Effect of suture material on postoperative astigmatism Howard V. Gimbel, M.D., F.R.C.S.(C), Marsha G. Raanan, M.S., Maryclare DeLuca ABSTRACT Two hundred patients were enrolled in a randomized, prospective clinical trial comparing the use of 10-0 nylon, 10-0 polypropylene (ProIene-), 11.0 poly~ster (Mersilene-), and 10-0 polyethylene (Noviifile ) sutUre materials on the amount and decay curves of sur. gicaUY.'indllced astigmatism following intraocular lens (IOL) surgery~ Patients with Mersilene and nylon sutures had the highest ll"'Olltl~ of induced with-the-rule (WTR) cylinder (significantly more than Prorene} at one day after surgery. However, the WTR cylinder decayed rapidly for nylon during the first three months hut more slowly for Mersilene because of its lack ofstretchahility. The Prolene group had the lowest level ofinduced WTR cylinder at one day, bi1tapinst~the~rule (ATR) drift occurred, .(eaving cases with ATRastigmatism by a year. The nylon group had the second highest alllsunt ofinduced WTR cylinder at one day, which had decayed to 'ATBcYliDderhyJ.1ve monthS. Between one and two years postoperatively, the nylon group experienced asignificant ATR shift. The ,'aIllonnt:ofearlyinducedWTR cylinder seemed to he related to the knoHying technique and tissue gripping characteristics, whereas .t heshapeofthe deCl,lY curve was related to the material characteristics of the'suture.
K~y' W~l'ds:induced astigmati~m, ,
intraocular lens surgery, nylon, polyester; polyethylene, polypropylene, suture, withthe-rule astigmatism
For approximately ten years, we have used 10-0 polypropylene (Prolene ill) sutures routinely at the Gimbel Eye Centre. Prolene does not hydrolyze and break as nylon has a tendency to do. The majority of surgeons, however, use 10-0 nylon sutures. Polyester (Mersilene®) is also used and, more recently, the synthetic substance polyethylene (N ovafil®). All suture materials have differing characteristics and elasticity. Nylon is generally the most stretchable suture, Mersilene does not stretch much, while Prolene and Novafil have similar handling characteristics. We conducted a prospective, four-group study comparing the use of 10-0 Prolene, 11-0 Mersi-
lene, 10-0 Novafil, and 10-0 nylon sutures following phacoemulsification with insertion of a posterior chamber intraocular lens (IOL) to determine if there were significant group differences in surgically induced cylinder and visual rehabilitation time. MATERIALS AND METHODS Two hundred patients were enrolled in a prospective, four-group study comparing the use of 10-0 Prolene, 11-0 Mersilene, 10-0 Novafil, and 10-0 nylon sutures following phacoemulsification with insertion of a posterior chamber IOL. Patients
From the Gimbel Eye Centre, Calgary, Alberta, Canada, and the Department of Ophthalmology, University of Illinois at Chicago . Analysis of the data was performed by the Center for Clinical Research, University of Illinois at Chicago. Reprint requests to Howard Gimbel, M.D. , Gimbel Eye Centre, Suite 450, 4935-40th Avenue N. W., Calgary, Alberta, Canada T3A 2Nl. 42
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
were randomized to receive one of the four suture types. Patients were eligible for enrollment if they were scheduled to have uncomplicated primary cataract/posterior chamber IOL surgery using phacoemulsification as the method of extraction. Patients also had to have less than 2.0 diopters (D) of primary keratometric cylinder preoperatively and had to agree to return to the Gimbel Eye Centre for a three-month postoperative visit. Upon enrollment, patients were randomized to receive one of the suture materials. The randomization schedule was generated using PROD AS (Professional Database Analysis System) software. l The wound was made superiorly centered about the 110 degree axis to facilitate right-handed phacoemulsification. A small, limbus-based conjunctival flap was formed. Very light cautery of episcleral vessels, using a Codman Mentor CMC II cautery set at 8 and Mentor bipolar eraser-type tip, was done. Using a diamond knife, a reverse 0.75 mm deep and 6.0 mm or 7.0 mm long groove was made 1.5 mm from the conjunctival reflection. The blade orientation was then changed and, from the depths of the groove, the incision was carried forward to leave a thin shelf of corneal scleral tissue over Schlemm's canal and the trabecular meshwork. A central 3 mm entry into the anterior chamber was made for phacoemulsification and then enlarged with the diamond knife to 6.5 mm or 7.5 mm for
either 6.0 mm or 7.0 mm diameter posterior chamber IOL implantation. Continuous shoelace closure was used in all cases. Suture adjustment was made after air was used to fill the anterior chamber but not overinflate it to cause the wound to gape. Because the nylon suture readily released upon attempting to close using a single throw on the first pass of the knot, a double throw was used. For all other sutures, a single throw on the first pass was used. Patients were generally examined at one day (0 to 3 days), two to three weeks (12 to 27 days), two months (6.0 to 11.4 weeks), three months (11.6 to 17.0 weeks), five to six months (4.5 to 7.5 months), one year (10.0 to 15.75 months), and two years postoperatively (18 to 27 months). At these visits, keratometry measurements and refractions were taken. All measurements were performed at the Gimbel Eye Centre. Four cases were lost to followup prior to the three-month visit and were replaced. The technicians performing keratometry measurements were unaware of the randomly assigned suture type group both prior to and following surgery. Keratometric readings were made using the Haag-Streit keratometer with the eyepiece adjusted for the operator. To quantify surgically induced cylinder fully, we used a vector analysis procedure that calculates the
Table 1. Patient characteristics. Characteristics Total number of patients Sex N (%) male N (%) female Age Mean (SD) Range Follow-up percentage 1 day 2-3 weeks 2 months 3 months 5-6 months 1 year 2 years Follow-up Mean (SD) Range
Prolene 52
Mersilene 48
Novafil
Nylon
50
50
16 (31 %) 36 (69%)
14 (29%) 34 (71 %)
19 (38%) 31 (62%)
17 (34%) 33 (66%)
71.7 (11.2) 47-94
70.7 (12.3) 38-93
72.3 (10.4) 44-87
71.1 (11.1) 42-90
100% 98% 100% 88%
100% 96%
100% 96% 96% 80%
100% 94% 98% 88%
63% 77% 52%
56% 90% 58%
78% 90% 50%
76% 88% 54%
16.8 (7.3) 2.5-26.75
18.0 (6.9)
17.1 (6.4)
17.5 (6.6)
2.0-25.5
3.0-25.25
0.75-26.5
90% 77%
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power and axis of the surgically induced cylinder based upon the equations developed by Naylor.2 Postoperative cylinder and surgically induced cylinder were both analyzed with a one-way analysis of variance using a Bonferroni adjustment for multiple comparisons. Surgically induced astigmatism was also broken down for each case into a with-the-rule (WTR) component and an against-the-rule (ATR) component. This method, developed by Cravy,3 allows examination of WTR cylinder decay. The mean changes in WTR cylinder within successive postoperative intervals were also analyzed using a oneway analysis of variance with a Bonferroni adjustment for multiple comparisons. Differences in sex of the patients in suture type groups were analyzed using the chi-square method. Age differences between suture types were analyzed with a one-way analysis of variance.
