Postkeratoplasty Astigmatism: Etiology, Management, and Femtosecond Laser Applications





Introduction


Penetrating keratoplasty (PK) involves resecting and suturing a donor corneal tissue to a recipient corneal bed. It requires meticulous attention to detail by the corneal surgeon. The surgeon’s task begins with communication with the eye bank and ends with lengthy postoperative management of the patient. Different degrees of graft astigmatism may arise postoperatively and can be attributed to poor surgical technique.




Pathogenesis of Postkeratoplasty Astigmatism


The factors that can contribute to postkeratoplasty astigmatism are listed below. Some are beyond the surgeon’s control, while others are totally or partially controlled by the surgeon:




  • Preoperative factors, recipient related




    • native astigmatism



    • topographic changes



    • recipient corneal scarring, vascularization, thinning



    • recipient uneven scleral rigidity



    • aphakia or pseudophakia (anterior chamber intraocular lens [ACIOL], prior vitrectomy, or scleral fixation)




  • Preoperative factors, donor related




    • nonuniform peripheral changes (scarring, thinning, vascularization)



    • donor topographic variations (undetected keratoconus, high astigmatism)




  • Intraoperative factors, recipient related




    • effect of pressure of eyelid speculum and Flieringa ring on trephination



    • dull trephine or overused trephine blade



    • trephine tilt



    • eccentric trephination scissors



    • uneven wound architecture, poor resection



    • asymmetric wound edges




  • Intraoperative factors, donor related




    • inadequate punch technique



    • dull trephine or overused trephine blade



    • eccentric trephination



    • oval trephination



    • punch tilt during freehand trephination



    • intraoperative donor–recipient relationship



    • donor–recipient diameter disparity



    • suture tension



    • donor–recipient torquing (improper suture orientation and location, wound override)



    • eccentric donor–recipient trephination



    • postoperative factors



    • wound microdehiscence, override



    • trauma with macrodehiscence



    • timing of suture removal or adjustment



    • graft rejection, melting, necrosis, infectious keratitis



    • pharmacologic agents



    • scleral fixated intraocular lens (IOL), ACIOL



    • other (contact lens, intraocular pressure [IOP] elevation)




Preoperative Factors


In general, surgeons have access to high-quality donor tissues that can be preserved and transported for days. Donors may have undetected astigmatism. Whether donor native astigmatism has any effect on final-graft astigmatism is not known.


Certainly, recipient pathologic conditions can lead to astigmatism. Sectorial vascular invasion, thinning, or uneven rigidity in keratoconus will lead to uneven donor-to-recipient wound apposition, wound healing, and the creation of uneven forces within the cornea. Irregular recipient corneas (scars, vessels, thinning, unequal rigidity) result in irregular opening after trephination and hence irregular astigmatism.


Operative Factors


Most donor corneal buttons today are trephined in an endothelial cell-up position in a concave Teflon block, as described by Vannas. This method yields a perfectly round edge graft. It is done in a more controlled fashion than when trephined epithelial cell-up in an intact globe. The donor corneoscleral rim is placed and fixated in a Teflon block to be punched with a piston-guided trephine or by a freehanded trephine. The piston-guided punch method is more precise and usually results in a circular tissue with vertical edges. If the trephine is slightly tilted, the freehanded technique may yield an oval button with shelved edges. A dull trephine will leave irregular edges, leading to irregular wound healing and astigmatism. More recent and sophisticated trephines, such as the Krumeich and Hanna-Moria trephines, cut donor corneas through the epithelial side, giving more reproducible results.


Although donor corneas can be correctly punched with reproducible results, recipient corneas are less likely to be trephined with the same accuracy, since more variables are involved and less predictable results are obtained. Freehand trephination is a common practice, but other trephines, such as the Hessburg-Barron, are more popular despite setting a reverse shelved wound. Other, more expensive trephines such as the Krumeich cam-guided trephine, automated Hans-Gueder trephine, and mechanized Hanna-Moria trephine, theoretically should give more reproducible results. The Krumeich cam-guided system cuts the cornea in an applanated shape, perpendicular to the limbal plane, and theoretically produces round grafts :




