4.1.5 Preoperative Medications

Preoperatively we prefer to give our patients i.v. mannitol if there are no systemic contraindications to doing so. Intravenous mannitol helps to dehydrate the vitreous cavity and lower the intraocular pressure (IOP), thus decreasing the incidence of forward prolapse of intraocular contents during the procedure. For younger patients 50 g is typically administered. For older individuals, 25 g may be appropriate. Mannitol is administered over 30 min to 1 h.

Unless a cataract extraction is planned along with PK, pilocarpine 1% ophthalmic drops are administered preoperatively to effect miosis of the pupil and decrease the forward prolapse of intraocular contents during the procedure.

If a cataract extraction is planned concurrent with PK, the pupil is dilated preoperatively in the typical fashion and intraoperative miochol may be administered following cataract removal to effect miosis.

An antibiotic is administered every 5 min for a total of three doses prior to surgery. Vigamox (Alcon, Fort Worth, TX) or Zymar (Allergan, Irvine, CA) are examples.

A Honan cuff may be used preoperatively to reduce IOP prior to beginning the procedure. A typical setting is 30 mmHg.

4.1.6Penetrating Keratoplasty Surgical Procedure

The technique of PK is as follows. Patients are taken to the operating room following informed consent and following administration of appropriate medications as detailed above. Anesthesia appropriate to the patient and of the surgeonÕs preference is undertaken. Most of our patients undergo general anesthesia unless there is a systemic contraindication. General anesthesia with complete paralysis at the time of the removal of the

host cornea ensures that the patient does not undergo a valsalva maneuver and expulse intraocular contents. The eye is prepped and draped in a sterile fashion and a lid speculum is placed. The lid speculum must be properly sized to Þt the patient and positioned to minimize pressure against the eye. Any pressure against the eye causes globe distortion that can lead to irregular trephination and resultant astigmatism due to poor suture alignment. Also, pressure against the eye can cause an expulsion of intraocular contents. So it cannot be emphasized enough that the lid speculum must Þt properly.

4.1.6.1 Placement of the Scleral Fixation Ring

When the patient has had a previous vitrectomy or when there is a history of prior lens removal, a scleral Þxation ring is sutured with four to six interrupted 7-0 vicryl sutures with half-thickness scleral bites, with care not to pass too deeply through the less than 1 mm deep sclera. These sutures are typically placed from the periphery toward the limbus. The Þxation ring is sized to measure slightly less than the interpalpebral opening as deÞned by the lid speculum. The ring functions as a potential scaffold to maintain scleral support once the eye is opened if scleral rigidity is insufÞcient.

Some surgeons have abandoned the use of the Þxation rings to avoid any associated globe distortion and astigmatism induced by ring placement. We prefer to suture a Flieringa ring to the episclera with 7Ð0 vicryl suture. This provides excellent stability.

4.1.6.2 Marking of the Host Cornea

Following placement of the Þxation ring if needed, measurement of the host corneal diameter is made in both vertical and horizontal directions with a caliper, and in cases of cornea ectasia, care is taken to measure around the base of the ectatic cornea. The optical axis is marked by the surgeon, using the center of the pupil if possible. Typically the donor graft is centered on the host cornea or over the pupillary axis. An 8- or 12-prong radial cornea marker is used by some surgeons to assist in donorÐhost suture alignment.

4.1.6.3 Sizing of the Trephine

Sizing of the host trephine is based on several factors that include cornea size and the diameter needed to excise all of the corneal pathology. Traditionally, sizing of the host was also based on minimizing the supposed risk of increased graft rejection and failure by staying as far back from the limbus as possible. But larger diameter grafts have some inherent advantages. These include the excision of the inciting pathology that may be in the cornea periphery as in cases of cornea ectasia, as well as reduced postoperative astigmatism with better corrected and uncorrected visual acuities (UCVA). The authors have presented a group of patients that had large diameter penetrating keratoplasty (LDPK), deÞned as 8.75 mm or larger, and did not experience an increase in the graft rejection or failure rate. In fact, no patients in our study experienced a graft failure. For this reason, we prefer to use a trephine size of 8.75 mm or larger. Further reasoning and outcome of our study will be discussed later. If an 8.75 mm or larger trephine is chosen, the same size trephine is chosen to cut the donor button. In general, if a smaller trephine is chosen, the donor tissue trephine is routinely sized 0.25 mm larger than the host trephine because donor corneal tissue cut with a trephine from the endothelial surface measures about 0.25 mm smaller in diameter than host corneal tissue cut with the same diameter trephine from the epithelial surface [1].

