From the Ophthalmology Department, IRCCS – “Casa Sollievo della Sofferenza” Hospital, San Giovanni Rotondo (Foggia), Italy.
The authors have no financial or proprietary interest in the materials presented herein.
Study concept and design (L.B.); data collection (L.B., G.P., A.L., E.M., M.G.); interpretation and analysis of data (L.B., G.P., P.V.); drafting of the manuscript (L.B.); critical revision of the manuscript (G.P., A.L., E.M., P.V., M.G., P.V.)
Correspondence: Luca Buzzonetti, MD, Ophthalmology Dept, IRCCS – “Casa Sollievo della Sofferenza” Hospital, Via Cappuccini – 71013 San Giovanni Rotondo (Foggia), Italy. Tel: 39 33 86899038; Fax: 39 06 3244986; E-mail: email@example.com
Penetrating keratoplasty often induces high astigmatism, which represents a significant limiting factor in the visual rehabilitation of patients. Several surgical techniques were proposed to treat postoperative keratoplasty astigmatism: excimer laser custom ablation,1,2 implantation of toric intraocular lenses,3 compression sutures, lamellar keratotomy, and astigmatic keratotomy.4
Merlin5 reported the efficacy of arc-shaped relaxing incisions performed in the corneal stroma to correct astigmatism. The arcuate incisions are commonly used to treat high astigmatism defects that cannot be corrected with spectacles or contact lenses.
The femtosecond solid-state laser6 has been used successfully in several corneal surgical procedures. This technology consists of an infrared Nd:Glass laser beam focused at a desired corneal depth that induces an optical breakdown (ie, photodisruption) without thermal or shockwave damage to the surrounding tissue, especially improving the predictability and optical quality of lamellar surgery.7–9 The femtosecond laser technology of the 60 kHz, the last version recently commercially introduced by the IntraLase system (Abbott Medical Optics, Santa Ana, Calif), enables flap creation in LASIK; the creation of channels for intracorneal rings; the preparation of donor and host tissue in anterior, posterior, and penetrating keratoplasty; and also enables the performance of arcuate incisions, theoretically increasing the precision, reproducibility, and safety of the cuts.
In this preliminary study, we evaluated the refractive and keratometric effect of arcuate keratotomy performed with IntraLase in patients with high astigmatism after keratoplasty.
Patients and Methods
Any experimental investigation reported in the manuscript was performed with informed consent and following all guidelines for experimental investigation with human subjects required by the institution with which all authors are affiliated. All surgeries and follow-up were performed at the Hospital “Casa Sollievo della Sofferenza,” San Giovanni Rotondo, Italy. Inclusion criteria in this retrospective study were high postoperative keratoplasty astigmatism and myopia, complete removal of sutures at least 10 months prior, contact lens intolerance, and a clear lens.
The study cohort comprised nine eyes of nine patients (mean age 45±7.5 years; mean spherical equivalent refraction −2.50±3.60 diopters [D]) who had undergone penetrating keratoplasty. The refractive parameters were measured by subjective refraction, corneal thickness, and keratometric parameters using the Pentacam (Oculus Optikgeräte, Wetzlar, Germany) (Fig 1). Corneal irregular astigmatism changes were evaluated by some of the Pentacam indices that indicate the regularity of the corneal surface: index of surface variance, value of curvature variation from the mean curvature; the index of vertical asymmetry, value of curvature symmetry comparison of the upper and lower area; and the index of height decentration, value of the decentration of height data in the vertical direction.
Figure 1. Comparison of A) Preoperative and B) 3-Month Postoperative Keratometric and Sagittal Anterior Maps Taken by the Pentacam (Oculus Optikgeräte). The Panel on the Right Shows the Difference Calculated for Each Map Between A Minus B.
All surgeries were performed under topical anesthesia using a 60 kHz IntraLase femtosecond laser loaded with the IntraLase Enabled Keratoplasty computer program without complication.
The IntraLase requires placement of a suction ring and applanation lens. After a proper vacuum seal was obtained, the applanation lens was applied to provide a uniform reference plane for the laser, and the laser procedure was performed. Only the “side-cut” function was used. No reference mark was performed.
Two simultaneous opposite incisions were created. Because the IntraLase software allows only one depth measure to be set for both simultaneous incisions, the cut depth was calculated at 80% of the thinnest corneal point measured by the Pentacam over the optical zone diameter.