  • freehand—disposable, Weck, Storz, Pharmacia



  • reusable—Grieshaber



  • disposable suction—Hessburg-Barron



  • automated—Hans-Gueder



  • mechanized—Hanna-Moria



  • cam-guided—Krumeich, Lieberman

Many factors can affect adequate trephination in the recipient cornea. Freehanded trephination is subject to more factors that may affect the outcome, such as external pressure by the eyelid speculum, Flieringa ring, or forceps stabilizing the globe. Dull trephines or trephines made by different manufacturers should not be used because they result in irregular edges. Hold trephines perpendicular to the plane of the globe, hugging the recipient tissue 360 degrees, because minor tilts can cause asymmetric corneal grooves and oval openings. Eccentric trephination results in a high degree of astigmatism with an axis toward the displacement direction. Finally, poor excision of the corneal tissue due to inadequate trephination or dull scissors may result in poor wound architecture, poor wound opposition and healing, and a high degree of astigmatism. Trephine tilts should be minimized during trephination of recipient corneas. This can be difficult to accomplish under the operating microscope, and a tilt more than 5 degrees can easily occur. Olson’s mathematic model has shown that buttons should be round, with 0 degrees of tilt. Ovality increases with higher degrees of tilt. Cohen and associates showed that maximum ovality appeared in 20 to 25 degrees tilt; however, buttons were oval and asymmetric at 0 to 15 degrees of tilt in eye bank eyes ( Fig. 42.1 ). They concluded that neither ovality nor asymmetry correlated with degrees of tilt and that factors other than tilt contribute to wound ovality and irregularity. Troutman and Buzard attempted to use intraoperative keratometry and rotate the donor button while inside the recipient wound to minimize disparity but without eliminating astigmatism.


Fig. 42.1


Side view of oblique trephination (top) showing pulling of corneal tissue. This results in oblong tissue resection in the recipient cornea (bottom) .

(From Cohen KL, Holman RE, Tripoli NK, et al. Effect of trephine tilt on corneal button dimensions. Am J Ophthalmol. 1986;101:722–725, with permission from Elsevier.)


The use of oversized donor buttons is believed to reduce hyperopia and glaucoma in aphakic patients. The donor button is punched from the endothelial surface and hence is smaller due to retraction of the tissue. Oversized is, in fact, superior to undersized and there is no difference in astigmatism, less wound leak, and less chance of collapse of the trabecular meshwork with oversized donor buttons.


Corneal wound disparity defined by mismatch between size and shape of donor button wound and its recipient wound determines degree, direction, and amount of astigmatism created. Multifactorial elements contribute to corneal wound disparity. Donor or recipient trephine, tilt, size, and eccentricity all contribute to disparity and ultimately astigmatism.


The donor button is punched from the donor endothelial side; approximately 0.2 mm in diameter is lost in this process. Thus a 0.2-mm oversized graft is practically the same size as the recipient. The use of oversized donor over recipient (0.25–1 mm) gained popularity because it is believed to reduce hyperopia and the incidence of glaucoma in aphakic patients. It also has the advantages of less wound leak, easier suturing, less collapse of the trabecular meshwork, and lower incidence of glaucoma. Most studies found that oversized grafts did not increase the incidence of astigmatism. However, the study by Perl and coworkers showed that oversized grafts compared with same-sized grafts did not affect intraocular pressure and refractive state and increased the amount of induced astigmatism. The extent of myopic error following PK in keratoconus patients can be decreased by reducing recipient–donor trephine disparity.




Preoperative Factors


In general, surgeons have access to high-quality donor tissues that can be preserved and transported for days. Donors may have undetected astigmatism. Whether donor native astigmatism has any effect on final-graft astigmatism is not known.


Certainly, recipient pathologic conditions can lead to astigmatism. Sectorial vascular invasion, thinning, or uneven rigidity in keratoconus will lead to uneven donor-to-recipient wound apposition, wound healing, and the creation of uneven forces within the cornea. Irregular recipient corneas (scars, vessels, thinning, unequal rigidity) result in irregular opening after trephination and hence irregular astigmatism.




