- Email: [email protected]
Corneal astigmatism after cataract surgery with 4.1 mm BENT scleral and 4.1 mm plus meridian corneal incisions
Corneal astigmatism after cataract surgery with 4.1 mm BENT sderal and 4.1 mm plus meridian corneal incisions Yasuo Kurimoto, MD, PhD, Yusuke Komurasaki, MD, Nagahisa Yoshimura, MD, PhD, Takehisa Kondo, MD, PhD
urgically induced astigmatism (SIA) after cataract surgery was once a serious problem. Various incisions and suturing methods were developed to mini-
S
Reprint requests to Ya.mo Kurimoto, MD, PhD, Department of Ophthalmolog~ Shinshu UniversitySchool of Medidne, 311 Asahi, Mauumoto, 390-8621, Japan.
mize this induced astigmatism. The techniques used to decrease postoperative corneal astigmatism are of 2 types: (1) those designed to make no change in pre-existing corneal astigmatism; (2) those designed to neutralize pre-existing corneal astigmatism by surgically inducing counteractive astigmatism. In the first type, the BENT (between 9 and 12 o'clock) sderal incision was deveb
j CATARACT REFRACT SURG---VOL 25, M A R C H 1999
427
ASTIGMATISM AFTER BENT SCLERAL A N D PLUS MERIDIAN INCISIONS
oped, I and in the second, the plus meridian corneal incision (PMCI). 2 It is well known that the smaller the surgical incision, the lower the SIA? '4 To implant a foldable intraoi:ular lens (IOL) of 6.0 m m diameter safely, an incision of about 4.0 mm is necessary. Thus, the neutralizing effect of the PMCI method on pre-existing astigmatism should be less effective after small incision cataract surgery. As a consequence, the final results of PMCI surgery with small incisions may be dose to astigmatically neutral surgery like the BENT scleral incision method. This study determined which of the 2 incisions better minimizes postoperative astigmatism by correlating pre-existing with postoperative astigmatism.
Patients and Methods This prospective study comprised 58 eyes of 29 consecutive patients with bilateral cataract who had bilateral small incision cataract surgery between November 1995 and October 1996 by the same surgeon (Y.K.). Full informed consent was obtained. All procedures were conducted in accordance with the principles embodied in the Declaration of Helsinki. One eye of each patient was randomly assigned to have cataract surgery with the 4.1 mm BENT scleral incision method and the other eye, with the 4.1 mm PMCI method. Patients who had other ocular disease were excluded. All surgeries were performed after the instillation of lidocaine 4% eyedrops. A fornix-based conjunctival flap was made superotemporally in the right eye or superonasaUy in the left eye. Sderal cautery was used for hemostasis in the BENT group, but no conjunctival or sderal treatment was necessary in the PMCI group. In the BENT group, a sderal tunnel incision was placed superotemporaUy in the right eye or superonasally in the left eye. The sderal dissection was initiated approximately 1.5 m m posterior to the surgical limbus, and a 4.1 m m 3-step incision was completed with a 4.1 m m wide metal blade (Alcon). In the PMCI group, a dear corneal incision, completed with the 4.1 m m wide metal blade, was placed on the steepest meridian of the corneal astigmatism. Except for the differences in location and construction ofindsions, the surgical procedures in both groups 428
were identical. In both groups, continuous curvilinear capsulorhexis and hydrodissection were performed, and the crystalline lens was removed using bimanual phacoemulsification and aspiration. A foldable 6.0 mm diameter optic IOL (Alcon MA60BM, AcrySof) was implanted in the capsular bag. The incisions were not sutured. No surgical complications occurred in either group. In all patients, automated keratometry was performed before surgery and 1, 3, 10, 30, and 100 days after surgery. The keratometric measurements were repeated 3 times at each examination. To enhance data reliability, the mean measurements were adopted as the corneal astigmatism value. The following factors were analyzed: (1) value of corneal astigmatism, (2) comparison of changes in corneal astigmatism between the 2 groups, (3) relationship between preoperative astigmatism and final postoperative changes in astigmatism within each group. The statistical significance of the differences between the preoperative and postoperative astigmatism within each group was determined using the Student 2-tailed paired t test. Differences in the postoperative changes in astigmatism between the 2 groups were analyzed using the Student 2-tailed unpaired t test. The relationship between the preoperative astigmatism and the final postoperative changes in each group was assessed by linear regression analysis. Before the linear regression analysis, it was confirmed that the distribution of the final postoperative changes in both groups was not significantly different from a normal distribution (P > .9; Kolmogorov-Smirnov test) and that the deviation of the final postoperative changes was much the same for any value of the independent variable with a residual analysis. Data are expressed as means _+ SD, and a Pvalue < .05 was considered statistically significant.