All statistical analyses were performed using PRODAS. 1 RESULTS Characteristics of the patients who received each suture type are presented in Table 1. Average age for the whole population was 72 years; approximately two thirds of the cases were female and one third was male. When analyzed separately, the suture type groups did not reveal significant differences in age or sex. Table 1 also shows the follow-up rates at the major postoperative periods. Follow-up was 94% to 98% at two to three weeks. Throughout the first year, at least 77% of patients returned for visits except at five to six months. Scheduling problems caused follow-up inconsistencies for the five to six month visit. Follow-up at two years decreased to
Table 2. Preoperative and postoperative cylinder. Interval Preoperative Mean (SD) Range 1 day* Mean (SD) Range 2-3 weeks
Prolene
Mersilene
Novafil
Nylon
N = 52 0.85 (0.56)
N = 50 0.93 (0.56)
0.25 to 2.5
N = 48 0.87 (0.54) 0.25 to 2.5
N = 50 0.85 (0.42) 0.25 to 2.25
N = 52 1.42 (1.00)
N = 48 2.23 (1.54)
0.0 to 4.25
0.25 to 2.75
N = 50 2.03 (1.69)
0.13 to 7.12
N = 50 1.65 (1.08) 0.12 to 5.0
0.13 to 8.5
Mean (SD) Range
N = 51 1.20 (0.83)
N = 46 1.58 (1.12)
N = 48 1.32 (0.65)
N = 47 1.34 (0.90)
0.25 to 4.5
0.13 to 5.13
0.0 to 2.75
0.0 to 4.5
2 monthst Mean (SD)
N = 51 1.01 (0.59)
N = 41 1.68 (1.30)
N = 47 1.14 (0.52)
0.13 to 2.63
0.25 to 6.62
0.12 to 2.63
N = 49 1.19 (0.76) 0.25 to 3.88
N = 46 0.90 (0.47) 0.0 to 2.0
N = 37 1.24 (0.91)
N = 44 1.12 (0.79)
0.12 to 4.75
N = 38 1.09 (0.59) 0.25 to 2.25
N = 31 1.01 (0.57)
N = 25 0.95 (0.61)
N = 38 1.01 (0.61)
0.12 to 2.37
0.0 to 2.75
N = 39 1.18 (0.58) 0.13 to 2.5
1 year Mean (SD) Range
N = 38 1.04 (0.62) 0.13 to 2.88
N = 43 1.03 (0.73) 0.12 to 3.13
N = 45 1.07 (0.66) 0.0 to 3.38
N = 43 0.95(0.62)
2 year
N = 26 1.23 (0.73) 0.12 to 2.88
N = 26 0.875 (0.50) 0.25 to 2.00
N = 25 1.16 (0.70) 0.25 to 3.50
N = 27 1.19 (0.81) 0.25 to 3.25
Range 3 months Mean (SD) Range 5-6 months Mean (SD) Range
Mean (SD) Range Significant differences: * Prolene and Mersilene (P < .01) t Mersilene and each group (P < .01) 44
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
0.0 to 3.75
0.12 to 2.75
0.0 to 2.5
50% to 58%. Each of the four groups had a mean follow-up of between 16.8 and 18.0 months. Preoperative and postoperative keratometric cylinder values are summarized in Table 2 and presented graphically in Figure 1. There were no significant differences among groups in distribution of wound sizes and mean preoperative cylinder. Because 6.0 mm and 7.0 mm diameter optic IOLs were used in this study, 6.5 mm or 7.5 mm incisions were made. The overall distribution of incision sizes for all suture material groups was 45% 6.5 mm incisions and 55% 7.5 mm incisions. The distribution within each suture material group was similar. Net postoperative and surgically induced cylinder was not significantly different (all P values ~ .3) between 6.5 mm and 7.5 mm incision sizes, overall or within any suture material group. Therefore no stratified analyses are presented. For the first three months after surgery, the Mersilene group consistently had the highest postoperative mean cylinder, while Prolene consistently had the lowest. The order of decreasing mean postoperative cylinder was Mersilene, nylon, Novafil, and Prolene. The largest difference between the Mersilene and Prolene groups occurred at the one-day visit. At one day, the 0.8 dioptric difference between Mersilene (2.2 D) and Prolene (1.4 D) was statistically significant (P < .01). At two months, cylinder in all groups except the Mersilene group regressed substantially, such that Mersilene had significantly (P< .01) more postoperative cylinder than each of the other suture type groups. After three months, the suture type groups were more comparable through one year. There were no
statistically significant differences among groups at subsequent times. At one year, cases in all groups still generally had about a diopter of postoperative cylinder, on the average, which was similar to preoperative levels. By two years, postoperative keratometric cylinder means were slightly higher than the one-year means (approximately 0.1 to 0.2 D higher) for all groups except the Mersilene group, in which the two-year mean was slightly lower (by 0.14 D) than the one-year mean. The means of the surgically induced cylinder values (as computed by vector analysis) at each postoperative period are presented in Table 3 and Figure 2. During the first three months, Mersilene had the highest amount of induced cylinder and Prolene had the lowest. At one day, the differences in induced cylinder between nylon and Prolene and between Mersilene and Prolene were statistically significant (P < .01). Initially, the induced cylinder regressed after one day for all groups, although Mersilene maintained significantly higher induced cylinder through two months. After two months postoperatively, there were no statistically significant differences among groups. However, while the Mersilene and Novafil groups maintained stable levels of induced cylinder (approximately 0.9 D) at one and two years, both the nylon and Prolene groups had two-year mean induced cylinders which were higher than at one year. The Prolene group mean increased by a third of a diopter (from 0.88 D to 1.23 D) and the nylon mean increased by approximately one half diopter (from 0.76 D to l.27 D). Figure 3 shows the WTR cylinder decay (as calculated by the Cravy method) for each of the four
M 2.5 E A N
~ PROLENE
-+- MERSILENE
-+- NOVAFIL
-a- NYLON
2
p
o S
T
o
1.5
Fig. 1.