Operative Factors


Most donor corneal buttons today are trephined in an endothelial cell-up position in a concave Teflon block, as described by Vannas. This method yields a perfectly round edge graft. It is done in a more controlled fashion than when trephined epithelial cell-up in an intact globe. The donor corneoscleral rim is placed and fixated in a Teflon block to be punched with a piston-guided trephine or by a freehanded trephine. The piston-guided punch method is more precise and usually results in a circular tissue with vertical edges. If the trephine is slightly tilted, the freehanded technique may yield an oval button with shelved edges. A dull trephine will leave irregular edges, leading to irregular wound healing and astigmatism. More recent and sophisticated trephines, such as the Krumeich and Hanna-Moria trephines, cut donor corneas through the epithelial side, giving more reproducible results.


Although donor corneas can be correctly punched with reproducible results, recipient corneas are less likely to be trephined with the same accuracy, since more variables are involved and less predictable results are obtained. Freehand trephination is a common practice, but other trephines, such as the Hessburg-Barron, are more popular despite setting a reverse shelved wound. Other, more expensive trephines such as the Krumeich cam-guided trephine, automated Hans-Gueder trephine, and mechanized Hanna-Moria trephine, theoretically should give more reproducible results. The Krumeich cam-guided system cuts the cornea in an applanated shape, perpendicular to the limbal plane, and theoretically produces round grafts :




  • freehand—disposable, Weck, Storz, Pharmacia



  • reusable—Grieshaber



  • disposable suction—Hessburg-Barron



  • automated—Hans-Gueder



  • mechanized—Hanna-Moria



  • cam-guided—Krumeich, Lieberman

Many factors can affect adequate trephination in the recipient cornea. Freehanded trephination is subject to more factors that may affect the outcome, such as external pressure by the eyelid speculum, Flieringa ring, or forceps stabilizing the globe. Dull trephines or trephines made by different manufacturers should not be used because they result in irregular edges. Hold trephines perpendicular to the plane of the globe, hugging the recipient tissue 360 degrees, because minor tilts can cause asymmetric corneal grooves and oval openings. Eccentric trephination results in a high degree of astigmatism with an axis toward the displacement direction. Finally, poor excision of the corneal tissue due to inadequate trephination or dull scissors may result in poor wound architecture, poor wound opposition and healing, and a high degree of astigmatism. Trephine tilts should be minimized during trephination of recipient corneas. This can be difficult to accomplish under the operating microscope, and a tilt more than 5 degrees can easily occur. Olson’s mathematic model has shown that buttons should be round, with 0 degrees of tilt. Ovality increases with higher degrees of tilt. Cohen and associates showed that maximum ovality appeared in 20 to 25 degrees tilt; however, buttons were oval and asymmetric at 0 to 15 degrees of tilt in eye bank eyes ( Fig. 42.1 ). They concluded that neither ovality nor asymmetry correlated with degrees of tilt and that factors other than tilt contribute to wound ovality and irregularity. Troutman and Buzard attempted to use intraoperative keratometry and rotate the donor button while inside the recipient wound to minimize disparity but without eliminating astigmatism.


Fig. 42.1


Side view of oblique trephination (top) showing pulling of corneal tissue. This results in oblong tissue resection in the recipient cornea (bottom) .

(From Cohen KL, Holman RE, Tripoli NK, et al. Effect of trephine tilt on corneal button dimensions. Am J Ophthalmol. 1986;101:722–725, with permission from Elsevier.)


The use of oversized donor buttons is believed to reduce hyperopia and glaucoma in aphakic patients. The donor button is punched from the endothelial surface and hence is smaller due to retraction of the tissue. Oversized is, in fact, superior to undersized and there is no difference in astigmatism, less wound leak, and less chance of collapse of the trabecular meshwork with oversized donor buttons.


Corneal wound disparity defined by mismatch between size and shape of donor button wound and its recipient wound determines degree, direction, and amount of astigmatism created. Multifactorial elements contribute to corneal wound disparity. Donor or recipient trephine, tilt, size, and eccentricity all contribute to disparity and ultimately astigmatism.


