Factors Influencing Long-term Regression After Posterior Chamber Phakic Intraocular Lens Implantation for Moderate to High Myopia KAZUTAKA KAMIYA, KIMIYA SHIMIZU, AKIHITO IGARASHI, AND HIDENAGA KOBASHI PURPOSE:
To evaluate the factors affecting the longterm regression after posterior chamber phakic intraocular lens (Visian ICL; STAAR Surgical) implantation for myopia. DESIGN: Retrospective observational case series. METHODS: We retrospectively examined 60 eyes of 35 consecutive patients (age, 38.4 ± 9.3 years [mean ± standard deviation]) with myopic refractive errors of L4.00 to L15.25 diopters (D) undergoing ICL implantation. We assessed the amount of myopic regression from 1 month to 6 years after surgery. Stepwise multiple regression analysis was used to assess the factors affecting the amount of myopic regression. RESULTS: The mean myopic regression from 1 month to 6 years after surgery was L0.33 ± 0.71 D (0.75 to L3.00 D). Explanatory variables relevant to the myopic regression were, in order of influence, patient age (partial regression coefficient B [ L0.042, P < .0001) and preoperative axial length (B [ L0.186, P [ .013) (adjusted R2 [ 0.300). No significant correlation was seen with other clinical factors such as sex, preoperative refraction, intraocular pressure, white-to-white distance, anterior chamber depth, central corneal thickness, or mean keratometric readings. CONCLUSIONS: Although the great majority of the variance remains unexplained, eyes of older patients and eyes with longer axial length are more predisposed to show greater myopic regression after ICL implantation. These results indicate that not only patient age but also axial length may play some role in predicting the long-term refractive outcomes of this surgical procedure. (Am J Ophthalmol 2014;-:-–-. Ó 2014 by Elsevier Inc. All rights reserved.)
T
HE VISIAN IMPLANTABLE COLLAMER LENS (VISIAN
ICL; STAAR Surgical, Nidau, Switzerland), a posterior chamber phakic intraocular lens (IOL), has been reported to be effective for the correction of moderate Accepted for publication Mar 20, 2014. From the Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan. Inquiries to Kazutaka Kamiya, Associate Professor, Department of Ophthalmology, University of Kitasato School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa, 252-0374, Japan; e-mail: [email protected] 0002-9394/$36.00 http://dx.doi.org/10.1016/j.ajo.2014.03.010
Ó
2014 BY
to high ametropia.1–11 In addition, this surgical procedure is largely reversible and the lens is replaceable, unlike laser in situ keratomileusis (LASIK), even when unexpected refractive changes occur after surgery. Recently, toric ICL has also been demonstrated to be effective for the correction of high myopic astigmatism.12–18 However, the myopic regression of the initial surgical effect can affect the predictability, efficiency, and stability of refractive surgery, leading to deterioration in visual performance and subsequent patient dissatisfaction. It is known that some long-term regression does occur, not only after LASIK but also after ICL implantation.19–23 Because of the prevalence of this surgical procedure, it is essential to evaluate the long-term refractive outcomes of ICL implantation. However, there has been no detailed report on the clinical factors behind the changes in manifest spherical equivalent after ICL implantation. Moreover, the longterm refractive changes after this surgical technique have not so far been fully elucidated. The purpose of this study is to retrospectively investigate the factors that influence myopic regression after ICL implantation.
