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Astigmatic keratotomy (AK) is an incisional procedure in which a diamond blade or a femtosecond laser is used for the correction of astigmatism.
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Radial keratotomy (RK) is a procedure in which radial incisions are used to correct myopic spherical refractive error. The lower predictability and higher complication rates of RK relative to laser refractive surgery have reduced its utility for the correction of myopia.
Key Features
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The effect of incisional keratotomy is influenced by corneal wound healing and patient age.
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For astigmatic keratotomy, the surgeon must screen surgical candidates for myopic or planospherical equivalents.
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The axis of astigmatism is important for the placement of AK incisions. Sound clinical judgment is needed when disparity occurs between corneal topography and clinical refraction.
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Complications of incisional keratotomy may be reduced by adequate marking of the visual axis, adequate corneal incision shape and depth, and avoidance of corneal perforations.
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Postoperative complications include progressive hyperopia, progressive wound gaping, induced astigmatism, and contact lens intolerance.
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Femtosecond laser arcuate keratotomy (femtosecond AK) is an effective alternative to AK. Laser arcuate resection (LAR) is an effective alternative to manual wedge resection for high postkeratoplasty astigmatism.
Historical Review
Incisional Keratotomy
The diamond knife, although still in use today, is being replaced in many centers by the femtosecond laser with its ultrashort pulses capable of chiseling precise incisions on the cornea. Femtosecond lasers have garnered acclaim in terms of generating corneal incisions with greater precision, accuracy, safety, predictability, and reproducibility.
Incisional keratotomy now is mainly limited to astigmatic keratotomy at the time of cataract surgery and, rarely, to two-incision radial keratotomy (RK) for patients with low-grade myopic astigmatism who are not good candidates for laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).
Several options are available today for femtosecond laser AK: femtosecond laser astigmatic keratotomy (femtosecond AK) and intrastromal astigmatic keratotomy (ISAK). Methods for astigmatism correction can be performed alone or in combination with other procedures such as cataract surgery. Further, the correction can be done for natural or surgery-induced astigmatism such as postkeratoplasty astigmatism.
Incisional Keratotomy
The diamond knife, although still in use today, is being replaced in many centers by the femtosecond laser with its ultrashort pulses capable of chiseling precise incisions on the cornea. Femtosecond lasers have garnered acclaim in terms of generating corneal incisions with greater precision, accuracy, safety, predictability, and reproducibility.
Incisional keratotomy now is mainly limited to astigmatic keratotomy at the time of cataract surgery and, rarely, to two-incision radial keratotomy (RK) for patients with low-grade myopic astigmatism who are not good candidates for laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).
Several options are available today for femtosecond laser AK: femtosecond laser astigmatic keratotomy (femtosecond AK) and intrastromal astigmatic keratotomy (ISAK). Methods for astigmatism correction can be performed alone or in combination with other procedures such as cataract surgery. Further, the correction can be done for natural or surgery-induced astigmatism such as postkeratoplasty astigmatism.
Surgical Techniques for Astigmatic and Radial Keratotomy
The surgical techniques for AK and RK (rarely used today) have many common aspects. AK remains a surgical option for correcting high astigmatism (e.g., postkeratoplasty astigmatism) that is beyond excimer laser correction or for the correction of smaller degrees of astigmatism in patients undergoing cataract surgery. Femtosecond AK has increased the predictability and improved the safety of AK. Although limbal relaxing incisions have become popular as well, they do not have the tensile strength of Descemet’s membrane, which fortifies a corneal incision.
Preoperative Considerations
Patient Selection
In selecting patients for AK, the surgeon must screen surgical candidates for myopic or planospherical equivalents. AK does not, as a general rule, benefit patients who have hyperopic astigmatism in which the spherical equivalents are relatively unaffected or associated with a further hyperopic shift.
Ideal candidates for AK, in addition to having a myopic or planospherical equivalent, have no keratoconus, are intolerant of contact lenses, and experience meridional magnification and distorted peripheral vision with high-cylinder spectacles.
Visual Axis Determination and Marking
The use of scanning slit topography or Scheimpflug-based topography along with clinical refraction will help confirm the positioning of the planned AK incisions. Modern topographers provide an estimate of pachymetry at the location of the intended AK incision sites.
A corneal light reflex used to guide procedure centration serves only to approximate the physiological visual axis location, because a coaxially aligned light reflex corresponds to the center of the corneal optical system and not the true visual axis. Studies have demonstrated this site to be associated most closely with the physiological visual axis.