P
c
y L I N
(Gimbel) Line graphs of mean preoperative and postoperative cylinders over time for each suture material group.
0.5
~
R PREOP
TIME
I J''"'1'''S--r 2'3-'S.--------1'Y-R---···----·. D W MM
M
,
2 YR
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Table 3. Surgically induced cylinder at the major periods. Postoperative Period
1 day· Mean (SO) 2-3 weekst Mean (SO) 2 monthst Mean (SO) 3 months Mean (SO) 5-6 months Mean (SO) 1 year
Prolene
Mersilene
Novafil
Nylon
N = 52 1.09 (0.72) N = 51 0.85 (0.81) N = 51 0.81 (0.53) N = 46 0.80 (0.44)
N= 48
1.85 (1.58)
N = 50 1.45 (0.93)
N = 50 1.74 (1.44)
N = 46 1.32 (1.05) N = 41 1.27 (1.23)
N = 48 1.04 (0.68) N = 47 0.88 (0.57)
1.05 (0.70) N = 49 0.90 (0.72)
N = 37 1.06 (1.02)
N = 38 0.97 (0.49)
N = 44 0.91 (0.53)
N = 31 0.89 (0.50)
N = 25 0.80 (0.81)
N = 39 0.94 (0.65)
N = 38 0.79 (0.49)
N = 43 1.10 (0.87)
N = 45 0.86 (0.57)
N = 26 0.97 (0.58)
N = 25 0.92 (0.64)
N
= 38
Mean (SO) 0.88 (0.52) 2 year N = 26 Mean (SO) 1.23 (0.67) Significant differences: • Prolene and Mersilene, Prolene and nylon P < .01 t Prolene and Mersilene P = .05 t Prolene and Mersilene P = .03
N
N
= 47
= 43
0.76 (0.48) N = 27 1.27 (0.65)
2 ~ PROLENE
M E A N
+
MERSILENE
+- NOVAFIL
-B- NYLON
I N
o
Fig. 2.
(Gimbel) Line graphs of mean surgically induced cylinder over time for each suture material group.
U C E
o C y L
1
N
0.5
o E
R
TIME O~-'-.--.-----------r-------------------r---
1 3 2 3 OW M M
groups. Mersilene and nylon had the largest amounts of induced WTR cylinder at one day. Significant decay occurred during the first three months and continued at a slower rate through one year. After one year, the Mersilene group had no significant decay in either direction. Between two 46
5 M
1
Y
2 Y
months and one year, mean ATR shift for this group was -1.07 D, with 43% of cases having> 1.0 D of ATR shift. However, between one and two years, mean shift was +0.09 D (WTR direction) and only 8% of cases had ATR cylinder axis shifts> 1 D (Figure 4).
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
1.5 -
PROLENE
-+- MERSILENE
--* NOVAFIL
-B- NYLON
0.5
WTR
0~:===5=~~
ATR
I
Fig. 3.
"
-0.5
(Gimbel) Line graph of mean Cravy-calculated induced astigmatism depicting WTR cylinder decay over time for each suture material group.
-1
TIME
-1.5~r-.-.---.-------------.----------------------.
1
o
3 W
2 3 M M
5 M
1 VR
The pattern ofWTR cylinder decay was different in the nylon group. Mean ATR shift between two months and one year was -0.47 D, with 21 % of cases shifting in the ATR direction by more than one diopter. Between one and two years, the mean ATR shift was -0.6 D and 32 % of nylon cases had ATR shifts greater than 1.0 D, showing that, unlike Mersilene, much of the overall ATR drift in the nylon group occurred after one year. The Prolene group also had a significant mean decrease in WTR cylinder (0.8 D) during the first three months (Figure 3), but this leveled off between three and six months. Since so little « 0.5 D) WTR was induced at one day, by six months these cases had mean ATR cylinder of over half a diopter. After six months, the decay stabi-
WTR
Fig. 4.
ATR
(Gimbel) Bar charts of percent of cases with> 1 D of ATR cylinder shifts between one year and two years for each suture material group.
2 VR
lized. Between one and two years, 16% of cases had ATR drift> 1.0 D (Figure 4). Although 1.45 D of total induced cylinder had been induced in the Novafil group at one day (Figure 2), it was almost evenly split between WTR and ATR, resulting in less than 0.5 D of mean WTR cylinder for this group at one day (Figure 3). By two months, the proportions ofWTR and ATR cylinder for these cases were similar. Through one year, there was no large preponderance of either WTR or ATR cylinder shifts in this group; however, between one and two years, 17% of cases did have > 1.0 D of ATR shift. To assess whether data indicating late shifts in axis from one to two years were affected by patient attrition between one and two years (see Table 1), the data were analyzed by the Cravy method using only cases that were examined at one day, two months, three months, one year, and two years. The pattern of late ATR drift was the same (Figure 5). Table 4 shows the percentage of cases with unaided visual acuities of 20/40 or better preoperatively and at the various postoperative visits. The unaided visual results were comparable, with no statistically significant differences among suture types at any period. The percentage of cases with 20/40 or better acuity at one day after surgery was 40% to 62%. By two months, 74% to 81 % had acuities of 20/40 or better. Twenty-nine percent to 42% had acuities of 20/25 or better. This level of visual acuity was maintained through one year. At two years, 68% to 81 % of the cases that returned had unaided visual
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
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acuities of20/40 or better and 32% to 43%,20/25 or better. DISCUSSION Commonly, when analyzing induced cylinder, the means and ranges of dioptric power changes from the preoperative to the postoperative measurements are compared without regard to any cylinder axis changes. Thus, surgically induced cylinder change will be accurately described only for cases in which the postoperative axis is exactly the same as the preoperative cylinder. For example, a preoperative cylinder of + 1.00 X 90, converted to a postoperative cylinder of + l.00 X 180, would appear to have no change in cylinder when, in fact, a +2.00 D change in cylinder occurred at axis 180. Computing induced cylinder is more complex if the postoperative axis is neither the same as nor 90 degrees away from the preoperative value . Naylor's equations are based upon a trigonometric solution for the resultant of two obliquely crossed cylinders (vector analysis programs). Instead of solving for the resultant cylinder, however, the equations find the second component (surgically induced cylinder power and axis) from a knowledge of the first component (preoperative cylinder power and axis) and the resultant (postoperative cylinder power and axis). Vector analysis is gaining more widespread use and has been used to evaluate induced cylinder following cataract extraction 4 ,5 and following astigmatic keratotomy. 6 , 7 Many factors determine the amount of surgically
I
Fig. 5 .
