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Effect of incision direction on refractive outcome after radial keratotomy
Effect of incision direction on refractive outcome after radial keratotomy Gerald W. Flanagan, OD, Perry S. Binder, MD
ABSTRACT Purpose: To determine whether the direction of radial keratotomy (RI<) incisions (centripetal versus centrifugal) affects refractive outcome. Setting: Private ophthalmology office. Methods: The database of a single surgeon was retrospectively reviewed. Stepwise regression was used to select significant predictors of refraction change in the population. In addition to incision direction, variables evaluated were optic zone diameter, number of incisions, patient age, corneal curvature, and planned incision depth. Results: All variables except planned incision depth and corneal power affected refractive outcome. After controlling for number of incisions, optic zone diameter, and patient age, centripetal incisions decreased myopia 0.87 diopters more than centrifugal incisions. Conclusion: Our results, consistent with previous investigations, found that number of incisions, optic zone diameter, and patient age were significant predictors of refractive outcome after RK. Incision direction was also a significant predictor by itself or coupled with optic zone diameter and number of incisions, with the centripetal incision decreasing myopia more. J Cataract Refract Surg 1996; 22: 915-923
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hen introduced in the United States, radial keratotomy (RK) was performed using razor blade fragments adjusted under the operating microscope against a linear block gauge; corneal thickness was measured by optical pachymetry. I As many as 32 incisions were made, beginning in the limbus, crossing the vascular arcade in a centripetal (uphill) direction, and continuing the incision to the edge of optical clear zone diameters as small as 2.5 mm. 1 In hopes of improving refractive outcome and under the influence of Leo Bores, MD, and the Prospective From the Ophthalmology Research Laboratory of the National Vision Research Institute, San Diego, California. Reprint requests to Perry S. Binder, MD, Vision Surgery and Laser Center, 8910 University Center Lane, San Diego, California 92122.
Evaluation of Radial Keratotomy (PERK) study,2 incision direction was changed to centrifugal (downhill). The goal was to increase safety by avoiding the risk of incisions in the visual axis. 2 The PERK and other studies 3- 6 found that patient age, number of incisions, and incision length and depth are the major determinants of refractive outcome. Centripetal incisions create significantly deeper wounds than centrifugal incisions performed by the same surgeon with the same blade extension? The deeper wounds are created by mechanical factors inherent in the cornea and the anatomy of the diamond blade,8,9 suggesting that refractive effect would be greater for the same number of incisions. Clinical studies have found that greater refractive results are obtained . hcentnpet . al'InCISlOns.' .. 10 II Berke1eyan d coauthors 12 Wit
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EFFECf OF RK INCISION DIRECfION
ceased randomizing cases after finding that centripetal incisions reduced patient refractive errors more than 2.00 diopters (D) compared with contralateral eyes that had RK using centrifugal incisions created with the same blade extension. In another study,13 91.6% of a single surgeon's RK cases achieved a refraction between :t2.00 0 with centripetal incisions compared with 63% with centrifugal incisions. To test the hypothesis that centripetal incisions create more refractive correction than similar-length centrifugal incisions, we did a retrospective study of one surgeon's RK cases. We compared the results of centripetal incisions with those of centrifugal incisions to assess the importance of incision direction as a predictor of outcome.
Subjects and Methods This retrospective study reviewed the database of RKs performed by one surgeon (P.S.B.) between January 1, 1983, and February 1, 1994. Excluded were enhancements, reoperations, and cases unavailable for or lost to follow-up. Also excluded were cases with two incisions only or an optical zone diameter larger than 5.0 mm because these operative parameters were used only for astigmatic keratotomies (AKs). Sixteen incision cases were excluded because only nine were done between 1983 and early 1985. The remaining database included AK (simultaneous correction of myopia and astigmatism) and simple RK procedures that met the eligibility criteria. Because the surgeon used similar surgical indications and techniques during the study period and all age ranges and myopic errors were included, the data analyzed were considered to be representative of all cases. From 1983 through 1986, RKs were done using centrifugal incisions (Figure O. By 1989, 95% were done using centripetal incisions.
Surgical Technique The centripetal incisions were made with three anatomically similar front- and back-cutting diamond knives (Mastel). Blade extension was set at 100% of the thinnest of four paracentral ultrasonic readings (Corneascan, Storz Instruments) obtained at the edge of a 3.0 to 4.0 mm optical clear zone. Blade extension was confirmed using one or a combination of the following: a 916
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Figure 1. (Flanagan) Proportion of all RK procedures using a centripetal or centrifugal incision by year of surgery.
coin gauge observed at maximum magnification with monocular viewing under a Week operating microscope equipped with a fixation light; the micrometer handpiece; a Mastel microscope. A back-cutting, oblique diamond knife was used to create centrifugal incisions with a blade extension set to 115 to 120% of the thinnest paracentral reading.
Data Collection Visual acuity was recorded in condensed 10 foot lanes using the standard Snellen projector acuity chart. Refractions were recorded as the spherical equivalent of the noncycloplegic refraction. Data were collected by the surgeon and entered into a computer using Microsoft Excel (version 4.0) spreadsheet software.
Data Analysis Data were analyzed to determine (1) the magnitude of the effect of incision direction on change in spherical equivalent refraction when placed in a regression model with other significant independent variables and (2) the order of importance of predictor variables relative to those other investigators found significant. In addition to patient age, the following variables were eligible for stepwise regression: optical zone, incision number, and incision direction. Centripetal incisions were coded as 1, centrifugal as o. No interaction or exponential terms were used. Blade extension was assessed as planned incision depth for use in the regression model. Achieved incision depth was not measured postoperatively with pachymetry as has been reported in other investigations. 14 Average corneal curvature was obtained with a Bausch & Lomb keratometer. The mean keratometry
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EFFECT OF RK INCISION DIRECTION
reading for each eye was used for stepwise regression. Spherical equivalent change, the dependent variable, was defined as postoperative change in spherical equivalent refraction. The surgeon predominantly used downhill incisions before 1986, switching to uphill incisions thereafter (Figure 1). Thus, the date of surgery would seriously confound the relationship between spherical equivalent change and incision direction if postoperative refractions changed as a function of time due to changes in technique or improvements in instrumentation. Therefore, an analysis of covariance was used to determine whether incision direction was significantly associated with spherical equivalent refractive change when adjusted for the continuous variable- date of surgery. Homogeneity of slopes assumption was met as the interaction term exceeded P = .05. When the model included incision direction adjusted for surgery date, P = .03. This showed (1) the mean refractive change in the two incision groups differed after controlling for date of surgery; (2) the slopes of the lines in a plot of refraction change versus surgery date for both incision techniques were equal and parallel; there was no significant interaction effect between date and incision direction. Stepwise regression was used to determine the importance of the variables in affecting spherical equivalent refraction change. Other RK investigators 4 - 6 used this method to determine the best subset of predictive variables from their respective RK data sets. More complete discussions of this technique have been published. 15-17
The first step in fotward stepwise regression calls for a separate F-statistic computation for each independent variable against the dependent variable. The independent variable with the highest F-statistic that meets the predetermined significance level of 0.15 is selected for step 1 of the stepwise model. An alpha level of 0.15 is commonly used in stepwise regression when the predictor variables are relatively independent. The alpha level is lowered when the variables are highly correlated. 16 A partial F-statistic is then recomputed separately for the remaining independent variables and added to the regression model at successive steps one variable at a time if the partial F-score meets the 0.15 alpha level criteria. If more than one variable meets the criteria at any single step, only the most significant enters the model. The stepping process is continued until no more variables meet the significance level. Occasionally, a variable may drop out when a new variable enters at a later step. This occurs when the partial F-statistic, which is recomputed for the variables inside the model as well as those outside, becomes insignificant (P > .15) for that previously entered variable. The final model includes all variables that have coefficients associated most strongly with the outcome variable. This subset of predictive variable coefficients should explain a large proportion of the variability of spherical equivalent change, which is estimated by the coefficient of determination, or R2.