For marking of the visual axis, the administration of a drop of fluid over the corneal apex may enhance an otherwise dull corneal light reflex. A Sinskey hook is used to indent gently the epithelium that overlies the visual axis. If the epithelial indentation is not visualized readily, a Weck cell may be applied to the central epithelium, which enhances the central epithelial mark.
Treatment alignment is important for successful correction of astigmatism. The results of vector analysis have indicated that treatment decentration by 5°, 15°, and 30° corresponds to losses of the flattening effect by 1.5%, 13.4%, and 50%, respectively. Moreover, complete loss of the flattening effect is seen with treatment misalignment of 45°.
Intraoperative Corneal Pachymetry
The paracentral corneal thickness (1.5 mm from the visual center, at the 3-mm central clear zone) is measured at both the temporal site and the thinnest paracentral site, as previously established by pachymetry at the screening examination. Most often, this thinnest paracentral site coincides with the paracentral temporal (or inferotemporal) site as the region closest to the anatomical corneal center. If the two sites do not coincide, the diamond blade is set to 100% of the thinner of the two intraoperatively measured sites using a calibration microscope.
Incision Technique
Astigmatic incisions, either arcuate or tangential, produce maximal flattening in the meridian of the incision when they are placed within 2.5–3.5 mm of the visual axis, which also is within the 5–7 mm optical zone. Incisions made closer than the 5-mm optical zone cause visual disturbances. Incisions beyond 7 mm (such as limbal relaxing incisions [LRIs]) have a diminished effect on central corneal flattening.
When arcuate incisions are lengthened, increasingly greater degrees of astigmatic correction are provided, up to an arc length of 90°. Beyond an arc length of 90°, no reliable additional flattening occurs. Stacking multiple rows of astigmatic incisions is neither productive nor advised. Incisions carried out at progressively smaller optical zones may result in global corneal flattening. Such incisions may be associated with increased incidence of irregular astigmatism.
Diamond Blade–Assisted AK
The diamond blade–assisted AK procedure is a manual process that requires marking the optical zone of the desired incision, marking the steep axis of astigmatism according to the appropriate arc length, and then determining the corneal thickness at the optical zone with ultrasound pachymetry before inserting the blade into the cornea to make the incision. A sterilized diamond knife blade then is mounted onto the sterile mounting block of the calibration microscope with the knife footplates set to zero and the diamond either extended to 550 mm or set at the thinnest paracentral screening pachymetric reading. Once real-time, intraoperative ultrasonic pachymetry is obtained, the diamond-tip extension is adjusted to the newly selected level. In this way, a minor adjustment is generally all that is needed, and it can be carried out in only a few seconds. When LRIs are being applied during cataract surgery, a preset blade is often used without the need for precalibration.
Although this AK procedure has the potential to reduce high degrees of astigmatism, the postoperative outcomes are often accompanied by complications such as wound gape, perforation, skin lesions, epithelial inclusion, higher order aberrations, and poor predictability. LRIs have become popular as well, but the tensile strength of Descemet’s membrane that strengthens a corneal incision is absent in laser arcuate resection (LAR).
Full Penetrating Femtosecond AK
Femtosecond AK corneal surgery creates arcuate incisions to flatten the cornea. AK incisions are generally placed in the steep corneal meridian and result in flattening of the steep meridian (often associated with a compensatory steepening “coupling” effect of the orthogonal flat meridian). A coupling ratio is defined as the ratio of the values of the flattening of the steep meridian and the steepening of the orthogonal flat meridian after placement of AK incisions. A coupling ratio equal to 1 indicates an unchanged spherical equivalent (SE) after the procedure due to equal flattening and steepening of the two meridians. A coupling ratio less than 1 indicates a SE shift toward myopia, and conversely, a coupling ratio greater than 1 indicates a SE shift toward hyperopia. Corneal topography or tomography, along with refraction, are performed in planning the femtosecond AK incisions. Nomograms are then used to determine the arc length and the optical zone to achieve the desired astigmatism correction.
Although prior nomograms existed, developing their own nomogram based on a series of cases was important for each surgeon as demonstrated by the different surgery results of the femtosecond procedure experienced by some surgeons compared with the manual AK technique. Donnenfeld and Nichamin LRI nomograms were most commonly used as the baseline for the laser nomograms. Some surgeons altered both the blade nomogram and the laser’s power output to come up with an effective approach for using the IntraLase laser. The modified Lindstrom nomogram is the most widely implemented nomogram for planning femtosecond AK incisions. Usually, when using the modified Lindstrom nomogram, surgeons recommend adding 0.05 diopters (D) to the planned astigmatism correction per year for patients less than 30 years old and subtracting 0.02 D per year for patients over 30 years old. Additional nomograms have been developed for femtosecond AK as well.