The mean depth of incisions was 475±59 μm. The mean optical zone was 5.9 mm (range: 4.8 to 6.8 mm). The optical zone was always centered on the pupil and its diameter was chosen to avoid the impact intersection of the femtosecond laser beam with the graft scar. The optical zone diameter decreased as a function of the amount of the astigmatic defect. Paired 70° arc length incisions were made and centered on the steep keratometric axis, as determined by the corneal topography, and the side cut was 90°. All incisions were performed in the graft itself. Immediately after treatment, the surgeon opened the full length of the side cut in an attempt to verify the effect of the femtosecond laser and thoroughly cleaned the area with balanced salt solution (BSS) using a 30-gauge cannula. All eyes received topical dexamethasone 0.1% and netilmicin 0.3% six times a day for 1 week, four times a day for 1 week, and two times a day for 1 week.
Mean change in best spectacle-corrected visual acuity (BSCVA), refractive and keratometric astigmatism, and spherical equivalent refraction were evaluated preoperatively and 3 months postoperatively.
The refractive and keratometric data were analyzed using vector analysis as described by Alpins10,11 that evaluated the effective change in astigmatism, with consideration of change in the astigmatic axis, measuring three vectors and different relationships among them: the target induced astigmatism vector (TIA), defined as the astigmatic change by magnitude and axis the surgery was intended to induce; the surgically induced astigmatism vector (SIA), defined as the amount and axis of astigmatic change the surgery actually induced; and the difference vector (DV), defined as the induced astigmatic change by magnitude and axis that would enable the initial surgery to achieve its intended target (it is an absolute measure of success and is preferably zero). Related to SIA are: the correction index, calculated by determining the ratio of the SIA to the TIA by dividing SIA by TIA, which is preferably 1.0 and indicates an overcorrection if greater than 1.0 or an undercorrection if less than 1.0; the magnitude of the error (ME), which is the arithmetic difference between the magnitudes of the SIA and TIA, positive for overcorrection and negative for undercorrection; the angle of error (AE), which is the angle described by the vectors of the achieved correction (SIA) versus the intended correction (TIA), positive if the achieved correction is on an axis counterclockwise to where it was intended and negative if the achieved correction is clockwise to its intended axis. Related to DV is the index of success, calculated by dividing the DV by the TIA, a relative measure of success and is also preferably zero.
A paired Student t test was used to compare the preoperative and 3-month postoperative data. A P value <.05 was considered statistically significant. Because keratometric and refractive data showed different degrees of skew, the geometric mean for each value of the index of success and correction index was computed. Therefore, by taking the logarithm of data and back-transforming, we obtained the following asymmetrical 95% confidence intervals: keratometric correction index 0.46<0.52<0.59 and index of success 0.38<0.44<0.51; refractive correction index 0.45<0.60<0.80 and index of success 0.20<0.29<0.43.
Mean BSCVA preoperatively was 20/30 and postoperatively was 20/25. The change was not statistically significant (P>.05). Without regard to the axis, the preoperative mean subjective cylinder was 9.10±3.90 D (range: 5.00 to 13.00 D) and the postoperative mean value was 3.10±1.50 D (range: 1.50 to 5.00 D); the cylinder error significantly decreased by 6.00 D (P<.05). Without consideration of the axis, the mean preoperative keratometric value was 9.80±1.90 D and postoperatively was 5.20±1.50 D, with a statistically significant reduction by 4.60 D (P<.05). The preoperative mean spherical equivalent refraction was −2.50±3.60 D (range: 2.25 to 6.00 D) and the postoperative mean value was −2.30±0.90 D (range: 1.25 to 3.50 D); the spherical equivalent refraction did not show significant change (P>.05).
The refractive and keratometric vectors were calculated for each eye (Tables 1 and 2). The mean refractive vectors were: TIA 9.1, SIA 6, DV 3.1, AE −33, ME −3.1, correction index 0.6, and index of success 0.3. The mean keratometric vectors were: TIA 9.8, SIA 5.2, DV 4.5, AE −49, ME −4.6, correction index 0.5, and index of success 0.4 (Table 3).
Table 1: Individual Refractive Data Analyzed Using Vector Analysis
Table 2: Individual Keratometric Data Analyzed Using Vector Analysis
Table 3: Mean Refractive and Keratometric Data Analyzed Using Vector Analysis
Preoperative index of surface variance, index of vertical asymmetry, and index of height decentration values resulted in the pathological range. Postoperatively, in individual cases, index of vertical asymmetry and index of height decentration decreased as a function of the optical zone diameter and incision depth, for optical zone larger than 6.0 mm and arcuate keratotomy deeper than 500 μm. In all other cases, postoperative indices got worse.
