Transverse incisions for mixed and myopic idiopathic astigmatism

Transverse incisions for mixed and myopic idiopathic astigmatism James P. Pulaski, MD ABSTRACT Purpose: To determine the efficacy and safety of the Casebeer transverse nomogram for correcting idiopathic astigmatism using transverse incisions (T-incisions) with variable optical zones. Setting: Rancho Bernardo Eye Center, San Diego, California. Methods: This retrospective study comprised 173 eyes of 100 consecutive patients who had T-incisions according to the Casebeer transverse nomogram to correct mixed or myopic astigmatism. Preoperative cylinder ranged from 0.75 to 6.00 diopters (D). The nomogram was modified for eyes with preoperative cylinder from 1.75 to 6.00 D. Principal outcome measures were net residual astigmatism for all eyes and uncorrected visual acuity in eyes with the highest preoperative cylinder (3.25 to 6.00 D). Results: A comparison of preoperative astigmatism (mean 3.07 D ::': 1.53 [SD]) and net residual astigmatism showed significant reduction in all diopter categories. Enhancements were done in 16.2% of eyes, mostly those with high preoperative cylinder. Patient age did not seem to affect results. No patient, including those with smaller optical zones, lost two or more Snellen lines of best corrected visual acuity. Conclusion: Use of the Casebeer transverse nomogram with T-incisions and variable optical zones effectively and safely reduced most idiopathic astigmatism between 0.75 and 5.50 D. Modifications to the nomogram yielded similar results in eyes with a higher preoperative cylinder. J Cataract Refract Surg 1996; 22:307-312

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any patients who choose refractive surgery have visually significant astigmatism. In the past 15 years, several of the corneal procedures used to eliminate astigmatism (e.g., trapezoidal keratotomy, parallel radial incisions, and oval optical zones) were abandoned because of unpredictable results.l(p1098),2 To correct idiopathic astigmatism, most surgeons use arcuate or transverse incisions (T-incisions) placed on the highest plus cylinder. Lans3 first reported using T-incisions to correct astigmatism nearly 100 years ago. More recently, renewed interest in incisional keratotReprint requests to James P. Pulaski, MD, 16766 Bernardo Center Drive, Suite 112A, San Diego, California 92128.

omy has produced many clinical and laboratory studies. 4- 10 In addition, computer simulations using the finite element model have been used to predict the theoretical effect of corneal incisions on idiopathic astigmatism. Although useful, these models are not precise because they cannot mimic the human eye's variable healing and the unknown biomechanical constants of corneal tissue. II Binder and Waring l (pI193) recommend that studies be conducted based on type of astigmatism (e.g., idiopathic, postkeratoplasty, postcataract), allowing better evaluation of surgical methods and results. Several investigators including Naylor, 12 Alpins, 13 and Holladay and 14 coauthors have presented the mathematical basis for

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calculating the vectoral surgically induced refractive change, which gives the true magnitude and axis of surgically induced cylinder. The Casebeer transverse nomogram for astigmatism I5 (pI75) (Table 1) calls for an opposing pair of T-incisions placed at a selected optical zone depending on the amount of preoperative cylinder. Several refractive courses, including Casebeer's, now favor arcuate incisions at fixed 7.0 mm optical zones, but the superiority of one method has not been demonstrated. This study evaluated the results of T -incisions to correct idiopathic mixed or myopic astigmatism from 0.75 to 6.00 D using the variable optical zone method of the Casebeer transverse nomogram.

