- Email: [email protected]
Evaluation of corneal biomechanical parameters after simultaneous phacoemulsification with intraocular lens implantation and limbal relaxing incisions
ARTICLE
Evaluation of corneal biomechanical parameters after simultaneous phacoemulsification with intraocular lens implantation and limbal relaxing incisions Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD, Fumiko Ohmoto, MD, Rie Amano, MD, PhD
PURPOSE: To determine whether alterations in architecture cause corneal biomechanical changes after simultaneous cataract surgery and limbal relaxing incisions (LRIs). SETTING: Department of Ophthalmology, Kitasato University, Kanagawa, Japan. DESIGN: Observational case series. METHODS: This study longitudinally assessed corneal hysteresis (CH) and the corneal resistance factor (CRF) using the Ocular Response Analyzer in eyes having cataract surgery with LRIs. The relationship between these biomechanical parameters and central corneal thickness (CCT), measured using an ultrasound pachymeter, was also assessed. RESULTS: The mean CH was 10.0 mm Hg G 1.2 (SD) preoperatively, 9.0 G 1.6 mm Hg 1 day postoperatively, 9.7 G 1.4 mm Hg at 1 week, 9.6 G 1.4 mm Hg at 1 month, and 10.2 G 1.3 mm Hg at 3 months. The mean CRF was 10.0 G 1.5 mm Hg, 8.9 G 1.6 mm Hg, 9.5 G 1.5 mm Hg, 9.5 G 1.4 mm Hg, and 9.5 G 1.4 mm Hg, respectively. There were significant differences between preoperative and 1-day postoperative measurements (PZ.005, CH; PZ.004, CRF). The CH and CRF were significantly correlated with CCT (r Z 0.33, PZ.04 and r Z 0.40, PZ.01, respectively) 3 months postoperatively. CONCLUSIONS: The CH and CRF values decreased 1 day after simultaneous cataract surgery with LRIs but soon recovered to preoperative levels, suggesting there were no significant changes in corneal biomechanical factors after 1 day. Corneal thickness may play a role in biomechanical factors even in such eyes. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2011; 37:265–270 Q 2011 ASCRS and ESCRS
Phacoemulsification with intraocular lens (IOL) implantation has been widely accepted as an effective means for the treatment of cataract, and good uncorrected visual acuity is essential to minimize spectacle dependence and maximize subsequent patient satisfaction after this procedure. Although surgical techniques, biometry, and IOL power calculation formulas have largely improved in recent years, corneal astigmatism can be a source of the remaining refractive errors after cataract surgery. Limbal relaxing incisions (LRIs) were developed to reduce preexisting corneal astigmatism during cataract surgery.1–4 Simultaneous phacoemulsification and LRI procedures require a clear corneal or sclerocorneal Q 2011 ASCRS and ESCRS Published by Elsevier Inc.
incision of approximately 2.0 to 3.0 mm as well as paired arcuate incisions on the limbus. Accordingly, it is possible that the postoperative biomechanical characteristics, which may play a role not only in refractive outcomes5–9 but also in the measurement of intraocular pressure (IOP),10–13 might be compromised with time. However, to our knowledge, the corneal biomechanics after simultaneous cataract surgery and LRIs has not been studied. Because of the prevalence of these combined surgical procedures, it is important to quantitatively and longitudinally assess corneal biomechanics postoperatively. The aim of this study was 2-fold; that is, to longitudinally assess the corneal biomechanical parameters in 0886-3350/$ - see front matter doi:10.1016/j.jcrs.2010.08.045
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eyes having simultaneous cataract surgery and LRIs and to evaluate the relationship between these parameters with corneal thickness. PATIENTS AND METHODS This observational study evaluated eyes of patients who had standard cataract surgery combined with LRIs and who returned regularly for postoperative examinations. All patients provided informed consent. The study adhered to the tenets of the Declaration of Helsinki. Institutional review board approval was not required for this retrospective study.
Surgical Technique To control for potential cyclotorsion, the horizontal axis was marked at the slitlamp preoperatively with the patient supine. All surgeries were performed by 1 of 3 experienced surgeons (K.S., K.K., R.A.). For the LRI portion of surgery, 2 paired arcuate incisions were created with a guarded micrometer diamond blade set at 500 mm. The incisions were made in the steepest corneal axis at the limbus just anterior to the palisades of Vogt. The incision length was 45 to 65 degrees of arc according to a modified Gills nomogram.14 Incisions were irrigated with a balanced salt solution. After the LRIs were made, standard cataract surgery was performed. The technique consisted of a capsulorhexis, nucleus and cortex extraction (Infinity Vision System, Alcon Laboratories, Inc.), and implantation of an aspheric silicone IOL (KS-3Ai, Staar Surgical Co.) through a temporal 2.8 mm clear corneal incision. Postoperatively, betamethasone 0.1%, (Rinderon), levofloxacin (Cravit), and diclofenac sodium 0.1% (Diclod) were administered topically 4 times daily for 1 month, after which the dose was steadily tapered.