PATIENTS AND METHODS STUDY POPULATION:
Sixty eyes of 35 consecutive patients (10 men and 25 women), who underwent ICL implantation for the correction of moderate to high myopia at Kitasato University Hospital and who regularly returned for 1-month and 6-year postoperative examinations, were included in this retrospective observational study. The inclusion criteria for this surgical technique were as follows: unsatisfactory correction with spectacles or con_ age < _ 55 years, stable refraction for at least tact lenses, 20 < 6 months, 4.0 to 20.0 diopters (D) of myopia, anterior chamber depth (measured from the corneal endothelium) > _2.8 mm, endothelial cell density > _1800 cells/mm2, and no history of ocular surgery, progressive corneal degeneration, cataract, glaucoma, or uveitis. Eyes with keratoconus were excluded from the study by using the keratoconus screening test of Placido disk videokeratography (TMS-2; Tomey, Nagoya, Japan). We selected non-toric ICL in 47 eyes (78%) with the manifest cylinder of 1.25 D or less and toric ICL in 13 eyes (22%) with manifest cylinder of 1.5 D or more. This retrospective review of data was
ELSEVIER INC. ALL
RIGHTS RESERVED.
1
approved by the Institutional Review Board at Kitasato University and followed the tenets of the Declaration of Helsinki. Our Institutional Review Board waived the requirement for informed consent for this retrospective study. IMPLANTABLE COLLAMER LENS POWER CALCULATION:
ICL power calculations were performed by the manufacturer (STAAR Surgical) using a modified vertex formula. Toric ICL power calculation was performed by the manufacturer using the astigmatism decomposition method. In 53 of 60 eyes, the preoperative manifest refraction was selected as the target myopic correction. In the remaining 7 eyes, it was intentionally selected as undercorrection for near vision. The size of the ICL was also chosen by the manufacturer based on the horizontal corneal diameter and anterior chamber depth measured with scanning-slit topography (Orbscan IIz; Bausch & Lomb, Rochester, New York, USA). IMPLANTABLE COLLAMER LENS SURGICAL PROCEDURE: The patients preoperatively underwent 2 peripheral
iridotomies with a neodymium–yttrium-aluminum-garnet laser. On the day of surgery, the patients were given dilating and cycloplegic agents (tropicamide and phenylephrine, Mydrin P; Santen, Osaka, Japan). After topical anesthesia, a model V4 ICL was inserted through a 3-mm clear corneal incision with the use of an injector cartridge (STAAR Surgical) after placement of a viscosurgical device (Opegan; Santen) into the anterior chamber. The ICL was placed in the posterior chamber, the remaining viscosurgical device was completely washed out of the anterior chamber with balanced salt solution, and a miotic agent (acetylcholine chloride, Ovisort; Daiichi-Sankyo, Tokyo, Japan) was instilled. For toric ICL implantation, to control for potential cyclotorsion in a supine position, the zero horizontal axis was marked preoperatively using a slit lamp. A Mendez ring was also used for measuring intraoperatively the required rotation from the horizontal axis. After the ICL had then been placed in the posterior chamber and rotated by 22.5 degrees or less using the manipulator. After surgery, steroidal (0.1% betamethasone, Rinderon; Shionogi, Osaka, Japan) and antibiotic (0.5% levofloxacin, Cravit; Santen) medications were administered topically 4 times daily for 2 weeks, and the dose was steadily reduced thereafter.