Results Mean age of the 29 patients (7 men, 22 women) was 69.4 years _ 13.7 (SD) (range 27 to 86 years). There were no significant differences between the 2 groups in pre-existing corneal astigmatism, axial length, and phacoemulsification energy and time (Table I). Although the mean surgical time was significantly different between groups, this arose from differences in preparations before the primary incision was made.
J CATARACTREFRACTSURG--VOL 25. MARCH 1999
ASTIGMATISMAFTERBENTSCLERALAND PLUSMERIDIANINCISIONS Table 1. Group results. Group (Mean _+ SD)
,~,:.~
~.mment Pre-existing astigmatism (D)
~'pUc,
0.99 -+- 0.66
1.14 ± 0.79
24.13 _ 2.08
23.82 _ 1.73
Phaco energy (I.~1)
0.31 +- 0.22
0.30 ± 0.27
Phaco time (minutes)
1.30 -'- 0.63
1.29 -'- 0.85
14.20 -" 2.30
12.60 ± 2.00
Axial length (mm)
(Diopters) 3.0-
=mm 4m~
2.0-
eml
Surgery time (minutes)*
< m
1.0-
*P = .005
L. O
L~ In the BENT scleral incision group, the mean corneal astigmatism preoperatively and 1, 3, 10, 30, and 100 days postoperatively was 0.99 + 0.66, 1.53 + 1.11, 1.12 + 0.72, 1.26 + 0.81, 1.16 + 0.73, and 1.09 + 0.64 diopters (D), respectively (Figure 1). Mean postoperative astigmatism was greater than preoperatively at each examination, but the differences were statistically significant only on days 1 and 10 after surgery (P < .03). In the PMCI group, the preoperative and 1, 3, 10, 30, and 100 days postoperative mean values of corneal astigmatism were 1.14 + 0.79, 1.38 + 0.98, 1.17 40.88, 1.31 4- 0.77, 1.01 4- 0.70, and 1.00 4- 0.60 D, respectively (Figure 2). Mean postoperative corneal astigmatism was higher on days 1, 3, and 10 but lower
0.0
preop i 3 1'0 3'0 Postoperative Days
I00
Figure 1. (Kurimoto) Preoperative and 1,3, 10, 30, and 100 day postoperative corneal astigmatism values in the BENT scleral incision group. Error bars represent the standard deviation (*P = .019: **P = .023).
on days 30 and 100. None of the differences was significant. In general, the changes in corneal astigmatism were less in the PMCI group than in the BENT group at every examination (Figure 3). The only time at which
(Diopters) (Diopters)
2.0~ T
3.0
i
+
S~'r
group
~
---I~-- PMCIgroup
.
0
I:
ell
N
2.0-
.oJD
=m
emm
1 I1
,< m
~J
1.0-
J
f~
0
0.0
m
Preop
J
3
1'0
Postoperative
3'0
I00
0
o~
<
-I.0~
-2.0
r
t'
I
1
Preop
3
I0
30
Days
Figure 2. (Kurimoto) Preoperative and 1, 3, 10, 30, and 100 days postoperative corneal astigmatism values in the PMCI group. Error bars represent the standard deviation.
~
100
Postoperative Days Figure 3. (Kurimoto) Changes in corneal astigmatism in the BENT scleral incision group and the PMCI group as a function of postoperative days. Error bars represent the standard deviation (*P = .049).
j CATARACTREFRACT$UI~VOL25,MARCH1999
429
ASTIGMATISMAFTER BENT SCLERALAND PLUS MERIDIAN INCISIONS (DiQpters) 2.0
(Diopters) 2.0 95% confidence intervals
95%te~al? dence 1.0
1.0' []
,, O)
0.0
~
0.0 0
i
.1.0 ¸
-2.0
0.0
~
1.0
2.0
3.0 (Diopters)
Preoperative Astigmatism
~
~
~
-1.0
-2.0 0.0
1.0
2.0
3.0
Preoperative Astigmatism
4.0 (Diopters)
F i g u r e 4. (Kurimoto) Relationship between preoperative and the final postoperative corneal astigmatism (day 100) in the BENT scleral incision group (y = - 0 . 3 4 x + 0.46, r = -0.41 [95% confidence interval, - 0 . 6 9 to -0.04; P = 0.032]; r 2 = 0.17, y-intercept = 0.46 [95% confidence interval, 0.12 to 0.81; P ffi .011]).