When femtosecond laser incisions are made, a series of spherical spots are created adjacent to each other tightly so that the final effect is similar to a wide-open incision made using the traditional diamond blade. Alternatively, this process can be adjusted by altering the laser energy and the spot size. Lowering the laser energy creates a weaker shot, making the spot sizes smaller. This way the spots never really connect, which reduces the effectiveness of the femtosecond laser–created AK incision compared with those created using the diamond blade. However, the incision is made tighter, ensuring that it does not open immediately. The laser pulses are typically placed around 3 µm on both the spot and layer separations.
Femtosecond Intrastromal Astigmatic Keratotomy (ISAK)
Intrastromal incision is conceptually different from a manual LRI or full penetration astigmatic incision in that there is no breakage of Bowman’s layer during the procedure. Compared with penetrating AK incisions, the optical zone is made a little smaller and the arc angle is made a little longer in ISAK. The complete intrastomal nature of the incision allows the healing of the realigned stroma without wound gaping or epithelial plug formation. To ensure proper alignment, limbal marks can be made with a sterile pen placed at the 3 o’clock and 9 o’clock positions at the slit lamp just before the procedure. The incision details such as the depth, optical zone, and arc length can be preprogrammed into the femtosecond laser post limbal marking, after which the procedure can be carried out in a very similar manner to LASIK flap creation with the femtosecond laser. Patients with mixed astigmatism with a plano spherical equivalent will best benefit from the ISAK procedure, because the AK incisions are neutral in relation to myopia or hyperopia. Thus, ISAK is a great additional potential option available for femtosecond laser users, especially for astigmatism ranging from 0.50 to 2.75 D of cylinder.
The variables in performing ISAK incisions are arc length on the nomograms and the surgical platform. As for femtosecond AK, Donnenfeld, Nichamin LRI, and modified Lindstrom nomograms are used as starting points for performing ISAK. However, additional nomograms have been determined for ISAK incisions combined with other procedures such as phacoemulsification. The surgeons suggest an incision depth of 80% or 90%.
ISAK confers a unique advantage to the process of astigmatism correction by virtue of its intrastromal nature. The intrastromal incisions can be titrated easily by opening of the incision, and they can be performed easily in a minor procedure room or right at the slit lamp. It is easiest to open the incision if the anterior edge of the incision is left just under the Bowman’s layer.
Wedge Resection Using Laser Arcuate Resection (LAR)
LAR is a standardized technique in which intersecting arcuate cuts are used to perform a wedge resection for the correction of high astigmatism through corneal steepening. The arcuate wedges to be excised are placed in the flat meridian. A simple formula is used to estimate the relative size and location of the arcuate cuts based on the radii of curvature and desired wedge width to be resected. The feasibility of the procedure was established in porcine corneas before treatment of a patient with 20.00 D of postkeratoplasty astigmatism. The astigmatism was reversed ( Fig. 3.8.1 ), and suture removal resulted in a 14.5 D reduction of astigmatism. LAR can be an effective alternative to manual wedge resection, allowing easier, more controlled, and more precise excision of tissue in width, length, and depth.
Surgical Protocol
When AK is planned, careful attention to quantitative keratography (corneal topography) may be valuable, because paired incisions based on either the standard keratometer measurements or the refraction alone may be inaccurate. If a manifest refraction fails to yield the patient’s potential acuity, irregular astigmatism may be present, and a topographical analysis is useful.
The visual axis is determined first, followed by selection of the appropriate optical zone, incision number, and length, as directed by the nomogram.
Axis of Astigmatism
When disparity occurs between corneal topography and clinical refraction, the surgeon needs sound clinical judgment on a case-by-case basis. In such circumstances, if topographical analysis demonstrates orthogonal astigmatism, the surgeon may proceed according to the manifest refraction, as this provides the physiological combined (lenticular plus corneal) astigmatism. Alternatively, in cases of nonorthogonal astigmatism (when the two steep hemimeridians differ by any angle other than 180°), if the spherocylindrical reconstruction of the topographical pattern is consistent with the refraction, the incisions are placed as indicated by the topographical map.