distance), anterior chamber depth, axial length, central corneal thickness, and mean keratometric readings. The Spearman rank correlation test was also performed to assess the relationships of this regression with other variables. The horizontal white-to-white distance and anterior chamber depth were measured using a scanning-slit topograph (Orbscan IIz; Bausch & Lomb, Rochester, New York, USA). The axial lengths were measured using the IOL Master (Carl Zeiss Meditec, Dublin, California, USA). The mean keratometric readings were measured using the autorefractometer (RK-5; Canon, Tokyo, Japan). Central corneal thickness was measured using an ultrasound pachymeter (DGH-500; DGH Technology, Exton, Pennsylvania, USA). The intraocular pressure was assessed with a noncontact tonometer (KT-500; Kowa, Tokyo, Japan). Each measurement was repeated 3 times and the mean value was used for the analysis. SUBGROUP ANALYSIS:
To investigate the effects of changes in axial length and of nuclear sclerosis of the crystalline lens on myopic regression in ICL-implanted eyes, we additionally measured the axial length with the IOL Master, and determined the density of the crystalline lens with the rotating Scheimpflug imaging system (Pentacam HR; Oculus, Wetzlar, Germany), both preoperatively and 6 years postoperatively, in 30 ICL-implanted eyes. The Scheimpflug imaging device collects 25 000 true elevation data points, which are processed to generate a 3-dimensional representation of the anterior eye. It also provides an image of the whole lens and an objective measurement of the lens density at the chosen point on a scale of 0–100 (0 ¼ no cloudiness; 100 ¼ completely opaque lens).24,25 On the 3-dimensional plot of the anterior segment, each section of which runs through the corneal vertex, the required lens density was taken as the peak value of the area of the nucleus on the image in the horizontal plane (0–180 degrees). STATISTICAL ANALYSIS:
All statistical analyses were performed using SPSS (SPSS Inc, Chicago, Illinois, USA). The relationship between 2 sets of data was analyzed by Spearman rank correlation test. The results are expressed as mean 6 standard deviation, and a P value less than .05 was considered statistically significant.
MULTIPLE REGRESSION ANALYSIS:
Stepwise multiple regression analysis was performed to investigate the relation between several variables and refractive regression after surgery. The dependent variable was the amount of myopic regression, which was determined as the change in manifest refraction (spherical equivalent) from 1 month to 6 years postoperatively. The explanatory variables included patient age, sex, preoperative refraction, intraocular pressure, horizontal corneal diameter (white-to-white
2
RESULTS THE PREOPERATIVE DEMOGRAPHICS OF THE STUDY POPULA-
tion are shown in Table 1. All surgeries were uneventful and no definite intraoperative complication was observed. Of the 60 eyes examined, 3 eyes (5.0%) developed asymptomatic anterior subcapsular cataracts, which lost 1 line in corrected visual acuity. Two eyes (3.3%) developed asymptomatic nuclear cataracts and showed regression of 0.75
AMERICAN JOURNAL OF OPHTHALMOLOGY
--- 2014
and 1.88 D without any change in corrected visual acuity. Simultaneous ICL extraction and cataract surgery was not necessary because all these eyes had 20/20 or more in corrected visual acuity. Two eyes (3.3%) required ICL repositioning because of a traumatic event 6 months postoperatively. The mean myopic regression from 1 month to 6 years after surgery was 0.33 6 0.71 D (0.75 to 3.00 D). The results of multiple regression analysis are shown in Table 2. The explanatory variables relevant to the myopic regression were the preoperative axial length (P < .0001, partial regression coefficient B ¼ 0.042) and patient age (P ¼ .013, B ¼ 0.186) (adjusted R2 ¼ 0.300). The multiple regression equation was expressed as follows: myopic regression (D) ¼ (0.042 3 patient age) þ (0.186 3 axial length) þ 2.086. There was no significant correlation shown with other clinical factors. The standardized partial regression coefficient was calculated in order to determine the magnitude of each variable’s influence. The patient age was the most relevant variable, and the preoperative axial length was the second. Similar results were obtained by Spearman rank correlation test, as shown in Table 2. The relationships of the myopic regression with the patient age and with the axial length are shown in Figures 1 and 2, respectively. With older age, a longer axial length, or both, the myopic regression of the initial surgical effect became significantly greater after ICL implantation. In the subgroup analysis, the changes in axial length and lens density were 0.29 6 0.43 mm (0.35 to 1.68 mm) and 0.88 6 3.19 (5.1 to 9.0), respectively. The amount of myopic regression was significantly correlated with the changes in both axial length (r ¼ 0.523, P ¼ .003) and lens density (r ¼ 0.390, P ¼ .033).
