Design of a Magnetically Integrated Microporous Implant FREE

BARTISCH FIRST described enucleation as a treatment of ocular disease in 1583.¹ The current surgical approach was introduced in 1841 by Farrell and Bonnet, and in 1884, Mules placed the first orbital implant.² In 1946, Ruedemann³ proposed the use of partially exposed, integrated implants with the attachment of the extraocular muscles to the implant to allow for better prosthetic motility. Complete tissue enclosure of the implant began in the 1950s, minimizing socket complications but limiting prosthetic motility. Troutman,⁴ in 1949, introduced a magnetic integrated implant. Magnetic implants were then evaluated by several investigators.^{5 - 8} Late complications associated with the magnetically integrated implants, including exposure and extrusion, led to the discontinuation of this reconstructive approach. The use of a microporous magnetic coupling system was postulated to eliminate the complications associated with prior magnetic implants.

The ideal orbital implant would offer excellent motility and cosmesis and few complications. To achieve this goal, various materials have been advocated, including cartilage, bone, fat, cork, rubber, gold, silver, silk, wool, aluminum, ivory, acrylics, silicone, quartz, glass, titanium, and, recently, porous materials, such as polyethylene and hydroxyapatite.^{2 ,9 - 13}

Tissue-covered, quasi-integrated implants derived from acrylic irregularly shaped spheres include the Allen implant (in which extraocular muscles are passed through tunnels) and the Iowa and Universal implants (in which extraocular muscles are passed through grooves within the implant).^{9 - 11 ,14 - 16}

Porous spherical implants are most widely used at the Bascom Palmer Eye Institute, Miami, Fla, and other institutions.^{17 - 18} Porous polyethylene is made from synthetic, high-density polyethylene powder that is easily molded into virtually any shape. Hydroxyapatite, derived from reef-building coral of the genus Porites, was introduced in 1985 by Perry^{19 - 20} as a microporous implant material. Microporous implants allow for fibrovascular ingrowth and offer the theoretical, and real, advantage of lower extrusion or migration rates.

Decreased local complication rates have clearly been achieved with the nonintegrated microporous implants.^{17 - 26} Unfortunately, attempts to increase motility through integration of the implant with the prosthesis have led to unacceptably high complication rates (approaching 28% of pegged implants).^{23 ,25 - 26}

This study was designed to investigate the potential advantages of magnetically coupling a buried microporous implant with an ocular prosthesis for tissue tolerance and enhanced motility.

All experiments in this study were conducted under the auspices of the Animal Care and Use Committee of the University of Miami School of Medicine, Miami, Fla. Experiments adhered to the guidelines of the Association for Research in Vision and Ophthalmology Statement for the Use of Animals in Ophthalmic and Vision Research.

Microporous polyethylene implants (MEDPOR; Porex Corp, College Park, Ga) were custom modified to integrate a medical-grade 2 × 3-mm stainless steel post embedded flush with the microporous sphere (Figure 1). Custom-fitted rabbit ocular prosthetics were manufactured integrating two 1-mm circular medical-grade rare earth magnets (Figure 2). A spacing interval of 1 mm was maintained, and the magnets were embedded 0.5 mm lateral to the midline (right and left of center at the horizontal meridian) (Figure 2). The prosthetics were vaulted to achieve a 0.5-mm elevation from the central conjunctiva, thereby eliminating direct central apposition of the conjunctiva and prosthesis. An identically designed prosthesis without incorporation of the magnets was used as a control.

Magnetic field strength (force) was determined using a hanging ball technique.²⁷ Briefly, the hanging ball technique measures the magnetic force by determining the amount of movement of a 0.32-cm steel ball as a test object placed at varying distances from the magnet test object. Field strength was determined in a single and combined magnet application at varying distances determined to approximate the clinical relation between the prosthetic and the implant.