Subjects and Methods This retrospective study comprised 173 eyes of 100 consecutive patients who had T -incisions to correct idiopathic astigmatism according to the Casebeer nomogram. The 61 men and 39 women ranged in age from 21 to 68 years (mean 43 years ± 11.1 [SD]). Follow-up was 1 to 32 months (mean 7.2 ± 4.9 months). In addition to T-incisions, 167 eyes (96.5%) required a four- or eight-incision radial keratotomy (RK) to correct preoperative myopia. In the four-incision RK, the radials were rotated 45 degrees; in the eight-incision RK, the radials jumped the T-incisions as specified by the nomogram. Eyes were divided according to preoperative cylinder: Group 1,0.75 to 1.50 D (n = 112); Group 2, 1.75 to 3.00 D (n = 47); Group 3, 3.25 to 6.00 D (n = 14). Table 1. Casebeer nomogram for astigmatism with myopia using T-incisions. Attempted Correction

(0)

T-Marker (mm)

Optical Zone (mm)

1.00 1.25 1.50 2.00 2.25 2.62 3.00 3.50 4.00 4.50 5.00 to 5.50 5.75 to 6.00

2.5 3.0 2.5 3.0 2.5 3.0 2.5 3.0 2.5 3.0 2.5 3.0

7.00 7.00 6.00 6.00 5.75 5.75 5.50 5.50 5.25 5.25 5.00 4.50

308

After the optical zone was outlined with a Hoffer crosshair marker and the steep axis of the cylinder identified with a Mendez gauge, T -incisions were placed as a single opposing pair made with a front-cutting Russianstyle blade. The nomogram calls for optical zones from 4.5 mm to 7.0 mm and incision lengths of 2.5 mm to 3.0 mm to correct astigmatism from 1.00 to 6.00 D. Incision depth was 100% of pachymetry measured 1.5 mm temporal to the visual axis. Two modifications were made to the nomogram: (1) astigmatism of 0.75 D was added and treated with a pair of 2.0 mm T -incisions, with incision depth as above; (2) in Groups 2 and 3, incision depths were increased empirically to 100% pachymetry at the incision sites because previous use of the nomogram in eyes with high cylinders had led to undercorrection. Enhancements were performed if patients had at least 0.75 D of residual refractive cylinder and desired additional surgery for clearer acuity. If the postoperative axis shift was fewer than 20 degrees, enhancements were made by deepening each T -incision to 100% pachymetry at the incision site in Group 1 and by extending the original incision in 0.5 mm increments up to a maximum of 5.0 mm in Groups 2 and 3. If the axis shifted more than 20 degrees and new T-incisions were required, a second pair was added at the new steep axis after 1 month stabilization in accordance with the Casebeer" astigmatism alone" nomogram. The T -incision results were analyzed by grouping eyes in 0.25 D increments of preoperative refractive cylinder, from 0.75 to 6.00 D. The mean of the postoperative net residual astigmatism was determined without regard to axis for each 0.25 D increment to assess optical function and patient satisfaction. The magnitude of surgically induced cylinder in diopters was determined using the Hall table. 16 Because the table does not provide the surgically induced cylinder axis, surgically induced cylinder must be viewed in conjunction with the postoperative results (net residual astigmatism) to evaluate the efficacy of the nomogram and the surgery. In addition, postoperative visual acuity was measured in eyes with T-incisions with optical zones of 4.5 to 5.5 mm to see of smaller optical zones led to induced irregular astigmatism. The percentage of enhancements in each group was recorded and further analyzed by age because the Casebeer nomogram is not age adjusted.

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Results Postoperative net residual cylinder was significantly lower than mean preoperative cylinder in all three groups. Of all 173 eyes, 145 (83.8%) achieved a postoperative net residual cylinder between 0 and 0.50 D (Table 2). The highest postoperative cylinder, + 2.00 D, was in an eye with a preoperative cylinder of +6.00 D that had the maximum amount of surgery under the nomogram plus two enhancements. The three other patients with a postoperative cylinder from 1.26 to 2.00 D did not have enhancements because they were satisfied with the results of the initial surgery. Mean cylinder by category dropped from 3.07 :±: 1.53 D to 0.52 :±: 0.36 D (range 0 to 2.00 D); mean surgically induced cylinder was 2.88 D (Table 3). Table 2. Astigmatism reduction from T-incisions without regard to change in postoperative axis (N = 173). Number of Eyes Cylinder (0)

Preop

Postop

Plano to 0.50 0.60 to 1.25 1.26 to 2.00 More than 2.00

0 100 30 43

145 24 4 0

Table 3.