F i g u r e 5. (Kurimoto) Relationship between preoperative and the final postoperative corneal astigmatism (day 100) in the PMCI group (y ffi - 0 . 4 3 x + 0.35, r -- - 0 . 6 5 [95% confidence interval, - 0 . 8 3 to -0.36; P = .0002]; r 2 = 0.42, y-intercept = 0.35 [95% confidence interval, 0.07 to 0.63; P ffi .017]).
the difference was significant was 30 days postoperatively (P < .05). A plot of the change in astigmatism (postoperative astigmatism on day 100 - preoperative astigmatism) as a function of the preoperative astigmatism showed a significant negative correlation (r = -0.41, P = .032) in the BENT group, and the value of the x-intercept of the regression line was 1.37 D (Figure 4). This means that for preoperative astigmatism higher than 1.37 D, there would be a negative change in astigmatism and the net postoperative astigmatism would be lower. Similarly, for preoperative astigmatism lower than 1.37 D, there would be a positive change in astigmatism and the net postoperative astigmatism would be higher. There was also a significant (r = -0.65, P -.0002) negative correlation between preoperative astigmatism and final postoperative change in corneal astigmatism in the PMCI group (Figure 5); the x-intercept of the regression line was 0.82 D. Thus, postoperative astigmatism would be lower in an eye that had a preoperative astigmatism of more than 0.82 D and "higher in an eye with preoperative astigmatism less than 0.82 D.
Discussion
430
We compared the course of corneal astigmatism in patients who had cataract surgery with a 4.1 m m BENT sderal incision in 1 eye and a 4.1 m m PMCI incision in the fellow eye. The 2 groups were wellmatched in the relevant parameters of the eye (i.e., no significant differences in the pre-existing corneal astigmatism, axial length, and phaco energy and time). The surgery was significantly longer in the BENT group, but this difference was inadvertent and arose from the treatment of the conjunctiva before the primary incision was made. Therefore, the 2 groups were equally matched, allowing comparison of the results of 2 incision techniques. Several methods are commonly used to evaluate postoperative changes in corneal astigmatism, and they can be divided into 2 categories: (1) simply compare the preoperative and the postoperative corneal astigmatism; 5'e (2) determine the SIA as a vector or scalar of astigmatism to evaluate the change in astigmatism. 7-9 Although the methods of determining SIA seem to be more useful, there are several problems. Some of the methods give erroneous values when the surgical inci-
J CATARACT REFRACTSURG~VOL 25. MARCH 1999
ASTIGMATISMAFTERBENTS C L E R A L sion is not located in the vertical or the horizontal meridian, such as with the BENT incision. I° In addition, there is a question whether determining SIA as a vector is valid when the cornea does not form a toric surface with the axes crossing at right angles. Therefore, we adopted the first method, which evaluates the preoperative and the postoperative corneal astigmatism, which is clearly of clinical importance. Throughout the postoperative period, the changes in corneal astigmatism and the absolute value of the postoperative astigmatism were less in the PMCI group. Current refinements in small incision cataract surgery have heightened patient expectations of immediate visual results. Therefore, the PMCI method, in which the postoperative changes in astigmatism and the degree of astigmatism were less, should have a better outcome than the BENT sderal incision method. In both groups, there was a statistically significant negative correlation between the preoperative values and the final postoperative change in corneal astigmatism, and the x- and y-intercepts of the regression line were positive. Therefore, in both the groups, postoperative astigmatism was better than preoperatively in eyes with considerable preoperative astigmatism but worse in eyes with low preoperative astigmatism. The critical point in determining whether astigmatism will be better or worse than preoperatively was 1.37 D in the BENT group and 0.82 D in the PMCI group. The lower value in the PMCI group indicates that the number of eyes in which postoperative astigmatism will be worse should be less in the PMCI group than in the BENT group. Thus, the PMCI method has an important benefit over the BENT scleral incision method.