Measurements of Corneal Biomechanical Parameters The biomechanical factors of the cornea were measured using a biomechanical waveform analysis system (Ocular Response Analyzer, Reichert Ophthalmic Instruments) preoperatively and 1 day, 1 week, and 1 and 3 months postoperatively. The biomechanical waveform device uses a sudden air impulse to deform the cornea and monitors the shape changes with an electro-optical system. The puff of air induces 2 applanations (inward and outward) of the cornea. The air stream deforms the cornea through an initial applanation event (peak 1) and then beyond it into concavity. The concavity then subsides, allowing the cornea to rebound through a second applanation (peak 2). Two biomechanical factorsdcorneal hysteresis (CH) and the corneal
Submitted: February 15, 2010. Final revision submitted: August 10, 2010. Accepted: August 25, 2010. From the Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan. Corresponding author: Kazutaka Kamiya, MD, PhD, Department of Ophthalmology, University of Kitasato School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa, 228-8555, Japan. E-mail: [email protected].
resistance factor (CRF)dwere measured. The CH parameter is calculated as the difference between pressure peak 1 and pressure peak 2 and is expressed in millimeters of mercury.15 The CRF is calculated as a linear function of peak 1 and peak 2. The device also provides the following 2 IOP measurements: Goldmann-correlated IOP, which is the mean of pressure peak 1 and pressure peak 2, and corneal-compensated IOP, which according to the manufacturer is less affected by corneal properties than Goldmann applanation IOP.A The measurements were taken at least 3 times to ensure that the signal quality, signal morphology, and measurement values were consistent. The mean of the measurements was used for statistical analysis according to the manufacturer’s instructions. After a topical anesthetic agent was placed in the patient’s eye, the central corneal thickness (CCT) was measured using an ultrasound pachymeter (DGH-500, DGH Technology, Inc.).
Statistical Analysis All statistical analyses were performed using StatView software (version 5.0, SAS Institute, Inc.). Repeatedmeasures analysis of variance (ANOVA) was used to evaluate the changes over time, and the Dunnett test was used for multiple comparisons. After normal distribution of the data was confirmed with the Kolmogorov-Smirnov test, the correlations between the CH and CRF values and the CCT values were assessed by calculating the Pearson correlation coefficient (r). Unless otherwise indicated, the results are expressed as the mean G SD. A P value less than 0.05 was considered statistically significant.
RESULTS The study enrolled 38 eyes of 29 consecutive patients (12 men, 17 women). The mean age of the patients was 69.4 G 9.4 years (range 50 to 83 years). The preoperative manifest refraction (spherical equivalent) was 2.41 G 4.65 diopters (D) (range 15.00 to 4.75 D). All surgeries were uneventful. Figures 1, 2, and 3 show the time courses of CH, CRF, and CCT, respectively. The variance in the CH data was statistically significant (PZ.006, repeatedmeasures ANOVA). Multiple comparisons showed a significant difference between preoperative and 1-day postoperative measurements (PZ.005, Dunnett test) but not between preoperative measurements and postoperative measurements at 1 week (PZ.45), 1 month (PZ.33), or 3 months (PZ.93). The variance in the CRF data was statistically significant (PZ.05, repeated-measures ANOVA). Multiple comparisons showed a significant difference between preoperative and 1-day postoperative measurements (PZ.004) but not between preoperative measurements and postoperative measurements at 1 week (PZ.19), 1 month (PZ.20), or 3 months (PZ.19). The variance in the CCT data was statistically significant (PZ.01, repeated-measures ANOVA). Multiple comparisons showed a significant difference between preoperative and 1-day postoperative measurements
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Figure 1. Time course of the mean CH (GSD) after cataract surgery with LRIs. The bar represents the SD (CH Z corneal hysteresis; Post Z postoperative; Preop Z preoperative).
Figure 2. Time course of CRF (GSD) after cataract surgery with LRIs. The bar represents the SD (CRF Z corneal resistance factor; Post Z postoperative; Preop Z preoperative).
(PZ.04) but not between preoperative measurements and postoperative measurements at 1 week (PZ.87), 1 month (PZ.98), or 3 months (PZ.95). The mean Goldmann-correlated IOP was 15.3 G 3.0 mm Hg preoperatively, 15.0 G 3.8 mm Hg 1 day postoperatively, 14.3 G 3.0 mm Hg at 1 week, 14.1 G 3.2 mm Hg at 1 month, and 13.4 G 3.1 mm Hg at 3 months. The variance of the data was not statistically significant (PZ.10, repeated-measures ANOVA). Multiple comparisons showed no significant differences between preoperative measurements and postoperative measurements at 1 day (PZ.99), 1 week (PZ.48), 1 month (PZ.29), or 3 months (PZ.05). The mean corneal-compensated IOP was 16.3 G 2.5 mm Hg preoperatively, 16.6 G 3.6 mm Hg 1 day postoperatively, 15.7 G 2.9 mm Hg at 1 week, 15.5 G 3.3 mm Hg at 1 month, and 14.7 G 3.2 mm Hg at
3 months. The variance of the data was not statistically significant (PZ.07, repeated-measures ANOVA). Multiple comparisons showed no significant differences between preoperative measurements and postoperative measurements at 1 day (PZ.96), 1 week (PZ.82), 1 month (PZ.64), or 3 months (PZ.09). The distributions of CH, CRF, and CCT preoperatively and at all postoperative examinations were normal (Kolmogorov-Smirnov test). There was a weak but significant correlation between CH and CCT preoperatively (r Z .33, PZ.04), 1 week postoperatively (r Z 0.33, PZ.04), at 1 month (r Z 0.43, PZ.006), and at 3 months (r Z 0.33, PZ.04) (Figure 4). There was no significant correlation between CH and CCT 1 day after surgery (r Z 0.20, PZ.23). There was also a weak but significant correlation between CRF and CCT preoperatively (r Z 0.42, PZ.008) and at 1
Figure 3. Time course of CCT (GSD) after cataract surgery with LRIs. The bar represents the SD (CCT Z central corneal thickness; Post Z postoperative; Preop Z preoperative).