(Gimbel) Line graph of mean Cravy-calculated induced astigmatism depicting WTR cylinder decay over time for each suture material group. Cases were examined at the one day, two week , two month, three month, one year, and two year visits.
N D U C
1.5
D
0.5
-+- MERSILENE
-+- NOVAFIL
-B- NYLON
E
A S T I
G
0 -0 .5
M
A
T I S M 48
induced astigmatism, the decay rate of that induced astigmatism, and the possible drift toward ATR astigmatism. In this study, one factor (the way the incision is made) was constant for all groups. The type of closure was also standard for all groups. However, the effect of suture type on the judgment of suture tension using a continuous suture can influence the amount of induced cylinder from tight or loose sutures. In this study, the surgeon's greatest experience and, therefore, probably best judgment on suture tension was with the Prolene material. Even though most surgeons use nylon, one of the authors (H.V.C.) does not because of its biodegradability; it can break and extend through conjunctiva, irritating the eye. Also, reports by Byes ("Nylon Biodegradation and Late Post-op Astigmatism Increase-the Mersilene Advantage," presented at the Welsh Cataract Congress, November 1988) and Cravy 8 indicate that use of nylon suture, which eventually dissolves causing the wound to relax late, may lead to ATR drift in astigmatism years after surgery. We, in fact, saw a dramatic ATR drift between one and two years postoperatively with the use of nylon. Prolene does not hydrolyze and biodegrade and is more wiry than nylon or Novafil. Prolene slides quite easily through the tissue so we did not see much early induced WTR cylinder, as with Mersilene. However, with the tension not maintained, the looser wound led to an early drift toward ATR astigmatism, which stabilized after six months. Mersilene is very wiry, more so than Prolene.
- 1
-1.5 1 3 2 3 D W MM
TIME 1 VR
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2 VR
Table 4. Percent of cases with postoperative VAse of 20/40 or better. Postoperative Period
Prolene
Mersilene
Novafil
Nylon
20/15-20/25 20/30-20/40 2-3 weeks
11 (21 %) 17 (33%)
7 (16%) 19 (42%)
8 (16%) 12 (24%)
5 (10%) 26 (52%)
20/15-20/25 20/30-20/40 2 months
11 (22%) 25 (51 %)
10 (23%) 18 (41%)
17 (36%) 20 (43%)
13 (28%) 21 (44%)
20/15-20/25 20/30-20/40 3 months
20 (38%) 21 (41 %)
18 (42%) 15 (34%)
18 (38%) 17 (36%)
14 (29%) 25 (52%)
20/15-20/25 20/30-20/40 5-6 months
15 (33%) 19 (41 %)
14 (38%) 16 (43%)
15 (38%) 13 (32%)
14 (33%) 19 (46%)
20/15-20/25 20/30-20/40 1 year
10 (30%) 13 (39%)
12 (44%) 7 (26%)
17 (46%) 15 (40%)
14 (38%) 15 (40%)
20/15-20/25
16 (41 %)
20/30-20/40 2 years
17 (44%)
17 (46%) 10 (27%)
11 (30%) 18 (49%)
17 (44%) 10 (26%)
20/1.5-20/25 20/30-20/40
9 (33%)
9 (35%)
10 (43%)
8 (32%)
12 (44%)
12 (46%)
6 (26%)
9 (36%)
1 day
When pulled tightly it induces a high level ofWTR cylinder. It then holds tightly because of its lack of stretchability, maintaining a higher level of WTR cylinder over time than do other suture types. This was the pattern we observed. Even though the decay rate was slower, eventually the level of induced cylinder came down, with levels comparable to the other suture types by a year. The manufacturer had recommended the use of 11-0 rather than 10-0 suture based on the belief that 11-0 Mersilene was equivalent in strength to 10-0 nylon. This thinner suture probably "cheese-wires," cutting through tissue to release the tension. One author (H.V.C.) had no previous clinical experience with Novafil and included it in the study for completeness. The rapid decay of induced WTR astigmatism in the Novafil group may relate to the stretchability of that material, which quickly forgives an overtight adjustment of the suture. Prolene and nylon seem to fall midway between Mersilene and N ovafil in this regard. In retrospect, the two weaknesses of the comparability of groups in the study were the use of 11-0 rather than 10-0 Mersilene and the unavoid-
able differences in knot-tying technique. The amount of early induced WTR cylinder is believed to be related to the knot-tying technique (in turn, due to the handling characteristics) and to tissue gripping characteristics (ease of sliding through tissue). The shape of the decay curve, however, is related to the material characteristics of the suture. This study, carried out over 21/2 years, resulted in absolute levels of surgically induced cylinder, which reflected the incision techniques used at the time. The surgeon's (H.V.C.) incision technique is currently very different. Nevertheless, the relative differences among suture materials do apply to current techniques. In general, we are concerned with apposing the external aspect of the wound with sutures and have not been concerned with the internal aspect. Not all patients develop ATR drift, even in groups in which the means clearly shift in that direction. Perhaps cases in which the internal aspect of the wound is well apposed are the ones that do not demonstrate the ATR drift. If this is true, there may be aqueous access to the large portion of the incision in cases in which the internal corneal aspect of
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
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internal architecture may prevent late ATR drift (Figure 6). REFERENCES
Fig. 6.