High astigmatism after penetrating keratoplasty is a common complication12 and many surgical procedures have been developed to correct it.1–5 Several studies reported the efficacy of arcuate keratotomy in correcting astigmatic errors with minimal changes in spherical equivalent refraction.4,13,14 Arcuate keratotomy, popularized by Merlin,5 is defined as one or more curved relaxing incisions performed in the corneal stroma or graft–host interface, perpendicular to the steep meridian of astigmatism. An arcuate incision produces a relative flattening of the cornea in the axis of the incision with relative steepening at 90° of the incision. This effect, called coupling effect, avoids a significant change in spherical equivalent refraction. Several elements influence the effect of arcuate keratotomy: number, depth, optical zone, and length.
Wilkins et al4 found that in postoperative keratoplasty eyes, the change in the magnitude of astigmatism induced by a pair of 60° arc length arcuate keratotomies performed with an optical zone of 6.0 mm for 600 μm depth, was proportional to the preoperative magnitude of astigmatism and that nomograms planned for congenital astigmatism are not helpful in the management of postoperative keratoplasty astigmatism. The mean cylinder decreased from −10.99±4.26 D to −3.33±2.18 D.
Recently, Zuberbuhler et al15 proposed a modified Ruiz procedure for reducing high astigmatism after penetrating keratoplasty. Two paired 60° arc length arcuate keratotomies with an optical zone ranging from 3.0 to 5.0 mm were performed within the graft and four radial cuts were made. All incisions were planned at 90% to 95% of the central corneal thickness. The preoperative mean subjective cylinder was 8.75±3.05 D and, without consideration of cylinder axis and its changes, the mean postoperative cylinder was 5.31±3.12 D. The reduction in cylinder of 3.44 D was statistically significant. This procedure resulted in decreased postoperative keratoplasty astigmatism, but the refractive outcome did not appear predictable with the method of vector analysis of Alpins (mean correction index was 1.95 and mean index of success was 1.27).
In this preliminary study, we evaluated the refractive and keratometric effect of arcuate keratotomy performed with IntraLase in patients with high postoperative keratoplasty astigmatism (Fig 2).
Figure 2. One Arcuate Keratotomy Performed with the IntraLase Photographed 7 Days After Surgery (original Magnification ×16).
We found an increase in BSCVA that was not statistically significant and a statistically significant reduction in refractive and keratometric astigmatism. The lack of a significant change in spherical equivalent refraction was due to the coupling effect. In individual cases, we found that the corneal irregular astigmatism amount was less with a larger optical zone and deeper incisions.
We also evaluated the predictability of the astigmatic correction using the method of vector analysis of refractive and keratometric data.11 Both refractive and keratometric analysis showed an undercorrection of the astigmatism with correction index values of 0.6 for refraction and 0.5 for keratometry. The negative magnitude of the error values, where the mean SIA magnitude is less than the mean TIA magnitude, confirmed a trend of undercorrection for the group.
The analysis of astigmatic correction suggested that the treatment was off axis clockwise, indicating a misalignment. The examination of the angle of error showed a less favorable outcome on alignment for the keratometric mean. The angle of error changes could be partially correlated to variable factors such as healing or alignment, as well as to the surgical procedure. The optical zone was always centered on the pupil because the IntraLase suction procedure makes it difficult for exact alignment of the applanation cone to the corneal apex, whereas the center of the pupil can be determined after cone docking using the options of the computer program. The procedure could induce a virtual decentration due to tilt of the cone. No reference mark was performed. Although the vacuum suction of the IntraLase maintains a constant position of the eye during the treatment, an axis mark will be necessary to optimize the procedure. All incisions were planned on the center of the steep keratometric axis as determined by corneal topography.