Figure 4. Scatterplot showing a significant correlation between CH and CCT 3 months after cataract surgery with LRIs (CCT Z central corneal thickness; CH Z corneal hysteresis).
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Figure 5. Scatterplot showing a significant correlation between CRF and CCT 3 months after cataract surgery with LRIs (CCT Z central corneal thickness; CRF Z corneal resistance factor).
week (r Z 0.36, PZ.03), 1 month (r Z 0.36, PZ.03), and 3 months (r Z 0.40, PZ.01) (Figure 5). There was no significant correlation between CRF and CCT 1 day after surgery (r Z 0.18, PZ.29). DISCUSSION In this study, the CH and CRF values briefly decreased 1 day after surgery but soon recovered to preoperative levels and were stable thereafter. The CCT also increased 1 day after surgery and soon recovered to preoperative levels. To our knowledge, ours is the first study to assess biomechanical changes after simultaneous cataract surgery and LRIs. Hager et al.16 reported the corneal biomechanic results in 101 eyes having phacoemulsification with IOL implantation through a 4.1 mm corneal incision. They found that the mean CH value decreased from 10.35 G 2.5 mm Hg preoperatively to 9.20 G 1.9 mm Hg 1 day postoperatively but that the CCT increased from 556.82 G 32.5 mm to 580.26 G 45.5 mm, respectively. In a study of 51 eyes having cataract surgery through a 2.4 mm corneal incision with a 3-month follow-up, Kucumen et al.17 found that the CH and CRF values basically returned to normal preoperative levels a short time after surgery. In a study by Falkenberg et al.,18 there was a mean increase in CCT of 37 mm 1 day after cataract surgery. In another study,19 we also found that CH and CRF decreased briefly 1 day after cataract surgery without LRIs but soon recovered to the preoperative levels. The biomechanical changes after simultaneous cataract surgery with LRIs in our study were similar to those after cataract surgery alone in the previous studies. Therefore, we assume that standard cataract surgery and combined cataract surgery and LRIs do not significantly affect these biomechanical parameters
except at 1 day. Although we did not directly compare the biomechanical changes of simultaneous cataract surgery and LRIs with those of cataract surgery alone in this study, our present and previous results suggest that LRIs alone do not induce a change in biomechanical parameters, presumably because these surgical techniques require only arcuate partial incisions on the limbus. The relationship between corneal biomechanical factors, such as CH and CRF, and CCT has been discussed. In a study by Lam et al.,20 CH was positively associated with CCT in normal eyes. Broman et al.21 found a significant correlation between CH and CCT and a modest correlation coefficient in patients presenting at a glaucoma clinic. Shah et al.22 also demonstrated that CH increased with increasing CCT in normal eyes, but the correlation was moderate. We previously demonstrated that CCT was the most relevant factor affecting CH and CRF in normal eyes.23 On the other hand, we found a significant decrease in CH and CRF in association with a significant increase in CCT 1 day after surgery in the current study. Lu et al.24 also found no significant correlation between CH and CCT induced by wearing soft contact lenses during eye closure. Although we previously showed that eyes with a higher IOP are more predisposed to a lower CH value,23 no eye in the present study had a significant IOP increase (O25 mm Hg) during the follow-up period. Accordingly, the increase in CCT and the decrease in CH and CRF may be attributed to the transient corneal edema induced by the surgical stress to the corneal endothelium, although measurements of these parameters 1-day postoperatively were not very reliable because of the presence of superficial punctuate keratitis, inflammatory responses, or difficulty opening the eye. The CH factor is a dynamic measure of the viscous damping in the corneal tissue and represents the energy absorption capability of the cornea. The CRF parameter is an indicator of the total corneal response, including the elastic resistance of the corneal tissue. Our results suggest that corneal edema immediately after the combined procedure decreases not only the energy absorption capability but also the elastic resistance of the corneal tissue. Dupps25 stated that CH is dependent on the type, time course, and magnitude of the perturbation and that the impact of variable air pressure is being studied as a source of error and as a means of eliciting additional biomechanical information from the Ocular Response Analyzer biomechanical waveform device. Using a viscoelastic model, Glass et al.26 also showed how changing viscosity and elasticity affects hysteresis measurements in various ways. The lack of a persistent decrease in CH and CRF based on 1 measurement technique does not imply that the corneal biomechanics were