The donor button is punched from the donor endothelial side; approximately 0.2 mm in diameter is lost in this process. Thus a 0.2-mm oversized graft is practically the same size as the recipient. The use of oversized donor over recipient (0.25–1 mm) gained popularity because it is believed to reduce hyperopia and the incidence of glaucoma in aphakic patients. It also has the advantages of less wound leak, easier suturing, less collapse of the trabecular meshwork, and lower incidence of glaucoma. Most studies found that oversized grafts did not increase the incidence of astigmatism. However, the study by Perl and coworkers showed that oversized grafts compared with same-sized grafts did not affect intraocular pressure and refractive state and increased the amount of induced astigmatism. The extent of myopic error following PK in keratoconus patients can be decreased by reducing recipient–donor trephine disparity.




Suturing Technique


Many uncontrollable factors discussed previously contribute to postkeratoplasty astigmatism. However, much attention has been focused on factors that the surgeon can manipulate operatively and postoperatively such as suture type, size, and technique. Different studies have shown that the final corneal astigmatism follows suture removal regardless of suture technique used. It is extremely difficult to draw conclusions from earlier studies because most studies (1) are retrospective, (2) lack controls, (3) involve different surgeons, (4) draw conclusions based on data before final suture removal, and (5) depend only on either refraction or keratometry. More recent studies, however, are prospective and employ vector analysis and computer-assisted topographic analysis. Although most established techniques to reduce postkeratoplasty astigmatism employ either selective suture removal or postoperative suture adjustment relying on keratometry or computer-assisted topographic analysis, a recent study advocated suture adjustment. Serdarevic and coauthors observed that after suture removal, at 15 months postoperatively, astigmatism was less in the intraoperative adjustment group (1.75 ± 1.04 diopters [D]) than in the postoperative adjustment group (2.23 ± 1.72 D), but this was not statistically significant. The authors concluded that low astigmatism and good visual results can be obtained with either intraoperative or postoperative running suture adjustment, but intraoperative suture adjustment permits more rapid visual rehabilitation, increased safety, and increased refractive stability.




Management of Significant Postkeratoplasty Astigmatism


The amount of postkeratoplasty astigmatism declines as the surgeon’s experience increases. Most surgeons feel that astigmatism greater than 3 D to 3.5 D is significant enough to warrant manipulation to minimize astigmatism. The most important factor is whether the sutures have been removed. Adjusting the sutures helps reduce the amount of astigmatism and possibly the final outcome. There is no definite postoperative time period when the graft becomes “fixed,” although some surgeons believe that this does not occur before 1 year. The final astigmatic stabilization occurs when all sutures are removed. For example, removing a running suture can theoretically change the amount and the axis of astigmatism up to 6 years after surgery.


Management of Astigmatism While Sutures Are In: Suture Manipulation


Surgeons try to manipulate corneal sutures to control and minimize astigmatism. To reduce suture tension at the steep meridian, sutures are adjusted either by selective removal or by adjusting the tension of single running sutures. Each approach has advantages and disadvantages. Both will reduce the amount of astigmatism for as long as the sutures are in. However, the cornea is not “fixed,” and removing the running sutures any time after surgery creates large degrees of astigmatism. Because 10-0 nylon is biodegradable, it may loosen, disintegrate, and/or break, possibly leading to potentially serious problems such as microbial keratitis, epithelial breakdown, suture abscesses, graft vascularization, and rejection. The complications of 10-0 nylon permanently left in corneal graft include:




  • spontaneous breakage or degradation



  • exposed knots and giant papillary conjunctivitis (GPC)



  • suture abrasion



  • suture erosion



  • vascularization and fibrosis along suture tract



  • infection



  • inflammation graft rejection after suture removal

Both techniques assume only compression suture effects on astigmatism, ignoring wound healing or other relevant variables that may affect astigmatism.


Selective Suture Removal


This technique was popularized by Binder and colleagues, and results of recent studies show a mean reduction of astigmatism from 2.5 D to 3.0 D after selective suture removal. It is based on the assumption that the cornea assumes a different curvature after the tight suture is selectively decompressed at a particular time. The change of astigmatism after long-term removal of the remaining sutures is less dramatic, suggesting that additional suture removal results in minimal curvature change after the corneal wound becomes fixed.