Several authors observed the high precision of IntraLase in corneal lamellar surgery,16,17 reporting the quality and precision of the latest femtosecond laser engine that permits a tight spot/line separation using low energy. The 60 kHz IntraLase femtosecond laser loaded with IntraLase Enabled Keratoplasty computer program makes it possible to use femtosecond technology in arcuate keratotomy using only the “side cut” function. The side-cut angle is the angle that the cylindrical side cut makes with the corneal surface (90° is perpendicular to the corneal anterior surface). The IntraLase creates corneal arcuate incisions whose length, depth, and optical zone diameter can be planned for each individual eye. Because the IntraLase software allows only one depth measure to be set for both simultaneous incisions, we calculated the cut depth at 80% of the thinnest corneal point measured by the Pentacam over the optical zone diameter. However, in an attempt to preserve the same residual corneal stroma thickness for each arcuate keratotomy, the surgeon could perform two distinct treatments, setting different stromal depth values. The effect of this approach was not evaluated by our study.
The use of the IntraLase femtosecond laser for the correction of high postoperative keratoplasty astigmatism seems to provide good refractive and keratometric results, better than those achieved by other authors who performed the incisional surgery with smaller optical zones and deeper incisions usually calculated at 90% to 95% of the corneal thickness.4,15 Our findings suggest that arcuate keratotomy performed with the IntraLase could reduce the risk of corneal perforation as well as the delayed changes in corneal curvature and the increased separation of the edges of the original wound occasionally observed after incisional surgery,18 hopefully producing a greater stability of the refractive effect.
One of the advantages related to the IntraLase technology7 is that, with proper spacing of the laser spots, no significant morphologic tissue bridges are present. This element could also provide a better and more stable wound healing process after arcuate keratotomy. With standardized arcuate keratotomy, the healing begins with an epithelial plug filling the gaping wound and it can take as long as 5 years for the plug to be replaced with disorganized hypercellular scar tissue.4
This preliminary study evaluated a small sample of patients with a brief follow-up; however, our findings suggest that a nomogram adjustment could be derived by the SIA magnitude that could represent the nominal value of TIA. In fact, a refractive and keratometric TIA of 9.00 D (the mean TIA value calculated in this study) appears as a nonrealistic surgical goal with the femtosecond laser. Alternatively, the SIA magnitude of approximately 6.00 D could be considered as the TIA, then as the amount of postoperative keratoplasty astigmatism that could really be corrected with the IntraLase.
These findings suggest that simultaneous arcuate keratotomy performed with the IntraLase femtosecond laser could be an effective and safe surgical procedure in the treatment of high postoperative keratoplasty astigmatism. The 3-month follow-up showed a good predictability of the procedure. However, more patients and longer follow-up are needed to confirm these early results and to improve treatment nomograms.
- Mularoni A, Laffi GL, Bassein L, Tassinari G. Two-step LASIK with topography-guided ablation to correct astigmatism after penetrating keratoplasty. J Refract Surg. 2006;22:67–74.
- Donnenfeld ED, Kornstein HS, Amin A, Speaker MD, Seedor JA, Sforza PD, Landrio LM, Perry HD. Laser in situ keratomileusis for correction of myopia and astigmatism after penetrating keratoplasty. Ophthalmology. 1999;106:1966–1974. doi:10.1016/S0161-6420(99)90410-4 [CrossRef]
- Viestenz A, Kuchle M, Seitz B, Langenbucher A. Toric intraocular lenses for correction of persistent corneal astigmatism after penetrating keratoplasty. Ophthalmologe. 2005;102:148–152. doi:10.1007/s00347-004-1090-5 [CrossRef]
- Wilkins MR, Mehta JS, Larkin DF. Standardized arcuate keratotomy for postkeratoplasty astigmatism. J Cataract Refract Surg. 2005;31:297–301. doi:10.1016/j.jcrs.2004.07.025 [CrossRef]
- Merlin U. Curved keratotomy procedure for congenital astigmatism. J Refract Surg. 1987;3:92–97.
- Kurtz RM, Liu X, Elner VM, Squier JA, Du D, Mourou GA. Photodisruption in the human cornea as a function of the laser pulse width. J Refract Surg. 1997;13:653–658.
- Sarayba MA, Ignacio TS, Tran DB, Binder PS. A 60 kHz IntraLase femtosecond laser creates a smoother LASIK stromal bed surface compared to a Zyoptix XP mechanical microkeratome in human donor eyes. J Refract Surg. 2007;23:331–337.
- Alió JL, Piñero DP. Very high-frequency digital ultrasound measurement of the LASIK flap thickness profile using the IntraLase femtosecond laser and M2 and Carriazo-Pendular microkeratomes. J Refract Surg. 2008;24:12–23.