AND
PLUS
MERIDIANINCISIONS
In summary, for cataract surgery using a 4.1 mm incision, the PMCI surpassed the BENT sderal incision method in minimizing postoperative corneal astigmatism.
Refc~nce.,s 1. Kawano K. Modified corneoscleral incision to reduce postoperative astigmatism after 6 mm dialneter intraocular lens implantation. J Cataract Refract Surg 1993; 19:387-392 2. Karickhoff JR. Plus meridian incision for secondary implantation. Ophthalmic Surg 1987; 18:658-660 3. Shepherd JR. Induced astigmatism in small incision cataract surgery.J Cataract Refract Surg 1989; 15:85-88 4. Oshika T, Tsuboi S, YaguchiS, et al. Comparative study ofintraocular lens implantation through 3.2- and 5.5-mm incisions. Ophthalmology 1994; 101:1183-1190 5. Singh D, Kumar K. Keratometric changes after cataract extraction. Br J Ophthalmol 1976; 60:638-641 6. Luntz MH, Livingston DG. Astigmatism in cataract surgery. Br J Ophthalmol 1977; 61:360-365 7. Jaffe NS, Clayman HM. The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalrnol Otolaryngo11975; 79:OP615-OP630 8. CravyTV~.Calculation of the change in corneal astigmatism following cataract extraction. Ophthalmic Surg 1979; 10(1):38-49 9. Holladay JT, Cravy TV, Koch DD. Calculating the surgically induced refractive change following ocular surgery.J Cataract Refract Surg 1992; 18:429 "!.'!.3 10. Oshika T, Samejirna T, Miyata N. [Calculation of surgically induced astigmatism]. [Japanese]J Jpn Ophthalrnol Soc 1995; 99:901-909 Acceptedfbr publication October16, 1998. From the Departmenu of Ophthalmolog~ Shimhu UniversitySchoolof Medicine, Matsumoto (Kurimoto, Komurasaki, Yoshimura),and Kobe City GeneralHo~ita~ Kobe (Kondo),Japan.
j CATARACT REFRACT SURG.--VOL 25,M A R C H 1999
~ I
urgically induced astigmatism (SIA) after cataract surgery was once a serious problem. Various incisions and suturing methods were developed to mini-
S
Reprint requests to Ya.mo Kurimoto, MD, PhD, Department of Ophthalmolog~ Shinshu UniversitySchool of Medidne, 311 Asahi, Mauumoto, 390-8621, Japan.
mize this induced astigmatism. The techniques used to decrease postoperative corneal astigmatism are of 2 types: (1) those designed to make no change in pre-existing corneal astigmatism; (2) those designed to neutralize pre-existing corneal astigmatism by surgically inducing counteractive astigmatism. In the first type, the BENT (between 9 and 12 o'clock) sderal incision was deveb
j CATARACT REFRACT SURG---VOL 25, M A R C H 1999
427
ASTIGMATISM AFTER BENT SCLERAL A N D PLUS MERIDIAN INCISIONS
oped, I and in the second, the plus meridian corneal incision (PMCI). 2 It is well known that the smaller the surgical incision, the lower the SIA? '4 To implant a foldable intraoi:ular lens (IOL) of 6.0 m m diameter safely, an incision of about 4.0 mm is necessary. Thus, the neutralizing effect of the PMCI method on pre-existing astigmatism should be less effective after small incision cataract surgery. As a consequence, the final results of PMCI surgery with small incisions may be dose to astigmatically neutral surgery like the BENT scleral incision method. This study determined which of the 2 incisions better minimizes postoperative astigmatism by correlating pre-existing with postoperative astigmatism.
