Final graft astigmatism remains a major shortcoming of penetrating keratoplasty for the corneal surgeon. One of the most important aspects of surgery is precise positioning of the donor tissue into the recipient opening. A simple modification of an existing technique has been developed that allows the surgeon to verify the correct positioning of the first eight cardinal sutures in the donor tissue and actual trephined wound, using a unique surgical reticule. This technique should minimize asymmetric placement of the graft within the host wound, which is one of the leading causes of corneal transplant astigmatism.
MATERIALS AND METHODS
The design of the reticule is quite simple (Fig. 1). The reticule (Archer Optics, Anaheim, CA, U.S.A.) has a 24 mm diameter, and fits into the optical holder that can be attached inside one of the objectives of the Leica M690 operating microscope (Leica, Heerbrugg, Switzerland). The reticule has an etched 5-mm circle with eight thin radial lines spaced 45 degrees around the circle. The lines (5 mm length) extend to the periphery and also extend inside the circumference of the circle by 0.5 mm. The circle is large enough that the clear central area permits normal visualization of the surgical field for all other operating maneuvers. The operating microscope is kept perfectly perpendicular to the surface of the cornea during donor tissue preparation and actual surgery on the patient to avoid an elliptical projection of the reticule onto the tissues.
The donor corneas would be marked as follows: A circular corneal button is prepared from either the endothelial side on a Teflon block, or the epithelial side in an artificial anterior chamber according to the standard technique of the surgeon. A viscoelastic material (Provisc, Alcon Laboratories, Fort Worth, TX, U.S.A.) is placed in a separate, clear storage block, and the donor button is positioned (endothelial side down) over the viscoelastic to protect the endothelial cells. The epithelial surface is dried with cellulose sponges. The magnifying power of the operating microscope is increased so that the circumference of the button matches the circumference of the reticule circle. The end of a sterile toothpick is inked with gentian violet (Visimark gentian violet dye pad, Visitec, Franklin Lakes, NJ, U.S.A.). Eight peripheral epithelial marks are placed, using the end of the radial lines inside the reticule marker as guides. The 6 o'clock axis can be marked with three dots so that when the donor tissue is transferred to the host cornea the donor tissue can be properly orientated to avoid any parallax error, i.e. the 6 o'clock donor and host marks will be aligned along the same axis prior to suturing (Fig. 2). The proper positioning of the marks can be verified by visual inspection or checked with surgical calipers, if necessary. The tissue is then turned epithelial side down, and covered with the Optisol storage media (Bausch & Lomb, Irvine, CA, U.S.A.) awaiting transfer to the surgical field.
The following technique simplifies marking the recipient tissue. The geometric central corneal epithelium relative to the limbus is scratched using surgical calipers in the horizontal and vertical axes, and this dimpled area is inked with a gentian-violet-coated, sterile toothpick. An unmarked, eight-sided radial keratotomy instrument with a central cross-hair (Mastel, Rapid City, SD, U.S.A.) is pressed against the cornea, and the sterile toothpick stained with gentian violet is used to place eight spots around the limbus (Fig. 3). The same central mark is used to center the Moria (Moria SA, Antony, France) or Barron radial vacuum (Katena, Denville, NJ, U.S.A.) trephine. A partial trephination (0.25 mm) is performed. The shallow trephine incision appears to remain circular as long as there is sufficient (greater than 50%) posterior stromal support. The surgeon then increases the magnification through the operating microscope so that the circumference of the reticule circle matches the partial circular trephine incision. The 6 o'clock limbal mark is used as the reference starting point. The microscope ocular may have to be rotated slightly to properly align the 6 o'clock reticule lines and host limbal mark. After this is accomplished, the surgeon can verify that the previously placed limbal marks on the recipient tissue coincide with the peripheral reticule lines, and make changes if the actual trephination does not perfectly match the intended incision in the x and y axes (Fig. 4). The verified limbal marks are visually inspected to determine if they are equidistant and can be checked with surgical calipers, if necessary. The recipient button is then removed with corneal scissors. In practice, this combined technique (confirmation and repositioning, if necessary, of pre-trephined limbal marks with the reticule) has been found to be simpler than trying to mark the recipient tissue after partial trephination using the reticule alone, without any preexisting limbal marks. There are two reasons why the combined technique is simpler. Usually, only a few marks have to be repositioned, and having some pre-placed marks present as guides speeds the verification process. Additionally, unlike a stationary donor button, there is always slight movement of the recipient eye at high magnification, which makes it difficult to simultaneously confirm that the recipient cut is aligned with the reticule circle and mark the limbus. It is easier to judge discrepancies in any peripheral marks and make the necessary change than attempt to mark the periphery without any guiding spots.