Six animals underwent implantation of the custom-modified microporous implants (Figure 1). All animals were anesthetized with intramuscular injections of ketamine hydrochloride and xylazine hydrochloride, retrobulbar bupivacaine hydrochloride, and topical proparacaine hydrochloride. Under appropriate anesthesia and monitoring, primary enucleation of the right globe was performed using a previously reported technique.²⁸ A custom 12-mm microporous implant with an integrated steel post was implanted to maintain the orientation of the post along the horizontal and vertical center of the socket (Figure 1). The recti muscles were reapproximated to the wrapped implant to maintain gross anatomic positioning. The deep and superficial Tenon tissues were closed with 5-0 polyglactin 910 sutures, and the conjunctiva was closed with a 7-0 polyglactin 910 suture. A small ophthalmic conformer was inserted, and an occlusive dressing was applied. The sockets were serially examined, and clinical photographs were obtained (Figure 3). At 6 weeks after enucleation, the magnetically integrated, custom-fitted prosthesis was placed in all animals (Figure 4). Animals were clinically evaluated for prosthetic motility using the eyes with a magnetically integrated prosthesis and the control eyes with a nonmagnetically integrated prosthesis. Control motility was determined by placement of the nonmagnetic prosthetic. Motility was graded, in a masked fashion, as good, fair, or poor, evaluating lateral prosthetic excursion during direct observation (poor indicates <1 mm; fair, 1-2 mm; and good, >2 mm).

Integration of the steel post within the microporous implant was accomplished without difficulty or compromise of the implant (Figure 1). Placement of the medical-grade magnets within the custom prosthesis did not impair the socket placement of the prosthesis (Figure 2 and Figure 4). Clinical and histopathologic evaluation of treated sockets, during a 20-month follow-up, disclosed no evidence of infection, inflammation, extrusion, or complication (Figure 3). Orbital histopathologic features at 6 and 12 weeks after placement of the prosthesis disclosed no evidence of conjunctival thinning or orbital complication. Magnetic movement forces achieved adequate field strengths to enable magnetic coupling of the buried implant and custom prosthesis (Table 1). All eyes with the magnetically integrated prosthesis were graded as having good motility, while all eyes with placement of the control, nonmagnetic prosthesis were graded as having poor motility.

Magnetic coupling of a buried, steel post–implanted, microporous implant appears to generate clinically effective force profiles for interaction with the modified magnetic prosthesis. No data are available to determine the ideal placement of the magnets within clinical practice. Data suggest that within the rabbit model, the spacing interval of 1 mm (0.5 mm lateral to the midline) for the 2 prosthetic magnets and a prosthetic vault of 0.5 mm will allow for enhanced movement of the prosthesis.

The design and production of these integrated devices are possible using medical-grade production items, including the microporous implant, steel post, and surgical-grade magnets. No intraoperative or postoperative complications were noted in any treated animal. Rapid socket healing and rehabilitation were associated with this surgical technique.

This technique appears to achieve the benefits of an invasively integrated implant-prosthetic combination without the associated concerns of socket-related complications, including epithelial breakdown, exposure, extrusion, or infection. Data suggest that these rates may be as high as 28% or greater for microporous implants undergoing "pegging" of the implant.^{23 ,25 - 26} These rates are concerning in light of the recently reported^{17 - 18} pediatric and adult complication rates in nonpegged microporous implants of less than 1.5%.

This study establishes the feasibility and efficacy of this magnetically coupled implant within this animal model. Advances in materials, surgical technique, and ocular prosthetic fitting allow for a reconsideration of a magnetically coupled implant.^{5 - 8 ,24 - 32} True evaluation of enhanced cosmesis will require clinical study in humans undergoing enucleation and implant placement.

A caveat to the use of this implant will be the restriction of magnetic resonance imaging studies for these patients. Fortunately, computed tomographic scanning analysis continues to allow for adequate orbital and central nervous system imaging in most patients.

Accepted for publication February 24, 2000.

This study was supported by the American Cancer Society, Tampa, Fla (Dr Murray); and Fight for Sight/Prevent Blindness America, New York, NY (Dr Murray).

Reprints: Timothy G. Murray, MD, Bascom Palmer Eye Institute, University of Miami School of Medicine, PO Box 016880, Miami, FL 33101 (e-mail: tmurray@bpei.med.miami.edu).