Of the 28 enhancements (16.2%) performed, most were in eyes with higher preoperative cylinders (Table 3, Figure l). Four eyes required more than one enhancement. Twenty-three enhancements were for undercorrections (axis within 20 degrees of original, residual cylinder of 0.75 D or more) and five for overcorrection (postoperative shift more than 20 degrees, residual cylinder of 0.75 D or more). Enhancements occurred across all age groups in a bell-shaped distribution similar to the distribution of the entire series. Mean age for all patients, 43.0 :±: 11.1 years, was similar to the enhancement group's mean age of 42.7 :±: 11.06 years (Figures 2 and 3). The five eyes that required additional T -incisions at a new axis were distributed equally across age groups. No serious complications occurred. Five micro perforations healed without complication. Edema around one incision induced 3.00 D of astigmatism; both the edema and astigmatism resolved after 3 weeks. Three inadvertent crossings of a radial and T -incision resolved without complication. Of eyes in which small optical zones (4.5 to 5.5 mm) were used, 11 had an uncorrected visual acuiry of20/25 or better, 2 of20/30, and 1 of20/50 (moderate

Results of T-incisions on 173 eyes by 0.25 D steps. Postoperative

Preop Cylinder + 0.75 +1 .00 +1 .25 +1 .50 +1.75 +2.00 +2.25 +2.50 + 2.75 +3.00 +3.25 +3.50 + 3.75 +4.00 + 4.25 +4.50 +5.00 +5.50 +6.00

Number of Eyes

Cylinder (Mean ± 50)

25 47 28 12 13 5 8 10 6 5 3 3

0.16 0.27 0.29 0.41 0.44 0.60 0.40 0.55 0.29 0.75 0.75 0.75

2

0.50 0.75 1.00

2.00

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± ::!: ± ::!: ± ± ± ± 0 ± ± ± 0 0 ±

0.22 0.34 0.39 0.43 0.42 0.28 0.32 0.46 0.40 0.75 0.43 0.25 0 0 0

0

Cylinder Range

Induced Cylinder (0) (Mean)

Enhancements Number (%)

0.68 0.96 1.25 1.34 1.69 2.10 1.99 2.30 2.64 2.41 2.60 2.90 3.75 4.50 4.30 4.90 5.00 5.50 4.00

1 (4) 3 (6) 0 2 (17) 1 (7) 1 (20) 2 (25) 2 (20) 2 (33) 4 (80) 3 (100) 2 (67) 0 1 (100) 1 (100) 1 (50) 0 0 2 (200)

o to 0.75 o to 1.25 o to 1.50 Oto 1.25

o to 1.00 0.25 to o to o to oto o to 0.50 to 0.50 to 0 0 0 0 0 0 0

1.00 0.75 1.50 1.00 1.50 1.25 1.00

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Discussion In recent years, there appears to be a shift toward arcuate incisions as the preferred method of correcting idiopathic astigmatism. Although Thornton 17 previously published a transverse nomogram, he now favors arcuate incisions. He postulates that arcuate incisions are T -incisions on an aspheric corneal surface and Tincisions are inverse arcuates with a transverse and radial component. Accordingly, T -incisions may reduce coupling through the radial component and produce an unwanted overall corneal flattening and hyperopia, especially with longer T-incisions. In the current study, 167 eyes (96.5%) required radial incisions for myopia in addition to T -incisions; thus, reduction in coupling was desired. In the six cases with mixed astigmatism and the five needing additional T-incisions, spherical equivalents were maintained at or close to preoperative levels using mostly 2.5 and 3.0 mm mCISIOns. Casebeer l5 has published transverse and arcuate nomograms but prefers arcuates because he believes T-incisions can be unpredictable, especially in pseudophakic and older patients. The arcuate incision does allow greater correction for a given optical zone because 310

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Ages (Years)

Preoperative Cylinder (D) Figure 1. (Pulaski) Number and percentage of enhancements related to the amount of preoperative cylinder by group.