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not changed at all by these surgical procedures. We also found that the CH and CRF parameters were significantly correlated with CCT 1 week and 1 and 3 months postoperatively, suggesting that postoperative corneal thickness may play a role in the biomechanical characteristics of the cornea after the transient corneal edema resolves. In the present study, we used the mean values of 3 consecutive CH and CRF measurements taken with the biomechanical waveform device for statistical analysis; however, the study may have been more robust and relevant if the best waveform signal had been used according to the manufacturer’s more recent instructions. In another study in which we used the same biomechanical waveform device,27 we assessed the repeatability of the measurements by taking 3 consecutive CH and CRF measurements in 18 normal eyes at the same time of day on 2 days. The mean difference between 2 consecutive measurements (G95% limits of agreement) was 0.1 G 0.5 mm Hg (range 1.1 to 1.0 mm Hg) for CH and 0.0 G 0.5 mm Hg (range 0.9 to 1.0 mm Hg) for CRF. Although a small amount of test–retest variability may not necessarily mean high accuracy, it indicates the reproducibility of the measurements and thus the applicability of data. Accordingly, we believe the Ocular Response Analyzer offers reasonable repeatability in the longitudinal assessment of these biomechanical factors. The limitations of this study are that the amount of data was limited and that the follow-up was short. We evaluated CH and CRF for up to 3 months postoperatively, when the biomechanical properties of the cornea would have stabilized based on the woundhealing responses of the cornea. A long-term study with a larger patient cohort is required to confirm our findings. In summary, we found that the CH and CRF biomechanical factors decreased transiently 1 day after surgery but soon recovered to the preoperative levels, suggesting that simultaneous cataract surgery with LRIs does change corneal biomechanical parameters except at 1 day after surgery. We also found a weak, but significant, correlation between these parameters and CCT except for 1 day after surgery, suggesting that corneal thickness plays a role in these biomechanical parameters, even after any transient edema resolves. REFERENCES
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Elizabeth Astigmatism Study Group. J Cataract Refract Surg 2005; 31:2261–2265 Arraes JC, Cunha F, Azevedo Arraes T, Cavalvanti R, ~es relaxantes limbares durante a cirurgia de Ventura M. Inciso s seguimento de um ano [Limbal relaxcatarata: resultados apo ing incisions during cataract surgery: one-year follow-up]. Arq Bras Oftalmol 2006; 69:361–364. Available at: http://www. scielo.br/pdf/abo/v69n3/30787.pdf. Accessed October 18, 2010 Nichamin LD. Astigmatism control. Ophthalmol Clin North Am 2006; 19(4):485–493 Roberts C. Biomechanics of the cornea and wavefront-guided laser refractive surgery. J Refract Surg 2002; 18:S589–S592 Kamiya K, Miyata K, Tokunaga T, Kiuchi T, Hiraoka T, Oshika T. Structural analysis of the cornea using scanning-slit corneal topography in eyes undergoing excimer laser refractive surgery. Cornea 2004; 23:S59–S64 Jaycock PD, Lobo L, Ibrahim J, Tyrer J, Marshall J. Interferometric technique to measure biomechanical changes in the cornea induced by refractive surgery. J Cataract Refract Surg 2005; 31:175–184 Deenadayalu C, Mobasher B, Rajan SD, Hall GW. Refractive change induced by the LASIK flap in a biomechanical finite element model. J Refract Surg 2006; 22:286–292 Dupps WJ Jr, Wilson SE. Biomechanics and wound healing in the cornea. Exp Eye Res 2006; 83:709–720 Orssengo GJ, Pye DC. Determination of the true intraocular pressure and modulus of elasticity of the human cornea in vivo. Bull Math Biol 1999; 61:551–572. Available at: http:// www.springerlink.com/content/t26248xv8p316637/fulltext.pdf. Accessed October 18, 2010 Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement; quantitative analysis. J Cataract Refract Surg 2005; 31:146–155 Herndon LW. Measuring intraocular pressure-adjustments for corneal thickness and new technologies. Curr Opin Ophthalmol 2006; 17:115–119 Bryant MR, McDonnell PJ. Constitutive laws for biomechanical modeling of refractive surgery. J Biomech Eng 1996; 118: 473–481 Gills JP, Gayton JL. Reducing pre-existing astigmatism. In: Gills JP, Fenzl R, Martin RG, eds, Cataract Surgery; the State of the Art. Thorofare, NJ, Slack, 1998; 53–66 Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005; 31:156–162 €llhas MO, Schroeder B, Grobherr M, Hager A, Loge K, Fu Wiegand W. Changes in corneal hysteresis after clear corneal cataract surgery. Am J Ophthalmol 2007; 144:341–346 Kucumen RB, Yenerel NM, Gorgun E, Kulacoglu DN, Oncel B, Kohen MC, Alimgil ML. Corneal biomechanical properties and intraocular pressure changes after phacoemulsification and intraocular lens implantation. J Cataract Refract Surg 2008; 34:2096–2098 Falkenberg B, Kutschan A, Wiegand W. Analyse der optisch wirksamen Parameter nach Kataraktoperation mit Faltlinsenimplantation [Analysis of optical parameters after cataract surgery and implantation of foldable lens]. Ophthalmologe 2005; 102:587–591 Kamiya K, Shimizu K, Ohmoto F, Amano R. Time course of corneal biomechanical parameters after phacoemulsification with intraocular lens implantation. Cornea 2010 Aug 5; [Epub ahead of print] Lam A, Chen D, Chiu R, Chui W-S. Comparison of IOP measurements between ORA and GAT in normal Chinese. Optom Vis Sci 2007; 84:909–914