Once the desired axis of astigmatic correction has been determined, this needs to be translated onto the cornea. Because the astigmatic axis is defined so carefully with the patient in an upright position without sedation or a lid speculum, one must not estimate the surgical axis intraoperatively with the patient in a supine position or sedated or with a lid speculum in place. Cyclotorsional rotation of the globe may occur and introduce significant error.
For control, with the patient seated at the slit lamp, epithelial marks are placed on either the vertical or horizontal axis. Using the slit beam for centration and with the contralateral eye covered, the patient fixates straight ahead first on the slit-lamp light source. Fixation on the slit-lamp filament at eye level and from head-on provides a virtual image of the light filament, which falls at the center of the corneal optical system and closely approximates the visual axis. The epithelium is abraded at the outer margins along the long axis of the beam using a Sinskey hook.
After true 90° or 180° is marked precisely at the slit lamp, the true visual axis is determined in the operating room under the operating microscope. With the 90° (or 180°) position determined precisely (reference axis), any desired axis may be marked using an axis marker and a surgical marking pen.
After appropriate marking of the astigmatic axis, pachymetry is carried out at the selected optical zone over the incision sites. The diamond blade is set at 100% of the measurement at the thinner of the two sites. With the globe fixated, the corneal marks are incised.
The use of guarded diamond knife blades is advisable. The conjunctiva is grasped close to the limbus, where it fuses with Tenon’s capsule and enables stable fixation; this region also is anesthetized more deeply than are the posterior conjunctiva and sclera.
The surgeon needs patience when secondary enhancements to AK are planned and performed. AK incisions require more time to stabilize than do radial incisions, so enhancement should be deferred for a minimum of 6 weeks after the primary procedure. A computerized videokeratography system is indispensable when the enhancement of an AK procedure is carried out. Caution is advised in treating overcorrected AK patients. If the resultant refractive error is one of hyperopic astigmatism, the original incisions may be reopened, the fibrous plug removed, and the wound margins approximated using a 10–0 nylon suture. Likewise, if an astigmatic incision is placed incorrectly in the flat axis, the wound margins are approximated using 10–0 nylon suture. Alternatively, if the residual refractive error is myopic astigmatism, further astigmatic incisions may be placed in the newly defined, steep hemimeridian. Such incisions are shorter than otherwise indicated, because the cornea has now demonstrated an excessive response to the initial incisions and could respond in like fashion to any further astigmatic incisions.
Postoperative Protocol
At the termination of the procedure, diclofenac sodium, mild corticosteroid, and anti-infective agent drops (such as tobramycin, ciprofloxacin, ofloxacin, or norfloxacin) can be given. The diclofenac drops are discontinued on postoperative day 2 to avoid masking early keratitis and to minimize the risk of a toxic response.
Preoperative Considerations
Patient Selection
In selecting patients for AK, the surgeon must screen surgical candidates for myopic or planospherical equivalents. AK does not, as a general rule, benefit patients who have hyperopic astigmatism in which the spherical equivalents are relatively unaffected or associated with a further hyperopic shift.
Ideal candidates for AK, in addition to having a myopic or planospherical equivalent, have no keratoconus, are intolerant of contact lenses, and experience meridional magnification and distorted peripheral vision with high-cylinder spectacles.
Visual Axis Determination and Marking
The use of scanning slit topography or Scheimpflug-based topography along with clinical refraction will help confirm the positioning of the planned AK incisions. Modern topographers provide an estimate of pachymetry at the location of the intended AK incision sites.
A corneal light reflex used to guide procedure centration serves only to approximate the physiological visual axis location, because a coaxially aligned light reflex corresponds to the center of the corneal optical system and not the true visual axis. Studies have demonstrated this site to be associated most closely with the physiological visual axis.
For marking of the visual axis, the administration of a drop of fluid over the corneal apex may enhance an otherwise dull corneal light reflex. A Sinskey hook is used to indent gently the epithelium that overlies the visual axis. If the epithelial indentation is not visualized readily, a Weck cell may be applied to the central epithelium, which enhances the central epithelial mark.
Treatment alignment is important for successful correction of astigmatism. The results of vector analysis have indicated that treatment decentration by 5°, 15°, and 30° corresponds to losses of the flattening effect by 1.5%, 13.4%, and 50%, respectively. Moreover, complete loss of the flattening effect is seen with treatment misalignment of 45°.
Intraoperative Corneal Pachymetry
The paracentral corneal thickness (1.5 mm from the visual center, at the 3-mm central clear zone) is measured at both the temporal site and the thinnest paracentral site, as previously established by pachymetry at the screening examination. Most often, this thinnest paracentral site coincides with the paracentral temporal (or inferotemporal) site as the region closest to the anatomical corneal center. If the two sites do not coincide, the diamond blade is set to 100% of the thinner of the two intraoperatively measured sites using a calibration microscope.