(Gimbel) Aqueous access to inner aspect of incision may cause permanent gaping and, therefore, late ATR.
the incision is not apposed. This could cause permanent gaping, wound slippage, and late ATR drift. If this hypothesis is correct, then self-sealing
.50
l. Conceptual Software, Inc. Professional Database Analysis System (PRODAS) Reference Manual, Version 3.0, Houston, 1985 2. Naylor EJ. Astigmatic difference in refractive error. Br J Ophthalmol1968; 52:422-424 3. Cravy TV. Calculation of the change in corneal astigmatism following cataract extraction. Ophthalmic Surg 1979; 10: 38-49 4. Jaffe NS, Clayman HM. The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol1975; 79:615-630 5. Neumann A, McCarty G, Sanders D, Raanan M. Small incisions to control astigmatism during cataract surgery. J Cataract Refract Surg 1989; 15:78-84 6. Thornton SP, Sanders DR. Graded nonintersecting transverse incisions for correction of idiopathic astigmatism. J Cataract Refract Surg 1987; 13:27-31 7. Neumann A, McCarty G, Sanders D, Raanan M. Refractive evaluation of astigmatic keratotomy procedures. J Cataract Refract Surg 1989; 15:25-31 8. Cravy TV. Long-term corneal astigmatism related to selected elastic, monofilament, nonabsorbable sutures. J Cataract Refract Surg 1989; 15:61-69
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K~y' W~l'ds:induced astigmati~m, ,
intraocular lens surgery, nylon, polyester; polyethylene, polypropylene, suture, withthe-rule astigmatism
For approximately ten years, we have used 10-0 polypropylene (Prolene ill) sutures routinely at the Gimbel Eye Centre. Prolene does not hydrolyze and break as nylon has a tendency to do. The majority of surgeons, however, use 10-0 nylon sutures. Polyester (Mersilene®) is also used and, more recently, the synthetic substance polyethylene (N ovafil®). All suture materials have differing characteristics and elasticity. Nylon is generally the most stretchable suture, Mersilene does not stretch much, while Prolene and Novafil have similar handling characteristics. We conducted a prospective, four-group study comparing the use of 10-0 Prolene, 11-0 Mersi-
lene, 10-0 Novafil, and 10-0 nylon sutures following phacoemulsification with insertion of a posterior chamber intraocular lens (IOL) to determine if there were significant group differences in surgically induced cylinder and visual rehabilitation time. MATERIALS AND METHODS Two hundred patients were enrolled in a prospective, four-group study comparing the use of 10-0 Prolene, 11-0 Mersilene, 10-0 Novafil, and 10-0 nylon sutures following phacoemulsification with insertion of a posterior chamber IOL. Patients
From the Gimbel Eye Centre, Calgary, Alberta, Canada, and the Department of Ophthalmology, University of Illinois at Chicago . Analysis of the data was performed by the Center for Clinical Research, University of Illinois at Chicago. Reprint requests to Howard Gimbel, M.D. , Gimbel Eye Centre, Suite 450, 4935-40th Avenue N. W., Calgary, Alberta, Canada T3A 2Nl. 42
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
were randomized to receive one of the four suture types. Patients were eligible for enrollment if they were scheduled to have uncomplicated primary cataract/posterior chamber IOL surgery using phacoemulsification as the method of extraction. Patients also had to have less than 2.0 diopters (D) of primary keratometric cylinder preoperatively and had to agree to return to the Gimbel Eye Centre for a three-month postoperative visit. Upon enrollment, patients were randomized to receive one of the suture materials. The randomization schedule was generated using PROD AS (Professional Database Analysis System) software. l The wound was made superiorly centered about the 110 degree axis to facilitate right-handed phacoemulsification. A small, limbus-based conjunctival flap was formed. Very light cautery of episcleral vessels, using a Codman Mentor CMC II cautery set at 8 and Mentor bipolar eraser-type tip, was done. Using a diamond knife, a reverse 0.75 mm deep and 6.0 mm or 7.0 mm long groove was made 1.5 mm from the conjunctival reflection. The blade orientation was then changed and, from the depths of the groove, the incision was carried forward to leave a thin shelf of corneal scleral tissue over Schlemm's canal and the trabecular meshwork. A central 3 mm entry into the anterior chamber was made for phacoemulsification and then enlarged with the diamond knife to 6.5 mm or 7.5 mm for
either 6.0 mm or 7.0 mm diameter posterior chamber IOL implantation. Continuous shoelace closure was used in all cases. Suture adjustment was made after air was used to fill the anterior chamber but not overinflate it to cause the wound to gape. Because the nylon suture readily released upon attempting to close using a single throw on the first pass of the knot, a double throw was used. For all other sutures, a single throw on the first pass was used. Patients were generally examined at one day (0 to 3 days), two to three weeks (12 to 27 days), two months (6.0 to 11.4 weeks), three months (11.6 to 17.0 weeks), five to six months (4.5 to 7.5 months), one year (10.0 to 15.75 months), and two years postoperatively (18 to 27 months). At these visits, keratometry measurements and refractions were taken. All measurements were performed at the Gimbel Eye Centre. Four cases were lost to followup prior to the three-month visit and were replaced. The technicians performing keratometry measurements were unaware of the randomly assigned suture type group both prior to and following surgery. Keratometric readings were made using the Haag-Streit keratometer with the eyepiece adjusted for the operator. To quantify surgically induced cylinder fully, we used a vector analysis procedure that calculates the
Table 1. Patient characteristics. Characteristics Total number of patients Sex N (%) male N (%) female Age Mean (SD) Range Follow-up percentage 1 day 2-3 weeks 2 months 3 months 5-6 months 1 year 2 years Follow-up Mean (SD) Range
Prolene 52
Mersilene 48
Novafil
Nylon
50
50
16 (31 %) 36 (69%)
14 (29%) 34 (71 %)
19 (38%) 31 (62%)
17 (34%) 33 (66%)
71.7 (11.2) 47-94
70.7 (12.3) 38-93
72.3 (10.4) 44-87
71.1 (11.1) 42-90
100% 98% 100% 88%
100% 96%
100% 96% 96% 80%
100% 94% 98% 88%
63% 77% 52%
56% 90% 58%
78% 90% 50%
76% 88% 54%
16.8 (7.3) 2.5-26.75
18.0 (6.9)
17.1 (6.4)
17.5 (6.6)
2.0-25.5
3.0-25.25
0.75-26.5
90% 77%
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
43
power and axis of the surgically induced cylinder based upon the equations developed by Naylor.2 Postoperative cylinder and surgically induced cylinder were both analyzed with a one-way analysis of variance using a Bonferroni adjustment for multiple comparisons. Surgically induced astigmatism was also broken down for each case into a with-the-rule (WTR) component and an against-the-rule (ATR) component. This method, developed by Cravy,3 allows examination of WTR cylinder decay. The mean changes in WTR cylinder within successive postoperative intervals were also analyzed using a oneway analysis of variance with a Bonferroni adjustment for multiple comparisons. Differences in sex of the patients in suture type groups were analyzed using the chi-square method. Age differences between suture types were analyzed with a one-way analysis of variance.