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21. Broman AT, Congdon NG, Bandeen-Roche K, Quigley HA. Influence of corneal structure, corneal responsiveness, and other ocular parameters on tonometric measurement of intraocular pressure. J Glaucoma 2007; 16:581–588 22. Shah S, Laiquzzaman M, Bhojwani R, Mantry S, Cunliffe I. Assessment of the biomechanical properties of the cornea with the Ocular Response Analyzer in normal and keratoconic eyes. Invest Ophthalmol Vis Sci 2007; 48:3026–3031. Available at: http://www.iovs.org/cgi/content/abstract/48/7/3026. Accessed October 18, 2010 23. Kamiya K, Hagishima M, Fujimura F, Shimizu K. Factors affecting corneal hysteresis in normal eyes. Graefes Arch Clin Exp Ophthalmol 2008; 246:1491–1494 24. Lu F, Xu S, Qu J, Shen M, Wang X, Fang H, Wang J. Central corneal thickness and corneal hysteresis during corneal swelling induced by contact lens wear with eye closure. Am J Ophthalmol 2007; 143:616–622 25. Dupps WJ Jr. Hysteresis: new mechanospeak for the ophthalmologist [editorial]. J Cataract Refract Surg 2007; 33:1499–1501 26. Glass DH, Roberts CJ, Litsky AS, Weber PA. A viscoelastic biomechanical model of the cornea describing the effect of viscosity and elasticity on hysteresis. Invest Ophthalmol Vis Sci 2008;
49:3919–3926. Available at: http://www.iovs.org/cgi/reprint/49/ 9/3919. Accessed October 18, 2010 27. Kamiya K, Shimizu K, Ohmoto F. Comparison of the changes in corneal biomechanical properties after photorefractive keratectomy and laser in situ keratomileusis. Cornea 2009; 28:765–769
OTHER CITED MATERIAL A. Luce DA. Methodology for cornea compensated IOP and corneal resistance factor for the Reichert Ocular Response Analyzer. IOVS 2006 47: ARVO E-Abstract 2266
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First author: Kazutaka Kamiya, MD, PhD Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan
Evaluation of corneal biomechanical parameters after simultaneous phacoemulsification with intraocular lens implantation and limbal relaxing incisions Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD, Fumiko Ohmoto, MD, Rie Amano, MD, PhD
PURPOSE: To determine whether alterations in architecture cause corneal biomechanical changes after simultaneous cataract surgery and limbal relaxing incisions (LRIs). SETTING: Department of Ophthalmology, Kitasato University, Kanagawa, Japan. DESIGN: Observational case series. METHODS: This study longitudinally assessed corneal hysteresis (CH) and the corneal resistance factor (CRF) using the Ocular Response Analyzer in eyes having cataract surgery with LRIs. The relationship between these biomechanical parameters and central corneal thickness (CCT), measured using an ultrasound pachymeter, was also assessed. RESULTS: The mean CH was 10.0 mm Hg G 1.2 (SD) preoperatively, 9.0 G 1.6 mm Hg 1 day postoperatively, 9.7 G 1.4 mm Hg at 1 week, 9.6 G 1.4 mm Hg at 1 month, and 10.2 G 1.3 mm Hg at 3 months. The mean CRF was 10.0 G 1.5 mm Hg, 8.9 G 1.6 mm Hg, 9.5 G 1.5 mm Hg, 9.5 G 1.4 mm Hg, and 9.5 G 1.4 mm Hg, respectively. There were significant differences between preoperative and 1-day postoperative measurements (PZ.005, CH; PZ.004, CRF). The CH and CRF were significantly correlated with CCT (r Z 0.33, PZ.04 and r Z 0.40, PZ.01, respectively) 3 months postoperatively. CONCLUSIONS: The CH and CRF values decreased 1 day after simultaneous cataract surgery with LRIs but soon recovered to preoperative levels, suggesting there were no significant changes in corneal biomechanical factors after 1 day. Corneal thickness may play a role in biomechanical factors even in such eyes. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2011; 37:265–270 Q 2011 ASCRS and ESCRS
Phacoemulsification with intraocular lens (IOL) implantation has been widely accepted as an effective means for the treatment of cataract, and good uncorrected visual acuity is essential to minimize spectacle dependence and maximize subsequent patient satisfaction after this procedure. Although surgical techniques, biometry, and IOL power calculation formulas have largely improved in recent years, corneal astigmatism can be a source of the remaining refractive errors after cataract surgery. Limbal relaxing incisions (LRIs) were developed to reduce preexisting corneal astigmatism during cataract surgery.1–4 Simultaneous phacoemulsification and LRI procedures require a clear corneal or sclerocorneal Q 2011 ASCRS and ESCRS Published by Elsevier Inc.