Patient Selection
In selecting patients for AK, the surgeon must screen surgical candidates for myopic or planospherical equivalents. AK does not, as a general rule, benefit patients who have hyperopic astigmatism in which the spherical equivalents are relatively unaffected or associated with a further hyperopic shift.
Ideal candidates for AK, in addition to having a myopic or planospherical equivalent, have no keratoconus, are intolerant of contact lenses, and experience meridional magnification and distorted peripheral vision with high-cylinder spectacles.
Visual Axis Determination and Marking
The use of scanning slit topography or Scheimpflug-based topography along with clinical refraction will help confirm the positioning of the planned AK incisions. Modern topographers provide an estimate of pachymetry at the location of the intended AK incision sites.
A corneal light reflex used to guide procedure centration serves only to approximate the physiological visual axis location, because a coaxially aligned light reflex corresponds to the center of the corneal optical system and not the true visual axis. Studies have demonstrated this site to be associated most closely with the physiological visual axis.
For marking of the visual axis, the administration of a drop of fluid over the corneal apex may enhance an otherwise dull corneal light reflex. A Sinskey hook is used to indent gently the epithelium that overlies the visual axis. If the epithelial indentation is not visualized readily, a Weck cell may be applied to the central epithelium, which enhances the central epithelial mark.
Treatment alignment is important for successful correction of astigmatism. The results of vector analysis have indicated that treatment decentration by 5°, 15°, and 30° corresponds to losses of the flattening effect by 1.5%, 13.4%, and 50%, respectively. Moreover, complete loss of the flattening effect is seen with treatment misalignment of 45°.
Intraoperative Corneal Pachymetry
The paracentral corneal thickness (1.5 mm from the visual center, at the 3-mm central clear zone) is measured at both the temporal site and the thinnest paracentral site, as previously established by pachymetry at the screening examination. Most often, this thinnest paracentral site coincides with the paracentral temporal (or inferotemporal) site as the region closest to the anatomical corneal center. If the two sites do not coincide, the diamond blade is set to 100% of the thinner of the two intraoperatively measured sites using a calibration microscope.
Incision Technique
Astigmatic incisions, either arcuate or tangential, produce maximal flattening in the meridian of the incision when they are placed within 2.5–3.5 mm of the visual axis, which also is within the 5–7 mm optical zone. Incisions made closer than the 5-mm optical zone cause visual disturbances. Incisions beyond 7 mm (such as limbal relaxing incisions [LRIs]) have a diminished effect on central corneal flattening.
When arcuate incisions are lengthened, increasingly greater degrees of astigmatic correction are provided, up to an arc length of 90°. Beyond an arc length of 90°, no reliable additional flattening occurs. Stacking multiple rows of astigmatic incisions is neither productive nor advised. Incisions carried out at progressively smaller optical zones may result in global corneal flattening. Such incisions may be associated with increased incidence of irregular astigmatism.
Diamond Blade–Assisted AK
The diamond blade–assisted AK procedure is a manual process that requires marking the optical zone of the desired incision, marking the steep axis of astigmatism according to the appropriate arc length, and then determining the corneal thickness at the optical zone with ultrasound pachymetry before inserting the blade into the cornea to make the incision. A sterilized diamond knife blade then is mounted onto the sterile mounting block of the calibration microscope with the knife footplates set to zero and the diamond either extended to 550 mm or set at the thinnest paracentral screening pachymetric reading. Once real-time, intraoperative ultrasonic pachymetry is obtained, the diamond-tip extension is adjusted to the newly selected level. In this way, a minor adjustment is generally all that is needed, and it can be carried out in only a few seconds. When LRIs are being applied during cataract surgery, a preset blade is often used without the need for precalibration.
Although this AK procedure has the potential to reduce high degrees of astigmatism, the postoperative outcomes are often accompanied by complications such as wound gape, perforation, skin lesions, epithelial inclusion, higher order aberrations, and poor predictability. LRIs have become popular as well, but the tensile strength of Descemet’s membrane that strengthens a corneal incision is absent in laser arcuate resection (LAR).
