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The procedure may be performed under general or local anesthesia, according to the surgeon's preference. In either case, the injection is given in the retrobulbar space. An antibiotic (e.g., cefazolin) and an anti-inflammatory (e.g., dexamethasone) are also given intravenously.
Isolation of the Levator Muscle
The levator muscle is then isolated by passing a 4-0 silk, double-armed suture full-thickness through the upper fornix, in a mattress fashion. This will protect the muscle and permit easy identification of the superior fornix during closure of anterior Tenon's capsule, thereby avoiding foreshortening of the fornix. (Soll 1972).
Peritomy, Muscle Identification, and Neurectomy
A 360° peritomy is then performed, with care taken to preserve as much of the conjunctiva and Tenon's capsule as possible. The extraorbital muscles are then tagged with 5-0 Vicryl sutures and are released by resection at their attachments to the globe. A neurectomy is then performed, according to the surgeon's preference, and the globe is delivered. Care should be taken to maintain hemostasis until the implant is placed in the orbit.
Evaluation and Preparation of Tenon's capsule
Tenon's capsule is then inspected for its readiness to accommodate the implant. In order to minimize the risk of implant exposure and subsequent infections, it is important to ensure that the tissues overlying the implant can be closed without tension. Traditionally, the orbital implant was placed within Tenon's capsule, as first described by Frost (Frost 1887), necessitating the use of a smaller implant. This technique usually required the use of implants with a diameter of 16 mm to 18 mm. However, due to the enophthalmos associated with these smaller implants, Soll recommended placement of the implant deeper within the muscle cone, posterior to Tenon's capsule (Soll 1972, Perry 1990). This posterior placement necessitates the use of a larger implant. As a variation of Soll's technique (Soll 1972), the implant should be placed within Tenon's capsule without closure of posterior Tenon's capsule. The rent in posterior Tenon's capsule, where the optic nerve penetrated, should be inspected; it will vary depending on how close to globe the nerve was cut. If necessary, enlarge the rent using a blunt hemostat. This will allow some portion of the implant to extend into the muscle cone, thereby alleviating pressure on the closure and the tissues overlying the implant.
Determination of Implant Size
Proper sizing of the orbit is essential to avoid complications and to achieve an optimum fit of the implant to both the orbit and the artificial eye. An undersized implant may result in enophthalmos or a deep superior sulcus, and may necessitate weight-increasing modifications to the artificial eye which, in turn, induce lower lid sag and restricted motility. Conversely, an oversized implant increases the risks of wound dehiscence, exposure, and infection, and may complicate the coupling of the artificial eye to the motility peg. An implant is of the proper size when its volume, added to the volume of the implant wrapping material and the volume of the artificial eye, equals the volume of the enucleated globe (Perry 1983). Refer to a sizing table for determining implant size from the size of the globe. A set of graduated sizing spheres should be used to properly size the orbit. A sizing sphere is of the proper size when it is the largest sphere that allows closure of the conjunctiva and anterior Tenon's capsule without tension. The diameter of the sizing sphere is noted from markings on its handle, although a calculation must be made to compensate for the thickness of the material used to wrap the implant. A scleral wrapping increases the diameter of the implant by an average of 1.5 mm. While it is known that the average orbit will accommodate an 18 mm or 20 mm implant wrapped in sclera, each orbit should nevertheless be individually sized.
Wrapping the Implant
There are several benefits to wrapping the implant. These include: increased motility, ease of insertion, ease of muscle attachment, and decreased risk of exposure. The use of an unwrapped implant allows the rough surface of the hydroxyapatite material to snag on orbital tissues during insertion, and may also cause abrasion of the overlying tissues during movement, thereby increasing the risk of exposure. Prior to wrapping, the implant is sterilized and soaked in an antibiotic solution (80 mg gentamicin in 100 cc saline). The implant is placed in the scleral shell and the shell is sutured closed with 5-0 Vicryl sutures. The excess tissue is trimmed and marks are made at the attachment sites of the four rectus muscles and at the anterior pole of the implant. Small windows (5 mm x 7 mm) are cut at the attachment sites of the rectus muscles and the implant is inserted into the orbit. The four rectus muscles are then brought into contact with the implant by passing a double-armed 5-0 Vicryl suture through the anterior lip of the window and tying the muscles snugly to the opening. This contact between the hydroxyapatite material and the highly vascular muscle tissues ensures good vascularization of the implant (Perry 1991c, DePotter 1992, Ferrone 1992). The oblique muscles can be attached, according to the surgeon's preference. Fornix-deepening sutures may be used at this point, as needed, to provide space for the artificial eye to move in concert with this highly motile implant.
The wound should be closed with separate closures of Tenon's capsule and the conjunctiva in order to minimize the risk of exposure. Anterior Tenon's capsule is closed with interrupted 5-0 Vicryl sutures; the conjunctiva is closed with a running 5-0 Vicryl suture. Antibiotic ointment is applied to the socket and an acrylic conformer is placed over the conjunctiva, with care taken to avoid undue pressure between the conformer and the wound closure. One or two temporary, intermarginal tarsorrhaphy sutures of 5-0 Vicryl can be used to control edema. Postoperative pain control is then initiated with 4 cc of 0.75% bupivacaine injected into the muscle cone.
Dressings and Postoperative Care
A firm pressure dressing is maintained for 4 to 6 days, oral antibiotics are given for 1 week, and steroids (prednisone 20 mg, adjusted according to patient status) are given 3 times daily for 4 days. Oral postoperative pain medication is also given as needed. The socket is evaluated after removal of the pressure dressing and, if the edema has subsided, the tarsorrhaphy sutures are removed. Topical antibiotics are applied 4 times daily for 4 weeks. The patient is generally ready for fitting with an artificial eye 6 to 8 weeks postoperatively.