Patients and Methods This prospective study comprised 58 eyes of 29 consecutive patients with bilateral cataract who had bilateral small incision cataract surgery between November 1995 and October 1996 by the same surgeon (Y.K.). Full informed consent was obtained. All procedures were conducted in accordance with the principles embodied in the Declaration of Helsinki. One eye of each patient was randomly assigned to have cataract surgery with the 4.1 mm BENT scleral incision method and the other eye, with the 4.1 mm PMCI method. Patients who had other ocular disease were excluded. All surgeries were performed after the instillation of lidocaine 4% eyedrops. A fornix-based conjunctival flap was made superotemporally in the right eye or superonasaUy in the left eye. Sderal cautery was used for hemostasis in the BENT group, but no conjunctival or sderal treatment was necessary in the PMCI group. In the BENT group, a sderal tunnel incision was placed superotemporaUy in the right eye or superonasally in the left eye. The sderal dissection was initiated approximately 1.5 m m posterior to the surgical limbus, and a 4.1 m m 3-step incision was completed with a 4.1 m m wide metal blade (Alcon). In the PMCI group, a dear corneal incision, completed with the 4.1 m m wide metal blade, was placed on the steepest meridian of the corneal astigmatism. Except for the differences in location and construction ofindsions, the surgical procedures in both groups 428
were identical. In both groups, continuous curvilinear capsulorhexis and hydrodissection were performed, and the crystalline lens was removed using bimanual phacoemulsification and aspiration. A foldable 6.0 mm diameter optic IOL (Alcon MA60BM, AcrySof) was implanted in the capsular bag. The incisions were not sutured. No surgical complications occurred in either group. In all patients, automated keratometry was performed before surgery and 1, 3, 10, 30, and 100 days after surgery. The keratometric measurements were repeated 3 times at each examination. To enhance data reliability, the mean measurements were adopted as the corneal astigmatism value. The following factors were analyzed: (1) value of corneal astigmatism, (2) comparison of changes in corneal astigmatism between the 2 groups, (3) relationship between preoperative astigmatism and final postoperative changes in astigmatism within each group. The statistical significance of the differences between the preoperative and postoperative astigmatism within each group was determined using the Student 2-tailed paired t test. Differences in the postoperative changes in astigmatism between the 2 groups were analyzed using the Student 2-tailed unpaired t test. The relationship between the preoperative astigmatism and the final postoperative changes in each group was assessed by linear regression analysis. Before the linear regression analysis, it was confirmed that the distribution of the final postoperative changes in both groups was not significantly different from a normal distribution (P > .9; Kolmogorov-Smirnov test) and that the deviation of the final postoperative changes was much the same for any value of the independent variable with a residual analysis. Data are expressed as means _+ SD, and a Pvalue < .05 was considered statistically significant.
Results Mean age of the 29 patients (7 men, 22 women) was 69.4 years _ 13.7 (SD) (range 27 to 86 years). There were no significant differences between the 2 groups in pre-existing corneal astigmatism, axial length, and phacoemulsification energy and time (Table I). Although the mean surgical time was significantly different between groups, this arose from differences in preparations before the primary incision was made.
J CATARACTREFRACTSURG--VOL 25. MARCH 1999
ASTIGMATISMAFTERBENTSCLERALAND PLUSMERIDIANINCISIONS Table 1. Group results. Group (Mean _+ SD)
,~,:.~
~.mment Pre-existing astigmatism (D)
~'pUc,
0.99 -+- 0.66
1.14 ± 0.79
24.13 _ 2.08
23.82 _ 1.73
Phaco energy (I.~1)
0.31 +- 0.22
0.30 ± 0.27
Phaco time (minutes)
1.30 -'- 0.63
1.29 -'- 0.85
14.20 -" 2.30
12.60 ± 2.00
Axial length (mm)
(Diopters) 3.0-
=mm 4m~
2.0-
eml
Surgery time (minutes)*
< m
1.0-
*P = .005
L. O
L~ In the BENT scleral incision group, the mean corneal astigmatism preoperatively and 1, 3, 10, 30, and 100 days postoperatively was 0.99 + 0.66, 1.53 + 1.11, 1.12 + 0.72, 1.26 + 0.81, 1.16 + 0.73, and 1.09 + 0.64 diopters (D), respectively (Figure 1). Mean postoperative astigmatism was greater than preoperatively at each examination, but the differences were statistically significant only on days 1 and 10 after surgery (P < .03). In the PMCI group, the preoperative and 1, 3, 10, 30, and 100 days postoperative mean values of corneal astigmatism were 1.14 + 0.79, 1.38 + 0.98, 1.17 40.88, 1.31 4- 0.77, 1.01 4- 0.70, and 1.00 4- 0.60 D, respectively (Figure 2). Mean postoperative corneal astigmatism was higher on days 1, 3, and 10 but lower
0.0
preop i 3 1'0 3'0 Postoperative Days
I00
Figure 1. (Kurimoto) Preoperative and 1,3, 10, 30, and 100 day postoperative corneal astigmatism values in the BENT scleral incision group. Error bars represent the standard deviation (*P = .019: **P = .023).