DISCUSSION IN THE PRESENT STUDY, OUR RESULTS DEMONSTRATED
that long-term myopic shift increases postoperatively in eyes of older patients or eyes with greater axial length, or both. Although the patient’s age and the axial length alone cannot provide sufficient explanation, as evidenced by the small R2 value (R2 ¼ 0.300), both patient age and axial length can affect the long-term refractive outcomes in ICL-implanted eyes. As far as we can ascertain, this is the first published study to assess the preoperative clinical factors influencing myopic regression after ICL implantation using multiple regression analysis. Moreover, there have been only a few studies on the long-term refractive changes after ICL implantation. We should be aware that older patients’ eyes and eyes with greater axial length are more predisposed to show greater long-term regressive changes in refraction after ICL implantation. Possible mechanisms for myopic regression after keratorefractive surgical procedures include corneal forward shift, VOL. -, NO. -
TABLE 1. Preoperative Demographics of the Study Population Undergoing Posterior Chamber Phakic Intraocular Lens Implantation Patient Demographics
Age (y) Sex (male:female) Manifest spherical equivalent (D) Intraocular pressure (mm Hg) White-to-white distance (mm) Anterior chamber depth (mm) Axial length (mm) Central corneal thickness (mm) Mean keratometric readings (D)
38.4 6 9.3 (range, 21–55) 10:25 10.64 6 2.61 (range, 4.00 to 15.25) 14.4 6 2.2 (range, 10.0–19.0) 11.6 6 0.4 (range, 10.9–12.5) 3.25 6 0.31 (range, 2.81–4.11) 27.60 6 1.18 (range, 24.40–29.55) 540.8 6 28.6 (range, 485–639) 44.2 6 1.4 (range, 41.1–46.5)
D ¼ diopter.
corneal hydration, stromal synthesis, compensatory epithelial hyperplasia, nuclear sclerosis of the crystalline lens, and elongation of the axial length.22 This surgical technique requires only a 3-mm horizontal corneal incision, and thus induces lesser changes in the central cornea. Therefore, possible mechanisms for myopic regression after ICL implantation include nuclear sclerosis of the lens and elongation of the axial length. We also investigated the effects of the changes in axial length and in lens density on myopic regression in the subgroup population. As the results indicate, myopic regression was significantly correlated with both of these changes. With regard to the axial length, Saka and associates demonstrated that it remained stable in 69% when measured by A-scan ultrasonography, but increased by more than 1 mm in 31% of high myopic eyes, after a mean follow-up period of 8.2 years.26 They also reported that the increase in the axial length determined with IOL Master was significantly correlated with the axial length at the initial examination.27 With regard to nuclear sclerosis of the lens, high myopic eyes requiring ICL implantation tended to develop nuclear cataracts, which are often accompanied with myopic shifts in refraction, in comparison with normal eyes.28–30 It has been reported that both age and axial length are risk factors for the development of vitreous liquefaction, which increases the risk of developing nuclear sclerotic cataract.31,32 In addition, ICL implantation itself may induce cataract formation because of the close proximity of the ICL to the crystalline lens.7–11,33 The U.S. Food and Drug Administration Trial demonstrated that the incidence of cataracts with ICL V4 was 2.7% 3 years postoperatively.7 Subsequently, Sanders also reported
LONG-TERM REGRESSION AFTER PHAKIC LENS IMPLANTATION
3
TABLE 2. Results of Correlation Analysis and Stepwise Multiple Regression Analysis to Select Variables Relevant to Myopic Regression After Posterior Chamber Phakic Intraocular Lens Implantation
Variables
Spearman Correlation Coefficient
P Value
Age (y) Axial length (mm) Sex (male ¼ 0, female ¼ 1) Preoperative refraction (D) Intraocular pressure (mm Hg) Horizontal corneal diameter (mm) Anterior chamber depth Central corneal thickness (mm) Mean keratometric readings (D)
0.330 0.265 0.090 0.093 0.152 0.062 0.202 0.138 0.127
.009 .041 .495 .468 .247 .637 .121 .293 .333
Partial Regression Coefficient
0.042 0.186
2.086
Standardized Partial Regression Coefficient
0.549 0.310 Not included Not included Not included Not included Not included Not included Not included Constant
P Value
