eye. Prior to implantation, the orbital implant was weighed and its volume measured by water displacement. A standardized posterior sclerostomy and insertion of the

Axial length measurements and weight of implants The mean axial length of the fellow eyes was 23.38mm 0.817 in the solid acrylic group and 23.34mm 1.424 in the hollow PMMA group (Table 1). The hollow PMMA implant weighed 2.5g, half the weight of the solid acrylic implant.

At 6 weeks, 11 of 12 implants remained in the socket; 1 in the solid acrylic group extruded. At 12 weeks, only 7 patients (4 solid acrylic, 3 hollow PMMA) completed the follow-up, and all implants were in place. Results of orbital CT scan at 12 weeks follow-up are shown in Table 2. A migrating implant is one that has been displaced from its original position but has not yet extruded from the socket on eye exam.

DISCUSSION

In the design of orbital implants, several factors, namely size, shape, composition, weight, and volume, must be considered to minimize migration. An implant smaller than the volume of the affected eye yields a volume deficit and poor aesthetic results. This study showed that for eyeballs that have an axial length of at least 21.98mm by A-scan ultrasound, a 20mm implant can be placed. These results are consistent with the findings in a 2000 study,

Modified technique of evisceration with anterior sclerotomy and double breasting of sclera along with primary placement of PMMA orbital implant was performed in 35 consecutive patients who presented with various causes of painful blind eye. The postoperative performance of the implant was assessed in terms of volume replacement, motility, extrusion rates and other complications.

In the present study, degree of volume replacement was found to be good in 13 patients and fair in 22 patients. 24 of the 35 patients had a good movement of prosthesis (>20 degrees of horizontal movement) and 11 patients had fair motility(10 to 20 degrees). Extrusion rate was nil.

Evisceration with modified technique followed in the present study minimised the extrusion rates of PMMA implants with successful retention of the implant. In conclusion, evisceration with PMMA orbital implant can be an effective treatmentfor painful blind eyes with excellent postoperative outcome.

Since eye contact is an essential part of human interaction, it is important for the artificial eye patient to maintain a natural, normal-appearing prosthetic eye. When someone loses an eye, two components are required: an orbital implant to replace the eye and help maintain the volume of the eye socket and an artificial eye (or prosthesis).

Over the last 100 years, a variety of synthetic (man-made) and natural materials have been used as orbital implants. Many of these materials, such as gold, silver, cartilage, bone, and cork among others, caused significant complications.

Non-porous, synthetic implants have been the mainstay of globe replacement for the past 50 or more years. These implants made of acrylic (polymethylmethacrylate, PMMA) or silicone are inert and well-tolerated by the human body. Non-porous implants typically do well for years following implantation. Because the surrounding orbital tissue does not grow into the implants (as with porous implants, see below), these standard implants occasionally may migrate (move) within the eye socket creating difficulties with fitting of a prosthesis.

Enucleation is the complete removal of the entire eye. This technique is required in all patients with known or suspected intraocular tumors. It may also be recommended if it can not be determined whether or not a tumor may be present in the back of the eye. In this technique the eye muscles are detached from the globe prior to removal of the eye, and the muscles are reattached to the orbital implant or to the material used to wrap or cover the implant.

Evisceration involves removal of the contents of the eye or globe leaving the sclera (white part of the eye) and the eye muscles in place. This technique is less disruptive to the orbit as dissection into the eye socket is minimized.

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