- Buzzonetti L, Petrocelli G, Valente P, Tamburrelli C, Mosca L, Laborante A, Balestrazzi E. Comparison of corneal aberration changes after laser in situ keratomileusis performed with mechanical microkeratome and IntraLase femtosecond laser: 1-year follow-up. Cornea. 2008;27:174–179. doi:10.1097/ICO.0b013e31815a50bf [CrossRef]
- Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg. 1993;19:524–533.
- Alpins N. Astigmatism analysis by the Alpins method. J Cataract Refract Surg. 2001;27:31–49 doi:10.1016/S0886-3350(00)00798-7 [CrossRef]
- Sharif KW, Casey TA. Penetrating keratoplasty for keratoconus: complications and long-term success. Br J Ophthalmol. 1991;75:142–146. doi:10.1136/bjo.75.3.142 [CrossRef]
- Akura J, Matsuura K, Hatta S, Otsuka K, Kaneda S. A new concept for the correction of astigmatism: full-arc, depth-dependent astigmatic keratotomy. Ophthalmology. 2000;107:95–104. doi:10.1016/S0161-6420(99)00021-4 [CrossRef]
- Koay PY, McGhee CN, Crawford GJ. Effect of a standard paired arcuate incision and augmentation sutures on postkeratoplasty astigmatism. J Cataract Refract Surg. 2000;26:553–561. doi:10.1016/S0886-3350(00)00324-2 [CrossRef]
- Zuberbuhler B, Agrawal A, Gale R, Kipioti T, Gauba V, Noble B. Modified Ruiz procedure for reducing high astigmatism after penetrating keratoplasty. Cornea. 2007;26:1171–1177. doi:10.1097/ICO.0b013e318153f241 [CrossRef]
- Stahl JE, Durrie DS, Schwendeman FJ, Boghossian AJ. Anterior segment OCT analysis of thin IntraLase femtosecond flaps. J Refract Surg. 2007;23:555–558.
- Binder PS. One thousand consecutive IntraLase laser in situ keratomileusis flaps. J Cataract Refract Surg. 2006;32:962–969. doi:10.1016/j.jcrs.2006.02.043 [CrossRef]
- Eiferman RA, Schultz GS, Nordquist RE, Waring GO III, . Corneal wound healing and its pharmacologic modification after refractive keratotomy. In: Waring GO III, , ed. Refractive Keratotomy for Myopia and Astigmatism. St Louis, Mo: Mosby Year Book; 1991:749–770.
Individual Refractive Data Analyzed Using Vector Analysis
|Target induced astigmatism, TIA||5||12||13||6.5||12||7.6||7.8||10.8||7.2|
|Surgically induced astigmatism, SIA||3.5||8||10.5||1.5||6.4||5.5||6.2||6.4||6.1|
|Difference vector, DV||1.5||4||2.5||5||5.6||2.1||1.6||4.4||1.1|
|Angle of error, AE||0||−95||0||−35||−28||0||−55||−11||−78|
|Magnitude of the error, ME||−1.5||−4||−2.5||−5||−5.6||−2.1||−1.6||−4.4||−1.1|
|Index of success||0.3||0.3||0.2||0.8||0.5||0.3||0.2||0.4||0.1|
Individual Keratometric Data Analyzed Using Vector Analysis
|Target induced astigmatism, TIA||9.9||11.8||10.5||7.1||10.4||9.5||8.4||12.2||8.9|
|Surgically induced astigmatism, SIA||4.3||5.9||3.9||4.1||6.1||5.6||6.1||6.5||4.8|
|Difference vector, DV||5.6||5.9||6.6||3||4.3||3.9||2.3||5.7||4.1|
|Angle of error, AE||−135.2||−21.6||−30.4||−34.1||−49||−29.1||−39||−58||−44|
|Magnitude of the error, ME||−5.6||−5.9||−6.6||−3||−4.3||−3.9||−2.3||−5.7||−4.1|
|Index of success||0.6||0.4||0.6||0.4||0.4||0.4||0.3||0.5||0.5|
Mean Refractive and Keratometric Data Analyzed Using Vector Analysis
|Refractive Analysis||Keratometric Analysis|
|Target induced astigmatism, TIA||9.1||9.8|
|Surgically induced astigmatism, SIA||6.0||5.2|
|Difference vector, DV||3.1||4.5|
|Angle of error, AE||−33||−49|
|Magnitude of the error, ME||−3.1||−4.6|
|Index of success||0.3||0.4|