To evaluate the factors affecting the longterm regression after posterior chamber phakic intraocular lens (Visian ICL; STAAR Surgical) implantation for myopia. DESIGN: Retrospective observational case series. METHODS: We retrospectively examined 60 eyes of 35 consecutive patients (age, 38.4 ± 9.3 years [mean ± standard deviation]) with myopic refractive errors of L4.00 to L15.25 diopters (D) undergoing ICL implantation. We assessed the amount of myopic regression from 1 month to 6 years after surgery. Stepwise multiple regression analysis was used to assess the factors affecting the amount of myopic regression. RESULTS: The mean myopic regression from 1 month to 6 years after surgery was L0.33 ± 0.71 D (0.75 to L3.00 D). Explanatory variables relevant to the myopic regression were, in order of influence, patient age (partial regression coefficient B [ L0.042, P < .0001) and preoperative axial length (B [ L0.186, P [ .013) (adjusted R2 [ 0.300). No significant correlation was seen with other clinical factors such as sex, preoperative refraction, intraocular pressure, white-to-white distance, anterior chamber depth, central corneal thickness, or mean keratometric readings. CONCLUSIONS: Although the great majority of the variance remains unexplained, eyes of older patients and eyes with longer axial length are more predisposed to show greater myopic regression after ICL implantation. These results indicate that not only patient age but also axial length may play some role in predicting the long-term refractive outcomes of this surgical procedure. (Am J Ophthalmol 2014;-:-–-. Ó 2014 by Elsevier Inc. All rights reserved.)
T
HE VISIAN IMPLANTABLE COLLAMER LENS (VISIAN
ICL; STAAR Surgical, Nidau, Switzerland), a posterior chamber phakic intraocular lens (IOL), has been reported to be effective for the correction of moderate Accepted for publication Mar 20, 2014. From the Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan. Inquiries to Kazutaka Kamiya, Associate Professor, Department of Ophthalmology, University of Kitasato School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa, 252-0374, Japan; e-mail: [email protected] 0002-9394/$36.00 http://dx.doi.org/10.1016/j.ajo.2014.03.010
Ó
2014 BY
to high ametropia.1–11 In addition, this surgical procedure is largely reversible and the lens is replaceable, unlike laser in situ keratomileusis (LASIK), even when unexpected refractive changes occur after surgery. Recently, toric ICL has also been demonstrated to be effective for the correction of high myopic astigmatism.12–18 However, the myopic regression of the initial surgical effect can affect the predictability, efficiency, and stability of refractive surgery, leading to deterioration in visual performance and subsequent patient dissatisfaction. It is known that some long-term regression does occur, not only after LASIK but also after ICL implantation.19–23 Because of the prevalence of this surgical procedure, it is essential to evaluate the long-term refractive outcomes of ICL implantation. However, there has been no detailed report on the clinical factors behind the changes in manifest spherical equivalent after ICL implantation. Moreover, the longterm refractive changes after this surgical technique have not so far been fully elucidated. The purpose of this study is to retrospectively investigate the factors that influence myopic regression after ICL implantation.
PATIENTS AND METHODS STUDY POPULATION:
Sixty eyes of 35 consecutive patients (10 men and 25 women), who underwent ICL implantation for the correction of moderate to high myopia at Kitasato University Hospital and who regularly returned for 1-month and 6-year postoperative examinations, were included in this retrospective observational study. The inclusion criteria for this surgical technique were as follows: unsatisfactory correction with spectacles or con_ age < _ 55 years, stable refraction for at least tact lenses, 20 < 6 months, 4.0 to 20.0 diopters (D) of myopia, anterior chamber depth (measured from the corneal endothelium) > _2.8 mm, endothelial cell density > _1800 cells/mm2, and no history of ocular surgery, progressive corneal degeneration, cataract, glaucoma, or uveitis. Eyes with keratoconus were excluded from the study by using the keratoconus screening test of Placido disk videokeratography (TMS-2; Tomey, Nagoya, Japan). We selected non-toric ICL in 47 eyes (78%) with the manifest cylinder of 1.25 D or less and toric ICL in 13 eyes (22%) with manifest cylinder of 1.5 D or more. This retrospective review of data was
ELSEVIER INC. ALL
RIGHTS RESERVED.