4.1.6.4 Trephination of the Host Cornea

Host trephination is performed with the hand-held disposable trephine held perpendicular to the cornea. Minimal pressure is exerted against the cornea as the trephine is progressively rotated until it has penetrated approximately 90% depth of the host cornea. It cannot be overemphasized that very little pressure needs to be exerted during this maneuver because the trephines are very sharp and too much exertion could cause an unwanted entry into the anterior chamber with a quick shallowing of the eye and a prolapse forward of intraocular contents. It is the rotation of the trephine that does the cutting, not the use of excessive pressure by the surgeon.

Some surgeons prefer to actually enter the anterior chamber with the initial host trephination and look for

a drop of aqueous to become visualized. Because the cornea is not of absolute uniform thickness, the initial opening using this method will often be only one or a few clock hours.

The authors prefer to trephine to approximately 90% thickness and then enter the anterior chamber with a sharp blade. This allows a more controlled entry. Abrupt entry into the anterior chamber can be associated with a higher risk of suprachoroidal hemorrhage.

4.1.6.5 Trephination of the Donor Cornea

The donor cornea is trephined with the endothelial side facing up using a sharp disposable trephine in a punch block apparatus. The authors prefer to use an Iowa P.K. press and cutting block. Brießy, the cornea tissue is brought to the back table by the circulating nurse and the surgeon uses a toothed forcep to grasp only the donor rim and place the tissue, endothelial side up, on the cutting block. At this time, the tissue is gently lifted by the rim and tilted up, and a Weck cell is used to remove excess ßuid from under the donor tissue so that it does not slip during the process of trephination. A red dot in the center of the cutting block helps the surgeon to center the donor cornea appropriately. Centration prior to trephination is essential. The cutting block is placed on the Iowa press which has been assembled with the trephine, and the central donor cornea button is punched. The donor tissue is kept submerged in storage medium and covered while the surgeon is completing preparation of the host stromal bed.

4.1.6.6 Removal of the Host Cornea

Following trephination of the donor cornea, attention is directed again to the patient and a sharp blade is used to enter the anterior chamber for 1 clock hour in the 8 oÕclock meridian. A small amount of viscoelastic is placed in the anterior chamber to push the iris back and beveled cornea transplantation scissors are used to remove the host cornea. This maneuver is aided by maintaining host button alignment with toothed forceps. Care is taken not to apply any downward pressure to the globe during this time so as to not expulse the lens. The tips of the scissors should be always be visualized, the scissors should be at an angle of about

10¡, and a slight amount of upward pressure against the host cornea should be maintained to avoid iris damage during the cut. Keeping the scissors at an angle of 10¡ helps create a slight inwardly beveled cut which makes a watertight wound closure easier. Furthermore, the scissors should be advanced around the host cornea in a continuous fashion with very little exit and reentry into the anterior chamber. This ensures that the cut is made uniform, which will help decrease postoperative astigmatism.

If the iris and the lens begin to bulge forward during host trephination, a number of maneuvers can be performed to decrease the positive posterior pressure. First, the surgeon should ensure that pressure is not being exerted while advancing the cornea transplantation scissors. The lid speculum can be loosened and the patient may be placed in reverse trendelenburg positioning. Putting a patient that is overweight in the reverse trendelenburg position can allieviate the positive pressure exerted on the head from the heavy abdomen.

4.1.6.7Placement of the Donor Cornea Tissue in the Host Stromal Bed

After removal of the host cornea, the donor cornea is brought onto the Þeld and gently grasped with a Þnetoothed forcep at the junction of the epithelium and the stroma with great care taken not to touch the endothelium. The donor cornea is transferred onto the host recipient bed and rotated until either the most spherical reßex is obtained with an intraoperative keratometer, or the surgeon is happy with the visualized alignment of the tissues [1]. This may help reduce postoperative astigmatism. If an area of arcus or residual scleral rim is present on the donor tissue, it should be placed superiorly if possible so that, it will be covered by the upper lid.

4.1.6.8 Placement of the Cardinal Sutures

The Þrst 10-0 nylon interrupted suture is placed at the 12 oÕclock position. The donor cornea is grasped as mentioned above at the epithelial-stromal junction with a Þne-toothed forcep. Some surgeons prefer to use a double-pronged forcep that allows for suture placement between the teeth of the forcep, and then