Results Table 1 shows the characteristics of the RK population analyzed. Mean follow-up (±SD) was 30.7 ±
Table 1. Patient demographics. Incision Type (Mean ± SO) Total Cases
Centrifugal
Centripetal
36.91 :!: 9.12
36.09 :!: 9.40
37.15:t 9.04
Preop
-4.30:!: 2.05
-4.06:!: 1.76
-4.38 :!: 2.14
Postop
-0.92 :!: 1.47
-1.10:!: 1.90
-0.85:!: 1.28
20/30
20/30
20/30
Age (years) Refraction (D)
Median postop BCVA Number of incisions
6.04:!: 2.00
6.48:t 1.95
5.91 :!: 2.00
Optical zone diameter (mm)
3.45 :!: 0.56
3.40:!: 0.49
3.46:t 0.58
Mean preop keratometry (D)
44.09:!: 1.46
44.06:!: 1.69
44.10:!: 1.38
BCVA = best corrected visual acuity
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EFFECT OF RK INCISION DIRECTION
Table 2.
Comparison of operative factors in four studies of RK predictor variables.
Study Sanders 6 Arrowsmith PERK
S
4
Current study
Number of Eyes
Incision Number
Optical Zone (mm)
Minimum Depth (%)
Incision Direction
Maximum Follow-up (months)
AK?
290
8,16
2.5-5.0
80
Centrifugal
24
No
0.87 to 10.25
Yes (2)
156
8, 16
3 minimum
85
Centripetal
12
No
1.50 to 12.00+
Yes
411
8
3.0, 3.5, 4.0
100
Centrifugal
12
No
2.00 to 8.00
No
427
4,8
3.0-5.0
100
Both
111
Yes
1.00 to 18.87
No
Table 3. Average change in spherical equivalent refraction (D) by incision direction as a function or preoperative refraction. Mean Change in Refraction ± SO
Preoperative Myopia
Centrifugal
Centripetal
1.00 to 2.87
1.80 ::+:: 2.05
1.73::+:: 0.67
3.00 to 4.87
2.92::+:: 1.46
3.05::+:: 1.04
5.00 to 6.87
4.19::+:: 1.94
4.81 ::+:: 1.08
7.00 to 8.87
4.64::+:: 2.61
6.83::+:: 1.98
31.4 months (range 1 to 77 months) in the centrifugal group and 8.2 ± 11.4 months (range 2 to 11 months) in the centripetal group. Table 2 compares our population with those in previous studies.
Stepwise Regression Analysis Centripetal incisions were used in 359 (73.4%) of primary RK procedures over the 10 year study period and centrifugal incisions in 130 (26.6%). Complete data were available for 427 cases in the final regression model from an estimated population of 900 procedures; RK, AK, or both were performed in 791 eyes, with 367 not meeting the inclusion criteria. Postoperatively, 54.87% of all patients achieved 20/40 or better visual acuity (52.13% centrifugal, 55.82% centripetal); 61.01 % of all patients attained a refraction between + 1.00 and -1.00 D (63.55% centripetal, 54.10% centrifugal). When categorized by preoperative refraction, centripetal incisions led to a greater effect than centrifugal incisions in all but the least myopic groups (Table 3). The difference between the groups increased with increasing baseline refraction (Figure 2). Table 4 shows the difference in refractive effect when the two incision groups are stratified by incision number and optical clear zone. Decreasing optical clear 918
Preop Myopia (D)
Reoperation?
zone diameter and increasing incision number increased refractive effect, suggesting that an interaction effect may have occurred between incision direction, optical zone, and incision number. The significance of incision direction in predicting the change in refraction is shown in Table 5, which describes the results of the stepwise regression for the surgeon's 1984 through 1994 RK database. Incision direction was the fourth variable selected and increased R2 by 0.04. The alpha level for entry and removal was set at 0.15, with corneal curvature (P = .50) and planned incision depth (P = .48) failing to enter the model. In addition to incision direction, the best subset of predictive variables included incision number, optic zone diameter, and patient age. Incision number was the first to enter the regression model and explains the greatest amount of variability in refractive change (23%). Patient age entered second and added 0.08 to the squared multiple correlation value. Optical zone diameter added 0.07 and incision direction 0.04 when they entered the model at steps 3 and 4, respectively.
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Figure 2. (Flanagan) Mean refractive change for both incision directions categorized by preoperative refraction (SE = spherical equivalent).
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EFFECT OF RK INCISION DIRECTION
Table 4. Reduction in myopic refraction as function of incision direction, stratified by incision number and optic zone. Reduction in Refraction (0) Centrifugal
Centripetal
Incision Number
Optical Zone (mm)
Mean ± SO
Number
Mean ± SO
Number
4
3.75-5.00
2.13 ± 1.39
19
1.78 ± 0.93
72
4
3.00-3.50
2.58 ± 1.72
27
2.76 ± 1.13
93
8
3.75-5.00
2.75 ± 1.32
19
3.60 ± 1.59
31
8
3.00-3.50
3.18 ± 2.45
65
4.22 ± 2.51
163
Table 5.