The donor tissue is brought over the central opening. The surgeon simply matches the dots on the donor graft with the peripheral recipient marks to ensure precise placement of the sutures in the graft and recipient tissues. The first eight cardinal sutures can be initially tied with a slipknot and then tied permanently to create more uniform suture tension. The surgeon should see an important visual cue after correctly placing the first four sutures at 12, 6, 3 and 9 o'clock; the four remaining marks in the oblique axes on the donor graft and recipient limbal tissue should be perfectly aligned to confirm that the marks have been properly placed. Once four or eight sutures are placed, the surgeon should see a regular square or octagon pattern, respectively, in the donor tissue to verify that the sutures are properly placed (Fig. 5). There is one additional maneuver the surgeon can add to the marking technique described above to guarantee that the important second suture is placed accurately in the donor tissue. After the eight peripheral marks have been placed in the donor tissue as described above, the center of the corneal donor tissue can be marked with gentian violet using the 12, 3, 6 and 9 o'clock marks as guides (Fig. 2). After the first cardinal suture is secured and the cornea grasped with Colibri forceps at the 6 o'clock location, the vertical tension line should pass through the central mark for verification of proper donor suture placement (Fig. 6). After the cardinal sutures have secured the graft symmetrically with even tissue distribution within the host trephination wound, the surgeon can close the wound with a running suture or additional interrupted sutures (Fig. 7).
This reticule should help to minimize graft astigmatism caused by asymmetric suture placement in both the donor and recipient tissue. It has several advantages over current techniques. The reticule is inexpensive. It can be used with any size trephine opening in the donor or recipient so that multiple surgical instruments are not required. This is important because corneal surgeons often use disparate donor and recipient trephines that vary from 7.0 to 8.5 mm. The surgeon can continue to use the trephine equipment he/she currently uses, and does not need to modify existing instruments or purchase new equipment. 1 The thin lines on the reticule with the clear central area allow normal visualization, for performing all other maneuvers during surgery. Additionally, the donor tissue is marked only in the periphery where the suture will be placed, which minimizes epithelial trauma and simplifies suture placement. The time to mark the donor tissue and verify the position of the host lines takes less than 5 minutes. The gentian violet marks remain visible for more than 1 hour after the donor tissue is placed back in the Optisol solution (Bausch & Lomb) to await trephination of the host cornea. Although not specifically measured in this study, the shallow trephination on the recipient tissue maintains a circular anterior corneal impression. This may decrease the ovality of the recipient opening, which has been well documented with deeper incisions using mechanical or excimer laser devices, and minimize “vertical tilt,” which can also produce graft astigmatism. 2–4 Although using scissors to cut the posterior cornea will cause some vertical deviation in the wound architecture, which can contribute to astigmatism, no existing technique, including deep trephination, yields precise vertical cuts because of tissue movement and blade vector forces during trephination. The critical second suture passes with precision through the donor and recipient tissues, as do the other cardinal sutures.
Other researchers have devised mechanical instruments that mark the donor cornea before the actual trephination to aid in future suture placement. 5,6 Inaccurate marks can be created by slippage of the tissue in the block, by a trephine system that has too much free play in its housing or the block well, or by loose epithelium that slides during or after the trephination. 6 For systems that use gentian violet to mark the cornea prior to trephination, the dye may run diffusely across the tissue if it is not adequately dried before placement in the trephine cutting block. Additionally, if the surgeon has trouble centering the tissue, multiple or eccentric marks will be placed on the donor tissue. By using the reticule, the fully trephined donor button and partially trephined recipient tissue are marked accurately after the trephination, which is the only way to avoid creating eccentric marks with mechanical instruments.
The ideal marking system probably would use nonmechanical devises to mark both the donor and recipient tissues from the epithelial surface. Such a system, combined with excision of stromal tissue, has been proposed by German researchers using the excimer laser. 7 Special matching “orientation teeth” that fit like a key into its keyhole were created in the donor and host tissues using special masks. While this is quite elegant, most hospitals and surgeons will not have such expensive technology readily available for routine corneal transplant surgery. The authors showed that final suture-out keratometric astigmatism was decreased in the excimer/notched donor and recipient tissues (3.0 +/− 2.1 D), compared with the manual mechanical trephination with no surgical markings on either the donor or host tissues (6.1 +/− 2.7 D). 8 These statistically significant differences may be related to the trephination (excimer versus mechanical) technique, but also highlight the importance of accurately marking both the donor and recipient tissues to minimize asymmetric suture placement.
Some transplant surgeons still do not mark the donor or host tissues. These surgeons must guestimate where their initial sutures should be placed. This is quite difficult after the trephine opening has been made and there is accompanying scleral collapse that changes the trephine opening from a circle to an ellipse, as is commonly seen in keratoconic patients and even in aphakic patients with a Flieringa ring placed prior to trephination. Surgeons who mark only the host tissue with a four or eight-sided radial keratotomy instrument must assume that these pre-placed marks have been accurately positioned in relation to the trephined wound. 1,5,6 The actual site of trephination, however, may be slightly displaced, invalidating these marks as useful guides for suture placement. Pflugfelder et al. mentioned the possibility that the donor marks may not coincide with the center of the donor button with their suction trephine block and thereby be inaccurate guides for cardinal suture placement. 6 The same situation easily can occur on the host tissue in relation to the actual trephine opening through slippage of the trephine blade when suction is applied, or by slightly misaligning the trephine blade on a wide gentian violet optical zone guide mark. 5 Additionally, radial impressions formed at the host limbus with newer trephine systems would function as correct post-trephination marks only if the blade did not slip or slide during the actual trephination.
The technique outlined in this paper centers the trephination on the recipient tissue relative to the limbus. Graft decentration relative to the limbus has been measured to be 0.58 +/− 0.23 mm with mechanical trephination and even 0.23 +/− 0.26 mm with nonmechanical excimer laser trephination. 4 Figure 8 highlights the discrepancies induced when the trephine opening is displaced vertically and horizontally by only 0.2 mm. If the proposed and actual centers of the patient's trephination do not match exactly, the surgeon will be suturing the correctly marked donor tissue to the wrong location on the pretrephination marked host cornea. Although it is easy to mark the host limbal tissue with the 8-sided marker, it is vitally important to verify that these marks are aligned properly along the actual trephine circumference. This verification step has not been stressed in the literature to date, and can be performed simply with the reticule as described above. Although a radial keratotomy marker could be used to mark the recipient tissue after partial trephination, it is easier and faster to use the reticule. With the RK marker, the central point must be reconfirmed and is difficult to pinpoint, if preexisting central marks have been made. Olson and Pflugfelder have shown that minor discrepancies in tissue alignment can produce tissue disparity and excessive astigmatism; 0.1 mm wound disparity (displacing the second cardinal suture by less than 5 degrees) produces 4 diopters of corneal astigmatism. 6,9 Most of the verification changes in moving the marks in the host tissue after trephination were 0.1–0.5 mm from the pretrephination gentian violet spots, which highlights the importance of verifying the correct position of the eight-sided radial keratotomy guide marks. Although the causes of suture-out astigmatism after penetrating keratoplasty are multifactorial, using this reticule for suture placement in the donor and recipient tissues should minimize the role of asymmetric suturing on graft astigmatism. Clinical studies are currently underway to determine if final sutures-out graft astigmatism will be decreased with this technique.
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Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
Keratoplasty; Penetrating method; Keratoplasty; Penetrating instruments; Astigmatism prevention and control; Corneal transplantation