It is not difficult to remove one or two 10-0 nylon sutures under a slit lamp using topical anesthesia, but it does require an increased number of patient office visits. It can also be unpredictable and irreversible, and it may result in wound dehiscence, suture-induced irritation, infection, vascularization, and graft rejection. This technique also regulates the amount of astigmatism at a specific meridian of that tight suture and not the whole corneal circumference.


Suture Adjustment


A single 10-0 nylon running suture is performed and usually left somewhat on the loose side so that suture adjustment can be done 1 day and up to 6 weeks postoperatively. The technique involves rotating the suture from an area of flat meridian (cool colors) to areas of steep meridian (hot colors; Fig. 42.2 ). Nabors and associates took a step further by opening the anterior third of the wound along the steep meridian.




Fig. 42.2


Corneal videokeratography of against-the-rule astigmatism. The arrows indicate direction of adjustment of running suture.


Suture adjustment achieves early visual rehabilitation and relatively regular keratometry mires. This is beneficial when an IOL is inserted 3 months after keratoplasty. It is a titrated procedure in which adjustment is continued and stopped until stable Ks are obtained. It is reversible and more predictable and results in regular astigmatism along the entire circumference of the cornea, not along a single meridian.


Suture adjustment can be performed under a slit lamp with a calm, cooperative patient. The procedure must be repeated and sometimes requires a loose running suture, which may compromise the graft wound, lead to wound disruption, leak, cause infection, or recess the wound. Suture breakage occurs rarely. If it occurs, the patient usually must go to the operating room for suture placement. McNeill and Wessels encountered a broken suture in five eyes of a total of 330 eyes studied.


Management of Astigmatism After Suture Removal


Relaxing Incisions ( )


The surgical approach using relaxing incisions to minimize graft astigmatism is undertaken in cases of astigmatism high enough to cause blurring of vision with or without spectacle correction. Although the eye can often be visually rehabilitated through special contact lenses, some patients require a surgical attempt to correct high astigmatism. Before relaxing incisions are entertained in postkeratoplasty patients, all sutures should be removed, and stable refractions, keratometry, and topographic analyses observed for at least 3 months. The above three methods for evaluating postkeratoplasty astigmatism are important and should be performed before relaxing incisions are made. In individuals with high degrees of astigmatic error (> 10 D), wound overrides should be suspected, ruled out, and corrected.


We recommend arcuate relaxing incisions made 1 mm on the graft itself and not in the graft–host interface ( Fig. 42.3 ). Compression sutures can be placed in the orthogonal meridian to enhance the surgical effect. The technique of arcuate relaxing incisions is simple to perform, requiring office setup and topical anesthesia. The postoperative recovery period is extremely short, and visual rehabilitation is rapid and dramatic.




Fig. 42.3


Top (A) and side (B) views of arcuate keratotomy to correct steep horizontal meridian in against-the-rule postkeratoplasty astigmatism.


Operative Technique


The eye is anesthetized with topical anesthetic drops. The particular meridian is marked on the slit lamp with a surgical blue marker, and the periocular skin is prepped with povidone iodine solution and draped with a plastic adhesive aperture drape. The lids are separated with a wire speculum, and the patient is asked to fixate using the surgical eye on a fixation mark built in the surgical microscope. Ultrasonic pachymetry is used to measure corneal thickness at different sites, especially at those where the cuts are planned. The Arc-T diamond knife’s depth is set at approximately 95% of corneal thickness. Two arcuate relaxing incisions are made at two hemi-meridians that the refraction, keratometry, or topographic analysis have depicted. Most of the time, the two hemi-meridians are 180° apart. While using the Arc-T diamond blade, the surgeon must roll his or her fingers to achieve an arcuate incision. Some corneal surgeons tended to perform relaxing incisions of three-quarters depth in the corneal recipient–host interface with the patient situated on the slit lamp. This technique became less popular recently because of lower predictability and effectiveness and increased risk of perforations and infections. If, after the first attempt, astigmatism is not reduced as desired, another double arcuate incision can be performed 1 mm inside the previous ones. Alternatively, the incisions can be deepened if slit lamp evaluation shows shallow incisions. Compression sutures can be used to augment the effect of the arcuate incisions ( Fig. 42.4 ).




Fig. 42.4


Surgeon’s view of horizontal arcuate incisions and “augmenting” vertical compression sutures.