Results of forward stepwise regression for RK population from 1984 through 1993; spherical equivalent change outcome variable. Variable Entered
Multiple R2
Change in R2
Incision number
0.23
0.23
Step Number
2
Age
0.31
0.08
3
Optical zone
0.38
0.07
4
Incision direction
0.42
0.04
Once the subset of predictor variables was isolated, the coefficients were estimated with an adjustment for the linear effects of the other model variables in a multiple linear regression model. Coefficient estimates were -0.47 for incision number, -0.06 for age, 1.07 for optical zone, and -0.87 for incision direction (P< .0001 in all cases). In addition to testing the relative importance of the independent variables, the estimated coefficients helped predict the decrease in nearsightedness. For example, since the coefficient for incision number was -0.47, a four-incision RK can be expected to reduce myopia approximately 1.88 D (4 X -0.47). An eight-incision RK would contribute another 1.88 D decrease for a total of 3.76 D (8 X -0.47). Going from a 3.0 to a 4.0 mm optical zone will yield about 1.00 D (1.07 X 1.00 mm) less refractive effect. A 10 year increase in a patient's age decreases myopia approximately 0.60 D (10 X -0.06). Using centripetal incisions instead of centrifugal incisions increases the RK effect by 0.87 D. These coefficients were estimated while controlling for the effects of the other variables. The predictive equation is SEChg = - 1.40 - (0.06)(age) - (0.47)(IN)
+ (1.06)(OZ)
- (0.87)(10)
where SEChg = spherical equivalent change; IN = incision number; OZ = optical zone; ID = incision direction.
Interaction Analysis Centripetal incisions appeared to potentiate the refractive effect of smaller optical zones and greater numbers of incisions more than centrifugal incisions (Table 4, Figure 3). To test this hypothesis, we performed another stepwise regression analysis with the eligible variables being only those found significant along with three incision-direction interaction terms. The incision-direction interaction terms were ID X IN, ID X OZ, and ID X age. The noninteraction terms entered the model in the same order described above with the exception of ID. In its place, ID X IN was added fourth and ID X OZ was added fifth. Incision direction as a univariate term and the ID X age interaction term did not enter the model. The addition of the two interaction terms in the place of incision direction alone added an
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Figure 3. (Flanagan) Change in spherical equivalent refraction for both incision direction techniques stratified by incision number and optic zone diameter (02 = optical zone; SE = spherical equivalent).
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EFFECT OF RK INCISION DIRECTION
additional 1% to the explained variability of the model (R2 = .43). Planned incision depth (blade extension) was significantly more shallow in the centripetal group (0.54 ± 0.04 mm) than in the centrifugal group (0.61 ± 0.08 mm) (P> .0001) because blade extension was set for 100% of thinnest pachymetry in the former and blade extension reached 120% in the latter.
Discussion When RK was first introduced, the recommended incision direction was centripetal. 1 The direction was changed with the PERK studl to avoid the optical complications of incisions within the visual axis documented to occur with centripetal incisions. Nonetheless, the enhanced refractive effect of centripetal incisions supports previous studies 1,7-9,18 that found centripetal incisions create deeper incisions than centrifugal incisions in animals and human pathologic specimens. Berkeley and coauthors 12 suggested centripetal incisions produce more than 2.00 D greater effect than centrifugal incisions in a randomized trial cut short for ethical reasons. A higher percentage of Lee's RK group13 attained postoperative refractions between +2.00 and -2.00 D when incisions were made centripetally. In that study, centripetal incisions were judged to be more uniform and deeper. Subsequent unpublished clinical reports suggested that fewer incisions were used when blade settings were increased and centripetal incisions used. The heightened effect may be caused by mechanical factors 9 and possibly by differences in the wound-healing response with different wound configurations. 18 The R2 for our model was 0.42, which agrees closely with PERK results of 0.44.4 The R2 value ranged from 0.54 (24 month follow-up, steel blade) to 0.80 (1 month follow-up, steel blade) in a study by Arrowsmith and Mark. 5 The R2 values in the diamond blade population was 0.74 after 2 years. In studies by Sanders and co authors 6 and Deitz and coauthors,19 R2 varied between 0.56 and 0.73 depending on the postoperative period and the inclusion of interaction terms in the model. Several factors can explain why R 2 was smaller in our study than in other studies using stepwise regression. First, mean follow-up in our study was longer than in the other studies (Table 2). Several studies have confirmed that a postoperative hyperopic shift occurs so 920
that cases with longer follow-up might seem to have an enhanced reduction of myopia. 19 - 23 The surgeon's change in incisional technique beginning in 1986 introduces a follow-up bias that creates this confounding and weakens the relationship between spherical equivalent change and incision direction, lowering R2. Second, 33.6% of our group included AKs with transverse incisions that may disturb the effect of the radial incisions. 24 The other reports do not mention usingAKs in their analyses. Our preliminary results suggest that AK cases do not achieve the same refractive results as simple RK cases. Third, if the blade extensions were the same for both directional techniques, the centripetal group would have shown a larger effect in reducing the myopic error. In our study population, however, blade extension was set to 110 to 120% of paracentral readings for centrifugal incisions and 100% for centripetal. If this confounding variable had been controlled for in the final model, it could have increased the explained variability. N onetheless, the results suggest centripetal incisions reduced the myopic refractive error 0.87 D more than centrifugal InCISIOns.
Finally, incision number and enhancements may have contributed to a reduced R2. We included procedures with 4 and 8 incisions, whereas the other investigators used 8 and/or 16 incisions in the analyses. We also excluded enhancement cases. Deitz and coauthors 21 and Sanders and coauthori used secondary procedures in only two cases. The PERK group performed enhancements but did not use the data in their regression analysis. 4 Arrowsmith and coauthors25 ,26 also found redeepening of incisions a consistent predictive factor; they redeepened incisions when uncorrected visual acuity was worse than 20/40. Most of the other reported surgical variables do not differ significantly from ours (Table 2). It has always been the surgeon's (P.S.B.) goal to leave eyes undercorrected after surgery. When the clinical indication(s) suggested a possibility of achieving close to the desired refractive result with four incisions, four incisions were used. In many cases of undercorrection, enhancements were performed when the uncorrected visual acuity was worse than 20/40. In the current study, 18.50/0 offour-incision cases (17.4% centripetal, 22.2% centrifugal) had enhancements compared with 14.0% of the eight-incision cases (15.5% centripetal,
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EFFECT OF RK INCISION DIRECTION
Table 6. Summary of four RK studies using stepwise regression-final models.