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every point on the arc is the same distance from the optical center. The T -incision is tangent to the selected optical zone, and the surgical effect will decrease as the incision extends from the astigmatic axis. Bores l8 believes the T-incision has the greatest effect where it is needed most (i.e., at the exact axis of the corneal astigmatism) and that it is desirable to have less effect away from the principal axis to avoid overcorrection. Another consideration is that arcuate incisions are more difficult to make than T -incisions. Whether this will make arcuate results more variable remains to be seen. Newer instruments such as the trifaceted diamond and arc T blades facilitate arcuate construction. In contrast, T-incisions are easy to construct and amenable to enhancements by deepening and adding length in 0.5 mm increments. In my series of mostly myopic astigmatism, there was a high correlation between attempted and achieved correction for cylinders from 0.75 to 5.50 D. The Casebeer nomogram, however, tended to underpredict for higher cylinders, and nomogram modifications were made to increase the effect in eyes with a preoperative cylinder higher than 1.50 D. Still, of the 22 eyes with a cylinder above 3.00 D, 10 (45.5%) required at least one enhancement. Although the enhancement results were good in eyes with the highest preoperative cylinder, it may be that arcuate incisions would be more effective at

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the initial surgery. In a prospective evaluation of arcuate keratotomy, Price and coauthors 19 found the maximum effect of arcuate incisions to be 6.70 0 compared with the 5.50 0 in this study ofT-incisions. In my study, I used the Hall table 16 to determine the mean surgically induced cylinder. The formula of Holladay and coauthors 14 provides a more detailed analysis of surgically induced cylinder. It also gives the axis of the surgically induced cylinder, allowing surgeons to determine how closely that axis agrees with the preoperative or intended surgical axis. The calculations to determine surgically induced cylinder are tedious and not as important to the patient or surgeon as a small net residual astigmatism. Computer software is being developed to simplifY the calculations. Knowing the surgically induced cylinder axis can, however, help surgeons determine whether their techniques of measuring, marking, and incising the steep cylinder axis are on target. In addition, the surgically induced cylinder magnitude can show whether a nomogram produces accurate dioptric corrections. Because there are valid concerns that T -incisions with smaller optical zones can induce irregular astigmatism, I analyzed postoperative uncorrected visual acuity of eyes with 4.5 to 5.5 mm optical zones. More than 78% had an acuity of 20/25 or better, and the lowest acuity was 20/50 (n = l). No eye lost two or more Snellen lines of best corrected visual acuity. Thus, it

appears that in this small group, the smaller optical zones of the Casebeer transverse nomogram did not produce a significant amount of irregular astigmatism. My series had a 16.2% enhancement rate. Although reporting these data may obscure the primary surgery results, postenhancement data most closely simulate clinical practice, giving refractive surgeons an idea of how often an enhancement will be needed to achieve optimum results. In this study, because cylinder refinement is easy with T -incisions, enhancements were offered to patients with 0.75 0 or more of residual cylinder who wanted clearer acuity. Other practices may have a lower rate if they require higher amounts of residual astigmatic error for enhancements. Although the nomogram used in this study was not weighted for age, age did not appear to affect the results of the correction of natural cylinder by T -incisions. Perhaps the influence of age on corneal incisions is related to incision length and is not clinically significant in eyes with small incisions (2.5 to 3.0 mm). These results, however, should not be extrapolated to the elderly pseudophakic patient, who often requires only a single Tincision for a satisfactory surgical result. Also, with longer arcuate incisions and certainly radial incisions, age weighting becomes more important. Using the Lindstrom arcuate nomogram and multiple regression analysis, Price and coauthors 19 found 0.0363 0 more effect with arcuate incisions for each additional year of age. In the multicenter study of Price and coauthors, 19 160 eyes of 95 patients had arcuate incisions by nine different surgeons according to the Lindstrom nomogram. Postoperatively, 39% of eyes had less than 1.000 of residual cylinder and 28% had 1.50 0 or more; 5% had between 3.00 and 5.00 0 of residual cylinder after arcuate keratotomy. The postoperative period was 1 month, and no enhancement results were reported. The authors state that the nomogram tended to underpredict for many patients (64%); the final mean cylinder was 1.20 0 for all patients. This study demonstrated that there is still considerable variability in the results of arcuate incisions, and the authors suggest further guidelines and nomogram refinements. In summary, the Casebeer transverse nomogram was useful in correcting idiopathic astigmatism from 0.75 to 5.50 D. Modifications in the nomogram were made to improve the results in eyes with higher preop-