on days 30 and 100. None of the differences was significant. In general, the changes in corneal astigmatism were less in the PMCI group than in the BENT group at every examination (Figure 3). The only time at which
(Diopters) (Diopters)
2.0~ T
3.0
i
+
S~'r
group
~
---I~-- PMCIgroup
.
0
I:
ell
N
2.0-
.oJD
=m
emm
1 I1
,< m
~J
1.0-
J
f~
0
0.0
m
Preop
J
3
1'0
Postoperative
3'0
I00
0
o~
<
-I.0~
-2.0
r
t'
I
1
Preop
3
I0
30
Days
Figure 2. (Kurimoto) Preoperative and 1, 3, 10, 30, and 100 days postoperative corneal astigmatism values in the PMCI group. Error bars represent the standard deviation.
~
100
Postoperative Days Figure 3. (Kurimoto) Changes in corneal astigmatism in the BENT scleral incision group and the PMCI group as a function of postoperative days. Error bars represent the standard deviation (*P = .049).
j CATARACTREFRACT$UI~VOL25,MARCH1999
429
ASTIGMATISMAFTER BENT SCLERALAND PLUS MERIDIAN INCISIONS (DiQpters) 2.0
(Diopters) 2.0 95% confidence intervals
95%te~al? dence 1.0
1.0' []
,, O)
0.0
~
0.0 0
i
.1.0 ¸
-2.0
0.0
~
1.0
2.0
3.0 (Diopters)
Preoperative Astigmatism
~
~
~
-1.0
-2.0 0.0
1.0
2.0
3.0
Preoperative Astigmatism
4.0 (Diopters)
F i g u r e 4. (Kurimoto) Relationship between preoperative and the final postoperative corneal astigmatism (day 100) in the BENT scleral incision group (y = - 0 . 3 4 x + 0.46, r = -0.41 [95% confidence interval, - 0 . 6 9 to -0.04; P = 0.032]; r 2 = 0.17, y-intercept = 0.46 [95% confidence interval, 0.12 to 0.81; P ffi .011]).
F i g u r e 5. (Kurimoto) Relationship between preoperative and the final postoperative corneal astigmatism (day 100) in the PMCI group (y ffi - 0 . 4 3 x + 0.35, r -- - 0 . 6 5 [95% confidence interval, - 0 . 8 3 to -0.36; P = .0002]; r 2 = 0.42, y-intercept = 0.35 [95% confidence interval, 0.07 to 0.63; P ffi .017]).
the difference was significant was 30 days postoperatively (P < .05). A plot of the change in astigmatism (postoperative astigmatism on day 100 - preoperative astigmatism) as a function of the preoperative astigmatism showed a significant negative correlation (r = -0.41, P = .032) in the BENT group, and the value of the x-intercept of the regression line was 1.37 D (Figure 4). This means that for preoperative astigmatism higher than 1.37 D, there would be a negative change in astigmatism and the net postoperative astigmatism would be lower. Similarly, for preoperative astigmatism lower than 1.37 D, there would be a positive change in astigmatism and the net postoperative astigmatism would be higher. There was also a significant (r = -0.65, P -.0002) negative correlation between preoperative astigmatism and final postoperative change in corneal astigmatism in the PMCI group (Figure 5); the x-intercept of the regression line was 0.82 D. Thus, postoperative astigmatism would be lower in an eye that had a preoperative astigmatism of more than 0.82 D and "higher in an eye with preoperative astigmatism less than 0.82 D.