Femtosecond Laser-Assisted Arcuate Keratotomy ( )


A femtosecond (FS) laser with standard hardware configuration can also be used to create the cuts in arcuate keratotomy. To create an arcuate cut in the donor cornea, a 5.0- or 6.0-mm cut (flap diameter) is performed with a side cut at 90 degrees and depth set at 400 µm. The suction ring is not applied to allow appropriate decentration of the cuts. Using a micropore tape applied to the applanating contact lens, the length of the laser cut can be mechanically restricted to the desired arc length.


Wedge Resections


In 1967, Troutman devised the corneal wedge-resection procedure for high astigmatism. Before this, a repeat keratoplasty was often the procedure of choice to correct excessive graft astigmatism. Today, wedge resection is reserved for cases of extremely high astigmatism and is recommended for astigmatism of 10 D or more. The procedure involves excising corneal tissue across the axis of the longer or flatter corneal meridian and suturing the two opposing ends of the resected tissue. The corneal meridian is shortened and the meridian steepened to correct the astigmatism. Although limited in number, previous clinical studies show an approximate 40% to 70% reduction of mean astigmatism after corneal wedge resection.


Compared with relaxing incisions, wedge resections can correct large amounts of astigmatism. Moreover, the wounds are sutured, unlike the gaping wounds of relaxing incisions, and astigmatism can be customized by selective suture removal. However, it may take months for the wound to stabilize and hence for stable keratometer readings.


Operative Technique


As with relaxing incisions, all sutures should be removed and stable refractions, keratometry, and topographic analyses should be observed for at least 3 months. A retrobulbar block is used for anesthesia and akinesia. Intraoperatively, the axis of the flattest meridian is identified. A keratometer-equipped surgical microscope is recommended. The surgeon should attend to intraoperative factors that may induce iatrogenic astigmatism, especially the lid speculum. Ultrasonic pachymetry is used in the axis where the resection is performed. Simultaneous partial penetrating incisions are made across the scar using a double-bladed diamond knife. The total excision area should be 90 degrees wide and at a depth of 90% to 95%. This wedge of tissue is excised with the diamond knife or by corneal scissors by freehand dissection. For approximately every 0.1 mm of resected tissue, a 1- to 2-D correction is obtained. Six to eight interrupted deep sutures are placed using 10-0 nylon. Suture loop tension is adjusted using slip knots under direct visualization of the surgical keratometer. Sutures are then tied down with square knots and buried. Tension should be placed to overcorrect the astigmatism by approximately 30% to 50%. To compensate for the astigmatic overcorrection in the meridian 90 degrees away from the wedge resection, two compression sutures may be placed during the immediate postoperative period until the wedge-resected sutures are removed. Selective suture removal can begin 8 to 10 weeks postoperatively. Sutures can be removed every 3 to 5 weeks until a satisfactory result is obtained. Sutures may be left indefinitely if the desired astigmatic correction occurs before all sutures are removed.


Femtosecond Laser-Assisted Wedge Resection ( )


FS laser-assisted wedge resection can overcome one important drawback of the manual technique: the difficulty in excising the exact amount of tissue in width and depth, which may account for the low predictability of the manual technique. The surgical technique using an FS laser is based on two arcuate cuts with different radii of curvature that intersect each other in the corneal periphery, creating a wedge-shaped wound. The depth of the FS laser ablation is set at 400 µm; the smaller-diameter cut (usually 6.0 mm) is performed with a side cut at 45 degrees and the larger-diameter cut (usually 7.0 mm) is performed with a side cut at 90 degrees. This creates a wedge resection as shown in Fig. 42.5 . A micropore tape attached to the applanating contact lens is used to limit the arc length for the laser cuts to approximately 90 degrees for the smaller-diameter area (6.0 mm) and to approximately 75 degrees for the larger-diameter area (7.0 mm). The contact lens can be rotated to create the wedge 180 degrees away. Feasibility of the technique was observed in porcine corneas and subsequently applied to treat high residual astigmatism after penetrating keratoplasty.


Oct 10, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Postkeratoplasty Astigmatism: Etiology, Management, and Femtosecond Laser Applications
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