All statistical analyses were performed using PRODAS. 1 RESULTS Characteristics of the patients who received each suture type are presented in Table 1. Average age for the whole population was 72 years; approximately two thirds of the cases were female and one third was male. When analyzed separately, the suture type groups did not reveal significant differences in age or sex. Table 1 also shows the follow-up rates at the major postoperative periods. Follow-up was 94% to 98% at two to three weeks. Throughout the first year, at least 77% of patients returned for visits except at five to six months. Scheduling problems caused follow-up inconsistencies for the five to six month visit. Follow-up at two years decreased to
Table 2. Preoperative and postoperative cylinder. Interval Preoperative Mean (SD) Range 1 day* Mean (SD) Range 2-3 weeks
Prolene
Mersilene
Novafil
Nylon
N = 52 0.85 (0.56)
N = 50 0.93 (0.56)
0.25 to 2.5
N = 48 0.87 (0.54) 0.25 to 2.5
N = 50 0.85 (0.42) 0.25 to 2.25
N = 52 1.42 (1.00)
N = 48 2.23 (1.54)
0.0 to 4.25
0.25 to 2.75
N = 50 2.03 (1.69)
0.13 to 7.12
N = 50 1.65 (1.08) 0.12 to 5.0
0.13 to 8.5
Mean (SD) Range
N = 51 1.20 (0.83)
N = 46 1.58 (1.12)
N = 48 1.32 (0.65)
N = 47 1.34 (0.90)
0.25 to 4.5
0.13 to 5.13
0.0 to 2.75
0.0 to 4.5
2 monthst Mean (SD)
N = 51 1.01 (0.59)
N = 41 1.68 (1.30)
N = 47 1.14 (0.52)
0.13 to 2.63
0.25 to 6.62
0.12 to 2.63
N = 49 1.19 (0.76) 0.25 to 3.88
N = 46 0.90 (0.47) 0.0 to 2.0
N = 37 1.24 (0.91)
N = 44 1.12 (0.79)
0.12 to 4.75
N = 38 1.09 (0.59) 0.25 to 2.25
N = 31 1.01 (0.57)
N = 25 0.95 (0.61)
N = 38 1.01 (0.61)
0.12 to 2.37
0.0 to 2.75
N = 39 1.18 (0.58) 0.13 to 2.5
1 year Mean (SD) Range
N = 38 1.04 (0.62) 0.13 to 2.88
N = 43 1.03 (0.73) 0.12 to 3.13
N = 45 1.07 (0.66) 0.0 to 3.38
N = 43 0.95(0.62)
2 year
N = 26 1.23 (0.73) 0.12 to 2.88
N = 26 0.875 (0.50) 0.25 to 2.00
N = 25 1.16 (0.70) 0.25 to 3.50
N = 27 1.19 (0.81) 0.25 to 3.25
Range 3 months Mean (SD) Range 5-6 months Mean (SD) Range
Mean (SD) Range Significant differences: * Prolene and Mersilene (P < .01) t Mersilene and each group (P < .01) 44
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
0.0 to 3.75
0.12 to 2.75
0.0 to 2.5
50% to 58%. Each of the four groups had a mean follow-up of between 16.8 and 18.0 months. Preoperative and postoperative keratometric cylinder values are summarized in Table 2 and presented graphically in Figure 1. There were no significant differences among groups in distribution of wound sizes and mean preoperative cylinder. Because 6.0 mm and 7.0 mm diameter optic IOLs were used in this study, 6.5 mm or 7.5 mm incisions were made. The overall distribution of incision sizes for all suture material groups was 45% 6.5 mm incisions and 55% 7.5 mm incisions. The distribution within each suture material group was similar. Net postoperative and surgically induced cylinder was not significantly different (all P values ~ .3) between 6.5 mm and 7.5 mm incision sizes, overall or within any suture material group. Therefore no stratified analyses are presented. For the first three months after surgery, the Mersilene group consistently had the highest postoperative mean cylinder, while Prolene consistently had the lowest. The order of decreasing mean postoperative cylinder was Mersilene, nylon, Novafil, and Prolene. The largest difference between the Mersilene and Prolene groups occurred at the one-day visit. At one day, the 0.8 dioptric difference between Mersilene (2.2 D) and Prolene (1.4 D) was statistically significant (P < .01). At two months, cylinder in all groups except the Mersilene group regressed substantially, such that Mersilene had significantly (P< .01) more postoperative cylinder than each of the other suture type groups. After three months, the suture type groups were more comparable through one year. There were no
statistically significant differences among groups at subsequent times. At one year, cases in all groups still generally had about a diopter of postoperative cylinder, on the average, which was similar to preoperative levels. By two years, postoperative keratometric cylinder means were slightly higher than the one-year means (approximately 0.1 to 0.2 D higher) for all groups except the Mersilene group, in which the two-year mean was slightly lower (by 0.14 D) than the one-year mean. The means of the surgically induced cylinder values (as computed by vector analysis) at each postoperative period are presented in Table 3 and Figure 2. During the first three months, Mersilene had the highest amount of induced cylinder and Prolene had the lowest. At one day, the differences in induced cylinder between nylon and Prolene and between Mersilene and Prolene were statistically significant (P < .01). Initially, the induced cylinder regressed after one day for all groups, although Mersilene maintained significantly higher induced cylinder through two months. After two months postoperatively, there were no statistically significant differences among groups. However, while the Mersilene and Novafil groups maintained stable levels of induced cylinder (approximately 0.9 D) at one and two years, both the nylon and Prolene groups had two-year mean induced cylinders which were higher than at one year. The Prolene group mean increased by a third of a diopter (from 0.88 D to 1.23 D) and the nylon mean increased by approximately one half diopter (from 0.76 D to l.27 D). Figure 3 shows the WTR cylinder decay (as calculated by the Cravy method) for each of the four
M 2.5 E A N
~ PROLENE
-+- MERSILENE
-+- NOVAFIL
-a- NYLON
2
p
o S
T
o
1.5
Fig. 1.
P
c
y L I N
(Gimbel) Line graphs of mean preoperative and postoperative cylinders over time for each suture material group.