1
approved by the Institutional Review Board at Kitasato University and followed the tenets of the Declaration of Helsinki. Our Institutional Review Board waived the requirement for informed consent for this retrospective study. IMPLANTABLE COLLAMER LENS POWER CALCULATION:
ICL power calculations were performed by the manufacturer (STAAR Surgical) using a modified vertex formula. Toric ICL power calculation was performed by the manufacturer using the astigmatism decomposition method. In 53 of 60 eyes, the preoperative manifest refraction was selected as the target myopic correction. In the remaining 7 eyes, it was intentionally selected as undercorrection for near vision. The size of the ICL was also chosen by the manufacturer based on the horizontal corneal diameter and anterior chamber depth measured with scanning-slit topography (Orbscan IIz; Bausch & Lomb, Rochester, New York, USA). IMPLANTABLE COLLAMER LENS SURGICAL PROCEDURE: The patients preoperatively underwent 2 peripheral
iridotomies with a neodymium–yttrium-aluminum-garnet laser. On the day of surgery, the patients were given dilating and cycloplegic agents (tropicamide and phenylephrine, Mydrin P; Santen, Osaka, Japan). After topical anesthesia, a model V4 ICL was inserted through a 3-mm clear corneal incision with the use of an injector cartridge (STAAR Surgical) after placement of a viscosurgical device (Opegan; Santen) into the anterior chamber. The ICL was placed in the posterior chamber, the remaining viscosurgical device was completely washed out of the anterior chamber with balanced salt solution, and a miotic agent (acetylcholine chloride, Ovisort; Daiichi-Sankyo, Tokyo, Japan) was instilled. For toric ICL implantation, to control for potential cyclotorsion in a supine position, the zero horizontal axis was marked preoperatively using a slit lamp. A Mendez ring was also used for measuring intraoperatively the required rotation from the horizontal axis. After the ICL had then been placed in the posterior chamber and rotated by 22.5 degrees or less using the manipulator. After surgery, steroidal (0.1% betamethasone, Rinderon; Shionogi, Osaka, Japan) and antibiotic (0.5% levofloxacin, Cravit; Santen) medications were administered topically 4 times daily for 2 weeks, and the dose was steadily reduced thereafter.
distance), anterior chamber depth, axial length, central corneal thickness, and mean keratometric readings. The Spearman rank correlation test was also performed to assess the relationships of this regression with other variables. The horizontal white-to-white distance and anterior chamber depth were measured using a scanning-slit topograph (Orbscan IIz; Bausch & Lomb, Rochester, New York, USA). The axial lengths were measured using the IOL Master (Carl Zeiss Meditec, Dublin, California, USA). The mean keratometric readings were measured using the autorefractometer (RK-5; Canon, Tokyo, Japan). Central corneal thickness was measured using an ultrasound pachymeter (DGH-500; DGH Technology, Exton, Pennsylvania, USA). The intraocular pressure was assessed with a noncontact tonometer (KT-500; Kowa, Tokyo, Japan). Each measurement was repeated 3 times and the mean value was used for the analysis. SUBGROUP ANALYSIS:
To investigate the effects of changes in axial length and of nuclear sclerosis of the crystalline lens on myopic regression in ICL-implanted eyes, we additionally measured the axial length with the IOL Master, and determined the density of the crystalline lens with the rotating Scheimpflug imaging system (Pentacam HR; Oculus, Wetzlar, Germany), both preoperatively and 6 years postoperatively, in 30 ICL-implanted eyes. The Scheimpflug imaging device collects 25 000 true elevation data points, which are processed to generate a 3-dimensional representation of the anterior eye. It also provides an image of the whole lens and an objective measurement of the lens density at the chosen point on a scale of 0–100 (0 ¼ no cloudiness; 100 ¼ completely opaque lens).24,25 On the 3-dimensional plot of the anterior segment, each section of which runs through the corneal vertex, the required lens density was taken as the peak value of the area of the nucleus on the image in the horizontal plane (0–180 degrees). STATISTICAL ANALYSIS:
All statistical analyses were performed using SPSS (SPSS Inc, Chicago, Illinois, USA). The relationship between 2 sets of data was analyzed by Spearman rank correlation test. The results are expressed as mean 6 standard deviation, and a P value less than .05 was considered statistically significant.
MULTIPLE REGRESSION ANALYSIS:
Stepwise multiple regression analysis was performed to investigate the relation between several variables and refractive regression after surgery. The dependent variable was the amount of myopic regression, which was determined as the change in manifest refraction (spherical equivalent) from 1 month to 6 years postoperatively. The explanatory variables included patient age, sex, preoperative refraction, intraocular pressure, horizontal corneal diameter (white-to-white
2
RESULTS THE PREOPERATIVE DEMOGRAPHICS OF THE STUDY POPULA-
tion are shown in Table 1. All surgeries were uneventful and no definite intraoperative complication was observed. Of the 60 eyes examined, 3 eyes (5.0%) developed asymptomatic anterior subcapsular cataracts, which lost 1 line in corrected visual acuity. Two eyes (3.3%) developed asymptomatic nuclear cataracts and showed regression of 0.75
AMERICAN JOURNAL OF OPHTHALMOLOGY
--- 2014
and 1.88 D without any change in corrected visual acuity. Simultaneous ICL extraction and cataract surgery was not necessary because all these eyes had 20/20 or more in corrected visual acuity. Two eyes (3.3%) required ICL repositioning because of a traumatic event 6 months postoperatively. The mean myopic regression from 1 month to 6 years after surgery was 0.33 6 0.71 D (0.75 to 3.00 D). The results of multiple regression analysis are shown in Table 2. The explanatory variables relevant to the myopic regression were the preoperative axial length (P < .0001, partial regression coefficient B ¼ 0.042) and patient age (P ¼ .013, B ¼ 0.186) (adjusted R2 ¼ 0.300). The multiple regression equation was expressed as follows: myopic regression (D) ¼ (0.042 3 patient age) þ (0.186 3 axial length) þ 2.086. There was no significant correlation shown with other clinical factors. The standardized partial regression coefficient was calculated in order to determine the magnitude of each variable’s influence. The patient age was the most relevant variable, and the preoperative axial length was the second. Similar results were obtained by Spearman rank correlation test, as shown in Table 2. The relationships of the myopic regression with the patient age and with the axial length are shown in Figures 1 and 2, respectively. With older age, a longer axial length, or both, the myopic regression of the initial surgical effect became significantly greater after ICL implantation. In the subgroup analysis, the changes in axial length and lens density were 0.29 6 0.43 mm (0.35 to 1.68 mm) and 0.88 6 3.19 (5.1 to 9.0), respectively. The amount of myopic regression was significantly correlated with the changes in both axial length (r ¼ 0.523, P ¼ .003) and lens density (r ¼ 0.390, P ¼ .033).
DISCUSSION IN THE PRESENT STUDY, OUR RESULTS DEMONSTRATED
that long-term myopic shift increases postoperatively in eyes of older patients or eyes with greater axial length, or both. Although the patient’s age and the axial length alone cannot provide sufficient explanation, as evidenced by the small R2 value (R2 ¼ 0.300), both patient age and axial length can affect the long-term refractive outcomes in ICL-implanted eyes. As far as we can ascertain, this is the first published study to assess the preoperative clinical factors influencing myopic regression after ICL implantation using multiple regression analysis. Moreover, there have been only a few studies on the long-term refractive changes after ICL implantation. We should be aware that older patients’ eyes and eyes with greater axial length are more predisposed to show greater long-term regressive changes in refraction after ICL implantation. Possible mechanisms for myopic regression after keratorefractive surgical procedures include corneal forward shift, VOL. -, NO. -
TABLE 1. Preoperative Demographics of the Study Population Undergoing Posterior Chamber Phakic Intraocular Lens Implantation Patient Demographics
Age (y) Sex (male:female) Manifest spherical equivalent (D) Intraocular pressure (mm Hg) White-to-white distance (mm) Anterior chamber depth (mm) Axial length (mm) Central corneal thickness (mm) Mean keratometric readings (D)
38.4 6 9.3 (range, 21–55) 10:25 10.64 6 2.61 (range, 4.00 to 15.25) 14.4 6 2.2 (range, 10.0–19.0) 11.6 6 0.4 (range, 10.9–12.5) 3.25 6 0.31 (range, 2.81–4.11) 27.60 6 1.18 (range, 24.40–29.55) 540.8 6 28.6 (range, 485–639) 44.2 6 1.4 (range, 41.1–46.5)
D ¼ diopter.
corneal hydration, stromal synthesis, compensatory epithelial hyperplasia, nuclear sclerosis of the crystalline lens, and elongation of the axial length.22 This surgical technique requires only a 3-mm horizontal corneal incision, and thus induces lesser changes in the central cornea. Therefore, possible mechanisms for myopic regression after ICL implantation include nuclear sclerosis of the lens and elongation of the axial length. We also investigated the effects of the changes in axial length and in lens density on myopic regression in the subgroup population. As the results indicate, myopic regression was significantly correlated with both of these changes. With regard to the axial length, Saka and associates demonstrated that it remained stable in 69% when measured by A-scan ultrasonography, but increased by more than 1 mm in 31% of high myopic eyes, after a mean follow-up period of 8.2 years.26 They also reported that the increase in the axial length determined with IOL Master was significantly correlated with the axial length at the initial examination.27 With regard to nuclear sclerosis of the lens, high myopic eyes requiring ICL implantation tended to develop nuclear cataracts, which are often accompanied with myopic shifts in refraction, in comparison with normal eyes.28–30 It has been reported that both age and axial length are risk factors for the development of vitreous liquefaction, which increases the risk of developing nuclear sclerotic cataract.31,32 In addition, ICL implantation itself may induce cataract formation because of the close proximity of the ICL to the crystalline lens.7–11,33 The U.S. Food and Drug Administration Trial demonstrated that the incidence of cataracts with ICL V4 was 2.7% 3 years postoperatively.7 Subsequently, Sanders also reported
LONG-TERM REGRESSION AFTER PHAKIC LENS IMPLANTATION
3
TABLE 2. Results of Correlation Analysis and Stepwise Multiple Regression Analysis to Select Variables Relevant to Myopic Regression After Posterior Chamber Phakic Intraocular Lens Implantation
Variables
Spearman Correlation Coefficient
P Value
Age (y) Axial length (mm) Sex (male ¼ 0, female ¼ 1) Preoperative refraction (D) Intraocular pressure (mm Hg) Horizontal corneal diameter (mm) Anterior chamber depth Central corneal thickness (mm) Mean keratometric readings (D)
0.330 0.265 0.090 0.093 0.152 0.062 0.202 0.138 0.127
.009 .041 .495 .468 .247 .637 .121 .293 .333
Partial Regression Coefficient
0.042 0.186
2.086
Standardized Partial Regression Coefficient
0.549 0.310 Not included Not included Not included Not included Not included Not included Not included Constant
P Value