Variable
Order
Incision number
Arrowsmith5 *
Sanders6
Current Study Coefficient
Order
Coefficient
Order
PERK4
Coefficient
Order
Coefficient
0.520
80nlyt
NA
-0.47
3
0.189
Optic zone
2
1.07
5
-0.289
6
IT
Age
3
-0.06
0.044
7
IT
2
-0.065
Incision direction
4
-0.87
NA
Entered
NA
Entered
NA
Entered
Planned depth
DNE
DNE
NA
NA
DNE
DNE
NA
NA
Achieved depth
NA
NA
2
0.077
2
-2.43
3
DNE
DNE
4
0.235
NA
NA
DNE
Average keratometry
-1.630
0.020 DNE
DNE = did not enter; NA = not applicable; IT = coupled as an interaction term * Diamond knife cases only; incision number dropped from model before final step
t Only 8 incision cases included
9.9% centrifugal). In a study by Salz and coauthors,27 15% of eyes had enhancements. The failure of planned incision depth (blade extension) to predict refractive outcome in this population may be a result of the systematic and random errors associated with setting the blade and performing the incisions. Arrowsmith and Marks 23 .28 found planned incision depth an inconsistent predictor in their populations, whereas optic zone diameter, age, and achieved incision depth consistently affected refractive outcome in both the steel blade and diamond blade populations. Neither Sanders and coauthors 6 not the PERK group4 used planned incision depth in their models; they did, however, find achieved depth to be associated with refraction change (Table 5). In addition to achieved depth, the PERK group 4 also established age and optical zone as important predictors of outcome. They controlled incision number by using only eight incisions and were thus unable to compare the effect on refractions. Table 6 shows the results of stepwise regression for these three other studies ofRK predictors compared with those of the current study. Sanders and coauthors6 concluded that keratometry (corneal power) was an important predictor of outcome in RK. They found that incision number, optic zone diameter, achieved incision depth, age, and keratometry readings entered their stepwise model at P < 0.15. In their data set, steep corneas (greater than 46.00 D) changed more than flat corneas (between 42.00 and 46.00 D) when all other variables were controlled. These
results are at odds with the PERK study,29 which found that flatter corneas changed more than steep corneas when curvature was measured with a corneascope. Recent studies of the accuracy and predictability of the corneascope have shown the instrument is too inaccurate for clinical measurement,30-33 casting doubt on the PERK study conclusions on predictability of preoperative corneal power on refractive outcome. Nevertheless, corneal curvature as measured with a Bausch & Lomb keratometer failed to reliably predict change in refraction in our population. The increased refractive effect of centripetal incisions is most evident in cases with smaller optical clear zones and more incisions (Figure 3). Construction of a second regression model that analyzed interaction supported the theory of an enhanced effect with centripetal incisions. The model with a univariate directional term is supplanted by bivariate interaction terms that coupled incision direction with diameter of the optical clear zone and incision number (ID X IN and ID X OZ). This finding suggests that the longer incisions are and the more incisions used, the greater their effect in reducing myopia. Thus, one can conclude that centripetal incisions potentiate the effect of smaller optical clear zones and greater numbers of incisions more than centrifugal incisions. The increase in R2 is modest, however, when the two models are compared. The univariate directional model increased R2 by 0.037 when it entered the model at the last step; it improved by only 0.045 after entry of the two interaction terms.
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EFFECT OF RK INCISION DIRECTION
Conclusion The direction of RK incisions is a more important predictor of outcome than planned incision depth and keratometry readings. Also, after adjusting for the effects of optic zone, incision number, and patient age, centripetal incisions increased the refractive change effect ofRK by 0.870 over centrifugal incisions in this population.
References
16. 17.
18.
1. Fyodorov SN, Durnev W. Operation of dosaged dissection of corneal circular ligament in cases of myopia of mild degree. Ann Ophthalmol1979; 11:1885-1890 2. Waring GO III, Moffitt SD, Gelender H, et al. Rationale for and design of the National Eye Institute Prospective Evaluation of Radial Keratotomy (PERK) study. Ophthalmology 1983; 90:40-58 3. Waring GO III. Refractive Keratotomy for Myopia and Astigmatism. St Louis, CV Mosby, 1992 4. Lynn M], Waring GO III, Sperduto RD, and the PERK Study Group. Factors affecting outcome and predictability of radial keratotomy in the PERK study. Arch Ophthalmol1987; 105:42-51 5. Arrowsmith PN, Marks RG. Evaluating the predictability of radial keratotomy. Ophthalmology 1985; 92:331338 6. Sanders DR, Deitz MR, Gallagher D. Factors affecting the predictability of radial keratotomy. Ophthalmology 1985; 92:1237-1243 7. Melles GR], Binder PS. Effect of radial keratotomy incision direction on wound depth. Refract Corneal Surg 1990; 6:394-403 8. Melles GR], Binder PS. Effect of wound location, orientation, direction, and postoperative time on unsutured corneal wound healing morphology in monkeys. Refract Corneal Surg 1992; 8:427-438 9. Melles GR], Wijdh RH], Cost B, et al. Effect of blade configuration, knife action, and intraocular pressure on keratotomy incision depth and shape. Cornea 1993; 12: 299-309 10. Hachet E. Etude comparative des incisions radiaires centrifuges et centripetes corneennes. Bull Soc Ophtalmol Fr 1989; 89:573-574 11. Haverbeke L. La Keratotomie radiaire doit-elle se pratiquer encentripete ou en centrifuge? Bull Soc Belge Ophtalmol1989; 234:1-8 12. Berkeley RG, Sanders DR, Piccolo MG. Effect of incision direction on radial keratotomy outcome. ] Cataract Refract Surg 1991; 17:819-823 13. Lee CPo Radial keratotomy: in-to-out and out-to-in. Preliminaty results in the Singapore general hospital. Ann Acad Med Singapore 1989; 18:141-150 14. Deitz MR, Sanders DR, Marks RG. Radial keratotomy: 922
15.
19.
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29.
30.
an overview of the Kansas City study. Ophthalmology 1984; 91:467-478 Kleinbaum DG, Kupper LL, Muller KE. Applied Regression Analysis and Other Multivariable Methods, 2nd ed. Boston, PWS-Kent Publishing, 1988; 324-327 Bendel D, Afifi A. Comparison of stopping rules in forward stepwise regression.] Am Stat Assn 1977; 72:46-53 Afifi M, Clark V. Computer-Aided Multivariate Analysis, 2nd ed. New York, Van Nostrand Reinhold, 1990; 196-203 Melles GR], Binder PS, Anderson]A. Variation in healing throughout the depth of long-term, unsutured, corneal wounds in human autopsy specimens and monkeys. Arch Ophthalmol 1994; 112: 100 -109 Deitz MR, Sanders DR, Raanan MG. Progressive hyperopia in radial keratotomy; long-term follow-up of diamond blade and metal blade series. Ophthalmology 1986; 93:1284-1288 Deitz MR, Sanders DR. Progressive hyperopia with longterm follow-up of radial keratotomy. Arch Ophthalmol 1985; 103:782-784 Deitz MR, Sanders DR, Raanan MG. A consecutive series (1982-1985) of radial keratotomies performed with the diamond blade. Am] Ophthalmol1987; 103:417422 Waring GO III, Lynn M], Kutner MH. Stability of refraction after refractive keratotomy. In: Waring GO III, ed, Refractive Keratotomy for Myopia and Astigmatism. St Louis, CV Mosby, 1992; 937-968 Arrowsmith PN, Marks RG. Visual, refractive, and keratometric results of radial keratotomy; five-year follow-up. Arch Ophthalmol1989; 107:506-511 Flanagan GW, Binder PS. Visual acuity in astigmatic keratotomy differs from radial keratotomy. ARVO abstract 2699. Invest Ophthalmol Vis Sci 1995; 36(suppl): S581 Arrowsmith PN, Sanders DR, Marks RG. Visual, refractive, and keratometric results of radial keratotomy. Arch Ophthalmol1983; 101:873-881 Arrowsmith PN, Marks RG. Visual, refractive and keratometric results of radial keratotomy; one-year follow-up. Arch Ophthalmol1984; 102:1612-1617 Salz]], Salz ]M, Salz M, Jones D. Ten years experience with a conservative approach to radial keratotomy. Refract Corneal Surg 1991; 7:12-22 Arrowsmith PN, Marks RG. Visual, refractive, and kerato metric results of radial keratotomy; a two-year followup. Arch Ophthalmol1987; 105:76-80 Rowsey]], Waring GO III, Monlux R, et al. Corneal topography as a predictor of refractive change in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Ophthalmic Surg 1991; 22:370-380 Zadnik K, Mutti DO, Adams A]. The repeatability of
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EFFECT OF RK INCISION DIRECTION
measurement of the ocular components. Invest Ophthalmol Vis Sci 1992; 33:2325-2333 31. Hannush SB, Waring GO III. Computer assisted corneal topography: accuracy and reproducibiliry with three instruments. ARVO abstract 39. Invest Ophthalmol Vis Sci 1988; 29(suppl):389 32. Hannush SB, Crawford SL, Waring GO III, et al. Accu-
racy and precision of keratometry, photokeratoscopy, and corneal modeling on calibrated steel balls. Arch Ophthalmol1989; 107:1235-1239 33. Hannush SB, Crawford SL, Waring GO III, et al. Reproducibiliry of normal corneal power measurements with a keratometer, photokeratoscope, and video imaging system. Arch Ophthalmol1990; 108:539-544
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ABSTRACT Purpose: To determine whether the direction of radial keratotomy (RI<) incisions (centripetal versus centrifugal) affects refractive outcome. Setting: Private ophthalmology office. Methods: The database of a single surgeon was retrospectively reviewed. Stepwise regression was used to select significant predictors of refraction change in the population. In addition to incision direction, variables evaluated were optic zone diameter, number of incisions, patient age, corneal curvature, and planned incision depth. Results: All variables except planned incision depth and corneal power affected refractive outcome. After controlling for number of incisions, optic zone diameter, and patient age, centripetal incisions decreased myopia 0.87 diopters more than centrifugal incisions. Conclusion: Our results, consistent with previous investigations, found that number of incisions, optic zone diameter, and patient age were significant predictors of refractive outcome after RK. Incision direction was also a significant predictor by itself or coupled with optic zone diameter and number of incisions, with the centripetal incision decreasing myopia more. J Cataract Refract Surg 1996; 22: 915-923
W
hen introduced in the United States, radial keratotomy (RK) was performed using razor blade fragments adjusted under the operating microscope against a linear block gauge; corneal thickness was measured by optical pachymetry. I As many as 32 incisions were made, beginning in the limbus, crossing the vascular arcade in a centripetal (uphill) direction, and continuing the incision to the edge of optical clear zone diameters as small as 2.5 mm. 1 In hopes of improving refractive outcome and under the influence of Leo Bores, MD, and the Prospective From the Ophthalmology Research Laboratory of the National Vision Research Institute, San Diego, California. Reprint requests to Perry S. Binder, MD, Vision Surgery and Laser Center, 8910 University Center Lane, San Diego, California 92122.
Evaluation of Radial Keratotomy (PERK) study,2 incision direction was changed to centrifugal (downhill). The goal was to increase safety by avoiding the risk of incisions in the visual axis. 2 The PERK and other studies 3- 6 found that patient age, number of incisions, and incision length and depth are the major determinants of refractive outcome. Centripetal incisions create significantly deeper wounds than centrifugal incisions performed by the same surgeon with the same blade extension? The deeper wounds are created by mechanical factors inherent in the cornea and the anatomy of the diamond blade,8,9 suggesting that refractive effect would be greater for the same number of incisions. Clinical studies have found that greater refractive results are obtained . hcentnpet . al'InCISlOns.' .. 10 II Berke1eyan d coauthors 12 Wit
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EFFECf OF RK INCISION DIRECfION
ceased randomizing cases after finding that centripetal incisions reduced patient refractive errors more than 2.00 diopters (D) compared with contralateral eyes that had RK using centrifugal incisions created with the same blade extension. In another study,13 91.6% of a single surgeon's RK cases achieved a refraction between :t2.00 0 with centripetal incisions compared with 63% with centrifugal incisions. To test the hypothesis that centripetal incisions create more refractive correction than similar-length centrifugal incisions, we did a retrospective study of one surgeon's RK cases. We compared the results of centripetal incisions with those of centrifugal incisions to assess the importance of incision direction as a predictor of outcome.
Subjects and Methods This retrospective study reviewed the database of RKs performed by one surgeon (P.S.B.) between January 1, 1983, and February 1, 1994. Excluded were enhancements, reoperations, and cases unavailable for or lost to follow-up. Also excluded were cases with two incisions only or an optical zone diameter larger than 5.0 mm because these operative parameters were used only for astigmatic keratotomies (AKs). Sixteen incision cases were excluded because only nine were done between 1983 and early 1985. The remaining database included AK (simultaneous correction of myopia and astigmatism) and simple RK procedures that met the eligibility criteria. Because the surgeon used similar surgical indications and techniques during the study period and all age ranges and myopic errors were included, the data analyzed were considered to be representative of all cases. From 1983 through 1986, RKs were done using centrifugal incisions (Figure O. By 1989, 95% were done using centripetal incisions.
Surgical Technique The centripetal incisions were made with three anatomically similar front- and back-cutting diamond knives (Mastel). Blade extension was set at 100% of the thinnest of four paracentral ultrasonic readings (Corneascan, Storz Instruments) obtained at the edge of a 3.0 to 4.0 mm optical clear zone. Blade extension was confirmed using one or a combination of the following: a 916
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Figure 1. (Flanagan) Proportion of all RK procedures using a centripetal or centrifugal incision by year of surgery.
coin gauge observed at maximum magnification with monocular viewing under a Week operating microscope equipped with a fixation light; the micrometer handpiece; a Mastel microscope. A back-cutting, oblique diamond knife was used to create centrifugal incisions with a blade extension set to 115 to 120% of the thinnest paracentral reading.
Data Collection Visual acuity was recorded in condensed 10 foot lanes using the standard Snellen projector acuity chart. Refractions were recorded as the spherical equivalent of the noncycloplegic refraction. Data were collected by the surgeon and entered into a computer using Microsoft Excel (version 4.0) spreadsheet software.
Data Analysis Data were analyzed to determine (1) the magnitude of the effect of incision direction on change in spherical equivalent refraction when placed in a regression model with other significant independent variables and (2) the order of importance of predictor variables relative to those other investigators found significant. In addition to patient age, the following variables were eligible for stepwise regression: optical zone, incision number, and incision direction. Centripetal incisions were coded as 1, centrifugal as o. No interaction or exponential terms were used. Blade extension was assessed as planned incision depth for use in the regression model. Achieved incision depth was not measured postoperatively with pachymetry as has been reported in other investigations. 14 Average corneal curvature was obtained with a Bausch & Lomb keratometer. The mean keratometry
J CATARACT REFRACT SURG-VOL 22, SEPTEMBER 1996
EFFECT OF RK INCISION DIRECTION
reading for each eye was used for stepwise regression. Spherical equivalent change, the dependent variable, was defined as postoperative change in spherical equivalent refraction. The surgeon predominantly used downhill incisions before 1986, switching to uphill incisions thereafter (Figure 1). Thus, the date of surgery would seriously confound the relationship between spherical equivalent change and incision direction if postoperative refractions changed as a function of time due to changes in technique or improvements in instrumentation. Therefore, an analysis of covariance was used to determine whether incision direction was significantly associated with spherical equivalent refractive change when adjusted for the continuous variable- date of surgery. Homogeneity of slopes assumption was met as the interaction term exceeded P = .05. When the model included incision direction adjusted for surgery date, P = .03. This showed (1) the mean refractive change in the two incision groups differed after controlling for date of surgery; (2) the slopes of the lines in a plot of refraction change versus surgery date for both incision techniques were equal and parallel; there was no significant interaction effect between date and incision direction. Stepwise regression was used to determine the importance of the variables in affecting spherical equivalent refraction change. Other RK investigators 4 - 6 used this method to determine the best subset of predictive variables from their respective RK data sets. More complete discussions of this technique have been published. 15-17
The first step in fotward stepwise regression calls for a separate F-statistic computation for each independent variable against the dependent variable. The independent variable with the highest F-statistic that meets the predetermined significance level of 0.15 is selected for step 1 of the stepwise model. An alpha level of 0.15 is commonly used in stepwise regression when the predictor variables are relatively independent. The alpha level is lowered when the variables are highly correlated. 16 A partial F-statistic is then recomputed separately for the remaining independent variables and added to the regression model at successive steps one variable at a time if the partial F-score meets the 0.15 alpha level criteria. If more than one variable meets the criteria at any single step, only the most significant enters the model. The stepping process is continued until no more variables meet the significance level. Occasionally, a variable may drop out when a new variable enters at a later step. This occurs when the partial F-statistic, which is recomputed for the variables inside the model as well as those outside, becomes insignificant (P > .15) for that previously entered variable. The final model includes all variables that have coefficients associated most strongly with the outcome variable. This subset of predictive variable coefficients should explain a large proportion of the variability of spherical equivalent change, which is estimated by the coefficient of determination, or R2.
Results Table 1 shows the characteristics of the RK population analyzed. Mean follow-up (±SD) was 30.7 ±
Table 1. Patient demographics. Incision Type (Mean ± SO) Total Cases
Centrifugal
Centripetal
36.91 :!: 9.12
36.09 :!: 9.40
37.15:t 9.04
Preop
-4.30:!: 2.05
-4.06:!: 1.76
-4.38 :!: 2.14
Postop
-0.92 :!: 1.47
-1.10:!: 1.90
-0.85:!: 1.28
20/30
20/30
20/30
Age (years) Refraction (D)
Median postop BCVA Number of incisions
6.04:!: 2.00
6.48:t 1.95
5.91 :!: 2.00
Optical zone diameter (mm)
3.45 :!: 0.56
3.40:!: 0.49
3.46:t 0.58
Mean preop keratometry (D)
44.09:!: 1.46
44.06:!: 1.69
44.10:!: 1.38
BCVA = best corrected visual acuity
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EFFECT OF RK INCISION DIRECTION
Table 2.
Comparison of operative factors in four studies of RK predictor variables.
Study Sanders 6 Arrowsmith PERK
S
4
Current study
Number of Eyes
Incision Number
Optical Zone (mm)
Minimum Depth (%)
Incision Direction
Maximum Follow-up (months)
AK?
290
8,16
2.5-5.0
80
Centrifugal
24
No
0.87 to 10.25
Yes (2)
156
8, 16
3 minimum
85
Centripetal
12
No
1.50 to 12.00+
Yes
411
8
3.0, 3.5, 4.0
100
Centrifugal
12
No
2.00 to 8.00
No
427
4,8
3.0-5.0
100
Both
111
Yes
1.00 to 18.87
No
Table 3. Average change in spherical equivalent refraction (D) by incision direction as a function or preoperative refraction. Mean Change in Refraction ± SO
Preoperative Myopia
Centrifugal
Centripetal
1.00 to 2.87
1.80 ::+:: 2.05
1.73::+:: 0.67
3.00 to 4.87
2.92::+:: 1.46
3.05::+:: 1.04
5.00 to 6.87
4.19::+:: 1.94
4.81 ::+:: 1.08
7.00 to 8.87
4.64::+:: 2.61
6.83::+:: 1.98
31.4 months (range 1 to 77 months) in the centrifugal group and 8.2 ± 11.4 months (range 2 to 11 months) in the centripetal group. Table 2 compares our population with those in previous studies.
Stepwise Regression Analysis Centripetal incisions were used in 359 (73.4%) of primary RK procedures over the 10 year study period and centrifugal incisions in 130 (26.6%). Complete data were available for 427 cases in the final regression model from an estimated population of 900 procedures; RK, AK, or both were performed in 791 eyes, with 367 not meeting the inclusion criteria. Postoperatively, 54.87% of all patients achieved 20/40 or better visual acuity (52.13% centrifugal, 55.82% centripetal); 61.01 % of all patients attained a refraction between + 1.00 and -1.00 D (63.55% centripetal, 54.10% centrifugal). When categorized by preoperative refraction, centripetal incisions led to a greater effect than centrifugal incisions in all but the least myopic groups (Table 3). The difference between the groups increased with increasing baseline refraction (Figure 2). Table 4 shows the difference in refractive effect when the two incision groups are stratified by incision number and optical clear zone. Decreasing optical clear 918
Preop Myopia (D)
Reoperation?
zone diameter and increasing incision number increased refractive effect, suggesting that an interaction effect may have occurred between incision direction, optical zone, and incision number. The significance of incision direction in predicting the change in refraction is shown in Table 5, which describes the results of the stepwise regression for the surgeon's 1984 through 1994 RK database. Incision direction was the fourth variable selected and increased R2 by 0.04. The alpha level for entry and removal was set at 0.15, with corneal curvature (P = .50) and planned incision depth (P = .48) failing to enter the model. In addition to incision direction, the best subset of predictive variables included incision number, optic zone diameter, and patient age. Incision number was the first to enter the regression model and explains the greatest amount of variability in refractive change (23%). Patient age entered second and added 0.08 to the squared multiple correlation value. Optical zone diameter added 0.07 and incision direction 0.04 when they entered the model at steps 3 and 4, respectively.
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Figure 2. (Flanagan) Mean refractive change for both incision directions categorized by preoperative refraction (SE = spherical equivalent).
J CATARACT REFRACT SURG-VOL 22, SEPTEMBER 1996
EFFECT OF RK INCISION DIRECTION
Table 4. Reduction in myopic refraction as function of incision direction, stratified by incision number and optic zone. Reduction in Refraction (0) Centrifugal
Centripetal
Incision Number
Optical Zone (mm)
Mean ± SO
Number
Mean ± SO
Number
4
3.75-5.00
2.13 ± 1.39
19
1.78 ± 0.93
72
4
3.00-3.50
2.58 ± 1.72
27
2.76 ± 1.13
93
8
3.75-5.00
2.75 ± 1.32
19
3.60 ± 1.59
31
8
3.00-3.50
3.18 ± 2.45
65
4.22 ± 2.51
163
Table 5.
Results of forward stepwise regression for RK population from 1984 through 1993; spherical equivalent change outcome variable. Variable Entered
Multiple R2
Change in R2
Incision number
0.23
0.23
Step Number
2
Age
0.31
0.08
3
Optical zone
0.38
0.07
4
Incision direction
0.42
0.04
Once the subset of predictor variables was isolated, the coefficients were estimated with an adjustment for the linear effects of the other model variables in a multiple linear regression model. Coefficient estimates were -0.47 for incision number, -0.06 for age, 1.07 for optical zone, and -0.87 for incision direction (P< .0001 in all cases). In addition to testing the relative importance of the independent variables, the estimated coefficients helped predict the decrease in nearsightedness. For example, since the coefficient for incision number was -0.47, a four-incision RK can be expected to reduce myopia approximately 1.88 D (4 X -0.47). An eight-incision RK would contribute another 1.88 D decrease for a total of 3.76 D (8 X -0.47). Going from a 3.0 to a 4.0 mm optical zone will yield about 1.00 D (1.07 X 1.00 mm) less refractive effect. A 10 year increase in a patient's age decreases myopia approximately 0.60 D (10 X -0.06). Using centripetal incisions instead of centrifugal incisions increases the RK effect by 0.87 D. These coefficients were estimated while controlling for the effects of the other variables. The predictive equation is SEChg = - 1.40 - (0.06)(age) - (0.47)(IN)
+ (1.06)(OZ)
- (0.87)(10)
where SEChg = spherical equivalent change; IN = incision number; OZ = optical zone; ID = incision direction.
Interaction Analysis Centripetal incisions appeared to potentiate the refractive effect of smaller optical zones and greater numbers of incisions more than centrifugal incisions (Table 4, Figure 3). To test this hypothesis, we performed another stepwise regression analysis with the eligible variables being only those found significant along with three incision-direction interaction terms. The incision-direction interaction terms were ID X IN, ID X OZ, and ID X age. The noninteraction terms entered the model in the same order described above with the exception of ID. In its place, ID X IN was added fourth and ID X OZ was added fifth. Incision direction as a univariate term and the ID X age interaction term did not enter the model. The addition of the two interaction terms in the place of incision direction alone added an
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.-
.1nCltlon 3.7S-5.00mmOZ
Incision Number and OZ Diameter
.-
3.00-3.50mmOZ
Figure 3. (Flanagan) Change in spherical equivalent refraction for both incision direction techniques stratified by incision number and optic zone diameter (02 = optical zone; SE = spherical equivalent).
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EFFECT OF RK INCISION DIRECTION
additional 1% to the explained variability of the model (R2 = .43). Planned incision depth (blade extension) was significantly more shallow in the centripetal group (0.54 ± 0.04 mm) than in the centrifugal group (0.61 ± 0.08 mm) (P> .0001) because blade extension was set for 100% of thinnest pachymetry in the former and blade extension reached 120% in the latter.
Discussion When RK was first introduced, the recommended incision direction was centripetal. 1 The direction was changed with the PERK studl to avoid the optical complications of incisions within the visual axis documented to occur with centripetal incisions. Nonetheless, the enhanced refractive effect of centripetal incisions supports previous studies 1,7-9,18 that found centripetal incisions create deeper incisions than centrifugal incisions in animals and human pathologic specimens. Berkeley and coauthors 12 suggested centripetal incisions produce more than 2.00 D greater effect than centrifugal incisions in a randomized trial cut short for ethical reasons. A higher percentage of Lee's RK group13 attained postoperative refractions between +2.00 and -2.00 D when incisions were made centripetally. In that study, centripetal incisions were judged to be more uniform and deeper. Subsequent unpublished clinical reports suggested that fewer incisions were used when blade settings were increased and centripetal incisions used. The heightened effect may be caused by mechanical factors 9 and possibly by differences in the wound-healing response with different wound configurations. 18 The R2 for our model was 0.42, which agrees closely with PERK results of 0.44.4 The R2 value ranged from 0.54 (24 month follow-up, steel blade) to 0.80 (1 month follow-up, steel blade) in a study by Arrowsmith and Mark. 5 The R2 values in the diamond blade population was 0.74 after 2 years. In studies by Sanders and co authors 6 and Deitz and coauthors,19 R2 varied between 0.56 and 0.73 depending on the postoperative period and the inclusion of interaction terms in the model. Several factors can explain why R 2 was smaller in our study than in other studies using stepwise regression. First, mean follow-up in our study was longer than in the other studies (Table 2). Several studies have confirmed that a postoperative hyperopic shift occurs so 920
that cases with longer follow-up might seem to have an enhanced reduction of myopia. 19 - 23 The surgeon's change in incisional technique beginning in 1986 introduces a follow-up bias that creates this confounding and weakens the relationship between spherical equivalent change and incision direction, lowering R2. Second, 33.6% of our group included AKs with transverse incisions that may disturb the effect of the radial incisions. 24 The other reports do not mention usingAKs in their analyses. Our preliminary results suggest that AK cases do not achieve the same refractive results as simple RK cases. Third, if the blade extensions were the same for both directional techniques, the centripetal group would have shown a larger effect in reducing the myopic error. In our study population, however, blade extension was set to 110 to 120% of paracentral readings for centrifugal incisions and 100% for centripetal. If this confounding variable had been controlled for in the final model, it could have increased the explained variability. N onetheless, the results suggest centripetal incisions reduced the myopic refractive error 0.87 D more than centrifugal InCISIOns.
Finally, incision number and enhancements may have contributed to a reduced R2. We included procedures with 4 and 8 incisions, whereas the other investigators used 8 and/or 16 incisions in the analyses. We also excluded enhancement cases. Deitz and coauthors 21 and Sanders and coauthori used secondary procedures in only two cases. The PERK group performed enhancements but did not use the data in their regression analysis. 4 Arrowsmith and coauthors25 ,26 also found redeepening of incisions a consistent predictive factor; they redeepened incisions when uncorrected visual acuity was worse than 20/40. Most of the other reported surgical variables do not differ significantly from ours (Table 2). It has always been the surgeon's (P.S.B.) goal to leave eyes undercorrected after surgery. When the clinical indication(s) suggested a possibility of achieving close to the desired refractive result with four incisions, four incisions were used. In many cases of undercorrection, enhancements were performed when the uncorrected visual acuity was worse than 20/40. In the current study, 18.50/0 offour-incision cases (17.4% centripetal, 22.2% centrifugal) had enhancements compared with 14.0% of the eight-incision cases (15.5% centripetal,
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EFFECT OF RK INCISION DIRECTION
Table 6. Summary of four RK studies using stepwise regression-final models.
Variable
Order
Incision number
Arrowsmith5 *
Sanders6
Current Study Coefficient
Order
Coefficient
Order
PERK4
Coefficient
Order
Coefficient
0.520
80nlyt
NA
-0.47
3
0.189
Optic zone
2
1.07
5
-0.289
6
IT
Age
3
-0.06
0.044
7
IT
2
-0.065
Incision direction
4
-0.87
NA
Entered
NA
Entered
NA
Entered
Planned depth
DNE
DNE
NA
NA
DNE
DNE
NA
NA
Achieved depth
NA
NA
2
0.077
2
-2.43
3
DNE
DNE
4
0.235
NA
NA
DNE
Average keratometry
-1.630
0.020 DNE
DNE = did not enter; NA = not applicable; IT = coupled as an interaction term * Diamond knife cases only; incision number dropped from model before final step
t Only 8 incision cases included
9.9% centrifugal). In a study by Salz and coauthors,27 15% of eyes had enhancements. The failure of planned incision depth (blade extension) to predict refractive outcome in this population may be a result of the systematic and random errors associated with setting the blade and performing the incisions. Arrowsmith and Marks 23 .28 found planned incision depth an inconsistent predictor in their populations, whereas optic zone diameter, age, and achieved incision depth consistently affected refractive outcome in both the steel blade and diamond blade populations. Neither Sanders and coauthors 6 not the PERK group4 used planned incision depth in their models; they did, however, find achieved depth to be associated with refraction change (Table 5). In addition to achieved depth, the PERK group 4 also established age and optical zone as important predictors of outcome. They controlled incision number by using only eight incisions and were thus unable to compare the effect on refractions. Table 6 shows the results of stepwise regression for these three other studies ofRK predictors compared with those of the current study. Sanders and coauthors6 concluded that keratometry (corneal power) was an important predictor of outcome in RK. They found that incision number, optic zone diameter, achieved incision depth, age, and keratometry readings entered their stepwise model at P < 0.15. In their data set, steep corneas (greater than 46.00 D) changed more than flat corneas (between 42.00 and 46.00 D) when all other variables were controlled. These
results are at odds with the PERK study,29 which found that flatter corneas changed more than steep corneas when curvature was measured with a corneascope. Recent studies of the accuracy and predictability of the corneascope have shown the instrument is too inaccurate for clinical measurement,30-33 casting doubt on the PERK study conclusions on predictability of preoperative corneal power on refractive outcome. Nevertheless, corneal curvature as measured with a Bausch & Lomb keratometer failed to reliably predict change in refraction in our population. The increased refractive effect of centripetal incisions is most evident in cases with smaller optical clear zones and more incisions (Figure 3). Construction of a second regression model that analyzed interaction supported the theory of an enhanced effect with centripetal incisions. The model with a univariate directional term is supplanted by bivariate interaction terms that coupled incision direction with diameter of the optical clear zone and incision number (ID X IN and ID X OZ). This finding suggests that the longer incisions are and the more incisions used, the greater their effect in reducing myopia. Thus, one can conclude that centripetal incisions potentiate the effect of smaller optical clear zones and greater numbers of incisions more than centrifugal incisions. The increase in R2 is modest, however, when the two models are compared. The univariate directional model increased R2 by 0.037 when it entered the model at the last step; it improved by only 0.045 after entry of the two interaction terms.
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EFFECT OF RK INCISION DIRECTION
Conclusion The direction of RK incisions is a more important predictor of outcome than planned incision depth and keratometry readings. Also, after adjusting for the effects of optic zone, incision number, and patient age, centripetal incisions increased the refractive change effect ofRK by 0.870 over centrifugal incisions in this population.
References
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