0.5
~
R PREOP
TIME
I J''"'1'''S--r 2'3-'S.--------1'Y-R---···----·. D W MM
M
,
2 YR
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
45
Table 3. Surgically induced cylinder at the major periods. Postoperative Period
1 day· Mean (SO) 2-3 weekst Mean (SO) 2 monthst Mean (SO) 3 months Mean (SO) 5-6 months Mean (SO) 1 year
Prolene
Mersilene
Novafil
Nylon
N = 52 1.09 (0.72) N = 51 0.85 (0.81) N = 51 0.81 (0.53) N = 46 0.80 (0.44)
N= 48
1.85 (1.58)
N = 50 1.45 (0.93)
N = 50 1.74 (1.44)
N = 46 1.32 (1.05) N = 41 1.27 (1.23)
N = 48 1.04 (0.68) N = 47 0.88 (0.57)
1.05 (0.70) N = 49 0.90 (0.72)
N = 37 1.06 (1.02)
N = 38 0.97 (0.49)
N = 44 0.91 (0.53)
N = 31 0.89 (0.50)
N = 25 0.80 (0.81)
N = 39 0.94 (0.65)
N = 38 0.79 (0.49)
N = 43 1.10 (0.87)
N = 45 0.86 (0.57)
N = 26 0.97 (0.58)
N = 25 0.92 (0.64)
N
= 38
Mean (SO) 0.88 (0.52) 2 year N = 26 Mean (SO) 1.23 (0.67) Significant differences: • Prolene and Mersilene, Prolene and nylon P < .01 t Prolene and Mersilene P = .05 t Prolene and Mersilene P = .03
N
N
= 47
= 43
0.76 (0.48) N = 27 1.27 (0.65)
2 ~ PROLENE
M E A N
+
MERSILENE
+- NOVAFIL
-B- NYLON
I N
o
Fig. 2.
(Gimbel) Line graphs of mean surgically induced cylinder over time for each suture material group.
U C E
o C y L
1
N
0.5
o E
R
TIME O~-'-.--.-----------r-------------------r---
1 3 2 3 OW M M
groups. Mersilene and nylon had the largest amounts of induced WTR cylinder at one day. Significant decay occurred during the first three months and continued at a slower rate through one year. After one year, the Mersilene group had no significant decay in either direction. Between two 46
5 M
1
Y
2 Y
months and one year, mean ATR shift for this group was -1.07 D, with 43% of cases having> 1.0 D of ATR shift. However, between one and two years, mean shift was +0.09 D (WTR direction) and only 8% of cases had ATR cylinder axis shifts> 1 D (Figure 4).
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
1.5 -
PROLENE
-+- MERSILENE
--* NOVAFIL
-B- NYLON
0.5
WTR
0~:===5=~~
ATR
I
Fig. 3.
"
-0.5
(Gimbel) Line graph of mean Cravy-calculated induced astigmatism depicting WTR cylinder decay over time for each suture material group.
-1
TIME
-1.5~r-.-.---.-------------.----------------------.
1
o
3 W
2 3 M M
5 M
1 VR
The pattern ofWTR cylinder decay was different in the nylon group. Mean ATR shift between two months and one year was -0.47 D, with 21 % of cases shifting in the ATR direction by more than one diopter. Between one and two years, the mean ATR shift was -0.6 D and 32 % of nylon cases had ATR shifts greater than 1.0 D, showing that, unlike Mersilene, much of the overall ATR drift in the nylon group occurred after one year. The Prolene group also had a significant mean decrease in WTR cylinder (0.8 D) during the first three months (Figure 3), but this leveled off between three and six months. Since so little « 0.5 D) WTR was induced at one day, by six months these cases had mean ATR cylinder of over half a diopter. After six months, the decay stabi-
WTR
Fig. 4.
ATR
(Gimbel) Bar charts of percent of cases with> 1 D of ATR cylinder shifts between one year and two years for each suture material group.
2 VR
lized. Between one and two years, 16% of cases had ATR drift> 1.0 D (Figure 4). Although 1.45 D of total induced cylinder had been induced in the Novafil group at one day (Figure 2), it was almost evenly split between WTR and ATR, resulting in less than 0.5 D of mean WTR cylinder for this group at one day (Figure 3). By two months, the proportions ofWTR and ATR cylinder for these cases were similar. Through one year, there was no large preponderance of either WTR or ATR cylinder shifts in this group; however, between one and two years, 17% of cases did have > 1.0 D of ATR shift. To assess whether data indicating late shifts in axis from one to two years were affected by patient attrition between one and two years (see Table 1), the data were analyzed by the Cravy method using only cases that were examined at one day, two months, three months, one year, and two years. The pattern of late ATR drift was the same (Figure 5). Table 4 shows the percentage of cases with unaided visual acuities of 20/40 or better preoperatively and at the various postoperative visits. The unaided visual results were comparable, with no statistically significant differences among suture types at any period. The percentage of cases with 20/40 or better acuity at one day after surgery was 40% to 62%. By two months, 74% to 81 % had acuities of 20/40 or better. Twenty-nine percent to 42% had acuities of 20/25 or better. This level of visual acuity was maintained through one year. At two years, 68% to 81 % of the cases that returned had unaided visual
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
47
acuities of20/40 or better and 32% to 43%,20/25 or better. DISCUSSION Commonly, when analyzing induced cylinder, the means and ranges of dioptric power changes from the preoperative to the postoperative measurements are compared without regard to any cylinder axis changes. Thus, surgically induced cylinder change will be accurately described only for cases in which the postoperative axis is exactly the same as the preoperative cylinder. For example, a preoperative cylinder of + 1.00 X 90, converted to a postoperative cylinder of + l.00 X 180, would appear to have no change in cylinder when, in fact, a +2.00 D change in cylinder occurred at axis 180. Computing induced cylinder is more complex if the postoperative axis is neither the same as nor 90 degrees away from the preoperative value . Naylor's equations are based upon a trigonometric solution for the resultant of two obliquely crossed cylinders (vector analysis programs). Instead of solving for the resultant cylinder, however, the equations find the second component (surgically induced cylinder power and axis) from a knowledge of the first component (preoperative cylinder power and axis) and the resultant (postoperative cylinder power and axis). Vector analysis is gaining more widespread use and has been used to evaluate induced cylinder following cataract extraction 4 ,5 and following astigmatic keratotomy. 6 , 7 Many factors determine the amount of surgically
I
Fig. 5 .
(Gimbel) Line graph of mean Cravy-calculated induced astigmatism depicting WTR cylinder decay over time for each suture material group. Cases were examined at the one day, two week , two month, three month, one year, and two year visits.