incision of approximately 2.0 to 3.0 mm as well as paired arcuate incisions on the limbus. Accordingly, it is possible that the postoperative biomechanical characteristics, which may play a role not only in refractive outcomes5–9 but also in the measurement of intraocular pressure (IOP),10–13 might be compromised with time. However, to our knowledge, the corneal biomechanics after simultaneous cataract surgery and LRIs has not been studied. Because of the prevalence of these combined surgical procedures, it is important to quantitatively and longitudinally assess corneal biomechanics postoperatively. The aim of this study was 2-fold; that is, to longitudinally assess the corneal biomechanical parameters in 0886-3350/$ - see front matter doi:10.1016/j.jcrs.2010.08.045
265
266
TIME COURSE OF CORNEAL BIOMECHANICS AFTER CATARACT SURGERY
eyes having simultaneous cataract surgery and LRIs and to evaluate the relationship between these parameters with corneal thickness. PATIENTS AND METHODS This observational study evaluated eyes of patients who had standard cataract surgery combined with LRIs and who returned regularly for postoperative examinations. All patients provided informed consent. The study adhered to the tenets of the Declaration of Helsinki. Institutional review board approval was not required for this retrospective study.
Surgical Technique To control for potential cyclotorsion, the horizontal axis was marked at the slitlamp preoperatively with the patient supine. All surgeries were performed by 1 of 3 experienced surgeons (K.S., K.K., R.A.). For the LRI portion of surgery, 2 paired arcuate incisions were created with a guarded micrometer diamond blade set at 500 mm. The incisions were made in the steepest corneal axis at the limbus just anterior to the palisades of Vogt. The incision length was 45 to 65 degrees of arc according to a modified Gills nomogram.14 Incisions were irrigated with a balanced salt solution. After the LRIs were made, standard cataract surgery was performed. The technique consisted of a capsulorhexis, nucleus and cortex extraction (Infinity Vision System, Alcon Laboratories, Inc.), and implantation of an aspheric silicone IOL (KS-3Ai, Staar Surgical Co.) through a temporal 2.8 mm clear corneal incision. Postoperatively, betamethasone 0.1%, (Rinderon), levofloxacin (Cravit), and diclofenac sodium 0.1% (Diclod) were administered topically 4 times daily for 1 month, after which the dose was steadily tapered.
Measurements of Corneal Biomechanical Parameters The biomechanical factors of the cornea were measured using a biomechanical waveform analysis system (Ocular Response Analyzer, Reichert Ophthalmic Instruments) preoperatively and 1 day, 1 week, and 1 and 3 months postoperatively. The biomechanical waveform device uses a sudden air impulse to deform the cornea and monitors the shape changes with an electro-optical system. The puff of air induces 2 applanations (inward and outward) of the cornea. The air stream deforms the cornea through an initial applanation event (peak 1) and then beyond it into concavity. The concavity then subsides, allowing the cornea to rebound through a second applanation (peak 2). Two biomechanical factorsdcorneal hysteresis (CH) and the corneal
Submitted: February 15, 2010. Final revision submitted: August 10, 2010. Accepted: August 25, 2010. From the Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan. Corresponding author: Kazutaka Kamiya, MD, PhD, Department of Ophthalmology, University of Kitasato School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa, 228-8555, Japan. E-mail: [email protected].
resistance factor (CRF)dwere measured. The CH parameter is calculated as the difference between pressure peak 1 and pressure peak 2 and is expressed in millimeters of mercury.15 The CRF is calculated as a linear function of peak 1 and peak 2. The device also provides the following 2 IOP measurements: Goldmann-correlated IOP, which is the mean of pressure peak 1 and pressure peak 2, and corneal-compensated IOP, which according to the manufacturer is less affected by corneal properties than Goldmann applanation IOP.A The measurements were taken at least 3 times to ensure that the signal quality, signal morphology, and measurement values were consistent. The mean of the measurements was used for statistical analysis according to the manufacturer’s instructions. After a topical anesthetic agent was placed in the patient’s eye, the central corneal thickness (CCT) was measured using an ultrasound pachymeter (DGH-500, DGH Technology, Inc.).
Statistical Analysis All statistical analyses were performed using StatView software (version 5.0, SAS Institute, Inc.). Repeatedmeasures analysis of variance (ANOVA) was used to evaluate the changes over time, and the Dunnett test was used for multiple comparisons. After normal distribution of the data was confirmed with the Kolmogorov-Smirnov test, the correlations between the CH and CRF values and the CCT values were assessed by calculating the Pearson correlation coefficient (r). Unless otherwise indicated, the results are expressed as the mean G SD. A P value less than 0.05 was considered statistically significant.