Discussion
430
We compared the course of corneal astigmatism in patients who had cataract surgery with a 4.1 m m BENT sderal incision in 1 eye and a 4.1 m m PMCI incision in the fellow eye. The 2 groups were wellmatched in the relevant parameters of the eye (i.e., no significant differences in the pre-existing corneal astigmatism, axial length, and phaco energy and time). The surgery was significantly longer in the BENT group, but this difference was inadvertent and arose from the treatment of the conjunctiva before the primary incision was made. Therefore, the 2 groups were equally matched, allowing comparison of the results of 2 incision techniques. Several methods are commonly used to evaluate postoperative changes in corneal astigmatism, and they can be divided into 2 categories: (1) simply compare the preoperative and the postoperative corneal astigmatism; 5'e (2) determine the SIA as a vector or scalar of astigmatism to evaluate the change in astigmatism. 7-9 Although the methods of determining SIA seem to be more useful, there are several problems. Some of the methods give erroneous values when the surgical inci-
J CATARACT REFRACTSURG~VOL 25. MARCH 1999
ASTIGMATISMAFTERBENTS C L E R A L sion is not located in the vertical or the horizontal meridian, such as with the BENT incision. I° In addition, there is a question whether determining SIA as a vector is valid when the cornea does not form a toric surface with the axes crossing at right angles. Therefore, we adopted the first method, which evaluates the preoperative and the postoperative corneal astigmatism, which is clearly of clinical importance. Throughout the postoperative period, the changes in corneal astigmatism and the absolute value of the postoperative astigmatism were less in the PMCI group. Current refinements in small incision cataract surgery have heightened patient expectations of immediate visual results. Therefore, the PMCI method, in which the postoperative changes in astigmatism and the degree of astigmatism were less, should have a better outcome than the BENT sderal incision method. In both groups, there was a statistically significant negative correlation between the preoperative values and the final postoperative change in corneal astigmatism, and the x- and y-intercepts of the regression line were positive. Therefore, in both the groups, postoperative astigmatism was better than preoperatively in eyes with considerable preoperative astigmatism but worse in eyes with low preoperative astigmatism. The critical point in determining whether astigmatism will be better or worse than preoperatively was 1.37 D in the BENT group and 0.82 D in the PMCI group. The lower value in the PMCI group indicates that the number of eyes in which postoperative astigmatism will be worse should be less in the PMCI group than in the BENT group. Thus, the PMCI method has an important benefit over the BENT scleral incision method.
AND
PLUS
MERIDIANINCISIONS
In summary, for cataract surgery using a 4.1 mm incision, the PMCI surpassed the BENT sderal incision method in minimizing postoperative corneal astigmatism.
Refc~nce.,s 1. Kawano K. Modified corneoscleral incision to reduce postoperative astigmatism after 6 mm dialneter intraocular lens implantation. J Cataract Refract Surg 1993; 19:387-392 2. Karickhoff JR. Plus meridian incision for secondary implantation. Ophthalmic Surg 1987; 18:658-660 3. Shepherd JR. Induced astigmatism in small incision cataract surgery.J Cataract Refract Surg 1989; 15:85-88 4. Oshika T, Tsuboi S, YaguchiS, et al. Comparative study ofintraocular lens implantation through 3.2- and 5.5-mm incisions. Ophthalmology 1994; 101:1183-1190 5. Singh D, Kumar K. Keratometric changes after cataract extraction. Br J Ophthalmol 1976; 60:638-641 6. Luntz MH, Livingston DG. Astigmatism in cataract surgery. Br J Ophthalmol 1977; 61:360-365 7. Jaffe NS, Clayman HM. The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalrnol Otolaryngo11975; 79:OP615-OP630 8. CravyTV~.Calculation of the change in corneal astigmatism following cataract extraction. Ophthalmic Surg 1979; 10(1):38-49 9. Holladay JT, Cravy TV, Koch DD. Calculating the surgically induced refractive change following ocular surgery.J Cataract Refract Surg 1992; 18:429 "!.'!.3 10. Oshika T, Samejirna T, Miyata N. [Calculation of surgically induced astigmatism]. [Japanese]J Jpn Ophthalrnol Soc 1995; 99:901-909 Acceptedfbr publication October16, 1998. From the Departmenu of Ophthalmolog~ Shimhu UniversitySchoolof Medicine, Matsumoto (Kurimoto, Komurasaki, Yoshimura),and Kobe City GeneralHo~ita~ Kobe (Kondo),Japan.
j CATARACT REFRACT SURG.--VOL 25,M A R C H 1999
~ I