Full Penetrating Femtosecond AK
Femtosecond AK corneal surgery creates arcuate incisions to flatten the cornea. AK incisions are generally placed in the steep corneal meridian and result in flattening of the steep meridian (often associated with a compensatory steepening “coupling” effect of the orthogonal flat meridian). A coupling ratio is defined as the ratio of the values of the flattening of the steep meridian and the steepening of the orthogonal flat meridian after placement of AK incisions. A coupling ratio equal to 1 indicates an unchanged spherical equivalent (SE) after the procedure due to equal flattening and steepening of the two meridians. A coupling ratio less than 1 indicates a SE shift toward myopia, and conversely, a coupling ratio greater than 1 indicates a SE shift toward hyperopia. Corneal topography or tomography, along with refraction, are performed in planning the femtosecond AK incisions. Nomograms are then used to determine the arc length and the optical zone to achieve the desired astigmatism correction.
Although prior nomograms existed, developing their own nomogram based on a series of cases was important for each surgeon as demonstrated by the different surgery results of the femtosecond procedure experienced by some surgeons compared with the manual AK technique. Donnenfeld and Nichamin LRI nomograms were most commonly used as the baseline for the laser nomograms. Some surgeons altered both the blade nomogram and the laser’s power output to come up with an effective approach for using the IntraLase laser. The modified Lindstrom nomogram is the most widely implemented nomogram for planning femtosecond AK incisions. Usually, when using the modified Lindstrom nomogram, surgeons recommend adding 0.05 diopters (D) to the planned astigmatism correction per year for patients less than 30 years old and subtracting 0.02 D per year for patients over 30 years old. Additional nomograms have been developed for femtosecond AK as well.
When femtosecond laser incisions are made, a series of spherical spots are created adjacent to each other tightly so that the final effect is similar to a wide-open incision made using the traditional diamond blade. Alternatively, this process can be adjusted by altering the laser energy and the spot size. Lowering the laser energy creates a weaker shot, making the spot sizes smaller. This way the spots never really connect, which reduces the effectiveness of the femtosecond laser–created AK incision compared with those created using the diamond blade. However, the incision is made tighter, ensuring that it does not open immediately. The laser pulses are typically placed around 3 µm on both the spot and layer separations.
Femtosecond Intrastromal Astigmatic Keratotomy (ISAK)
Intrastromal incision is conceptually different from a manual LRI or full penetration astigmatic incision in that there is no breakage of Bowman’s layer during the procedure. Compared with penetrating AK incisions, the optical zone is made a little smaller and the arc angle is made a little longer in ISAK. The complete intrastomal nature of the incision allows the healing of the realigned stroma without wound gaping or epithelial plug formation. To ensure proper alignment, limbal marks can be made with a sterile pen placed at the 3 o’clock and 9 o’clock positions at the slit lamp just before the procedure. The incision details such as the depth, optical zone, and arc length can be preprogrammed into the femtosecond laser post limbal marking, after which the procedure can be carried out in a very similar manner to LASIK flap creation with the femtosecond laser. Patients with mixed astigmatism with a plano spherical equivalent will best benefit from the ISAK procedure, because the AK incisions are neutral in relation to myopia or hyperopia. Thus, ISAK is a great additional potential option available for femtosecond laser users, especially for astigmatism ranging from 0.50 to 2.75 D of cylinder.
The variables in performing ISAK incisions are arc length on the nomograms and the surgical platform. As for femtosecond AK, Donnenfeld, Nichamin LRI, and modified Lindstrom nomograms are used as starting points for performing ISAK. However, additional nomograms have been determined for ISAK incisions combined with other procedures such as phacoemulsification. The surgeons suggest an incision depth of 80% or 90%.
ISAK confers a unique advantage to the process of astigmatism correction by virtue of its intrastromal nature. The intrastromal incisions can be titrated easily by opening of the incision, and they can be performed easily in a minor procedure room or right at the slit lamp. It is easiest to open the incision if the anterior edge of the incision is left just under the Bowman’s layer.
Wedge Resection Using Laser Arcuate Resection (LAR)
LAR is a standardized technique in which intersecting arcuate cuts are used to perform a wedge resection for the correction of high astigmatism through corneal steepening. The arcuate wedges to be excised are placed in the flat meridian. A simple formula is used to estimate the relative size and location of the arcuate cuts based on the radii of curvature and desired wedge width to be resected. The feasibility of the procedure was established in porcine corneas before treatment of a patient with 20.00 D of postkeratoplasty astigmatism. The astigmatism was reversed ( Fig. 3.8.1 ), and suture removal resulted in a 14.5 D reduction of astigmatism. LAR can be an effective alternative to manual wedge resection, allowing easier, more controlled, and more precise excision of tissue in width, length, and depth.