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erative cylinders. Although there has been a recent trend toward arcuate incisions, further clinical studies comparing these surgical methods are needed. It is important to know through what dioptric range each method is effective, the ease of construction and reproducibility of arcuate versus T -incisions, and the potential of each to manage undercorrections and overcorrections. Complications related to wound gape, overcorrection, and irregular astigmatism must be assessed for each surgical method.

10.

11.

12.

References 1. Binder PS, Waring GO III. Keratotomy for astigmatism. In: Waring GO III, ed, Refractive Keratotomy for Myopia and Astigmatism. St Louis, Mosby-Year Book, 1992 2. Fenzl RE. Control of astigmatism using corneal incisions. In: Sanders DR, Hofmann RF, eds, Refractive Surgery: A Text of Radial Keratotomy. Thorofare, NJ, Slack, Inc, 1985; 160 3. Lans LJ. Experimentelle Untersuchungen tiber Entstehung von Astigmatismus durch nicht-perforirende Corneawunden. Albrecht Von Graefes Arch Ophthalmol 1898; 45:117-152 4. Merlin U. Curved keratotomy procedure for congenital astigmatism. J Refract Surg 1987; 3:92-97 5. Duffey RJ, Jain VN, Tchah H, et al. Paired arcuate keratotomy: a surgical approach to mixed and myopic astigmatism. Arch Ophthalmol1988; 106:1130-1135 6. Thornton SP, Sanders DR. Graded nonintersecting transverse incisions for correction of idiopathic astigmatism. J Cataract Refract Surg 1987; 13:27-31 7. Lindstrom RL. The surgical correction of astigmatism: a clinician's perspective. Refract Corneal Surg 1990; 6:441-454 8. Binder PS. Surgical correction of astigmatism. In: Cor-

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nea, Refractive Surgery, and Contact Lenses; Transactions of the New Orleans Academy of Ophthalmology. New York, Raven Press, 1987; 1-9 Nordan LT. Quantifiable astigmatism correction: concepts and suggestions, 1986. J Cataract Refract Surg 1986; 12:507-518 Neumann AC, McCarty GR, Sanders DR, Raanan MG. Refractive evaluation of astigmatic keratotomy procedures. J Cataract Refract Surg 1989; 15:25-31 Hanna KD, Jouve FE, Waring GO III. Preliminary computer simulation of the effects of radial keratotomy. Arch Ophthalmol1989; 107:911-918 Naylor EJ. Astigmatic difference in refractive errors. Br J Ophthalmoll968; 52:422-425 Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg 1993; 19:524-533 Holladay JT, Cravy TV, Koch DO. Calculating the surgically induced refractive change following ocular surgery. J Cataract Refract Surg 1992; 18:429-443 Casebeer J C. Casebeer Incisional Keratotomy. Thorofare, NJ, Slack, Inc, 1995 Waring GO III, Holladay JT. Optics and topography of corneal astigmatism. In: Waring GO III, ed, Refractive Keratotomy for Myopia and Astigmatism. St Louis, Mosby-Year Book, 1992; 1080-1081 Thornton SP. Astigmatic keratotomy: a review of basic concepts with case reports. J Cataract Refract Surg 1990; 16:430-435 Bores LD. Refractive Eye Surgery. Boston, Blackwell Scientific Publications, 1993; 319 Price FW, Grene RB, Marks RG, Gonzales JS. Astigmatism reduction clinical trial: a multicenter prospective evaluation of the predictability of arcuate keratotomy; evaluation of surgical nomogram predictability. Arch Ophthalmol1995; 113:277-282; correction 577

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