N D U C
1.5
D
0.5
-+- MERSILENE
-+- NOVAFIL
-B- NYLON
E
A S T I
G
0 -0 .5
M
A
T I S M 48
induced astigmatism, the decay rate of that induced astigmatism, and the possible drift toward ATR astigmatism. In this study, one factor (the way the incision is made) was constant for all groups. The type of closure was also standard for all groups. However, the effect of suture type on the judgment of suture tension using a continuous suture can influence the amount of induced cylinder from tight or loose sutures. In this study, the surgeon's greatest experience and, therefore, probably best judgment on suture tension was with the Prolene material. Even though most surgeons use nylon, one of the authors (H.V.C.) does not because of its biodegradability; it can break and extend through conjunctiva, irritating the eye. Also, reports by Byes ("Nylon Biodegradation and Late Post-op Astigmatism Increase-the Mersilene Advantage," presented at the Welsh Cataract Congress, November 1988) and Cravy 8 indicate that use of nylon suture, which eventually dissolves causing the wound to relax late, may lead to ATR drift in astigmatism years after surgery. We, in fact, saw a dramatic ATR drift between one and two years postoperatively with the use of nylon. Prolene does not hydrolyze and biodegrade and is more wiry than nylon or Novafil. Prolene slides quite easily through the tissue so we did not see much early induced WTR cylinder, as with Mersilene. However, with the tension not maintained, the looser wound led to an early drift toward ATR astigmatism, which stabilized after six months. Mersilene is very wiry, more so than Prolene.
- 1
-1.5 1 3 2 3 D W MM
TIME 1 VR
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
2 VR
Table 4. Percent of cases with postoperative VAse of 20/40 or better. Postoperative Period
Prolene
Mersilene
Novafil
Nylon
20/15-20/25 20/30-20/40 2-3 weeks
11 (21 %) 17 (33%)
7 (16%) 19 (42%)
8 (16%) 12 (24%)
5 (10%) 26 (52%)
20/15-20/25 20/30-20/40 2 months
11 (22%) 25 (51 %)
10 (23%) 18 (41%)
17 (36%) 20 (43%)
13 (28%) 21 (44%)
20/15-20/25 20/30-20/40 3 months
20 (38%) 21 (41 %)
18 (42%) 15 (34%)
18 (38%) 17 (36%)
14 (29%) 25 (52%)
20/15-20/25 20/30-20/40 5-6 months
15 (33%) 19 (41 %)
14 (38%) 16 (43%)
15 (38%) 13 (32%)
14 (33%) 19 (46%)
20/15-20/25 20/30-20/40 1 year
10 (30%) 13 (39%)
12 (44%) 7 (26%)
17 (46%) 15 (40%)
14 (38%) 15 (40%)
20/15-20/25
16 (41 %)
20/30-20/40 2 years
17 (44%)
17 (46%) 10 (27%)
11 (30%) 18 (49%)
17 (44%) 10 (26%)
20/1.5-20/25 20/30-20/40
9 (33%)
9 (35%)
10 (43%)
8 (32%)
12 (44%)
12 (46%)
6 (26%)
9 (36%)
1 day
When pulled tightly it induces a high level ofWTR cylinder. It then holds tightly because of its lack of stretchability, maintaining a higher level of WTR cylinder over time than do other suture types. This was the pattern we observed. Even though the decay rate was slower, eventually the level of induced cylinder came down, with levels comparable to the other suture types by a year. The manufacturer had recommended the use of 11-0 rather than 10-0 suture based on the belief that 11-0 Mersilene was equivalent in strength to 10-0 nylon. This thinner suture probably "cheese-wires," cutting through tissue to release the tension. One author (H.V.C.) had no previous clinical experience with Novafil and included it in the study for completeness. The rapid decay of induced WTR astigmatism in the Novafil group may relate to the stretchability of that material, which quickly forgives an overtight adjustment of the suture. Prolene and nylon seem to fall midway between Mersilene and N ovafil in this regard. In retrospect, the two weaknesses of the comparability of groups in the study were the use of 11-0 rather than 10-0 Mersilene and the unavoid-
able differences in knot-tying technique. The amount of early induced WTR cylinder is believed to be related to the knot-tying technique (in turn, due to the handling characteristics) and to tissue gripping characteristics (ease of sliding through tissue). The shape of the decay curve, however, is related to the material characteristics of the suture. This study, carried out over 21/2 years, resulted in absolute levels of surgically induced cylinder, which reflected the incision techniques used at the time. The surgeon's (H.V.C.) incision technique is currently very different. Nevertheless, the relative differences among suture materials do apply to current techniques. In general, we are concerned with apposing the external aspect of the wound with sutures and have not been concerned with the internal aspect. Not all patients develop ATR drift, even in groups in which the means clearly shift in that direction. Perhaps cases in which the internal aspect of the wound is well apposed are the ones that do not demonstrate the ATR drift. If this is true, there may be aqueous access to the large portion of the incision in cases in which the internal corneal aspect of
J CATARACT REFRACT SURG-VOL 18, JANUARY 1992
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internal architecture may prevent late ATR drift (Figure 6). REFERENCES
Fig. 6.
(Gimbel) Aqueous access to inner aspect of incision may cause permanent gaping and, therefore, late ATR.
the incision is not apposed. This could cause permanent gaping, wound slippage, and late ATR drift. If this hypothesis is correct, then self-sealing
.50
l. Conceptual Software, Inc. Professional Database Analysis System (PRODAS) Reference Manual, Version 3.0, Houston, 1985 2. Naylor EJ. Astigmatic difference in refractive error. Br J Ophthalmol1968; 52:422-424 3. Cravy TV. Calculation of the change in corneal astigmatism following cataract extraction. Ophthalmic Surg 1979; 10: 38-49 4. Jaffe NS, Clayman HM. The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol1975; 79:615-630 5. Neumann A, McCarty G, Sanders D, Raanan M. Small incisions to control astigmatism during cataract surgery. J Cataract Refract Surg 1989; 15:78-84 6. Thornton SP, Sanders DR. Graded nonintersecting transverse incisions for correction of idiopathic astigmatism. J Cataract Refract Surg 1987; 13:27-31 7. Neumann A, McCarty G, Sanders D, Raanan M. Refractive evaluation of astigmatic keratotomy procedures. J Cataract Refract Surg 1989; 15:25-31 8. Cravy TV. Long-term corneal astigmatism related to selected elastic, monofilament, nonabsorbable sutures. J Cataract Refract Surg 1989; 15:61-69
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