RESULTS The study enrolled 38 eyes of 29 consecutive patients (12 men, 17 women). The mean age of the patients was 69.4 G 9.4 years (range 50 to 83 years). The preoperative manifest refraction (spherical equivalent) was 2.41 G 4.65 diopters (D) (range 15.00 to 4.75 D). All surgeries were uneventful. Figures 1, 2, and 3 show the time courses of CH, CRF, and CCT, respectively. The variance in the CH data was statistically significant (PZ.006, repeatedmeasures ANOVA). Multiple comparisons showed a significant difference between preoperative and 1-day postoperative measurements (PZ.005, Dunnett test) but not between preoperative measurements and postoperative measurements at 1 week (PZ.45), 1 month (PZ.33), or 3 months (PZ.93). The variance in the CRF data was statistically significant (PZ.05, repeated-measures ANOVA). Multiple comparisons showed a significant difference between preoperative and 1-day postoperative measurements (PZ.004) but not between preoperative measurements and postoperative measurements at 1 week (PZ.19), 1 month (PZ.20), or 3 months (PZ.19). The variance in the CCT data was statistically significant (PZ.01, repeated-measures ANOVA). Multiple comparisons showed a significant difference between preoperative and 1-day postoperative measurements
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Figure 1. Time course of the mean CH (GSD) after cataract surgery with LRIs. The bar represents the SD (CH Z corneal hysteresis; Post Z postoperative; Preop Z preoperative).
Figure 2. Time course of CRF (GSD) after cataract surgery with LRIs. The bar represents the SD (CRF Z corneal resistance factor; Post Z postoperative; Preop Z preoperative).
(PZ.04) but not between preoperative measurements and postoperative measurements at 1 week (PZ.87), 1 month (PZ.98), or 3 months (PZ.95). The mean Goldmann-correlated IOP was 15.3 G 3.0 mm Hg preoperatively, 15.0 G 3.8 mm Hg 1 day postoperatively, 14.3 G 3.0 mm Hg at 1 week, 14.1 G 3.2 mm Hg at 1 month, and 13.4 G 3.1 mm Hg at 3 months. The variance of the data was not statistically significant (PZ.10, repeated-measures ANOVA). Multiple comparisons showed no significant differences between preoperative measurements and postoperative measurements at 1 day (PZ.99), 1 week (PZ.48), 1 month (PZ.29), or 3 months (PZ.05). The mean corneal-compensated IOP was 16.3 G 2.5 mm Hg preoperatively, 16.6 G 3.6 mm Hg 1 day postoperatively, 15.7 G 2.9 mm Hg at 1 week, 15.5 G 3.3 mm Hg at 1 month, and 14.7 G 3.2 mm Hg at
3 months. The variance of the data was not statistically significant (PZ.07, repeated-measures ANOVA). Multiple comparisons showed no significant differences between preoperative measurements and postoperative measurements at 1 day (PZ.96), 1 week (PZ.82), 1 month (PZ.64), or 3 months (PZ.09). The distributions of CH, CRF, and CCT preoperatively and at all postoperative examinations were normal (Kolmogorov-Smirnov test). There was a weak but significant correlation between CH and CCT preoperatively (r Z .33, PZ.04), 1 week postoperatively (r Z 0.33, PZ.04), at 1 month (r Z 0.43, PZ.006), and at 3 months (r Z 0.33, PZ.04) (Figure 4). There was no significant correlation between CH and CCT 1 day after surgery (r Z 0.20, PZ.23). There was also a weak but significant correlation between CRF and CCT preoperatively (r Z 0.42, PZ.008) and at 1
Figure 3. Time course of CCT (GSD) after cataract surgery with LRIs. The bar represents the SD (CCT Z central corneal thickness; Post Z postoperative; Preop Z preoperative).
Figure 4. Scatterplot showing a significant correlation between CH and CCT 3 months after cataract surgery with LRIs (CCT Z central corneal thickness; CH Z corneal hysteresis).
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Figure 5. Scatterplot showing a significant correlation between CRF and CCT 3 months after cataract surgery with LRIs (CCT Z central corneal thickness; CRF Z corneal resistance factor).
week (r Z 0.36, PZ.03), 1 month (r Z 0.36, PZ.03), and 3 months (r Z 0.40, PZ.01) (Figure 5). There was no significant correlation between CRF and CCT 1 day after surgery (r Z 0.18, PZ.29). DISCUSSION In this study, the CH and CRF values briefly decreased 1 day after surgery but soon recovered to preoperative levels and were stable thereafter. The CCT also increased 1 day after surgery and soon recovered to preoperative levels. To our knowledge, ours is the first study to assess biomechanical changes after simultaneous cataract surgery and LRIs. Hager et al.16 reported the corneal biomechanic results in 101 eyes having phacoemulsification with IOL implantation through a 4.1 mm corneal incision. They found that the mean CH value decreased from 10.35 G 2.5 mm Hg preoperatively to 9.20 G 1.9 mm Hg 1 day postoperatively but that the CCT increased from 556.82 G 32.5 mm to 580.26 G 45.5 mm, respectively. In a study of 51 eyes having cataract surgery through a 2.4 mm corneal incision with a 3-month follow-up, Kucumen et al.17 found that the CH and CRF values basically returned to normal preoperative levels a short time after surgery. In a study by Falkenberg et al.,18 there was a mean increase in CCT of 37 mm 1 day after cataract surgery. In another study,19 we also found that CH and CRF decreased briefly 1 day after cataract surgery without LRIs but soon recovered to the preoperative levels. The biomechanical changes after simultaneous cataract surgery with LRIs in our study were similar to those after cataract surgery alone in the previous studies. Therefore, we assume that standard cataract surgery and combined cataract surgery and LRIs do not significantly affect these biomechanical parameters
except at 1 day. Although we did not directly compare the biomechanical changes of simultaneous cataract surgery and LRIs with those of cataract surgery alone in this study, our present and previous results suggest that LRIs alone do not induce a change in biomechanical parameters, presumably because these surgical techniques require only arcuate partial incisions on the limbus. The relationship between corneal biomechanical factors, such as CH and CRF, and CCT has been discussed. In a study by Lam et al.,20 CH was positively associated with CCT in normal eyes. Broman et al.21 found a significant correlation between CH and CCT and a modest correlation coefficient in patients presenting at a glaucoma clinic. Shah et al.22 also demonstrated that CH increased with increasing CCT in normal eyes, but the correlation was moderate. We previously demonstrated that CCT was the most relevant factor affecting CH and CRF in normal eyes.23 On the other hand, we found a significant decrease in CH and CRF in association with a significant increase in CCT 1 day after surgery in the current study. Lu et al.24 also found no significant correlation between CH and CCT induced by wearing soft contact lenses during eye closure. Although we previously showed that eyes with a higher IOP are more predisposed to a lower CH value,23 no eye in the present study had a significant IOP increase (O25 mm Hg) during the follow-up period. Accordingly, the increase in CCT and the decrease in CH and CRF may be attributed to the transient corneal edema induced by the surgical stress to the corneal endothelium, although measurements of these parameters 1-day postoperatively were not very reliable because of the presence of superficial punctuate keratitis, inflammatory responses, or difficulty opening the eye. The CH factor is a dynamic measure of the viscous damping in the corneal tissue and represents the energy absorption capability of the cornea. The CRF parameter is an indicator of the total corneal response, including the elastic resistance of the corneal tissue. Our results suggest that corneal edema immediately after the combined procedure decreases not only the energy absorption capability but also the elastic resistance of the corneal tissue. Dupps25 stated that CH is dependent on the type, time course, and magnitude of the perturbation and that the impact of variable air pressure is being studied as a source of error and as a means of eliciting additional biomechanical information from the Ocular Response Analyzer biomechanical waveform device. Using a viscoelastic model, Glass et al.26 also showed how changing viscosity and elasticity affects hysteresis measurements in various ways. The lack of a persistent decrease in CH and CRF based on 1 measurement technique does not imply that the corneal biomechanics were
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not changed at all by these surgical procedures. We also found that the CH and CRF parameters were significantly correlated with CCT 1 week and 1 and 3 months postoperatively, suggesting that postoperative corneal thickness may play a role in the biomechanical characteristics of the cornea after the transient corneal edema resolves. In the present study, we used the mean values of 3 consecutive CH and CRF measurements taken with the biomechanical waveform device for statistical analysis; however, the study may have been more robust and relevant if the best waveform signal had been used according to the manufacturer’s more recent instructions. In another study in which we used the same biomechanical waveform device,27 we assessed the repeatability of the measurements by taking 3 consecutive CH and CRF measurements in 18 normal eyes at the same time of day on 2 days. The mean difference between 2 consecutive measurements (G95% limits of agreement) was 0.1 G 0.5 mm Hg (range 1.1 to 1.0 mm Hg) for CH and 0.0 G 0.5 mm Hg (range 0.9 to 1.0 mm Hg) for CRF. Although a small amount of test–retest variability may not necessarily mean high accuracy, it indicates the reproducibility of the measurements and thus the applicability of data. Accordingly, we believe the Ocular Response Analyzer offers reasonable repeatability in the longitudinal assessment of these biomechanical factors. The limitations of this study are that the amount of data was limited and that the follow-up was short. We evaluated CH and CRF for up to 3 months postoperatively, when the biomechanical properties of the cornea would have stabilized based on the woundhealing responses of the cornea. A long-term study with a larger patient cohort is required to confirm our findings. In summary, we found that the CH and CRF biomechanical factors decreased transiently 1 day after surgery but soon recovered to the preoperative levels, suggesting that simultaneous cataract surgery with LRIs does change corneal biomechanical parameters except at 1 day after surgery. We also found a weak, but significant, correlation between these parameters and CCT except for 1 day after surgery, suggesting that corneal thickness plays a role in these biomechanical parameters, even after any transient edema resolves. REFERENCES
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First author: Kazutaka Kamiya, MD, PhD Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan